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