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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

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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

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Undercutting

• Convex corners bounded by {111} planes are attacked

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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

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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

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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

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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

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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

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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.