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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

54.7

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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

42

2

He2H2

cathodeAtH2SiFHF2SiF

SiFh2F2Si

anodeAt

====++++

++++++++

++++++++

++++

++++

++++

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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Doping Dependence : Heavily doped silicon etches faster

through electrochemical etching n+

n-epi

Electrochemicaletch

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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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Cantilever Fabrication:

Depositsacrificial layer& pattern

Deposit structurallayer and pattern

Etch awaysacrificial layer

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anchor

cantilever

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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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Lateral Resonator Fabrication:

Microstructure Release

HF to etch PSG

Water rinse

Dry, avoiding surface tension ofwater

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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

d s i d e w a l

l

S c a l l o p e d

s i d e w a l l

S c a l l o p e d

s i d e w a l l

S c a l l o p e

d s i d e w a l

l

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

S c a l l o p e

d s i d e w a l

l

S c a l l o p e d

s i d e w a l l

S c a l l o p e d

s i d e w a l l

S c a l l o p e

d s i d e w a l

l

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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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-turbine.

An object made of three imbricated springs. This structureconsists of 1000 layers of 5mm each, built along the axisdirection.


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Plastic watch gear, total height:1.4 mm.

Two level SU-8 structure with anadded axle.


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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

- structure

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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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F o r u s e o

f B P U T M

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h .

S t u d e n

t s o n

l y

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

12.memsfab

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