MEMS Monocrystalline-Silicon Based Thermal Devices

for Chemical and Microfluidic Applications

Marko Mihailović

MEMS Monocrystalline-Silicon Based Thermal Devices

for Chemical and Microfluidic Applications

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus prof. ir. K. C. A. M. Luyben, voorzitter van het College voor Promoties, in het openbaar te verdedigen op

woensdag 29 juni 2011 om 15.00 uur

door

Marko MIHAILOVIĆ

Diplomirani inženjer elektrotehnike Universiteit van Belgrado, Servië geboren te Belgrado, Joegoslavië

Dit proefschrift is goedgekeurd door de promotor:

Prof. dr. ir. P. M. Sarro

Samenstelling promotiecommissie:

Rector Magnificus, voorzitter Prof. dr. ir. P. M. Sarro, Technische Universiteit Delft, promotor Prof. dr. P. J. French, Technische Universiteit Delft Prof. dr. R. A. M. Wolters, Universiteit Twente Prof. dr. Z. Đurić, Univerzitet u Beogradu, Servië Prof. dr. S. Franssila, Aalto University, Finland Dr. ir. J. F. Creemer, Technische Universiteit Delft Dr. ir. C. M. Rops, TNO Eindhoven Prof. dr. ir. C. I. M. Beenakker, Technische Universiteit Delft, reservelid

The research presented in this thesis was financially supported by The Dutch Technology Foundation STW (project DET.7213).

Marko Mihailović, MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications, Ph.D. thesis, Delft University of Technology, with a summary in Dutch.

Keywords: MEMS, monocrystalline silicon, bulk micromachining, wafer bonding, resistive heating, two-phase flow, micro-evaporation, micro-propulsion, high- temperature resistance measurements.

An electronic version of this dissertation is available at http://repository.tudelft.nl/.

ISBN: 978-90-5335-430-8

Copyright © 2011 by Marko Mihailović, All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means without the prior written permission of the copyright owner.

Cover: sunset at Porto Giunco, near Villasimius, Sardinia; photo by: Marko Mihailović

Printed on FSC-certified, chlorine-free, environmentally-friendly Biotop paper.

Printed by Ridderprint B.V., Ridderkerk, the Netherlands.

MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Table0B of contents

TABLE OF CONTENTS 1

1. INTRODUCTION 5

1.1. MINIATURIZATION 6 1.2. THERMAL MICRO DEVICES 7 1.2.1. HEATERS 7 1.2.2. MATERIAL CONSIDERATIONS FOR HEATERS 8 1.2.3. HEAT SINKS AND HEAT SPREADERS 8 1.2.4. MATERIAL CONSIDERATION FOR HEAT SINKS 10 1.2.5. MONOCRYSTALLINE SILICON 11 1.3. THESIS OUTLINE 11 REFERENCES 13

2. HEAT AND HEAT TRANSFER 15

2.1. HEAT 16 2.2. MODES OF HEAT TRANSFER 16 2.2.1. CONDUCTION 17 2.2.2. CONVECTION 18 2.2.3. RADIATION 19 2.3. RESISTIVE (JOULE) HEATING 20

1 Table of contents

2.4. CONCLUSIONS 22 REFERENCES 22

3. TEMPERATURE DEPENDENCE OF SILICON ELECTRICAL AND THERMAL PROPERTIES 23

3.1. THERMAL CONDUCTIVITY OF SILICON 24 3.2. ELECTRICAL RESISTIVITY OF SILICON 25 3.3. ELECTRICAL RESISTIVITY CHARACTERIZATION 27 3.3.1. SAMPLE PREPARATION 28 3.3.2. MEASUREMENT SET-UP 30 3.3.3. MEASUREMENT RESULTS AND DISCUSSION 31 3.4. CONCLUSIONS 34 3.5. REFERENCES 35

4. MICRO-HOTPLATE HEATER 37

4.1. INTRODUCTION 38 4.2. DESIGN 39 4.3. FABRICATION 41 4.3.1. PACKAGING 44 4.4. MEASUREMENTS 45 4.4.1. MEASUREMENT SET-UP 45 4.4.2. ELECTRICAL CHARACTERIZATION OF THE HEATERS 46 4.4.3. RESISTANCE CALIBRATION OF SILICON HEATERS 48 4.4.4. CALIBRATION OF TISI/TIN STACK 49 4.5. DISCUSSION 50 4.5.1. DEVICE PERFORMANCE 50 4.5.2. STRESS AND BUCKLING CONSIDERATIONS 51 4.6. CONCLUSIONS 52 REFERENCES 53

5. MICRO-EVAPORATOR 55

5.1. HEAT SINKS WITH LIQUID FLOW AND THEIR APPLICATIONS 56 5.2. DESIGN OF THE MICRO-EVAPORATOR 57 5.2.1. WORKING PRINCIPLE 58 5.2.2. THE CHANNEL-FIN STRUCTURE 58 5.2.3. EMBEDDED HEATER 62 5.3. FABRICATION AND PACKAGING 62 5.4. MEASUREMENTS 66 5.4.1. MEASUREMENT SET-UP FOR HEATER CALIBRATION 66

2 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

5.4.2. FLUIDIC MEASUREMENT SET-UP 66 5.4.3. FLOW RATE AND POWER DISSIPATION 68 5.5. ALL-SILICON DEVICES 69 5.5.1. FABRICATION 69 5.5.2. CHARACTERIZATION OF TYPE A DEVICES 72 5.6. SILICON-GLASS DEVICES 74 5.6.1. FABRICATION 74 5.6.2. CHARACTERIZATION OF TYPE B DEVICES 76 5.7. SILICON-GLASS DEVICES WITH THERMAL CAVITY 80 5.7.1. FIN-CHANNEL DESIGN 80 5.7.2. THERMAL CAVITY 80 5.7.3. FABRICATION OF TYPE C DEVICES 81 5.7.4. CHARACTERIZATION OF TYPE C DEVICES 83 5.8. CONCLUSIONS 84 REFERENCES 85

6. RESISTOJET MICRO-THRUSTER 87

6.1. INTRODUCTION 88 6.2. DESIGN 89 6.2.1. WORKING PRINCIPLE 89 6.2.2. CHANNEL AND NOZZLE 90 6.2.3. HEATER 91 6.3. FABRICATION 92 6.4. MEASUREMENT SETUP 96 6.5. MEASUREMENT RESULTS 98 6.6. DISCUSSION 99 6.7. CONCLUSIONS 100 REFERENCES 101

7. CONCLUSIONS 103

7.1. CONCLUSIONS 104 7.1.1. SILICON USED FOR HEATING 104 7.1.2. SILICON USED FOR SPREADING THE HEAT 105 7.1.3. COMMON TECHNOLOGY PLATFORM FOR MICROFLUIDIC DEVICES 106 7.2. OUTLOOK 108

APPENDIX A 111

A.1. MICRO-HOTPLATE FABRICATION FLOW CHART 111 A.2. MICRO-EVAPORATOR FABRICATION FLOW CHART 112

3 Table of contents

A.3. MICRO-THRUSTER FABRICATION FLOW CHART 114

LIST OF ABBREVIATIONS 116

SUMMARY 117

SAMENVATTING 119

LIST OF PUBLICATIONS 123

ACKNOWLEDGEMENTS 125

ABOUT THE AUTHOR 129

4 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

1.1B Introduction

In this chapter applications that need thermal micro devices are briefly introduced. The limitations of commonly used materials for micro heaters and micro heat exchangers are addressed. Monocrystalline silicon is identified as suitable choice for these devices. The layout of this thesis is also given.

5 1. Introduction

1.1.B Miniaturization

Downscaling of microelectronic device dimensions results in faster, more functional and cheaper devices. This downscaling follows the well-known Moore’s law [1]. Next to the advances in (nowadays mainly digital) electronics, which are driven by the industry, there is a big need to expand the functionalities of these electronic systems. Adding new functions like sensing non-electrical quantities in the environment opens new applications with broad industrial and social impact. The combined need for digital and non- digital functionalities in an integrated system is translated as a dual trend in the International Technology Roadmap for Semiconductors (ITRS): miniaturization of the digital functions (“more Moore”) and functional diversification (“more than Moore”), as shown in figure 1.1 [2]. Combining digital electronics with different functional elements, such as passive components, microfabricated sensors and actuators, analog and RF electronics modules, leads to the so-called system-in-package (SiP) solutions. All of those non-digital functions are also taking the road of miniaturization. Smaller physical dimensions lead to downscaled extensive quantities of the systems (e.g. response time and power consumption are drastically reduced) compared to their larger-scale equivalents. Smaller devices usually have lower production costs, due to the higher throughput possibility related to batch fabrication.

Figure 1.1. “More Moore” (miniaturization of the digital functions) and “more than Moore” (functional diversification) concepts (source: [2])

6 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Miniaturized SiP solutions are very powerful and efficient in automotive sensing, consumer electronics, data storage, healthcare products, (bio)chemical analysis and more. Among the numerous microsystems and devices, there are many that possess distinct requirements for heat generation or heat removal. E.g. actuation of microscale devices and structures can be achieved by injecting and removing heat. Changes in temperature profile generate mechanical displacement or force output through thermal expansion, contraction or phase change. Miniaturization brings different constrains on how the required heat generation and/or removal can be done. To address these problems, a list of design adaptations, from material and geometry selection to choosing the right fabrication sequence, need to be performed [3].

1.2.16B Thermal micro devices

Within this thesis, the term thermal micro devices is employed to describe miniaturized devices meant to generate or sense an increase or decrease of temperature. Temperature of a microstructure can be raised by absorption of electromagnetic waves, resistive heating, conductive heating, convective heating and exothermic chemical processes. Cooling can be achieved by conductive, convective and radiant dissipation, or by active thermoelectric cooling.

1.2.1.6B Heaters

Many micro systems and devices require elevated temperatures at localized spots in the chip, such as gas sensors, flow sensors, calorimeters, infrared sources, thermal actuators and nano-reactors [4-11]. Heaters are employed to provide the desired operating conditions [6] or to simulate and/or compensate for some specific external conditions [10]. For practical reasons, resistive heaters are very suitable for most of the applications as they are reliable and simple to realize in a broad set of well- established microfabrication processes. They can also be employed to

7 1. Introduction substitute a heat source in devices and systems when it is too complex or too expensive to use other types of heaters for testing a certain configuration or concept. After the principle is proven, resistive heaters can be replaced with the heaters used in real applications. Temperature differences achieved by on-chip integrated heaters can vary from very low (fractions of 1 °C) to very high (hundreds or thousands of °C). A common characteristic to most of the employed micro heaters is fast response time (order of μs-ms) and reduced power consumption, due to the small mass and optimized geometry to minimize thermal losses.

1.2.2.57B Material considerations for heaters

Typical materials, either semiconductors or metals, used for micro-heaters are in thin film form and have a polycrystalline structure [6,8,11]. Most of these thin films exhibit some drawbacks. First, due to their polycrystalline nature, they suffer from different degradation problems at elevated temperatures, which cause deviations from the expected material properties. Furthermore, their material properties are different from the properties of the same materials in a bulk form. Those properties are generally more difficult to model (compared to the ones of single crystals), and on top of that, they are usually not known, and are fabrication process dependent. That means that if those layers are formed (grown or deposited) under different process conditions, although the material structure is nominally the same, the material properties can vary significantly. Additionally, some of the used materials are not (fully) compatible with mainstream silicon IC-technology, which makes it difficult to integrate them with e.g. on-chip electronic circuitry. Therefore, it is important to identify a heater material which maintains its stable, well defined properties when in a thin film form and for a wide temperature range and that can be integrated with electronics.

1.2.358B Heat sinks and heat spreaders

There are many devices that, as side-effect of their applications, increase the local or global temperature of the system. In addition to the power dissipation of digital ICs, EM radiation sources (such as lasers and high-power

8 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

LEDs) and modern displays, there are processes in non-electronic devices that can elevate the temperature as well. Benefits of smaller scale processing (such as laminar flow and easier process modeling, less consumption of frequently expensive or dangerous reactants, faster processing times, possibility of parallelization) brought to the recent development of microreactors for chemical and pharmaceutical applications which very often release heat during the reactions [12]. Although (due to miniaturization) the total power dissipation of the microsystems is being lowered or kept at the same level, there is a big increase in the energy density, leading to elevated temperatures on the chip. In most of the applications, it is very important to efficiently remove the excess heat in order to prevent overheating and thus to allow proper functioning of the microsystems. Keeping temperature within certain limits is particularly challenging for e.g. on-board space applications, due to the absence of the ambient medium. Heat sinks and heat spreaders are heat exchanger type of devices that help in draining the heat away from a “hot spot”, i.e. they are “cooling” devices. They are usually made of solid materials with a high thermal conductivity. They are designed in a way to increase the surface area in contact with the surrounding cooling fluid, resulting in arrays of (high aspect ratio) fins (figure 1.2). To improve cooling performance, usually a fan is operated above the heat sink, exploiting the benefits of forced convection cooling.

Figure 1.2. Different finned heat sink designs, applied for cooling microprocessors [source: 13]

Using larger-scale heat sinks and ambient air as cooling fluid is a simple, cheap and reliable solution. However, as illustrated in figure 1.3, further miniaturization (“more Moore”) and stacking chips on top of each other (“more than Moore”) lead to much higher power densities which require new heat sink solutions. The only way to address this problem and achieve more efficient cooling is to employ liquids as coolants, instead of air or gas. If the phase change of the coolant is utilized, even more efficient cooling may be achieved.

9 1. Introduction

Figure 1.3. Examples of devices and configurations for which a large amount of heat needs to be dissipated [source: 14]

However, miniaturizing systems that utilize liquid coolants is not trivial. For example there is great interest in miniaturization of evaporators like the one shown in figure 1.4, for cooling compact reactors for e.g. pharmaceutical applications.

Figure 1.4. Demonstrator of a compact (~6cm x1 cm) evaporator fabricated in steel [source: 15]

1.2.4.59B Material consideration for heat sinks

Larger-scale heat sinks, which are in millimeter-centimeter scale, are successfully fabricated in metals such as steel, copper or aluminum. Miniaturization of such devices faces severe fabrication difficulties. Therefore, it is not possible, or not practical, to directly scale down the dimensions of the structures produced in these materials. An alternative solution is provided by the use of a material in which accurate, well defined microstructures can be

10 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications fabricated. These microstructures should be placed very close to or even in the devices itself. In this way, the heat transfer rate will be significantly increased.

1.2.5.60B Monocrystalline silicon

Monocrystalline silicon is a single crystal material, with known and stable properties, even if thinned down to micrometer thin sheets. Employing monocrystalline silicon as an active functional layer allows better reproducibility of the devices and better modeling and theoretical prediction of its properties. Moreover, monocrystalline silicon is the most common material in microelectronics and microsystems industry, for active components as well as structural material. Being a semiconductor, it is possible to significantly alter its electrical properties. Its resistivity can be changed within more than six orders of magnitude, a characteristic which is well exploited in a wide range of microelectronic devices. To obtain the desired electrical properties of the silicon layers, as needed for heating and sensing applications, specific dopant types can be added and different concentrations achieved by doping of silicon (most often by ion implantation and epitaxial growth, but also possible by thermal diffusion). Also, very thin monocrystalline silicon layers are available in silicon-on- insulator (SOI) wafers. It is also possible by means of wafer bonding to transfer silicon structures to other types of substrates, such as glass or polymers. These considerations – monocrystalline nature, wide availability of the material for microfabrication processes, and versatile silicon microstructuring processes – qualify monocrystalline silicon as a very suitable functional material for various thermal micro devices investigated in this thesis.

1.3.17B Thesis outline

This thesis exploits the use of monocrystalline silicon in microfabricated devices intended for resistive heating or cooling by heat conduction through silicon.

11 1. Introduction

In chapter 1 applications that need thermal micro devices, micro heaters and micro heat exchangers, are briefly introduced. The shortcomings of commonly used materials are listed, and monocrystalline silicon is identified as an appropriate choice for several thermal micro devices. Chapter 2 briefly presents the basic theory on resistive heating and heat transfer and how they relate to the devices and structures presented in the following chapters. Chapter 3 summarizes the temperature dependence of the electrical and thermal properties of monocrystalline silicon in a wide temperature range, from room temperature to almost 1000 °C. The experimental electrical characterization of As-doped thin silicon layers is also described. Such layers can be employed in silicon sensors and specifically in heaters with sensing capability. Chapter 4 introduces a micro-hotplate heater capable of reaching temperatures up to 800 °C. The used materials are epitaxial Si for the heater and TiSi for the contact lines, which provides compatibility with CMOS technology. Devices with differently doped silicon layers were fabricated and tested. Chapter 5 presents a silicon-based micro-evaporator. This is a cooling device intended for dealing with high heat fluxes caused by intensive local heating. Four different proposed fin-channel structures, to be compared in respect to operation stability of the device, were fabricated in silicon with MEMS technology. In order to mimic an external heat source in real-life application, these demonstrator devices contain monocrystalline silicon heaters. Chapter 6 presents a miniaturized resistojet thruster device with an integrated thin-film heater, capable of delivering thrusts in the micronewton– millinewton range. In this device, silicon acts as a heat spreader from the integrated aluminum heater to the propellant flow inside the microchannel etched in silicon, to reduce propellant consumption. In chapter 7, concluding remarks are given, together with recommendations for further research.

12 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

References18B

[1] R.W. Keyes, "The Impact of Moore's Law," Solid-State Circuits Newsletter, IEEE , vol.20, no.3, pp.25-27, Sept. 2006 [2] G.Q. Zhang, M. Graef, A. van Roosmalen, More than Moore - Creating High Value Micro/Nanoelectronics Systems, 1st Edition., 2008, Springer Verlag, Berlin [3] R. Ponnappan, "A novel micro-capillary groove-wick miniature heat pipe," Energy Conversion Engineering Conference and Exhibit, 2000. (IECEC) 35th Intersociety, vol.2, pp.818-826, 2000 [4] S. Franssila, S. Marttila, K. Kolari, P. Ostman, T. Kotiaho, R. Kostiainen, R. Lehtiniemi, C.-M. Fager, J. Manninen, "A Microfabricated Nebulizer for Liquid Vaporization in Chemical Analysis”, Journal of Microelectromechanical Systems, vol.15, no.5, pp. 1251- 1259, Oct 2006 [5] M. Dijkstra, “Low-drift micro flow sensors”. PhD thesis (2009), University of Twente. [6] J.F. Creemer, S. Helveg, P.J. Kooyman, A.M. Molenbroek, H.W. Zandbergen, P.M. Sarro, “A MEMS Reactor for Atomic-Scale Microscopy of Nanomaterials Under Industrially Relevant Conditions”, Journal of Microelectromechanical Systems, vol.19, no.2, pp.254-264, April 2010 [7] G. Hetsroni, A. Mosyak, E. Pogrebnyak, R. Rozenblit, “Infrared temperature measurements in micro-channels and micro-fluid systems”, International Journal of Thermal Sciences, Volume 50, Issue 6, June 2011, Pages 853-868 [8] G. Guenther, E. Aulbach, H. Hahn, O. Guillon, “High-temperature chip calorimeter with atmosphere control”, Thermochimica Acta, March 2011, doi:10.1016/j.tca.2011.03.013 [9] A. Jain, K.E. Goodson, “Thermal microdevices for biological and biomedical applications”, Journal of Thermal Biology, March 2011 [10] J.C. Salvia, R. Melamud, S.A. Chandorkar, S.F. Lord, T.W. Kenny, “Real-Time Temperature Compensation of MEMS Oscillators Using an Integrated Micro-Oven and a Phase-Locked Loop”, Journal of Microelectromechanical Systems, vol.19, no.1, pp.192-201, Feb. 2010 [11] W. Qu, W. Wlodarski, “A thin-film sensing element for ozone, humidity and temperature”, Sensors and Actuators B: Chemical, Volume 64, 2000, Pages 42-48 [12] G. Kolb, V. Hessel, “Micro-structured reactors for gas phase reactions”, Chemical Engineering Journal, Volume 98, Issues 1-2, 15 March 2004, Pages 1-38, ISSN 1385- 8947 [13] http://commons.wikimedia.org/wiki/File:Pin_fin,_straight_fin_and_flared_heat_sinks.png [14] ElectroIQ – The Portal for Electronics Manufacturing: “IBM to use water cooling

13 1. Introduction

for future 3D IC processors”, March 18, 2011 [15] Courtesy of C.M. Rops, TNO Science & Industry

14 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

2.2B Heat and heat transfer

In this chapter the basic theory on resistive heating and heat transfer is briefly presented. Material properties (electrical resistivity and thermal conductivity) and other parameters (heat transfer coefficient) are related to the processes of heat generation and heat removal used in the devices described in this thesis.

15 2. Heat and heat transfer

2.1.19B Heat

Heat (or heat transfer) is defined as a process of energy transfer from one body or system due to thermal contact and temperature gradient or differences. However, heat is sometimes defined as the energy of a body that increases with its temperature. In case heat is referred to the “dynamic” part of energy (assuming there is a process of energy transfer between one body and another), the term internal energy (or thermal energy) is used to define the “static” quantity [1]. Internal energy is the sum of all forms of energy of a system. It is related to the molecular structure and the degree of molecular activity and may be viewed as the sum of kinetic and potential energies of the molecules. The total energy can be decomposed in two components. One component is directly proportional to the temperature of the body or the system, and the other can be released or absorbed during the phase change that occurs without changing the temperature, such as condensation of water vapor or melting of ice. The latter component is usually referred to as latent energy as it cannot be sensed by monitoring temperature. The temperature dependent component is the product of the body mass, its specific heat capacity and its temperature above a reference temperature. The amount of heat added or removed can be measured by a change of temperature, e.g. in a calorimeter [2]. Temperature is a measure of the average energy of motion, or kinetic energy, of particles in matter. When particles, whether in solids, liquids or gases, move faster or have greater mass, they carry more kinetic energy, and the material appears warmer than a material with slower particles or particles with lower mass. Heat always flows from regions of higher temperature to regions of lower temperature [1].

2.2.20B Modes of heat transfer

There are three basic modes of heat transfer: heat conduction, heat

16 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications convection and thermal radiation (see figure 2.1).

Figure 2.1. Illustration of different modes of heat transfer (source: [3]).

2.2.1.61B Conduction

Conduction is heat transfer through a matter due to the temperature gradient. It can be present in solids, liquids and gasses, but does not involve any bulk motion of the matter. The phenomenological law, at a macroscopic level, governing heat conduction is the Fourier’s law of heat conduction [1]. Its most simple form, for one-dimensional problem and linear and uniform heat distribution determines the heat Q transferred through the surface area A along the distance ΔL due to the temperature difference ΔT: QkAT= ⋅⋅ΔΔ/ L (2.1) The constant of proportionality k is named thermal conductivity, and is a material property. It depends on the microscopic structure of the material and is temperature dependent. Materials with high thermal conductivity values can be used for removing heat by conduction and can be used for the cooling devices Table 2.1 reports the thermal conductivity at room temperature for different (classes of) materials. Generally speaking, thermal conductivity is related to the electrical conductivity, and good electrical conductors are good thermal conductors. As observed in table 2.1, fluids, polymers, wood and glass are distinct thermal insulators, while metals are good thermal conductors. One of the few exceptions is diamond, electrical insulator but an excellent thermal

17 2. Heat and heat transfer conductor. Also, silicon (monocrystalline) has high thermal conductivity parameter and is positioned in this table among various metals. Therefore, silicon is suitable for heat spreading applications and bad for thermal insulation. Values for thermal conductivities of materials often used in microsystem fabrication are given in section 3.1 of this thesis, when the temperature dependence of this parameter for silicon is discussed.

Table 2.1 Values of thermal conductivity at 300 K for selected materials (source: [1])

Material k [W m-1 K-1] Material k [W m-1 K-1] Air 0.027 Mercury 8.4 Fiberglass 0.038 Lead 35 Cork 0.043 Steel 64 PVC 0.092 Brass (70% Cu, 30% Zn) 111 Pine wood 0.1 Silicon 148 Engine oil 0.145 Aluminum 204 Water 0.611 Copper 386 Pyrex glass 1.09 Silver 429 Concrete 1.4 Diamond 1300

2.2.2.62B Convection

Strictly speaking, convection is the transport of energy by the bulk motion of a medium, and therefore not exactly a distinct mode of heat transfer. However, common engineering practice is to use the term convection more broadly and describe heat transfer from a surface to a moving fluid also as convection or convective heat transfer, even though conduction and radiation play a dominant role [1]. The rate of heat transfer by convection is usually a complicated function of temperature, surface geometry and fluid properties. However, this dependence can be reduced to the simple relation between the heat Q (transferred through the surface area A), and the temperature difference ΔT between the surface and the fluid:

QhAT= c ⋅⋅Δ (2.2)

This relation is often called Newton’s law of cooling, with the constant of proportionality named the convective heat transfer coefficient (hc):

18 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

So-called natural (or free, or passive) convection is the transfer from a surface into a stationary fluid. Temperature gradient in fluid induces local flow movement through buoyancy. The movement of fluid mass facilitates heat transfer. As opposed to that, forced convection is the heat transfer to a moving fluid. The bulk fluid movement provides enhanced heat transfer compared with that of natural convection. Heat transfer coefficients for free and forced convection in air and water are given in table 2.2, together with heat transfer coefficients for some of the phase changes.

Table 2.2. Comparison of the heat transfer coefficient for liquids and gasses (source:[1])

Material and process h [W m-2 K] Air, free convection 3 – 25 Air, forced convection 10 – 200 Water, free convection 15 – 1000 Water, forced convection 50 – 10000 Condensing steam 5000 – 50000 Boiling water 3000 – 100000

From table 2.2 it is clear that, by employing liquids instead of gasses, (forced) convective heat transfer is 1-2 orders of magnitude more efficient. Additional improvement in heat transfer is achieved by phase change, as employed in the device described in chapter 5 of this thesis. Therefore, using liquids as coolants with the sustained flow, increasing the aspect ratio of cooling/heating fins (in order to increase the surface area while preserving the footprint) is a preferred way for addressing cooling challenges.

2.2.3.63B Radiation

All matter emits thermal radiation, originating from thermal motion of charged particles in matter. The radiant energy emitted from a surface of grey body depends on surface temperature T, surface area A, emissivity of the material ε, and is given by the Stefan-Boltzmann law: EAT= ε σ 4 (2.3) where σ is Stefan-Boltzmann constant (σ = 5.67∙10-8 Js-1m-2K-4). Energy incident to a surface is also proportional to the forth power of the temperature of its source. The net heat transfer from surface 1 to surface 2 is therefore proportional to the difference of temperatures to the forth power:

19 2. Heat and heat transfer

44 QATT12=⋅⋅⋅εσ 1 1() 1 − 2 (2.4) The flux of radiant heat transfer can be viewed in terms of electromagnetic waves or in terms of photons. Anyway, it is clear that there is no medium required for this mode of heat transfer, making it the only heat transfer mode in vacuum. In most of practical applications, in ambient and for temperature range up to 300 °C, radiant heat transfer is negligible compared to conductive and convective heat transfer.

2.3.21B Resistive (Joule) heating

Resistive heating (or Joule heating, named after James Prescott Joule who was first to study this phenomenon) is the process by which the flow of an electric current through a medium releases heat. In his studies, Joule deduced that the heat Q generated by a constant current I flowing through a conductor of electrical resistance R, for a time t equals:

2 QIRt= ⋅⋅ (2.5) This relationship is known as Joule's law. The SI unit of energy was subsequently named the joule and given the symbol J. The commonly known unit of power, the watt, is equivalent to one joule per second. Joule heating is caused by interactions between the moving particles that form the current and the atomic ions of the conductive medium. Joule's law and Ohm's law are equivalent and derivable from each other, though they were discovered independently and experimentally. Joule heating forms the basis for the myriad of practical applications involving electric heating. However, in applications where heat generation is an unwanted by-product of the current flow, the dissipation of energy is often referred to as resistive losses (or heat losses). Resistance of the resistor/conductor is a macroscopic parameter, depending on the material properties, but also on the geometry of the device. Material property which is responsible for electrical resistance is called resistivity (most commonly denoted as ρ). In a resistor (for simplicity, of prismatic shape), the relation between resistance (R), resistivity (ρ) and geometry (L – length, W – weight, H – height) is given as:

20 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

R = ρ·L / (W·H). (2.6) In microfabrication, where thin films are often used, film thickness is fixed to a certain value (defined by deposition or growth process) and the remaining variable quantity is L/W (the ratio between the length and width of the resistor) which can be altered in the layout design. The fixed term is then corresponding to the resistance of the square resistor and is therefore named square resistance (or sheet resistance, as it is related to a sheet of determined thickness):

R□ = ( ρ / H ) ∙( L / W ) = ( ρ / H ) ∙ 1 = ρ / H . (2.7) Change of the (sheet) resistance with temperature is influenced by the temperature dependence of resistivity. In general, electrical resistivity of materials is a complex function of temperature and can be expanded to a Taylor’s series form. However, for most of the materials of practical interest, they can be modeled by a linear model:

RT()= R0 (1+⋅Δα T ) (2.8) where α is a parameter named temperature coefficient of resistance – TCR: 1 ⎡δ R ⎤ α = (2.9) RT⎢δ ⎥ 0 ⎣ ⎦TT= 0 In table 2.3 TCR values for different materials are given. A higher TCR means that the resistance change is larger for the same temperature increase and can, therefore, be beneficial for temperature sensing.

Table 2.3 Comparison of the temperature coefficient of resistance for metals (source: [4])

Material TCR [ppm / K] Aluminum 3600 Chromium 3000 Copper 3900 Gold 8300 Nickel 6900 Platinum 3927

Semiconductors have a more complex temperature dependence of the resistivity than metals. Electrical resistivity of silicon and its temperature dependence is discussed in chapter 3.

21 2. Heat and heat transfer

2.4.2B Conclusions

The electrical resistance of the heater is the parameter responsible for heat generation by resistive heating. If a material exhibits a measureable change of resistance with temperature, it can be used for temperature sensing. Combining these two effects, it is possible to use a single resistor for heating and temperature sensing of the heater, as employed in the devices described in chapters 4 and 5. Excess heat, generated by resistive heating or some other mean, can be removed by conduction (using highly thermally conductive solids, such as metals or silicon), convection (especially using liquid coolants and sustained flow, and preferably employing the phase change) and radiation. The devices employing heat transfer by conduction and convection are presented in chapters 5 and 6.

References23B

[1] A. F. Mills, “Basic Heat and Mass Transfer”, 2nd Edition, Prentice Hall, 1999 [2] M. Elwenspoek, R. Wiegerink, “Mechanical microsensors”, Springer, 2001 [3] Taken from lecture notes from University of Idaho, College of Science: http://www.sci.uidaho.edu/scripter/geog100/lect/03-atmos-energy-global-temps/03-08- heat-energy-xfer.htm [4] C. Liu, “Foundations of MEMS”, Pearson Prentice Hall, 2006

22 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

3.3B Temperature dependence of silicon electrical and thermal properties

In this chapter, a theoretical overview of the electrical and thermal properties of mono-crystalline silicon in a wide temperature range is given. The temperature range of interest is from room temperature to almost 1000 °C. The temperature dependence of electrical resistivity and thermal conductivity is briefly introduced. Experimental confirmation is provided by electrical characterization of As-doped thin silicon layers. Such layers can be employed in silicon sensors and specifically in heaters with sensing capability, such as the heaters in chapters 4 and 5. The good thermal conductivity of silicon qualifies it as a suitable heat spreading medium, as implemented in the devices described in chapters 5 and 6.

23 3. Temperature dependence of silicon electrical and thermal properties

3.1.24B Thermal conductivity of silicon

Thermal conductivity (k) is the property of a material ability to conduct heat (see section 2.2.1). It is measured in watts per kelvin-meter (W·K−1·m−1). Multiplied by the temperature difference (in K) and the area (in m2), and divided by the thickness (in m), the thermal conductivity predicts the rate of heat transfer (in W) through a piece of material. Thermal conductivity of mono-crystalline Si is high compared to the most of other materials often used in micro-fabrication (table 3.1).

Table 3.1 Thermal conductivity (at room temperature) for different materials commonly used in microfabrication

Material k [W m-1 K-1] Ref.

Pyrex glass SiO2 1.1 [1]

Silicon dioxide SiO2 1.17 [2]

Silicon nitride Si3N4 1.55 [2] Silicon carbide SiC 70 [1] Polycrystalline silicon Si 34.5 [2] Monocrystalline silicon Si 148 [3]

Aluminum oxide Al2O3 10-35 [1] Aluminum nitride AlN 130-170 [1] Aluminum Al 204 [4] Copper Cu 386 [4]

At low temperatures, as depicted in figure 3.1, thermal conductivity of silicon has drastically higher values (with the peak value exceeding 3000 W/Km at ~50 K). That makes silicon perfect for heat transfer applications at cryogenic temperatures [5]. Although thermal conductivity of silicon is decreased above room temperature (figure 3.1, table 3.2), it is still higher than for most other materials available in an IC environment and therefore can be used in several heat spreading applications at elevated temperatures as well.

24 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Table 3.2 Thermal conductivity of Si at different temperatures [6]

T [K] k [W m-1 K-1]

300 148

350 119

400 98.9 10000 500 76.2 )

1 - 600 61.9 K

1

- range of interest within this thesis 800 42.2 1000 (W m 1000 31.2 1200 25.7 1400 23.5 100 1600 22.1

10

Figure 3.1. Thermal conductivity of silicon as a function of temperature (source:[7]).

3.2.25B Electrical resistivity of silicon

The electrical conductivity (σ) of semiconductor materials depends on the concentration and mobility of the available charge carriers (electrons in conduction band and holes in the valence band). The electrical resistivity (ρ), which is later used for sensing temperature, is the inverse of the conductivity:

−1 ρσ==qn() μμnp + p. (3.1) In the above equation, q is the elementary charge; n, p are the concentrations of electrons and holes, respectively; and μn, μp the mobilities of electrons and holes, respectively. Both concentrations and mobilities of the carriers depend on the temperature, and therefore the electrical resistivity of silicon is temperature dependent [8].

In addition to the thermally generated charge carriers (ni – the intrinsic

25 3. Temperature dependence of silicon electrical and thermal properties concentration of the charge carriers), carrier concentration can be significantly altered by introducing dopant ions: donors (negatively charged carriers), or acceptors (positively charged carriers). The carrier concentrations for silicon doped with donors may be expressed as:

2 NNDA−−⎛⎞ NN DA 2 nn≈+⎜⎟ +i , 22⎝⎠ (3.2) 2 NNDA−−⎛⎞ NN DA 2 p ≈ −+⎜⎟ +ni 22⎝⎠

where ni is the intrinsic concentration of the carriers, and (ND-NA) is the nett difference between the donor concentration (ND) and the acceptor concentration (NA). It is assumed that the dopant atoms are completely ionized (which can be held true for temperatures above room temperature). The intrinsic concentration has a very modest value at room temperature (in the order of 1010 cm-3), but it grows exponentially with temperature:

−EkTg /2 nNNeiCV=⋅ (3.3)

Eg is the temperature dependent energy gap of silicon, NC and NV are temperature dependent effective densities of states in the conduction and valence band, respectively, and k is the Boltzmann constant. At temperatures slightly higher than room temperature, the temperature- independent donor concentration (extrinsic) is much higher than intrinsic carrier concentration and the resistivity/conductivity is determined by the temperature dependence of the electron mobility:

−1 NND −⇒==⋅⋅−Ai n ρσ q μ n ( TNN ) ( DA ) . (3.4) Electron mobility has a complex temperature dependence [8] and leads to an increase of the resistivity with temperature. With increasing temperature, thermally generated carriers (equation 3.3) contribute more and more to the conductivity, thus lowering the increase of the resistivity. At a certain temperature the resistivity reaches a maximum value, after which it starts to decrease. The temperature, at which the resistivity peak occurs, is called intrinsic temperature (Ti) and is depending on doping concentration and type (e.g. ~300 °C for 1016 cm-3). For temperatures above this intrinsic temperature, the number of free charge carriers is

26 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications dominated by the intrinsic concentration, and increases exponentially. The Si layer is in the so-called intrinsic region and differently doped layers have similar behavior:

−1 nNNiDA −⇒==+⋅ ρσ q ( μμ npi ) nT ( ) . (3.5)

In the equation 3.5, the term (μn + μp) also depends on temperature, but the temperature dependence of the intrinsic concentration is much more pronounced [8]. The consequence is an almost exponential decrease of the resistivity. If layers with equal geometries but different doping concentrations of the same dopant are compared, several conclusions can be drawn. Layers with lower dopant concentrations have higher resistivity at room temperature. Layers with lower dopant concentrations have lower intrinsic temperature. This is qualitatively depicted in figure 3.2. At temperatures above the intrinsic temperatures, all silicon mono-Si layers have almost the same values of resistivity.

ρSi ρSi

n1

n2 n3 n1 < n2 < n3

Ti T T a) b)

Figure 3.2. a) Typical curve representing the temperature dependence of the electrical resistivity of silicon. b) Typical curve for layers with different doping concentrations

3.3.26B Electrical resistivity characterization

In order to confirm theoretical predictions and acquire proper values for the thermal dependency of electrical parameters of silicon, we performed material characterization on thin silicon samples. The electrical resistance of differently doped mono-crystalline silicon samples was characterized in the

27 3. Temperature dependence of silicon electrical and thermal properties temperature range from 50 – 800 °C. Arsenic was chosen as dopant because silicidation of titanium (by high temperature annealing) is better for arsenic- doped layers (see chapter 4). The in-situ doped layers in concentrations from 1.0∙1016 to 6.6∙1017 cm-3 were subjected to temperature cycles up to 800 °C. The sheet resistance of the layers was measured during heating and cooling cycles. Measured results can help in proper selection of doping concentrations for sensing different temperature ranges. Additionally, the obtained results provide a deeper insight into the behavior of the hotplate heater devices at high temperatures (chapter 4). The fact that silicon behavior at highly elevated temperatures is independent of doping means that even the silicon bulk substrate would behave in the same way as the doped thin layers. Electrical insulation by the inversely polarized p-n junction is of no use in this case. Insulation needs to be performed by physical separation with an insulating material. Silicon-on- insulator (SOI) wafers can provide mono-Si layer on top (thus allowing epitaxial growth of silicon with the desired thickness) and good electrical insulation below it. Electrical insulation in lateral direction can be performed by selective etching of the top Si layer. In case of continuous blanket layers, insulation in the lateral direction is not needed.

3.3.1.64B Sample preparation

To prepare the test samples, we used SOI wafers 100 mm in diameter, with 400 nm buried oxide and 340 nm top silicon layer (figure 3.3.a). The silicon seed layer was low-doped by boron (7·1014 cm-3). The 5 μm epitaxially grown silicon layers were in-situ doped with arsenic, in different concentrations (table 3.3, figure 3.3.b). Table 3.3. Dopant concentrations for the studied samples Wafer #1 #2 #3 #4 Doping concentration [cm-3] 1·1016 5·1016 9.5·1016 6.6·1017

After the epitaxial growth, the wafers were annealed in a furnace, in an inert nitrogen atmosphere at 1100 °C for 120 minutes, in order to let the n-type dopants diffuse into the initial 340 nm thick silicon layer above the buried oxide. In this way a uniformly doped layer is obtained (figure 3.3.c). In order to have low-ohmic contacts, regions to be used as contacts for the four-point

28 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications probe measurements were implanted with As (energy: 50 keV, dose: 1·1014 cm-2). The implantation was performed through a thin wet thermal silicon dioxide of 30 nm (figure 3.3.c-d). After the ion implantation, a short annealing step at 950 °C was performed, to re-crystallize the lattice and to activate the dopants.

The thin SiO2 layer that acted as a dirt barrier was removed by dipping the wafers in a 0.5% HF solution (figure 3.3.d). Initial top silicon layer (340 nm)

Buried SiO2 (400 nm) a)

Si substrate

As-doped mono-Si layer (5 μm) Initial top silicon layer (340 nm)

Buried SiO2 (400 nm) b)

Si substrate

Thermal SiO2 (30 nm) As-doped top mono-Si layer (5.34 μm)

Buried SiO2 (400 nm) c)

Si substrate

Thermal SiO2 (30 nm)

Implanted highly-doped Si regions As-doped top mono-Si layer (5.34 μm)

d) Buried SiO2 (400 nm)

Si substrate dicing 20 mm lines 10 mm e) implanted regions 4 mm

Figure 3.3. Cross-sectional preparation sequence of the silicon test sample for material characterization: a) starting material; b) epitaxial growth of in-situ doped Si with As; c) annealing and thermal growth of 30 nm silicon oxide; d) implantation through the thin thermal oxide and stripping of oxide. e)Top-view with the designated implanted regions.

29 3. Temperature dependence of silicon electrical and thermal properties

At the end of fabrication process, the wafers were diced in 20 mm x 20 mm samples, with the implanted contact regions centered, as depicted in figure 3.3.e.

3.3.2.65B Measurement set-up

Measurements were performed in a vacuum chamber, on a hot chuck with temperature control using a thermocouple placed inside the aluminum nitride hot chuck. The samples were glued to the chuck by silver paste, providing good thermal contact to it (figure 3.4.a). Four tungsten needles were used to probe the samples according to the four-point measurement method. Measurements were taken by forcing the current (I) through two of the neighboring contacts and by measuring the voltage drop (V) across the other two contacts (figure 3.4.b). measurement vacuum probes chamber I V

device under test hot chuck with the R = V / I temperature sensor a) b) Figure 3.4. a) Schematics of the measurement setup, b) the principle of the electrical measurements

Customized LabVIEW code was used to control the system (heater and thermocouple in the chamber, digital current source and digital multimeter) and read the data. The current was adjusted to the value which provides constant voltage reading. The mean value of several V/I readings is used as one measurement point. The measured resistance is actually not the sheet resistance itself, but it differs only by a constant factor f. The sheet resistance is therefore calculated as RS = f ·V/I. For this system f = 5.65 . The system was programmed with a heating and cooling rate of 5 degrees per minute. Measurements were performed after each minute, providing sheet resistance values at 5 degrees intervals.

30 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

3.3.3.6B Measurement results and discussion

The obtained measurement data are shown in figure 3.5. Two different temperature regions are clearly observable: extrinsic in which the layer resistance is determined by dopant, and intrinsic – in which al. The two temperature regions are separated by the peak value appearing at the so-called intrinsic temperature. Measured intrinsic temperature for different layers is given in table 3.4 and plotted in figure 3.6. By fitting the measured values, an equation for shifting the intrinsic temperature with different As dopant concentration was derived -3 (Ti in °C, ND in cm ):

0.076 TNiD=⋅−39.65 334.4 . (3.6)

Figure 3.5. Comparison of the measured resistance as a function of temperature for four differently As-doped Si-layers.

Table 3.4. Measured intrinsic temperatures for different donor concentrations -3 16 16 16 17 Concentration nD [cm ] 1·10 5·10 9.5·10 6.6·10 Intrinsic temperature Ti [°C] 313 394 435 554

In the extrinsic region (equation 3.4), measurement results showed the good matching with the complex model from literature. Figure 3.7 compares the results from one of the wafers with the resistance model derived from the detailed mobility model that is dependent on temperature and donor concentration [9]. This model is not taking intrinsic carriers into account and is not valid at temperatures closer to the intrinsic temperature. In the depicted case, maximal deviation of measured data from the model, in the valid range (300-500 K), was 1.4 °C. It is interesting to observe that second-order

31 3. Temperature dependence of silicon electrical and thermal properties polynomial fit matches well with the results even in wider temperature range compared to the mentioned complex model. This assures us that quadratic equation is good enough approximation for temperature-induced resistance changes of silicon in the extrinsic temperature range.

Figure 3.6. Intrinsic temperature of As-doped mono-Si layers as the function of As doping concentration

Figure 3.7. The measured normalized resistance change for wafer #1, compared with the theoretical model [9] and the quadratic fit

In the intrinsic region, measured resistance was fitted by the exponential function: R = 0.014 ∙ e6860/T, with the temperature expressed in K (figure 3.8). From these data, it appears that an exponential function with only two parameters is sufficiently good approximation of the more complex temperature dependence behind equation 3.6. At temperatures above 700 °C (1/T ≈ 0.001 1/K), the deviation from this ideal curve is visible. Due to this deviation, which is probably caused by the limitations of the measurement set-

32 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications up, the highest doped sample (wafer #4) in this study could not reach the intrinsic region (figure 3.8).

Figure 3.8. Change of the measured resistance in the intrinsic region for four differently As-doped Si-layers.

To test repeatability and to check whether a hysteresis behavior is present, samples were heated from room temperature to 200 °C, cooled down to 50 °C, and then repeatedly heated in subsequent cycles to 400 °C, 600 °C and 800 °C. Figure 3.9 depicts the measurement data retrieved after several heating and cooling cycles of the samples. No hysteresis behavior was noticed, except in the regions around the intrinsic temperature. The measured resistance values at the intrinsic temperature decreases (up to 10%) among repeated heating cycles.

Figure 3.9. Measurements cycles revealed hysteresis around the intrinsic temperature

Additionally, due to the peak profile of resistivity, temperatures lower and higher than the intrinsic temperature could have the same resistance readings. Despite the absence of hysteresis in the extrinsic and the intrinsic regions, a

33 3. Temperature dependence of silicon electrical and thermal properties slight change in the resistance readings among the different heating cycles is observed. This change, less than 0.5 Ω for all samples tested, corresponds to ~1% of the measured values. After reaching 800 °C the behavior of the layers resistivity during cooling and further heating shows some permanent alterations. The cause of this effect is still not clear, but as the problem was experienced also with samples made of other materials, it is most likely related to some limitations in the measurement set-up.

3.4.27B Conclusions

Mono-crystalline silicon is a material with a pronounced temperature dependence of its electrical resistivity. Above room temperature, resistivity first rises almost quadratically with temperature and after reaching the peak value (at a so-called intrinsic temperature), it drops exponentially. The intrinsic temperature can be shifted to higher or lower values by altering the doping concentration of silicon. The analysis here presented indicates that thin layers of mono-crystalline Si behave similarly to bulk silicon and are therefore suitable for temperature sensing. The obtained characterization results match well with theoretical predictions and provide an overview of silicon resistivity behavior in a broad temperature range between 300 and 1000 K. In temperature ranges below and above intrinsic temperature, measured curves are stable and easy to model. However, the transitional region around the intrinsic temperature is not very suitable for temperature sensing. Therefore, identifying the intrinsic temperature of a specific doping level is the key factor determining the potential operating range of the device fabricated with these layers. The dependence of the thermal conductivity of mono-crystalline Si is high compared to the most of other materials often used in micro-fabrication. Although the thermal conductivity drops with the temperature, it is still high enough for a wide range of heat spreading applications. All this makes mono-Si as a material for heaters, like the ones reported in chapters 4 and 5, and for heat spreaders/heat exchangers, as implemented in the devices reported in chapters 5 and 6.

34 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

References28B

[1] H. Reichl, “Packaging and interconnection of sensors”, Sensors and Actuators A: Physical, Volume 25, Issues 1-3, (1991) [2] A. Irace, P.M. Sarro, “Measurement of thermal conductivity and diffusivity of single and multilayer membranes”, Sensors and Actuators A: Physical, Volume 76, Issues 1-3, (1999), Pages 323-328 [3] T.M. Tritt, “Thermal Conductivity: Theory, Properties, and Applications (Physics of Solids and Liquids)”, Springer, 2005 [4] A. F. Mills, “Basic Heat and Mass Transfer”, 2nd Edition, Prentice Hall, 1999 [5] P.P.P.M. Lerou, Micromachined Joule-Thompson cryocooler, PhD thesis, University of Twente, 2007 [6] eFunda website, http://www.efunda.com/materials/elements/TC_Table.cfm?Element_ID=Si [7] C.J. Glassbrenner, G. A. Slack, “Thermal conductivity of silicon and germanium from 3 K to the melting point”, Physical Review, 134, 4A (1964) A1058-A1069 [8] S.M. Sze, “Physics of Semiconductor Devices”, 2nd edition, Wiley-Interscience, 1981 [9] R. Hull, Properties of Crystalline Silicon, The Institution of Engineering and Technology, 1999

35 3. Temperature dependence of silicon electrical and thermal properties

36 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

4.4B Micro-hotplate heater

In this chapter we present a micro-hotplate heater capable of reaching temperatures up to 800 °C. The used materials, epitaxial Si for the heater and TiSi/TiN for the contact lines, provide compatibility with CMOS technology. Devices were fabricated using silicon-on-insulator (SOI) wafers, and were released by wet etching of the Si substrate in KOH. Heater hotplates, 1-5 µm thick and 300 µm × 500 µm in size, can be brought to visible glowing with an electrical power in order of 100 mW. Temperature dependent resistance of the devices was calibrated in the calibration furnace up to 775 °C.

37 4. Micro-hotplate heater

4.1.29B Introduction

As mentioned in chapter 1, there is often a need for localized heating on a chip. Many infrared sources, thermal actuators, gas sensors or micro-reactors [1-3] require elevated temperatures on the chip. The desired temperature increase is in a broad range, spanning across six orders of magnitudes: from millikelvins [4] to thousand of kelvins [5]. High temperatures (>500 °C) are often demanded for some of the phenomena studied with chemical micro- reactors and gas sensors, and thus, reliable devices capable of reaching high temperatures are required. To maintain a localized temperature increase without significantly disturbing the temperature of the rest of the chip, and to minimize power losses, a thermal insulation of the heater is required. Available microheaters commonly use the hotplate geometry with a resistive thin-film heating element placed on a thermally insulating membrane (membrane thickness typically in order of hundreds of nm to a few μm), or as fully released structures (cantilevers or bridges). The temperature-dependent resistance of the heater is often used as mechanism for sensing the heater temperature, or another temperature sensor is placed directly next to the heater. The most commonly used material for reaching high temperatures is platinum. The disadvantage of this configuration is that platinum is not very well compatible with conventional CMOS electronics processing. In addition, there is a limitation for using thin-film platinum heaters in temperature ranges above 600 °C due to degradation phenomena [6]. Quite often, polycrystalline silicon is selected for the heater. This material is limited in practical use to 500 °C, because above this temperature the grain boundaries of silicon start to move. This results in a slow uncontrollable and unpredictable change of electrical resistance of the heater, which makes it no longer suitable to sense the temperature by monitoring the change of electrical resistance. Monocrystalline silicon does not have grain boundaries and it has a high melting point (1414 °C) [7]. It is therefore an interesting candidate material for microheaters, like for the heaters embedded in the tip of a micro-cantilever, with a stable behavior for temperatures up to 800 °C or 1000 °C depending on

38 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications the chosen configuration [8]. These promising results encouraged us in 2008 to consider monocrystalline silicon for microheaters [9] that can be employed in for high temperature operation (>600 °C) in highly miniaturized chambers, like the recently reported MEMS nanoreactors [10]. The necessary modification of the heater geometry and of the fabrication process, especially the introduction of a different metallization scheme, are presented in the following sections. In 2010, successful usage of similar configuration was shown in micro-CVD system for graphene synthesis [11].

4.2.30B Design

A challenge in making heaters out of silicon is the high electrical resistivity of the undoped material. If no special care is taken, high voltage is required in order to reach high enough levels of dissipated power and thereby temperature. For intended power dissipation levels in the order of 100 mW, and limiting the voltage to values not higher than 10 V, the electrical resistance of the heater should not exceed 1 kΩ. As the peak level of silicon resistivity at intrinsic temperature can be increased up to 4 times [12], we would like to keep the nominal room temperature value of the heater resistor below 200 Ω. By optimizing the heater geometry and doping of the silicon, it is possible to achieve such values for the electrical resistance of the heater.

Si heater bulk

supporting/ contact beams

Figure 4.1. Concept of the device

In order to achieve high efficiency, i.e. to use the input power for heating only the heater and not the surrounding volume, the heater has to be thermally

39 4. Micro-hotplate heater insulated from the rest of the system. Insulation is done by eliminating substrate material, i.e. to release a freestanding hotplate (figure 4.1). However, electrical contacts to the heater are needed, connecting it with the cold parts of the device. These contact wires form thermal leaks and also generate heat by themselves. This leads to increased power consumption and to heating of the unwanted regions. If the electrical connection from the heater to the bulk is provided by silicon, it means that contacts between the silicon and the conductive wiring metal are formed at a “cold spot”, above the bulk. In this case, however, more power will be dissipated by the supporting connections than by the heater itself. If another, highly conductive material is used for connection to the heater, contacts with silicon are formed at the “hot spot”. Therefore, the selected conductive material needs to be stable at high temperatures and also to have a good adhesion to the silicon. The first demand automatically disqualifies the most common IC metallization material – aluminum, due to its low melting point of only 660 °C [8]. As suggested in literature, however, some silicides would be adequate for this purpose [13]. A good choice is titanium silicide, as it fulfils the above mentioned requirements. In addition, titanium is a common metal in CMOS processes. The most desired and the most stable phase of titanium-silicide (in general, TiSi), is titanium disilicide (TiSi2) with a melting point of 1540 °C and low electrical resistivity (in the order of 15 μΩcm) [14].

Table 4.1. The design parameters for different heater designs

Heater design A B C Hotplate dimensions 500 μm × 300 μm 300 μm × 300 μm 500 μm × 300 μm

Heater resistance (RH) 180 Ω 110 Ω 180 Ω Contact beam width 30 μm 30 μm 30 μm Contact beam length 550 μm 350 μm 550 μm

Contact beam resistance (RC) ~20 Ω ~12 Ω ~20 Ω

The heater is a solid rectangular sheet of doped silicon with a rectangular footprint. The dimensions of the heater and of the three supporting beams configurations (A, B and C) are given in table 4.1. The shape of the silicide- silicon contacts are arranged in such a way to make the distribution of the electric field across the heater during operation as homogeneous as possible, providing uniform heating. The suspending beams are not straight, but curved to allow some thermal expansion in plane, preventing buckling and potential breaking of the structure. As depicted in figure 4.2, there are four such beams,

40 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications symmetrically placed. Two of them at the opposite sides of the heater are used for the energy flow, whereas the other two serve for sensing the voltage. For heat generation uniformity, the best way is to provide the current flow diagonally. The equivalent electrical circuit for all device types is given in figure 4.2.d. The desired sheet resistances of the silicon layer and titanium silicide are 110 Ω/□ and 1 Ω/□, respectively.

Iout

Iout V+ V+

V- V- Iin

a) Iin b) hotplate

RC1 RH RC3

RC2 RC4 IIN V

+ - Iin V V Iout d) c)

Figure 4.2. The three device configurations: a) bridge structure with curved beams (type A); b) bridge structure with straight beams (type B); c) cantilever structure (type C) and d) the equivalent electrical circuit

4.3.31B Fabrication

Devices were fabricated starting from silicon-on-insulator (SOI) wafers, using only three masks. With slight modifications, this process can be made

41 4. Micro-hotplate heater

CMOS compatible. The SOI wafers were 100 mm in diameter, with 400 nm thick buried silicon oxide and 340 nm thick initial top silicon layers. The fabrication process starts with the epitaxial growth of monocrystalline silicon, to achieve the desired layer thickness and doping (table 4.2). As suggested in literature [15], arsenic is chosen as a dopant, because of the best formation of titanium silicide layer. Epitaxial growth is followed by low pressure chemical vapor deposition (LPCVD) of silicon nitride (figure 4.3.a) which is later used as a mask for back-side etching of the silicon substrate in potassium hydroxide (KOH) solution.

Table 4.2. The design parameters for different epitaxial Si layers

Type of dopant and Thickness of the Nominal sheet Layer doping concentration epitaxial layer resistance at 300 K I As, 1018 cm-3 1 μm ~110 Ω/□ II As, 6.6 ∙ 1017 cm-3 3 μm ~110 Ω/□ III As, 9.5 ∙ 1016 cm-3 5 μm ~110 Ω/□

Titanium silicide, a compound formed out of titanium and silicon, is used for the interconnects. Titanium silicide thin films can be obtained by so-called co-sputtering [14], i.e. sputtering by using two targets simultaneously. Co- sputtering leads to the direct deposition of the desired material. However, it is possible to obtain titanium silicide layers by depositing only titanium directly on silicon, followed by high temperature annealing. The annealing initiates the chemical reaction between titanium and silicon. This method consumes silicon underneath the metal. The additional annealing step makes the fabrication process slightly more complex, but was chosen as the formed contact possesses a lower contact resistance than in the case of co-sputtering. In addition, it can be performed with a wider range of sputtering tools. The stack of 120 nm thick titanium layer capped with 80 nm thick titanium nitride layer was sputter-deposited. Titanium nitride was added as it can serve as a protection layer for Ti, preventing oxidation of Ti. The Ti/TiN layer stack was patterned after deposition and before annealing, so that the silicide is formed only at predefined areas: the contact pads, the interconnects, and at the contacts with the heater (figure 4.3.b). Before silicidation, unwanted silicon is removed on the front side by plasma etching, providing the electrical insulation between the contacts and defining the heater (figure 4.3.c). Titanium silicide is formed by rapid thermal annealing (RTA), in nitrogen atmosphere, for 1 minute at 900 °C. Under the described process conditions, the entire titanium layer should have reacted with silicon, resulting in ~300 nm thick

42 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications silicide layer [14]. The titanium nitride layer remains unchanged. This RTA step was performed at MiPlaza, High-tech campus in Eindhoven.

a) c)

b) d)

monocrystalline silicon buried oxide titanium silicon nitride titanium nitride titanium silicide Figure 4.3. Schematic fabrication sequence: a) epitaxial growth, SiN deposition and etching; b) titanium sputtering and etching; c) etching silicon and annealing (silicidation); d) KOH etching and removing buried oxide

Deep silicon wet etching is performed to release the heater structures from the bulk. This is done by etching the silicon substrate from the back side of the wafer in a 33wt% KOH solution while the front side is protected by a vacuum holder. The buried oxide layer of SOI wafers acts as a stop layer for this etching. The last step (figure 4.3.d) is to remove the buried oxide layer under the heater by wet etching in buffered hydrofluoric acid (BHF). Top view of one of the fabricated structures is given in figure 4.4.

supporting beams

mono-Si heater

bulk etched-through (air) a) b)

Figure 4.4. a) Optical photograph of the heater structure before the releasing by wet etching in KOH; b) Top view optical photograph of the fully processed device with the light coming from the bottom.

43 4. Micro-hotplate heater

In the supporting arms under the titanium silicide, there is silicon left. This material gives extra mechanical robustness to the structure. It is electrically inactive (due to much higher resistivity as compared to TiSi) but it does conduct some heat.

4.3.1.67B Packaging

In order to provide electrical connections to the chip which are suitable for operation at high temperatures or for calibration of the chip at elevated temperatures, bond pads are needed. On the used layer stack (TiSi/TiN), it is not possible to directly wire-bond platinum or gold wires. Therefore, we need another conductive layer on top. We opted for a platinum layer with a thin layer of tantalum for adhesion enhancement. The Ta/Pt layer stack was deposited by evaporation. As platinum is very difficult to etch, typical platinum patterning is done by a so-called lift-off process. Prior to deposition, a thick photoresist is coated and patterned. During the evaporation step, metal is deposited on the wafer surface only where the photoresist was previously removed. Removing the rest of the photoresist removes (lifts off) also the unwanted metal.

Evaporated particles

Shadow mask Deposited layer Process wafer

Figure 4.5. The principle of using the shadow mask for the platinum evaporation process that does not require lithography steps.

Considering that we need to deposit platinum only for large contact pads (300 μm × 300 μm), and in order to avoid difficulties that can arise with the lift- off process on a non-planar surface (because of the ~ 5 μm topography on the top surface), evaporation through a hard mask (so-called “shadow mask”) is used. This method requires no photoresist on the process wafers. The “shadow mask” is another silicon wafer that was patterned by etching through the entire wafer and later interposed between the device wafer and the evaporation source (figure 4.5). In this way, we fully eliminate the complex lift- off process. Patterned Ta/Pt contact pads can be seen in figure 4.7.

44 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Ta/Pt bond pad

TiSi/TiN interconnnects a) b) Figure 4.6. a) Chips after deposited platinum bond pads. b) A magnified pair of bond pads.

4.4.32B Measurements

4.4.1.68B Measurement set-up

Electrical measurements were performed on the released heaters at wafer level using an Agilent 4156C precision semiconductor parameter analyzer and a Cascade probe station. Two contacts were used to provide the power to the device, and the other two to sense the resistance, as explained in figure 4.2.d. In this way, the resistance can be measured as a function of the input current or input power, but it does not contain the information on the temperature. Therefore, a temperature calibration needs to be performed, as well. To calibrate the resistance of the heater, a setup with a Fluke 9150 calibration furnace was used. Diced chips with fabricated devices were attached with a glue, resistant up to 800 °C, to a custom-designed ceramic probes (figure 4.7.a) supplied by Xensor Integration. Electrical contacts to the device are provided by wire-bonding the contact pads on the chip to the platinum wires on the ceramic probes. Probes were inserted into the furnace kept at stable and controllable temperature. Resistance of the heaters was then measured by Keithley 2611 source-monitor unit, by applying a small electrical power (in the order of μW). To avoid excessive handling of released devices, furnace calibration was performed with the non-released devices. This means that below the hotplate, both the buried oxide of the SOI wafer and the silicon bulk remained. This has

45 4. Micro-hotplate heater no effect on the electrical behavior of the heater, only on thermal losses. However, the measurements were performed in the controlled and stable temperature ambient of the calibration furnace. Ceramic probe inserted Source- into the furnace measurement Platinum Chip with the high-T unit (SMU) Calibration furnace bond pads ceramic glue Keithley 2601 Fluke 9150 (150 – 1200 °C) Ceramic probe Control PC with the customized Platinum LabVIEW script wires Bond-wires to connect the chip to the ceramic probe

a) b) Figure 4.7. a) Diced chips glued to customized ceramic probes. b) Measurement setup for temperature calibration of the resistance of the heaters.

4.4.2.69B Electrical characterization of the heaters

The measured sheet resistance of the epitaxial silicon layers at room temperature was in the range 110-112 Ω/□ for all the different layers, whereas sheet resistance of the TiSi/TiN stack was 1 Ω/□. During the testing of the released heaters, the current forced trough one diagonal pair of the contacts was in the range from 0 to 35 mA. Change of the heater electrical resistance as a function of power dissipated in heaters made of layer I (see table 4.2) is depicted in figure 4.8.a. On the horizontal axis the power that is dissipated across the heater body (calculated as the product of the forced current and voltage across the other two contacts) is reported. The total power consumption (dissipated at the access wires on the bulk and the suspended contact arms) is 2-3 times higher. Figure 4.8.b shows the glowing of the heater, indicating reached temperatures above 600 °C [16].

46 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

a) b) Figure 4.8. a) Graph showing the measured electrical resistance of the heater versus input power. b) Optical image of the device in operation. Hot regions emit red-orange visible radiation, indicating temperatures above 600 °C

Figure 4.9. Relative change of the resistance of the heater as a function of the power dissipated across the heater a) for silicon layer I (1018 cm-3) b) for silicon layer II (6.6 ·1017 cm-3)

Figure 4.9 depicts the relative change of resistance induced by the power dissipated to heaters employing two different layers. Heaters were brought to the intrinsic region of temperatures, exceeding intrinsic temperatures of the used silicon layers (554 °C for layer I, and 590 °C for layer II). Effects of long-term operation are shown in figures 4.10 and 4.11. First, a drop of about 5000 ppm/h of the heater resistance was noticed. After 10 hours of continuous performance, silicon layer looked intact while contact beams were showing signs of corrosion (figure 4.11), i.e. the conductive layer stack was oxidizing. Most likely the oxidation of the contact beams is interfering with the resistance measurements. This can be avoided by operating in a non- corrosive ambient, or by providing some oxidation protection of the devices.

47 4. Micro-hotplate heater

Figure 4.10. Time change of the resistance for a constant power (80 mW, above 600 °C) supplied to the heaters for several hours

intact Si heater

Corroded contact beams

Figure 4.11. Degradation of TiSi material after 10 hours of continuous operation at elevated temperature >500 °C

4.4.3.70B Resistance calibration of silicon heaters

Devices were successfully measured at temperatures up to 775 °C. For temperatures above 700 °C, packaging problems (such as detaching of the glue, breaking of the bond-wires) can limit the operating range. Measured temperature dependence of the resistance of the heaters with the layer II is shown in figure 4.12.

350

300 250

200

150

100

50 Heater resistance [Ohm ] resistance Heater 0 150 250 350 450 550 650 750 Temperature [C]

Figure 4.12. Temperature dependence of the measured electrical resistance of the fabricated microheater devices utilizing layer II (6.6 ·1017 cm-3).

48 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

4.4.4.71B Calibration of TiSi/TiN stack

In order to obtain the temperature dependence of the resistivity of TiSi/TiN stack, a test resistor was characterized using two set-ups for two different temperature ranges. For temperatures up to 450 °C, the electrical behavior of the measured layer stack is stable and repeatable, with no hysteresis observed (figure 4.13). The observed TCR value of ~4000 ppm/K makes this layer stack also suitable for sensing/heating applications.

Figure 4.13. Resistance of the resistor structures made with TiSi/TiN stack as a function of temperature

At temperatures of 500 °C and higher, a constant drift of the measured resistance is clearly noticed. The most probable cause of this permanent change of the resistance behavior is the thermal oxidation of the top titanium nitride. However, because of the used four-probe measurement method to sense the resistance of the heater, as long as silicon is not oxidized, the heater resistance measurements should be unaffected.

49 4. Micro-hotplate heater

4.5.34B Discussion

4.5.1.72B Device performance

The measured resistance of the hotplate heaters follows the trend obtained from the continuous layers described in section 3.3. The heater resistance first increases with the power supplied to the heater body. This power has a direct relation with the temperature of the heater. The resistance keeps following the increasing trend until reaching the intrinsic point of silicon, followed by a decline of the resistance for further temperature increase. On the horizontal axis of figure 4.8.a the power that is dissipated across the heater body (calculated as the product of the forced current and voltage across the other two contacts) is reported. The total input power is approximately doubled and the difference is dissipated within the suspending beams and on the interconnects placed on top of the substrate. The heater starts to glow for a supplied power of ~300 mW, corresponding to 96 mW on figure 4.8.a. For that dissipation, it is needed to apply a voltage of 11.7 V, which is slightly above the projected limits of the desired maximal voltage level of 10 V. Dissipating only a third of the total power in the hotplate region is something that needs to be improved in order to increase the efficiency of the design. Most of the unwanted dissipation is happening in the contact beams. In connection to that, during heating, the resistance of titanium silicide interconnects is increasing, even when the resistance of the silicon heater itself is decreasing (at high temperatures). At a certain point, the suspending contact arms become less conductive than the heater body, which results in contact arms being shinier than the heater. This is a problem resulting in hotter supportive beams than the hotplate itself. It can be addressed by, for example, layout redesign. In the studied heaters, there is no encapsulation or protective coating. Therefore, if the heater operates at very high temperatures (> 800 °C) for a sufficiently long time in an oxygen containing atmosphere, it is expected to fail by oxidation of the silicon. However, if the heater is to be operated in an oxygen free atmosphere (e.g. in vacuum, or some special ambient), or if coated with a layer that can serve as oxidation barrier (e.g. LPCVD SiN), a proper functioning of the heater should be expected even in the higher temperature range.

50 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

4.5.2.73B Stress and buckling considerations

The released structures exhibit rather large stress phenomena, like the bucking of the buried silicon oxide after release (figure 4.14.a). This can lead to the fracture of the released structures (figure 4.14.b).

a)

b) Figure 4.14. a) Distinct buckling of the buried oxide layer. b) Cracking of the released structures due to buckling of buried oxide.

Also titanium silicide exhibits stress, tending to bend the silicide-silicon stack upwards. During operation, as the temperature is being increased, the bending decreases. This is especially pronounced in silicon layers of type I (which are only 1 μm thick), leading to very low yield (~20%). Thicker layers (such as types II and III – 3 and 5 μm thick) are more stable and less prone to deformations, providing much higher yield (above 70%).

51 4. Micro-hotplate heater

a) b) c) d) Figure 4.15. Bending of the cantilever structures due to stress: a) fabricated structures (with buried oxide still present); b) after removal of buried oxide, the cantilever is bent (and thus out of focus; c) elevated temperature reduces stress and d) cantilever hotplate is almost planar.

As for buckling caused by thermal expansion of the released and heated structure, it proved beneficial to have “longer curves” in the designed contact arms. In figure 4.16, a comparison is given between two fabricated structures: one with 200 μm long folds and the other with folds of 80 μm.

a) b) c) d) Figure 4.16. Effect of the curved beams on buckling. a) Devices with longer “curves” without and b) with heating applied. c) Devices with shorter “curves” without and d) with heating. Longer “curves” help the hotplate staying in plane by bending the “curves”, while shorter “curves” cannot prevent buckling deformation.

4.6.35B Conclusions

We have designed, fabricated and characterized a microhotplate heater device for high temperatures, capable of reaching 800 °C. This device is fabricated with a process that is basically CMOS compatible (requiring only one non-compatible step as post-processing). With a total electrical power of 300 mW the heater was brought to glow, indicating its temperature to be above 600 °C. Direct temperature calibration of the device showed very good matching with the blanket layer characterization of the same mono-Si layers, which indicates that the performance of the thin mono-Si layers is in good agreement with the predicted behavior.

52 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

References36B

[1] J. Spannhake, O. Schulz, A. Helwig, G. Müller, and T. Doll, “Design, development and operational concept of an advanced MEMS IR source for miniaturized gas sensor systems”, Proceedings 4th IEEE Sensors Conference, Irvine, 2005, pp. 762-765 [2] D. Briand, A. Krauss, B. van der Schoot, U. Weimar, N. Barsan, W. Göpel, N. F. de Rooij, “Design and fabrication of high-temperature micro-hotplates for drop- coated gas sensors”, Sensors and Actuators B, vol. 68, pp. 223-233, 2000 [3] C. Zhang, K. Najafi, L. P. Bernal, P. D. Washabaugh, “An integrated combustor- thermoelectric micro power generator”, Dig. Transducers ’01, Munich, 2001,pp. 34- 37 [4] M. Graf, D. Barretino, M. Zimmermann, A. Hierlemann, H. Baltes, “CMOS monolithic metal-oxide sensor system comprising a microhotplate and associated circuitry”, IEEE Sensors Journal, vol 4, no. 1, pp. 9-16, 2004. [5] J. Tu, D. Howard, S.D. Collins and R.L. Smith, “Micromachined, silicon filament light source for spectrophotometric microsystems”, Journal of Applied Optics, vol. 42 (2003), pp. 2388–2397 [6] L. Firebaugh, K. F. Jensen, and M. A. Schmidt, “Investigation of High-Temperature Degradation of Platinum Thin Films with an In Situ Resistance Measurement Apparatus”, Journal of Microelectromechanical Systems, Vol. 7, No. 1, pp. 128-135, (1998) [7] Handbook of Chemistry and Physics. CRC Press, 87th edition, 2006-2007 [8] J. Lee, W. P. King, “Microcantilever hotplates: Design, fabrication, and characterization”; Sensors and Actuators A 136 (2007), pp 291–298. [9] M. Mihailovic, J.F. Creemer, P.M. Sarro, “Monocrystalline silicon microhotplate heater”, Proceedings of the EUROSENSORS XXII (pp. 1611-1614), 2008, Dresden, Germany [10] J.F. Creemer, S. Helveg, P.J. Kooyman, A.M. Molenbroek, H.W. Zandbergen, P.M. Sarro, “A MEMS Reactor for Atomic-Scale Microscopy of Nanomaterials Under Industrially Relevant Conditions”, Journal of Microelectromechanical Systems,, vol.19, no.2, pp.254-264, April 2010 [11] Q. Zhou, L. Lin, “Synthesis of Graphene Using Micro Chemical Vapor Deposition” Proceedings of 23th IEEE Micro Electro Mechanical Systems Conference, pp. 43-46, Hong Kong, Jan. 2010. [12] M. Grundmann; The Physics of Semiconductors. Springer, 2006 [13] P. Furjes, Zs. Vizvary, M. Adam, I. Barsony, A. Morissey, Cs. Ducso “Materials and processing for realization of micro-hotplates operated at elevated temperature”, Journal of Micromechanics and Microengineering, 12 (2002), pp. 425–429.

53 4. Micro-hotplate heater

[14] S. P. Murarka; Silicides for VLSI applications. New York: Academic Press, 1983 [15] J.F. Jongste; Formation and properties of titanium disilicide. TU Delft dissertation, 1994 [16] R.M. Tiggelaar, J.W. Berenschot, J.H. de Boer, R.G.P. Sanders, J.G.E. Gardeniers, R.E. Oosterbroek, A. Van Den Berg, M.C. Elwenspoek, “Fabrication and characterization of high-temperature microreactors with thin film heater and sensor patterns in silicon nitride tubes”, Lab on a Chip - Miniaturisation for Chemistry and Biology, 3(2005), p.326

54 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

5.5B Micro-evaporator

The increase of energy density due to miniaturization of devices and intensification of chemical process generates intense local heating. Properly designed devices for heat absorption, such as evaporators, are essential for dealing with high heat fluxes. In this chapter a miniaturized evaporator with fin structure fabricated in silicon with MEMS technology is introduced. The aim of this micro- evaporator is to achieve a maximum cooling capacity and operation stability at very small liquid flow rates (in the order of 1-5 ml/h). Devices with high aspect ratio channels (10 μm and 20 μm wide, 100 μm deep) are sealed with silicon or glass by wafer bonding and tested with de-ionized water as coolant. The embedded bulk silicon heater (footprint 2.7 mm × 2.7 mm) mimics external heat sources and, at the same time, acts as temperature sensor. Absorbed power fluxes are up to 40 W/cm2 for a fluid flow of 5 ml/h. Optimizations of the fin-channel structure lead to a more stable operation in a broader range of set-point conditions.

55 5. Micro-evaporator

5.1.37B Heat sinks with liquid flow and their applications

In many advanced chemical, pharmaceutical and biomedical processes, miniaturized devices and microsystems are employed [1,2]. In these processes a significant amount of excess heat can be generated. Also for electronic devices the energy density increases due to the continuous downscaling. In case of high energy density levels, even small power dissipation can significantly increase the temperature on the chip locally. Consequently, cooling the devices during operation is needed to limit this temperature increase. One method to cool devices is heat absorption by forced convection of fluid flow that is in contact with the hot surface. Although the forced convection by external flow of the ambient gas is successfully applied in many larger-scale applications [3], there are micro-scale applications in which this approach is not sufficient (because of the too high power dissipation), or not possible (e.g. in space, where there is no ambient gas) [3,4]. Internal forced convection by the flow of liquids is more promising in these cases, as the heat transfer coefficient for liquids is one to two orders of magnitude larger than for gasses, and can be applied also in vacuum. Currently, there are numerous micro heat exchangers and micro heat sinks [5,6], capable of reaching very intensive cooling rates, for applications such as liquid cooling of ICs (one of the first reported absorbed power fluxes were in order of 790 W/cm2 achieved with 600 ml/min of water) [7]. However, in applications that require very low pumping powers limited by power consumption, mass or volume (such as in some aerospace applications), this approach might also be insufficient [8]. Nevertheless, by heating up the liquid coolant to its saturation temperature (boiling point), it is possible to absorb additional energy needed for the phase change, so-called latent heat. Boiling is very efficient in heat absorption, as the absorbed latent heat can be much larger than the energy required for heating up the coolant from the ambient temperature to the saturation temperature. As an example, the energy needed to heat up 1 g of liquid water from 0 °C to 100 °C is ~0.3 kJ, whereas eight times that energy is needed to transform that 1 g of liquid water at 100 °C to the gas phase [9]. Evaporators are heat exchangers which apply the described approach and deal with high heat fluxes by evaporating the liquid flowing in channels [2]. Boiling mechanisms in mini- and micro-channels have been thoroughly studied

56 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications and modeled, addressing the specific phenomena related to the channel dimensions [10-12]. However, still much fundamental experimental and analytical work remains to be done to reach the state-of-the-art of two-phase flow and heat transfer in micro-channels [10]. Additionally, in order to implement micro-channel arrays in the miniaturized cooling devices some practical aspects need to be addressed, such as unstable operation due to rapid bubble expansion and occasional flow reversal [12]. Silicon is an ideal material for miniaturizing evaporators, due to its good thermal conductivity and wide range of microstructuring processes, such as high aspect ratio etching, integration of thin film electrical and electronic components. Moreover, when the excess heat generation is required, silicon can be used as a material for resistive heater devices [13]. In this chapter, a new silicon based micro-evaporator with high-aspect ratio channels [14-16] is introduced. This device combines the potential of silicon fabrication technology with the good heat conduction capability of silicon (see section 3.2) and silicon resistive heating/sensing applications (see sections 3.3 and 4.5).

5.2.38B Design of the micro-evaporator

Several configurations of the micro-evaporator are studied. They can be grouped in three types: − type A: all-silicon device − type B: silicon-glass device − type C: silicon-glass device with the with integrated “thermal” cavity These devices have different micrometer sized channel structures (addressed in more details in the coming sections) and an embedded bulk silicon heater which mimics an external heat source. The silicon fin structures are sealed with another silicon wafer by direct (fusion) wafer-to-wafer bonding (type A devices), or with a glass wafer by anodic wafer-to-wafer bonding (type B and C devices).

57 5. Micro-evaporator

5.2.1.74B Working principle

A conceptual design of the micro-evaporator is shown in figure 5.1. Coolant is forced into the evaporator through the inlet, and through divider manifold delivered into the mesh of channels before reaching the heated zone. In real- life applications, this heated zone receives energy from e.g. exothermal chemical reaction in a micro-reactor, or through the power dissipation in the integrated electrical circuits. In our demonstrator devices, this external energy source is mimicked by the embedded resistive heater. Excess heat is absorbed in the water flow, by means of forced convection heat transfer. If heated above the boiling point, water changes phase to vapor. Water as the liquid and/or vapor is then collected in the collector manifold. From there, water vapor leaves the device through the outlet.

Fin-channel Outlet structure Heated region Divider manifold

Inlet Outlet Top part Bottom part Collector manifold

a) b) Inlet

Figure 5.1. Schematic drawing (3D) of the micro-evaporator: (a) the conceptual device; (b) internal structure shown in the flipped-over top part of the device

5.2.2.75B The channel-fin structure

Instead of a single channel structure, arrays of parallel straight or connected channels are used. Individual channels are separated by silicon walls that serve as fins, therefore improving the heat transfer function. The finned structures increase the contact surface area without increasing the total footprint of the device. Therefore, channels of high aspect ratio (height/width>>1) are of interest. As a measure of how beneficial the fin structure is, compared to the case without the fins, the structure effectiveness parameter E can be used. The effectiveness E of a fin structure (figure 5.2.a) is defined as the ratio of the total

58 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications heat transfer of one single unit (Q’fin_unit), and the heat transfer of the same unit without the fin structure (Q’d_unit) [17], and can be calculated as:

Q' Hkd+ ⋅ fin_ unit , (5.1) E ==' Qkddunit_ (1)+ ⋅ where H is the height of the fin (i.e. the depth of the channel), d is the fin width and k∙d is the distance between two adjacent fins (i.e. the channel width).

T ∞

h

a) H h

h

k∙ d d 1 unit Tbase

T

Tbase

b)

T∞ Heff H

Figure 5.2. a) Schematic drawing of an array of fins and a unit cell of such an array; b) Distribution of temperature along the fin.

To determine the fin height (channel depth), another parameter - the effective height of the fin (Heff) is of importance. Heat flux in the fins consists of

59 5. Micro-evaporator two components: heat conduction down the fin and the forced convection by the fluid flow. For the best effectiveness, the fin height should not exceed the value that will cause the temperature of the fin to drop below the boiling temperature. The largest distance from the base of the fin at which the fin temperature is still higher than the saturation temperature of a coolant, is defined as the effective height of the fin (Heff). This is calculated as:

d ⋅λ H = , (5.2) eff 2⋅ h where λ is the thermal conductivity of the fin material and h is the heat transfer coefficient from the fin to the surroundings. Both λ and h are dependent on Tbase (the base temperature of the fin), and T∞ (the surroundings temperature). The temperature distribution along the fin is given in figure 5.2.b. In our case the surroundings temperature is 100 °C, the saturation temperature of water under standard atmospheric pressure. For a fin width d of 10 or 20 μm, the related effective fin height is 112 and 158 μm, respectively. Four different designs of the channel structure are implemented: two structures with straight parallel channels with different channel width (10 μm and 20 μm), and two structures with cross-linked channels and closely spaced fins. One cross-linked design employs inline rectangular fins. The other uses staggered diamond-shaped fins, based on the design reported in [18] and an optimized geometry. More details on the dimensions of the fin-channel structure are provided in tables 5.1 and 5.2, and figure 5.3. The fin height (channel depth) was fixed to 100 μm, which provides a high aspect ratio (AR = 5-10) and is still within the limits of the effective fin height. The length of the fin structure is designed as 4 mm in total. The total size of the micro- evaporator chips is the same for all of the devices: 10 mm × 3.33 mm × 1 mm. For the design I and II, the channel entry is restricted to the width of 5 μm to prevent vapor bubbles that are formed in the channels to go back to the divider manifold, as suggested in [19-20]. The presence of the vapor bubbles in the divider manifold can block the water flowing into the channels. In the connected channels (design III and IV), no inlet restriction is needed because of the alternative paths provided for the vapor bubbles by the connections between the channels.

60 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Table 5.1. Main characteristics of the different design structures Design No. of Channel depth Effec- Channel structure Channel width Fin width type channels (fin height) tiveness I straight 90 10 µm 20 µm 100 µm 3.7 II straight 36 20 µm 50 µm 100 µm 1.7 10 µm 20 µm connected III 90 (cross-linked (length of each 100 µm 5.0 (rectangular fins) with 20 µm ch.) block 40 µm) connected 0 – 40 µm IV (diamond-shaped 45 20 µm – 60 µm (length of each 100 µm 3.1 fins) block 160 µm)

I II III IV

Figure 5.3. Schematic top views (not to scale) of the four channel designs. For clarity, only few channels per design are shown. The meaning and values of the indicated geometric parameters are in table 5.2. The big dotted arrow indicates the intended flow direction.

Table 5.2. The dimensions of the geometrical parameters shown in figure 5.3 (symbols valid only for the figure 5.2). Inlet/outlet width a = 350 µm Channel length h = 4000 µm Width of the divider/ b = 2680 µm Length of the rectangular fins (III) i = 40 µm collector manifolds Width of the channel c = 5 µm, Width of the cross-links (III) j = 20 µm inlet restriction d = d =10 µm Length of the Channel width I III k = 160 µm d = 10 µm diamond-shaped fins (IV) II e = e = 20 µm Width of the Fin width I III l = 40 µm eII = 20 µm diamond-shaped fins (IV) Inlet/outlet length f = 1500 µm Minimal channel width (IV) m = 20 µm Length of the divider/ g = 1500 µm Maximal channel width (IV) n = 60 µm collector manifolds

61 5. Micro-evaporator

5.2.3.76B Embedded heater

A heater is integrated to mimic processes which cause heating. It consists of a bulk silicon resistor, placed directly above and surrounding the channels. This heater also serves as a temperature sensor, by employing the temperature dependence of the electrical resistivity of silicon (see chapter 3). The heater geometry is defined by the aluminum contacts (figure 5.4). The footprint of the resistor heater is a square of 2.7 mm × 2.7 mm which covers the entire width of the channel structure, maximizing the uniformity of heat generation across the channels.

Al contacts

heater a) b) channel

Figure 5.4. Aluminum contacts defining the heater area. a) 3D view; b) cross-section along the A-A’ axis (figure 5.3)

The heater is designed to have an electrical resistance of 100-200 Ω at room temperature. Expecting an increase of the resistance by a factor 2 at most, a power dissipation of 10 W can be realized by supplying a voltage up to 60 V.

5.3.39B Fabrication and packaging

Fabrication involves processing on both sides of the top silicon wafer, using front-to-back alignment (FTBA). Lithography systems used in Dimes cleanroom provide the FTBA accuracy below 1 μm. The most important fabrication step is the two-step deep reactive ion etching (DRIE) of silicon, to form the fluidic channels and inlet/outlet regions. The channels are sealed by wafer bonding. Figures 5.5 and 5.6 show SEM images with the etched channel structures. Some specific steps in the fabrication process are different in each device type (A, B and C) and will be explained in more detail in the coming sections. Process flow for type C (the most comprehensive one) is given in Appendix A.

62 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

a)

b)

Figure 5.5. SEM images of the etched channel structures a) I and b) II. The channel entrance is restricted to 5 μm to prevent bubbles going back from the channels to the manifold.

63 5. Micro-evaporator

a)

b)

Figure 5.6. SEM images of the etched channel structures a) III and b) IV.

64 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

The bonded wafers with sealed channels were diced in 10 mm × 3.33 mm dies (figure 5.7).

Figure 5.7. All-silicon (top image)and silicon-glass (bottom image) devices. The size of all dies is 10 mm × 3.33 mm × 1 mm.

As all microfluidic devices, the micro-evaporator needs inlet and outlet connections with the rest of the microfludic system (i.e. flow supply). For this purpose, stainless steel tubes were glued to the access holes at the sides of the device to allow the injection of de-ionized water and the dispensing of liquid/vapor. The outer diameter of the tubes used was 500 μm for type A devices, and 350 μm for types B and C.

(a) (b) Figure 5.8. (a) Device with the attached steel inlet/outlet tubes and glued to the PCB. (b) The hole in the PCB (back side) to allow optical access to the channels.

65 5. Micro-evaporator

To provide electrical connections and mechanical interface, dies with glued steel tubes are mounted on a custom-designed printed circuit board (PCB), as shown in figure 5.8.a. After gluing to the PCB, the dies were wire-bonded with aluminum wires to the contact pads on the PCB. Type B and type C devices have channels sealed by glass wafer which allows visual inspection. For that reason, a hole in the PCB was drilled (figure 5.8.b).

5.4.40B Measurements

5.4.1.7B Measurement set-up for heater calibration

Before characterizing devices with the sustained flow of a coolant, the heater resistance needs to be calibrated as a function of the temperature. This is done by measuring the heater resistance using the four-point measurement method. The calibration was performed at known and stable temperature levels in the 30-130 °C range using a Cascade Microtech probe station (model 12971B), with a temperature controlled thermochuck (model Temptronic TP0315B-2). The input current and output voltage signals were supplied and measured by an Agilent 4156C semiconductor parameter analyzer.

5.4.2.78B Fluidic measurement set-up

Fluidic measurements were performed at TNO Science & Industry, in Delft. The used set-up is shown in figure 5.9. An electronic source HP E3631A supplies the voltage (limited to 25 V). The current through the heater was measured by a Keithley 2700 multimeter; and the voltage across the heater by a Fluke 8840A multimeter (figure 5.9). Custom-made software, written by researchers from TNO, was used to control the temperature of the device at a constant value. The flow rate of the coolant (de-mineralized water) was supplied by a syringe and a programmable syringe pump Aladdin AL1000 (capable of producing flow in 0.1 – 1700 ml/h range), which was connected to the device with flexible silicone fluidic tubes.

66 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

The assembled device was mechanically clamped at one side to the measurement stage (figure 5.10). During characterization of the type B and C devices, an optical microscope with a camera was placed directly above the chip to observe and record the flow behavior during operation.

Figure 5.9. Schematic of the measurement set-up.

Figure 5.10. Packaged device connected and clamped to the measurement stage. The optical microscope is positioned directly above the device.

67 5. Micro-evaporator

5.4.3.79B Flow rate and power dissipation

Flow rate measurements show that the device and fluidic set-up are functioning properly. All device types sustained a flow rate of 1700 ml/h without breaking or leaking (the flow rate was limited by the syringe pump performance). However, as announced in the introduction, much lower flow rates were employed (in order of few ml/h) during characterization. The set-up shown in figure 5.10 allowed not only the control of the heater temperature set-point, but also the reading of the power dissipated into the system. Due to thermal losses – heat conduction to the rest of the chip, to the PCB and to the measurement stage; heat convection to the ambient and negligible radiation – a certain power is required to maintain the heater at elevated temperature even when there is no coolant flow. This power is referred to as P0. Power provided into the system with the sustained coolant flow is partly lost to the ambient (as mentioned above) and partly absorbed by the flow. The net absorbed power is then calculated as Pabs = Ptotal – P0 . Typical heat losses (for any type of the devices) without sustained flow are shown in figure 5.11.

Figure 5.11. Power losses without the coolant flow as function of the heater temperature.

All other measurement results and observations are given in the following sections of this chapter, as they are related to the specific device types.

68 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

5.5.41B All-silicon devices

The first type of developed micro evaporators (thereafter referred to as type A devices) were all-silicon devices, with the channels sealed by another silicon wafer (figure 5.12). Type A devices were made with three variations of the channel-fin design, using structures I, II and III (see subsection 5.5.2).

Figure 5.12. 3D sketch of the type A micro-evaporator: (a) the conceptual device; (b) internal structure shown in the flipped-over top part of the device; (c) top part of the device with the designated heater contacts; (d) bottom half with the inlet and outlet.

5.5.1.80B Fabrication

The type A micro-evaporators were fabricated in a four masks process, starting with a double-side polished wafer (further denoted as the top wafer) that is later bonded to a single-side polished wafer (bottom wafer). Both wafers were p-type <100> single crystal wafers, 525 μm thick and 100 mm in diameter. The process flow is schematically depicted in figure 5.13.

69 5. Micro-evaporator

(a) Top wafer after patterning aluminum contacts.

(b) PECVD oxide protection layer and DRIE of silicon to define inlet and outlet.

(c) DRIE of silicon to form inlet, outlet, divider manifold, collector manifold and channel structure.

(d) Bottom wafer after DRIE of inlet and outlet and PECVD oxide on the back side.

(e) Stripping of oxide on the bonding surfaces and direct wafer-to-wafer bonding.

(f) Bonded wafers in transversal cross-section.

Figure 5.13. Schematic of the fabrication sequence (dashed lines outline original wafer surface).

70 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

To perform direct wafer bonding between the two silicon wafers, the surfaces should be very smooth and free of particles. Therefore, as a first fabrication step, a 360 nm thick wet thermal oxide layer was grown on both wafers to protect the bonding surfaces during processing. After etching the alignment marks, contact holes were opened in the silicon oxide on the top wafer. The silicon was doped by boron implantation (dose: 1.0∙1015 cm-2, energy: 50 keV), to make low-ohmic contact between the aluminum thin film and the silicon bulk. After annealing for dopant activation (at 950 °C, for 20 minutes), a 1.5 μm thick aluminum layer was sputter-deposited and patterned by aluminum wet etching (figure 5.13.a). A 2.5 μm thick PECVD (plasma enhanced vapour chemical deposition) silicon oxide layer was then deposited on the back side to serve as a mask during etching of the structures. The inlet, outlet, divider and collector manifolds and channels were processed in a two-step DRIE process. The total etch-depth for inlet/outlet was 550 μm, i.e. 275 μm per wafer. First, the inlet and outlet were etched 175 μm deep (figure 5.13.b) and the remaining 100 μm of silicon were etched together with the 100 μm deep divider manifold, collector manifold and channels (figure 5.13.c). Close-up SEM images of the etched channels are shown in figures 5.5 and 5.6. A 2.5 μm thick PECVD oxide layer was deposited on the top side of the bottom wafer as a mask for etching 275 μm deep inlet and outlet (figure 5.13.d). PECVD oxide layer was deposited on the bottom side as well, for electrical insulation during the measurements.

Figure 5.14. Infrared image of the bonded silicon wafers. The image in the inset is the enlarged view of one of the devices.

71 5. Micro-evaporator

Before direct wafer bonding, the protective oxide layer on the bonding surfaces was removed by wet silicon oxide etching. After cleaning the wafers in

RCA SC-1 solution (mixture of NH4OH, H2O2 and H2O in volumetric ratio 4:1:1 [21]), wafers were aligned and bonded in an EV420 mask aligner. The bonded wafers (figure 5.13.e, 5.13.f) were annealed at 400 °C for 1 hour to increase the bond strength. An infrared image of two bonded silicon wafers is shown in figure 5.14. The image is clear, indicating successful wafer bonding. The heater contacts (the structure on the top of the wafer) appear as two parallel dark lines, whereas the channels structure (the structure inside the bonded wafers) as the shaded rectangle (enlarged in the inset at the right lower corner of the figure). The bonded wafers were cut in 10 mm × 3.33 mm dies (figure 5.15.a). The chips were then glued and wire-bonded to the PCB, as shown in figure 5.15.b.

(a) (b) Figure 5.15. (a) Diced devices of the type A. (b) Device glued to the PCB and wire-bonded.

5.5.2.81B Characterization of type A devices

Temperature calibration was performed by measuring the resistance of the heater using the four-probe method in a temperature range from 30 °C to 130 °C. As expected (see subsection 3.3.3), in that temperature range the resistance has a quadratic change with temperature and the measured data were nicely fitted (R2>0.9999) with the second order polynomial. Typical thermal dependence of the electrical resistance of the heaters is shown in figure 5.16 and compared with the curve for type B devices.

72 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

For the devices of type A, it was not possible to look in the channel structure and to see what is happening inside the channels during the measurements. However, a qualitative test has been performed in order to demonstrate the operation of the device. By heating the water flow and converting it completely to the vapour phase, no liquid should come out of the device, even with the sustained flow. To visualize the initially invisible water vapour, a glass plate was placed above the device. Condensed water droplets on the cold glass plate became visible (figure 5.17). The image is blurred above the outlet, while far from the outlet the image is sharp (no condensed vapour).

Figure 5.16. Temperature dependence of the electrical resistance of the heaters.

Figure 5.17. Condensed water vapour on the glass plate above the working chip.

The evaporator successfully converts water into steam. Out of three

73 5. Micro-evaporator different channel structures tested, the one with the cross-linked channels showed a more stable operation for the widest set of the applied measurement conditions. For flow rates higher than 1 ml/h, the heated device cannot cope with the mixed flow (liquid and vapour bubbles) and the system becomes unstable, not being able to follow the set point. For the single-phase flow (heater temperature < 100° C), the temperature set-point is nicely followed. However, for the heater temperature > 100° C, the actual dissipated power and the average heater temperature are not stable anymore, but oscillate significantly. Most likely, the heat conducted along the device, from the heater to the inlet part, makes the divider manifold as hot as the heater part. This leads to bubble formation and expansion already in the divider manifold. Such vapor bubbles can easily block the entrance of the channels, preventing the water to flow further. One way to significantly reduce conductive heat transfer towards the inlet is to employ a glass wafer for the bottom part of the device. Moreover, this allows visual monitoring of the processes in the device. These considerations lead us to consider a different type of device, device type B.

5.6.42B Silicon-glass devices

The main difference between type A and type B devices is the replacement of the bottom silicon wafer (employed in type A) with a glass wafer (type B). As stated in 5.5.3, motivation for this is to lower the heat conduction losses towards the inlet (with the goal of preventing premature boiling) and to allow optical access to the channel structure. The design of the channel-fin structure is the same as in type A and three different configurations: I, II and III (see subsection 5.2.2.) were implemented.

5.6.1.82B Fabrication

The silicon-glass micro-evaporators were fabricated in a four masks process, using the same mask set as for type A devices. The devices were processed starting from a double-side polished p-type silicon wafer and a sodium-containing glass wafer. Both wafers were 500 mm thick and 100 mm in

74 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications diameter. The glass wafer was not processed prior to the wafer bonding.

SiO2

Si substrate SiO2 a) Thermal oxidation of the double-side polished silicon wafer Al

doped region

b) Opening of contact holes, ion implantation, metal patterning PECVD silicon oxide

c) PECVD silicon oxide (on both sides) and first Si DRIE step on backside to define inlet and outlet

original wafer edge

d) Second Si DRIE step to complete the etching of inlet and outlet and define channels

glass substrate along A-A’ (Fig 5.3) along B-B’ (Fig. 5.3)

e) Stripping of oxide on the bonding surfaces and anodic bonding to unprocessed glass wafer Figure 5.18. Schematic of the fabrication sequence

The process flow is very similar to the process described in subsection 5.5.1 and is schematically depicted in figure 5.18. For these devices, the process is

75 5. Micro-evaporator simplified as the bottom wafer is not processed and thus no alignment is required during wafer bonding. A small design alteration is needed to adjust the etching for the inlet the inlet and outlet that are now placed entirely in the top silicon wafer. In type A devices, metal tubes used to provide the fluidic contact with the rest of fluidic system were placed in the 500 μm deep inlet/outlet holes. The etch depth was split among the two wafers (top and bottom). In devices of types B and C, the entire inlet/outlet hole is etched in the top wafer only, to avoid deep etching of glass. Considering that silicon wafer is about 525 μm thick and to insure good mechanical stability of the device, the etching depth for inlet/outlet is increased to 350 μm, but the tubes of smaller outer diameter are employed for the type B and C devices. Additionally, as the anodic bonding is less demanding regarding the cleanliness of the bonding surface, the last cleaning step before bonding was performed in the HNO3 only and not in RCA SC-1 [21] solution. Fabricated type B devices (shown in figure 5.19) were then packaged following the packaging procedure explained in section 5.3.

Figure 5.19. Fabricated device of the type B

5.6.2.83B Characterization of type B devices

As with type A devices, temperature calibration was performed by measuring the resistance of the heater using the four-point measurement method in a temperature range from 30 °C to 130 °C. Figure 5.20 depicts the typical thermal dependence of the electrical resistance of the heater (type A is included as well for comparison). The lower electrical resistance of the type A devices is due to the conductive bottom part of the device that is made of silicon. In type B devices, although the heater has the same footprint as in type A, the bottom part is made of glass, an electrical insulator, which decreases the

76 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications thickness of the resistive heater and increases the resistance.

Figure 5.20. Temperature dependence of the electrical resistance of the heaters.

The temperature dependence of the electrical resistivity is clearly measurable, with an increase of roughly 100% in the range 30 °C – 130 °C. Standard deviation of measured resistance was in order of 0.1 Ω which translates to roughly 0.1 °C. Measurements with sustained water flow showed that all the devices successfully cooled down the heated zone by the internal flow of de- mineralized water. Heat absorbed by the fluid flow heated the liquid coolant and led to its evaporation. The behavior of the flow was visualized by the optical microscope placed above the device (see figure 5.10). The boiling in the channels of structures I and III is shown in figure 5.21. The liquid-vapor front is clearly visible. The water flow is sustained from left to right. The maximum voltage the used set-up can supply to the heater was 25 V which limited power dissipation in the heater. Under these conditions, flows between 4 ml/h and 5 ml/h could be evaporated. Figure 5.22 depicts the typical absorbed power for flow rates in the range from 0.5 ml/h until 50 ml/h. For flow rates up to 1 ml/h, the absorbed power is almost negligible until 100 °C, when boiling occurs. Increase of the absorbed power due to boiling is clearly observable at temperatures above 100 °C. Absorbed power for the flow rate of 5 ml/h at temperatures higher than 100 °C was in the same order as with the ten times higher flow rate but in the single-phase flow (50 ml/h at 70 °C). The maximal measured absorbed power for the 5 ml/h was slightly less than 3 W (limited by the measurement set-up) which corresponds to an absorbed power per unit area of ~40 W/cm2.

77 5. Micro-evaporator

(a)

(b) Figure 5.21. Boiling in the channels of structure (a) I and (b) III. The flow is sustained from left to right. The dashed white line represents liquid-vapor front.

3.5 50 ml/h 3 Absorbed powerlinear [W] fit 5 ml/h 2.5 PCHIP fit 2 1 ml/h PCHIP fit 0.5 ml/h Two-phase 1.5 regime PCHIP fit 1 0.5 0 20 40 60 80 100 120 Average heater temperature° C][ Figure 5.22. Absorbed power in the device with the connected channel structure for different flow rates. High flow rate (50 ml/h) results were linearly fitted because of the single-phase regime, while the other data sets were fitted using piecewise cubic Hermite data interpolation (PCHIP).

For the devices type I and II, which contain 4 mm long parallel channels,

78 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications the backflow of vapor has a limiting effect. When the vapor bubble forms inside a channel, it is pushing back the liquid to the entrance of the channel. The employed inlet restrictions are not performing well enough, causing the vapor bubbles to enter the divider manifold where bubbles accumulate and grow, blocking the entrance of some of the channels. In case of structures I and II this has significant consequences. Some channels remain blocked, changing the flow resistance of the available channels and the liquid velocity through them. This process is of random nature. It does not give repeatable results and stable operation for the two-phase flow. This makes the analysis and prediction of their behavior more difficult. In case of design III, lateral links between the channels play an important role and make the liquid flow through all the longitudinal channels, independently from the possible blockage of the entrance. The backflow was suppressed, though still present. When the temperature of the heater is elevated to above 100 °C and the mixed flow regime is reached, instabilities occur. First, the entire channels become fully evaporated and dried out due to the backflow. The heat transfer coefficient of the vapor flowing along the channel walls is at least one order of magnitude lower, causing the channel walls to overheat. This leads to the increased temperature sensed by the heater. The feedback loop reduces the power supplied to the heater, channel walls cool down and the liquid enters back into the channels. The liquid is then rapidly heated up and evaporated again and the device is in an oscillatory mode, with an instable temperature swinging from ~90 °C to ~115 °C. In order to better stabilize operating conditions and to better address the problem of backflow, a few modifications are implemented in a third design, type C. First, as additional measure to reduce heating of the divider manifold, a cavity on the top side (between the heater and the inlet) is introduced (thereafter refereed to as thermal cavity). Further, an optimized fin-channel design with staggered diamond-shaped fins is implemented.

79 5. Micro-evaporator

5.7.43B Silicon-glass devices with thermal cavity

5.7.1.84B Fin-channel design

As the back-flow of the water vapor bubbles was identified as limiting effect, an improvement of the existing fin-channel was implemented in type C devices. Staggered arrays of fins with diamond-shaped footprints were used for the new design (design IV). The sharp tips of the diamond-shaped fins prevent formation and growth of bubbles in the channels, thus contributing to the suppression of the back-flow. In design IV, divider/collector manifolds are of trapezoidal footprint and also filled with the diamond-shaped fins, to additionally support the prevention of the vapor back-flow. Based on the results from devices type A and B, channel designs I and II were not included in type C. Only design III (to compare with devices type B) and the new configuration (channel design IV) were fabricated.

5.7.2.85B Thermal cavity

Silicon is a good thermal conductor and, therefore, a suitable structure material for fins in the used configuration. However, heat conduction is also present in the direction of the liquid flow which can lead to the unwanted overheating of the divider manifold, as mentioned in the previous subsections. In order to reduce heat conduction towards the inlet, an additional cavity, a so- called “thermal” cavity [15-16], is located on the top side of the silicon wafer, physically separating the heater from the inlet. Only a thin silicon ceiling above the channels is left (figure 5.23).

80 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Al pads thermal (a) inlet cavity Si heater outlet thin ceiling 100 µm channels

glass

(b)

Figure 5.23. Schematic cross section (a) and top view (b). The arrow indicates the intended flow

5.7.3.86B Fabrication of type C devices

The fabrication sequence for the type C devices is based on the one for type B devices (figure 5.18). There are few additional steps to pattern and etch the thermal cavity. These steps are performed after completing the etching of inlet/outlet and the channels on the back side of the wafer. The thickness of the silicon ceiling above the channels is determined by controlling the etching from both sides of the wafer. That requires measurement of the wafer thickness before the etching, and measurements of the etched depth (figure 5.24). The etch depth at the back side is fixed by the channel design to 100 μm. The target thickness of the silicon ceiling was 10-15 μm. Such thin silicon layer becomes transparent for the visible radiation. Figure 5.25 illustrates this. Dies of type C devices are shown in figure 5.26. Thermal cavity is clearly visible in the top view and the side view. Packaging of the type C devices is exactly the same as for type B devices (described in subsection 5.6.1).

81 5. Micro-evaporator

PECVD silicon oxide

a) First Si DRIE step (at the backside of the wafer) to define inlet and outlet (steps preceding this one are shown in figure 5.20.a-b)

original wafer edge

b) Second Si DRIE step (backside) to complete the etching of inlet and outlet and define channels

thermal cavity

c) Third Si DRIE step (front side) to define the thermal cavity

glass substrate along A-A’ (Fig 5.3) along B-B’ (Fig. 5.3)

d) Stripping of oxide on the bonding surfaces and anodic bonding to unprocessed glass wafer

Figure 5.24. Abbreviated schematic process flow for type C devices showing three DRIE steps and anodic bonding

82 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Figure 5.25. Part of the wafer after etching the thermal cavity. The wafer is placed above the lamp. Visible light can penetrate through ~10-15 µm thick silicon and orange-red regions disclose thinned silicon ceiling.

Heater Top Side view view

Bottom view

Thermal cavity

Figure 5.26. Type C micro-evaporator devices after dicing.

5.7.4.87B Characterization of type C devices

The temperature dependence of the heater is the same as for the type A and B devices, and can be modeled by a second-order polynomial. With the sustained flow, improvement in the operation stability in the

83 5. Micro-evaporator mixed flow regime was observed. Devices of the design IV showed the best suppression of the backflow of the vapor bubbles due to the sharp tips arranged in the staggered layout. Some instability at high temperatures remained, but the oscillations were now significantly reduced, and the devices provided a sufficiently stable operation to get quantitative data. The temperature set- point was followed with a standard deviation of 0.1% for temperatures below 100 °C, and 0.3% for temperatures above 100 °C, respectively. The measured absorbed power was less stable than the average heater temperature, with a standard deviation of 0.5% for T<100 °C and of 10-25% for T>100 °C. The effect of thermal cavity was clear when the performance was compared to devices with no thermal cavity (type B). The heat conduction towards the divider manifold and the inlet was reduced, resulting in a noticeably more pronounced temperature gradient. As can be clearly seen in figure 5.27, the vapor-liquid front is now limited to the line that correlates with the edge of thermal cavity (see figure 5.22).

Figure 5.27. Boiling in the channels of design option IV. The flow is sustained from left to right.

5.8.4B Conclusions

The proposed concept of miniaturized evaporators for heat absorption (cooling) is demonstrated. Devices with high aspect ratio channels etched in silicon by DRIE and sealed by anodic bonding to a glass wafer, were fabricated,. They were tested with de-ionized water as the coolant with flow rates of 1-5 ml/h and successfully turned water into vapor, absorbing up to 40 W/cm2 for a flow rate of 5 ml/h. Two-step deep reactive ion etching of silicon to define channels and lateral fluidic inlet/outlet, followed by anodic bonding to glass in order to seal the fluidic channels proved to be a reliable method for fabricating

84 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications leak-proof micro-evaporators. Silicon-glass devices showed better performance as compared to all-silicon devices, and allowed visual inspection of the processes in the channels. Different channel designs were studied, with connected channels layout showing better performance in comparison to the straight parallel channels layout. Diamond-shaped fins with staggered layout showed more stable behavior and less problems with the backflow compared to the rectangular fins. Introducing a thermal cavity and leaving only a 15μm thin silicon ceiling above the channels improves the stability of the water vapor front and thus leads to more stable operation in the mixed flow regime. The fabrication and functional concept developed can be applied (with minimal design and fabrication modifications) in other devices for various applications that require heating (or cooling) the fluid flowing in micrometer sized channels. One such example is a micro-thruster device, presented in Chapter 6.

References45B

[1] K.F. Jensen, “Microchemical systems: Status, challenges, and opportunities”, AIChE Journal, 45: 2051–2054. (1999) [2] C.M. Rops, F. van der Graaf, J.F.M. Velthuis, “Micro Evaporators” MEMS, NANO and Smart Systems, ICMENS Proc. Int.Conf. on 421-426 (2004) [3] L. Zhang, K.E. Goodson, T.W. Kenny, Silicon Microchannel Heat Sinks - Theories and Phenomena, Springer-Verlag, Berlin, 2004 [4] M. Marengo, S. Zhdanov, L. Chignoli, G. E. Cossali, Micro-Heat-Sinks for Space Applications, ASME Conference Proceedings. 2004, 87 [5] J.D. Heppner, D.C. Walther, A.P. Pisano, “The design of ARCTIC: A rotary compressor thermally insulated μcooler”, Sensors and Actuators A: Physical 134 47- 56, (2007) [6] J.-Y. Jung, H.S. Oh, D.K. Lee, K.B. Choi, S.K. Dong, H.-Y. Kwak, “A capillary-pumped loop (CPL) with microcone-shaped capillary structure for cooling electronic devices”, Journal of Micromechanics and Microengineering 18 017002, (2008) [7] D.B. Tuckerman, R.F.W. Pease, “High-performance heat sinking for VLSI”, Electron Device Letters, IEEE, 2 (5), 1981, pp. 126 - 129 [8] G.C. Birur, T.W. Sur, A.D. Paris, P. Shakkottai, A.A. Green, S.I. Haapanen,

85 5. Micro-evaporator

“Micro/nano spacecraft thermal control using a MEMS-based pumped liquid cooling system”, Proc. SPIE 4560, 196 (2001) [9] D.R. Lide, Handbook of Chemistry and Physics 79th Edition CRC Press (1998) [10] J.R. Thome, “Boiling in microchannels: a review of experiment and theory”, International Journal of Heat and Fluid Flow 25 128-139 (2004) [11] M.B. Bowers, I. Mudawar, “High flux boiling in low flow rate, low pressure drop mini-channel and micro-channel heat sinks”, International Journal of Heat and Mass Transfer 37 321-332 (1994) [12] S.G. Kandlikar, S. Garimella, D. Li, S. Colin, M. King Heat Transfer and Fluid Flow in Minichannels and Microchannels, Elsevier (2006) [13] M. Mihailovic, J.F. Creemer, P.M. Sarro, “Monocrystalline silicon microhotplate heater”, Proceedings of EUROSENSORS XXII, Dresden, Germany, 2008, pp 1611-1614 [14] J. Hao, M. Mihailovic, C.M. Rops, J. F. Creemer, P.M. Sarro, “Silicon-based MEMS micro-evaporator”, Proceedings of 20th Micromechanics Europe Workshop (MME), Toulouse, France, 2009 [15] M. Mihailovic, C. Rops, J.F. Creemer, P.M. Sarro, “MEMS silicon-based micro- evaporator with diamond-shaped fins”, Procedia Engineering, Volume 5, Eurosensor XXIV Conference, 2010, Pages 969-972 [16] C.M. Rops, M. Mihailovic, P.M. Sarro, “Design and Development of an Ultra Compact Silicon Phase-Change Heat Exchanger”, Proceedings of Second European Conference in Microfluidics, Toulouse, France, 8-10 December 2010 [17] A.F. Mills, Basic Heat and Mass Transfer 2nd Edition Prentice Hall (1999) [18] G. Tanda, “Heat transfer and pressure drop in a rectangular channel with diamond-shaped elements”, International Journal of Heat and Mass Transfer, 44, 3529-3541 (2001) [19] C.M. Rops, Two phase flow and phase change heat transfer in small structures, PhD thesis TU Delft, 2009 [20] A. Mukherjee, S.G. Kandlikar, “The effect of inlet constriction on bubble growth during flow boiling in microchannels”, International Journal of Heat and Mass Transfer 52 5204-5212 (2005)

86 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

6.6B Resistojet micro-thruster

This chapter presents a miniaturized resistojet thruster device with an integrated thin-film heater, capable of delivering thrusts in the micronewton–millinewton range. Its small size (25 mm × 5 mm × 1 mm), low mass (162 mg), low power and propellant consumption make it very attractive for attitude control of nano-satellites (mass 1-10 kg). Based on the pressure measurements with the fabricated devices, thrust is calculated to be in the 20 μN – 1 mN range, depending on the propellant (cold gas nitrogen) flow rate. By heating the propellant flow to 350 °C, thrust was increased, resulting in a 30% reduction of propellant consumption.

87 6. Resistojet micro-thruster

6.1.46B Introduction

A constant desire for lowering costs in expensive space satellite missions is driving extensive research aiming at miniaturization of the relevant components and systems [1]. Small spacecrafts can be launched as the secondary load of huge spacecrafts, practically not increasing the total mass of the entire system and at very limited additional costs. Significant effort is being placed into the development of nano- (mass 1- 10 kg), pico- (0.1-1 kg) and femto-satellites (less than 100 g). The main focus is on providing these miniaturized spacecrafts with a set of performance capabilities similar to today's larger satellites. One of the open issues for these small satellites is propulsion (figure 6.1). Traditionally, propulsion for spacecrafts in orbit is provided by chemical rocket thrusters or cold gas systems [1]. For the small and lightweight nano- spacecrafts, however, new concepts need to be developed that provide low- thrust propulsion and efforts are made on miniaturization of the on-board propulsion systems and devices [2-9]. In addition, it is essential not only to scale-down the dimensions of the thruster device itself, but also to reduce the propellant consumption as very limited volume and mass is allowed for the propellant container in the small satellites.

Direction of propellant flow nozzle propulsion heater devices channel Direction of (nano)satellite cold propellant movement a) b) Figure 6.1: a) Conceptual sketch of a small spacecraft with the micro-propulsion devices indicated. b) Schematic drawing of one type of such propulsion devices: the resistojet.

To be used in nano-satellites, such propulsion systems should be small, lightweight, have a limited power consumption (<1 W), and capable of

88 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications delivering very small thrust levels in the millinewton and micronewton range. Resistojet is an option often used in larger scale systems. It is a propulsion device in which a propellant is heated by a heater element and the propellant is exhausted through a nozzle (figure 6.1.b) [2]. A miniaturized resistojet, fabricated in silicon MEMS technology which can provide thrust levels of 1 mN and less, as required for fine attitude control of nano-satellites, is designed, fabricated and tested.

6.2.47B Design

6.2.1.8B Working principle

The MEMS resistojet micro-thruster we developed consists of a channel in which the propellant flows, a heater to heat up the propellant, and a nozzle to expand the hot gas to produce thrust (figure 6.2). The propellant enters the device through the inlet and is heated by the integrated thin-film heater. Silicon has good thermal conduction properties and acts as heat spreader, providing the elevated temperature at the channel walls. The hot, pressurized, gas is expanded through a convergent-divergent channel, i.e. nozzle, delivering thrust to the system.

Outlet of the pressurized hot gas through the nozzle

Integrated thin-film Silicon top

heater Glass Si inlet bottom Entrance of the cold gas glass channel through the inlet hole Cross-section at inlet

Figure 6.2: Sketch of the MEMS Resistojet. The cold nitrogen gas, used as propellant enters the device through the inlet and is heated by the integrated thin-film heater.

89 6. Resistojet micro-thruster

Resistojet type of micro-thrusters is suitable for attitude control of geostationary nano-satellites [2,3]. In such applications required thrust levels are typically below 1 mN.

6.2.2.89B Channel and nozzle

Based on calculations from [10], selected design details are shown in figure 6.3. The channel is of rectangular profile with a width of 50 μm. The height (depth) of the channel is fixed to 150 μm. The length of the channel part that is exposed to heating is 2 cm, to provide conditions for thermally fully developed flow. The heated channel is connected to a linear (slit) nozzle. The nozzle (figure 6.3.c, figure 6.4) has the same depth as the channel and is designed with an area expansion ratio of 25:1. Three different configurations were considered: single-channel devices with nozzle throat width of 10 μm and 5 μm, and a three-channel device with nozzle throat of 10 μm.

250 μm nozzle

single 2 cm channel

3 parallel 10 μm channels

Si 50 μm inlet

a) b) c) Figure 6.3. a) Schematic top view of the channel geometry. b) An alternative geometry (with 3 parallel channels to reduce the pressure drop is also considered. c) Close-up sketch of the nozzle throat. All the channels and nozzles are 150 μm deep.

90 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

nozzle throat

18° 20° P c Converging Pamb 50 μmn Diverging part 10 μm part

75 μm 430 μm

Figure 6.4. Schematic diagram of a nozzle (top view)

6.2.3.90B Heater

To provide heating of the propellant flow, an embedded thin-film heater is provided on top of the silicon channel. To keep the process flow and packaging simple and straight-forward, aluminum was used as a heating material. Utilizing aluminum limits the magnitude of the temperature elevation, but can allow a faster processing of the first device to confirm the proper working of the design concept proposed in this thesis. Temperature dependent electrical resistivity of aluminum is used to monitor the average heater temperature. Considering the good electrical conduction properties of aluminum, to achieve the desired resistance value of ~50 Ω, a meandered design is chosen (figure 6.5). The heater footprint is about 2200 μm × 250 μm. To electrically insulate the heater from the substrate, the heater is residing on a thin layer of insulator material (silicon oxide). 50 µm 50 µm

1000 µm

Figure 6.5. Design and geometrical parameters of the meandered heater.

91 6. Resistojet micro-thruster

Heat dissipated at the heater is then transferred to the fluidic channel through the silicon. As silicon is a good thermal conductor, and the geometry of the system is such that heat is transferred through a big heater footprint over a very short distance (less than 500 μm), the temperature gradient between the heater surface and the channel wall is negligible.

6.3.48B Fabrication

The micro-thruster was fabricated in a four-mask process, starting with a double-side polished silicon wafer that was later anodically bonded to a glass wafer. Both wafers were 550 μm thick and 100 mm in diameter. The glass wafer was not processed prior to wafer bonding. The process flow is schematically depicted in figure 6.6. To perform anodic wafer bonding, the bonding surface should be smooth and free of particles. Therefore, as a first fabrication step, a 360 nm thick wet thermal oxide layer was grown on the silicon wafer to protect the bonding surface during subsequent process steps (figure 6.6.a). The same silicon oxide on the top side of the wafer serves as an insulating layer between the silicon substrate and the heater film. A 675 nm thick aluminum layer was sputter- deposited and patterned by aluminum wet etching (figure 6.6.b) to define the thin film heater. A 6 μm thick PECVD (plasma enhanced vapour chemical deposition) silicon oxide layer was then deposited on both sides of the silicon wafer (figure 6.6.c). On the back side, it serves as a mask during deep reactive ion etching (DRIE) of the structures, and on the front side, it serves as an etch stop layer for the structures that need to be etched through the wafer and as a heater protection during fabrication. There were three different depths to be etched on the wafer: the channel with the nozzle (150 μm), the inlet (350 μm) and the etched-through area (>550 μm) to define the resistojet. This was accomplished with a three-step DRIE process (figure 6.6.c-6.6.e). To transfer the pattern of the second and third etching step, photoresist was spray-coated due to the high surface topography. During the last etching step, an etching recipe that can provide aspect ratio independent etch rates needs to be used. Close-up optical and SEM images of the etched channel and the nozzle are shown in figure 6.7.

92 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

SiO2

a) Si

Thermal oxidation of Si at both surfaces Al

b)

Al deposition and patterning PECVD

SiO2

c)

PECVD SiO2 PECVD SiO2 at both surfaces, first DRIE step from wafer backside

d)

Patterning the inlet, second DRIE step

e)

Patterning the channel and nozzle and third DRIE step

f)

glass glass

Removing SiO2 from the bonding surface and sealing of the channels by anodic Si-glass wafer bonding Figure 6.6. Schematic view of the fabrication sequence along longitudinal (left) and lateral (right) cross-section

93 6. Resistojet micro-thruster

a) b) Figure 6.7. a) Optical and b) SEM close-ups of the fabricated nozzle The throat width is 10 µm and the etched depth is 150 µm

Before wafer bonding, the protective oxide layer on the bonding surfaces was removed by wet silicon oxide etching. The glass and silicon wafer were inserted in an EV420 bonder. The wafer stack (figure 6.6.f) was bonded at 400 °C for 1 hour, with an applied voltage of 1000 V. Figure 6.8 depicts etched devices with three channels, while figure 6.9 shows details of the fabricated structures, before and after anodic bonding.

a) b) Figure 6.8. SEM micrographs of the etched three-channel design. a) channel entrance; b) channel endings and the nozzle

94 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Si etched-through (glass only) Si island with Etched- the bond-pads through on the top side region

Channels Etched- through region Al heater with thin

SiO2 still remaining Figure 6.9. Optical photographs of a) meandered heater structure; b) sealed channels (circular artifacts are coming from some contamination in the glass wafer)

The bonded wafers were diced in 25 mm × 5 mm dies (figure 6.10.a). Individual thruster dies are very lightweight, weighing only 162 mg. The dies were glued and wire-bonded with aluminum wires to the custom-designed printed circuit board (PCB), as shown in figure 6.10.b. Fluidic adapters, with stainless steel tips were glued into the inlet of the device with an epoxy glue, to provide the delivery of the propellant.

Device glued and wire-bonded to PCB

Fluidic connection

Electrical connections

a) b) Figure 6.10. A micro-thruster device (25 mm x 5 mm x 1 mm) a) after fabrication and dicing and b) after packaging to the customized PCB.

95 6. Resistojet micro-thruster

6.4.49B Measurement setup

The devices were tested in a testing facility of the Space Systems Engineering laboratory. The setup is schematically shown in figure 6.11. The chip was placed inside the vacuum chamber capable of lowering pressure to 10- 20 mbar absolute. Cold gas nitrogen is kept pressurized in a bottle, and the flow is regulated by Clippard solenoid valve and Brooks mass flow controller. The used mass flow controller (MFC) can deliver mass flows in the range of 0.15-3 mg/s. The Omega pressure transducer (sensing range: 0-6 bar), through a T- junction, senses the pressure of the flow at the inlet of the device (system pressure - PS). The pressure at the inlet of the nozzle (PC), which governs the mass flow rate and hence the thrust, is then calculated as the difference between the measured system pressure and the calculated pressure drop:

PC = PS – ΔP. The pressure drop ΔP is exhibited in the device itself and the used adapter to connect the device to the rest of the system (figure 6.11). More details on how pressure drop was calculated in this case can be found in [10]. For the hot gas testing, the integrated heater was used to deliver the power needed for heating up the gas flow. The heater was controlled by Keithley SMU (source-measurement unit) 2611. The heater resistance was monitored using the four-point measurements method. Based on the measured resistance and the known TCR value (temperature coefficient of resistance) of the heater material, the average temperature of the heater was acquired.

valve MFC P sensor 0.15-3 mg/s 0-6 bar

PS

pressurized ΔP gas bottle

PC DUT

PC = PS - ΔP Vacuum chamber (50 mbar)

Figure 6.11. Schematic drawing of the measurement setup, consisting of vacuum chamber, mass flow controller and pressure transducer.

Figures 6.12 and 6.13 contain photos of the measurement set-up.

96 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Pressure sensor

T-junction

Micro-thruster device

Figure 6.12. Photograph of the mounted device inside the vacuum chamber.

Gas bottle and the controlling valves

Vacuum chamber with the mounted device LabVIEW interface Electric source controlling the and metering system instruments

Figure 6.13. Photograph of the entire measurement setup

97 6. Resistojet micro-thruster

6.5.50B Measurement results

First measurements were performed without heating, with the propellant at room temperature (figure 6.14). This temperature is in the range of the normal satellite on-board temperatures (-10 to +40 °C). 6 5 μm nozzle 4

2

Pressure [bar] Pressure 1-ch. device 10 μm nozzle 3-ch. device 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Mass flow rate [mg/s] Figure 6.14. Measured PS (data points) and calculated and fitted PC (lines) for different geometries and different flow rates of cold nitrogen propellant gas.

5

4

3

2 Pressure [bar]

1 0 50 100 150 200 250 300 350 400 Temperature [°C] Figure 6.15. Change of pressure (points refer to PS ; lines are the fitted PC) caused by heating, for flow rates of 0.3 mg/s (×) and 0.85 mg/s (○), for the device with the 10 μm wide nozzle throat.

For the hot gas test, the mass flow rate was kept constant and the integrated heater was employed to elevate the temperature. Figure 6.15 shows the change of pressure when heating from room temperature to 350 °C, for two fixed mass flow rates: 0.3 mg/s and 0.85 mg/s.

The expected thrust was calculated based on the PC values calculated from

98 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

the measured PS, the known ambient pressure in the vacuum chamber (50 mbar), and assuming isentropic expansion of the gas in the nozzle. Figure 6.16 depicts the predicted thrust for the device with the 10 μm wide nozzle throat in the form of the line fitted through the points available from the measured data shown in figure 6.14. The calculated thrust is in the range from 20 μN (for the lowest available flow of 0.15 mg/s) to 960 μN (for a 1.5 mg/s flow which causes the PS to hit the maximal sensitive value of 6 bar). In addition, figure 6.16 indicates the expected increase of thrust by heating. Data points at 350 °C for mass flow rates of 0.3 mg/s and 0.85 mg/s are also shown. 1 at 350 °C

0.5 Thrust [mN] Thrust

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Mass flow rate [mg/s] Figure 6.16. Thrust calculated for gas flow without heating, from the measured PS, and ambient pressure (50 mbar). Increase of the thrust by heating (as in figure 6.15) is also shown.

Conductive and radiant heat losses for reaching heater temperatures of 350 °C were in order of 2 W which can be reduced by optimization of the packaging scheme. In order to heat the gas flow to the desired working temperature, additional heating power is required. This is in order of only ~1 mW/K for mass flow rates of 1 mg/s.

6.6.51B Discussion

The fabricated devices exhibit pressure increase for mass flow increasing

(figure 6.14). As expected, multi-channel devices have the same values of PC, compared to the single-channel devices with the same nozzle size, but exhibit lower pressure drop. A higher pressure could be obtained for the same mass

99 6. Resistojet micro-thruster flow rates in the devices with the narrower nozzle throat (5 μm), but the achieved thrust levels are not higher compared to the wider throats, as the thrust is proportional to the cross-sectional area of the nozzle throat. The increase of pressure can also be induced by keeping the mass flow rate constant and by heating the gas flow. As indicated in figure 6.16, by heating the flow to 350 °C, the predicted thrust is the same as by using ~45% higher mass flow and no heating. Therefore, propellant consumption is clearly decreased (~30%) when heating is applied. Based on the calculations from the measured system pressure, the presented micro-thruster devices are capable of producing thrust in the 20 μN – 1 mN range. Upper limit is determined from the maximal system pressure of 6 bar, and this pressure level is not desired to be increased. However, the lower thrust limit can be further decreased, by applying mass flow controller capable of providing lower flow rates.

6.7.52B Conclusions

The presented resistojet devices can successfully provide flow of propellant through the micro-channel and the nozzle, with or without heating. From the measured system pressure, these micro-thruster devices are capable of producing thrust in the 20 μN – 1 mN range. Fabricated devices are very lightweight (only 162 mg) and minute (outer dimensions: 25 mm × 5 mm × 1 mm). The thrust produced and the significant reduction in size and weight make them very suitable for implementation in propulsion systems for attitude control of nano-satellites. Heating the gas flow clearly indicates that propellant consumption is reduced (~30% if heated to 350 °C). The proven concept makes silicon-based resistojets suitable also for the use of other propellants, such as hydrogen or water. Water is particularly interesting as it can be stored on-board in a liquid phase, requiring less storage volume, and can be gasified employing the integrated heater, for an increased thruster performance. With the proper selection of the heater material, it is possible to increase the heating temperatures of the propellant flow, further reducing the propellant consumption. The use of Si MEMS technology allows integration of the presented concept

100 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications with a MEMS flow control valve and other onboard electronic components. Additionally, its small size and weight make it suitable for applications also in pico-satellites.

References53B

[1] R. Fleeter, “The logic of microspace – technology and management of minimum- cost space missions”, Microcosm Inc. 2000 [2] J. Mueller, “Thruster options for microspacecraft: a review and evaluation of existing hardware and emerging technologies”, in: AIAA97-3058. [3] Lecture notes, Olivia Billett, Micropropulsion for Nanosatellites, http://ssdl.stanford.edu/ssdl/images/stories/AA236/A06Fall/Lectures/lecture-17.pdf [4] M.S. Bartsch, M.H. McCrink, R.W. Crocker, B.P. Mosier, K.A. Peterson, K. Wally, K.D. Patel, “Electrokinetically Pumped Liquid Propellant Microthrusters For Orbital Station Keeping”, Transducers 2007, Lyon, France [5] R. Krpoun, H.R. Shea, “Microfabricated Out-of-Plane Arrays of Integrated Capillary Nano-Electrospray Emitters”, Transducers 2009, Denver, USA [6] D. Briand, L. Guillot, U. Bley, S. Danninger, V. Gass, N.F. de Rooij, “Digital Micro- thrusters with simplified architecture and reliable ignition and combustion”, PowerMEMS 2008, Sendai, Japan [7] M.S. Bartsch, M.H. McCrink, R.W. Crocker, B.P. Mosier, K.A. Peterson, K. Wally, K.D. Patel, “Electrokinetically Pumped Liquid Propellant Microthrusters For Orbital Station Keeping”, Transducers 2007, Lyon, France [8] R. Krpoun, H.R. Shea, “Microfabricated Out-of-Plane Arrays of Integrated Capillary Nano-Electrospray Emitters”, Transducers 2009, Denver, USA [9] D. Briand, L. Guillot, U. Bley, S. Danninger, V. Gass, N.F. de Rooij, “Digital Micro- thrusters with simplified architecture and reliable ignition and combustion”, PowerMEMS 2008, Sendai, Japan [10] T.V. Mathew, MSc. Thesis, “Design of a MEMS Micro-Resistojet”, TU Delft, June 2011

101 6. Resistojet micro-thruster

102 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

7.7B Conclusions

In this chapter conclusions derived from the study presented in this thesis are summarized and recommendations for future work are given.

103 7. Conclusions

7.1.54B Conclusions

This thesis investigated the relevance of monocrystalline silicon as the functional material in thermal micro devices such as micro heaters and micro heat sinks. The use of the very versatile silicon-based MEMS technology proved to be an effective way to realize innovative and efficient microfluidic devices, such as micro-evaporators and resistojets.

7.1.1.91B Silicon used for heating

Doped silicon in a thin layer form was used in the hotplate heater presented in chapter 4, while undoped bulk silicon was employed for resistive heating in the device described in chapter 5. From the material study and the fabrication and characterization of these devices the following conclusions can be drawn:

ƒ Monocrystalline silicon is a material with a pronounced temperature dependence of its electrical resistivity which makes it suitable for temperature sensors and heaters with sensing capabilities. ƒ Above room temperature, resistivity first rises with temperature almost as a second-order polynomial function and after reaching a peak value (at the so-called intrinsic temperature), it drops exponentially. The intrinsic temperature can be shifted to higher or lower values by altering the doping concentration of silicon. ƒ The obtained characterization results match well with theoretical predictions and provide an overview of the silicon resistivity behavior in a broad temperature range (between 300 and 1000 K). In temperature ranges below and above intrinsic temperature, measured curves are stable and easy to model. ƒ The transitional region around the intrinsic temperature is not very suitable for temperature sensing. Therefore, determining a specific doping level (to obtain layers with proper intrinsic temperature) is the key factor enabling operation of the device fabricated with these layers in the desired temperature range.

104 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

ƒ Silicon heaters can be used to mimic heat sources to promptly test new concepts and principles, as described in chapter 5, and could be employed as alternative to metal heaters in devices like the resistojet described in chapter 6.

7.1.2.92B Silicon used for spreading the heat

Monocrystalline silicon possesses good thermal conduction properties which qualifies it as a suitable heat spreading material. Considering the findings with respect to monocrystalline silicon employed for cooling or heat spreading, a few concluding remarks are drawn:

MEMS micro-evaporator - cooling device ƒ Miniaturized, ultra compact evaporators for cooling regions with intensive heat density generation, by utilizing low flow rates of coolant can be fabricated using silicon MEMS technology. Measured data reveals that they are capable of absorbing at least 3 W on the 2.7 mm × 2.7 mm footprint area (~40 W/cm2) with water flowing at only 5 ml/h. ƒ To improve cooling performance, it is preferable to use arrays of high aspect ratio fins, use liquids as coolant, and preferably involve a phase change (in this case, evaporation). For cooling devices that follow above mentioned recommendations it is essential to successfully address the vapor backflow problem. Interconnected mesh of channels allows the vapor bubbles to grow also in lateral direction, which leads to better performance compared to the straight parallel channels. Furthermore, staggered layout of diamond-shaped fins with sharp tips provides a more stable operation than the rectangular fins. ƒ Because of the good thermal conductivity properties of silicon, heat transfer through the substrate can also be significant to the unwanted regions of the chip (such as inlet area). Therefore, thermal insulation is an important aspect, and can be achieved (as described in chapter 5) by replacing part of the substrate with glass and by introducing a “thermal cavity”. ƒ Heat transfer to fluid flow is significantly increased at temperatures above 100 °C because of evaporation, as expected. If combined with micrometer- sized high-aspect-ratio channels, considerable heat absorption rates can be achieved, even for very low coolant flow rates.

105 7. Conclusions

MEMS micro-resistojet - thruster device for space applications ƒ Miniaturized resistojets capable of producing thrust in μN range (up to 1 mN), with small mass (160 mg) and small dimensions (25 mm × 5 mm × 1 mm), suitable for application in nano-satellites, are realized using silicon MEMS technology. They can be integrated with miniaturized control valves and containers for storing pressurized propellant. ƒ In the micro-thruster silicon was successfully employed for spreading the heat from an integrated thin film heater to the propellant flow. It was convenient to structure silicon in such way that the channels were etched directly into heat spreading material. By heating the flow from room temperature to 350 °C, a 30% reduction in propellant consumption was observed.

7.1.3.93B Common technology platform for microfluidic devices

It is interesting to observe that although the micro-evaporator described in chapter 5 and the micro-thruster presented in chapter 6 are designed for different applications, they possess quite some similarities (see figure 7.1 and table 7.1). Both devices contain fluidic channels etched in silicon and sealed by silicon-to-glass anodic bonding. Channels were fabricated with a DRIE process and were designed to provide the flow of the fluid in-plane with the device. The fluidic connection to the rest of the test set-up is enabled through widened and deepened inlet and outlet regions etched in a two-step DRIE process.

heater fluid in and/or fins fluid out

chip bond wires El. instr. PCB

Figure 7.1. Common working principle and packaging scheme of the two devices

After wafer bonding, devices were diced in chips of the required size. Such

106 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications dies were glued to the same customized PCB following an identical gluing procedure (using the same glue and curing steps, see Appendix), and the metal tubes were glued to the inlet. Electrical connection between the metal pads on the PCB and the contact pads of the heater on the die was provided by wire- bonding with a 25 μm thick aluminum wire. Table 7.1. Comparison of the processing steps for micro-evaporators and micro-thrusters

Micro- Micro- Fabrication step Purpose evaporat thruster or

Thermal oxidation Protecting the bonding surface 9 9

Opening contact holes in SiO 2 + ion implantation of silicon ~ 9 Sputter-deposition of a metal Conductive layer to be used as layer + patterning by wet etching heater and/or interconnections 9 9

PECVD of SiO2 on both sides of the Mask for DRIE and protection of wafer the metal layer 9 9 Patterning the oxide mask Definition of to-be-etched- + DRIE of silicon (backside) through structures 9 Patterning the oxide mask Definition of inlet (+outlet) + DRIE of silicon (back side) 9 9 Patterning the oxide mask Definition of channels + DRIE of silicon (back side) 9 9 Patterning the oxide mask Definition of thermal cavity + DRIE of silicon (front side) 9 Wet stripping of the thermal “Unwrapping” the bonding oxide from the bonding surface surface 9 9 Anodic bonding to a glass wafer Sealing the channels 9 9

Stripping PECVD SiO2 Reaching conductive metal layer 9 9 Dicing Separating individual chips 9 9 Gluing the metal tubes Providing fluidic connection to the chip 9 9 Gluing the chip to the PCB Providing mechanical support 9 9 Wire-bonding the chip to PCB Providing electrical connection 9 9 ~ means that the step is not performed for the device described, but for a modified version the proposed step is also possible (in case Si is also used as a heater)

107 7. Conclusions

The one essential difference in the fabrication process (opening the contact holes in silicon oxide and implantation of the silicon through the contact holes) was due to the only functional difference: in the micro-evaporator device the bulk silicon had the role of both heater and heat spreader, whereas in the micro-thruster device, the bulk silicon was employed as heat spreader only and the heat was generated in the metallic thin film on the surface of the silicon wafer, on top of the silicon channel. In conclusion, we designed and fabricated devices for two quite different applications using almost the same fabrication process and the same packaging scheme to provide both electrical and fluidic connections. Both types of devices require one specific DRIE step, not needed for the other device (to define the thermal cavity in micro-evaporators, and to define the etch-through areas in micro-thruster, respectively). Those DRIE steps can be easily added or removed without affecting the remaining process flow. This indicates that the developed technology can be considered as a technology platform for fluidic devices. By optimizing the layout design, choosing proper etching recipes and altering the etching time in DRIE processes, this process module and testing scheme can be applied to other fluidic devices requiring heated liquid flow.

7.2.5B Outlook

The results presented in this thesis underline the large potential of monocrystalline silicon for thermal microdevices. However, further research and development is needed to address additional applications areas. Some interesting aspects of the devices treated in this thesis that need to be further addressed are indicated in this section.

Hotplate heaters At very high temperatures (above the intrinsic temperature of the used silicon layer, typically above 300-500 °C), there is a conflicting trend between the (increasing) resistance of the interconnects and the (decreasing) resistance of the silicon heater. This leads to a situation in which the interconnects are more resistive than the heater itself, and therefore more hot, which should be avoided. This can be solved by geometry optimization and/or selecting an alternative interconnect material. If the envisioned application would benefit

108 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications from the use of silicon also as a material for the interconnects, having different doping level (between the heater block and the connecting arms) does not solve the problem. In fact, at very high temperatures, both regions (with higher and lower doping) will be in the intrinsic regime and have the same resistivity. Consequently, as the interconnects are longer than the heater, they will again be more resistive. A possible solution could be to alter the design. If a thicker silicon layer is employed for the interconnects and a thinner one for the heater, the square resistance of interconnects will be reduced by a factor corresponding to the thickness ratio (at least a factor 10 should be considered). The technology used makes it possible to have arrays of such hotplate heaters that can be individually controlled so to operate at different temperatures. This is of interest in applications for calibration/ characterization of, for example, thermal cameras (i.e. IR detectors).

MEMS micro-evaporator The concept of cooling capabilities of the micro-evaporator demonstrator devices was successfully proven. In those devices, the embedded heater mimics an external heat source. They can obviously be coupled with a real external heat source. One such case is thermal management of light emitting diodes (LEDs), as depicted in figure 7.2. In such configuration, the heat produced by the LED during operation can be removed by the microevaporator, and the embedded Si heater in the micro-evaporator can now be used as a temperature sensor only.

LED

LED contact pads

Micro-evaporator

Figure 7.2. LED attached by thermally conductive glue on top of the micro-evaporator

Further steps would include the development of a closed-loop fluidic system. Such a system should be designed to avoid premature evaporation (in the regions before the coolant enters the intended fin-channel structure). The

109 7. Conclusions thermal insulation of the heated zone (removal of silicon, such as etching of the thermal cavity described in chapter 5) is an important aspect to consider. However, in some applications this insulation is not possible or not desirable. Then, the fin-channel structure should be designed with a sufficiently larger footprint than the footprint of the heated zone.

MEMS micro-resistojet For micro-thrusters applied in space satellites is very important to reduce propellant consumption as the propellant storage mass and volume are very limited on-board. A way to achieve that is to significantly increase the working temperature of the propellant gas. If the bulk silicon is employed as a heater (instead of the currently proposed thin-film heater on top of the device), heat generation would take place next to the channel, and propellant temperatures of 1000 °C or even higher can be reached. For temperatures above 1500 °C silicon carbide should be considered as a heater and substrate material. Although very practical for laboratory testing, resistive heating implemented on-board requires electrical energy stored in batteries (which increases the total power consumption of the satellite. If solar radiation can be focused (by specially designed shields and mirrors) to the micro-resistojet, it can be directly used for heating the propellant flow. The designed integrated resistor (thin-film or bulk) can then be used as a temperature sensor for better control of the propulsion system. As mentioned in section 6.7, it is advantageous to store the propellant on- board in liquid phase and then heat it up to evaporation during operation. The very high liquid density requires much less storage volume for the same propellant mass. The higher energy needed for evaporation (as compared to heat the gas) could be provided by solar radiation (abundant in space).

110 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Appendix8B A

A.1.94B MICRO-HOTPLATE FABRICATION FLOW CHART

Substrate: 1 SOI Si wafer, <100>, p-type, 100mm, Epitaxial growth: Epsilon epitaxial reactor, in-situ doped with arsenic, 950 °C, doping levels and thicknesses mentioned in table 4.2 Lithography: ASML wafer stepper; define the alignment marks

Silicon plasma etching: Omega plasma etcher, Cl2 = 80 sccm, HBr = 40 sccm

Cleaning: Oxygen plasma and HNO3 wet based

Thermal oxidation: Tempress furnace, T=1100°C, flow O2: 2.25 l/min, H2: 3.85 l/min, d=300 nm (d – thickness)

Silicon nitride LPCVD deposition: Tempress furnace, flow SiH2Cl2 / NH3 = 169.5 / 30.5 sccm, p = 150 mTorr, T= 850 °C, d=300 nm Lithography: ASML wafer stepper; define openings for etching in KOH [back side] Silicon nitride and silicon oxide combined plasma etching: Drytek 384T

plasma etcher, flow C2F6 = 65 sccm, p=130 mTorr, P=300W, [back side]

Cleaning: Oxygen plasma and HNO3 wet based Metallization: Trikon Sigma, sputtering Ti/TiN, d=120/80 nm, T=350 °C

Alloying: Tempress furnace, T=400°C , N2: 3.0 l/min, H2: 0.3 l/min, 20 min Lithography: ASML wafer stepper; define the interconnects + contact pads Titanium/titanium nitride plasma etching: Omega plasma etcher, recipe: TiN_Ti

Cleaning: Oxygen plasma and HNO3 wet based Lithography: ASML wafer stepper; define the heater+ interconnects + contact pads Silicon plasma etching: Adixen plasma etcher DRIE Bosch process (1-5 μm)

111 Appendix

Cleaning: Oxygen plasma and HNO3 wet based

RTA annealing: T=900°C, N2 atmosphere, 2 min

Cleaning: HNO3 wet based Metal evaporation: CHA evaporator, Ta/Pt, d=10/100 nm, evaporation through the hard mask (see section 4.3.1) Silicon wet etching: in KOH, 33%wt, 85°C (protection of the front side by vacuum holder), stop on buried oxide Silicon oxide wet striping: in BHF (1:7) (removing buried oxide of SOI wafer) Dicing: chip size 10 mm × 3.3 mm Packaging: gluing the chip to customized ceramic probe (see section 4.4.1)

A.2.95B MICRO-EVAPORATOR FABRICATION FLOW CHART

(Type C micro-evaporator)

Substrates: 1 double-side polished Si wafers, <100>, p-type, 100mm, 1 Borofloat glass wafer, 100 mm

Thermal oxidation: Tempress furnace, T=1100°C, O2: 2.25 l/min, H2: 3.85 l/min, d=300 nm (d – thickness) Lithography: ASML wafer stepper; define the alignment marks

Silicon plasma etching: Omega plasma etcher, flow Cl2 = 80 sccm, HBr = 40 sccm

Cleaning: Oxygen plasma and HNO3 wet based Lithography: ASML wafer stepper; define the contact openings

Silicon oxide plasma etching: Drytek 384T plasma etcher, flow C2F6/CHF3 = 36/144 sccm, p = 180mTorr, P = 300W

Cleaning: Oxygen plasma and HNO3 wet based Ion implantation: boron, dose: 1.0∙1015 cm-2, energy: 50 keV

Cleaning: Oxygen plasma and HNO3 wet based Silicon oxide wet etching: removal of native oxide, dipping in HF 0.55%, 4 min

112 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Metallization: Trikon Sigma, sputtering Al/Si(1%), d=1400 nm, T=350 °C

Alloying: Tempress furnace, T=400°C , N2: 3.0 l/min, H2: 0.3 l/min, 20 min Lithography: EVG contact aligner; define the heater + contact pads Aluminum wet etching: in phosphoric acid heated up to 35 degrees + 30 sec dip in mixture of nitric and hydrofluoric acids to remove (1%)-silicon grains

Cleaning: Acetone and HNO3 wet based

Silicon oxide deposition: PECVD Novellus, flow N2/SiH4/N2O = 3150/205/6000 sccm, p = 2.2 Torr, T = 400 °C, d = 6 μm [both sides] Lithography: ASML wafer stepper; define thermal cavity

Silicon oxide plasma etching: Drytek 384T plasma etcher, flow C2F6/CHF3 = 36/144 sccm, p = 180mTorr, P = 300W

Cleaning: Oxygen plasma and HNO3 wet based Lithography: ASML wafer stepper; define the inlet/outlet [this and further steps performed on the back side]

Silicon oxide plasma etching: Drytek 384T plasma etcher, flow C2F6/CHF3 = 36/144 sccm, p = 180mTorr, P = 300W

Cleaning: Oxygen plasma and HNO3 wet based Silicon DRIE 1: Adixen plasma etcher DRIE Bosch process ~250 μm

Cleaning: Oxygen plasma and HNO3 wet based Lithography: (spray coating), EVG contact aligner; define the channels

Silicon oxide plasma etching: Drytek 384T plasma etcher, recipe C2F6/CHF3 = 36/144 sccm, p = 180mTorr, P = 250W Silicon DRIE 2: Adixen plasma etcher DRIE Bosch process, recipe with aspect- ratio independent etch rate, ~100 μm Silicon DRIE 3: Adixen plasma etcher DRIE Bosch process ~385 μm (thermal cavity) [front side] Silicon oxide wet striping: in BHF (1:7) (unwrapping the bonding surface) Anodic bonding to a glass wafer: AML wafer bonder, 400 °C, 1000V, 1h

Silicon oxide plasma etching: Drytek 384T plasma etcher, flow C2F6/CHF3 = 36/144 sccm, p = 180mTorr, P = 300W, [front side] (to access the Al layer) Dicing: chip size 10 mm × 3.3 mm Packaging: gluing the metal tube, gluing to PCB (epoxy glue Araldite AV 138M and hardener HV 998, proportion 1:4, curing in the oven for 20 min at 80 °C), wire bonding with Al wires

113 Appendix

A.3.96B MICRO-THRUSTER FABRICATION FLOW CHART

Substrates: 1 double-side polished Si wafers, <100>, p-type, 100mm, 1 Borofloat glass wafer, 100 mm

Thermal oxidation: Tempress furnace, T=1100°C, O2: 2.25 l/min, H2: 3.85 l/min, d=300 nm (d – thickness) Lithography: EVG contact aligner; define the alignment marks

Silicon oxide plasma etching: Drytek 384T plasma etcher, flow C2F6/CHF3 = 36/144 sccm, p = 180mTorr, P = 300W

Silicon plasma etching: Omega plasma etcher, flow Cl2 = 80 sccm, HBr = 40 sccm

Cleaning: Oxygen plasma and HNO3 wet based Metallization: Trikon Sigma, sputtering Al/Si(1%), d=675 nm, T=350 °C Lithography: EVG contact aligner; define the heater + contact pads Aluminum wet etching: in phosphoric acid heated up to 35 degrees + 30 sec dip in mixture of nitric and hydrofluoric acids to remove (1%)-silicon grains

Cleaning: Acetone and HNO3 wet based

Silicon oxide deposition: PECVD Novellus, flow N2/SiH4/N2O = 3150/205/6000 sccm, p = 2.2 Torr, P=1000W, T = 400 °C, d = 6 μm [both sides] Lithography: EVG contact aligner; define etch-through area [this and further steps performed on the back side]

Silicon oxide plasma etching: Drytek 384T plasma etcher, flow C2F6/CHF3 = 36/144 sccm, p = 180mTorr, P = 300W

Cleaning: Oxygen plasma and HNO3 wet based Silicon DRIE 1: Adixen plasma etcher DRIE Bosch process, ~200 μm

Cleaning: Oxygen plasma and HNO3 wet based Lithography: (spray coating), EVG contact aligner; define the inlet

Silicon oxide plasma etching: Drytek 384T plasma etcher, flow C2F6/CHF3 = 36/144 sccm, p = 180mTorr, P = 300W

Cleaning: Oxygen plasma and HNO3 wet based Silicon DRIE 2: Adixen plasma etcher DRIE Bosch process ~200 μm

Cleaning: Oxygen plasma and HNO3 wet based Lithography:(spray-coating),EVG contact aligner; to define channel and nozzle

Silicon oxide plasma etching: Drytek 384T plasma etcher, flowC2F6/CHF3 = 36/144 sccm, p = 180mTorr, P = 300W

114 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Silicon DRIE 3: Adixen plasma etcher DRIE Bosch process, recipe with aspect- ratio independent etch rate ~150 μm Silicon oxide wet striping: in BHF (1:7) (unwrapping the bonding surface) Anodic bonding to a glass wafer: AML wafer bonder, 400 °C, 1000V, 1h

Silicon oxide plasma etching: Drytek 384T plasma etcher, flow C2F6/CHF3 = 36/144 sccm, p = 180mTorr, P = 300W, [front side] (to access the Al layer) Dicing: chip size 25 mm × 5 mm Packaging: gluing the metal tube, gluing to PCB (epoxy glue Araldite AV 138M and hardener HV 998, proportion 1:4, curing in the oven for 20 min at 80 °C), wire bonding with Al wires

115 Appendix

List9B of abbreviations

BHF buffered hydrofluoric acid CMOS complementary metal–oxide–semiconductor CVD chemical vapor deposition DRIE deep reactive ion etching EM electromagnetic FTBA front-to-back alignment IC integrated circuit LED light emitting diode LPCVD low pressure chemical vapor deposition KOH potassium hydroxide MEMS microelectromechanical systems PCB printed circuit board PECVD plasma enhanced chemical vapor deposition ppm parts per million RTA rapid thermal annealing SEM scanning electron microscopy SiP system in package SOI silicon-on-insulator

116 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Summary10B

MEMS monocrystalline-silicon based thermal devices for chemical and microfluidic applications by Marko Mihailović

This thesis explores the employment of monocrystalline silicon in microsystems as an active material for different thermal functions, such as heat generation and heat transfer by conduction. In chapter 1 applications that need thermal micro devices, micro heaters and micro heat exchangers, are briefly introduced. The shortcomings of commonly used materials are listed, and monocrystalline silicon is identified as an appropriate choice for several thermal micro devices. Chapter 2 briefly presents the basic theory on resistive heating and heat transfer (by conduction, convection and radiation) and how they relate to the devices and structures presented in the following chapters. Chapter 3 summarizes the temperature dependence of the electrical and thermal properties of monocrystalline silicon in a wide temperature range. Thermal conductivity of silicon places silicon among good thermal conductors at room temperature and even better at cryogenic temperatures. In spite of the declining value of thermal conductivity at higher temperatures, silicon is still among the better thermal conductors compared to other materials typically employed in microsystem fabrication. The temperature dependence of the electrical resistivity is in chapter 3 both derived theoretically and confirmed experimentally on specially prepared samples that used differently-doped silicon layers of the same thickness. Electrical resistivity of silicon first increases with temperature, and after reaching the peak value (at so-called intrinsic temperature) it decreases with further temperature increase. This intrinsic temperature is also dependent on the doping level. Such doped silicon layers can be employed in sensors and specifically in heaters with sensing capability.

117 Summary

Chapter 4 introduces a micro-hotplate heater capable of reaching temperatures up to 800 °C. The used materials are epitaxial Si for the heater and TiSi/TiN layer stack for the interconnects, which provides compatibility with CMOS technology. Differently doped silicon layers of different thicknesses were released by bulk micromachining to fabricate free-standing structures. The temperature dependent resistance of silicon was used for temperature monitoring. A silicon-based micro-evaporator is introduced in chapter 5. This is a cooling device intended for dealing with high heat fluxes caused by intensive local heating (e.g. by electrical power dissipation or exothermal chemical reaction). The aim of this micro-evaporator is to achieve a maximum cooling capacity and operation stability at very small liquid flow rates (in the order of 1-5 ml/h). Four different proposed fin-channel structures, with high aspect ratio channels (10 μm and 20 μm wide, 100 μm deep) are sealed with silicon or glass by wafer bonding and tested with de-ionized water as coolant. Silicon fins enhance the heat transfer to the coolant. The embedded bulk silicon heater mimics external heat sources and, at the same time, acts as temperature sensor. Measured absorbed power fluxes were up to 3 W (which corresponds to 40 W/cm2 for a heater footprint of 2.7 mm × 2.7 mm), for a fluid flow of 5 ml/h. Optimizations of the fin-channel structure and removing material by etching a cavity for thermal insulation led to a more stable operation in a broader range of set-point conditions. Chapter 6 presents a miniaturized resistojet thruster device with an integrated thin-film heater, capable of delivering thrusts in the micronewton– millinewton range. Such devices can be applied for fine attitude control of nano-satellites. Miniaturized resistojet comprises a microchannel (width: 50 μm, height: 150 μm, length: 2 cm) and a nozzle throat narrowed to 10 μm. Both channel and the nozzle were etched in silicon and sealed by anodic bonding to glass. In this device, silicon acts as a heat spreader from the integrated aluminum heater to the propellant flow inside the etched microchannel, to reduce propellant consumption. Based on the pressure measurements, calculated thrust is in 20-960 μN range, which complies with the desired range, and a 30% reduction of propellant consumption is observed when propellant flow is heated from room temperature to 350 °C. Reducing the propellant consumption is essential as the propellant storage mass and volume are very limited on-board. Finally, concluding remarks are given in chapter 7, together with recommendations for further research.

118 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Samenvatting1B

MEMS thermische apparaten gebaseerd op monokristallijn-silicium voor de chemische en microfluïdische toepassingen door Marko Mihailović

Dit proefschrift onderzoekt de werking van monokristallijn silicium in microsystemen als een actief materiaal voor verschillende thermische functies, zoals warmteontwikkeling en warmteoverdracht door geleiding. In hoofdstuk 1 worden de toepassingen, die thermische microapparaten (microverwarmers en microwarmtewisselaars) nodig hebben, kort geïntroduceerd. Verder worden de tekortkomingen van de veel gebruikte materialen vermeld en wordt toegelicht waarom het monokristallijn silicium een geschikte keuze is voor diverse thermische microapparaten. Hoofdstuk 2 presenteert kort de fundamentele theorie over weerstandsverwarmings- bronnen en warmteoverdracht (door geleiding, convectie en straling) en hoe deze betrekking hebben op de apparaten en structuren, die in de volgende hoofdstukken beschreven worden. Hoofdstuk 3 geeft een overzicht weer van de temperatuur afhankelijke elektrische en thermische eigenschappen van monokristallijn silicium in een breed temperatuurbereik. Door de thermische geleidbaarheid van silicium valt silicium onder de categorie van goede thermische geleiders bij kamer- temperatuur en zelfs beter bij cryogene temperaturen. Ondanks afname van de thermische geleidbaarheid bij hogere temperaturen, is silicium nog steeds een betere thermische geleider ten opzichte van andere materialen die doorgaans in de fabricage van microsystemen worden gebruikt. De temperatuurafhankelijkheid van de elektrische weerstand is in hoofdstuk 3 zowel theoretisch afgeleid als experimenteel bevestigd op speciaal geprepareerde monsters, welke verschillend gedoteerde silicium lagen gebruiken van dezelfde dikte. Elektrische weerstand van silicium neemt eerst toe met de temperatuur, na het bereiken van de piekwaarde (de zogenaamde intrinsieke temperatuur) vermindert dit bij verdere toename van de

119 Samenvatting temperatuur. De intrinsieke temperatuur is ook afhankelijk van het doping- niveau. Dergelijke gedoopte silicium lagen kunnen worden gebruikt in sensoren en specifiek in verwarmers die tegelijkertijd de temperatuur kunnen meten. Hoofdstuk 4 introduceert een micro-kookplaat verwarmer die temperaturen tot 800 °C kan bereiken. De gebruikte materialen zijn epitaxiale Si voor de verwarmer en een stapel van TiSi-/TiN-lagen voor de verbindingen, waardoor integratie met CMOS-technologie mogelijk is. Verschillende gedoteerde silicium lagen van verschillende dikten werden vrijgegeven door bulk micromachining om hiermee vrijstaande structuren te fabriceren. De temperatuur afhankelijke weerstand van silicium werd gebruikt voor het monitoren van de temperatuur. Een op silicium-gebaseerde microverdamper wordt geïntroduceerd in hoofdstuk 5. Dit is een koelinrichting die bestemd is voor het omgaan met hoge warmtestromen veroorzaakt door intensieve lokale verwarming (bijvoorbeeld door een verlies van elektrisch vermogen of een exotherme chemische reactie). Het doel van deze microverdamper is een maximale koelcapaciteit en een operationele stabiliteit te bereiken bij zeer lage doorstroomtempo’s van vloeistoffen (in de orde van 1-5 ml/h). Vier verschillende voorgestelde vinkanaal structuren, met hoge aspect ratio kanalen (10 μm en 20 μm breed, 100 μm diep) worden met silicium of glas door middel van wafer bonding samengebracht en met gedeïoniseerd water als koelmiddel getest. Silicium vinnen verbeteren de warmteoverdracht naar de koelvloeistof. De omsloten bulk silicium verwarmer simuleert een externe warmtebron en fungeert tegelijkertijd als een temperatuursensor. Er werden geabsorbeerde vermogens tot 3 W gemeten (wat overeenkomt met 40 W/cm2 voor een voetafdruk van een verwarmer van 2,7 mm × 2,7 mm) voor een vloeiende stroom van 5 ml/h. Optimalisaties van het vinkanalen structuur en het verwijderen van het materiaal door een holte voor thermische isolatie te etsen, heeft geleid tot een meer stabiele werking in een breder bereik van set-point voorwaarden. Hoofdstuk 6 presenteert een geminiaturiseerde resistojet boegschroef toestel met een geïntegreerde dunne film verwarmer die waarden in de micronewton-millinewton bereik kan leveren. Dergelijke apparaten kunnen worden toegepast voor de nauwkeurige controle van de houding/positionering van nano-satellieten. Geminiaturiseerde resistojet bestaat uit microkanaal (breedte: 50 μm, hoogte: 150 μm, lengte: 2 cm) en een straalpijphals teruggebracht tot 10 micrometer. Zowel het kanaal als het mondstuk zijn geëtst in silicium en verzegeld door anodische bonding op het glas. In dit apparaat, fungeert het silicium als een warmteverspreider van de geïntegreerde

120 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications aluminium verwarmer naar de voortbewegende stroming binnen het geëtst microkanaal, om de consumptie van drijfgas te verminderen. Op basis van de drukmetingen ligt de berekende stuwkracht in een bereik van 20-960 μN en voldoet aan het gewenste bereik. Een reductie van 30% van het verbruik van drijfgas wordt waargenomen wanneer voortbewogen stroom wordt verwarmd van een kamer-temperatuur tot 350 °C. Vermindering van het verbruik van brandstof is van essentieel belang omdat de massa en het volume van brandstofopslagsystemen aan boord zeer beperkt zijn. Tot slot staat in hoofdstuk 7 de slotbeschouwing, samen met de aanbevelingen voor verder onderzoek.

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MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

List12B of publications

Journal papers

[1] M. Mihailović, C.M. Rops, J. Hao, L. Mele, J.F. Creemer, P.M. Sarro, “MEMS silicon-based micro-evaporator”, Journal of Micromechanics and Microengineering (JMM), vol. 21, no. 7 (2011) 075007 [2] M. Mihailović, T.V. Mathew, J.F. Creemer, B.T.C. Zandbergen, P.M. Sarro, “MEMS slicon-based micro-thruster for attitude control of nano-satellites”, Sensors and Actuators A: Physical, to be submitted [3] M. Mihailović, J.F. Creemer, E. Iervolino, A.W. van Herwaarden, R.A.M. Wolters, P.M. Sarro, “MEMS hotplate heaters based on epitaxial silicon layers for targeted temperature ranges”, Sensors and Actuators A: Physical, to be submitted

Conference proceedings

[1] M. Mihailović, T.V. Mathew, J.F. Creemer, B.T.C. Zandbergen and P.M. Sarro, “MEMS silicon-based resistojet micro-thruster for attitude control of nano- satellites”, Proceedings of the Transducers ’11 conference (pp. 262-265), 2011, Beijing, China [2] L. Mele, F. Santagata, E. Iervolino, M. Mihailović, T. Rossi, A.T. Tran, H. Schellevis, J.F. Creemer and P.M. Sarro, “Sputtered molybdenum as conductive material for high-temperature microhotplates”, Proceedings of the Transducers ’11 conference (pp. 2690-2693), 2011, Beijing, China [3] B. Morana, F. Santagata, L. Mele, M. Mihailović, G. Pandraud, J.F. Creemer and P.M. Sarro, “A silicon carbide MEMS microhotplate for nanomaterial characterization in TEM”, Proceedings of IEEE MEMS 2011 conference (pp. 380-383), 2011, Cancun, Mexico [4] T.V. Mathew, M. Mihailović, B. Zandbergen, P.M. Sarro, J. F. Creemer, “A silicon-based MEMS resistojet for propelling cubesats”, accepted for

123 List of publications

presentation at 62nd International Astronautical Congress (IAC-11.C4.3.2), October 2011, Cape Town, South Africa [5] C.M. Rops, M. Mihailović, P.M. Sarro, “Design and development of an ultra compact silicon phase-change heat exchanger”, Proceedings of the 2nd European Conference in Microfluidics, 2010, Toulouse, France [6] M. Mihailović, J.F. Creemer, P.M. Sarro, “Electrical characterization of TiSi/TiN layer stack in temperature range from 0 – 500 °C”, Proceedings of 13th Annual Workshop on Semiconductor Advances for Future Electronics and Sensors (SAFE) (pp. 114-117), 2010, Veldhoven, the Netherlands [7] M. Mihailović, C.M. Rops, J.F. Creemer, P.M. Sarro, “MEMS silicon-based micro-evaporator with diamond-shaped fins”, Proceedings of the EUROSENSORS XXIV (pp. 969-972), 2010, Linz, Austria [8] F. Santagata, L. Mele, M. Mihailović, B. Morana, J.F. Creemer, P.M. Sarro, “Wafer level encapsulation techniques for a MEMS microreactor with integrated heat exchanger”, Proceedings of IEEE Sensors 2009 Conference (pp. 799- 802), 2009, Christchurch, New Zealand [9] C.M. Rops, J. Hao, R. Lindken, M. Mihailović, J.F.Creemer “Heat transfer enhancement using fin structures: ultra-compact heat exchanger design”, Proceedings of 2nd European Process Intensification Conference, 2009, Venice, Italy [10] M. Mihailović, J.F. Creemer, P.M. Sarro, “Sheet resistance of As-doped monocrystalline silicon in temperature range up to 1100 K”, Proceedings of 12th Annual Workshop on Semiconductor Advances for Future Electronics and Sensors (SAFE) (pp. 36-39), 2009, Veldhoven, the Netherlands [11] J. Hao, M. Mihailović, C.M. Rops, J.F. Creemer, P.M. Sarro, “Silicon-based MEMS micro-evaporator”, Proceedings of 20th Micromechanics Europe Workshop (MME) (pp. 1-4), 2009, Toulouse, France [12] M. Mihailović, J.F. Creemer, P.M. Sarro, “Electrical behaviour of mono-Si based microhotplate heater”, Proceedings of 11th Annual Workshop on Semiconductor Advances for Future Electronics and Sensors (SAFE) (pp. 411-414), 2008, Veldhoven, the Netherlands [13] M. Mihailović, J.F. Creemer, P.M. Sarro, “Monocrystalline silicon microhotplate heater”, Proceedings of the EUROSENSORS XXII (pp. 1611-1614), 2008, Dresden, Germany [14] M. Mihailović, J.F. Creemer, P.M. Sarro, “Monocrystalline Si-based microhotplate heater”, Proceedings of 10th Annual Workshop on Semiconductor Advances for Future Electronics and Sensors (SAFE) (pp. 608-611), 2007, Veldhoven, The Netherlands

124 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Acknowledgements13B

There is no single achievement that is a purely individual accomplishment. Indirectly, each time an author uses part of our common heritage, such as enjoying the peace and prosperity, gradually achieved by the ancestors in the past, or profiting from knowledge and experience acquired through the system of public education or through interaction with other people… The same goes for this PhD thesis, but in addition to this general acknowledgment to humanity, I am very grateful to many people who have helped me complete this dissertation. My apologies to anyone whom I might have forgotten to mention. First of all, I would like to thank my promoter Lina Sarro for endless support in the past four and half years. I am grateful not only for her technical supervision, but also for general mentoring and especially for providing such a great positive atmosphere in the group that it was always very pleasant to be a member of. There are no words to thank you for all your patience with me, so I will stop here. Lina, thanks for everything!!! I am also very grateful to Fredrik Creemer for all his effort and patience in dealing with me during the first two and a half years as my supervisor. I learned a lot during our meetings and discussions. To my friends Miloš and Jelena Popadić I owe great thanks for being so helpful and supportive in all ways imaginable, both at work and in our free time, and especially for being the ones who persuaded me to come to Delft. Ever since the extensive cleanroom training (the first serious technical activity for all PhD candidates in Dimes) given by Silvana Milosavljević, she was a person I could always rely on. As compatriots speaking the same language, it was natural that I referred to her even for non-research related topics. She and her husband Slobodan Mijalković helped me significantly to feel more at home in the Netherlands, which is very appreciated when home is far, far away. I would like to emphasize the help I received from my colleagues in the MEMS group: Fabio Santagata, Luigi Mele, Jia Wei, Elina Iervolino, Hoa Pham, Gregory Pandraud and Bruno Morana. Many short brainstorming sessions we had and all your assistance in fabrication/packaging/measurements definitely made it more

125 Acknowledgements comfortable and enjoyable for me to perform my research and complete this thesis. Especially, thank you for being such very good friends! ECTM and Dimes are very big family. I had a chance and pleasure to collaborate and socialize with so many people and I would like to thank all of them: - scientific and managerial staff: Lis Nanver, Miro Zeman, Kees Beenakker, Ronald Dekker, Ryoichi Ishihara, Marian Bartek, Bert Goudena, Paul de Wit, thank you all for managing and providing pleasant working environment! - support staff: I would like to start with John Slabbekoorn and Mario Laros, who were always available in the long evenings when I was fabricating my first device and experienced many troubles with the cleanroom equipment. Without you two, it would have taken me much longer to gain self-confidence in cleanroom processing! The list goes on with Joost Berendse, Charles de Boer, Alex van den Bogaard, Johan van der Cingel, Jan Groeneweg, Koos van Hartingsveldt, Suzanne van Herp, Ruud Klerks, Sebastiaan Maas, Hugo Schellevis, Tom Scholtes, Loek Steenweg, Peter Swart, Martin Tijssen, Wim Tiwon, Robert Verhoeven, Ron van Viersen, Cassan Visser, Wim van der Vlist, Jan Warmerdam, Wim Wien, Johannes van Wingerden, Jan Cornelis Wolf, Michiel van der Zwan, and the magnificient Henk van Zeijl! Thank you all for all your support, assistance in processing/packaging/measurements, short and long trainings and explanations, but also general life discussions... - administrative staff: Marian Roozenburg, Marysia Lagendijk, Bianca Knot, Tamara den Hartog and Rosario Salazar Lozano. And although he was supposed to have a more technical function within Dimes, I feel adding Rino Martilia to this part of the list, and I am sure he would prefer this company anyway. Thank you all for your always cheerful and supportive attitude! - fellow researchers (post-docs, PhD candidates, MSc students, guest researchers): This is a big group of people from ECTM and other groups with whom I shared many good and bad moments, people with whom I always felt like a member of team. Tuncay Alan, Aslıhan Arslan, Solomon Agbo, Alessandro Baiano, Cleber Biasotto, Ellen Christopherson, Trin Chu Duc, Yann Civale, Jaber Derakhshandeh, Pablo Estevez, Gennaro Gentile, Negin Golshani, Victor Gonda, Lei Gu, Vincent Henneken, Andrea Ingenito, Olindo Isabella, Klaus Jäger, Vladimir Jovanović, Faruk Krečinić, Luigi La Spina, Ruoxuan Li, Gianpaolo Lorito, Sabrina Magnani, Parastoo Maleki, Mauro Marchetti, Benjamin Mimoun, Thomas Moh, Vahid Mohammadi, Caroline Mok, Sharma Mokkapati, Amir Naeimi, Sander Paalvast, Rene Poelma, Marcello Porta, Vijayekumar Rajaraman, Salvatore Russo, Amir Sammak, Francesco Sarrubi, Chenggang Shen, Lei Shi, Sebastian Sosin, Marco Spirito, Michele Squillante, Agata Šakić, Daniel Tajari Mofrad, Jiagi Tang, Sima Tarashioon, An Tran, Genni Testa, Ana Vieira da Silva, Daniel Vidal, Francesco Vitale, Sten Vollebregt,

126 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

Michael Wank, Joke Westra, Bo Yan, Huaiyu Ye and Theodoros Zoumpoulidis, thank you all for your support and assistance and for the fact that I can honestly say that I had a very enjoyable working environment! Special thanks go to my office mates. I really enjoyed the time spent in amusing talks with Nobuyuki Matsuki, Mohamed Saadaoui and Yujian Huang who were in the office with me for short time one after another. And of course, I am very thankful to already mentioned Jia Wei with whom I had a pleasure to share the office for more than three years. I am also indebted to Sander van Herwaarden, managing director of Xensor Integration, Delfgauw, and colleagues from that company, for all of our fruitful discussions and always receiving express help in chip assembly and high– temperature measurements. Big thanks go to external partners in research on some of the devices presented in this thesis. The micro-evaporator would not be possible without Cor Rops from TNO. I would also like to acknowledge the contribution of Wim Peterse, also from TNO Science and Industry, Delft, and Jun Hao, a student who was working on his MSc thesis in MEMS group of ECTM. As for the micro-thruster, I would like to acknowledge the contribution of Barry Zandbergen from the Space Systems Engineering Group at Aerospace engineering at TU Delft, and Tittu Mathew who was working on his MSc thesis at that group. Rob Wolters from NXP helped by providing a set-up and performing high- temperature measurements of the epitaxial layers described in chapter 3, and through several very interesting and useful discussions. I would like to thank the Dutch Technology Foundation STW for financing the project and many useful discussions during the user-committee meetings. For this physical book to appear, I am indebted to Željko Matić for the cover design, to Martin Mulder and Milena Jovanović for correcting my translation of the summary into Dutch, and to Veronica Pišorn and Miloš Popadić for proofreading some parts of the text. A big role in getting me interested to start PhD research in the MEMS field was performed by teaching staff and fellow students at the Electrical Engineering Faculty of University of Belgrade where I received my technical knowledge, as well by people from Physical Electronics Laboratory at ETH Zurich (namely, Andreas Hierlemann, Kay-Uwe Kirstein, Petra Kurzawski and all other colleagues from that group) where I stepped into my first practical engineering MEMS experience. So, thank you all! During my PhD project, I spent almost whole four years as a board member of PromooD - an association of PhD candidates at TU Delft. I enjoyed the time spent

127 Acknowledgements organizing and participating in various PromooD activities that helped me better understand and live the life of a PhD candidate. I would like to thank all of my fellow board members in the past four years (Rudy, Tom, Gwen, Anand, Herdis, Cathal, Aleksandro, Vincent, Ipek, Irem, Alberto, Rajen, Sylvie…) for collaboration, inspiration and the pleasant time I had. Special thanks are owed to Frederik de Wit, president of PromooD at the moment when I joined TU Delft, who got me interested in joining the PromooD board. Outside TU Delft, I did not meet many Dutch people, but certainly cherished the multinational community in this university town and its surroundings. Coming from abroad was a uniting factor for many of us, no matter from which part of the planet we arrived. I enjoyed the company of numerous people gathered around the ex-Yugoslavian and ex-SSSR mailing lists in Delft - “Delfćani” and “Ura Tovarishi!”, but also of people belonging to Italian, Greek, Turkish, Spanish, Latin American and other, mainly “southern” communities. Just to name a few of them who were in Delft for longer or shorter period and with whom I shared many pleasant moments: Darko Simonović, Jasmina Omić, Aleksandar Borisavljević, Veronica Pišorn, Stevan Nađ-Perge, Vladimir Milovanović, Stevan Rudinac, Maja Rudinac, Filip Miletić, Miloš Vulović, Mihailo Obućina, Elena Rius Garcia, Markoslav Petrović, Nemanja Filipović, Dalibor Čvorić, Dubravka Aranđelović, Jovana Ripić, Milena Jovanović, Biljana Kaitović, Milena Milovanović, Milica Živković, Karmen Schäfer, Tobias Schenk, Deniz Ugur, Arkady Kustov, Vadim Sidorkin, Oksana Sidorkina, Oleg Guziy, Maria Rudneva, Slava Savenko, Yefheniy Pivak, Anton Gryzlov, Teresa Adrega, Clemencia de Abreu, Igor Rust... Thank you all for allowing me to have a more enjoyable and richer life, not only limited to my work environment. I am especially grateful to my flatmate Ivan Lazić, for constant caring and support, for being superb friend and great company for long “meaning-of-life” discussions, for his entertaining guitar performances and much more. I would like to express my gratefulness also to all the friends from Serbia and all over Europe who visited me in Delft during the past years and enriched my stay in the Netherlands. I am also very grateful to all the friends who offered me hospitality during many, many (prolonged) weekends I spent around Europe. All these played a role in completing this thesis, either by distracting me, or by motivating and inspiring me when it was needed. Many thanks go to all the friends and family in Serbia with whom I was in constant or less regular contact. All of you (school-friends, scouts, neighbors from my home town Mladenovac, colleagues from university, EESTEC-ers, roommates and friends from Soba 143…) made me smile with each message or phone call I received from you. At the end, special thanks go to my parents, Milka and Dobrivoje Mihailović, for their comprehensive contribution and support. Мама и тата, хвала вам на свему!

128 MEMS Monocrystalline-Silicon Based Thermal Devices for Chemical and Microfluidic Applications

About14B the author

Marko Mihailović was born in Belgrade, Serbia (Yugoslavia) in 1981. He received a graduate engineering degree from the Faculty of Electrical Engineering, University of Belgrade, in January 2007, graduating from the Department of Physical Electronics (Group for Optoelectronics and Laser Engineering). During his studies, in 2006, as an intern in the Physical Electronics Laboratory of Prof. dr. Andreas Hierlemann, at the Department of Physics at the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland, he performed characterization of CMOS-based calorimetric chemical micro- sensors. The results of this research were presented in his diploma thesis at the University of Belgrade. In February of 2007, he joined the Laboratory of Electronic Components, Technology and Materials (ECTM) at the Delft University of Technology, and the Delft Institute of Microsystems and Nanoelectronics (Dimes), in the Microsystem technology/MEMS group of Prof. dr. Lina Sarro, as a PhD candidate. His research focused on monocrystalline-silicon-based thermal devices for chemical and microfluidic applications (hotplate heaters, micro- evaporators, micro-thrusters). The results of his PhD research are presented in this doctoral dissertation. As of August 2011, he will be appointed with Océ-Technologies B.V. in Venlo, The Netherlands, working as a developer of MEMS inkjet printheads.

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