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MEMS FabricationProcesses
Micro machiningBulk Micro machining, Surface MicromachiningDeep RIE, Advanced LithographyHEXIL & SCREAM ProcessPolymer molding and LIGA Process
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Bulk, Surface, DRIE
Bulk micromachining involves removingmaterial from the silicon wafer itself Typically wet etched Traditional MEMS process Artistic design Inexpensive equipment Issues with IC compatibility
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Bulk, Surface, DRIE Surface micromachining leaves the wafer
untouched, but adds/removes additionallayers above the wafer surface, First widelyused in 1990s Typically plasma etched IC-like design philosophy, relatively expensive
equipment Different issues with IC compatibility
Deep Reactive Ion Etch (DRIE) removessubstrate but looks like surfacemicromachining!
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Bulk Micromachining
Many liquid etchants demonstrate dramatic etchrate differences in different crystal directions etch rate is slowest, and faster Fastest:slowest can be more than 400:1 KOH, EDP (ethylene diamine pyrocatechol), TMAH (tetra
methyl ammonium hydroxide) most common anisotropicsilicon etchants
Isotropic silicon etchants Acids
HF, nitric, and acetic acids Lots of neat features, tough to work with
XeF 2, BrF 3 gas phase, gentle
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Bulk Micromachining
Choosing a method
Desired shapes
Etch depth and uniformity
Surface roughness
Process compatibility
Safety, cost, availability, environmental impact
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Anisotropic Etching of Silicon
Anisotropic etches have direction dependent etch rates in crystals Typically the etch rates are slower perpend icularly to the crystalline
planes with the highest density Commonly used anisotropic etches in silicon include Potassium
Hydroxide (KOH), Tetramethyl Ammonium Hydroxide (TMAH), andEthylene Diamine Pyrochatechol (EDP)
Silicon Substrate
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Anisotropic Etching of Silicon
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
Etching of Si with KOH
Si + 2OH - Si(OH) 22+ + 4e -
4H2O + 4e - 4(OH) - + 2H 2
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Undercutting
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TMAH, Tetramethyl ammonium hydroxide, 10-40 wt.% (90C )Etch rate (100) = 0.5-1.5 m/minAl safe, IC compatibleEtch ratio (100)/(111) = 10-35Etch masks: SiO 2 , Si 3N4 ~ 0.05-0.25 nm/minBoron doped etch stop, up to 40 slower
EDP (115C)Carcinogenic, corrosiveEtch rate (100) = 0.75 m/minAl may be etchedR(100) > R(110) > R(111)Etch ratio (100)/(111) = 35Etch masks: SiO 2 ~ 0.2 nm/min, Si 3N4 ~ 0.1 nm/minBoron doped etch stop, 50 slower
Other Anisotropic Etchants
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Electrochemical Etching ofSilicon
Application +ve voltage to Si makes holesavailable at Si-electrolyte interface.
2
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He2H2
cathodeAtH2SiFHF2SiF
SiFh2F2Si
anodeAt
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H2 bubbles at the ve Pt electrode. Etch rate depend on the doping
level. Heavily doped silicon etches faster
through electrochemical etching Shinning light on the silicon can
increase the etch rate.
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Electrochemical Etching ofSilicon
Doping Dependence : Heavily doped silicon etches faster
through electrochemical etching n+
n-epi
Electrochemicaletch
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Electrochemical Etching ofSilicon
p & n type Si For voltages higher than Passivation Point (PP), SiO 2 is
formed on surface and the dissolution stops. PP for p-Si is higher than that of n-Si.
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Etch Stops in AnisotropicSilicon Etching
High boron doping Control etch depth precisely with
boron doping (p ++) [B] > 10 26 m -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 unavailablefor reactions to give OH -
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Lithography &B implant
Diffusion
orifice
membrane
Use of B Etch Stops
AnisotropicEtch
Stripping
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Micro-nozzle
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Micromachining Ink Jet Nozzles
Microtechnologygroup, TU Berlin
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Electrochemical Etch Stops
Etch stops at the p-n junction.
Such stop defines thethickness of the membrane ofthickness of the n-Si epilayer.
Electrochemical etch stop
n-type epitaxial layer grown on p-type wafer forms p-n diode
p-n diode in reverse bias Passivation potential potential
at which thin SiO 2 layer forms,different for p- and n-Si
p-substrate floating etched
n-layer above passivationpotential not etched
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Etch Stops in AnisotropicSilicon Etching
Electrochemical etching on preprocessed CMOS wafers N-type Si well with circuits suspended from SiO 2 support
beam Thermally and electrically isolated TMAH etchant, Al bond pads safe
Oxide Beam Support
Al Metallization
Circuit OxidePassivation
Pit etched in substrate
Suspended well
P-type substrate
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MEMS Pressure Sensor
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MEMS Pressure Sensor
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Bulk Micromachining Anisotropic etching allows
very precise machining ofsilicon
Silicon also exhibit a strongpiezoresistive effect
These properties, combinedwith silicons exceptionalmechanical characteristics,and well-developedmanufacturing base, makesilicon the ideal material forprecision sensors
Pressure sensors andaccelerometers were the firstto be developed
Silicon pressure sensor chip
Packaged pressure sensor
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Bulk micromachined cavities
Anisotropic KOH etch(Upperleft)
Isotropic plasma etch (upperright)
Isotropic BrF 3 etch with
compressive oxide stillshowing (lower right)
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Bulk, Surface, DRIE Surface micromachining leaves the wafer
untouched, but adds/removes additionallayers above the wafer surface, First widelyused in 1990s Typically plasma etched IC-like design philosophy, relatively expensive
equipment Different issues with IC compatibility
Deep Reactive Ion Etch (DRIE) removessubstrate but looks like surfacemicromachining!
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Surface Micromachining
Deposit sacrificial layer Pattern contacts
Deposit/pattern structural layer
Cantilever Fabrication:
Etch sacrificial layer
anchor
cantilever
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Surface Micromachining
Cantilever Fabrication:
Depositsacrificial layer& pattern
Deposit structurallayer and pattern
Etch awaysacrificial layer
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Surface Micromachining
anchor
cantilever
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Surface Micromachining
1. Electrostatic forceis applied by adrive comb to asuspended shuttle
2. Motion is detectedcapacitively by asense comb
3. Operated atresonantfrequency
Anchor
Drive combcontact pad
Sensecombcontactpad
Sense
comb
Drive comb
Suspendedshuttle
Flexure
y
x
C. T.-C. Nguyen and R. T. Howe, IEEE IEDM , 1993
Lateral Resonator:
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Surface MicromachiningLateral Resonator:
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Surface MicromachiningLateral Resonator:One structural poly and one oxide process
Lateral resonator with electrostatic comb drives, Sandia Labs
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Surface Micromachining
Lateral Resonator Fabrication:
Microstructure Release
HF to etch PSG
Water rinse
Dry, avoiding surface tension ofwater
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Surface Micromachining
Meshing gears on amoveable platform, SandiaLab
Digital Micromirror Device,Texas Instruments
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Surface MicromachiningHinges:
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MEMS Hinged Mirror
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MEMS Hinged Mirror
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Make structures with hinges as in nature hinge is asoft material in butterfly
Polyimide hinges have been made ( butterfly wing)---
movable structures E = 3 GPa against E = 140 GPafor poly-Si
MEMS Hinged StructurePolyimide Hinges :
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Surface MicromachiningMaterial Systems
Structural Sacrificial Etchantlayer layer
Polysilicon SiO 2 /BPSG HF SiO 2 Polysilicon XeF 2 Aluminum Photoresist oxygen
/plasma Photoresist Aluminum Al etch Poly-SiGe Poly-Ge H 2O2+hot H 2O
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100-150Cl2 + SiCl 4660H3PO 4:HNO 3:CH3COOH
Al
35-3500O2>4000AcetonePhotoresist
150-250SF 65H3PO 4Si3N4
--40KIGold
50-150CHF 3 + O 220-2000HF:NH 4FSiO 2
170-920SF 6 + He120-600HNO 3:H2O:HFPoly Si
Etch rate(nm/min)
Dry etchantEtch rate(nm/min)
Wet etchantMaterial
Etch Systems
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Micromotor
Nitride Poly 0 Oxide 1 Oxide 2Poly 1 Poly 2 Metal
Deposit Poly 0On nitride
MUMPSFoundryProcess
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Micromotor
DepositOxide 1
Pattern Poly 0
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Micromotor
Etch foranchordefinition
Etch dimplesInto Oxide 1
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Micromotor
Poly 1patterned &etched
Deposit Poly 1
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Micromotor
PatternOxide 2
DepositOxide 2
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Micromotor
Deposit Poly 2
Etch thrufor anchor
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Micromotor
Depositmetal bylift-off
Pattern Poly 2
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Micromotor
Etch sacrificial oxide layer to give thestator and rotor of the micromotor
rotorstator
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Required: high etch rates, high aspect ratios requires very high degrees of anisotropy
Bosch process (German patent: Larmer and Schilp, 1994) uses recombinant species and side-wall polymer formation
Sequential etch / polymer deposition high bias reactive ion etch: SF 6 / Ar typical low bias polymerization: C 4F 8, CHF 3 repeat
Usually need high density plasma source inductively coupled/ECR
Aspects ratios up to about 30:1 Etch rate: few microns per minute Selectivities
to PR : 50-100 to oxide 100-200
DRIE
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Deep Reactive Ion Etch
BOSCH PatentSTS, Alcatel, Trion, Oxford Instruments
Uses high density plasma to alternativelyetch silicon and deposit a etch-resistantpolymer on side walls
Silicon etch usingSF 6 chemistry
Polymer deposition
Polymer
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Deep Reactive Ion Etch
Inductively Coupled Plasma : Intense plasma generation using
induction power
Pressure 1-10 mtorr
Negative substrate bias - 1KV
-Vbias
Quartzvessel
plasma
Inductioncoil
To pump
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1111 mmmm
Scalloping and Footing in DRIE
S c a l l o p e
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Top wafer surfaceTop wafer surfaceTop wafer surfaceTop wafer surface
cathode cathode cathode cathode Top wafer surfaceTop wafer surfaceTop wafer surfaceTop wafer surface
anode anode anode anode
Tip precursorsTip precursorsTip precursorsTip precursors
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S c a l l o p e d
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Top wafer surfaceTop wafer surfaceTop wafer surfaceTop wafer surface
cathode cathode cathode cathode Top wafer surfaceTop wafer surfaceTop wafer surfaceTop wafer surface
anode anode anode anode
Tip precursorsTip precursorsTip precursorsTip precursors
10 micron gap
microgrid Footing at the bottom ofdevice layer
Milanovic et al, IEEE TED, Jan. 2001.
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DRIE StructuresAdvantages : Increased capacitance
for actuation andsensing
Low-stress structures single-crystal Si only
structural material
Highly stiff in verticaldirection isolation of motion to
wafer plane
flat, robust structures2DoF Electrostatic actuator
Thermal Actuator
Comb-drive Actuator
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SCREAM
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Extreme UV Photolithography
Immersion Lithography
E-beam lithography
X-ray lithography
Soft Lithographic Processes
Stereo Lithography
Advanced Lithography
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Microcontact Printing(Developed by Whitesides, et. al. at Harvard) Elastomerics tamp Patterns of self-assembled monolayers (SAMs) and proteins SAMs allow a variety of surface modifications
Thickness variation by changing tail lengthModification of tail group changes surface propertiesVariety available for different substrate materials
Other SAM advantagesSelf healing and defect rejectingUltrathin resists and seed layersDo not require clean room facilitiesLow cost
Fabricated using a PDMS mold of photoresist structure
Soft Lithography
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Imprint Lithography
Molecular Imprints Co.
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Imprint Lithography
Advantages: Resolution not limited by wavelength of light or numerical aperture Tools have longer life High throughput at high resolution
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Imprint LithographyEtch Process :
University of Texas
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Imprint Lithography
10nm DiaPillar Mold 10nm Dia Resist Hole byImprint
10nm Dia Metal Dotsby Imprint and Lift-off
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Stereo Lithography
Light beam photo-polymerizes the liquid resin solidifying it. After one scan of the beam, the work is lowered to deposit the
next layer. A 3-D pattern can be written.
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Stereo Lithography
One can invert the processby shining the light from thebottom.
A set of masks can be usedin place of scanning beamfor exposure.
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Sub-Micron Stereo Lithography
Layer build up
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Sub-Micron Stereo Lithography
Micro Electro Mechanical SystemsJan., 1998 Heidelberg, Germany
gear wit h f our t ee th ge ar wit h eight t eet h
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Micro Electro Mechanical SystemsJan., 1998 Heidelberg, Germany
micro-turbine.
An object made of three imbricated springs. This structureconsists of 1000 layers of 5mm each, built along the axisdirection.
Sub-Micron Stereo Lithography
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Micro Electro Mechanical SystemsJan., 1998 Heidelberg, Germany
Plastic watch gear, total height:1.4 mm.
Two level SU-8 structure with anadded axle.
Sub-Micron Stereo Lithography
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Microfabrication Technology
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Polymers for Microfabrication
Advantages over silicon Inexpens ive Flexible Transparent to visible/ UV Easily molded Surface properties easily modified Improved biocompatibility or bioactivity
Disadvantages Low thermal stability Low thermal and electrical conductivity Techniques for fabrication on microscale
not as well developed
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Polymers for MicrofabricationExamples diverse polymers used
PDMS PMMA Polyurethane Polyimide Polystyrene
Polydimethylsiloxane (PDMS)Advantages Deforms reversibly Can be molded with high fidelity Optically transparent down to ~300 nm Durable and chemically inert Non-toxic Inexpens ive
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LIGA ( lithographie, g alvanoformung, abformtechnik)uses x-ray l ithography (PMMA), electro-deposition andmolding to produce very high aspect ratio (>100)microstructures up to 1000 m tall (1986)
LIGA Process
LI thographie LithographyGalvanoformung ElectroformingA bformung Moulding
Dr. Ehrfeld, Karlsruhe Nuclear Research Centre, Germany (1986)
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LIGA Process
X-ray
Mask
Resist
Substrate
DevelopResist
Electro-plate
Metal Mold
Embossing
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LIGA Process
1st electroforming: X-ray exposure
(irradiation) developing electroforming for
final metal productor for mold insert
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LIGA ProcessPlastic molding
and 2 ndElectroforming/casting slip
Plastic finalstructures or lostmold
Metal or ceramicfinal parts
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LIGA Process
Exposure station and masks Mask :
low Z membrane
high Z absorber
Alignment of substrate with mask is
difficult since no visible light can
pass through the mask membrane
Sample is moved vertically through
the irradiation band with a precision
scanner
Absorber structure Bemembrane
MaskFrame
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LIGA Process
Mask Materials: Handling ring Pyrex,
glass, metal
Carrier Si(B), SiC,
SiN, Si, Be, Ti, C thickness from
~2-5 m up to 200 m
Absorber Au,W(Si,N), Ta(Si,N)
thickness from
~0.5-1 m up to 50 m
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Molding Reaction injection molding
(RIM): Mixed reagents pumped into
the mold Injection molding:
Mold is kept above the glasstransition temperature andmolten plastic is injected (e.g.,CDs)
Compression molding (also hotembossing):
A molding tool is pressed intothe plastic material attemperatures above the glasstransition temperature
LIGA Process
Demolding :Demolding requires extra smooth walls and internal mold release agents