Microfabrication / Micromachining

Nanoscience and Nanotechnology180/198-534A

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Microfabrication / Micromachining

References

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This portion of the course will make use primarily of the following text:Fundamentals of Microfabrication, 1st EditionMarc MadouCRC 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 SourcebookGregory T.A. KovacsWCB/McGraw-Hill, United States, 1998

The MEMS Handbookedited by Mohamed Gad-el-HakCRC Press LLC, United States, 2002

Microsystem DesignStephen D. SenturiaKluwer Academic Publishers, United States, 2001

Nano- and Microelectromechanical SystemsSergey Edward LyshevskiCRC Press LLC, United States, 2001

Section Outline

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1. Introduction2. Examples of Micromachined Devices3. Micromachining Technologies

• Bulk Micromachining• Surface Micromachining• LIGA

4. Overview of Basic Fabrication Processes• Lithography• Wet etching• Dry etching• Deposition• Oxidation• Doping• Wafer bonding• etc. (time permitting)

5. Fabrication Environment• Cleanroom• Gowning• Chemicals / Gases / Water Purity

NOTE:

A visit of McGill’s NanotoolsMicromachining Fab is planned for the last session

1. Introductiona) History

– 1970’s: beginnings of silicon-based Microelectromechanical Systems (MEMS) (MJM.2.184-5).

• pressure transducers (Sensym, National Semiconductor)

• thermal print heads (Texas Instruments – 1977)

• ink jet nozzle arrays (IBM – 1977) (MJM.2.184-5)

MOEMS: Micro-Opto-Electromechanical Systems

– 10 to 15 years ago, nearly all commercial MEMS applications werelimited 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 integrated circuit (IC) fabrication technologies.

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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.

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– 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.

2. Micromachined Devices

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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.

a) Electrostatically-Actuated Micro-Mirror

Reference: Korea Advanced Institute of Science and TechnologySee 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.

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a-1) Schematic & Basic Function

2-axis mirror schematic

Response of devices of different dimensions

Elevated & side view of actual mirror

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a-2) Fabrication Overview

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Key Words of interest:

Seed metal

Electroplating

Exposure

Thick PR

Fabrication process making use of thick resist and electroplating

b) Microfluidic Circuitboard

Reference: University of Southampton, Southampton, UKSee 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 linkfor more information):

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

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b-1) Schematic & Basic Function

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Block diagram of microfluidicsystems incorporated onto the

“fluidic” board.

Photograph of system as-built

Schematic of Microfluidic board

b-2) Fabrication Overview - Micropump

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b-3) Fabrication Overview - Circuitboard

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Fabrication steps for front-to-backside

alignment. IR alignment


Pyrex

Alignment marks

Resist

Alignment window

IR Alignment

Fabrication steps for channels

b-4) Droplet Manipulation via ElectroWetting

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CJ Kim, UCLA

c) Cantilevers with Tips – NanolithographyReference: 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.

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c-1) Basic Function

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AFM setup for Dip-Pen Nanolithography

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

moved from left to right.

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

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Lithography

Patterning

EDP Etching

Wet Oxidation

Silicon Nitride Deposition

Fabrication overview

c-3) Passive Probe Array – Silicon Tip

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Fabrication overview – using SOI (Silicon On Insulator) wafer as starting material

SEM photo of 10 tip probe array. 100um tip-to-tip spacing. Radius of

curvature of probe tips ~100nm


SOI Wafer

Deep RIE

Oxidation

Evaporation

Oxide Removal

c-4) Active Probe Array – Thermally Actuated

Probe deflection vs. (a) change in T and (b)

heater power6um x 4um patterns generated via 10 pass sequence (80nm

linewidths)

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

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3. Micromachining Technologies

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

A. Bulk Micromachining

B. Surface Micromachining

C. LIGA

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• 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:


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

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• 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

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• 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.

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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).

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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.

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• 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.


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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):

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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

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C. LIGA

Photo Mask: Glass plate with transparent & opaque regions

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PMMA

X-Rays

Photoresist typically used:

PMMA (plexiglass)

C. LIGA

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Microfabrication / Micromachining

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