MASSUIUC

Chang Liu

A brief history of MEMS fabrication

Chang LiuMicro Actuators, Sensors, Systems Group

University of Illinois at Urbana-Champaign

MASSUIUC

Chang Liu

To Do …

• Get a better diagram of MOS process flow.

MASSUIUC

Chang Liu

Outline

• Traditional silicon micromachining technology– Common microfabrication technology for IC– Bulk micromachining

• Etching, bonding, planarization– Surface micromachining

• Suspended structures, antistiction methods, 3D microstructures– Methods for merging micromechanics and IC

• Extended microfabrication technology in 90’s– LIGA– Deep reactive ion etching– Polymer based microfabrication

• Future foundry based processes

MASSUIUC

Chang Liu

A Standard IC Process• Draw a diagram of a circuit.

http://www.chips.ibm.com/gallery/

MASSUIUC

Chang Liu

Basic Fabrication Processes

• Deposition (Material addition)– spin coating, evaporation, electroplating, reactive growth, CVD,

sputtering• Lithography

– various wavelengths, mask making, alignment, exposure• Etching (Material removal)

– wet chemical etching, dry plasma etching, gas phase etching, • Wafer bonding

– Silicon on insulator wafers (SOI)• Packaging

– adhesion, wire bonding

MASSUIUC

Chang Liu

From wafer to device

MASSUIUC

Chang Liu

MASSUIUC

Chang Liu

Processing Equipments

Wafer alignerand exposure tool

Metal Evaporator Plasma etcher

A tour of lab is arranged in the middle of semester

MASSUIUC

Chang Liu

Micro Fabrication Technology

Startingwafer

pattern

Adding(deposition)

subtracting(etching)

MEMS

start Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6

MASSUIUC

Chang Liu

Silicon Bulk Etching

• Anisotropic Etching• Isotropic etching

MASSUIUC

Chang Liu

MASSUIUC

Chang Liu

MASSUIUC

Chang Liu

First Idea for Surface Micromachining …

• Physicist Richard Feynman– “There is plenty of room at the bottom”

• Excerpt– How can we make such a device? What kind of manufacturing

processes would we use? One possibility we might consider, since we have talked about writing by putting atoms down in a certain arrangement, would be to evaporate the material, then evaporate the insulator next to it. Then, for the next layer, evaporate another position of a wire, another insulator, and so on. So, you simply evaporate until you have a block of stuff which has the elements--- coils and condensers, transistors and so on---of exceedingly fine dimensions.

MASSUIUC

Chang Liu

Polysilicon as a Mechanical Material

• Invented by Dr. Muller and Dr. Howe of Berkeley

• Established sacrificial etching process using– polysilicon as a mechanical structural material– oxide as a sacrificial material

MASSUIUC

Chang Liu

Surface MicromachiningFabrication Process for Micro Motor

(1st pass description)

Learning objectives:How to represent process using cross-sectional view?Build ability to correlate mask and sideviews.

MASSUIUC

Chang Liu

Step 1: Starting wafer

• Mask top view • Side view

mask

Start with blank silicon wafer (one side polished with optical finish). Wafer orientation is not critical. The thickness of the wafer is not drawn to scale- the typical thickness of 0.3-0.5 mm.

MASSUIUC

Chang Liu

Step 2: Deposition of sacrificial layer

• Top view (mask) • Side view

mask

Deposit silicon oxide film (with phosphorous doping) as the sacrificial layer.- conformal coating- thickness 1-3 micrometers

MASSUIUC

Chang Liu

Step 3: Deposition of structural layer

• Top view (mask) • Side view

mask

Deposit polycrystalline silicon film (without phosphorous doping) as the structural layer.- conformal coating

MASSUIUC

Chang Liu

Step 4: Pattern the top polysilicon layer

• Top view (mask) • Side view

Pattern the silicon layer with the first mask to form the shape of the rotor and the hole for the anchor.

MASSUIUC

Chang Liu

Step 5: Deposit a second sacrificial layer

• Mask top view • Side view

Conformal deposition of P-doped oxide again.

mask

MASSUIUC

Chang Liu

Step 6: Pattern and Etch the sacrificial layers

• Top view (mask) • Side view

Pattern the wafer with the photoresist layer and the first mask.Using HF solutions to etch through the two oxide layers. Lateral etching will occur and the dimension control is critical.

MASSUIUC

Chang Liu

Step 7: Deposit polysilicon structural layer.

• Top view (mask) • Side view

Conformal deposition of polysilicon again.

mask

MASSUIUC

Chang Liu

Step 8: Pattern Polysilicon.

• Top view (mask) • Side view

Pattern the top layer polysilicon to form the confinement structure and anchor.

mask

MASSUIUC

Chang Liu

Step 9: Sacrificial layer removal and freeing of structures

• Top view (mask) • Side view

Remove the oxide using 49% HF solutions, which etches oxide fast (1 micron/minute) and the polysilicon slowly.

mask

MASSUIUC

Chang Liu

High Aspect Ratio Devices

*: Lithographie, galvanoformung, abformung

Thick photoresist

MASSUIUC

Chang Liu

New Materials and Processes

Inorganic Materials and processes• High temperature materials

processing (SiC)• Silicon Ge (SiGe)• Diamond (electrical

conductance and mechanical toughness)

• Laser Micromachining• Deep Reactive Ion Etching• Focused ion beam etching• Chemical mechanical polishing• Permanent magnet and

electromagnetic materials• Rapid prototyping

Organic Materials• Silicone elastomer• Elastic polymer• Chemical vapor deposition of

plastic films (Parylene)• Electroactive Materials • …

MASSUIUC

Chang Liu

Micro guitar – Cornell University

IBM Superconetip

Sandia Photonic lattice

MASSUIUC

Chang Liu

Man-made Submarine … in your arteryMicroTEC Inc.

• RMPD is a micro stereo lithography method for rapid creation of 3-D micro structures of any shape as prototypes or for series production

www.microTEC-D.com Duisburg, Germany

Das micro-U-Boot, das kleinste U-Boot der Welt!

MASSUIUC

Chang Liu

MASSUIUC

Chang Liu

Focused Ion Beam Etching

• http://www.iis-b.fhg.de/en/arb_geb/technology_an_fib.htm

• energy: 30 keV• current: 6 pA - 7 nA• resolution: 16 nm

MASSUIUC

Chang Liu

FIB process (continued)• http://www.msm.cam.ac.uk/dmg/research/fib/micromachine/index.html

MASSUIUC

Chang Liu

Ferrari MEMS vs. Suzuki MEMS

Suzuki Swift 1997Suzuki Swift 1997

Ferrari 348 1989Ferrari 348 1989

MASSUIUC

Chang Liu

MUMPS

• 3 poly surface micromachining• Process• One process, different devices.

MASSUIUC

Chang Liu

MEMS Foundry -MEMS Exchange

• Distributed, Virtue fab– UC Berkeley– Cornell Nanofabrication

Facility (CNF)

www.mems-exchange.org

MASSUIUC

Chang Liu

Polymer MEMS

• Polymer materials as substrate– Replaces silicon– Lower costs– Examples: liquid crystal polymer, polyimide, glass– Drawbacks: cannot integrate circuitry. However, circuits can be

wire-bonded to the polymer chip• Polymer as structures

– Replaces silicon, silicon nitride, silicon oxide, etc– Lower costs, greater mechanical flexibility– Examples: Parylene, photoresist, polyimide

MASSUIUC

Chang Liu

LCP for MEMS packaging

• Copper-LCP laminates for flexible circuit boards

• LCP thermal bonding for environmental encapsualtion

• LCP substrates for robust devices

15mm

MASSUIUC

Chang Liu

Tactile Sensor Fabrication

• Double-sided alignment, deposition and patterning of NiCr Strain gauges and Al RIE mask on 2mil (50μm) thick LCP

• Dry etching (RIE O2 plasma) of 35μm deep, 500μm square backside cavity, remove Al

• Deposition and patterning of Au interconnects

• Spin and pattern 20μm tall polyimide tactile bumps

MASSUIUC

Chang Liu

Tactile Sensor Operation

• Converts normal applied load into change in resistance

• Array can image tactile contact• Similar fabrication techniques can

provide shear data

Strain GaugeArea

Tactile Bump

Applied Load

MembranePerimeter

Tensile Strain (x-dir)

Compressive Strain (x-dir)

1400μm

MASSUIUC

Chang Liu

Conclusions

• Microfabrication technology is a dynamically advancing field.– Technology push

• New microfab processes and materials are developed in response to application needs

– Technology pull• New fabrication techniques enables new devices and new applications

• Micromachining involves silicon, glass, and polymer materials, not just silicon alone.

• The microfabrication process is an integral part of the device design and material selection. The capability and practicality of microfabrication must be taken into consideration when considering candidate designs.