2.008 design & manufacturing iiweb.mit.edu/2.008/www/lectures/2.008-2005-mems-2.pdfby thin film...
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2.008, S.G. Kim, MIT
2.008 Design & Manufacturing II
Spring 2005Sang-Gook Kim
MEMS, Tiny Products
2.008, S.G. Kim, MIT
Wireless Sensor Nodes
Millennial's i-Bean nodes
Crossbow Technology
Intel, Berkeley
Low Power ConsumptionEnergy density, weightStorageHarvesting
Ensuring security & diversity of supplyPromoting sustainable developmentConversion efficiency$$$
Energy Research
Portable Energy Research
2.008, S.G. Kim, MIT
Portable Energy
Modified with SOURCE: Carlin, Richard, ONR, Electric Power Sources for the Navy and Marine Corps, Presented at the Navy Grand Challenges Workshop, November 16-18, 1999.
1 mW
1 µW
Piezo MEMS energy harvesting
MobilityWirelessSensing
?
Mobile Computing
EnergyHarvesting
2.008, S.G. Kim, MIT
Harvesting Ambient Vibration EnergyBy Thin Film Piezo
PZT
ZrO2
+ - + - + - +
membrane
Inter-digitated Electrode
E3
MIT PMPG (PiezoelectricMicro Power Generators)
Y. Jeon, R. Sood, and S.G. Kim, Hilton Head Conference 2004, and will appear Sensors and Actuators A Physical
L=170µm
t=1µm
2.008, S.G. Kim, MIT
(a) Films deposition (CVD, spinning)
(b) Top electrode deposition (e-beam)and then lift-off
(c) Films patterning (RIE)
(d) Proof mass (SU-8, spinning)
Device Fabrication
PZT (Pb(Zr,Ti)O3)ZrO2
Membrane
Si
Interdigitated Electrode (Ti/Pt)Proof mass
2.008, S.G. Kim, MIT
(e) Cantilever release (XeF2 etcher)
(f) Packaging & PZT poling
Device Fabrication
2.008, S.G. Kim, MIT
• First mode of resonance at 13.9 kHz with 14 nm amplitude• Rectifying the AC signal, then store the charge• Variation of load resistance
Power Generation
2.28V
Load
vo
ltag
e (V
c)
Time (sec)
0.4µA
So
urc
e cu
rren
t (µ
V)
Time (sec)
Schottky diodes: smallest forward voltage drop10 nF mylar cap: low leakage current
2.008, S.G. Kim, MIT
Structural Health Monitoring
2.008, S.G. Kim, MIT
Research/Educational Opportunities
Monitoring of
Large-scale Remote Systems
Gas line monitoring
Underground beacons
Measuring rotational machines
Smart bearings
Helicopter blades
Structural health monitoring
Multi-functional structures
2.008, S.G. Kim, MIT
Process Flow
Deposition Lithography Etch
Wafers DevicesPhoto resist coatingPattern transferPhoto resist removal
OxidationSputteringEvaporationCVDSol-gelEpitaxy
Wet isotropicWet anisotropicPlasmaRIEDRIE
2.008, S.G. Kim, MIT
Deposition processesChemical
CVD(Chemical Vapor Deposition)Thermal OxidationEpitaxyElectrodeposition
PhysicalPVD
EvaporationSputtering
Casting
2.008, S.G. Kim, MIT
Thermal OxidationSilicon is consumed as the silicon dioxide is grown.Growth occurs in oxygen and/or steam at 800-1200 C.Compressive stress ~2um films are maximum practically. Simple, easy process for electrical insulation, intentional warpage, etc.
Silicon
O2
SiliconSiO2
2.008, S.G. Kim, MIT
Thermal OxidationOxidation can be masked with silicon nitride, which prevents O2 diffusion
Silicon nitride
Silicon
SiO2
2.008, S.G. Kim, MIT
Chemical Vapor DepositionThermal energy to dissociate gases and deposit thin films on surfaces, high productivity, better step coveragelow pressure (LPCVD), atmospheric pressure (APCVD), plasma enhanced (PECVD), horizontal, verticalLPCVD pressures around 300mT (0.05% atmosphere)Moderate Temperatures
450oC SiO2 : PSG, LTO580-650oC polysilicon800oC SixNy – SiH4 + NH3
Very dangerous gasesSilane: SiH4Arsine, phosphine, diborane: AsH3, PH3, B2H6
>25
2.008, S.G. Kim, MIT
Physical Vapor Deposition
Evaporated metals in a tungsten crucibleAluminum, gold, Pt, W
Evaporated metals and dielectrics by electron-beam or resistance heatingTypically line-of-sight depositionVery high-vacuum required to prevent oxidation, load lock
Evaporation
E-beam evaporator
Shadowing
2.008, S.G. Kim, MIT
Physical Vapor Deposition –Sputtering
Sputtered metals and dielectricsArgon ions bombards targetEjected material takes ballistic path to wafers
Typically line-of-sight from a distributed sourceRequires high vacuum, but low temperature
Evaporation vs. Sputtering
RF sputter
2.008, S.G. Kim, MIT
Spin CastingViscous liquid is poured on center of waferWafer spins at 1,000-5,000 RPM for ~30sBaked on hotplates 80-500oC for 10-1000sApplication of etchants and solvents, rinsingDeposition of polymers, sol-gel PZT
dispenser
vacuum chuck
PR
wafer
ω
t
slowcoat
spindown
level out
2.008, S.G. Kim, MIT
Deposition Issues - Compatibility
Thermal compatibilityThermal oxidation and LPCVD films Thermal oxidation and LPCVD vs. polymers (melting/burning) and most metals (eutectic formation, diffusion)
Topographic compatibilitiySpin-casting over large step heightsDeposition over deep trenches-key hole
2.008, S.G. Kim, MIT
Deposition Issues - Conformality
A conformal coating covers all surfaces to a uniform depthA non-conformal coating deposits more on top surfaces than bottom and/or side surfaces
Conformal Non-conformal Non-conformal
2.008, S.G. Kim, MIT
Photo 1 . Cracking of sol-geldeposited PZT after 650C firing
Photo 2. Poor step coverage and high stress evolved at corner of a step
2.008, S.G. Kim, MIT
Process Flow
Deposition Lithography Etch
Wafers DevicesPhoto resist coatingPattern transferPhoto resist removal
OxidationSputteringEvaporationCVDSol-gelEpitaxy
Wet isotropicWet anisotropicPlasmaRIEDRIE
2.008, S.G. Kim, MIT
Isotropic silicon etchantsHNA (“poly-etch”) -wet
Mix of HF, nitric acid (HNO3), and acetic acids (CH3COOH)
Difficult to control etch depth and surface uiformityXeF2 -dry
gas phase, etches silicon, polysiliconDoes not attack SiO2, SiNx,metals, PR
Etching
Wet
Dry
2.008, S.G. Kim, MIT
Anisotropic wet etching
Many liquid etchants demonstrate dramatic etch rate differences in different crystal directions
<111> etch rate is slowest, <100> fastestFastest: slowest can be more than 100:1KOH, EDP, TMAH most common anisotropic silicon etchantsPotasium Hydroxide (KOH), TetramethylAmmonium Hydroxide (TMAH), and Ethylene Diamine Pyrochatecol (EDP)
2.008, S.G. Kim, MIT
KOH EtchingEtches PR and Aluminum instantly(100) to (111) 100 to 1 etch rate
V-grooves, trenchesConcave stop, convex undercutCMOS incompatible
Masks:SiO2: for short period
SixNy: Excellent
heavily doped P++ silicon: etch stop
<111>
<100>
Silicon Substrate
54.7
a 0.707a
2.008, S.G. Kim, MIT
Anisotropic wet etching
When a (100) wafer with mask featuresOriented to <110> direction is placed in an anisotropic etchant.
A square <110> oriented mask feature results in a pyramidal pit.
2.008, S.G. Kim, MIT
Anisotropic wet etching
When a (100) wafer with mask featuresOriented to <110> direction is placed in an anisotropic etchant.
A square <110> oriented mask feature results in a pyramidal pit.
2.008, S.G. Kim, MIT
A
A’
BB’
w
θh
Etch mask
(100)silicon
Top-viewA-A’ cross section
V-grooved Optical Bench
R
2.008, S.G. Kim, MIT
Dry etchingRIE (reactive ion etching)
Chemical & physical etching by RF excited reactive ionsBombardment of accelerated ions, anisotropicSF6 Si, CHF3 oxide and polymersAnisotropy, selectivity, etch rate, surface roughness by gas concentration, pressure, RF power, temperature control
Plasma etchingPurely chemical etching by reactive ions, isotropic
Vapor phase etchingUse of reactive gases, XeF2No drying needed
sticktion
2.008, S.G. Kim, MIT
DRIE (Deep RIE)Alternating RIE and polymer deposition process for side wall protection and removalEtching phase: SF6 /ArPolymerization process: CHF3/Ar forms Teflon-like layerInvented by Bosch, process patent, 1994
-1.5 to 4 µm/min-selectivity to PR 100 to 1
2.008, S.G. Kim, MIT
1 µm
Scalloping and Footing issues of DRIE
Scalloped sidewall
Top wafer surfacecathode Top wafer surface
anode
Tip precursors
Scalloped sidewall
Top wafer surfacecathode Top wafer surface
anode
Tip precursors
<100 nm silicon nanowireover >10 micron gap
Footing at the bottom of
device layerMilanovic et al, IEEE TED, Jan. 2001.
2.008, S.G. Kim, MIT
Deep Reactive Ion EtchSTS, Alcatel, Trion, Oxford Instruments …
Most wanted by many MEMS students
High aspect ratio 1:30
Easily masked (PR, SiO2)
2.008, S.G. Kim, MIT
Wet or dry?
Low resolution feature size Low costUndercut for isotropicWider area needed for anisotropic wafer etchSticking
High resolution feature sizeExpensiveVertical side wallAvoid sticking
Wet Dry
2.008, S.G. Kim, MIT
Etching Issues - Anisotropy
Isotropic
An-isotropicmask
-Structural layer-Sacrificial layer
2.008, S.G. Kim, MIT
Etching Issues - SelectivitySelectivity is the ratio of the etch rate of the target materialbeing etched to the etch rate of other materials Chemical etches are generally more selective than plasma etchesSelectivity to masking material and to etch-stop is important
Masktarget
Etch stop
2.008, S.G. Kim, MIT
Process Flow
Deposition Lithography Etch
Wafers DevicesPhoto resist coatingPattern transferPhoto resist removal
OxidationSputteringEvaporationCVDSol-gelEpitaxy
Wet isotropicWet anisotropicPlasmaRIEDRIE
2.008, S.G. Kim, MIT
Lithography (Greek, “stone-writing”)Pattern Transfer
Appication of photosensitive PROptical exposure to transfer image from mask to PRRemove PR - binary pattern transfer
2.008, S.G. Kim, MIT
Photo resistSpin coat phto-resist
3000 – 6000 rpm, 15-30 secViscosity and rpm determine thicknessSoft bake ->alignment ->exposure
Develop PR after exposureHardbake
Positive Negative
dispenser
vacuum chuck
PR
wafer
ω
t
slowcoat
spindown
level out
2.008, S.G. Kim, MIT
PhotomasksMaster patterns to be transferredTypes:
Photographic emulsion on soda lime glass (cheap)Fe2O3 or Cr on soda lime glass
Cr on quartz (expensive, for deep UV light source)Polarity
Light field: mostly clear, opaque featureDark field: mostly opaque, clear feature
2.008
2.008
2.008, S.G. Kim, MIT
Types of AlignerContact Proximity Projection
Reduction ratio 1:1Array of the same pattern Stepper
2.008, S.G. Kim, MIT
Double sided Alignment
2.008, S.G. Kim, MIT
Pattern transfer by lift-off
2.008, S.G. Kim, MIT
VGA640 X 480307,200 pixels
50 µm
Human Hair
XGA1024 X 768
786,432 pixels
Micromirror Arrays
2.008, S.G. Kim, MIT
Light
50 µm
Active Matrix
PiezoelectricActuator
Mirror
TMA(Thinfilm Micromirror Array)
Mirror Array on
Piezoelectric
Actuator Array
Daewoo Electronics
2.008, S.G. Kim, MIT
Evolution of TMA Pixels
00 02 04 06 08
4
8
12
16
20
24
Opt
ical
Effi
cien
cy(%
)
Time from the start of project, year
1st1st
2nd2nd
3rd3rd
Daewoo Electronics
2.008, S.G. Kim, MIT
Clean RoomGowning with bunny suitClass 1, 10, 100
A salt grain on a chip
2.008, S.G. Kim, MIT
Class of clean roomsClass 1 means one speckle of 0.5 µpartical in one ft3. Class 10, 100, 1000HEPA filter, AHU
2.008, S.G. Kim, MIT
Air FiltersHEPA (High Efficiency Particulate Air) filtersHigh efficiency, low ∆p, good loading characteristicsGlass fibers in a paper like medium97% retainment of incident particles of 0.3 µm or larger
2.008, S.G. Kim, MIT
Class of clean rooms
Class 0.5 µ particle
Temp tolerance
RH tolerance
$/ft2
10,000 10,000 +/- 3oF +/- 5% $250-300
1,000 1,000 +/- 2oF +/- 5% $350-400
100 100 +/- 1oF +/- 5% $1200
10 10 +/- 0.5oF +/- 3% $3500
1 1 +/- 0.3oF +/- 2% $10,000
2.008, S.G. Kim, MIT
Particlesclass 0.1µ 0.2µ 0.3µ 0.5µ 5.0µ
1 35 7.5 3 1 N/A
10 350 75 30 10 N/A
100 N/A 750 300 100 N/A
1000 N/A N/A N/A 1000 7
Federal Standard 209; Number of particles per cubic foot
2.008, S.G. Kim, MIT
ToxicityTLV (Threshold Limit Value)
Upper limit material concentration that an average healthy person can be exposed without adverse effects, ppm or mg/m3
Notorious PoisonsCO (100 ppm), CO2 (5000 ppm), HCN (110 ppm), H2S (10 ppm)
SO2 (5 ppm), NH3 (50 ppm)
Arsenic trioxide AS2O3 (0.1g fatal)
Hg (0.1 ppm via skin contact)
All material are toxic in sufficient quantity, 5g caffein is fatal.
2.008, S.G. Kim, MIT
Design DomainsDesign is a mapping processFrom “What” to “How”Small scale systems design
What How