Surface Micromachining Dr. Marc Madou, Fall 2012, UCI Class 10.
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Transcript of Surface Micromachining Dr. Marc Madou, Fall 2012, UCI Class 10.
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Surface Micromachining
Dr. Marc Madou,
Fall 2012, UCI
Class 10
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Blanket n+ diffusion of Si substrate (ground plane)
Passivation layer (e.g. SiO2 , Si3N4 , LPCVD Si3N4 on top of SiO2)
Opening up the passivation layer for contacts (observe color change or hydrophobic/hydrophilic behavior):
– wet (BHF)
– dry (SF6)
Strip resist in piranha (adds some oxide in the window)
Short BHF etch to remove thin oxide
n+
n
n+
ntSiO2
Lm
Lm + 2tSiO2Photoresist
Si3N4
Surface MicromachiningBasic Process Sequence (poly-Si)
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Deposition of a base, spacer or sacrificial layer-phosphosilicate glass (PSG)-CVD
Densification at 950 °C for 30-60 min in wet oxygen
Base window etching in BHF for anchors
Structural material deposition e.g. poly-Si (doped or undoped) from (CVD at about 600°C , 73 Pa and 125 sccm (standard centimeter cube per minute); at about 100Å/min) e.g.
Anneal of the poly-Si at 1050°C for 1 hour to reduce stress in the structure
PSG
Structural layer
SiH4 → Si + 2H2
nitride
Surface Micromachining
Basic Process Sequence (poly-Si).
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Basic Process Sequence (poly-Si)
Doping of poly-Si: in-situ, PSG sandwich and ion implantation
Patterning of structural material e.g. RIE in , say, CF4-O2
Release step, selective etching of spacer layer e.g. in diluted HF
RS(μm / )min >> Rm(μm / )min >> R i(μm )min
RS
Ri Rm
nitride
Surface Micromachining
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Etchants-Spacer and Microstructural LayerEtchant Buffer/Isolation Spacer MicrostructureBuffered HF(5:1, NH4F:conc. HF)
LPCVD Si3N4/thermal
SiO2
PSG Poly-Si
RIE us ing CHF3BHF (6:1)
LPCVDSi3N4
LP CVDSiO2
CVD Tungsten
KOH LPCVDSi3N4/thermal SiO2
Poly-Si Si3N4
Ferri c Chloride Thermal SiO2 Cu Polyimide
HF LPCVDSi3N4/thermal SiO2
PSG Polyimide
Phosphoric/Aceti cAcid/Ni tri c Acid(PAN or5:8:1:1water:phosphoric:aceti c:ni tri c)
Thermal SiO2 Al PE CVD Si3N4Nickel
Ammonium Iodide/ Iodine Alcohol
Thermal SiO2 Au Ti
EDP Thermal SiO2 Poly-Si SiO2
Generic principle of surface micromachining
Si
Si
Al
Etch access
Si
Sacrificial layer definition
Releasing diaphragm:phosphoric/acetic acid/nitric acid (PAN)
Polyimide diaphragm deposition
Surface Micromachining
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LPCVD of poly-Si Hot wall, horizontal reactor Reaction rate controlled--at lower
pressures and well controlled temperatures (100 to 200 wafers)
Poly-Si deposits everywhere requiring periodic cleaning (e.g. every 20 runs if each run deposits 0.5 µm)
Visit: http://mems.eeap.cwru.edu/shortcourse/partII_2.html and http://www-mtl.mit.edu:800/htdocs/tutorial.html
Surface Micromachining
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ABCDESi Phosphosilicate glass PolysiliconStiction Stiction during release:– Surface tension during drying pulls movable
members together (See also room temperature bonding of Si to Si and glass to Si)
– Solutions:» Stand-off bumps» Sacrificial polymer » Sacrificial poly-Si links to stiffen the structures» HF vapor» Freeze-drying water/methanol mixtures» Super critical cleaning
Stiction after release:– Hydrophobic monolayers– Rough surfaces– Bumps
Surface Micromachining
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Control of film stress With L=150 µm and W=t=2 µm, fo=10 to 100 kHz.
Annealing at high temperature (900-1150°C)
Fine-grained tensile vs large grained compressive
Doping elements Sandwich doping and annealing. Vary material composition e.g Si
rich Si3N4
In PECVD: change the RF power and frequency
In sputtering: gas pressure and substrate bias
fo ≈12π
4EtW3
ML3 +24σ rtW5ML
Polysilicon Drive combSense combAnchorFlexure ( length L, width W, thickness t)Drive combcontact padSense contact padyxSuspended shuttle (mass M)
Surface Micromachining
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Control of film stress
MMMM
Folding flexures makes the resonant frequency independent of the residual stress but warpage becomes an issue Corrugated structural members (see above)
Y
X
ky >> kx( )force constant
fx =kx
M ⎛ ⎝
⎞ ⎠
12
Surface Micromachining
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Sealing processes Microshells a wafer level
packaging strategy Thin gaps (e.g. 100 nm) are etched
out and then sealed:– Reactive sealing by oxidation
– LPCVD deposition
Surface Micromined Sealed ResonatorSiO2Si Polysilicon VacuumResonatorAEtched spacerCavityO2B
COMPOSITE SI3 N4 /POLYSILICONPOLYSILICON PIEZORESISTOR
Surface Micromachining
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IC compatibility
CMOS SurfaceMicromachining
Common Features Silicon based processes Same materials Same etching principles
Process flow Standard Application specificVertical dimens ion ~ 1 µm ~1-5 µmLateral dimension <1 µm 2-10 µmComplexity (# masks) >10 2-6
Temperature (°C) MaterialLP CVD Depos ition 450 Low Temperature Oxide
(LTO)/PSG" " 610 Low stress poly Si" " 650 Doped poly Si" " 800 NitrideAnnealing 950
1050PSG densificationPoly Si stress annealing
Comparison of CMOS and Surface Micromachining
Critical Process Temperatures for Microstructures
- Junction migration at 800 to 950°C- Al interconnect suffers at 400-450 °C- Topography
Surface Micromachining
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Poly-Si surface micromachining modifications: porous poly-Si
Just like we can make porous Si from single crystal Si we can do the same with poly-Si (low currents densities in highly concentrated HF)
Applications:– Channels parallel to a flat surface
(switch from porous to polishing and back--chambers with porous plugs)
– Enclosed chambers (blisters of free poly-Si)
– Hollow resonators (higher Q)
CVD Si3N4
CVD poly-Si
CVD Si3N4
Surface Micromachining
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Poly-Si surface micromachining modifications: hinged poly-Si
Make structures horizontally and erect them on a poly-Si hinge (probe station)---rigid structures (Prof. Pister, UCB)
Polyimide hinges also have been made ( butterfly wing)---movable structures
Poly 1OxidePoly 2Poly 2Poly 1Poly 2 staple holdingpoly 1 plate
polyimide hinge (E= 3 GPa) poly-Si hinge (E= 140 GPa)
Surface Micromachining
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Poly-Si surface micromachining modifications:hinged poly-Si
Pister et al., UCB
Micromachined integrated optics for free space interconnections
Surface Micromachining
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Poly-Si surface micromachining modifications: thick poly-Si and HEXSIL
Thick poly-Si--10 µm in 20 ‘ with SiH2Cl2 at 1000°C has become possible (low tensile stress)
HEXSIL (Dr. Keller, UCB):– Deep dry etching of trenches in SCS
(e.g. 100 µm deep)-short isotropic etch to smooth the walls
– Deposition of sacrificial and structural materials (undoped, doped poly-Si and metal e.g. Ni)
– Demolding by etching away the sacrificial material
Surface Micromachining
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Poly-Si surface micromachining modifications: HEXSIL
Dr. Keller, UCB
Membrane filter with stiffening rib HEXSIL tweezers
Surface Micromachining
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Poly-Si surface micromachining modifications: SIMOX
Types of Silicon On Insulator (SOI) processes:
– SIMOX (Separated by IMplanted OXygen)
– Si fusion bonded wafers
– Zone-melt recrystallized polysilicon (ZMR)
SIMOX substrate
0.2 μ -m epi Si
0.4 μ m buried oxide
Thicken up epi Si to
4 μ m
Dry etch access hole
S acrificial layer etching Hermetic sealing of etching hole by plasma CVD of
non stressed dielectric plug
Metallization and diaphragm definition
PP
Surface Micromachining
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Non-poly-Si surface micromaching.
Polyimide: e.g. SRI flat panel display
UV depth lithography– AZ-4000 (high viscosity, many
layers)
– SU-8 (IBM)
Surface Micromachining
Capp Spindt