Lecture2.BulkMicromachining

8/11/2019 Lecture2.BulkMicromachining

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U. Srinivasan

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Dr. Thara Srinivasan

Lecture 2

MEMS Fabrication I

Process Flows and Bulk

Micromachining

Picture credit: Alien Technology

U. Srinivasan

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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.

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


3/23

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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

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MEMS Devices

Microoptomechanicalswitches, Lucent

Analog Devices

Integratedaccelerometer Microturbine, Schmidt group MIT

Thermally isolated RMSconverter Reay et al.

Caliper


4/23

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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

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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

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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

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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

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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


12/23

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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

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Micronozzle

Maluf


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Microneedles

Ken Wise group,

University of Michigan

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Microneedles

Wise group,



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Microneedles

Wise group,


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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.

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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


18/23

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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


19/23

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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)

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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.

Documents

Lecture2.BulkMicromachining