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8/11/2019 Lecture2.BulkMicromachining
1/23
U. Srinivasan
EE
C245
Dr. Thara Srinivasan
Lecture 2
MEMS Fabrication I
Process Flows and Bulk
Micromachining
Picture credit: Alien Technology
U. Srinivasan
EE
C245
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.
Todays Lecture Tools Needed for MEMS Fabrication Photolithography Review Crystal Structure of Silicon Bulk Silicon Etching Techniques
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IC 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 VaporDeposition
Diffusion
Ion Implantation
Evaporation andSputtering
Epitaxy
Complementary Metal-Oxide-SemiconductorJaeger
deposit
patternetch
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MEMS Devices
Staple
Polysilicon level 2
Polysilicon level 1
Silicon substrate
Polysilicon level 1
Polysilicon level 2
Hinge staple
Plate
Silicon substrate
Support arm
Prof. Kris Pister
U. Srinivasan
EE
C245
MEMS Devices
Microoptomechanicalswitches, Lucent
Analog Devices
Integratedaccelerometer Microturbine, Schmidt group MIT
Thermally isolated RMSconverter Reay et al.
Caliper
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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
Package
Dice
Release
sacrificial layerstructural layer
U. Srinivasan
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Photolithography:
Masks and Photoresist
dark-fieldlight-field
Photolithography steps Photoresist spinnning, 1-10 m 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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Photoresist Application
Spin-casting photoresist Polymer resin, sensitizer, carriersolvent
Positive and negative photoresist
Thickness depends on Concentration Viscosity Spin speed Spin time
www.brewerscience.com
U. Srinivasan
EE
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Photolithography Tools
Contact or proximity Resolution: Contact - 1-2 m,
Proximity - 5 m
Depth of focus poor
Projection Reduce 5-10, stepper mode Resolution - 0.5 (/NA) ~ 1 m
Depth of focus ~ Few ms
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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Materials for MEMS
Substrates Silicon
Glass
Quartz
Thin Films Polysilicon
Silicon Dioxide,Silicon Nitride
Metals Polymers
Wolf and Tauber
Silicon crystal structure = 5.43
U. Srinivasan
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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],
x x x
yy y
z z z
(100) (110) (111)
{111}
[001]
[100]
[010]
(110)
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Silicon Crystallography
Angles between planes, between [abc] and [xyz] 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 1/2 0
0 1/2 0
3/41/4
1/43/4
01/2 1/2
))3)(1/()001((1
)111(),100( ++=
Cos
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Silicon Crystal Origami
Silicon fold-up cube Adapted from Profs. KrisPister and Jack Judy
Print onto transparency
Assemble inside out
Visualize crystal planeorientations, 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)
[ 01 0] [ 01 0]
[001]
[001]
[100][100]
[101][101]
[011][011]
[110][110]
Judy, UCLA
Judy
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Silicon Wafers
Location of primaryand secondary flatsshows Crystal orientation Doping, n- or p-type
Maluf
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Mechanical Properties of Silicon
Crystalline silicon is a hard and brittle material thatdeforms elastically until it reaches its yield strength,at which point it breaks. Tensile yield strength = 7 GPa (~1500 lb suspended from 1
mm)
Youngs 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 500C
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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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
U. Srinivasan
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Anisotropic Etching of Silicon
Etching of Si with KOHSi + 2OH- Si(OH)2
2+ + 4e-
4H2O + 4e- 4(OH) - + 2H2
Maluf
Crystal orientation relative etchrates {110}:{100}:{111} = 600:400:1
{111} plane has three of its bondsbelow the surface
{111} may form protective oxidequickly
{111} smoother than other crystalplanes
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KOH Etch Conditions
1 KOH : 2 H2O (wt.), stirred bath @ 80C
Si (100) 1.4 m/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
Convexcornersbounded by{111} planesare attacked
Maluf
Ristic
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Undercutting
Convexcornersbounded by{111} planesare attacked
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Corner Compensation
Protect corners with compensationareas in layout
Mesa array for self-assembly teststructures, Smith and coworkers (1995)
Alien TechnologyHadley
Chang
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Corner Compensation
Self-assembly microparts,Alien Technology
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Other Anisotropic Etchants
TMAH, Tetramethyl ammonium hydroxide, 10-40 wt.% (90C) Etch rate (100) = 0.5-1.5 m/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 (115C) Carcinogenic, corrosive Etch rate (100) = 0.75 m/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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Boron-Doped Etch Stop
Control etch depth precisely withboron 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 invalence band and are unavailable forreactions to give OH-
Results Beams, suspended films
1-20 m layers possible p++ not compatible with CMOS Buried p++ compatible
U. Srinivasan
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Micronozzle
Maluf
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Microneedles
Ken Wise group,
University of Michigan
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Microneedles
Wise group,
University of Michigan
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Microneedles
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 layerforms, different for p- and n-Si
Set-up p-n diode in reverse bias
p-substrate floating etched
n-layer above passivationpotential 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.
U. Srinivasan
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Pressure Sensors Bulk micromachined pressure
sensors Piezoresistivity change in
electrical resistance due tomechanical stress
In response to pressure load onthin Si film, piezoresistiveelements change resistance
Membrane deflection < 1 m
Maluf
n-typeepilayer,p-typesubstrate
(111)
R1R3
Bondpad(100) Sidiaphragm
P-type diffusedpiezoresistor
n-typeepitaxiallayer
Metalconductors
AnodicallybondedPyrexsubstrate
Etchedcavity
Backsideport
(111)
R2 R1
R3
Depositinsulator
Diffusepiezoresistors
Deposit &pattern metal
Electrochemicaletch of backsidecavity
Anodicbondingof glass
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Only 150 400 900 m3
Pressure Sensors
Catheter-tippressure 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 m/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, 10s to 1000s W Pressure, mTorr >100 Torr
sheath
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Plasma 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)
U. Srinivasan
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Deep reactive ion etching (DRIE) withinhibitor film Inductively-coupled plasma
Bosch method for anisotropic etching,1.5 - 4 m/min
Etch cycle (5-15 s)
SF6 (SFx+) etches Si
Deposition cycle (5-15 s)
C4F8 deposits fluorocarbon protectivepolymer (-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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Etching 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; Si
converts to SiCl4.
Etch rate: 100,000 m3/s; 3 min toetch 500500125 m3 trench
Surface roughness: 30 nm RMS
Serial process: patterned directlyfrom CAD file
Revise, Inc.
Laser-assisted etchingof a 500500 m2
terraced silicon well.Each step is 6 mdeep.
Recommended