mems fabrication i : process flows and bulk micromachining · pdf filemems fabrication i :...
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Dr. Thara SrinivasanLecture 2
MEMS Fabrication I :Process Flows and Bulk
Micromachining
Picture credit: Alien Technology
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Lecture Outline
• Reading• Reader is in! (at South side Copy Central)• Kovacs, “Bulk Micromachining of Silicon,” pp. 1536-43.• Williams, “Etch Rates for Micromachining Processing,” pp.
256-60.• Senturia, Chapter 3, “Microfabrication.”
• Today’s Lecture• Tools Needed for MEMS Fabrication• Photolithography Review• Crystal Structure of Silicon• Bulk Silicon Etching Techniques
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5IC Processing
Cross-section
Jaeger
Masks Cross-section Masks
N-type Metal Oxide Semiconductor (NMOS) process flow
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CMOS Processing
• Processing steps• Oxidation• Photolithography• Etching• Chemical Vapor
Deposition• Diffusion• Ion Implantation• Evaporation and
Sputtering• Epitaxy
Complementary Metal-Oxide-SemiconductorJaeger
deposit
patternetch
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5MEMS Devices
StaplePolysilicon level 2
Polysilicon level 1
Silicon substrate
Polysilicon level 1
Polysilicon level 2
Hinge staple
Plate
Silicon substrate
Support arm
Prof. Kris Pister
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MEMS Devices
Microoptomechanicalswitches, Lucent
Analog DevicesIntegrated
accelerometer Microturbine, Schmidt group MIT
Thermally isolated RMS converter Reay et al.
Caliper
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5MEMS 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
PackageDice
Release
sacrificial layerstructural layer
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Photolithography: Masks and Photoresist
dark-fieldlight-field
• 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 O2 plasma
• Photomasks• Layout generated from CAD file• Mask reticle: chrome or emulsion on fused silica• 1-3 $k
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5Photoresist 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
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Photolithography Tools
• Contact or proximity• Resolution: Contact - 1-2 µm,
Proximity - 5 µm• Depth of focus poor
• Projection• Reduce 5-10×, stepper mode• Resolution - 0.5 (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
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5Materials for MEMS
• Substrates• Silicon• Glass• Quartz
• Thin Films• Polysilicon• Silicon Dioxide,
Silicon Nitride• Metals• Polymers
Wolf and Tauber
Silicon crystal structureλ = 5.43 Å
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Silicon Crystallography
• 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>
x x x
yy y
z z z
(100) (110) (111)
{111}
[001]
[100]
[010]
(110)
<100>
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5Silicon Crystallography
• Angles between planes, ∠• ∠ between [abc] and [xyz] given by:
ax+by+cz = |(a,b,c)|*|(x,y,z)|*cos(Θ)
• {100} and {110} – 45°• {100} and {111} – 54.74°• {110} and {111} – 35.26, 90 and 144.74°
0 1/2 0
0 1/2 0
3/41/4
1/43/4
01/2 1/2
))3)(1/()001((1)111(),100( ++= −Cosθ
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Silicon Crystal Origami
• 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
{111}(111)
{111}(111)
{111}(111)
{111}(111)
{111}(111)
{111}(111)
{111}(111)
{111}(111)
{100}(100)
{110}(110){100}
(010)
{110}(011)
{110}(011)
{110}(110)
{110}(110){100}
(010)
{110}(011)
{110}(011)
{110}(110)
{110}(101)
{100}(001)
{100}(100)
{110}(101)
{110}(101)
{100}(001)
{110}(101)
[010] [010]
[001][001]
[100][100]
[101
][101
]
[011
][011
]
[110][110]
Judy, UCLA
Judy
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5Silicon Wafers
• Location of primary and secondary flats shows• Crystal orientation• Doping, n- or p-type
Maluf
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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; {110} = 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
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What is Bulk Micromachining?
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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”
• Choosing a method • Desired shapes• Etch depth and uniformity• Surface roughness• Process compatibility• Safety, cost, availability,
environmental impact
adsorption desorptionsurfacereaction
slowest step controls rate of reaction
Maluf
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5Wet 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
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Anisotropic Etching of Silicon
• Etching of Si with KOHSi + 2OH- → Si(OH)2
2+ + 4e-
4H2O + 4e- → 4(OH) - + 2H2
<100>Maluf
• Crystal orientation relative etch rates• {110}:{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
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5KOH Etch Conditions
• 1 KOH : 2 H2O (wt.), stirred bath @ 80°C• Si (100) → 1.4 µm/min• Etch masks
• Si3N4 → 0• SiO2 → 1-10 nm/min• Photoresist, Al ~ fast
• “Micromasking” by H2 bubbles leads to roughness• Stirring displaces bubbles• Oxidizer, surfactant additives
Maluf
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Undercutting
• Convex corners bounded by {111} planes are attacked
Maluf
Ristic
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5Undercutting
• Convex corners bounded by {111} planes are attacked
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Corner Compensation• Protect corners with “compensation”
areas in layout• Mesa array for self-assembly test
structures, Smith and coworkers (1995)
Alien TechnologyHadley
Chang
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5Corner Compensation
• Self-assembly microparts, Alien Technology
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Other Anisotropic Etchants• TMAH, Tetramethyl ammonium hydroxide, 10-40 wt.% (90°C)
• Etch rate (100) = 0.5-1.5 µm/min• Al safe, IC compatible • Etch ratio (100)/(111) = 10-35• Etch masks: SiO2 , Si3N4 ~ 0.05-0.25 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: SiO2 ~ 0.2 nm/min, Si3N4 ~ 0.1 nm/min • Boron doped etch stop, 50× slower
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5Boron-Doped Etch Stop
• Control etch depth precisely with boron doping (p++)• [B] > 1020 cm-3 reduces KOH etch
rate by 20-100ו 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
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Micronozzle
Maluf
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5Microneedles
Ken Wise group, University of Michigan
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Microneedles
Wise group, University of Michigan
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5Microneedles
Wise group, University of Michigan
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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 SiO2 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
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• Electrochemical etching on preprocessed CMOS wafers• N-type Si well with circuits suspended from SiO2 support beam• Thermally and electrically isolated• TMAH etchant, Al bond pads safe
Electrochemical Etch Stop
Reay et al. (1994)Kovacs group, Stanford U.
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Pressure Sensors• 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
Maluf
n-type epilayer, p-typesubstrate
(111)
R1R3
Bondpad(100) Sidiaphragm
P-type diffusedpiezoresistor
n-typeepitaxiallayer
Metalconductors
Anodicallybonded Pyrexsubstrate
Etchedcavity
Backsideport
(111)
R2 R1R3
Depositinsulator
Diffusepiezoresistors
Deposit &pattern metal
Electrochemicaletch of backsidecavity
Anodicbondingof glass
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• Only 150 × 400 × 900 µm3
Pressure Sensors
Catheter-tip pressure sensor, Lucas NovaSensor
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Isotropic Etching of Silicon• HNA: hydrofluoric acid (HF),
nitric acid (HNO3) and acetic (CH3COOH) or water• HNO3 oxidizes Si to SiO2
• HF converts SiO2 to soluble H2SiF6
• Acetic prevents dissociation of HNO3
• Etch masks• SiO2 etched at 30-80 nm/min• Nonetching Au or Si3N4
Robbins
pure HNO3diffusion-limited
pure HFreaction-limited
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• 5% (49%) HF : 80% (69%) HNO3 : 15% H2O (by volume)• Half-circular channels for chromatography• Etch rate 0.8-1 µm/min• Surface roughness 3 nm
Isotropic Etching Examples
• Pro and Con• Easy to mold from rounded channels• Etch rate and profile are highly agitation sensitive
Tjerkstra, 1997
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Dry Etching of Silicon
e - + CF4 → CF3+ + F + 2e-
• Dry etching • Plasma phase• Vapor phase
• Parameters• Gas and species generated ~
ions, radicals, photons• RF frequency, 13.56 MHz• RF power, 10’s to – 1000’s W• Pressure, mTorr – >100 Torr
sheath
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5Plasma Etching of Silicon
• Crystalline silicon• Etch gases ~ fluorine, chlorine-
based• Reactive species ~ F, Cl, Cl2• Products ~ SiF4, SiCl4
• 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)
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• Deep reactive ion etching (DRIE) with inhibitor film• Inductively-coupled plasma• Bosch method for anisotropic etching,
1.5 - 4 µm/min
• Etch cycle (5-15 s) SF6 (SFx
+) etches Si• Deposition cycle (5-15 s)
C4F8 deposits fluorocarbon protective polymer (-CF2-)n
• Etch mask selectivity: SiO2 ~ 200:1, photoresist ~ 100:1
• Sidewall roughness: scalloping < 50 nm• Sidewall angle: 90 ± 2°
High-Aspect-Ratio Plasma Etching
Maluf
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• 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 SiO2
• Lateral undercut at Si/SiO2 interface ~“footing”
DRIE Issues
Maluf
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DRIE Examples
Comb-drive Actuator
Keller, MEMS Precision Instruments
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5Electrospray Nozzle
Advanced BioAnalytical ServicesG. A. Schultz et al., 2000.
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Vapor Phase Etching of Silicon• Vapor-phase etchant XeF2
2XeF2(v) + Si(s)→ 2Xe(v) + SiF4(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, SiO2, Si3N4, Al,
metals• Issues
• Etched surfaces have granular structure, 10 µm roughness
• Hazard: XeF2 reacts with H2O in air to form Xe and HF
Xactix
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5Etching with Xenon Difluoride
• Post processed CMOS inductor
Pister group
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Laser-Driven Etching• Laser-Assisted Chemical Etching
• Laser creates Cl radicals from Cl2; Siconverts to SiCl4.
• Etch rate: 100,000 µm3/s; 3 min to etch 500×500×125 µm3 trench
• Surface roughness: 30 nm RMS• Serial process: patterned directly
from CAD file
Revise, Inc.
Laser-assisted etching of a 500×500 µm2
terraced silicon well. Each step is 6 µm deep.