MEMS Fabrication I : Process Flows and Bulk Micromachining

Transcription

1 MEMS Fabrication I : Process Flows and Bulk Micromachining Dr. Thara Srinivasan Lecture 2 Picture credit: Alien Technology Lecture Outline Reading Reader is in! (at South side Copy Central) Kovacs, Bulk Micromachining of Silicon, pp Williams, Etch Rates for Micromachining Processing, pp Senturia, Chapter 3, Microfabrication. Today s Lecture Tools Needed for MEMS Fabrication Photolithography Review Crystal Structure of Silicon Bulk Silicon Etching Techniques

2 IC Processing Cross-section Masks Cross-section Masks N-type Metal Oxide Semiconductor (NMOS) process flow Jaeger CMOS Processing Processing steps Oxidation Photolithography Etching Chemical Vapor Deposition Diffusion Ion Implantation Evaporation and Sputtering Epitaxy Jaeger Complementary Metal-Oxide-Semiconductor deposit etch pattern

3 MEMS Devices Polysilicon level 2 Polysilicon level 1 Plate Polysilicon level 2 Staple Polysilicon level 1 Silicon substrate Hinge staple Support arm Silicon substrate Prof. Kris Pister MEMS Devices Caliper Microoptomechanical switches, Lucent Thermally isolated RMS converter Reay et al. Analog Devices Integrated accelerometer Microturbine, Schmidt group MIT

4 MEMS Processing Unique to MEMS fabrication Sacrificial etching Mechanical properties critical Thicker films and deep etching Etching into substrate Double-sided lithography 3-D assembly Wafer-bonding Molding Integration with electronics, fluidics Unique to MEMS packaging and testing Delicate mechanical structures Packaging: before or after dicing? Sealing in gas environments Interconnect - electrical, mechanical, fluidic Testing electrical, mechanical, fluidic sacrificial layer structural layer 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 Bake to drive off solvents Remove using solvents (acetone) or O 2 plasma Photomasks Layout generated from CAD file Mask reticle: chrome or emulsion on fused silica 1-3 $k light-field dark-field

5 Photoresist Application Spin-casting photoresist Polymer resin, sensitizer, carrier solvent Positive and negative photoresist Thickness depends on Concentration Viscosity Spin speed Spin time Photolithography Tools Contact or proximity Resolution: Contact µm, Proximity - 5 µm Depth of focus poor Projection Reduce 5-10, stepper mode Resolution (λ/na) ~ 1 µm Depth of focus ~ Few µms Double-sided lithography Make alignment marks on both sides of wafer Use IR imaging to see through to back side Store image of front side marks; align to back

6 Materials for MEMS Substrates Silicon Glass Quartz Thin Films Polysilicon Silicon Dioxide, Silicon Nitride Metals Polymers Silicon crystal structure λ = 5.43 Å Wolf and Tauber Silicon Crystallography [001] z z z (110) [100] (100) x x (110) x (111) <100> y [010] Miller Indices (h k l) Planes Reciprocal of plane intercepts with axes Intercepts of normal to plane with plane (unique), {family} Directions Move one endpoint to origin [unique], <family> y {111} y

7 Silicon Crystallography 0 1/2 0 3/4 1/4 1/2 0 1/2 1/4 3/4 Angles between planes, between [abc] and [xyz] given by: ax+by+cz = (a,b,c) * (x,y,z) *cos(θ) 1 θ = Cos (( ) /(1)( ( 100),(111) + {100} and 45 {100} and {111} and {111} 35.26, 90 and /2 0 3)) Silicon Crystal Origami {111} (111) (101) {111} (111) {100} (100) {111} (111) (101) [101][101] {111} (111) Silicon fold-up cube Adapted from Profs. Kris Pister and Jack Judy Print onto transparency Assemble inside out Visualize crystal plane orientations, intersections, and directions Judy (110) (011) {100} (010) [100] [100] (011) {111} (111) [001] [001] (110) {111} (111) {100} (001) (101) {100} (100) (101) [010] [010] {100} (001) {111} (111) (110) {111} (111) (011) {100} (010) (011) [011][011] [110] [110] (110) Judy, UCLA

8 Location of primary and secondary flats shows Crystal orientation Doping, n- or p-type Silicon Wafers Maluf Mechanical 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; = 169 GPa; {111} = 188 GPa Mechanical properties uniform, no intrinsic stress Mechanical integrity up to 500 C Good thermal conductor, low thermal expansion coefficient High piezoresistivity

9 What is Bulk Micromachining? Bulk Etching of Silicon Etching modes Isotropic vs. anisotropic Reaction-limited Etch rate dependent on temperature Diffusion-limited Etch rate dependent on mixing Also dependent on layout and geometry, loading Maluf Choosing a method Desired shapes Etch depth and uniformity Surface roughness Process compatibility Safety, cost, availability, environmental impact adsorption surface reaction slowest step controls rate of reaction desorption

10 Wet Etch Variations, Crystalline Si Etch rate variation due to wet etch set-up Loss of reactive species through consumption Evaporation of liquids Poor mixing (etch product blocks diffusion of reactants) Contamination Applied potential Illumination Etch rate variation due to material being etched Impurities/dopants Etch rate variation due to layout Distribution of exposed area ~ loading Structure geometry Anisotropic Etching of Silicon Etching of Si with KOH Si + 2OH - Si(OH) e - 4H 2 O + 4e - 4(OH) - + 2H 2 Crystal orientation relative etch rates :{100}:{111} = 600:400:1 {111} plane has three of its bonds below the surface {111} may form protective oxide quickly {111} smoother than other crystal planes <100> Maluf

11 KOH Etch Conditions 1 KOH : 2 H 2 O (wt.), stirred 80 C Si (100) 1.4 µm/min Etch masks Si 3 N 4 0 SiO 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

12 Undercutting Convex corners bounded by {111} planes are attacked Corner Compensation Protect corners with compensation areas in layout Mesa array for self-assembly test structures, Smith and coworkers (1995) Hadley Chang Alien Technology

13 Corner Compensation Self-assembly microparts, Alien Technology Other Anisotropic Etchants TMAH, Tetramethyl ammonium hydroxide, wt.% (90 C) Etch rate (100) = µm/min Al safe, IC compatible Etch ratio (100)/(111) = Etch masks: SiO 2, Si 3 N 4 ~ nm/min Boron doped etch stop, up to 40 slower EDP (115 C) Carcinogenic, corrosive Etch rate (100) = 0.75 µm/min Al may be etched R(100) > R(110) > R(111) Etch ratio (100)/(111) = 35 Etch masks: SiO 2 ~ 0.2 nm/min, Si 3 N 4 ~ 0.1 nm/min Boron doped etch stop, 50 slower

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

15 Microneedles Ken Wise group, University of Michigan Microneedles Wise group, University of Michigan

16 Microneedles 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 current Passivation potential potential at which thin SiO 2 layer forms, different for p- and n-si Set-up p-n diode in reverse bias p-substrate floating etched n-layer above passivation potential not etched Maluf

17 Electrochemical Etch Stop Electrochemical etching on preprocessed CMOS wafers N-type Si well with circuits suspended from SiO 2 support beam Thermally and electrically isolated TMAH etchant, Al bond pads safe Reay et al. (1994) Kovacs group, Stanford U. Bulk micromachined pressure sensors Piezoresistivity change in electrical resistance due to mechanical stress In response to pressure load on thin Si film, piezoresistive elements change resistance Membrane deflection < 1 µm (100) Si diaphragm Bondpad R 2 RR 1 1 R 3 3 (111) (111) Pressure Sensors P-type diffused piezoresistor Metal conductors n-type epitaxial layer n-type epilayer, p-type substrate Deposit insulator Diffuse piezoresistors Deposit & pattern metal Electrochemical etch of backside cavity Backside port Etched cavity Anodically bonded Pyrex substrate Anodic bonding of glass Maluf

18 Pressure Sensors Only µm 3 Catheter-tip pressure sensor, Lucas NovaSensor 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 SiO 2 HF converts SiO 2 to soluble H 2 SiF 6 Acetic prevents dissociation of HNO 3 Etch masks SiO 2 etched at nm/min Nonetching Au or Si 3 N 4 Robbins pure HNO 3 diffusion-limited

19 Isotropic Etching Examples Tjerkstra, % (49%) HF : 80% (69%) HNO 3 : 15% H 2 O (by volume) Half-circular channels for chromatography Etch rate 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 sheath Parameters Gas and species generated ~ ions, radicals, photons RF frequency, MHz RF power, 10 s to 1000 s W Pressure, mtorr >100 Torr e - + CF 4 CF 3+ + F + 2e -

20 Plasma Etching of Silicon Plasma phase etching processes Sputtering Physical, nonselective, faceted Plasma etching Chemical, selective, isotropic Reactive ion etching (RIE) Physical and chemical, fairly selective, directional Inductively-coupled RIE Physical and chemical, fairly selective, directional (physical) Crystalline silicon Etch gases ~ fluorine, chlorinebased Reactive species ~ F, Cl, Cl 2 Products ~ SiF 4, SiCl 4 High-Aspect-Ratio Plasma Etching Deep reactive ion etching (DRIE) with inhibitor film Inductively-coupled plasma Bosch method for anisotropic etching, µm/min Etch cycle (5-15 s) SF 6 (SF x + ) etches Si Deposition cycle (5-15 s) C 4 F 8 deposits fluorocarbon protective polymer (-CF 2 -) n Etch mask selectivity: SiO 2 ~ 200:1, photoresist ~ 100:1 Sidewall roughness: scalloping < 50 nm Sidewall angle: 90 ± 2 Maluf

21 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 SiO 2 Lateral undercut at Si/SiO 2 interface ~ footing Maluf DRIE Examples Comb-drive Actuator Keller, MEMS Precision Instruments

22 Electrospray Nozzle Advanced BioAnalytical Services G. A. Schultz et al., Vapor Phase Etching of Silicon Vapor-phase etchant XeF 2 2XeF 2(v) + Si (s) 2Xe (v) + SiF 4(v) Set-up Xe sublimes at room T Closed chamber, 1-4 Torr Pulsed to control exothermic heat of reaction Etch rates: 1-3 µm/min (up to 40), isotropic Etch masks: photoresist, SiO 2, Si 3 N 4, Al, metals Issues Etched surfaces have granular structure, 10 µm roughness Hazard: XeF 2 reacts with H 2 O in air to form Xe and HF Xactix

23 Etching with Xenon Difluoride Post processed CMOS inductor Pister group Laser-Driven Etching Laser-Assisted Chemical Etching Laser creates Cl radicals from Cl 2 ; Si converts to SiCl 4. Etch rate: 100,000 µm 3 /s; 3 min to etch µm 3 trench Surface roughness: 30 nm RMS Serial process: patterned directly from CAD file Laser-assisted etching of a µm 2 terraced silicon well. Each step is 6 µm deep. Revise, Inc.