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Sacrificial material: Silicon oxide

Structural material: polycrystalline Si (poly-Si)

Isolating material (electrical/thermal): Silicon Nitride

How a cantilever is made:

Silicon oxide deposition

For deposition at lower temperatures, useLow Pressure Chemical Vapor Deposition (LPCVD)

SiH4 + O2 SiO2 + 2 H2 : 450 oC

Other advantages:

Can dope Silicon oxide to create PSG (phospho-silicate glass)

SiH4 + 7/2 O2 + 2 PH3 SiO2:P + 5 H2O : 700 oC

PSG: higher etch rate, flows easier (better topography)

SiH4 + O2

425-450 oC0.2-0.4 Torr

LTO: Low Temperature Oxidation process

Case study: Poly-silicon growth

- by Low Pressure Chemical Vapor Deposition- T: 580-650 oC, P: 0.1-0.4 Torr

Effect of temperature

Amorphous Crystalline: 570 oCEqui-axed grains: 600 oCColumnar grains: 625 oC

(110) crystal orientation: 600 – 650 oC (100) crystal orientation: 650 – 700 oC

SiH4

Amorphous film570 oC

Crystalline film620 oC

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Poly-silicon growthTemperature has to be very accurately controlledas grains grow with temperature, increasing surface roughness, causing loss of pattern resolution and stresses in MEMS

Mechanisms of grain growth:

1. Strain induced growth- Minimize strain energy due to mechanical deformation, doping … - Grain growth time

2. Grain boundary growth- To reduce surface energy (and grain boundary area)- Grain growth (time)1/2

3. Impurity drag- Can accelerate/prevent grain boundary movement- Grain growth (time)1/3

Grains control properties• Mechanical properties Stress state: Residual compressive stress (500 MPa)

- Amorphous/columnar grained structures: Compressive stress- Equiaxed grained structures: Tensile stress- Thick films have less stress than thinner films

-ANNEALING CAN REDUCE STRESSES BY A FACTOR OF 10-100

•Thermal and electrical properties Grain boundaries are a barrier for electrons

e.g. thermal conductivity could be 5-10 times lower (0.2 W/cm-K)

• Optical properties Rough surfaces!

Silicon Nitride

Is also used for encapsulation and packaging

Used as an etch mask, resistant to chemical attack

High mechanical strength (260-330 GPa) for SixNy, provides structural integrity (membranes in pressure sensors)

Deposited by LPCVD or Plasma –enhanced CVD (PECVD)

LPCVD: Less defective Silicon Nitride filmsPECVD: Stress-free Silicon Nitride films

(for electrical and thermal isolation of devices)

1016 cm, Ebreakdown: 107 kV/cm

SiH2Cl2 + NH3

x SiH2Cl2 + y NH3 SixNy + HCl + 3 H2

700 - 900 oC0.2-0.5 Torr

Depositing materialsPVD (Physical vapor deposition)

• Sputtering: DC (conducting films: Silicon nitride) RF (Insulating films: Silicon oxide)

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Depositing materialsPVD (Physical vapor deposition)

• Evaporation (electron-beam/thermal)

Commercial electron-beam evaporator (ITL, UCSD)

Electroplating

e.g. can be used to form porous Silicon, used for sensors due to the large surface to volume ratio

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Issues: •Micro-void formation• Roughness on top surfaces• Uneven deposition speeds

Used extensively for LIGA processing

Depositing materials –contd.-

• Spin-on (sol-gel)

e.g. Spin-on-Glass (SOG) used as a sacrificial molding material, processing can be done at low temperatures

Si wafer

Dropper

Surface micromachining- Technique and issues- Dry etching (DRIE)

Other MEMS fabrication techniques- Micro-molding- LIGA

Other materials in MEMS- SiC, diamond, piezo-electrics,magnetic materials, shape memory alloys …

MEMS foundry processes- How to make a micro-motor

Surface micromachiningCarving of layers put down sequentially on the substrate by using selective etching of sacrificial thin films to form free-standing/completely released thin-film microstructures

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HF can etch Silicon oxide but does not affect Silicon

Release step

Release of MEMS structures A difficult step, due to surface tension forces:

Surface Tension forces are greater than gravitational forces ( L) ( L)3

Release of MEMS structures To overcome this problem:

(1) Use of alcohols/ethers, which sublimate, at release step

(2) Surface texturing

(3) Supercritical CO2 drying: avoids the liquid phase

35oC, 1100 psi

Si substrate

Cantilever

A comparison of conventional vs. supercritical drying

Reactive Ion Etching (RIE) DRY plasma based etching

Deep RIE (DRIE): • Excellent selectivity to mask material (30:1)• Moderate etch rate (1-10 m/minute)• High aspect ratio (10:1), large etch depths possible

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Deep Reactive Ion Etching (DRIE)

Bosch Process Alternate etching (SF6) +Passivation (C4F8)

• Bowing: bottom is wider• Lag: uneven formation

A side effect of a glow discharge polymeric species created

Plasma processes: Deposition of polymeric material from plasma vs. removal of material

Usual etching processes result in a V-shaped profile

Gas phase Silicon etching

XeF2 BrF3

Developed at IBM (1962) Developed at Bell labs (1984)

2 XeF2 + Si 2 Xe + SiF4 4 BrF3 + 3 Si 2 Br2 + 3 SiF4

Cost: $150 to etch 1 g of Si $16 for 1 g of Si

• Room temperature process• No surface tension forces• No charging effects• Isotropic

Etching rate: 1-10 m/minute

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-For thick films (> 100 m)

- HEXSIL/PDMS, compatible with Bio-MEMS

- loss of feature definition after repeated replication

- Thermal and mechanical stability

LIGA (LIthographie, Galvanoformung, Abformung)

For high aspect ratio structures

• Thick resists (> 1 mm)• high –energy x-ray lithography ( > 1 GeV)

Millimeter/sub-mm sized objects which require precision

Mass spectrometer with hyperbolic armsElectromagnetic motor

Capability Bulk Surface LIGA

Max. structural thickness Wafer thickness < 50 m 500 m

Planar geometry Rectangular Unrestricted Unrestricted

Min. planar feature size 2 depth < 1 m < 3 m

Side-wall features 54.7o slope Limited by dry etch 0.2 m

Surface & edge

definitions

Excellent Adequate Very good

Material properties Very well controlled

Adequate Well controlled

Integration with electronics

Demonstrated Demonstrated Difficult

Capital Investment Low Moderate High

Published knowledge Very high High Moderate

Technology Comparison

Bulk vs. Surface micromachining vs. LIGA

Materials in MEMS

Bio-MEMS (micro-electrode arrays, DNA probes) enzymes, antigen/antibody pairs, DNA, polyimides, hydrogels, plastics, porous Si, C, AgCl…

Mechanical MEMS (for micro-motors etc.) Si, quartz (SiO2), Si3N4, Ti, Ni, permalloy (NiFe), polycrystalline Si …

RF-MEMS (for wireless communications): Compound semiconductors: GaAs, InP, GaN Si, SiO2 …

METALSused for wiring (Al, Cu), etch masks (Cr), structural elements (Al, W) - excellent electrical conductors - prone to fatigue

SMA : Shape memory alloys (NiTi: Nitinol)Reversible temperature induced transformation from a stiff austenite phase (Y.S.: 550 MPa) to a ductile martensite (Y. S.: 100 MPa) phase.

- used for thermal actuation - Can exert stresses of up to 100 MN/m2

- Maximum operating temperature ~ 70 oC - very slow actuation mechanism

Polymers: poly-norbornene

Magnetic materials

• prevalent: Ni, NiFe (permalloy), Co alloys- Not as widely used as electrostatic actuation- Needs thick films (10-20m); using electro-deposition

Cr/AuPhotoresist (PR)

Glass substratePattern PR

Etch Cr/Au

Etch Glass

Remove Cr/Au

Si SiO2Si oxidation

Si etch(KOH)Pattern & deposit NiFe

RIE to releasecantilever

A magnetically actuated cantilever

New applications demand new materials Silicon Carbide (SiC): structural & isolating layer - mechanically robust, E(500 GPa) higher resonance frequency - high temperature material (>200 oC) - difficult to shape (chemically inert) - used in micro-gas turbines

Diamond: very hard, for electrical isolation - E: 1035 GPa - excellent thermal conductor, easy heat dissipation - difficult to machine, needs oxygen-plasmas

- used in Atomic Force Microscope cantilevers

GaAs/InP: opto-electronics - good combination of electrical and mechanical properties - high piezo-electric coefficients - sophisticated manufacture for GaAs and InP substrates (Molecular Beam Epitaxy)

Polymers: structurally compliant - 50 times lower E compared to Si/Silicon nitride- Can withstand large strains (100%)- Polyimide: used in force sensor, shear stress sensor skin

Piezoelectrics: have a mechanical response to an electric field: ZnO, (Pb,Zr)TiO3

- Large mechanical transduction, force sensors

• Silicon is comparable to steel

MaterialElastic

ModulusYield

Strength DensityKnoop

Hardness

Thermal Expansion Coefficient

Thermal Conductivity

(GPa) (GPa) (g/cm3) (kg/mm2) (10-6/K) (W/cm-K)Diamond 1035 53 3.5 7000 1.0 20

SiC 700 21 3.2 2480 3.3 3.5Al2O3 530 15.4 4 2100 5.4 0.5TiC 497 20 4.9 2470 6.4 3.3W 410 4.0 19.3 485 4.5 1.78

Si3N4 385 14 3.1 3486 0.8 0.19Mo 343 2.1 10.3 275 5.0 1.38

Steel (max) 210 4.2 7.9 1500 12.0 0.97Stainless Steel 200 2.1 7.9 660 17.3 0.329

Fe 196 12.6 7.8 400 12.0 0.803

Si 190 7.0 2.3 850 2.3 1.57SiO2 73 8.4 2.5 820 0.6 0.014

Al 70 0.2 2.7 130 25.0 2.36