thara srinivasan lecture 2 mems fabrication: process flows and bulk silicon etching picture credit:...
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Thara SrinivasanLecture 2
MEMS Fabrication: Process Flows and Bulk
Silicon Etching
Picture credit: Alien Technology
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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• 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• Diffusion• Evaporation and
Sputtering• Chemical Vapor
Deposition• Ion Implantation• Epitaxy
Complementary Metal-Oxide-SemiconductorJaeger
deposit
patternetch
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MEMS Devices
Integrated accelerometer chipFord Microelectronics
Micromachined turbine Schmidt group, MIT
Angular rate sensorDelphi-Delco Electronic Systems
Microoptomechanical switches, Lucent
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MEMS Devices
StaplePolysilicon level 2
Polysilicon level 1
Silicon substrate
Polysilicon level 1
Polysilicon level 2
Hinge staple
Plate
Silicon substrate
Support arm
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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
• 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
• Photomasks• Layout generated from CAD file
• Chrome or emulsion on glass
• 1-3 $k
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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
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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
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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 Å
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Silicon Crystallography
• Miller Indices (hkl)• Normal to plane
• Reciprocal of plane intercepts with axes• (unique), {family}
• Direction • Move one endpoint to origin• [unique], <family>
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] 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 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. 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
][10
1]
[011
][01
1]
[110][110]
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Silicon Wafers
• Location of primary and secondary flats shows• Crystal orientation
• Doping, n- or p-type
Maluf
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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
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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• Layout and uniformity• Surface roughness• Process compatibility• Safety, cost, availability
adsorption desorptionsurfacereaction
slowest step controls rate of reaction
Maluf
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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
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Anisotropic Etching of Silicon
• Etching of Si with KOH
Si + 2OH- Si(OH)2 2+ + 4e-
4H2O + 4e- 4(OH) - + 2H2
• 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
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KOH 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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Undercutting
• Convex corners bounded by {111} planes are attacked
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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
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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.% (90°C)
• Al safe, IC compatible • Etch rate (100) = 0.5-1.5 µm/min• 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• Al may be etched• Etch rate (100) = 0.75 µm/min• 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 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 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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Microneedles
Ken Wise group, University of Michigan
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Microneedles
Wise group, University of Michigan
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Microneedles
Ken 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”
• passivation potential – potential at which thin SiO2 layer forms
• 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)
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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
Maluf
Integrated Pressure Sensor, Bosch
p-typesubstrate & frame
(111)
R1
R3
Bondpad(100) Sidiaphragm
P-type diffusedpiezoresistor
n-typeepitaxiallayer
Metalconductors
Anodicallybonded Pyrexsubstrate
Etchedcavity
Backsideport
(111)
R2 R1
R3
Depositinsulator
Diffusepiezoresistors
Deposit &pattern metal
Electrochemicaletch of backsidecavity
Anodicbondingof glass
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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 300-800 /min
• Nonetching Au or Si3N4
Robbins
pure HNO3
diffusion-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• 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
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Plasma Etching of Silicon
• SF6 • Plasma phase• Vapor phase
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• Deep reactive ion etching (DRIE)• Inductively-coupled plasma• Bosch method for anisotropic
etching, 1.5 - 4 µm/min
• Etch cycle (5-15 s)
SF6 (SFx+) etches Si
• Deposition cycle (5-12 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
Fig 3.15 p.68 MalufMaluf
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DRIE Examples
Comb-drive Actuator
Keller, MEMSPI
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Vapor Phase Etching of Silicon
• Vapor-phase etchant XeF2
2XeF2(v) + Si(s) 2Xe(v) + SiF2(v)
• Etch rates: 1-3 µm/min (up to 40)• Etch masks: photoresist, SiO2, Si3N4, 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: XeF2 reacts with H2O in air to
form Xe and HF Xactix
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Etching with Xenon Difluoride
• Example
Pister group
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Laser-Driven Etching
• Laser-Assisted Chemical Etching• Mechanism• Etch rate: 100,000 µm3/s; 3 min to
etch 500500125 µm3 trench• Surface roughness: 30 nm RMS• Serial process: patterned directly
from CAD file
.
Revise, Inc.
Laser-assisted etching of A 500500 µm2 terraced silicon well. Each step is 6 µm deep.