MEMS Fabrication. Beyond Integrated Circuits. MEMS Basic Concepts

Transcription

1 MEMS Fabrication Beyond Integrated Circuits MEMS Basic Concepts Uses integrated circuit fabrication techniques to make mechanical as well as electrical components on a single chip. Small size 1µm 1mm Typically a batch fabrication process, i.e. make many devices simultaneously. 2 MEMS pressure sensors in the eye of a needle Photo courtesy of Integrated Sensing Systems (ISSYS) Inc.

2 MEMS Differs from ICs Etching the silicon substrate away Bulk micromachining Etching selected layers away entirely Surface micromachining Additional processing methods used LIGA, spin-casting, molding Additional materials used Gold, Titanium, SiC, polymers, epoxies Computer Aided Design (CAD) Idea CAD Simulate Masks (for fabrication) Comb Drive Resonator

3 MEMS Layout CAD Tools Mechanical considerations: cross section, mechanical simulation models, etc. MEMS component libraries Simulate fluid flow, stress gradients, etc. Predict how processing parameters affect mechanical performance Can check cross-section of structures Bulk Micromachining

4 Silicon Crystal Structure Silicon has a covalently bonded diamond cubic (zinc blende) structure. Unit cell can be imagined as 2 FCC cells with 1 cell offset by [a/4, a/4, a/4], where a = length of unit cell = 5.431Å. (top view) a/4 a/4 a = FCC cell = offset FCC cell Silicon unit cell Miller Indices Used to label crystal planes and directions. Miller indices of a plane: Determine the intercepts (x,y,z) of the plane along each of the three crystallographic directions. Take the reciprocals of the intercepts (1/x, 1/y, 1/z) If fractions result, multiply each by the denominator of the smallest fraction, resulting in (h, k, l). Plane and Direction symbols: Plane: (hkl) Family of planes: {hkl} Direction: [hkl] Family of directions: <hkl>

5 Example: Miller Directions Imagine you are looking down the Z axis of the crystal Y X [210] [110] [120] [010] [110] [100] [310] Example: Miller Planes Z a=5.431å (110) (111) (111) (111) (111) Y (111) (111) X (100) (111) NOTE: Miller directions are perpendicular to Miller planes! Example: [100] vs. (100)

6 Isotropic Silicon Wet Etching Isotropic etchants etch equally in all crystallographic directions Examples: HNO 3 (nitric) and CH 3 COOH (acetic), HNA Etching done at or around room temperature (< 50 C) Etching is very fast (e.g. up to 100 µm/min) Undercuts mask Masks materials include SiO 2, Au/Cr, or Si 3 N 4 Anisotropic Silicon Wet Etching Anisotropic etch rates depend on crystal orientation Examples: KOH, EDP, TMAH Etching done at higher temperatures (> 50 C e.g. 85 to 115 C) Etching is slow 1 µm/min (for <100> direction) Does not undercut the mask Etch rates differ: (100) > (110) >> (111) Masking materials include SiO 2, Si 3 N 4 [100] (100) Silicon 54.7 [111]

7 Anisotropic Silicon Wet Etching Trenches V-groove Inverted pyramid w ~0.7w Through-wafer vias Anisotropic Silicon Wet Etching NOTE: In (100) silicon, etchants will seek out the {111} planes, regardless of mask alignment or shape

8 MEMS Microblades Silicon micro-blades etched via chemical etching May be used for delicate microsurgery Microneedles fabricated in a similar fashion Fabricated at Standard MEMS, Inc. Etch Stop Technology: Wet Silicon Etching boron Resulting doped region silicon Requires ion-implanted regions in substrate. Typical doping level greater than cm -3 boron in silicon. Etch rate in TMAH, KOH, decreases up to 40:1 doped:undoped (electrochemical etch stop). Good technique for forming membranes, needles.

9 Etch Stop Technology - Wet Microelectrode array for central nervous system. Drawing and photo courtesy of K. D. Wise, University of Michigan. Isotropic Silicon Dry Etch: XeF 2 XeF 2 crystals sublimate at vacuum, vapor etches silicon. Masking materials include SiO 2, photoresist, metals (Aluminum, etc.) Gentle no meniscus forces etch mask (100) Silicon

10 Anisotropic Silicon Dry Etch: Reactive Ion Etching (RIE) Typical gases used: O 2, SF 6, CF 4 For silicon etching: CF 4 + O 2 Drawing courtesy of G. O Brien, U. Michigan Deep Reactive Ion Etch (DRIE) High aspect ratio structures via RIE! Drawing courtesy of G. O Brien, U. Michigan

11 DRIE Passivation/Etch Cycle Each cycle = 4 12 seconds Drawing courtesy of G. O Brien, U. Michigan RIE vs. DRIE RIE DRIE Diffusion of reacting species causes lateral etching, nonvertical sidewalls as etch progresses beyond a few µm. Best aspect ratio 5:1 Etch rate of silicon 0.3µm/min Fairly selective to oxide, PR Passivation prevents lateral etching. Nanometer scalloping. Aspect ratio > 40:1 Etch rate of silicon > 3µm/min Extremely selective to oxide, PR

12 DRIE High Aspect Ratios 20µm deep, 1µm wide walls. Scalloping on nm scale, decreases with etch depth 15µm deep, 300nm wide! Photos courtesy of G. O Brien, U. Michigan Example Leaf Spring Tether Capacitive plates Produced using Deep Reactive Ion Etching of Silicon (DRIE) Photo courtesy of Lucas NovaSensor

13 High Aspect Ratio - LIGA LIGA = lithography, galvanoformung/electroplating, abformung/plastic molding (Ehrfeld et al., Karlsruhe Center, Germany) Lines/spaces: 5µm Aspect ratios of 10:1 to 20:1 Requires X-Ray (synchrotron) source!! 1. Deposit oxide, Cu, Ti, PMMA 2. x-ray lithography 3. Electroplate Ni Ti Cu PMMA oxide Ni Ni Si substrate High Aspect Ratio - LIGA 4. Mill back to planarize 5. etch PMMA 6. Release Ni (sacrificial Ti) Photos courtesy of U. Wisconsin

14 High Aspect Ratio with SU-8 800µm thick Su-8 100µm Ni plated via Su-8 mold Photosensitive epoxy, processing similar to PR Standard equipment (no X-ray source required) Thickness up to 700µm with single spin application Smooth sidewalls, aspect ratios up to 20:1 Photos courtesy of Micro Resist Technology, Berlin, Germany.