mems fabrication: process flows and bulk silicon etching

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

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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)

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.