3.Microfabrication_Techniques_for_MEMS.pdf

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Microfabrication techniques for MEMS
Hyun Soo Kim
Department of Electronic Engineering

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Prof. Jaewon Park, MicroChip BioSystems Lab.
Department of Electrical and Electronic Engineering, Southern University of Science and Technology
MEMS Processing
https://www.youtube.com/c/samsungsemicond
https://www.youtube.com/c/samsungsemiconductor/featured

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MEMS Processing
▪ Thin film deposition
− Dielectric material deposition
− Metal deposition
▪ Lithography
▪ Etching
− Dry etching
− Wet etching
▪ Electrodeposition, LIGA, Lift-off

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Photolithography Equipment

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Typical Microfabrication Process
▪ Deposition
− Spin-coating, reactive growth, chemical vapor deposition, evaporation,
electroplating
▪ Lithography
− Various wavelength (UV, X-ray, electron beam)
▪ Etching
− Wet chemical etching
− Dry etching
New wafer
Finished
wafer
Cutting & dicing
Packaging
Adding
(deposition)
Patterning
(lithography)
Subtracting
(etching)

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Deposition/Lithography/Etching

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Thin Film Deposition – Thermal Oxidation
▪ Growing or depositing SiO2
− For insulation, masking, sacrificial layer
▪ Thermal oxidation (on Si wafer)
− Dry oxidation: Si + O2-> SiO2
✓ High quality but slow deposition time
✓ 120A/hour at 1000℃
− Wet oxidation: Si + 2H2O-> SiO2 + 2H2
✓ Faster deposition time but lower quality
✓ 1200A/hour at 1000℃
− High temperature (800-1100℃)
− Dry/Wet/Dry oxidation step
www.gla.ac.uk
http://design.lbl.gov/

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Thin Film Deposition – CVD
▪ Chemical Vapor Deposition (CVD)
− Plasma-enhanced CVD (PECVD), Low-pressure CVD (LPCVD)
− SiO2,PSG (phosphosilicate glass), BSG(borosilicate glass), LTO (Low temperature
oxide, combination of phosphorus and boron doped)
− Each type has different composition, step coverage, density, refractive index,
stress, dielectric strength, and etch rate
− SiN is also commonly deposited using CVD
✓ 3SiH4 + 4NH3-> Si3N4 + 12H2
✓ Excellent dielectric material
✓ Typically dry-etched

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Metal Deposition – Evaporation (PVD)
▪ Heating methods: Resistive, E-beam, inductive, RF, Laser
− Thermal (Filament) evaporation
✓ (+) Cheap
✓ (-) Precise control of thickness and evaporation rate is hard
✓ (-) Contamination form filament
✓ (-) Hard to evaporation certain metals (e.g. platinum)
− Electron beam (E-beam) evaporation
✓ (+) Good thickness and evaporation rate control
✓ (+) Higher quality film
✓ (+) Fewer contamination problems
✓ (+) Less substrate heating
✓ (-) Expensive
✓ (-) Radiation damage

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Metal Deposition – Thermal Evaporation
▪ Low vacuum
▪ Higher temperature compared to E-beam
▪ Often uses metal boats
vacaero.com http://angstromengineering.com

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Metal Deposition – E-beam Evaporation
▪ Properties
− Physical Vapor Deposition
− Ultra high vacuum (< 10-6 Torr)
− Small metal source, Point source
− No or little step coverage
− Can be utilized for metal patterning (Lift-off process)

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Metal Deposition – Sputtering (PVD)
▪ Sputtering
− Pressure: 10s mTorr
− Ar ions bombard the metal source to drive out aggregation of metal atoms
− Sputtered metal atoms expect more collision
− Almost any material can be sputtered (e.g., metal, alloy, dielectric compound)
− Deposition < 1-2 µm (< 10 Å/s)
− Low temperature
− Better step coverage (side wall deposition)

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Metal Deposition – Sputtering (PVD)

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Thin Film Deposition – Electroplating
▪ Properties
− Metal ions move toward the negatively charged “Target” and bond to the surface
(e.g., Au, Ag, Cu, Ni, Pt, etc.)
▪ Advantages
− Thick deposition: 10s µm
− High deposition rate: 10s µm/hr
− Process done at room temperature
▪ Disadvantages
− “Seed layer” required
− Uniformity issue for large deposition area
http://areeweb.polito.it/

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Electroplating

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Photolithography

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Spin Coating
▪ Photoresist coating
− For uniform thickness coating.
− Typically 3000 - 6000 rpm for 15-60 seconds.
− Resist thickness primarily depends on resist viscosity and spin speed
− Resist thickness
𝑡 =
𝑘𝑝2
𝑤1/2
k = spinner constant, typically 80-100,
p = resist solids content in percent
w = spinner rotational speed
Photoresist
Si wafer
Spin-coated PR layer

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Spin Coating
▪ Spinning Artifacts
− Edge Bead
✓ Residual ridge in resist at edge of wafer
✓ Can be up to 20-30 times the nominal thickness of the resist
✓ Edge bead removers are solvents that are spun on after resist coating and which
partially dissolve away the edge bead
2009.igem.org
sensorsmag.com/
elveflow.com

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Spin Coating
▪ Spinning Artifacts
− Striations: Variations in resist thickness (~30 nm) due to non-uniform drying
− Streaks: Radial patterns caused by hard particles
− Etc.
http://large.stanford.edu

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Photoresist
▪ Photo-sensitive polymer
▪ Positive Photoresist
− Exposure to light destroys the
polymer crosslink and becomes more
soluble in developer
(e.g. S1818, S1805)
▪ Negative photoresist
− Exposure to light forms crosslink and
becomes less soluble in developer
(e.g. SU-8, Futurrex)
www.microchem.com

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Photoresist Coating
▪ Wafer priming
− Adhesion promoters are used to assist resist coating.
− Resist adhesion factors:
✓ Moisture content on surface
✓ Wetting characteristics of resist
✓ Delay in exposure and prebake
✓ Resist chemistry
✓ Surface smoothness
✓ Surface contamination
− Ideally want no H2O on wafer surface
✓ 15 minutes in 80-90ᵒC convection oven

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Photoresist Coating
▪ Wafer priming
− Silicon wafers
✓ Primers form bonds with surface and produce a polar (electrostatic) surface
✓ 1,1,1,3,3,3-hexamethyldisilazane (HMDS)
✓ Trichlorophenylsilane (TCPS)
✓ Bistrimethylsilylacetamide (BSA)
✓ Omnicoat for SU-8 series
− Gallium arsenide wafers
✓ GaAs already has a polar surface
✓ Monazoline C
✓ Trichlorobenzene
✓ Xylene

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Photoresist Coating
▪ Soft bake
− Used to evaporate the coating solvent and to densify the resist after spin coating.
− Baking inside a convection ovens or hot plates
✓ Convection ovens:
• Solvent at surface of resist is evaporated first, which can cause resist to develop
impermeable skin, trapping the remaining solvent inside
• Heating must go slow to avoid solvent burst effects
✓ Conduction (hot plate):
• Need an extremely smooth surface for good thermal contact and heating uniformity
• More thoroughly evaporating the coating solvent
• Generally much faster and more suitable for automation
− Commercially, microwave heating or IR lamps are also used in production lines.
− The thickness of the resist usually decrease by 25 %
− Less prebake increases the development rate

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Photoresist Coating
▪ Hard bake (Post bake)
− Used to stabilize and harden the developed photoresist
− Main parameter is the plastic flow or glass transition temperature
− Removes any remaining traces of the coating solvent or developer
− Introduces some stress into the photoresist
− Some shrinkage of the photoresist may occur
− Longer or hotter post bake makes resist removal much more difficult
▪ Firm post bake is needed for acid etching, e.g. BOE.
▪ Post bake is not needed for processes in which a soft resist is
desired (e.g. metal liftoff patterning)
▪ Resist reflow
− With sufficient time and/or temperature:
− Can be used for tailoring sidewall angles.

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Photoresist Removal
▪ Want to remove the photoresist and any of its residues
▪ Simple solvents are generally sufficient for non- post baked
photoresists
− Positive photoresists: Acetone, trichloroethylene (TCE), phenol-based strippers
(Indus-Ri-Chem J-100)
− Negative photoresists: methyl ethyl ketone (MEK), methyl isobutyl ketone
(MIBK),
▪ Plasma etching with O2 (ashing) is also effective
▪ Shipley 1165 stripper (contains n-methyl-2-pyrrolidone)
− Effective on hard, post baked resist.

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Photolithography
▪ Lithography
− Pattern definition process
− Generation and transfer of patterns by exposing radiation on a substrate
− UV light, X-ray, E-beam, Ion-beam, Laser
▪ Photolithography (Optical Lithography)
− Transfer of patterns from a mask
to a substrate by exposing UV light
through a mask on a substrate
− Mask
✓ Created by direct writing (E-beam)
or lattice exposure on a
photoresist/Cr coated glass
− Series of pattern transfer using
alignment marks
− UV light source:
✓ I line (365 nm)
✓ H line (405 nm)
✓ G line (436 nm)
http://www.nano-ou.net/

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Photolithography
▪ Lithography
− Pattern definition process
− Generation and transfer of patterns by exposing radiation on a substrate
− UV light, X-ray, E-beam, Ion-beam, Laser
▪ Photolithography (Optical Lithography)
− Transfer of patterns from a mask
to a substrate by exposing UV light
through a mask on a substrate
− Mask
✓ Created by direct writing (E-beam)
or lattice exposure on a
photoresist/Cr coated glass
− Series of pattern transfer using
alignment marks
− UV light source:
✓ I line (365 nm)
✓ H line (405 nm)
✓ G line (436 nm)

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Mask Aligner
Photo Mask
Si wafer
Photoresist
PR Spin-coat
UV Light
UV Exposure

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Photomask
▪ Mask polarity
− Bright field:
✓ Mostly clear
✓ Drawn feature = Opaque
− Dark field
✓ Mostly opaque
✓ Drawn feature = Clear
www.mems-exchange.org

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Photomask
▪ Alignment marks
− Use alignment marks on mask and wafer to register patterns prior to exposure.
− Modern process lines (steppers) use automatic pattern recognition and
alignment systems.
− Human operators usually take 30-45 seconds at least
− Normally requires at least two alignment mark sets on opposite sides
− Use a split-field microscope to make alignment easier
www.memnet.org

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Photomask
▪ Alignment marks
− Use alignment marks on mask and wafer to register patterns prior to exposure.
− Modern process lines (steppers) use automatic pattern recognition and
alignment systems.
− Human operators usually take 30-45 seconds at least
− Normally requires at least two alignment mark sets on opposite sides
− Use a split-field microscope to make alignment easier
www.memnet.org

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Etching
▪ Etching
− Processing step to remove materials that is not covered by a protective mask
− Selectivity: Depending on the target material to be etched, masking material and
etching process has to be decided
▪ Etching methods
− Wet Etching : Liquid etchant removes the target material
− Dry Etching : Chemically active species in gaseous state removes the target
material
▪ Etching profile
− Isotropic etching : Etching in all direction
− Anisotropic etching: Etching in certain direction

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Etching – Wet Etching
▪ Advantages
− Simple
− Stable and reliable
− Low-cost compared to dry etching
− High uniformity over the whole sample
▪ Disadvantage
− Mostly isotropic etching
− Causes ‘Under cut’
− Difficult to precisely control the process
Under-etch Perfect-etch Over-etch
http://iopscience.iop.org

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▪ Silicon Isotropic etching
− HNA: Hydrofluoric acid + Nitric acid + Acetic acid
▪ Silicon anisotropic etching
− KOH or TMAH ( Tetramethyl ammonium hydroxide)
− Etch rate is high in <100> plane
− Etch rate slowest in <111> plane
▪ Etch Stop control
− Etching time
− Doping
G. T. A. Kovacs, “Micromachined Transducer Sourcebook”
S. M. Sze, “Semiconductor Sensors”

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▪ Dielectric material
− SiO2: Diluted HF
− SiN: Heated H3PO4 (120-200⁰C)
▪ Metal
− Titanium: HF
− Chromium: CR-7, HCI, Phosphoric acid
− Gold: HCI + HNO3,KI + I2 + H2O
− Platinum: HCI + HNO3
− Copper: HNO3 + H2O, H2SO4, CR-7
− Aluminum: Phosphoric acid, HCI + HNO3
 Selectivity is important in etching!!!

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Etching – Dry Etching
▪ Reactive Ion Etching (RIE)
− Plasma etching + ion sputtering
− Chemical + physical etching
− SiO2,SiN, Polymers
− Anisotropic etching (relatively)
− Gas: CF4, SF6, CHF3, CCI4 + O2
− Advantages
✓ Process precisely controllable
✓ Relatively anisotropic etching
✓ Capable of etching materials difficult
with wet etching (e.g., SiN, Teflon)
− Disadvantages
✓ Expensive equipment and toxic gases involved
✓ Characterization not easy

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Etching – Dry Etching
▪ Deep Reactive Ion Etching (DRIE)
− High aspect ratio silicon etching method
− Inductive Coupled Plasma (ICP)
✓ Provides directionality to the etching process
− Bosch Process
✓ Alternate between etching (SF6) and sidewall passivation (flurocarbon polymer) steps
✓ High-aspect ratio, deep etching, high selectively
http://www.semiconductor-technology.com/
http://www.iue.tuwien.ac.at/

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Metal Patterning – Lift-off Process
▪ Method to create patterned metal structures without direct etching
▪ Advantages
− No etching involved
− No compatibility issues
− Useful when suing metals that are hard to etch
− Do not require expensive dry etching equipment
▪ Disadvantages
− More complicated lithography process
− Resolution as not as good as direct patterning/etching

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LIGA Process
▪ LIGA (Lithographie, Galvanofomung, Abformung)
− Lithography, electroplating, injection molding
− Capable of generating extremely thick (> 500 µm) straight wall structures
− Requires synchrotron radiation X-ray source and special mask
www. lafayette.edu

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Things to Consider I
▪ Photolithography
− Smallest feature size
− Type of exposure tool (UV, X-ray, e-beam)
− Type of photoresist
− Thickness of photoresist
− Backing temperature and time
− Exposure dosage
− Development time
▪ Deposition
− Deposition quality
− Deposition rate
− Deposition profile
− Deposition temperature
− Examples
✓ Metal deposition: Evap[oration, sputtering
✓ SiO deposition: Oxidation, LPCVD, PECVD, spin-on
✓ SiN deposition: LPCVD, PECVD

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Things to Consider II
▪ Etching
− Etching rate
− Etching selectivity
✓ Compatibility with deposition materials on wafer
✓ Selectivity with photoresist
− Etching profile
− Etching condition (temperature)
− Sensitivity to over-etch

3.Microfabrication_Techniques_for_MEMS.pdf

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