MEMS Fabrication Process Flows and Bulk Silicon Etching

MEMS Fabrication: Process Flows and Bulk Silicon Etching Thara Srinivasan Lecture 2 Picture credit: Alien Technology

Lecture Outline • Reading • Reader: Kovacs, pp. 1536 -43, Williams, pp. 256 -60. • Senturia, Chapter 2. • Today’s Lecture • • Tools Needed for MEMS Fabrication Photolithography Review Crystal Structure of Silicon Etching Techniques

IC Processing Cross-section Masks N-type metal oxide semiconductor (NMOS) process flow Jaeger

CMOS Processing • Processing steps • • • Oxidation Photolithography Etching Diffusion Evaporation and Sputtering • Chemical Vapor Deposition • Ion Implantation • Epitaxy Jaeger Complementary Metal-Oxide-Semiconductor deposit etch pattern

MEMS Devices Microoptomechanical switches, Lucent Micromachined turbine Schmidt group, MIT Integrated accelerometer chip Ford Microelectronics Angular rate sensor Delphi-Delco Electronic Systems

MEMS Devices Polysilicon level 1 Plate Polysilicon level 2 Staple Polysilicon level 1 Silicon substrate Hinge staple Support arm Silicon substrate

MEMS Processing • Unique to MEMS fabrication • • Sacrificial etching Thicker films and deep etching Mechanical properties critical Etching into substrate 3 -D assembly Wafer-bonding Molding • Unique to MEMS packaging and testing • Delicate mechanical structures sacrificial layer structural layer • Packaging: before or after dicing? • Sealing in gas environments • Interconnect - electrical, mechanical, fluidic • Testing – electrical, mechanical, fluidic Package Dice Release

Photolithography: Masks and Photoresist • Photolithography steps • Photoresist spinnning, 1 -10 µm spin coating • Optical exposure through a photomask • Developing to dissolve exposed resist • Photomasks • Layout generated from CAD file • Chrome or emulsion on glass • 1 -3 $k light-field dark-field

Photoresist Application • Spin-casting photoresist • Polymer resin, sensitizer, carrier solvent • Positive and negative photoresist • Thickness depends on • • Concentration Viscosity Spin speed Spin time www. brewerscience. com

Photolithography Tools • Contact or proximity • Resolution: Contact - 1 -2 µm, Proximity - 5 µm • Depth of focus • Projection • Resolution - 0. 5 ( /NA) ~ 1 µm • Depth of focus ~ Few µms

Materials for MEMS • Substrates • Silicon • Glass • Quartz • Thin Films • Polysilicon • Silicon Dioxide, Silicon Nitride • Metals • Polymers Silicon crystal structure = 5. 43 Å Wolf and Tauber

Silicon Crystallography [001] z z z (110) y y y [010] [100] x • (100) x Miller Indices (hkl) (110) x (111) • Normal to plane • Reciprocal of plane intercepts with axes • (unique), {family} • Direction • Move one endpoint to origin • [unique], <family> {111}

Silicon Crystallography 0 1/2 1/4 3/4 1/2 0 1/4 0 • Angles between planes, 3/4 1/2 • between [abc] and [xyz] is given by: ax+by+cz = |(a, b, c)|*|(x, y, z)|*cos( ) • {100} and {110} – 45° • {100} and {111} – 54. 74° • {110} and {111} – 35. 26, 90 and 144. 74° 0

Silicon Crystal Origami {111} (111) {110} {111} (111) (101) {100} {111} (111) {110} (101) [101] (100) {111} (111) {100} • Silicon fold-up cube {111} (111) (011) {110} (101) {111} (111) {110} (011) [110] (010) [100] {110} (011) {111} (111) {110} (101) [010] {100} (001) {111} (111) {110} (011) (110) {100} {110} (100) (110) {100} {110} [001] [100] [001] (010) (110) {100} {110} (110) {110} • Adapted from Profs. Kris Pister and Jack Judy • Print onto transparency • Assemble inside out • Visualize crystal plane orientations, intersections, and directions {110} [011] (001)

Silicon Wafers • Location of primary and secondary flats shows • Crystal orientation • Doping, n- or p-type Maluf

Properties of Silicon • Crystalline silicon is a hard and brittle material that deforms elastically until it reaches its yield strength, at which point it breaks. • Tensile yield strength = 7 GPa (~1500 lb suspended from 1 mm²) • Young’s Modulus near that of stainless steel • {100} = 130 GPa; {110} = 169 GPa; {111} = 188 GPa • Mechanical properties uniform, no intrinsic stress • Good thermal conductor • Mechanical integrity up to 500°C

Bulk Etching of Silicon 1. Etching modes 1. Isotropic vs. anisotropic 2. Reaction-limited 1. Etch rate dependent on temperature • Diffusion-limited • Etch rate dependent on mixing • Also dependent on layout and geometry, “loading” 2. Choosing a method 1. Desired shapes 2. Layout and uniformity 3. Surface roughness 4. Process compatibility 5. Safety, cost, availability Maluf adsorption surface reaction desorption slowest step controls rate of reaction

Wet Etch Variations • Etch rate variation due to wet etch set-up • • • Loss of reactive species Evaporation of liquids Poor mixing (etch product blocks diffusion of reactants) Contamination Applied potential Illumination

Anisotropic Etching of Silicon • Etching of Si with KOH Si + 2 OH- Si(OH)2 2+ + 4 e 4 H 2 O + 4 e- 4(OH) - + 2 H 2 • Crystal orientation relative etch rates • {110}: {100}: {111} = 600: 400: 1 • {111} plane has three backbonds • • below the surface Energy explanation {111} may form protective oxide quickly <100> Maluf

KOH Etch Conditions • 1 KOH : 2 H 2 O (wt. ), stirred bath @ 80°C • Si (100) 1. 4 µm/min • Etch masks • Si 3 N 4 0 • Si. O 2 1 -10 nm/min • Photoresist, Al ~ fast • “Micromasking” by H 2 bubbles leads to roughness • Stirring displaces • bubbles Oxidizer, surfactant additives Maluf

Undercutting • Convex corners bounded by {111} planes are attacked Maluf Ristic

Undercutting • Convex corners bounded by {111} planes are attacked

Corner Compensation • Protect corners with “compensation” • areas in layout, Buser et al. (1986) Mesa array for self-assembly test structures, Smith and coworkers (1995) Alien Technology Hadley Chang

Corner Compensation • Self-assembly microparts, Alien Technology

Other Anisotropic Etchants • TMAH, Tetramethyl ammonium hydroxide, 10 -40 wt. % (90°C) • • • Al safe, IC compatible Etch rate (100) = 0. 5 -1. 5 µm/min Etch ratio (100)/(111) = 10 -35 Etch masks: Si. O 2 , Si 3 N 4 ~ 0. 05 -0. 25 nm/min Boron doped etch stop, up to 40 slower • EDP (115°C) • • Carcinogenic, corrosive Al may be etched Etch rate (100) = 0. 75 µm/min R(100) > R(111) Etch ratio (100)/(111) = 35 Etch masks: Si. O 2 ~ 0. 2 nm/min, Si 3 N 4 ~ 0. 1 nm/min Boron doped etch stop, 50 slower

Boron-Doped Etch Stop • Control etch depth precisely with boron doping (p++) • [B] > 1020 cm-3 reduces KOH etch rate by 20 -100 • Gaseous or solid boron diffusion • At high dopant level, injected electrons recombine with holes in valence band are unavailable for reactions to give OH- • Results • • Beams, suspended films 1 -20 µm layers possible p++ not compatible with CMOS Buried p++ compatible

Microneedles Ken Wise group, University of Michigan

Microneedles Wise group, University of Michigan

Microneedles Ken Wise group, University of Michigan

Electrochemical Etch Stop • Electrochemical etch stop • • n-type epitaxial layer grown on p-type wafer forms p-n diode p>n electrical conduction p<n “reverse bias” passivation potential – potential at which thin Si. O 2 layer forms • Set-up • p-n diode in reverse bias • p-substrate floating etched • n-layer above passivation potential not etched Maluf

Electrochemical Etch Stop • Electrochemical etching on preprocessed CMOS wafers • N-type Si well with circuits suspended from Si. O 2 support beam • Thermally and electrically isolated • TMAH etchant, Al bond pads safe Reay et al. (1994)

Pressure Sensors • Bulk micromachined pressure sensors • In response to pressure load on thin Si film, piezoresistive elements detect stress • Piezoresistivity – change in electrical resistance due to mechanical stress • Membrane deflection < 1 µm (100) Si diaphragm Bondpad R 2 (111) Backside port Etched cavity P-type diffused piezoresistor RR 11 RR 3 3 Diffuse piezoresistors p-type substrate & frame Metal conductors n-type epitaxial layer Anodically bonded Pyrex substrate Deposit insulator Deposit & pattern metal Electrochemical etch of backside cavity Anodic bonding of glass Maluf Integrated Pressure Sensor, Bosch

Isotropic Etching of Silicon pure HF reaction-limited • HNA: hydrofluoric acid (HF), nitric acid (HNO 3) and acetic (CH 3 COOH) or water • HNO 3 oxidizes Si to Si. O 2 • HF converts Si. O 2 to soluble H 2 Si. F 6 • Acetic prevents dissociation of HNO 3 • Etch masks • Si. O 2 etched at 300 -800 /min • Nonetching Au or Si 3 N 4 pure HNO 3 Robbins diffusion-limited

Isotropic Etching Examples Tjerkstra, 1997 • 5% (49%) HF : 80% (69%) HNO 3 : 15% H 2 O (by volume) • Half-circular channels for chromatography • Etch rate 0. 8 -1 µm/min • Surface roughness 3 nm • Pro and Con • Easy to mold from rounded channels • Etch rate and profile are highly agitation sensitive

Dry Etching of Silicon • Dry etching • Plasma phase • Vapor phase • Plasma set-up and parameters • RF power • Pressure • Nonvolatile etch species • Plasma phase etching processes • Plasma etching • Reactive ion etching (RIE) • Inductively-coupled plasma RIE

Plasma Etching of Silicon • SF 6 • Plasma phase • Vapor phase

High-Aspect-Ratio Plasma Etching • Deep reactive ion etching (DRIE) • Inductively-coupled plasma • Bosch method for anisotropic etching, 1. 5 - 4 µm/min • Etch cycle (5 -15 s) SF 6 (SFx+) etches Si • Deposition cycle (5 -12 s) C 4 F 8 deposits fluorocarbon protective polymer (-CF 2 -)n • Etch mask selectivity: Si. O 2 ~ 200: 1, photoresist ~ 100: 1 • Sidewall roughness: scalloping < 50 nm • Sidewall angle: 90 ± 2° Maluf

DRIE Issues • Etch rate is diffusion-limited and drops for narrow trenches • Adjust mask layout to eliminate large disparities • Adjust process parameters (etch rate slows to < 1 µm/min) • Etch depth precision • Etch stop ~ buried layer of Si. O 2 • Lateral undercut at Si/Si. O 2 interface ~ “footing” Fig 3. 15 p. 68 Maluf

DRIE Examples Comb-drive Actuator Keller, MEMSPI

Vapor Phase Etching of Silicon • Vapor-phase etchant Xe. F 2 2 Xe. F 2(v) + Si(s) 2 Xe(v) + Si. F 2(v) • Etch rates: 1 -3 µm/min (up to 40) • Etch masks: photoresist, Si. O 2, Si 3 N 4, Al, metals • Set-up • Closed chamber, 1 torr • Pulsed to control exothermic heat of reaction • Issues • Etched surfaces have granular structure, 10 µm roughness • Hazard: Xe. F 2 reacts with H 2 O in air to form Xe and HF Xactix

Etching with Xenon Difluoride • Example Pister group

Laser-Driven Etching • Laser-Assisted Chemical Etching . • Mechanism • Etch rate: 100, 000 µm 3/s; 3 min to etch 500 125 µm 3 trench • Surface roughness: 30 nm RMS • Serial process: patterned directly from CAD file Laser-assisted etching of A 500 µm 2 terraced silicon well. Each step is 6 µm deep. Revise, Inc.