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
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
IC Processing
Cross-section
Jaeger
Masks Cross-section Masks
N-type metal oxide semiconductor (NMOS) process flow
CMOS Processing
• Processing steps• Oxidation• Photolithography• Etching• Diffusion• Evaporation and
Sputtering• Chemical Vapor
Deposition• Ion Implantation• Epitaxy
Complementary Metal-Oxide-SemiconductorJaeger
deposit
patternetch
MEMS Devices
Integrated accelerometer chipFord Microelectronics
Micromachined turbine Schmidt group, MIT
Angular rate sensorDelphi-Delco Electronic Systems
Microoptomechanical switches, Lucent
MEMS Devices
StaplePolysilicon level 2
Polysilicon level 1
Silicon substrate
Polysilicon level 1
Polysilicon level 2
Hinge staple
Plate
Silicon substrate
Support arm
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
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
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
Wolf and Tauber
Silicon crystal structure = 5.43 Å
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)
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
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]
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
• 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
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 + 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
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
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: 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
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
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 SiO2 layer forms
• Set-up• p-n diode in reverse bias
• p-substrate floating etched
• n-layer above passivation potential not etched
Maluf
• 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)
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
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
• 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
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
• SF6 • Plasma phase• Vapor phase
• 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
• 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
DRIE Examples
Comb-drive Actuator
Keller, MEMSPI
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
Etching with Xenon Difluoride
• Example
Pister group
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.