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A brief history of MEMS fabrication Chang Liu Micro Actuators, Sensors, Systems Group University of Illinois at Urbana-Champaign Chang Liu MASS UIUC To Do … • Get a better diagram of MOS process flow. Chang Liu 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 Chang Liu MASS UIUC A Standard IC Process • Draw a diagram of a circuit. Chang Liu 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 Chang Liu MASS UIUC From wafer to device Chang Liu MASS UIUC Chang Liu MASS UIUC Processing Equipments A tour of lab is arranged in the middle of semester Wafer aligner and exposure tool Chang Liu Metal Evaporator Plasma etcher MASS UIUC Micro Fabrication Technology start Cycle 1 Cycle 2 Starting wafer Cycle 3 Cycle 4 Cycle 5 Cycle 6 MEMS subtracting (etching) pattern Adding (deposition) Chang Liu MASS UIUC Silicon Bulk Etching • Anisotropic Etching • Isotropic etching Chang Liu MASS UIUC Chang Liu MASS UIUC Chang Liu 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. Chang Liu 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 Chang Liu MASS UIUC 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. Chang Liu 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 0.30.5 mm. Chang Liu 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 Chang Liu 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 Chang Liu 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. Chang Liu MASS UIUC Step 5: Deposit a second sacrificial layer • Mask top view • Side view mask Conformal deposition of P-doped oxide again. Chang Liu 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. Chang Liu MASS UIUC Step 7: Deposit polysilicon structural layer. • Top view (mask) • Side view mask Conformal deposition of polysilicon again. Chang Liu MASS UIUC Step 8: Pattern Polysilicon. • Top view (mask) • Side view mask Pattern the top layer polysilicon to form the confinement structure and anchor. Chang Liu 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. Chang Liu MASS UIUC High Aspect Ratio Devices Thick photoresist *: Chang Liu 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) Organic Materials • Silicone elastomer • Elastic polymer • Chemical vapor deposition of plastic films (Parylene) • Electroactive Materials • … • • • • • Laser Micromachining Deep Reactive Ion Etching Focused ion beam etching Chemical mechanical polishing Permanent magnet and electromagnetic materials • Rapid prototyping Chang Liu MASS UIUC Micro guitar – Cornell University Sandia Photonic lattice IBM Supercone tip Chang Liu 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! Chang Liu www.microTEC-D.com Duisburg, Germany MASS UIUC Chang Liu 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 Chang Liu MASS UIUC FIB process (continued) • Chang Liu http://www.msm.cam.ac.uk/dmg/research/fib/micromachine/index.html MASS UIUC Ferrari MEMS vs. Suzuki MEMS Ferrari 348 1989 Suzuki Swift 1997 Chang Liu MASS UIUC MUMPS • 3 poly surface micromachining • Process • One process, different devices. Chang Liu MASS UIUC MEMS Foundry MEMS Exchange • Distributed, Virtue fab – UC Berkeley – Cornell Nanofabrication Facility (CNF) Chang Liu www.mems-exchange.org MASS UIUC 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 Chang Liu MASS UIUC LCP for MEMS packaging • Copper-LCP laminates for flexible circuit boards • LCP thermal bonding for environmental encapsualtion • LCP substrates for robust devices 15mm Chang Liu 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 Chang Liu 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 Applied Load Tactile Bump Membrane Perimeter Compressive Strain (x-dir) Strain Gauge Area Tensile Strain (x-dir) Chang Liu 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. Chang Liu MASS UIUC
