A brief history of MEMS fabrication Chang Liu Micro Actuators, Sensors, Systems Group University of Illinois at Urbana-Champaign MASS UIUC

To Do … Get a better diagram of MOS process flow. MASS UIUC

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 MASS UIUC

A Standard IC Process http://www.chips.ibm.com/gallery/ Draw a diagram of a circuit. http://www.chips.ibm.com/gallery/ MASS UIUC

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 MASS UIUC

From wafer to device MASS UIUC

MASS UIUC

Processing Equipments A tour of lab is arranged in the middle of semester Wafer aligner and exposure tool Metal Evaporator Plasma etcher MASS UIUC

Micro Fabrication Technology start Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6 Starting wafer MEMS subtracting (etching) pattern Adding (deposition) MASS UIUC

Silicon Bulk Etching Anisotropic Etching Isotropic etching MASS UIUC

MASS UIUC

MASS UIUC

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. MASS UIUC

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 MASS UIUC

How to represent process using cross-sectional view? Surface Micromachining Fabrication Process for Micro Motor (1st pass description) Learning objectives: How to represent process using cross-sectional view? Build ability to correlate mask and sideviews. MASS UIUC

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. MASS UIUC

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 MASS UIUC

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 MASS UIUC

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. MASS UIUC

Step 5: Deposit a second sacrificial layer Mask top view Side view mask Conformal deposition of P-doped oxide again. MASS UIUC

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. MASS UIUC

Step 7: Deposit polysilicon structural layer. Top view (mask) Side view mask Conformal deposition of polysilicon again. MASS UIUC

Step 8: Pattern Polysilicon. Top view (mask) Side view mask Pattern the top layer polysilicon to form the confinement structure and anchor. MASS UIUC

Step 9: Sacrificial layer removal and freeing of structures Top view (mask) Side view mask Remove the oxide using 49% HF solutions, which etches oxide fast (1 micron/minute) and the polysilicon slowly. MASS UIUC

High Aspect Ratio Devices Thick photoresist *: Lithographie, galvanoformung, abformung MASS UIUC

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 … MASS UIUC

How to ? Micro guitar – Cornell University Sandia Photonic lattice IBM Supercone tip MASS UIUC

Man-made Submarine … in your artery MicroTEC 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 Das micro-U-Boot, das kleinste U-Boot der Welt! www.microTEC-D.com Duisburg, Germany MASS UIUC

MASS UIUC

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 MASS UIUC

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

Ferrari MEMS vs. Suzuki MEMS Suzuki Swift 1997 MASS UIUC

MUMPS 3 poly surface micromachining Process One process, different devices. MASS UIUC

MEMS Foundry - MEMS Exchange Distributed, Virtue fab UC Berkeley Cornell Nanofabrication Facility (CNF) www.mems-exchange.org MASS UIUC

Polymer MEMS Polymer materials as substrate Polymer as structures 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 MASS UIUC

LCP for MEMS packaging 15mm Copper-LCP laminates for flexible circuit boards LCP thermal bonding for environmental encapsualtion LCP substrates for robust devices 15mm MASS UIUC

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 MASS UIUC

Tactile Sensor Operation Converts normal applied load into change in resistance Array can image tactile contact Similar fabrication techniques can provide shear data 1400μm Applied Load Compressive Strain (x-dir) Tactile Bump Strain Gauge Area Membrane Perimeter Tensile Strain (x-dir) MASS UIUC

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. MASS UIUC