Nanoscience and Nanotechnology 180/198-534A

Microfabrication / Micromachining

Vito Logiudice 1 References

This portion of the course will make use primarily of the following text: Fundamentals of , 1st Edition Marc Madou CRC Press LLC, United States, 1997 NOTE: Unless otherwise indicated, the figures and schematics presented in the course notes originate from this textbook.

Links to relevant papers will also be found in the class notes.

Other interesting references: Micromachined Transducers Sourcebook Gregory T.A. Kovacs WCB/McGraw-Hill, United States, 1998

The MEMS Handbook edited by Mohamed Gad-el-Hak CRC Press LLC, United States, 2002

Microsystem Design Stephen D. Senturia Kluwer Academic Publishers, United States, 2001

Nano- and Microelectromechanical Systems Sergey Edward Lyshevski CRC Press LLC, United States, 2001

Vito Logiudice 2 Section Outline

1. Introduction 2. Examples of Micromachined Devices 3. Micromachining Technologies • Bulk Micromachining • Surface Micromachining • LIGA 4. Overview of Basic Fabrication Processes • Lithography • Wet etching • Dry etching • Deposition NOTE: • Oxidation A visit of McGill’s Nanotools • Doping Micromachining Fab is planned for • Wafer bonding the last session • etc. (time permitting) 5. Fabrication Environment • Cleanroom • Gowning • Chemicals / Gases / Water Purity Vito Logiudice 3 1. Introduction a) History – 1970’s: beginnings of -based Microelectromechanical Systems (MEMS) (MJM.2.184-5). MOEMS: Micro-Opto- • pressure transducers (Sensym, National Semiconductor) Electromechanical • thermal print heads (Texas Instruments – 1977) Systems

• ink jet nozzle arrays (IBM – 1977) (MJM.2.184-5) – 10 to 15 years ago, nearly all commercial MEMS applications were limited to pressure sensing (ex: engine management) and accelerometers (air bag deployment). – 1979: 1st mass-produced commercial device released to market • silicon-based Manifold Absolute Pressure (MAP) sensor for automotive applications (MJM.1.461) – MEMS designs were largely based on well-established silicon (IC) fabrication technologies.

Vito Logiudice 4 b) Present and Future trends

– Wide range of applications spanning automotive, medical, biotechnology, environmental and telecommunications markets.

– Use of substrate types other than silicon.

– Moving away from strictly IC-based silicon process technologies.

– Extending beyond the “micro” realm into the “nano” realm via manufacturing methods making use of: • Self-assembled monolayers (SAM) • Atom manipulation tools •Biomimetics • DNA-assisted micro-assembly •Etc. – Cooperation between disciplines steadily increasing.

Vito Logiudice 5 2. Micromachined Devices

This chapter presents a brief overview of several micromachined devices.

These devices are created via the integration of several microfabrication processes, some of which will be discussed in greater detail in the class sessions to follow.

Vito Logiudice 6 a) Electrostatically-Actuated Micro-Mirror

Reference: Korea Advanced Institute of Science and Technology See following link for copy of related publication: http://www.coventor.com/media/papers/MEMS03_kaistmirror.pdf

– Over the last eight to ten years, many Micro-Mirror designs have been created and the two most common applications of these has been for: • the redirection or switching of optical telecommunications signals • imaging systems

– A vast assortment of designs have been designed and manufactured, each offering numerous advantages and drawbacks.

– A somewhat complex, 2-axis design is presented with the aim of introducing some of the process technologies used in MEMS fabrication.

Vito Logiudice 7 a-1) Schematic & Basic Function

Elevated & side view of actual mirror Response of devices 2-axis mirror of different schematic dimensions

Vito Logiudice 8 a-2) Fabrication Overview

Key Words of interest: 9Seed metal 9Electroplating 9Exposure 9Thick PR

Fabrication process making use of thick resist and electroplating

Vito Logiudice 9 b) Microfluidic Circuitboard

Reference: University of Southampton, Southampton, UK See following link for copy of related publication: http://www.iop.org/EJ/abstract/0960-1317/9/2/318

See also: http://ej.iop.org/links/65/Fh341cTS5JE1a7uTerrXVw/jm2413.pdf

– The following is an interesting example of a system designed to pump minute amounts of fluids in a controlled manner within a contained environment. The design incorporates flow sensors and a reaction chamber which allow two fluids to be precisely metered and mixed.

– Devices such as these are being devised in an effort to incorporate many of the features needed to sample, mix and analyze various fluids in bio-medical applications (“Lab-on-a-Chip”).

– “ElectroWetting” is also being used towards similar aims (See the following link for more information):

http://www.mems2004.org/withjs/pdf/DropletManip.pdf

Vito Logiudice 10 b-1) Schematic & Basic Function

Block diagram of microfluidic systems incorporated onto the “fluidic” board.

Photograph of system as-built

Schematic of Microfluidic board

Vito Logiudice 11 b-2) Fabrication Overview - Micropump

Vito Logiudice 12 b-3) Fabrication Overview - Circuitboard

Key Words of interest: 9Pyrex 9Alignment marks 9Resist 9Alignment window 9IR Alignment

Fabrication steps for Fabrication steps for front-to-backside channels alignment. IR alignment

Vito Logiudice 13 b-4) Droplet Manipulation via ElectroWetting

CJ Kim, UCLA

Vito Logiudice 14 c) Cantilevers with Tips – Nanolithography

Reference: University of Illinois at Urbana-Champaign See following link for greater detail & relevant publications: http://www.google.com/u/illinoisedu?q=Dip+Pen+Nanolithography&hl=en&lr=&ie=UTF-8&filter=0

– Dip-Pen Nanolithography (DPN) is a new form of chemical lithography that can be used to directly deposit biochemical substances in an orderly fashion with sub-100 nm resolution.

– The types of substances that may be deposited (onto suitable surfaces) may include DNA, protein, self-assembled monolayers, etc.

– A DPN process is typically implemented using an Atomic Force Microscope (AFM).

– Conventional DPN applications use a single probe (slow).

– New work in the field of both passive and active probe arrays is increasing both the speed and flexibilty of these applications.

Vito Logiudice 15 c-1) Basic Function

“Ink” molecules being transferred from DPN-AFM Tip to substrate, as tip is moved from left to right.

AFM setup for Dip-Pen Nanolithography

Vito Logiudice 16 c-2) Passive Probe Array – Silicon Nitride Tip

SEM photo of Si3N4 based 32-tip DPN array. 100um tip-to-tip spacing. Radius of curvature of probe tips ~600nm

Key Words of interest: 9Lithography 9Patterning 9EDP Etching Fabrication overview 9Wet Oxidation 9Silicon Nitride Deposition Vito Logiudice 17 c-3) Passive Probe Array – Silicon Tip

SEM photo of 10 tip probe array. 100um tip-to-tip spacing. Radius of curvature of probe tips ~100nm

Key Words of interest: 9SOI Wafer 9Deep RIE 9Oxidation 9Evaporation Fabrication overview – using SOI (Silicon 9Oxide Removal On Insulator) wafer as starting material

Vito Logiudice 18 c-4) Active Probe Array – Thermally Actuated

SEM photo of 10-probe array. 100um tip-to-tip spacing. Radius of curvature of probe tips ~900nm

Probe deflection vs. (a) 6um x 4um patterns generated change in T and (b) via 10 pass sequence (80nm heater power linewidths)

Vito Logiudice 19 3. Micromachining Technologies

Micromachining technologies tend to be classified under one of three separate categories:

A. Bulk Micromachining

B. Surface Micromachining

C. LIGA

Vito Logiudice 20 A. Bulk Micromachining

• As the name implies, bulk micromachining focuses on the creation of patterns or features within the bulk of some sort of starting material. In doing so, we rely on the physical structure of the material in question (amongst other variables) to control the shape of these features.

• Although materials such as quartz, pyrex, GaAs, Ge, etc. are sometimes used as the starting material for this technology, the material that is most commonly used is silicon, owing to its well-understood diamond lattice structure which is composed of two interpenetrating face- centered-cubic (FCC) lattices:

Vito Logiudice 21 A. Bulk Micromachining

• Silicon crystal orientation: Miller Indices are symbolic vector representations of the orientation of the atomic planes that make up the crystal lattice

Vito Logiudice 22 A. Bulk Micromachining

• Some planes will have more atoms per given area, depending on “how you slice it”. Thus, etch rates will vary as a function of the number of atoms located on the surface that is exposed to the etchant. Greater density of atoms slower etch rates

Vito Logiudice 23 A. Bulk Micromachining

• Silicon boules are grown using a seed material of known crystal orientation. The boule is subsequently sliced into wafers that will have this same orientation.

• Silicon etchants and silicon wafer orientations are selected to create the necessary features within the bulk of the wafer.

Vito Logiudice 24 A. Bulk Micromachining

• Anisotropic etchants such as KOH:H2O (alkaline) tend to etch different crystal planes at different rates, thus giving rise to structures having well defined sidewalls with precise and predictable angles of inclination (very little undercutting).

Vito Logiudice 25 A. Bulk Micromachining

• Isotropic etchants such as mixtures of HF:HNO3:CH3COOH (acidic) tend to etch different crystal planes at the same rate, thus giving rise to rounded structures with much undercutting.

Vito Logiudice 26 B. Surface Micromachining

• In the case of surface micromachining, a different approach is taken in that rather than etching into the bulk of the starting material, structures are built up on the substrate surface. These structures are created via the repetitive addition of layers selected for their various material properties, followed by the selective removal of these layers in a specific sequence.

• The vast range of materials that can be “deposited” includes polysilicon, silicon nitride, oxide, polyimide, metals, etc.

Vito Logiudice 27 B. Surface Micromachining

©M.J, Madou

Vito Logiudice 28 B. Surface Micromachining

Vito Logiudice 29 Bulk vs. Surface - Comments

• Both bulk and surface micromachining concepts are most often combined to create both intricate and simple devices and systems. • Other observations (M.J. Madou):

Vito Logiudice 30 C. LIGA

• LIGA is the German acronym for X-ray lithography (LIthographie), electrodeposition (Galvanoformung) and molding (Abformtechnik). • Allows the creation of 3-D structures with excellent tolerances and extremely high aspect ratios. • Main drawback - very expensive to implement:

©M.J, Madou

Vito Logiudice 31 C. LIGA

X-Rays

Photoresist typically used: PMMA PMMA (plexiglass) Photo Mask: plate with transparent & opaque regions

Vito Logiudice 32 C. LIGA

Vito Logiudice 33