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3-DIMENSIONAL MICROSTRUCTURAL

FABRICATION OF FOTURANTM GLASS WITH

FEMTOSECOND LASER IRRADIATION

TEO HONG HAI

NATIONAL UNIVERSITY OF SINGAPORE 2009

3-DIMENSTIONAL MICROSTRUCTURAL

FABRICATION OF FOTURANTM GLASS WITH

FEMTOSECOND LASER IRRADIATION

TEO HONG HAI

(B. Eng. (Hons.), Nanyang Technological University)

A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009

Acknowledgement

ACKNOWLEDGEMENTS

I would like to take this opportunity to express my appreciation to my supervisor,

Associate Professor Hong Minghui for his guidance during the entire period of my

Masters studies. He has been encouraging particularly in trying times. His suggestions and advice were very much valued.

I would also like to express my gratitude to all my fellow co-workers from the DSI-NUS

Laser Microprocessing Lab for all the assistance rendered in one way or another.

Particularly to Caihong, Tang Min and Zaichun for all their encouragement and assistance as well as to Huilin for her support in logistic and administrative issues.

Special thanks to my fellow colleagues from Data Storage Institute (DSI), in particular,

Doris, Kay Siang, Zhiqiang and Chin Seong for all their support.

To my family members for their constant and unconditioned love and support throughout these times, without which, I will not be who I am today.

Table of Contents

TABLE OF CONTENTS

ACKNOWLEDGEMENTS i

TABLE OF CONTENTS ii

SUMMARY v

LIST OF FIGURES viii

LIST OF SYMBOLS x

LIST OF PUBLICATIONS xi

CHAPTER 1 INTRODUCTION

1.1 General Background 1

1.2 Recent Progress in Photosensitive Glass and Femtosecond Laser 2 Microprocessing

1.3 Limitations of Conventional Glass and Challenges on Laser Microprocessing 9 Techniques of Glass Substrates

1.4 Fabrication by Femtosecond Laser on FoturanTM Glass 12

1.5 Research Objectives and Contributions 17

1.5.1 Fabrication of a Monolithic 2-level 3-dimensional Micro-mixer 17

1.5.2 Optimization of a Monolithic 2-level 3-dimensional Micro-mixer 18

1.6 Thesis Outline 20

Table of Contents

CHAPTER 2 THEORETICAL BACKGROUND

2.1 Multi-photon Absorption by Photosensitive Glass 27

2.1.1 Photosensitive Glass under Femtosecond Laser Irradiation 31

2.2 Mechanism during Thermal Annealing of Photosensitive Glass under 35 Femtosecond Laser Irradiation

2.2.1 Reactions Involved during Thermal Annealing of FoturanTM Glass 35 after Laser Exposure

2.3 Chemical Reactions during Wet Chemical HF etching of Photosensitive 38 Glass under Femtosecond Laser Irradiation

CHAPTER 3 EXPERIMENTAL SETUP

3.1 Femtosecond Laser Fabrication of Microstructures In-situ of FoturanTM 44 Glass

3.1.1 Introduction 44

3.1.2 Experimental Setup of Femtosecond Laser Irradiation of FoturanTM 46 Glass

3.1.3 Application of the Femtosecond Laser 48

3.1.4 Thermal Annealing Cycle and Wet Chemical Etching 50

3.2 Characterization Techniques for the Monolithic 2-level 3D Internal Micro- 51 mixer

3.2.1 Optical Microscopy 51

3.2.2 JEOL JSM 7401F Scanning Electron Microscopy (SEM) 54

3.2.3 3D Surface Profilometry 56

3.2.5 Characterizing the Mixing Capability of the Fabricated Micro-mixer 58

iii

Table of Contents

CHAPTER 4 3-DIMENSIONAL MICROSTRUCTURAL FABRICATION BY

FEMTOSECOND LASER MICROPROCESSING

4.1 Fabrication Process Flow 62

4.1.1 Sample Preparation 64

4.1.2 Multi-Photon Absorption 66

4.1.3 Principles of Two-Photon Excitation 69

4.2 Thermal Annealing 71

4.3 Etching 72

4.3.1 Isotropy / Anisotropy 77

4.3.2 Selectivity 79

4.4 Characterizing the 2-level 3D Internal Micro-mixer 79

CHAPTER 5 RESULTS AND DISCUSSION

5.1 Introduction 88

5.2 Influence of Laser Irradiation 91

5.3 Influence of Thermal Cycling 99

5.4 Influence of Wet Chemical Etching 102

5.5 Design Optimization 104

CHAPTER 6 CONCLUSIONS

6.1 Conclusions 112

6.2 Suggestions for Future Work 115

Summary

SUMMARY

Fabrication of real 3-dimensional (3D) microstructures embedded inside a monolithic

FoturanTM glass is a very attractive and promising technology in the field of life sciences and biotechnology. It offers a wide range of opportunities and opens up many potential applications in the studies of photonics, medicine as well as aerospace engineering. This technique exploits the unique optical, chemical and physical properties of microstructuring inside glass.

The research reported in this thesis primarily aims to fabricate real 3D microstructures, achieves multiple micro-channels and multi-level connectivity as well as to investigate the process optimization by making use of different methods and varying different parameters. Currently, the difficulties of fabricating microstructures inside glass are evident in the wide variety of non-conventional techniques employed. The most commonly used approach in glass patterning is based on conventional lithography, however, this technique is limited by slow etch rates with majority of the patterning performed on the surfaces of the samples. Micro-cracking and other collateral damages further introduce additional unnecessary stresses to the glass substrates. In addition, this approach has many limitations in numerous industrial applications attributing from the high cost of the masks, low throughput and the many tedious repeating steps.

Therefore, our direct laser application, a fast and maskless technique, has been used in this thesis for the patterning and fabrication of micro-architectures and microstructures

Summary

inside the photosensitive glass, FoturanTM. This technique involves only a femtosecond laser and a 3 axes X-Y-Z stage. Its main advantages lie in its low cost, high speed and being a simple operation. After the patterns are formed inside the FoturanTM glass, a

thermal annealing cycle will follow and subsequent etching is employed to fabricate the

required design of the microstructures. The utilization of femtosecond laser irradiation

employs multi-photon absorption (MPA), which permits the fabrication of embedded

intricate 3D microstructures and integrates all these complex microstructures within the

material. This technique has the capability to create undercut and freestanding microstructures without resorting to ablation, thereby minimizing residual stresses as well

as impending issues of surface morphology. These monolithic ‘all-in-one’ devices are

highly desirable because of their potentials in meeting unique and individually customized requirements in various applications. In this thesis, a monolithic real 3D ‘all- in-one’ analytical device with 2 levels of micro-reservoirs and interconnecting micro- channels was fabricated. The mixing capability of the device is demonstrated by mixing individual colored dyes to obtain a single homogeneous dye solution. The repeatability and reliability of the fabricated device were further demonstrated with further mixing of other dyes. Since FoturanTM glass is highly sensitive to the dosage of irradiation,

exposure time, annealing temperature and time, etching concentration and time, the

desired embedded microstructures have been achieved by varying these parameters.

Different approaches were undertaken in order to optimize the mixing capability of the

monolithic 2-level 3D embedded micro-mixer. First and foremost, the laser irradiation

power was investigated and determined to eradicate the issue of surface ablation, an issue

Summary

experienced by most international research groups. Subsequent process of thermal annealing time and temperature were further optimized to initiate the photo-chemical reaction. Insufficient heating does not initiate the nucleation and agglomeration process while extended heating will result in the warpage of the FoturanTM glass, which has a

major detrimental effect towards the etching process. The etching times were also

affected by the internal surface roughness of the microstructures. This in turn is

determined by the geometries of the microstructures as well as gravity-assisted reactions.

In conclusion, a monolithic real 3D microstructure has been fabricated by a simple

process of direct maskless laser processing capable of customization for various unique

requirements in different applications. Several approaches were undertaken in

optimizing the fabrication process and maintaining the functionality and structural

integrity of the device. This enables the successful demonstration of the mixing capability

of the micro-mixer. Detailed mechanism is also carefully investigated in this thesis.

vii

List of Figures

LIST OF FIGURES

Fig. 2.1. Structure of the monolithic 2-level 3D micro-mixer: (a) micro-mixer 37 fabricated inside FoturanTM glass by femtosecond laser, (b) micro-mixer after thermal annealing cycle and (c) 3D micro-mixer after the final etching process.

Fig. 3.1. Experimental setup for the femtosecond laser irradiation of FoturanTM 46 glass.

Fig. 3.2. (a) Setup of the irradiation process, comprising of the femtosecond laser 47 together with the optical setup as well as the 3 axes stage. Red arrows depict the path of the laser. (b) and (c) Closed up views of the magnifying lens and 3 axes stage.

Fig. 3.3. Design of the microsctructure within the FoturanTM samples from 4 49 different viewing angles; (a) Top view. (b) Isometric view. (a) Front view. (b) Side view.

Figure 3.4. HP4284A Precision LCR Meter Dielectric Tester oven employed in the 50 2-stage annealing process.

Figure 3.5. Olympus MX-50 optical microscope with magnification lens of 10, 20, 52 50, 80, 100 and 150.

Figure 3.6. Magnification module of the Olympus MX-50 optical microscope. 53

Figure 3.7. JEOL JSM 7401F. 54

Figure 3.8. Mitutoyo Surftest Extreme SV-3000CNC 3D surface profiler. 56

Fig. 3.10. (a) Structure of the fabricated monolithic 2-level 3D micro-mixer. (b) 59 Mixing of the yellow and blue dyes in the central micro-reservoir resulting in a green dye solution. (c) Mixing of the yellow and pink dyes.

viii

List of Figures

Fig. 4.1. Flow chart of the 2-level 3D micro-mixer fabrication process. 63

Fig. 4.2. Design of the microstructure within the FoturanTM samples from 4 65 different viewing angles; (a) Top view. (b) Isometric view. (a) Front view. (b) Side view.

Fig. 4.3. 2-photon absorption of Cesium. 68

Fig. 4.4. Two-photon Jablonski Energy Diagram. 68

Fig. 4.5. Temperature-time plot of the annealing cycle. 71

Fig. 4.6. Typical parallel-plate reactive ion etching system. 76

Fig. 4.7. Difference between anisotropic and isotropic wet etching. 78

Fig. 4.8. (a) Optical images of the entrance of yellow dye through the right inlet. (b) 81 Entrance of blue dye through the left inlet. (c) Cross-sectional view of the mixing process for the blue and yellow dyes. (d) Onset of mixing between the yellow and blue dyes.

Fig. 4.8. (e) Closed-up view of the sample during the mixing process. (f) Closed-up 82 on the mixing process within the micro-reservoir. (g) Mixing process of the yellow and pink dyes. (h) Cross-sectional view during mixing of the yellow and pink dyes.

Fig. 4.9. (a) to (n) Sequence of events during the mixing process. (o) A close up 85 view of the mixing within the central micro-reservoir.

Fig. 5.1. Etch rate ratios vs. laser irradiance for (a) λ = 266 nm and (b) λ = 355 nm. 96 The etch rate ratios are represented by the solid squares.

List of symbols

LIST OF SYMBOLS

λ Wavelength h Planck's constant (6.626068 × 10-34 m2 kg / s) v Frequency e- Electron

H Enthalpy

E Energy l Angular momentum t Time g Local acceleration due to gravity (9.81 m / s2)

List of Publications

LIST OF PUBLICATIONS

1. H. H. Teo and M. H. Hong, A Monolithic 2-level 3-Dimensional Micro-mixer

Fabrication by Femtosecond Laser and its Characterization, 5th International Congress

on Laser Advanced Materials Processing.

2. H. H. Teo, M. H. Hong, L.P. Shi and T.C. Chong, Optical Diagnostics of

Femtosecond Laser Processing of Foturan Glass, 10th International Conference on

Laser Ablation. (Abstract submitted)

3. P. B. Phua, W. J. Lai, Y. L. Lim, K. S. Tiaw, B. C. Lim, H. H. Teo, and M. H. Hong,

" Mimicking optical activity for generating radially polarized light," Optics Letters,

Vol. 32, p 376-378, 2007

4. W. J. Lai, B. C. Lim, P. B. Phua, K. S. Tiaw, H. H. Teo, and M. H. Hong, Generation

of radially polarized beam with a segmented spiral varying retarder, Optics Express,

Vol. 16, Issue 20, pp. 15694-15699 (2008)

5. H.L. Seet, X.P. Li, M.H. Hong, K.S. Lee, K.H. Teh and H.H. Teo, Electrodeposition

of Ni–Fe micro-pillars using laser drilled templates, Journal of Materials Processing

Technology, Volumes 192-193, 1 October 2007, Pages 346-349

Chapter 1 Introduction

CHAPTER 1

INTRODUCTION

1.1 General Background

Glass ceramic (GC) materials have a distinguished history in numerous industrial, commercial, and scientific disciplines and are widely employed in a broad spectrum of applications. GC materials are a unique class of material that combines the special properties of sintered ceramics with the characteristics of glasses and represents an important technological capability in various applications, such as aerospace engineering

[1], optics, photonics [2], biology, biochemistry [3], computer technology [4] and commercial architecture [5]. This diverse array of applications is attributed to the attractive and versatile chemical and physical properties of these glass ceramics, such as corrosion resistance, high temperature stability, optical transparency and biocompatibility. GC materials can be further synthesized to be virtually defect-free with no porosity [4]. They can be optically transparent or opaline, and can also be designed for applications requiring limited shrinkage and electronic- and bio-compatible [4]. By altering the chemical compositions within the GC materials, the optical, mechanical and electrical properties of glass ceramics can also be further tailored to meet a wider and more diverse set of application requirements.

Chapter 1 Introduction

1.2 Recent Progress in Photosensitive Glass and Femtosecond Laser

Microprocessing

The first commercially viable glass ceramics were utilized in the aerospace industry to

fabricate radomes (radar domes) that shielded sensitive antenna equipment in aircraft and

rocket nose cones [6]. Glass ceramics, for instance cordierites, were ideally suited for radome technology and atmospheric re-entry issues due to their high bending strength,

low thermal expansion, and high thermal shock resistance [7]. More recently, glass

substrates have been utilized in the development and construction of micro-total-analysis- systems (μTAS) or “lab-on-a-chip” systems [3]. These devices require complex integration of sub-systems to facilitate efficient sample transfer, chemical separation, control of chemical and biological reactions and species monitoring and detection. The fabrication technology employs to manufacture the individual components that accomplish these tasks must be versatile to realize all the required devices that contain multiple depth structures, chemical inertness at desired locations, and surface roughness that can be fashioned for each application. The ability to co-fabricate micro-fluidic systems, such as valves, nozzles and channels, along with other integrated optical, electrical, and mechanical structures either on the surface or in the interior of the glass,

has helped to realize the μTAS system as a complete analytical device that can accomplish all phases of the sample analyses. The myriad of technological applications

that are possible with glass ceramic materials necessitate that the fabrication methods

must be able to achieve component and sub-system integration by producing functional

Chapter 1 Introduction structures that range in the scale from the micro (< 100 μm) to the meso- (100 μm–10 mm) and to the macro-dimensional (> 10 mm) domains and beyond.

Besides silicon, glass is a widely used substrate material in micro-system technology, in particular in the manufacture of micro-fluidic devices for biological analyses and biotechnical applications [7] as it provides beneficial structural and functional material properties. In comparison to silicon, the use of glass in micro-total-analysis-systems

(µTAS) applications is advantageous with regards to its optical transparency which allows for easy instantaneous visual monitoring and real-time optical inspection and detection due to its good fluorescence properties as well as its good dielectric properties used in a number of applications, thereby allowing it to withstand the high voltages in electro-kinetically driven flows and separations, particularly in capillary electrophoresis.

Other beneficial properties of glass are its good chemical resistance, high thermal stability, chemical inertness and combined with other matured and established schemes for surface modification and fictionalization (silane modification), make glass the most widely used substrate in the fabrication of DNA arrays. The use of glass substrates further improves the long term chemical stability of the devices in comparison with silicon-based systems. Many applications also require the high mechanical strength and the good mechanical stability of glass.

The growing needs for micro-devices with high efficiency and performance for biochemical analyses and medical inspection drive the three-dimensional (3D) fabrication and integration of versatile micro-components on a single chip [8]. Currently, the

Chapter 1 Introduction development of “all-in-one” microchips in which micro-components, such as those of fluidics, mechanics, and optics, are integrated into a single chip still pose numerous challenges from the conceptual stage to the mass production phase [9]. In the manufacture of micro-devices, the material selection is of utmost importance for “all-in- one” fabrication.

This concept of lab-on-a-chip has created a revolution in chemical, biological, and medical sciences. A lab-on-a-chip device, which is a palm sized chip, integrated with functional components, such as micro-fluidics, micro-optics, microelectronics, and micromechanics, offers several advantages over the traditional chemical and biological analytical techniques [12]. Particularly its ultra-fine dimensions, the lab-on-a-chip allows the performing of chemical and biological analyses with the ease of use, low sample and reagent consumption, low waste production, high speed analysis, and high reproducibility due to standardization and automation. Recently, significant progress has been made in the incorporation of optical circuits into the fluidic circuits, which would eventually enable enhanced functionality of the lab-on-a-chip devices. A variety of optical structures, including optical waveguides [13], micro-optical gratings [14], optical fibers

[15], micro-optical mirrors [11], and micro-optical cylindrical lenses [16], have been used in the construction of fluidic photonic integrated devices. The fabrication of three- dimensional (3D) micro-optical cylindrical and hemispherical lenses vertically embedded inside glass by a femtosecond (fs) laser micromachining system, together with micro- optical lenses, can focus laser beams into very small and tight beam spots. This holds great promises for numerous practical applications in life sciences.

Chapter 1 Introduction

In the “lab-on-a-chip” or micro-total-analysis-system (μ-TAS) devices used for photonic

bio-sensing, a light beam is often required to illuminate a liquid sample in a micro-fluidic

chamber to realize photo-absorption or fluorescence detection. The use of a waveguide is a highly efficient way to guide the light to the liquid sample. The waveguide can be

connected to an optical spectrometer or photo-detector to collect the light transmitted by

or emitted from the liquid sample. Therefore, some research groups are attempting to

integrate optical waveguides with micro-fluidic components in a single chip [17]. Further

integration of micro-optical components, such as a micro-mirror and a microlens, is

expected to be able to enhance the efficiencies of inducing and / or collecting signals in

optical analyses. For this purpose, micro-mirrors have been fabricated inside FoturanTM glass [18]. Furthermore, the fabrication of cylindrical and hemispherical microlenses made of FoturanTM glass by fs laser processing has been reported [19].

In recent years, we have witnessed a steady progress in the fabrication of micro-fluidic

structures, for example, micro-reactors for chemical analyses [10] and a number of

micro-optical structures, such as micro-fluidic dye laser [11] in a photosensitive

FoturanTM glass chip by femtosecond (fs) laser microprocessing. The ability to directly

fabricate 3D microstructures in FoturanTM glass by using the femtosecond laser, along

with its resistance to high temperature and corrosion as well as high optical transparency,

has made it a particularly attractive platform for bio-photonic micro-devices [3].

More recently, near infra-red (NIR) femtosecond lasers have been utilized for the

manufacturing of embedded 3D structures in GCs. Complex micro-fluidic channels [21],

Chapter 1 Introduction

micro-optical components [22], and compliant micro-plate structures [23] for lab-on-a-

chip applications have been formed in photo-structurable glass ceramics using

femtosecond laser irradiation at λ = 775 nm and λ = 800 nm. Some present studies are

investigating the ability to merge pulsed UV laser direct-write patterning with batch

chemical processing. The laser fabrication technique involves the controlled variation of

the laser irradiance with high precision during pattern formation in the photo-structurable

glass; we refer to this method as variable laser exposure processing [24].

Pulsed high repetition rate ultra-violet (UV) and infra-red (IR) lasers have also been

incorporated into the photo-processing of these GC materials. Using direct write

patterning techniques, where masking steps are not necessary, pulsed lasers have

permitted the fabrication of true three-dimensional (3D) microstructures, embedded

channels, and interconnected microstructures [5]. Recently, a novel UV laser processing

technique has been developed that significantly enhances the capability and versatility of

traditional materials fabrication and current laser processing. The new technique

combines the advantages of direct-write volumetric laser patterning and batch chemical

processing. The merged non-thermal laser processing approach involves the controlled

variation of the laser irradiance (mW/μm2) with high precision during pattern formation

in the photo-structurable glass. Variation in the laser exposure dose permits the creation

of variegated and proximal high (30 : 1) and low (< 1 : 1) aspect ratio structures on a

common substrate. These structures can be co-fabricated in a single batch chemical etch step without the need for a complex masking sequence or a post-processing ablation step.

This novel technique facilitates rapid prototype processing, while maintaining pattern and

Chapter 1 Introduction

component uniformity and integrity, achieves material processing over large areas

without incurring high cost. Under controlled variation of the laser irradiation, this

technique also permits site-specific or “fine tuning” of other GC material properties, such

as the optical transmission, mechanical strength and compliance, and electrical

conductivity [20].

One type of GC material that is receiving significant attention is the photo-structurable or photosensitive glass ceramic material class. The key attribute that separates these

photosensitive glass ceramic materials from the other material classes is the ability to control the precipitation of a soluble ceramic phase from the base glass. A photo-active compound (photo-initiator) and metal ions (nucleating agents) are added to the glass matrix to permit local spatial control of the precipitation process [25]. This attribute permits two-dimensional (2D) and three-dimensional (3D) micro-shaping and microstructuring of PSGC materials via optical lithographic patterning and chemical etching processes; these techniques are analogous to the processing of photo-resist and SU8 materials. Traditional glass ceramics are typically shaped and sculptured in 2D using pyrolitic volume-crystallization processes and standard milling operations. For example,

commercially available GCs, such as MacorTM and DicorTM (Corning Corp.), can be

mechanically machined, while PyroceramTM (Corning 9608, Corning Corp.) and

ZerodurTM (Schott Corp.) are processed using pyrolytic techniques. ZerodurTM is a GC material with a very low coefficient of thermal expansion (0 ± 0.02 x 10-6 K-1) [4] and is

used in precision optical equipment, for instance, the 8.2 m telescope mirror developed

for the European Southern Observatory Very Large Telescope. There are many

Chapter 1 Introduction

photosensitive glass ceramic compositions, but the most commercially successful

photosensitive glass ceramic has a non-stoichiometric composition near the lithium

disilicate system (e.g., phyllosilicate crystals, Li2Si2O5). Non-stoichiometry implies a

SiO2:Li2O molar ratio that deviates from 2:1. The present study utilizes a photosensitive

glass ceramic material obtained from the Schott Corporation under the trade name

FoturanTM.

FoturanTM is a photosensitive glass manufactured by Schott Mikroglas which can be directly patterned using conventional UV lithography, femtosecond lasers [22, 26], and

MeV protons [27]. Unlike other glasses that require masks in order for the formation of

patterns, irradiated regions of FoturanTM have an etch rate which is 20 times higher after heat treatment with HF solution. The ability to directly fabricate microstructures in this

glass along with its resistance to high temperature and corrosion has made it a

particularly attractive platform for micro-fluidic applications such as micro-

electrochemical reactors [28]. Furthermore, its high transparency and the ability to

modify the refractive index of this glass make it more attractive in the fabrication of

micro-optical components, such as waveguides [29], gratings [30], and micro-mirrors

[26]. Although femtosecond lasers are commonly used for waveguide fabrication in many

types of glasses [31], only a limited number of studies have been concentrated on the

fabrication of multi-level 3D microstructures within FoturanTM.

Chapter 1 Introduction

1.3 Limitations of Conventional Glass and Challenges on Laser Microprocessing

Techniques of Glass Substrates

Two-dimensional (2D) and extruded prismatic (2.5D) micro- and meso-scale structure fabrication in traditional glass ceramic materials have been achieved using a variety of conventional processing techniques, including mechanical micro-milling [32], direct thermal ablation [33], and lithographic patterning and chemical etching [20]. Mechanical milling and laser machining (ablation) techniques offer high material removal rates for rapid production and can create structures and features down to micrometer regime.

Despite the recent advancements in laser technology (high repetition rate and short pulse

length < 10 ps) and micro-tool development (e.g., polycrystalline diamond, PCD), these

machining techniques, unfortunately, suffer from percussion and thermal-induced effects

that lead to fatigue and reduced cycle life. These adverse modifications can appear as

brittle fracture, micro-cracks and surface roughness, along with optical defect formation.

Optical lithography and chemical etching methods provide other routes to fabricate

structures in glass ceramics with predefined geometry and high precision. However, the

formation of each structure with distinct aspect ratios requires a corresponding mask step

during photo-patterning and etching. Consequently, fabrication using multiple masks can

be very costly and entails a concomitant reduction in overall resolution. Furthermore,

optical mask-based techniques are generally not amenable to rapid prototype

manufacturing.

Chapter 1 Introduction

Fabrication of precise microstructures in a controlled fashion made out of glass, in particular in glass for micro-fluidics [7] is very challenging. The difficulty of fabricating structures in glass is evident in the wide variety of non-conventional techniques for glass micromachining along with some conventional micro-fabrication technologies. Glass micro-fabrication technologies include photolithography used in tandem with chemical etching are often limited by a slow etch rate of the structural depth. Glass is an isotropic material that is wet-etched with buffered HF in a non-directional manner. Therefore, structures with curved sidewalls and relatively low aspect ratio are produced by isotropic wet etching [34]. Dry chemical etching of glass is also possible in typically a SF6 plasma

[35], however is also limited by a slow etch rate. There are many problems in etching materials which contain atoms of lead or sodium (glass, PZT, etc.) as they yield non- volatile halogen compounds (PbF2, NaF, etc.) as the reaction products. High speed

directional etching of silicon by deep reactive ion etching (DRIE) with inductively

coupled plasma source, which produces high-density plasma at low pressure, can be used to fabricate silicon channels but is still not sufficiently developed for producing similar structures in glass or quartz (DRIE). Laser micromachining of glass is hindered by the brittleness and poor thermal properties of most glasses, resulting in the possibilities of micro-cracks and other collateral damages, such as debris formation and poor surface qualities [36]. Two main ways to overcome this limitation are to use short wavelengths

(UV) lasers that can be focused down to smaller spot sizes or use lasers with ultra-short pulse duration that reduce thermal effect. Brittle materials are difficult to be mechanically micro-machined by cutting processes like milling due to damage resulting from material removal by brittle fracture which leads to rough surfaces and requires subsequent

Chapter 1 Introduction polishing steps. Material removal by ductile regime instead of brittle fractures is made possible by using polycrystalline diamond tools [37]. Mechanical sawing, though limited to simple straight patterns, has also proved successful [38]. Mechanical machining with techniques specialized in brittle materials, such as powder blasting, also known as abrasive jet machining, is based on the mechanical removal from a substrate by a jet of particulates [38] which induce additional and unnecessary stresses in the glass substrates.

Powder blasting allows us to get complex and controlled shapes of the eroded structure.

Moreover, the erosion rate is much higher than that of standard wet-etching processes.

Other techniques include micro-ultra-sonic machining (MUSM) which exploits the ultrasonic frequency vibration of a tool to force abrasive grains to erode a substrate [39], thermal molding [40] and photo-structuring. There also exist a variety of silica-based oxide glass materials, such as soda-lime glass, borosilicate glass and pure silica glass

(quartz glass). Some special varieties of glasses are amenable to anisotropic photo- structuring so they do not require an intermediate photoresist layer for patterning. It is commercially available through various suppliers and is patterned by photolithography using a mask [20] or by direct laser writing [41, 42]. Typical approaches, like wet chemical etching and mechanical structuring, are not suitable to achieve fine (<10 µm) and high aspect ratio (>10) structures.

Chapter 1 Introduction

1.4 Fabrication by Femtosecond Laser on FoturanTM Glass

In micro-systems technology, glass components are required with well defined shapes and have strict tolerance. Typically, conventional moulding methods used in the manufacturing of glass cannot fulfill these requirements. Subsequent mechanical operations, such as drilling, milling, sandblasting, are expensive and limited in their possibilities. A solution to this problem is offered by glasses from the basic Li2O/Si2O family containing traces of noble metals. After exposure to UV light and subsequent heat treatment, these glasses will partially crystallize. The crystalline phase is lithium silicate, which is much more soluble in hydrofluoric acid than the surrounding unexposed amorphous glass. This makes the production of complicated and high precision components possible via an etching process. For over two years, the Schott Glaswerke, with their know-how in the fields of glass and glass ceramics together with the IMM

Institute of Microtechnology GmbH (Mainz), with its technical facilities and knowledge in the field of microtechnology, are working together on the field of photo-structurable glass. The trade name of this glass, made by Schott, is FoturanTM. All results mentioned

here are made with this glass. Similar glasses produced by Corning and Hoya are under

development at the Technical University Ilmenau. In the following chapters, the

properties of the photo-structurable glass FoturanTM as well as the structurization method

will be explained

FoturanTM is a photosensitive glass. This property enables it to be structured for a variety

of purposes. The main difference between FoturanTM and ceramic materials is that it has

Chapter 1 Introduction

absolutely no pores. Its temperature stability and chemical resistance are notably higher

than those of plastics. In comparison to metals, FoturanTM shows better corrosion

resistance, is electrically isolating and has a lower thermal conductivity. Its advantages

over silicon are its availability in a wide variety of dimensions and the fact that it can be

structured in various geometries and, above all, its higher breaking strength.

A major advantage of our fabrication technique is the utilization of femtosecond laser

irradiation with the principle of non-linear multi-photons absorption. This permits the

fabrication of embedded intricate 3D structures within the material and the ability to create undercut and free-standing structures without resorting to ablation, thereby,

minimizing undue residual stress created as well as impending issues of surface

morphology. Conventional processing of glass materials relies on lithography and UV

lamp illumination, which allow only patterning on the surface and are time consuming.

Given the present reliability and availability of the new laser processing techniques, they

show particular promise in applications that require microstructures with the properties of a glass. For example, the fabrication of true 3D micro-fluidic structures and micro-total-

analysis-systems (µTAS) in glass materials substrates could greatly simplify design

protocols and remove costly bonding steps during assembly and packaging. Using this

technique, we are able to fabricate a true 3D micro-mixer with 2 levels of micro-

reservoirs with interconnecting micro-channels. The fabricated resulting micro-mixer was

characterized by mixing 2 individual coloured dyes to obtain a single homogeneous dye

solution.

Chapter 1 Introduction

This alternative approach exists for the fabrication of structures on the surface of glass

and inside the glass bulk that can span the micro- to macro-scale domains. It relies on a

class of amorphous glass ceramic materials that are directly photo-structurable [5]; these

GC materials are referred to as photositalls, photocerams, or photosensitive glass

ceramics (PSGCs). The key attribute that separates PSGC materials from other material

classes is the ability to control the precipitation of a soluble ceramic phase from the base glass. The photosensitive properties result from the addition of a photo-active compound

(photo-initiator) and metal ions (nucleating agents) to the glass matrix that permit local spatial control of the precipitation process [5]. Photo-activation typically occurs at ultraviolet (UV) wavelengths < 350 nm [43]. The irradiated sample is baked to transform the latent image into a visible or “fixed” permanent image. Subsequently, preferential material removal and structure fabrication are performed by isotropic chemical etching of the developed and highly crystalline regions that comprise of the “fixed” image. The remaining glass structure can be further laser processed and thermally treated to create a fully ceramic structure without compromising the fidelity of the patterned features. These attributes have enabled the formation of patterned 2D structures and extruded prismatic shapes in photo-structurable glass using a high power UV lamp and a photo-mask [20].

Recent experiments have also shown that pulsed and high repetition rate nanosecond UV lasers can be integrated into a direct-write patterning scheme to facilitate the fabrication of true 3D structures [44] and embedded channels [45]. For example, pulsed nanosecond

UV laser processing at λ = 248 nm and λ = 355 nm has enabled the formation of micro- cavities and micro-nozzles in PSGCs for cold gas micro-thruster applications [44].

Nanosecond UV laser processing techniques have also been employed to fabricate

Chapter 1 Introduction

intricate components and integrated support structures for miniature satellite systems,

such as the 100 g class co-orbital satellite assistant (COSA) spacecraft [46].

The current work represents the experimental analysis of the effect of the laser exposure

dosage on the chemical etch rate of a commercial photosensitive glass (FoturanTM, Schott

Corporation, Germany). The premise of this fabrication approach is related to the photo- physics of the exposure process. The incident actinic irradiation induces the generation of photoelectrons, which can then reduce trapped nascent silver ions in the glass matrix

[43]. Upon thermal treatment, the neutralized silver species diffuse and agglomerate, and the metallic clusters can then serve as nucleation sites for the formation of the soluble crystalline phase. Therefore, the photon density should be a valid experimental parameter that can be utilized to control the local density of soluble crystallites in the irradiated volume. The variation in crystallite density will lead to a concomitant change in the exposed glass etch rate [47]. By locally altering the PSGC chemical etch rate, the UV laser direct-write patterning technique permits the concurrent fabrication of structures from micro- to meso-scale on a common substrate. The structures can be formed in close proximity, and retain variegated heights with high and low aspect ratios. The total patterned ensemble can be chemically etched in a single step without the need for a complex sequential, protective masking scheme. The ability to vary the laser exposure also permits site specific modification or “fine-tuning” of other PSGC material properties, such as the optical transmission, mechanical strength, and electrical conductivity. For example, the crystallite density can be controlled to adjust the glass ceramic transmissivity [48] and impart color gradations [49] in PSGCs to form band pass

Chapter 1 Introduction

and cut-off filters in the UV and near-IR wavelength regions. The mechanical strength

can be locally adjusted via laser exposure to create structures based on a material that has relative compliance for spring and high shock applications.

Chapter 1 Introduction

1.5 Research Objectives and Contributions

In response to the current challenges in both the glass fabrication techniques and

repeatability and reliability mentioned in the previous sections, we have investigated the

feasibility of using femtosecond laser to fabricate a monolithic 2-level 3 dimensional

(3D) multi-channel structures in-situ of FoturanTM glass. Design fabrication has been

proposed and demonstrated. The mechanism of the process behind the fabrication of

these structures is also discussed in the following chapters.

1.5.1. Fabrication of a Monolithic 2-level 3-dimensional Micro-mixer

Currently, the main fabrication method makes use of lithography technology incurring

high cost in the making of the photolithographic masks. Furthermore, most of the

structures can only be fabricated on the surface of the samples. Owning to the low speed

and high cost, this fabrication technique is unsuitable for rapid proto-typing. In our present study, we have used a fast and maskless fabrication technique – direct laser writing / patterning. With this technique, we are able to fabricate any designs of varying depths inside the FoturanTM samples. This direct laser writing within FoturanTM makes use of the multi-photon absorption principle of the femtosecond laser. Only at the focused point will there be a structural change, this enables microstructures to be fabricated within the sample, regardless of their shapes and sizes. The main setup involves only a femtosecond laser source and a 3 axes stage, which is very much more economical in present financial situation as well as offering the ease and flexibility in operation. By

Chapter 1 Introduction

varying the laser intensities, irradiation time and the motion of the stage, different microstructures of varying shapes, sizes and depths can be easily fabricated within the

FoturanTM samples. In previous publications and reports, most of the studies have been

focused on fabricating waveguides and structures on the surface of the FoturanTM samples. Our work is based on fabricating micro-channels and micro-reservoirs in-situ of

FoturanTM samples. Besides in-situ fabrication, we have extended this technique further

in designing and achieving multi-levels fabrication of various shapes, regular and

irregular, such as rods and cylinders. We are able to successfully fabricate and

demonstrate the mixing capability of this micro-mixer by using a fast, easy, simple,

flexible and most importantly economical method.

1.5.2. Optimization of a Monolithic 2-level 3-dimensional Micro-mixer

Precise control of the shapes, sizes and depths of the fabricated microstructures has been

the focus of much research interest recently. Even until today, extensive researches have

been made to realize these variables, including the selection of appropriate materials, the

roughness between interface, the geometry and the size of the microstructures. Therefore,

we present our study mainly in the fabrication of multi-levels microstructures in-situ of

FoturanTM. The mixing capability is highly desirable because of their great potentials in many industrial applications, particularly in the biotechnical analyses. Therefore, the main objective of our study is to demonstrate the successful mixing capability of the

multi-level micro-mixer by fabricating multi-level microstructures followed by the

changing the other parameters of the structure, including the shapes, sizes and locations.

Chapter 1 Introduction

We have successfully fabricated and demonstrated the mixing capabilities of the multi- level, multi-channel microstructures and investigated their properties by using scanning emission microscopy, 3D surface profiler as well as demonstrated the mixing capability of the fabricated monolithic 2-level 3D micro-mixer. With the above-mentioned studies, the present work provides information on a multi-level multi-channel microstructure fabrication and the properties of these microstructures. It further provides information for fundamental research to the improvement in the performance of these microstructures and their potential applications in biotechnology.

Chapter 1 Introduction

1.6 Thesis Outline

This thesis is divided into 6 chapters and their contents are briefly described below:

Chapter 1 gives a fundamental introduction of Glass Ceramics (GC) and their significances and applications, and reviews the recent progress and major challenges in their research. Fabrication techniques of 3D microstructures in photosensitive glasses are presented, and the feasibility of tuning these structures by adjusting the parameters of the processes is highlighted. The objective and contributions of this study are also addressed.

Chapter 2 introduces the basic mechanism and theoretical background of the processes.

Different techniques supported by various parameters are briefly described.

Chapter 3 introduces the experimental setup. The principles of the femtosecond irradiation in photosensitive glass and its setup are presented. The major optical

characterization tools are also briefly described.

Chapter 4 describes the detail procedures of the fabrication of 3D microstructures in

photosensitive glass by femtosecond irradiation. The factors that affect the

microstructures are studied and discussed with different operation parameters, such as

temperature, time of exposure and etching rate. Experimental results are shown as the

complementary statement of the above discussion.

Chapter 1 Introduction

Chapter 5 investigates the structure integrity of the fabricated microstructures and the mixing capabilities of the microstructures. The fabricated monolithic 3D multi-channel microstructures were the test subjects. Several new and different designs are modeled, conceptualized and fabricated for characterization by changing the parameters during the laser procedures. Subsequently, thermal annealing of the samples were further investigated at different etching parameters

Chapter 6 concludes the results of the study and suggests some future research proposals in this area.

Chapter 1 Introduction

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in borosilicate glass using micro EDM combined with micro ultrasonic vibration

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micromachining technology for structuring borosilicate glass substrates. In: IEEE

proceedings of the 12th international conference on solid state sensors, actuators

and microsystems, Boston, June 2003, pp 258–261

[41] Gimkiewicz C, Gerhard E (1997) High-aspect micro-wing wheels in

photosensitive glass. Microsyst Technol 4:40–45

[42] Masuda M, Sugioka K, Cheng Y, Aoki N, Kawachi M, Shihoyama K, Toyoda K,

Helvajian H, Midorikawa K (2003) 3D microstructuring inside photosensitive

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[43] S.D. Stookey: Indust. Eng. Chem. 41, 856 (1949)

[44] W.W. Hansen, S.W. Janson, H. Helvajian: SPIE Proc. 2991, 104 (1997)

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Chapter 1 Introduction

[47] F.E. Livingston, P.M. Adams, H. Helvajian: (2005), submitted to J. Appl. Phys.

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Co.: Weinheim, Germany 2005), in press

Chapter 2 Theoretical Background

CHAPTER 2

THEORETICAL BACKGROUND

2.1 Multi-photon Absorption by Photosensitive Glass

In recent years, three-dimensional multi-layers micro-structural patterning in glasses and

polymers has attracted an increasing interest due to their prospective applications in the

fields of optical memory [1], photonic crystals [2], micro-fluidic and MEMS devices [3].

In the case of channel fabrication, a direct patterning by a tightly focused laser beam [3] or a post-exposure wet etching can be implemented [4]. This anisotropic wet etching of the irradiated patterns in silicate or polymeric glasses is usually applicable for the fabrication of only low aspect ratio structures where the aspect ratio is defined by the length and diameter of the channel as fa.r. = length/diameter. The anisotropy of etching is

a universal principle based on an increased chemical reactivity of optically modified

regions as compared to un-modified areas, with the reactivity difference between 1 to 5

times. This causes problems and renders the technique impractical for most applications.

In contrast, special glasses, called photo-etchable glasses, such as FoturanTM (by Schott

Corp.), when properly irradiated by laser and annealed, show up to 30 times of contrast in

etching speed between irradiated and unexposed areas in diluted solutions of hydrofluoric

(HF) acid. High anisotropy of etching is due to the precipitation of a lithium-silicate

phase, which has a high etching rate in HF solutions. Recently, this glass was utilized to

fabricate embedded structures under femtosecond exposure [5].

Chapter 2 Theoretical Background

Femtosecond (fs) lasers have recently been extensively used in the three-dimensional

patterning of numerous transparent materials. The interactions between the fs laser and

the material occur only in the vicinity of the focal spot due to the multi-photon absorption

process, leaving the surface of the material completely intact. One of the most important

applications of this ability is to modify the refractive index of bulk glasses [6 - 8]. A

variety of photonic devices, such as waveguides [6], couplers [9], and gratings [3, 10],

have also been fabricated using this technique inside the glasses with two-dimensional

(2D) or three-dimensional (3D) configurations.

A femtosecond laser pulse can have an electric field strength which approaches or

exceeds the strength of the electric field that holds valence electrons in a transparent

material to their ionic cores [11]. In this regime, the interaction between the laser pulse and the material becomes highly nonlinear. Laser energy can be nonlinearly absorbed by the material, leading to optical breakdown [12], self-focusing [13], multi-photon ionization [14] and soft x-ray generation [15], etc. Photosensitive glasses, which were developed at Corning Glass Works in 1947 [16], have very interesting properties, such as a high Young’s modulus, a low absorption coefficient in the visible wavelengths, and good chemical stability and biocompatibility, for the fabrication of micro-systems. They are currently being widely used in many technological applications, including GEM-Type detectors [17], hydrodynamic microelectrochemical reactor for voltammetric sensing of chemical species [18], nanotube-based field emission flat panel display [19], ultra-long glass tips for atomic force microscopy [20], and miniaturized satellites [21].

Chapter 2 Theoretical Background

Conventionally, almost all photosensitive glasses and FoturanTM are irradiated using

either an Hg lamp [22] or an ultraviolet (UV) laser [23]. Due to the low transmittivity for

UV photons in FoturanTM glasses, the engraved pattern is normally fabricated on the glass

surface. Therefore, the microstructuring of FoturanTM glass by UV lithography is

generally limited to the surface of the sample because of the resonant absorption of UV light. Thus, these methods permit only fabrication of microstructures near the surface of the sample.

To overcome this limitation, lasers working at non-resonant wavelengths, including nanosecond (ns) lasers at 355 nm [9] and fs lasers at 400 nm and 755 nm [8, 10, 11], were employed to fabricate 3D microstructures buried deep inside the FoturanTM glass without

much damage to the surface. Since these wavelengths are outside the absorption band of

FoturanTM glass, photo-modification of glass can only be initiated near the focal point

where the extremely high laser intensity induces nonlinear optical effects through multi-

photon absorption. It has been found that the photochemistry of fs laser modification of

FoturanTM glass is different from the established theory for UV light processing,

primarily due to the photo-reduction of silver with fs laser irradiation [12, 13].

In order to successfully fabricate three-dimensional (3D) microstructures in-situ of the

FoturanTM glasses, Kondo et al [24] and Cheng et al [5] used an infra-red femtosecond laser for which the FoturanTM is transparent except at the focusing point where nonlinear

multi-photon absorption occurs. This leads to a local phase transformation in the glass.

Chapter 2 Theoretical Background

With this process, deeply embedded 3D structures, such as Y- or U-shaped inter-

channels, have been produced [24, 25].

The mechanism of fs laser micro-machining of FoturanTM glass will be briefly introduced

here [5, 25, 26]. The photosensitive glass FoturanTM is a lithium aluminosilicate glass

which is doped with a trace amount of silver and cerium. Cerium (Ce3+) ions are added as

photosensitizers, which release an electron to become Ce4+ by exposure to UV light

irradiation. The silver ions capture the free electrons to form silver atoms. During the following heat treatments, the silver atoms diffuse and subsequently agglomerate to form clusters at a temperature around 500oC; then the crystalline phase of the lithium

metasilicate grows around the silver clusters which act as nucleating sites in the

amorphous glass matrix at a temperature of around 600oC. Since this crystalline phase of

lithium metasilicate has a much higher etching rate in a dilute solution of hydrofluoric

(HF) acid than the base glass matrix, it can be preferentially etched, leaving behind the

engraved patterns.

In this work, we attempt to modify the optical properties and reflective index of selective

areas in-situ of the glass, in particular, a photosensitive glass FoturanTM, by the photochemical reaction of an fs laser with small pulse energy which sows the seeds of the modification by first laser irradiation followed by post-baking and chemical wet etching.

Chapter 2 Theoretical Background

2.1.1 Photosensitive Glass under Femtosecond Laser Irradiation

FoturanTM glass is UV sensitive in the wavelength range from 290 nm to 330 nm. It is an alkali-aluminosilicate glass and consists primarily of silica (SiO2: 75–85 wt%) along with

various stabilizing oxide admixtures, such as Li2O (7–11 wt.%), K2O and Al2O3 (3–6 wt.%), Na2O (1–2 wt%), ZnO (<2 wt.%) and Sb2O3 (0.2–0.4 wt.%). The photo-active

component is cerium (0.01–0.04 wt% admixture Ce2O3) and the nucleating agent is silver

(0.05–0.15 wt% admixture Ag2O) which makes the glass photosensitive. This sensitizer

is used to facilitate the optical exposure process. The primary role of the sensitizer is to absorb incident radiation and generate photo-electrons. The process consists of a UV-

exposure, followed by a thermal annealing treatment and wet chemical HF etching.

Exposure to UV light results in the formation of 3D latent images within the glass (silver

atoms). These are confined to zones where the UV is absorbed. The photo-initiation

process (latent image formation) and subsequent “fixing” of the exposure (permanent

image formation) proceed via several generalized equations (1) – (3). In most

photosensitive glass ceramic materials, the exposure process is initiated by the photo-

induced generation of free electrons from the photo-active species. For FoturanTM, a

lithium aluminosilicate glass, the photosensitive character is derived primarily from the

3+ ionization of the nascent cerium ions Ce (admixture Ce2O3) upon exposure to actinic

radiation λ < ~350 nm, the nascent cerium ions are photo-ionized and trapped (defect)

electron states are formed in the glass matrix (Eq. (1)). These trapped (defect) states

correspond to the latent image and have been associated with impurity hole centers and

Chapter 2 Theoretical Background

electron color centers. The ionization process leads to the formation of trapped (defect) electron states; impurity hole centers (Ce3+) and electron color centers (Ce4+ + e-) in the glass matrix [27].

Ce3+ + hv → Ce4+ + e- (1)

The renewed interest in this technique and class of photosensitive material are a result of

a series of experiments that demonstrated the fabrication of intricate true 3D patterns in

the PSGC material using pulsed UV nanosecond (ns) [28] and near-IR femtosecond (fs)

[29] lasers without resorting to ablation. Conventional processing of PSGC materials relies on lithography and UV lamp illumination, but recent experiments have shown that the photolytic mechanism for pulsed laser excitation might be different [30]. These pulsed laser experiments revealed that the photo-induced absorption process that initiates the precipitation of the chemically soluble ceramic phase proceeds via a multiple-photon excitation mechanism [31]. For pulsed UV nanosecond lasers (λ = 266 nm and 355 nm), the photochemical process is quadratic with respect to incident laser fluence [30], while a six-photon dependence has been shown with pulsed near-IR femtosecond lasers (λ = 775 nm) [32]. Both studies support the existence of either long-lived intermediate states or impurity defect states that permit a high-order excitation process. A major implication of these previous experiments is that pulsed lasers can successfully and efficiently expose the PSGC material at wavelengths that are outside the absorption band of the photo- initiator [28]. This unique property permits the fabrication of embedded structures in the media and the ability to create undercut and freestanding structures [30]. Given the

Chapter 2 Theoretical Background

reliability and availability of the new laser processing techniques, PSGC materials show

particular potential applications that require microstructures with the properties of a glass

and/or a ceramic.

For example, the fabrication of true 3D micro-fluidic structures and micro-total-analysis-

systems (µTAS) in PSGC materials could simplify design protocols and remove costly

bonding steps during assembly and packaging. PSGC materials have high flexural

strength (100–300 MPa) [33] and could also be utilized to fabricate mechanically compliant structures that are suitable for high shock and vibration environments. The mechanical compliance of PSGCs is related to the stiffness of the material and can be tailored on size domains ranging from the microscale (<100 mm) to the meso-scale (100 mm–10 mm) via additional laser exposure and thermal processing.

PSGCs are also associated with the commercially profitable, low-temperature co-fired

ceramic (LTCC) materials. These LTCC materials have a wide RF bandwidth and are

amenable to the 3D packaging of passive electronic components. Consequently, LTCCs

are beginning to replace printed circuit board (PCB) technology in a variety of electronic

devices, such as cellular phones. These LTCC materials could be fused with a micro-

fabricated PSGC materials and facilitate the development of complex instruments with

wireless communications. In addition, we have been utilizing all of the above aspects of

the PSGC materials at The Aerospace Corporation to develop miniature (<250 g weight)

disposable satellites [34].

Chapter 2 Theoretical Background

Advancements in laser processing techniques have provided new approaches for the implementation of PSGC materials in novel applications. As a consequence, interesting scientific questions emerge. For instance, pulsed lasers operating at a wavelength outside the absorption bandwidth of the chromophore (i.e., photo-initiator) can induce photo- electron excitation that leads to exposure and subsequent metasilicate phase precipitation.

The intensity of the pulsed lasers can excite the chromophore via non-resonant absorption processes that are driven by multi-photon or multiple absorption events.

Chapter 2 Theoretical Background

2.2 Mechanism during Thermal Annealing of Photosensitive Glass under

Femtosecond Laser Irradiation

2.2.1 Reactions Involved during Thermal Annealing of FoturanTM Glass after Laser Exposure

The subsequent heat treatment promotes a good crystallization within the exposed areas.

Thermal treatment is then used to convert the latent image into a fixed permanent image.

During the initial thermal treatment step at approximately 500oC, these trapped defect

+ states promote the reduction of nascent silver ions Ag (admixture Ag2O). The nucleation

process is initiated by the scavenging of the trapped electrons by impurity silver ions as

described by Eq. (2). During thermal processing, the atomic silver clusters agglomerate

and nucleate to form nanometer-scale Ag clusters as shown in Eq. (3). The formation of

metallic clusters corresponds to the ‘‘fixing’’ of the exposure and permanent image

formation in the glass matrix. Subsequently, the neutralized silver species diffuse and

0 agglomerate to form microscopic metallic silver clusters (Ag )x .

Ag+ + e- + ∆H → Ag0 (2)

In the following post-baking, the silver atoms diffuse and agglomerate into clusters at

around 500oC, and then the crystalline phase of lithium metasilicate grows into an

amorphous glass matrix using the silver clusters as nuclei at around 600oC.

Chapter 2 Theoretical Background

The growth of the soluble ceramic crystalline phase (lithium metasilicate, Li2SiO3) is initiated when the temperature is further increased to about 600oC. During this second

0 thermal treatment step, the (Ag )x clusters serve as efficient nucleation sites and promote

the heterogeneous formation of the lithium metasilicate (Li2SiO3) crystalline phase from

the amorphous glass matrix [35]. Thermal treatment is used to convert these latent images

into fixed permanent images. The nucleation process is initiated by the scavenging of the

trapped electrons by impurity silver ions as described by equation (2).

o 0 During the second thermal treatment step at 600 C, the (Ag )x clusters serve as efficient

nucleation sites which promote heterogeneous formation of the lithium metasilicate

(Li2SiO3) crystalline phase from the amorphous glass matrix as shown in equation (3)

[36-38].

0 0 x Ag + ∆H → (Ag )x (3)

The figures below show different stages of the monolithic 2-level 3D micro-mixer.

Figure 1(a) shows the FoturanTM sample right after the femtosecond irradiation. The

change in refractive index of the glass is evident from the change in color, from

transparent to yellowish-brown, between the exposed and un-exposed areas. After the 2-

stage annealing cycle, the irradiated areas show a greater contrast from the rest of the

glass matrix as shown in Fig. 1(b). Figure 1(c) is the fabricated 2-level 3D micro-mixer

after wet chemical HF etching.

Chapter 2 Theoretical Background

(a) (b)

Fig. 2.1. Structure of the monolithic 2-level 3D micro-mixer: (a) micro-mixer fabricated inside FoturanTM glass by femtosecond laser, (b) micro-mixer after thermal annealing cycle and (c) 3D micro-mixer after the final etching process.

(c)

Chapter 2 Theoretical Background

2.3 Chemical Reactions during Wet Chemical HF Etching of Photosensitive

Glass under Femtosecond Laser Irradiation

The dissolution rate of the lithium metasilicate crystalline phase in HF (reaction (4)) is

significantly faster when compared to the dissolution rate of the native amorphous glass

(reactions (5) and (6)), with differences in the etch rate ranging from a factor of 20 to 50

[39]. This marked difference in the reaction rates facilitates the preferential isotropic

etching of the Li2SiO3 crystallites and permits the selective removal of materials, thereby, allowing the fabrication of structures in FoturanTM. The reaction scheme that has been

proposed follows these elementary reactions [40]:

Li2SiO3 + 8HF → 2LiF + H2SiF6 + 3H2O (6)

SiO2 + 4HF → SiF4 + 2H2O (7)

SiF4 + 2HF → H2SiF6 (8)

Since the crystalline phase of lithium metasilicate is much more soluble in a dilute

solution of hydrofluoric (HF) acid than the glass matrix, it can be preferentially etched

away. Recently, it was found that the photosensitivity of FoturanTM glass can also be

activated with infra-red fs laser pulses by a multi-photon absorption process [25], and 3D

microstructures, such as micro-fluidic structures and micro-optical structures have been

embedded inside the glass [5, 25]. Since it has been pointed out that a dielectric medium with embedded metallic nanoparticles changes its permittivity as well as the refractive index [41], modification of optical properties of FoturanTM glass becomes possible if the

Chapter 2 Theoretical Background

post-baking of the sample is conducted at a temperature below that for crystallization.

The required intensity of the fs laser power is only the threshold of the multi-photon absorption which is much smaller than that for direct refractive index change by the fs laser [25]. In this case, the refractive index change is actually realized by the post-baking, and the modified region is confined only in the latent image region written by fs laser irradiation. In this manner, a high spatial resolution of modification, controllable refractive index change, and smooth internal structure could be achieved.

The variable laser exposure processing technique relies on the ability to precisely and selectively alter the chemical etch rate of the photo-structurable glass ceramic by varying the laser exposure during pattern formation. Previous experiments conducted in our laboratory have determined that the chemical etch rate of a commercial photosensitive glass ceramic (FoturanTM, Schott Corp., Germany) in dilute aqueous hydrofluoric (HF)

acid is strongly dependent on the incident laser irradiance during patterning at λ = 266 nm

and λ = 355 nm [42, 43]. For low laser irradiances, the etch rate ratio increases nearly

linearly with laser irradiance. The near linear increase in the etch rate ratio can be fit

using linear least-squares regression analysis. At high laser irradiances, the measured etch

rate ratios reach a plateau. The value of the maximum etch rate contrast is dependent on

the thermal treatment and chemical etching procedures, as well as the concentration of

ingredients in the base glass. In the plateau region, the average maximum etch rate for the

exposed and heat treated glass was determined to be independent of the exposure

wavelength.

Chapter 2 Theoretical Background

References:

[1] H.Misawa: Three-Dimensional Optical Memory Element and Method of Writing

Information, Jpn. patent app. 023 614 (1995), acceptance No. 2 961 126,

[2] H. Sun, Y. Xu, S. Juodkazis, K. Sun, M. Watanabe, S. Matsuo, H. Misawa, J.

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[3] K. Yamasaki, S. Juodkazis, S. Matsuo, H. Misawa: Appl. Phys. A 77, 371 (2003)

[4] M. Watanabe, Thesis of Mech. Eng., Tokushima Univ., Japan (BSc. 1996, MSc.

1998)

[5] Y. Cheng, K. Sugioka, K. Midorikawa, M. Masuda, K. Toyoda, M. Kawachi, K.

Shihoyama: Opt. Lett. 28, 55 (2003)

[6] K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass

with a femtosecond laser,” Opt. Lett. 21, 1729-1731 (1996).

[7] Y. Li, W. Watanabe, K. Yamada, T. Shinagawa, K. Itoh, J. Nishii, and Y. Jiang,

“Holographic fabrication of multiple layers of grating inside soda-lime glass with

femtosecond laser pulses,” Appl. Phys. Lett. 80, 1508-1510 (2002).

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[9] K. Minoshima, A. M. Kowalevicz, I. Hartl, E. P. Ippen, and J. G. Fujimoto,

“Photonic device fabrication in glass by use of nonlinear materials processing

with a femtosecond laser oscillator,” Opt. Lett. 26, 1516-1518 (2001).

Chapter 2 Theoretical Background

[10] S.H.Cho, H. Kumagai, and K. Midorikawa, “Fabrication of internal diffraction

gratings in planar silica plates using low-density plasma formation induced by a

femtosecond laser,” Nucl. Instrum. Methods Phys. Res. B 197, 73-82 (2002).

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788.

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Microeng. 6, 254 (1996).

Chapter 2 Theoretical Background

[21] H. Helvajian, P. D. Fuqua, W. W. Hansen, and S. Janson, Proc. SPIE 4088, 319

(2000).

[22] P.J. Brock, M.D. Levenson, J.M. Zavislan, J.R. Lyerla, J. Vac. Sci. Technol. B 9,

3155 (1991)

[23] J. Kim, H. Berberoglu, X. Xu, J. Microlith. Microfab. Microsyst. 3, 478 (2004)

[24] Y. Kondo, J. Qiu, T. Mitsuyu, K. Hirao, and T. Yoko, Jpn. J. Appl. Phys., Part 2

38, L1146 (1999).

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Toyoda, H. Helvajian, and K. Midorikawa, Appl. Phys. A: Mater. Sci. Process.

76, 857 (2003).

[26] H. Helvajian, P.D. Fuqua, W.W. Hansen, S. Janson, RIKEN Rev. 32, 57 (2001)

[27] V. Arbuzov, J. Non-Cryst. Solids 253, 37 (1999)

[28] W.W. Hansen, S.J. Janson, H. Helvajian, Proc. SPIE 2991 (1997) 104; P.D.

Fuqua, S.W. Janson, W.W. Hansen, H. Helvajian, Proc. SPIE 3618 (1999) 213.

[29] Y. Cheng, K. Sugioka, K. Midorikawa, M. Masuda, K. Toyoda, M. Kawachi, K.

Shihoyama, Opt. Lett. 28 (2003) 1144.

[30] P. D. Fuqua, D. P. Taylor, H. Helvajian, W. W. Hansen, and M. H. Abraham,

Mater. Res. Soc. Symp. Proc. 624, 79 (2000).

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Academic Press, NY, 2002, p. 415;

[32] Y. Cheng, K. Sugioka, M. Masuda, K. Midorikawa, M. Kawachi, K. Shihoyama,

K. Toyoda, Proc. SPIE 4977 (2003) 314.

Chapter 2 Theoretical Background

[33] P. Hing, P.W. McMillan, J. Mater. Sci. 8 (1973) 1041; M.P. Borom, A.M. Tukalo,

R.H. Doremus, J. Am. Ceram. Soc. 58 (1975) 385.

[34] A. Huang, W.W. Hansen, S.W. Janson, H. Helvajian, Proc. SPIE 4559 (2001)

164;

A. Huang, W.W. Hansen, S.W. Janson, H. Helvajian, Proc. SPIE 4637 (2002)

297.

[35] S.D. Stookey, Indust. Engin. Chem. 41, 856 (1949)

[36] F. E. Livingston, P. M. Adams and H. Helvajian, Appl. Phys. A 89, 97-107 (2007).

[37] Ya Cheng, Koji Sugioka and Katsumi Midorikawa, Applied Surface Science 248,

172-176 (2005).

[38] Ya Cheng, Koji Sugioka, Masashi Masuda, Kazuhiko Shihoyama, Koichi Toyoda

and Katsumi Midorikawa, Optics Express 11(15), 1809-1816 (2003).

[39] S.D. Stookey, Indust. Engin. Chem. 45, 115 (1953)

[40] A. Berezhnoi, Glass-Ceramics and Photo-Sitalls (Plenum Press, New York,

1970)

[41] N. Garcia, E. V. Ponizovskaya, and J. Q. Xiao, “Zero permittivity materials: Band

gaps at the visible,” Appl. Phys. Lett. 80, 1120-1122 (2002).

[42] F.E. Livingston, H. Helvajian, Proc. SPIE 4830, 189 (2003)

[43] F.E. Livingston, H. Helvajian, Appl. Phys. A 81, 1569 (2005)

Chapter 3 Experimental Setup

CHAPTER 3

EXPERIMENTAL SETUP

3.1 Femtosecond Laser Fabrication of Microstructures In-situ of FoturanTM

Glass

3.1.1 Introduction

The fabrication of femtosecond laser micro-machining of FoturanTM glass is briefly

introduced [8–10]; FoturanTM is a photosensitive glass composed of lithium

aluminosilicate doped with some traces of silver and cerium. In this case, Cerium (Ce3+)

ions are the photosensitizers which absorb the incident radiation and release electrons to become Ce4+ from exposure to UV light irradiation. The silver ions capture these free

electrons to form silver atoms.

The sample after laser irradiation will be subjected to thermal annealing cycle. During the thermal annealing process, the silver atoms will diffuse and agglomerate to form clusters at a temperature of around 500oC. The crystalline phase of lithium metasilicate then

grows around the silver clusters serving as efficient nucleation sites which promote

heterogeneous formation of the lithium metasilicate (Li2SiO3) crystalline phase from the

amorphous glass matrix at a temperature of about 600oC.

Chapter 3 Experimental Setup

This crystalline phase of lithium metasilicate has a higher etching rate in a dilute solution

of hydrofluoric (HF) acid than the glass matrix. This marked difference in the reaction

rates facilitates the preferential isotropic etching of the Li2SiO3 crystallites and permits

the selective removal of materials, thereby, allowing the fabrication of structures in

FoturanTM. [1 – 17]

The present experimental investigations elucidate the underlying chemical and physical

processes that influence the observed laser-induced variations in the chemical etch rate.

Several complimentary analytical research tools were employed at various stages of the

laser exposure and thermal treatment steps. Optical microscopy and scanning electron

microscopy were used to identify major crystalline phases that were formed in FoturanTM following the respectively laser irradiation and heat treatment as well as monitoring the crystalline content as a function of incident laser irradiance. A 3 dimensional surface profiler was further utilized to examine the microscopic structural characteristics and morphology of the soluble metasilicate crystallites. These results help elucidate the sequential processes that correspond to the initial laser excitation event and eventually lead to the precipitation of a crystalline phase that is soluble in dilute HF. Furthermore, these results allow the optimization of the processing parameters for FoturanTM to yield both common and unique material states for specific technological applications.

Chapter 3 Experimental Setup

3.1.2 Experimental Setup of Femtosecond Laser Irradiation of FoturanTM Glass

The schematic diagram of the setup of the femtosecond laser fabrication of the monolithic 2-level 3D micro-mixer is shown in the figure below. Laser irradiation is the first step in the fabrication of 3D microstructures in-situ of the photosensitive FoturanTM glass.

Mirror Mirror

Femtosecond laser Lens

Mirror

Sample

3 axes stage controlled by PC

Fig. 3.1. Experimental setup for the femtosecond laser irradiation of FoturanTM glass

Chapter 3 Experimental Setup

(a)

(b) (c)

Fig. 3.2. (a) Setup of the irradiation process, comprising of the femtosecond laser together with the optical setup as well as the 3 axes stage. Red arrows depict the path of the laser. (b) and (c) Closed up views of the magnifying lens and 3 axes stage.

Chapter 3 Experimental Setup

The laser employed here is a Ti:Sapphire femtosecond laser (Spectra Physics Tsunami,

Model 3960) with a regenerative amplifier (Spectra Physics Spitfire, Model 9769A with a

central wavelength λ of 800 nm, pulse duration τ of 100 fs and repetition rate at 1000

Hz). The setup of the fabrication procedure is shown in Figs. 3.1 and 3.2. Laser beams

from the femtosecond laser are directed from the laser to the samples by 3 mirrors. The

laser beam is subsequently focused using a 20 times magnification Mitutoyo lens.

3.1.3 Application of the Femtosecond Laser

The basic instrument to generate short pulses is the femtosecond laser. Two fundamental

elements of a laser are the amplifier and a cavity which produces a positive feedback

between the light and the amplifying medium. In the following section, a short

description of the application of the femtosecond laser is briefly mentioned.

The directions and shapes of the 3D structures are controlled by the movements of the stage. The samples were positioned onto a 3 axes XYZ stages from Aerotech with a

Mitutoyo lens of magnification of 20 times. The designed micro-channels and micro- reservoirs were fabricated directly into the FoturanTM samples. The designs of the

structures were created using a 3D software, MasterCam. This process employs a direct

write method, eliminating the use of masks and any subsequent lithography steps, thus,

making it a direct and maskless process. The following figure shows the designs of the

microstructures within the FoturanTM sample from 4 different viewing angles. (Fig. 3.3)

Chapter 3 Experimental Setup

(a) (b)

Fig. 3.3. Design of the microstructure within the FoturanTM samples from 4 different viewing angles; (a) Top view. (b) Isometric view. (a) Front view. (b) Side view.

Chapter 3 Experimental Setup

3.1.4 Thermal Annealing Cycle and Wet Chemical Etching

The laser irradiated samples were subsequently subjected to a 2-stage thermal annealing

cycle. In the first stage, the samples were annealed from an ambient temperature to

500oC. Subsequently, the FoturanTM samples will ramp to 600oC. The figure below shows the HP4284A Precision LCR Meter Dielectric Tester oven used in the annealing process

(Fig. 3.4).

Figure 3.4. HP4284A Precision LCR Meter Dielectric Tester oven employed in the 2- stage annealing process

The annealed samples will be chemically etched by 10% HF solution for an hour in an

ultrasonic bath. The agitation process improves the flow of the 10% HF solution in areas

which are difficult to reach and improves capillary actions as well as the uniformity in

Chapter 3 Experimental Setup

etching. Areas exposed to laser irradiation will have higher etching rates as compared to

unexposed areas.

3.2 Characterization Techniques for the Monolithic 2-level 3D Internal Micro-

mixer

3.2.1 Optical Microscopy

The Olympus MX-50 optical microscope is employed to examine the FoturanTM sample after the fabrication process. The microscope is equipped for up to 150 times of magnification and connected to a CCD camera which feeds real-time images into an image-acquisition software, analysis 3.0 in the computer.

Chapter 3 Experimental Setup

Figure 3.5. Olympus MX-50 optical microscope with magnification lens of 10, 20, 50, 80, 100 and 150.

Chapter 3 Experimental Setup

(a)

Figure 3.6. Magnification module of the Olympus MX-50 optical microscope.

An optical microscope creates a magnified image of an object specimen with an objective lens and magnifies the image further with an eyepiece to allow the user to observe it by the naked eye.

Chapter 3 Experimental Setup

3.2.2 JEOL JSM 7401F Scanning Electron Microscopy (SEM)

The JSM-7401F, one of JEOL's highestFigure resolution 3.7. JEOL field JSM em 7401Fission scanning electron

(FESEM) incorporating a cold cathode field emission gun, ultra high vacuum, and sophisticated digital technologies for high resolution, high quality imaging of microstructures. The JOEL JSM-7401F FESEM has a minimum resolution of 1 nanometer was used in this study to characterize the high resolution topography images.

The accelerating voltage is in a range of 5 kV to 30 kV, and the magnification factor can vary from 20 to 300,000. Using a patented Gentle Beam™ method, the JSM-7401F

Chapter 3 Experimental Setup produces ultra high resolution at low kV (0.1 kV) and enables observation of true surface structures while helping to reduce charging of non-conductive samples. An r-filter allows variable energy filtering of secondary electrons and backscattered electrons to observe varying contrast from surface morphology images to compositional images. The JSM-

7401F also has an available "in-lens" solid state backscatter detector which provides the ultimate in resolution and low kV performance. The JSM-7401F with its automatic, fully eucentric stage (An auto loading stage is optionally available) is also equipped with EDX analysis.

Chapter 3 Experimental Setup

3.2.3 3D Surface Profilometry

Figure 3.8. Mitutoyo Surftest Extreme SV-3000CNC 3D surface profiler.

The morphology and topology of the FoturanTM samples are measured with a Mitutoyo

Surftest Extreme SV-3000CNC 3D surface profiler. The X & Y axes have a resolution of

50 nm and a maximum measuring range of 200 mm. The z axis has a measuring range between 300 mm (slow) to 500 mm (high). The resolution is 0.05 um for the z axis. This equipment has the capability of tilt between -45 to +10 degrees, with a tilt angle

resolution of 0.000225 deg. The rotating speed is of 1 rpm. The driving speed during

Chapter 3 Experimental Setup measurement by the equipment is at a maximum of 200 mm/s while manually with a joystick is 60 mm/s. The measuring speed is 0.02 to 2 mm/s. The Mitutoyo Surftest

Extreme SV-3000CNC 3D surface profiler makes use of a reflection type linear encoder for all the morphology and topology analyses.

Chapter 3 Experimental Setup

3.2.4 Characterizing the Mixing Capability of the Fabricated Micro-mixer

The monolithic 2-level 3D micro-mixer is tested for its mixing capability with hydrophilic water-based liquid dyes (Fig. 3.01 (a)). 2 different coloured dyes were introduced to the 2 individual inlets. The two dyes flow through their respective inlets and subsequently the 2 micro-channels, which meet and mix within the central micro- reservoir before exiting through the outlet. The mixer is employed in the mixing of the yellow and blue dyes. Fig. 3.10 (b) shows the mixing of the blue and yellow dyes which gives a green dye solution.

The mixing ability of the micro-mixer is further verified and confirmed for its repeatability and reliability by mixing yellow and pink dyes. The resultant mixed dye was an orange coloured dye. A cross-sectional view showing the entrance of the yellow and pink dyes from their respective designated first level micro-reservoirs into the second level central micro-reservoir during the mixing procedure is shown in Fig. 3.10(c).

Chapter 3 Experimental Setup

(a) (b)

2 mm 2 mm

(c)

Fig. 3.9. (a) Structure of the fabricated monolithic 2-level 3D micro-mixer. (b) Mixing of the yellow and blue dyes in the central micro-reservoir resulting in a 2 mm green dye solution. (c) Mixing of the yellow and pink dyes.

Chapter 3 Experimental Setup

References:

[1] Chantal Khan Malek, Laurent Robert, Jean-Jacques Boy and Pascal Blind,

Microsyst. Technol 13, 447-453 (2007).

[2] A. A. Bettiol, C. N. B. Udalagama, E. J. Teo, J. A. van Kan and F. Watt, Nuclear

Instruments and Methods in Physics Research B 260, 357-361 (2007).

[3] F. E. Livingston, P. M. Adams and H. Helvajian, Appl. Phys. A 89, 97-107 (2007).

[4] Y. Cheng, H. L. Tsai, K. Sugioka and K. Midorikawa, Appl. Phys. A 85, 11-14

(2006).

[5] A. A. Bettiol, S. Venugopal rao, E. J. Teo, J. A. van Kan and Frank Watt, Applied

Physics Letters 88, 171106 (2006).

[6] A. A. Bettiol, S. Venugopal rao, T. C. Sum, J. A. van Kan and F. Watt, Journal of

Crystal Growth 288, 209-212 (2006).

[7] S. Juodkazis, K. Yamasaki, V. Mizeikis, S. Matsuo and H. Misawa, Appl. Phys. A

79, 1549-1553 (2004).

[8] F. E. Livingston, P. M. Adams and H. Helvajian, Applied Surface Science 247,

526-536 (2005).

[9] Ya Cheng, Koji Sugioka and Katsumi Midorikawa, Applied Surface Science 248,

172-176 (2005).

[10] Ya Cheng, Koji Sugioka, Masashi Masuda, Kazuhiko Shihoyama, Koichi Toyoda

and Katsumi Midorikawa, Optics Express 11(15), 1809-1816 (2003).

[11] R. An, Y. Li, D. Liu, Y. Dou, F. Qi, H. Yang and Q. Gong, Appl. Phys. A 86,

343-346 (2007).

Chapter 3 Experimental Setup

[12] B. Fisette, F. Busque, J. –Y. Degorce and M. Meunier, Applied Physics Letters 88,

091104 (2006).

[13] Zhongke Wang, Koji Sugioka and Katsumi Midorikawa, Appl. Phys. A 89, 951-

955 (2007).

[14] F. E. Livingston and H. Helvajian, Appl. Phys. A 81, 1569-1581 (2005).

[15] V. R. Bhardwaj, E. Simova, P. B. Corkum, D. M. Rayner, C. Hnatovsky, R. S.

Taylor, B. Schreder, M. Kluge and J. Zimmer, Journal of Applied Physics 97,

083102 (2005).

[16] Young-Rae Choo, Jae-Yeol Oh, Heui-Soo Kim, Hyo-Soo Jeong, Materials

Science and Engineering B64, 79-83, (1999)

[17] T. R. Dietrich, W. Ehrfeld. M. Lacher, M. Krämer and B. Speit, Microelectronic

Engineering 30, 497-504 (1996).

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

CHAPTER 4

3-DIMENSIONAL MICROSTRUCTURAL FABRICATION

BY FEMTOSECOND LASER MICROPROCESSING

4.1 Fabrication Process Flow

The entire process of the fabrication of the monolithic 2-level 3D micro-mixer is described below and shown in the flow chart in Fig. 4.1. From the sample preparation stage to the laser irradiation to thermal annealing and finally to the HF etching of the samples. [1 – 10]

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

A flow chart of the entire process is shown in the following figure.

Sample Preparation Cutting and cleaning of samples

Mirror Mirror

Femtosecond laser Lens Mirror Sample 3 axes stage controlled by PC

Temperature (oC)

600 500

Ambient

Time (hours)

Isotropic preferential HF etching

Fig. 4.1. Flow chart of the 2-level 3D micro-mixer fabrication process.

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

4.1.1 Sample Preparation

FoturanTM samples measuring 3 cm by 1.5 cm with a thickness of 6 mm were cut from 10 cm diameter discs using a diamond saw. Additional post polishing was performed to smoothen out the edges of the samples. The FoturanTM samples were of quality B, which implies that the samples only have a maximum of two bubbles within each sample. The sizes of the bubbles range from 50 µm to 1 mm / 100 cm² or up to a maximum of 1 edge effect of defect size of 1 mm / 40 cm and up to a maximum of 2 scratches of 50 µm to 20 mm. These samples were subsequently cleaned using acetone, isopropyl alcohol (IPA) and deionized (DI) water in ultrasonic baths (Branson 1510) for a few minutes to remove any possible presence of organic contaminants.

The design of the micro-mixer was conceptualized and modeled using a design software,

MasterCam. The figure below shows the modeling stage of the design from MasterCam.

(a) (b)

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

Fig. 4.2. Design of the microstructure within the FoturanTM samples from 4 different viewing angles: (a) Top view. (b) Isometric view. (a) Front view. (b) Side view.

The samples were positioned on a XYZ stage from Aerotech with a Mitutoyo lens of a magnification of 20 times. The designed micro-channels and micro-reservoirs were fabricated into the FoturanTM samples. The designs of the structures were created using the 3D design software MasterCam. This process uses direct-write technique, eliminating any use of masks and any subsequent lithography steps, thus making it a direct and maskless process.

The fabricated micro-reservoir is approximately 3 mm in diameter and 1200 µm in thickness. The inlets are 300 um deep, 1 mm wide and 6 mm long. Outlets are 1 mm in diameter and 800 µm deep. The dimensional tolerance of the proposed fabricated method is approximately 10 µm and is very much dependent on the various factors during the laser irradiation, annealing and etching. The finest effective micro-channel is about 100

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

µm in dimension while the smallest effective micro-reservoir is approximately 500 µm and both are inter-dependent on the various processing factors. The design of the microstructure was first focused inside the FoturanTM sample. The laser spot from the

femtosecond laser was directed and focused inside the FoturanTM sample, an algorithm

for the fabrication of the microstructures was written using MasterCam. The focused

laser spot mapped out the micro-channels and micro-reservoirs within the FoturanTM samples in 3 dimensions. The laser beam irradiates the sample in a X-Y direction parallel to the length of the sample, subsequently it moves vertically down

in the Z direction to achieve the required height and dimensions of the designed structures. The inlets, micro-reservoirs and micro-channels on the first level will be written by the laser beam, followed by the second level central micro-reservoir and the outlets. In this way, true 3D structures were totally fabricated within a single monolithic sample. This maskless process significantly reduces the amount of time required for the various lithographic steps.

4.1.2 Multi-Photon Absorption

Photons with energy greater than a threshold energy, the ionization energy of the atom, can cause the ionization of the atoms. Absorption of two photons simultaneously by a single atom would cause an electron transition to very high-energy states unreachable by

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

single photon absorption1. Once an atom was excited to a high state, photons of lower

energy than the threshold energy could ionize that atom.

If a photon collides with a ground state atom, the atom may absorb the photon via

electronic transitions to energy levels higher than the ground state by δE=hv. Selection rules determined by quantum mechanical calculation restrict these transitions such that the change in angular momentum of single photon transitions is δl = +1, -1. For example, an electron with a ground state in an s-state may transition by one photon only to a higher p-state. If an atom absorbs two photons simultaneously, since each photon has angular momentum of +1 or –1, the electron may only change angular momentum by δl = +2, 0,

or –2. The s-state electron may then transit to another s-state or to a d-state: states out-of- reach by single photon absorption. Absorption of a third photon may then lead to ionization, as theorized by Göppert-Mayer.

When absorbing multiple photons simultaneously, an atom will proceed to an intermediate state corresponding to a characteristic eigenstate or to a "virtual state."

These virtual states do not exist within the atom; they correspond to no electronic n or l state. They are superpositions of waves. No electron transitions to the virtual state. The lifetime of a virtual state is incredibly short, relative to eigenstates. The closer the energy of a virtual state is to that of an eigenstate, the longer the lifetime of said virtual state.

This can be thought of as a result of Heisenburg's Uncertainty Principle: δEδt > h. Since

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

what has been termed “simultaneous” two-photon absorption is not literally simultaneous but instead is the absorption of two photons a very, very short time apart, a longer lifetime of the single photon excited state (virtual or real eigenstate) increases the possibility of a second photon being absorbed before the state decays. [11]

Fig. 4.3. 2-photon absorption of Cesium.

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

Fig. 4.4. Two-photon Jablonski Energy Diagram.

4.1.3 Principles of Two-Photon Excitation

The phenomenon of two-photon excitation arises from the simultaneous absorption of

two photons in a single quantitized event. Since the energy of a photon is inversely

proportional to its wavelength, the two absorbed photons must have a wavelength about

twice that required for one-photon excitation. For example, a fluorophore that normally absorbs ultraviolet light (approximately 350 nm wavelength) can also be excited by two photons of near-infra-red light (approximately 700 nanometers wavelength) if both reach the fluorophore at the same time (see Fig. 4.4). In this case, "the same time" means within an interval of about 10-18 seconds.

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

Because two-photon excitation depends on simultaneous absorption, the resulting fluorescence emission varies with the square of the excitation intensity. This quadratic relationship between excitation and emission gives rise to many of the significant advantages associated with two-photon excitation microscopy. In order to produce a significant number of two-photon absorption events, the photon density must be approximately one million times that required to generate the same number of one-photon absorptions. The consequence is that extremely high laser power is required to generate significant two-photon-excited fluorescence. This power level is easily achieved by focusing mode-locked (pulsed) lasers, in which the power during the peak of the pulse is high enough to generate significant two-photon excitation, while the average laser power remains fairly low. In this situation, the resulting two-photon-excited state from which emission occurs is the same singlet state that is populated when carrying out a conventional fluorescence experiment. Thus, fluorescent emission following two-photon excitation is exactly the same as emission generated in normal one-photon excitation. Fig.

4.3 presents a Jablonski diagram illustrating absorption of a single (ultraviolet) photon

(Figure 1(a)) and the simultaneous absorption of two near infra-red photons (Figure 1(b)), producing the identical excited state.

Another closely related nonlinear optical process, three-photon excitation, may also prove to be beneficial for biological experiments. Three-photon excitation occurs in much the same way as the two-photon process except that three photons must interact with the

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

fluorophore simultaneously to produce emission. Due to the quantum-mechanical properties of fluorescence absorption, the photon density required for three-photon excitation is only about tenfold greater than the density needed for two-photon absorption

(rather than another million-fold greater). Three-photon excitation can, therefore, be considered an attractive option for some experiments. Higher-order non-linear effects, such as four-photon absorption, have been experimentally demonstrated, although it is unlikely that these phenomena will find any immediate application in biological research.

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

4.2 Thermal Annealing

Temperature (oC)

600

500

Ambient Temperature

Time (hours)

Fig. 4.5. Temperature-time plot of the annealing cycle.

The laser irradiated samples were subsequently subjected to a 2-stage thermal annealing cycle in an oven (HP4284A Precision LCR Meter Dielectric Tester). In the first stage, the

samples were annealed from an ambient temperature of 25oC to 500oC at a ramping rate

of 5oC/minute. At the first pre-determined temperature of 500oC, the FoturanTM samples

dwelt for an hour before ramping to 600oC at the same ramping rate of 5oC/minute. The

samples were kept for another hour at this temperature before ramping down to the

ambient temperature with a ramping rate of 5oC/minute. This ramp down procedure is

necessary to prevent excessive warpage to the glass substrates as well as to minimize any

thermal shock due to the difference in the coefficient of thermal expansion between the

glass samples and the surrounding ambient. The entire annealing process took

approximately 5 hours and 30 minutes. The figure below shows the complete thermal

annealing cycle (Fig. 3.). [1, 2, 10]

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

4.3 Etching

The annealed samples will be chemically etched by 10% HF solution for an hour in an ultrasonic bath. The agitation process improves the flow of the 10% HF solution into areas which are difficult to reach and improves capillary actions as well as the uniformity in etching. Areas exposed to laser irradiation will have higher etching rates as compared to unexposed areas. [12]

There are two major types of etching processes: wet and dry etching. In the following, we will briefly discuss the most popular technologies for wet and dry etching.

Wet Etching is an etching process that utilizes liquid chemicals or etchants to remove materials for example from the wafer, usually in specific patterns defined by photoresist masks on the wafer. Materials not covered by these masks are 'etched away' by the chemicals while those covered by the masks are left almost intact. These masks were deposited on the wafer in an earlier wafer fabrication step known as 'lithography.'

A basic wet etching process may be broken down into three basic steps: 1) diffusion of the etchant to the surface for removal; 2) reaction between the etchant and the material being removed and 3) diffusion of the reaction byproducts from the reacted surface.

Reduction-oxidation (redox) reactions are commonly encountered in wet etching processes, i.e., an oxide of the material to be etched is first formed, which is then

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

dissolved, leading to the formation of new oxide, which is again dissolved, and so on until the material is consumed.

Wet etching is a simple technology, which will give good results if the right combination of etchant and mask material can be found to suit the required applications. Wet etching works very well for etching thin films on substrates, and can also be used to etch the substrate itself. The problem with substrate etching is that isotropic processes will cause undercutting of the mask layer by the same distance as the etch depth (small features in thin films comparable to the film thickness, undercutting will be at least equal to the film thickness). Anisotropic processes allow the etching to stop on certain crystal planes in the substrate, but still results in a loss of space, since these planes cannot be vertical to the surface when etching holes or cavities. If this is exists as a limitation, dry etching can be considered for the substrate instead. With dry etching, it is possible to etch almost straight down without undercutting, which provides much higher resolution. However, the cost of dry etching will be higher than for wet etching.

Dry etching technology can be split into three separate classes called reactive ion etching

(RIE), sputter etching and vapor phase etching.

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

The most common form of dry etching is reactive ion etching (RIE). In RIE, the substrate is placed inside a reactor in which several gases are introduced. A plasma is struck in the gas mixture using an RF power source, breaking the gas molecules into ions. The ions are accelerated towards the materials to be etched and react at the surface of the materials being etched, forming other gaseous materials. The etching reaction is enhanced in the direction of travel of the ion. This is known as the chemical part of reactive ion etching.

There is also a physical part which is similar in nature to the sputtering deposition process. If the ions have high enough energy, they can knock atoms out of the material to be etched without a chemical reaction. It is a very complex task to develop dry etch processes that balance chemical and physical etching, since there are many parameters to adjust. RIE is an anisotropic etching technique. Deep trenches and pits (up to ten or a few tens of microns) of arbitrary shape and vertical walls can be etched in a variety of materials including silicon, oxide and nitride. Unlike anisotropic wet etching, RIE is not limited by the crystal planes in the silicon. By changing the balance it is possible to influence the anisotropy of the etching, since the chemical part is isotropic and the physical part is highly anisotropic. The combinations can form sidewalls that have shapes from rounded to vertical. A schematic of a typical reactive ion etching system is shown in the figure below.

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

A special sub-class of RIE which continues to grow rapidly in popularity is deep RIE

(DRIE). In this process, etch depths of hundreds of microns can be achieved with almost

vertical sidewalls. Two different gas compositions are alternated in the reactor. The first

gas composition creates a polymer on the surface of the substrate, and the second gas

composition etches the substrate. The polymer is immediately sputtered away by the

physical part of the etching, but only on the horizontal surfaces and not the sidewalls.

Since the polymer only dissolves very slowly in the chemical part of the etching, it builds

up on the sidewalls and protects them from etching. As a result, etching aspect ratios of

50 to 1 can be achieved.

Sputter etching is essentially RIE without reactive ions. The systems used are very similar in principle to sputtering systems. The big difference is that the substrate is now subjected to the ion bombardment instead of the material target used in sputtering deposition.

Vapor phase etching is another dry etching method, which can be done with simple equipment. In this process the substrate to be etched is placed inside a chamber, in which

one or more gases are introduced. The material to be etched is dissolved at the surface in

a chemical reaction with the gas molecules. The two most common vapor phase etching technologies are silicon dioxide etching using hydrogen fluoride (HF) and silicon etching

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

using xenon diflouride (XeF2), both of which are isotropic in nature. Usually, care must be taken in the design of a vapor phase process to not have bi-products form in the chemical reaction that condense on the surface and interfere with the etching process.

Fig. 4.6. Typical parallel-plate reactive ion etching system.

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

4.3.1 Isotropy / Anisotropy

Anisotropic etching, in contrast to isotropic etching, means different etch rates in different directions in the material. The classic example of this is the <111> crystal plane sidewalls that appear when etching a hole in a <100> silicon wafer in a chemical such as potassium hydroxide (KOH). The result is a pyramid shaped hole instead of a hole with rounded sidewalls with a isotropic etchant. The principle of anisotropic and isotropic wet etching is illustrated in the figure below.

Wet etchants fall into two broad categories; isotropic etchants and anisotropic etchants.

Wet etching is generally isotropic, i.e., it proceeds in all directions at the same rate. An etching process that is not isotropic is referred to as 'anisotropic.' An etching process that proceeds in only one direction (e.g., vertical only) is said to be 'completely anisotropic'.

Isotropic etchants attack the material being etched at the same rate in all directions.

Anisotropic etchants attack the silicon wafer at different rates in different directions, and so there is more control of the shapes produced. Some etchants attack silicon at different rates depending on the concentration of the impurities in the silicon (concentration dependent etching). A high degree of anisotropy is desired in etching when a more reliable and consistent copy of the mask pattern is required.

When an isotropic etchant eats away a portion of the material under the mask, the etched film is said to have 'undercut' the mask. The amount of 'undercutting' is a measure of an etching parameter known as the 'bias.' Bias is simply defined as the difference between

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

the lateral dimensions of the etched image and the masked image. Thus, the mask used in etching must compensate for whatever bias an etchant is known to produce, in order to create the desired feature on the wafer. Because of the isotropic nature of wet etching, it results in high bias values that are not practical for use in pattern images that have features measuring less than 3 microns. Thus, feature sizes that are smaller than 3 microns must not be wet-etched, and should instead be subjected to other etching techniques that offer a higher degree of anisotropy.

Isotropic etchants are available for oxide, nitride, aluminium, polysilicon, gold, and silicon. Since isotropic etchants attack the material at the same rate in all directions, they remove material horizontally under the etch mask (undercutting) at the same rate as they etch through the material.

Anisotropic etchants are available which etch different crystal planes in silicon at different rates. The most popular anisotropic etchant is potassium hydroxide (KOH), since it is the safest to use.

Fig. 4.7. Difference between anisotropic and isotropic wet etching.

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

4.3.2 Selectivity

Selectivity, S, is measured as the ratio between the different etch rates of the etchant for

different materials. Thus, a good etchant needs to have a high selectivity value with

respect to both the mask and the substrate, for instance, its etching rate for the film being

etched must be much higher than its etching rates for both the mask and the substrate.

An etchant not only attacks the materials being removed, but the mask and the substrate

(the surface under the materials being etched) as well. The 'selectivity' of an etchant

refers to its ability to remove only the materials intended for etching, while leaving the

mask and substrate materials intact.

4.4 Characterizing the 2-level 3D Internal Micro-mixer

The monolithic 2-level 3D internal mixer was tested for its mixing capability with hydrophilic water-based liquid dyes. The sample was cleaned under agitation in deionized (DI) water with the ultrasonic cleaner for 8 hours to remove any residual HF etchant. DI water was changed after every 2 hours in order to expose the sample to consistently fresh supply of DI water. A drop of 2 different coloured dyes was introduced to the 2 individual inlets. The two dyes flow through the respective inlets and subsequently the 2 micro-channels which meet and mix within the central micro-reservoir before exiting through the outlet. The mixer was employed in mixing of the yellow and

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

blue dyes as well as pink and yellow dyes. The results obtained were green and orange

coloured mixtures of dyes respectively.

The yellow dye enters through its designated micro-inlet located on the first level of the

2-level mixer. Figure 4.8(a) shows the entry of the yellow dye into the 2-level mixer from the top view. The blue dye subsequently enters its designated first level micro-reservoir as shown in Fig. 4.8(b). Figure 4.8(c) shows the positions of both the yellow and blue dyes in their first level micro-reservoirs of the micro-mixer just before the commencement of the mixing process. The yellow and blue dyes meet and commence mixing in a central micro-reservoir on the second level of the 2-level micro-mixer. Figure

4.8(d) shows the mixing process from the top view while Fig. 4.8(e) shows the mixing from the cross-sectional view. The blue dye flows from its first level micro-reservoir from the left of the 2-level micro-mixer while the yellow dye flows from its first level micro-reservoir on the right of micro-mixer. Both dyes meet and begin mixing in the second level central micro-reservoir. The resulting mixed green dye can be seen flowing out of the second level micro-reservoir and exiting through a connecting tube. A closed- up view during the mixing process in the second level central micro-reservoir is also

shown in Fig. 4.8(f). The blue dye enters from the left micro-channel while the yellow

dye enters from the right micro-channel into the second level central micro-reservoir to

commence mixing. An entire mixing sequence of events is further shown in Figs. 4.9(a)

to (n) while Fig. 4.9(o) is a closed-up view showing what is going on within the central

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

micro-reservoir during the actual mixing process. The entire mixing process justified that it is a fast method as it took only about a few seconds for the mixture to exit the outlet.

(a) (b)

2 mm 2 mm

Fig. 4.8. (a) Optical images of the entrance of yellow dye through the right inlet. (b) Entrance of blue dye through the left inlet. (c) Cross-sectional view of the mixing process for the blue and yellow dyes. (d) Onset of mixing between the yellow and blue dyes.

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

(e) (f)

2 mm 1 mm

(g) (h)

2 mm 2 mm

Fig. 4.8. (e) Closed-up view of the sample during the mixing process. (f) Closed-up on the mixing process within the micro-reservoir. (g) Mixing process of the yellow and pink dyes. (h) Cross-sectional view during mixing of the yellow and pink dyes.

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

(a) (b)

(e) (f)

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

(g) (h)

(i) (j)

(k) (l)

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

(m) (n)

(o)

Fig. 4.9. (a) to (n) Sequence of events during the mixing process. (o) A close up view of the mixing within the central micro-reservoir.

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

References:

[1] Chantal Khan Malek, Laurent Robert, Jean-Jacques Boy and Pascal Blind,

Microsyst. Technol 13, 447-453 (2007).

[2] Ya Cheng, Koji Sugioka and Katsumi Midorikawa, Applied Surface Science 248,

172-176 (2005).

[3] A. A. Bettiol, C. N. B. Udalagama, E. J. Teo, J. A. van Kan and F. Watt, Nuclear

Instruments and Methods in Physics Research B 260, 357-361 (2007).

[4] Y. Cheng, H. L. Tsai, K. Sugioka and K. Midorikawa, Appl. Phys. A 85, 11-14

(2006).

[5] A. A. Bettiol, S. Venugopal rao, E. J. Teo, J. A. van Kan and Frank Watt, Applied

Physics Letters 88, 171106 (2006).

[6] A. A. Bettiol, S. Venugopal rao, T. C. Sum, J. A. van Kan and F. Watt, Journal of

Crystal Growth 288, 209-212 (2006).

[7] S. Juodkazis, K. Yamasaki, V. Mizeikis, S. Matsuo and H. Misawa, Appl. Phys. A

79, 1549-1553 (2004).

[8] R. An, Y. Li, D. Liu, Y. Dou, F. Qi, H. Yang and Q. Gong, Appl. Phys. A 86,

343-346 (2007).

[9] B. Fisette, F. Busque, J. –Y. Degorce and M. Meunier, Applied Physics Letters 88,

091104 (2006).

[10] F. E. Livingston and H. Helvajian, Appl. Phys. A 81, 1569-1581 (2005).

[11] Multi-Photon Absorption and Ionization , Department of Physics, Davidson

College

Chapter 4 3‐Dimensional Microstructural Fabrication by Femtosecond Laser Microprocessing

[12] F. E. Livingston, P. M. Adams and H. Helvajian, Applied Surface Science 247,

526-536 (2005).

Chapter 5 Results and Discussion

CHAPTER 5

RESULTS AND DISCUSSION

5.1 Introduction

The increasing demands for micro-devices with superior and efficient performance for

biochemical analyses and medical imaging impel the integration of versatile micro-

components on a single chip [1–5]. Currently, the development of “all-in-one”

microchips in which micro-components, such as those of fluidics, mechanics, and optics, are integrated into a single chip pose numerous challenges from the conceptual stage to the manufacturing phase [6]. In the production of these micro-devices, the selection of the substrate materials is of great importance for this “all-in-one” fabrication.

Three-dimensional patterning in glasses and polymers has always attracted enormous interest due to their prospective applications in the fields of optical memory [7–10], photonic crystals [11], micro-fluidic and MEMS devices [12]. In the case of channel fabrication, direct writing and patterning by a tightly focused laser beam [12] or a post- exposure wet etching, is implemented [13, 14].

Recently, a steady progress in the fabrication of micro-fluidic devices, for example, micro-reactors for chemical analyses [13] and a number of micro-optical structures, such as micro-fluidic dye laser [14], have been fabricated in FoturanTM glass using

Chapter 5 Results and Discussion

femtosecond (fs) laser. The ability to directly create embedded 3D microstructures in

FoturanTM glass using the fs laser, along with its many unique advantages of resistance to high temperature and corrosion as well as high optical transparency, has made it a particularly attractive platform for bio-photonic micro-devices [15, 16].

Recent advances in laser processing techniques have opened up numerous new methodologies for the implementation of photosensitive materials in many novel applications. As a consequence, many interesting scientific questions have emerged. It is now possible that pulsed lasers can operate at a wavelength outside the absorption bandwidth of a chromophore (i.e., photo-initiator). This induces photo-electron excitation that leads to exposure and subsequent metasilicate phase precipitation. The intensity of pulsed lasers can excite the chromophore via non-resonant absorption processes that are driven by multi-photon or multiple absorption events. Although data exists to support the multi-photon theory, this issue is very complex since many other constituents in the base glass can act as efficient electron donors. Nascent silver ions (nucleating agent) can act as their own sensitizing agent upon absorption of laser radiation. Absorption of laser radiation by the silica host (glass matrix) can also lead to the generation of photo-

electrons [17].

The fabrication of true 3D micro-fluidic structures and micro total analysis systems

(µTAS) in PSGC materials not only simplify design procedures, it further allows the

removal of costly intermittent steps during both the assembly and packaging phases.

PSGC materials have high flexural strength (100–300 MPa) [18] and could also be

Chapter 5 Results and Discussion

utilized to fabricate mechanically compliant structures that are suitable for high shock

and vibration environments. The mechanical compliance of PSGCs is related to the

stiffness of the materials and can be tailored on size domains ranging from the microscale

(<100 microscale (<100 µm) to the meso-scale (100 µm–10 mm) via additional laser

exposure and thermal processing.

We will briefly introduce the mechanism of fs laser micro-machining of FoturanTM glass

[19–21]. FoturanTM is a photosensitive glass which is composed of lithium

aluminosilicate and doped with a trace amount of silver and cerium. The cerium (Ce3+) ion plays the role of the photosensitizer, which releases an electron to become Ce4+ by exposure to UV light irradiation. Some silver ions then capture the free electrons to form silver atoms. During heat treatments, the silver atoms diffuse before agglomerating to form clusters. Lithium metasilicate crystallites grow around the silver nuclei clusters in the amorphous glass matrix. This crystalline phase of the lithium metasilicate has a higher etching rate than the glass matrix in a dilute aqueous solution of hydrofluoric (HF) acid and is preferentially etched away, leaving behind the required patterns.

Chapter 5 Results and Discussion

5.2 Influence of Laser Irradiation

In this work, we aim to modify the optical properties of the glass substrate, in particular,

a photosensitive glass FoturanTM, by photochemical reaction using an fs laser with a

small pulse energy, which initiates the modification for subsequent annealing cycle.

FoturanTM glass has been successfully used for microstructuring using UV light [20, 22].

It is composed of lithium aluminosilicate glass doped with trace amounts of silver and cerium. By exposure to UV light, the cerium ions (Ce3+) release electrons to the silver ions (Ag+) to induce silver atoms.

Ce3+ + hv → Ce4+ + e- (1)

The photo-active component is cerium (0.01–0.04 wt% admixture Ce2O3) and the

nucleating agent is silver (0.05–0.15 wt% admixture Ag2O) which makes the glass

photosensitive. This sensitizer is used to facilitate the optical exposure process. The primary role of the sensitizer is to absorb incident radiation and generate photo-electrons.

Exposure to UV light results in the formation of 3D latent images within the glass (silver

atoms). These are confined to zones where the UV light is absorbed. The photo-initiation

process (latent image formation) and subsequent “fixing” of the exposure (permanent

image formation) proceed via several generalized equations. In most photosensitive glass

ceramic materials, the exposure process is initiated by the photo-induced generation of

free electrons from the photo-active species. For FoturanTM, a lithium aluminosilicate

Chapter 5 Results and Discussion

glass, the photosensitive character is derived primarily from the ionization of the nascent

3+ cerium ions Ce (admixture Ce2O3) upon exposure to actinic radiation λ < ~350 nm, the nascent cerium ions are photo-ionized and trapped (defect) electron states are formed in the glass matrix. These trapped (defect) states correspond to the latent image and have been associated with impurity hole centers and electron color centers. The ionization process leads to the formation of trapped (defect) electron states; impurity hole centers

(Ce3+) and electron color centers (Ce4+ + e-) in the glass matrix.

Since this crystalline phase of lithium metasilicate has an etching rate in a dilute solution

of hydrofluoric (HF) acid which is much higher than the glass matrix, it can be

preferentially etched away, leaving behind the engraved pattern. However, the

microstructuring of FoturanTM glass by UV lithography is limited to the sample surface

because of the resonant absorption of UV light starting at the sample surface.

In order to resolve these laser-induced surface damages, nanosecond (ns) and fs lasers

working outside the resonant wavelengths [15, 19, 21] were employed to fabricate the 3D

microstructures buried in FoturanTM glass. Since these wavelengths are outside of the

absorption band of FoturanTM glass, photo-modification of the glass can only be initiated

near the focal point where the extremely high laser intensity induces non-linear optical effects like multi-photon absorption. The photochemistry of fs laser modification of

FoturanTM glass is found to be different from the established theory for UV light processing, because photo-reduction of silver with fs laser irradiation is possible even without Ce3+ ions [23, 24].

Chapter 5 Results and Discussion

One of the most important applications of this ability is the internal modification of the

refractive index of transparent bulk glasses such as pure and boron-doped fused silica glasses [8, 25], soda-lime glass [26], and phosphate glass [35]. A variety of photonic devices such as waveguides [8], couplers [25, 28], and gratings [26, 29] have been fabricated inside the glasses with two-dimensional (2D) or three-dimensional (3D) configurations. In general, the refractive index change increases with laser intensity [29] or exposure time [28], until the refractive index change reaches a saturation value.

However, high laser intensity or long exposure time usually makes the modified area large, resulting in a low spatial resolution of modification. Moreover, high intensity laser

irradiation causes high roughness of the internal structure, and thereby induces the

scattering of light. This conflict in simultaneously obtaining a high spatial resolution with

a high refractive index change has been an important issue for the fabrication of photonic

devices with fine structures, such as photonic crystals [11] and optical gratings. Although

the mechanism is still under investigation, some authors have suggested that a

densification from local melting and the subsequent rapid quenching after the optical

breakdown is responsible for the refractive index change [8, 30]. In this sense, the direct

induction of the refractive index change in glasses by fs laser irradiation is a heat-induced process. Therefore, the resolution of fabrication is not only determined by the area where the multi-photon absorption occurs, but is also dependent on the area of heat diffusion. In many cases, the latter factor becomes the bottleneck for obtaining high spatial resolution even for extremely short pulse duration of the fs laser. To overcome the above difficulty, a new strategy is to use the fs laser merely for initiation of a secondary technological process.

Chapter 5 Results and Discussion

This variable laser irradiation processing technique relies on the ability of the fs laser to

precisely and selectively alter the chemical etch rate of the photo-structurable glass ceramic locally during pattern formation. The chemical etch rate of a commercial

photosensitive glass ceramic (FoturanTM, Schott Corp., Germany) in dilute aqueous

hydrofluoric (HF) acid has been conducted. The etch rate of FoturanTM is strongly

dependent on the incident laser irradiance during patterning at λ = 266 nm and λ = 355

nm [31]. Fig. 5.1. shows the etch rate contrast (etch rate ratio) values as a function of

laser irradiance for FoturanTM, where the etch rate ratio is defined as the etching rate of

the laser exposed and crystallized material (Rexposed) divided by the nascent etching rate of

the unexposed amorphous material (Runexposed). For low laser irradiances, the etch rate ratio increases nearly linearly with laser irradiance. The near linear increase in the etch rate ratio can be fit using linear least-squares regression analysis to yield the following values: slope = 435.9 ± 46.7 μm2 / mW for λ = 266 nm and slope = 46.2 ± 2.3μm2 / mW

for λ = 355 nm. At high laser irradiances, the measured etch rate ratios reach a plateau of

30 : 1. The value of the maximum etch rate contrast is dependent on the thermal

treatment and chemical etching procedures, as well as the concentration of its

constituents in the base glass. In the plateau region, the average maximum etch rate for

the exposed and heat treated glass was determined to be Rexposed = 18.62 ± 0.30 μm / min

and was independent of the exposure wavelength.

Chapter 5 Results and Discussion

(a)

2 Laser Irradiation (mW/µm )

Chapter 5 Results and Discussion

(b)

2 Laser Irradiation (mW/µm )

Fig. 5.1. Etch rate ratios vs. laser irradiance for (a) λ = 266 nm and (b) λ = 355 nm. The etch rate ratios are represented by the solid squares.

Pulsed lasers operating at a wavelength outside the absorption bandwidth of the chromophore (i.e., photo-initiator) can induce photo-electron excitation that leads to exposure and subsequent metasilicate phase precipitation as the intensity of pulsed lasers can excite the chromophore via non-resonant absorption processes that are driven by multi-photon or multiple absorption events. Although data exists to support the multi- photon theory, this issue is very complex since many other constituents in the base glass can act as efficient electron donors. Nascent silver ions (nucleating agent) can act as their

Chapter 5 Results and Discussion

own sensitizing agent upon absorption of laser radiation as shown in Eq. (4) [32].

Absorption of laser radiation by the silica host (glass matrix) can also lead to the

generation of photo-electrons.

2Ag+ + hv → Ag2+ + Ag0 (4)

The photosensitivity of FoturanTM glass can also be activated with infra-red fs lasers and

3D microstructures have been patterned inside the glass [19, 33, 34]. A dielectric medium

with embedded metallic nanoparticles changes the permittivity as well as the refractive

index of the glass [35, 36]. Modification of the optical properties of the FoturanTM glass is possible if the post-baking temperature is below the crystallization temperature of the glass. The required intensity of the fs laser is only the threshold of the multi-photon absorption which is much smaller than that for direct refractive index change by the fs laser [19]. In this case, the refractive index change is actually realized by the post-baking, and the modified region is confined in the latent image region written by fs laser irradiation. In this circumstances, a high spatial resolution of modification with controllable refractive index change and smooth internal structure is attainable.

Recent experimental discoveries have shown that the photolytic mechanism for pulsed laser excitation might be different from conventional glass processing [37]. These pulsed laser experiments revealed that the photo-induced absorption process initiates the precipitation of the chemically soluble ceramic phase via a multiple-photon excitation mechanism [38]. For pulsed UV nanosecond lasers (λ = 266 nm and 355 nm), the

Chapter 5 Results and Discussion

photochemical process is quadratic with respect to incident laser fluence [37], while a

six-photon dependence has been shown with pulsed near-IR femtosecond lasers (λ = 775

nm) [39]. Both studies support the existence of either long-lived intermediate states or impurity defect states that allow a high-order excitation process. A major implication of these experiments is the demonstration that pulsed lasers can successfully and efficiently

expose the PSGC material at wavelengths that are outside the absorption band of the

photo-initiator [40]. This unique property permits the fabrication of embedded structures

in the media and the ability to create undercut and freestanding structures [37]. With the

recent advancement in laser processing techniques, in terms of the reliability and

availability, PSGC materials show great promises in applications that require

microstructural properties of a glass and a ceramic.

Chapter 5 Results and Discussion

5.3 Influence of Thermal Cycling

During the thermal annealing process, considerations were on the temperature ramping

rate as well as the dwell temperature and time. A steep ramping rate will not allow

sufficient agglomeration of the metal clusters (Equations (2) and (3)). On the other hand,

a gentle ramping rate will require a much longer annealing time. Too long an annealing

process will cause the glass sample to warp and melt eventually. Insufficient time does not allow the necessary chemical reactions to initialize and occur. Temperature greater than 500oC is required for the onset of the initial reaction while temperature greater than

600oC is required to initialize the subsequent reactions, without which, no reaction will

occur.

The subsequent heat treatment promotes a good crystallization within the exposed areas.

Thermal treatment is then used to convert the latent image into a fixed permanent image.

During the initial thermal treatment step at 500oC, these trapped defect states promote

+ the reduction of nascent silver ions Ag (admixture Ag2O). The nucleation process is

initiated by the scavenging of the trapped electrons by impurity silver ions as described

by Eq. (2). During thermal processing, the atomic silver clusters agglomerate and nucleate to form nanometer-scale Ag clusters as shown in Eq. (3). The formation of metallic clusters corresponds to the ‘‘fixing’’ of the exposure and permanent image formation in the glass matrix. Subsequently, the neutralized silver species diffuse and

0 agglomerate to form microscopic metallic silver clusters (Ag )x . Without this initiation

stage, reduction of the nascent silver ions Ag+ will not occur, which will results in no

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Chapter 5 Results and Discussion

agglomeration and nucleation process of the Ag clusters, which form the permanent

images inside the glass substrate.

Ag+ + e- + ∆H → Ag0 (2)

In the following post-baking, first the silver atoms diffuse and agglomerate to clusters at around 500oC, and then the crystalline phase of lithium metasilicate grows into an

amorphous glass matrix using the silver clusters as nuclei at around 600oC. Since the crystalline phase of lithium metasilicate is much more soluble in a dilute solution of hydrofluoric (HF) acid than the glass matrix, it can be preferentially etched away.

The growth of the soluble ceramic crystalline phase (lithium metasilicate, Li2SiO3) is initiated when the temperature is further increased to ~ 600oC. During the second thermal

o 0 treatment step at 600 C, the (Ag )x clusters serve as efficient nucleation sites and promote the heterogeneous formation of the lithium metasilicate (Li2SiO3) crystalline

phase from the amorphous glass matrix [32]. Thermal treatment is used to convert these

latent images into fixed permanent images. With no Ag clusters as effective nucleation

sites, the Ag clusters cannot grow and diffuse to form any permanent images.

0 0 x Ag + ∆H → (Ag )x (3)

However, high laser intensity or long exposure time usually makes the modified area

large, resulting in a low spatial resolution of modification. Moreover, high intensity laser

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Chapter 5 Results and Discussion

irradiation causes high roughness of the internal structure, and thereby induces the

scattering of light. The conflict in simultaneously obtaining the high spatial resolution

and the high refractive index change has been an important issue for the fabrication of

photonic devices with fine structures, such as photonic crystals [11] and optical gratings.

Although the mechanism is still under investigation, some authors have suggested that a

densification from local melting and the subsequent rapid quenching after the optical

breakdown is responsible for the refractive index change [8, 30]. In this sense, the direct

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Chapter 5 Results and Discussion

5.4 Influence of Wet Chemical Etching

The HF etching step requires sufficient concentration of the F- ions, which will be

exhausted due to consumption during the etching process. Insufficient agitation as well as

residues within the etchant resulting from saturation will hinder the etching process and

reduce the rate of etching dramatically. A long etching duration will result in the loss of

design integrity. Capillary flow where the etchant cannot flow into the micro-reservoirs,

inlets and outlets further affects the structural integrity as well as the desired designed

structure. This could be the result of insufficient annealing or laser irradiation. Potential

pits will result in areas with fast onset of etching as well as high initial etching. Sizes of

holes were larger than anticipated as well as the lost of structural integrity were due to

long etching time in the 10% HF etchant.

At high laser irradiances, the etch rate ratios reach a plateau of about 30 : 1. The value of

the maximum etch rate contrast is dependent on the thermal treatment and chemical

etching procedures, as well as the concentration of ingredients in the base glass. In the

plateau region, the average maximum etch rate for the exposed and heat treated glass was

independent of the exposure wavelength.

The variable laser exposure processing technique relies on the ability to precisely and selectively alter the chemical etch rate of the photo-structurable glass ceramic by varying the laser exposure during pattern formation. Previous experiments have determined that the chemical etch rate of FoturanTM in dilute aqueous hydrofluoric (HF) acid is strongly

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Chapter 5 Results and Discussion

dependent on the incident laser irradiance during patterning at λ = 266 nm and λ = 355

nm [31].

The anisotropic wet etching of irradiated patterns in silicate or polymers glasses is

usually applicable for fabrication of only low-aspect-ratio structures (the aspect ratio is defined by the length and diameter of the channel as fa.r. = length/diameter). The

anisotropy of etching is a universal principle based on an increased chemical reactivity of

optically modified regions as compared to unmodified areas, with the reactivity

difference is typically 1–5 times. This causes problems and renders the technique

impractical for the most applications. In contrast, special glasses, called photo-etchable,

such as FoturanTM (Schott), when properly exposed and annealed shows up to 30 times a contrast of etching speed in water solutions of hydrofluoric (HF) acid. High anisotropy of etching is due to the precipitation of a lithiumsilicate phase, which has a higher etching rate in HF aqueous solutions. The crystalline phase of lithium metasilicate will be preferentially etched away, leaving behind the required pattern.

Recently, it was found that the photosensitivity of FoturanTM glass can also be activated

with infra-red fs laser pulses by a multi-photon absorption process [19], and 3D

microstructures such as micro-fluidic structures and micro-optical structures have been

embedded inside the glass [15, 19]. Since it has been pointed out that a dielectric medium with embedded metallic nanoparticles changes its permittivity as well as the refractive index [35], modification of optical properties of FoturanTM glass becomes possible if the

post-baking of the sample is conducted at a temperature below that for crystallization.

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Chapter 5 Results and Discussion

The required intensity of the fs laser is only the threshold of the multi-photon absorption

which is much smaller than that for direct refractive index change by the fs laser [19]. In

this case, the refractive index change is actually realized by the post-baking, and the

modified region is confined only in the latent image region written by fs laser irradiation.

In this manner, a high spatial resolution of modification, controllable refractive index

change, and smooth internal structure could be achieved.

5.5 Parameters to be Considered in the Design

A 2-level 3D internally embedded micro-mixer was fabricated for the mixing of 2

hydrophilic water-based solutions. The final design went through a series of design

optimization process. Variations in the fluence values in the laser irradiation process to the capillary actions during the etching process were considered.

The diameter of the reservoir is 3 mm and 1200 µm deep. Inlets are 300 um deep, 1 mm wide and 6 mm long. Outlets are 1 mm in diameter and 800 µm deep. The design of the microstructure was first focused inside the FoturanTM sample. During direct laser

irradiation process, the concern was over the selection of appropriate power value.

Maximum power will result in ablation. This will result in a potential notch or crack,

which will initial a faster etching at that particular defective site during the etching

process. Too low a power, on the other hand, will result in insufficient laser fluence to

initiate the irradiation process. This will subsequently result in insufficient annealing and

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Chapter 5 Results and Discussion

etching will not be possible for the desired design. Initial fabrication results in occurrence

of ablation. Power was significantly reduced to minimize any possibilities of ablation.

Capillary action is the ability of a substance to draw another substance into it. It occurs

when the adhesive intermolecular forces between the liquid and a substance are stronger

than the cohesive intermolecular forces inside the liquid. Capillary action is the result of

adhesion and surface tension. Adhesion of liquid to the walls of a vessel will cause an

upward force on the liquid at the edges and result in a meniscus which turns upward. The

surface tension acts to hold the surface intact, so instead of just the edges moving

upward, the whole liquid surface is dragged upward. The height to which capillary action

will take water in a uniform circular tube is limited by surface tension. Capillary flow can

be calculated as follows:

2 Fupward = 2Tπr = ρg(hπr )

Where T is the surface tension, ρ is the density of liquid and r is the radius of tube. The height h, to which capillary action will lift the liquid depends upon the weight of the liquid which the surface tension will lift.

106

Chapter 5 Results and Discussion

The height to which the liquid can be lifted is given by:

Since it is weight limited, it will rise higher in a smaller tube.

The initial design of the cylindrical micro-reservoir was conceptualized with 2 straight,

horizontal inlets and a single outlet. The micro-inlets entered from both sides, left and

right of the sample, with the outlet in the centre. However, in order to facilitate the

etching process as well as the mixing of the fluids, a Y-shaped design was employed. The

2 inlets were represented as the arms of the letter ‘Y’ with a vertical central outlet.

Improvement was implemented on this first design to a Y-shaped structure to improve the flow of the fluids so as to improve the mixing process. Having horizontal inlets create impedance and turbulence to the initial flow of the fluids to be mixed. A gentle sloping inlet into the micro-reservoir allows easier flow as well as ensures more homogeneous mixing. Modifications to the outlet design were also made to allow gravity assisted flow.

A narrower diameter in the outlet also improves the pump out throughput. A larger width offset in the design to the entrance of the inlet was further optimized as these areas are the last to come into contact with the etchant. This implies that the other areas will be

subjected to further etching, thus, with an increased in width just before the entrance of

the micro-reservoir as compared to the entrance on the surface of the inlet, capillary

actions of the etchant will be improved to facilitate the HF etching process. Furthermore,

107

Chapter 5 Results and Discussion

the overall etching time will be substantially reduced. This gives a tapered appearance at

the joint segments.

All the micro-reservoirs and micro-channels were fabricated inside a single monolithic

glass sample in order to reduce the required reagent consumption as well as sample

wastage. This in term will dramatically cut down the time involved in analysis. Multi-

layers and multi-channels greatly enlarge the surface areas which increase the probability

of reaction. In terms of the mixing process, the dyes will be mixed in a more uniform and homogeneous manner. Surface roughness of the numerous micro-channels and micro- reservoirs were also considered due to the possible turbulence flow within them.

108

Chapter 5 Results and Discussion

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112

Chapter 6 Conclusions

CHAPTER 6

CONCLUSIONS

6.1 Conclusions

This research is emphasized on the fabrication of a monolithic 2-level 3 dimensional (3D)

internally embedded micro-mixer for micro-mixing applications with a simple setup using femtosecond laser. The fabricated micro-mixers have successfully demonstrated its capability and functionality in allowing 2 different coloured hydrophilic water-based dyes to mix uniformly within the embedded central micro-reservoir on a lower second level.

Further advantage of this technique is its ability to create embedded free-standing intricate 3D micro-structures within glass materials without ablation. This is a maskless fabrication process which greatly simplifies design protocols and removes costly intermittent steps during assembly and packaging processes in comparison to conventional methodology of glass materials processing (lithography and UV lamp illumination), which is time consuming. Furthermore, this technique improves the throughput significantly and is suitable for mass rapid prototyping. Its niche market lies in the biotechnology and biomedical applications, which make use of the micro-fluidic properties of these devices.

Expanding on this technique, fabrication of multi-levels and multi-channels micro- devices can become a reality in the near future. Microstructures of irregular shapes with

113

Chapter 6 Conclusions

different sizes can be easily fabricated inside the FoturanTM glass without going through

multiple process steps required in current manufacturing. Intricate and complicated 3

dimensional designs embedded inside a single monolithic glass can be easily fabricated

reliably and repeatedly. This integrated and versatile device allows smaller reagent

consumptions with lower wastage as well as all-in-one analyses for all conditions, which

are the main requirements in the field of biomedical applications. The main contributions

and results are summarized as follows together with some suggested future works:

• Embedded microstructures are easily fabricated on multiple levels using

femtosecond laser within the bulk of FoturanTM glass via multi-photon absorption.

Multiple micro-channels, micro-reservoirs and free-standing microstructures are

fabricated with no surface damage from laser ablation. This maskless technique

significantly reduces expenditures incurred for photo-masks used in conventional

lithography.

• Microstructures of different and / or irregular geometrical shapes and dimensional

sizes are fabricated with great ease inside the FoturanTM glass. Detailed

specifications of the various micro-reservoirs and micro-channels are

accomplished with ease using programmed algorithm. This allows rapid

prototyping which increases the throughput significantly. Furthermore, replication

of reliable and repeatable designs and patterns are ensured as compared to parts

produced by the traditional chemical wet etching.

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Chapter 6 Conclusions

• The flexibility of the process, in terms of a simple setup with minimal

involvement of complicated chemistry, allows a faster time-to-market production

of these devices. This inherent flexibility allows accessibility of this technique to

the industries more readily. Moreover, a higher production throughput of these

devices is essential for various biological applications in their analytical testing.

• The effects of threshold power, annealing time and temperature as well as etching

time have been investigated and optimized. Different geometrical aspects of

various embedded microstructures are also compared. Inter-linking factors, such

as geometry, flow turbulence, surface roughness, gravity-assisted reactions,

further affect the rate of etching Thermal annealing effect is also considered.

Annealing has a beneficial effect of smoothening the surface areas, however,

prolong annealing results in warpage to the structural integrity of the device, thus,

creating problems during etching. It is concluded that the resulting turbulence to

the flow of the fluids within these embedded microstructures is a direct

consequence of the surface roughness of these microstructures.

• Demonstrations of the functionality of the monolithic 2-level 3 dimensional (3D)

internally embedded micro-mixers are reliable and repeatable. Blue and yellow

dyes as well as the red and yellow dyes are homogeneously and uniformly mixed.

These devices have also been tested by mixing organic and other biological

samples, such as the mixing of different types of amino acids, proteins and DNAs.

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Chapter 6 Conclusions

6.2 Suggestions for Future Work

Despite the recent advancements in the laser processing of photo-structurable glass

ceramic materials, a detailed understanding of the underlying photochemical and kinetic

processes as a function of incident laser irradiance are very limited.

The exact mechanism of exposure is still under discussion, because the six-photon

absorption for irradiation at 775 nm (1.59 eV) does not comply with the following facts.

The absorption band of Ce3+ at 304 ±15 nm, which is responsible for initiating a latent

image formation in photo-chromic glasses, should be excited by only a three photon

process. Also, the edge of the color center formation in silicate glasses as well as in

FoturanTM glass is typically at 212 nm (5.8 eV), which is again, less than four photons

should be sufficient. Thus, judgment about six-photon absorption, which was based on

the exposure power law, probably does not reveal the actual mechanism of photo-

modification. More in-depth investigations are still required for building a thorough

physical understanding for fs laser modifications and interactions of FoturanTM glass.

Multi-levels and multi-channels micro-fluidic analytical devices are in great demand for many specific applications. More studies can be performed in the addition of different applications-specific constituents for customed-made photosensitive glasses. Laser localized annealing can also be explored. This technique can widen the scope of many current applications as un-activated regions can, for instance, act as etch-stop during the wet chemical etching process. Etching in a volumetric-controlled chamber with a pump

116

Chapter 6 Conclusions to regulate the flow of the etchant will further promote the etching process in hard-to- reach regions as well as improve the efficiency of the process.

117

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Microsyst. Technol 13, 447-453 (2007).

[2] Ya Cheng, Koji Sugioka and Katsumi Midorikawa, Applied Surface Science 248,

172-176 (2005).

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Instruments and Methods in Physics Research B 260, 357-361 (2007).

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(2006).

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Physics Letters 88, 171106 (2006).

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Crystal Growth 288, 209-212 (2006).

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79, 1549-1553 (2004).

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343-346 (2007).

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091104 (2006).

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[11] Multi-Photon Absorption and Ionization , Department of Physics, Davidson

College

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References for Chapter 5 Results and Discussion