Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
1
Surface Micromachining: Techniques and Materials
Dr. Thara SrinivasanLecture 3
Picture credit: Texas Instruments
2
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
2
3
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
4
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
3
5
Bulk Micromachining Review
• Bulk etching and metallization
• Possible structures
6
Lecture Outline
• Today’s Lecture• Introduction
• Thin film deposition
• Thin film etching techniques
• Material combinations
• Lateral resonator process flow
4
7
Surface Micromachining
8
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
5
9
One Structural Poly, One Oxide Process
Lateral resonator with electrostatic comb drives, Sandia Labs
10
Surface Micromachining
• Basic process sequence• Structural layer
• Sacrificial layer
• Release etch
Meshing gears on a moveable platform, Sandia
Digital Micromirror Device, Texas Instruments
6
11
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)
12
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
7
13
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
14
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
8
15
Today’s Lecture
• Introduction
• Thin film deposition techniques
• This film etching techniques
• Material combinations
• Lateral resonator process flow
16
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
9
17
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
=
18
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
10
19
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
20
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
11
21
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)
22
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
12
23
• 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
24
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.
13
25
Stress Gradients
• Stress gradient: (+) or (-)compressive
tensile
+–
26
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
14
27
Controlling PolySi Stress
• Resonant frequency for lateral resonator, L = 150, W = 2 µm.
ML
tW
ML
tWEf ry
5244
21
3
3
0σ
π+≈
Stress term dominates
28
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
15
29
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
30
• 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
16
31
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
32
• 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
17
33
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
34
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
18
35
Today’s Lecture
• Introduction
• Thin film deposition
• Thin film etching techniques
• Material combinations
• Lateral resonator process flow
36
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
19
37
Wet etchants, Williams etch table
38
Today’s Lecture
• Introduction
• Thin film deposition
• Thin film etching techniques
• Material combinations
• Lateral resonator process flow
20
39
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.
40
Today’s Lecture
• Introduction
• Thin film deposition
• Thin film etching techniques
• Material combinations
• Lateral resonator process flow
21
41
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
42
Cross Section
anchor
comb finger
dimple
suspended plate
22
43
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
44
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
23
45
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
46
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