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Surface Micromachining: Techniques and Materials

Dr. Thara SrinivasanLecture 3

Picture credit: Texas Instruments

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Reading and Project Info.• Reading

• Senturia, sections from Chapter 3. • From reader

• Bustillo et al., “Surface Micromachining of MicroelectromechanicalSystems,” pp. 1552-6, 1559-63.

• Williams, “Etch Rates for Micromachining Processing,” pp. 256-69.

• Print out lecture notes before lecture• http://www-bsac.eecs.berkeley.edu/projects/ee245/index.htm

• For the project later on in the semester, you will…• Pick a fabrication method and materials for your MEMS device• Explain fabrication decisions

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Bulk Micromachining Review

• Definition: Etching pits, trenches or all the way through the silicon wafer to make mechanical structures.

• Methods• Wet etching• Dry etching (plasma or vapor)

• Etching modes• Isotropic: etches all crystal directions equally• Anisotropic: etches certain directions faster than others• Diffusion (or transport) -limited vs. Reaction-limited

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Bulk Micromachining Review

• Etch stops for wet etching

• Plasma etching• Etching species: ions and neutrals

(radicals)• “Plasma” etching: neutrals• Reactive ion etching: ions and neutrals

• Deep reactive ion etch (DRIE)• Vertical, deep trenches• Alternate between etch step and

protective Teflon deposition step

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Bulk Micromachining Review

• Bulk etching and metallization

• Possible structures

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Lecture Outline

• Today’s Lecture• Introduction

• Thin film deposition

• Thin film etching techniques

• Material combinations

• Lateral resonator process flow

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Surface Micromachining

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Poly-Si

Si substrate

Deposit & pattern poly

Oxide

Si substrate

Deposit & pattern oxide

10 µmCantileverAnchor

Si substrate

Sacrificial etch. This step “releases” the ca

Micromachining a Cantilever

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One Structural Poly, One Oxide Process

Lateral resonator with electrostatic comb drives, Sandia Labs

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Surface Micromachining

• Basic process sequence• Structural layer

• Sacrificial layer

• Release etch

Meshing gears on a moveable platform, Sandia

Digital Micromirror Device, Texas Instruments

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History• History of surface micromachining

• 1984: Howe and Muller used polysilicon and oxide to make beam resonator as gas sensor

• 1988: Pin joints, springs, gears, rotary electrostatic side drive motors (Fan, Tai, Muller)

• 1989: Lateral comb drive (Tang, Nguyen, Howe)

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History

• History of surface micromachining• 1991: Polysilicon hinge (Pister, Judy, Burgett, Fearing)

• 1992: MCNC starts MUMPS (a MEMS foundry)

• Early 1990’s: First surface micromachined accelerometer sold (Analog Devices, ADXL50)

StaplePolysilicon level 2

Polysilicon level 1

Silicon substrate

Polysilicon level 1

Polysilicon level 2

Hinge staple

Plate

Silicon substrateSupport arm

Prof. Kris Pister

Analog DevicesIntegrated accelerometer

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Polysilicon Mechanical Properties

• Mechanical properties of polycrystalline silicon (polysilicon or “poly”) • Stronger than stainless steel: fracture strength of

poly ~ 2-3 GPa, steel ~ 0.2GPa-1GPa

• Young’s Modulus ~ 140-190 GPa

• Extremely flexible: maximum strain before fracture ~ 0.5%

• Does not fatigue readily

• Compatible with IC fabrication processes, process parameters well-known

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C. T.-C. Nguyen and R. T. Howe, IEEE IEDM, 1993

• Electrostatic force is applied by a drive comb to a suspended shuttle

• Motion is detectedcapacitively by a sense comb

• Operated at resonant frequency

Lateral Resonator

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Today’s Lecture

• Introduction

• Thin film deposition techniques

• This film etching techniques

• Material combinations

• Lateral resonator process flow

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Thin Film Deposition

• Chemical Vapor Deposition • Polysilicon

• Silicon nitride

• Silicon dioxides

• Thermal oxidation• Silicon dioxide

• Physical Vapor Deposition• Evaporation of metals

• Sputtering of metals, dielectrics

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Mean Free Path

• Definition: distance a molecule travels before hitting another molecule

• Mean free path for gases

• Atmospheric 760 torr → λ = 40 nm

• Low vacuum 0.76 torr → λ = 40 µm

• Medium vacuum 7.6 mtorr → λ = 4 mm

• High vacuum 7.6 µtorr → λ = 4 m

760 Torr = 101 kPa = 1 atm

1 mTorr = 0.13 Pa = 1.3×10-6 atm

PM

RT ηπλ2

=

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Chemical Vapor Deposition

• Gases react at hot wafer surface to create solid films• Materials: polysilicon, silicon nitride, phosphosilicate glass (PSG),

low temperature oxide (LTO)

• Parameters: T, P, gas flowrates

Jensen

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Chemical Vapor Deposition

Pressure Energy Source

APCVD 100-760 torr 350-400°C Transport-limited

LPCVD 100-500 mtorr 500-800°C Reaction-limited

PECVD 2-5 torr plasma + Reaction-limited300-400°C

APCVD and PECVD furnaces

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Chemical Vapor Deposition

LPCVD furnace

pum

p

gate valve

N2

SiH

4

Si2H

6

B 2H6

PH3

mass flowcontroller

mass flowcontroller

GeH

4

injector boats wafers bafflescantilever

tube

exhaust

door

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CVD and Film Conformality• Film coverage

• Low pressure = long mean free path

• Molecules hit surface with extra energy to migrate

• Conformal coverage, case (a)

• If there is no surface migration,

• Coverage depends on range of arrival angles, case (c)

a) b) c)

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LPCVD Polysilicon

Deposited at 590°C, 5×5 µm2, rms ~ 5 nm

• Undoped polycrystalline silicon (poly)• Uses: IC layers, interconnect, MEMS structural material

• Pyrolysis of silane, SiH4, and dichlorosilane, SiH2Cl2• T = 550-700°C, P = 100’s mTorr

• Deposition rate: 10 nm/min at 630°C, 70 nm/min at 700°C (undoped)

• Conformal coverage (aspect ratio < 10)

• Large stresses (500 MPa) and stress gradients

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• Doping “in situ”• n-type: 1 vol % phosphine (PH3), p-type: diborane (B2H6)

• [ ] = 1020 cm-3, R = 1-10 mΩ/cm

• n-type, dep. rate ↓• p-type dep. rate ↑• Large stresses ~ 500 MPa

• Diffusion• Use PSG layers

• 900-1000°C, hours

• Heavy doping possible, R = 0.1 mΩ/cm

• Ion implantation

• As-deposited doped films also have high stress (500 MPa)

Doped LPCVD Polysilicon

Deposited at 590°C, 5×5 µm2, rms ~ 12 nm

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Residual Stress in Thin Films

• Residual film stress• Microstructure

• Thermal mismatch

• Compressive vs. tensile stress

Under compressive stress, film wants to expand.

Constrained to substrate, bends it in convex way.

Under tensile stress, film wants to shrink

Constrained to substrate, bends it in concave way.

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Stress Gradients

• Stress gradient: (+) or (-)compressive

tensile

+–

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Stress in Polysilicon Films

• Stress depends on crystal structure:• ≤ 600°C ~ films are initially amorphous,

then crystallize• Equiaxed crystals, isotropic

• Crystals have higher density → tensile stress

• Small stress gradient

• ≥ 600°C ~ Columnar crystals grow during deposition

• As crystals grow vertically and in-plane they push on neighbors → compressive stress

• Positive stress gradient T

P

amor

phou

s/eq

uiax

ed

colu

mna

r

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Controlling PolySi Stress

• Resonant frequency for lateral resonator, L = 150, W = 2 µm.

ML

tW

ML

tWEf ry

5244

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3

3

0σ

π+≈

Stress term dominates

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Annealing out PolySi Stress

• Control polySi stress by annealing at high temperatures, 900-1150°C• Grain boundaries move, relax• Annealing between similarly doped oxides (symmetric dopant

drive-in)• Also, rapid thermal anneal (RTA)

Biebl & Howe

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LPCVD Oxides• Phosphosilicate glass (PSG) and low-

temperature oxide (LTO)• Uses: sacrificial layer, insulator

• Pyrolysis of silane, and/or phosphine, diborane

• T = 400-450°C, P = 100’s mTorr

• Deposition rate: LTO ~ 150 nm/min at 450°C

• Poor conformality

• Annealing

• T = 600-1000°C → “densification”

• T = 1000°C → softens and flows to conform to underlying topography

• Stress: compressive 100-300 MPa

• TEOS: conformal oxide

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• LPCVD deposition• Uses: etch mask, passivation layer, coating for structural

layers

• Dichlorosilane (SiCl2H2), ammonia (NH3)

• T = 700-900°C, P = 100’s mTorr

• Types• Stoichiometric, Si3N4 ~ up to 1000 MPa tensile

• Non-stoichiometric, SixNy ~ Si excess ≤100 MPa tensile, even compressive

• PECVD < 400°C → pinholes, high H2 content, can also control stress

Silicon Nitride Deposition

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Thermal Oxidation

• Oxidize silicon in dry O2 or steam (850-1150°C)• Dry oxidation

• Si + O2 SiO2

• Wet oxidation

• Si + 2H2O SiO2 + 2H2

• Uses: electrical isolation, sacrificial layer, etch mask, biocompatible

• SiO2 volume greater than Si • To grow 1 um SiO2 layer, 0.44 um

Si consumed

• Compressive stress: 100’s MPa

Silicon

O2

Silicon

SiO2

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• Local heating of target material generates vapor that condenses on substrate• Materials: Al, Ti, W, Au, Pd, Pt, Cr, Al2O3

• Types• Thermal evaporation: resistively heat source

• Electron beam evaporation: accelerated electrons strike and melt target

• Deposition rate ~ 0.05 - 1 µm/min

• Depends on material’s vapor pressure Vp,measure of volatility

• Pressure: µTorrs

Evaporation

Madou

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Evaporation Issues• Step coverage

• Film evaporates in a straight line to wafer, highly “directional”

• Rotate substrate for film continuity with “planetary” holder

• Amenable to lift-off process

• Pros and cons• Resistive

+ Simple

– Contamination from filament, filament size limits film thickness

• E-beam

+ Purer films, higher deposition rates, better adhesion

– Complex system requires target cooling and x-ray shielding

photoresistmetal

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Sputtering• Process

• Target material at high (-) potential is bombarded by (+) inert gas ions (Ar, 0.5 to 3 kv) created in plasma

• Target atoms ejected and deposited onto wafer

• P > 0.01 Torr

• Materials: metals, PZT, dieletcrics

PumpGasSource

Target (Material to be Deposited)Cathode (Negative Voltage, DC or RF)

Substrate HolderAnode (Grounded, +V(bias), Floating, Hot, Cold)

Substrate

Plasma

Low Pressure: 1-100 mTorr

Ar+

Cooling water

• Pros and Cons + Production technique, uniformity

+ Wider materials choice, better film adhesion

+ Good step coverage (some thinning near corners)

+ Control of film properties (bias, pressure, substrate T)

– Complex, substrate heating

Judy

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Today’s Lecture

• Introduction

• Thin film deposition

• Thin film etching techniques

• Material combinations

• Lateral resonator process flow

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Film Etching

n/an/a40KIGold

35-3500O2>4000AcetonePhotoresist

100-150Cl2 + SiCl4660H3PO4:HNO3:CH3COOH

Aluminum

50-150CHF3 + O220-2000HFSilicon dioxide

150-250SF65H3PO4Silicon nitride

170-920SF6 + He120-600HNO3:H2O:NH4F

Polysilicon

Etch rate [nm/min]

Dry etchantEtch rate [nm/min]

Wet etchantMaterial

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Wet etchants, Williams etch table

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Today’s Lecture

• Introduction

• Thin film deposition

• Thin film etching techniques

• Material combinations

• Lateral resonator process flow

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Material Combinations

• Etchant, concentrated HF (49%)• Polysilicon etch rate ~ 0

• Silicon nitride etch rate ~ 3-14 nm/min

• Thermal oxide (wet) ~ 1.8-2.3 µm/min

• Annealed PSG ~ 3.6 µm/min

• Aluminum ~ 4 nm/min

Structure Sacrificial Etchant

polySi SiO2, PSG, LTO HF, BHF

Al photoresist O2 plasma

SiO2 polySi XeF2

Al Si EDP, TMAH, XeF2

poly-SiGe poly-Ge H2O2, hot H2O

Muhlstein et al.

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Today’s Lecture

• Introduction

• Thin film deposition

• Thin film etching techniques

• Material combinations

• Lateral resonator process flow

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Lateral Resonator

• Electrostatic force is applied by a fixed drive comb to a suspended shuttle

• Motion is detected capacitively by a fixed sense comb

• Operated at resonance

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Cross Section

anchor

comb finger

dimple

suspended plate

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Lateral Resonator Process Flow

• Substrate passivation and poly ground plane• n+ diffusion, 0.5 µm thermal oxide, 0.15 µm LPCVD nitride

• 0.3 µm phosphorus-doped LPCVD poly

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Lateral Resonator Process Flow

• Sacrificial layer deposition and patterning• 2 µm LPCVD PSG

• Densify and reflow PSG at 1000°C, 1 h• Timed etch to create dimples, wet etch• Etch anchors, reactive ion etch

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Lateral Resonator Process Flow

• Structural poly deposition, doping, and anneal• 2 µm LPCVD poly (undoped), 610°C

• 0.3 µm PSG on top

• Symmetric doping occurs during anneal at 1050°C in N2 for 1 hour

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Lateral Resonator Process Flow

• Microstructure release• HF to etch PSG• Water rinse• Dry, avoiding surface tension of water

anchorcomb finger

dimple

suspended plate

Sand

ia La

bs