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Surface micromachining. How a cantilever is made:. Sacrificial material : Silicon oxide Structural material : polycrystalline Si (poly-Si) Isolating material (electrical/thermal): Silicon Nitride. Silicon oxide deposition. SiH 4 + O 2.

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Surface micromachining

How a cantilever is made:

Sacrificial material: Silicon oxide

Structural material: polycrystalline Si (poly-Si)

Isolating material (electrical/thermal):Silicon Nitride


Silicon oxide deposition

SiH4 + O2

LTO: Low Temperature Oxidation process

  • For deposition at lower temperatures, use
  • Low Pressure Chemical Vapor Deposition (LPCVD)
  • SiH4 + O2 SiO2 + 2H2 : 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)

425-450 oC

0.2-0.4 Torr


Case study: Poly-silicon growth


Amorphous film

570 oC

Crystalline film

620 oC

  • by Low Pressure Chemical Vapor Deposition
  • T: 580-650 oC, P: 0.1-0.4 Torr
      • Effect of temperature
  • Amorphous  Crystalline: 570 oC
  • Equi-axed grains: 600 oC
  • Columnar grains: 625 oC
  • (110) crystal orientation: 600 – 650 oC
  • (100) crystal orientation: 650 – 700 oC

Kamins,T. 1998 Poly-Si for ICs and diplays, 1998


Poly-silicon growth

  • Temperature has to be very accurately controlled
  • as grains grow with temperature, increasing surface
  • roughness, causing loss of pattern resolution and stresses in
  • MEMS
  • Mechanisms of grain growth:
  • 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 (500MPa)
  • - Amorphous/columnar grained structures: Compressive stress
  • - Equiaxed grained structures: Tensile stress
  • Thick films have less stress than thinner films
  • 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

SiH2Cl2 + NH3

(for electrical and thermal isolation of devices)

r: 1016W cm, Ebreakdown: 107 kV/cm

  • 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 films

PECVD: Stress-free Silicon Nitride films

x SiH2Cl2 + y NH3 SixNy + HCl + 3 H2

700 - 900 oC

0.2-0.5 Torr


Depositing materialsPVD (Physical vapor deposition)

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


Depositing materialsPVD (Physical vapor deposition)

  • Evaporation (electron-beam/thermal)

Commercial electron-beam evaporator (ITL, UCSD)



Courtesy: Jack Judy

  • Issues:
  • Micro-void formation
  • Roughness on top surfaces
  • Uneven deposition speeds
  • Used extensively for LIGA processing

e.g. can be used to form porous Silicon, used for

sensors due to the large surface to volume ratio


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


Surface micromachining

- Technique and issues

- Dry etching (DRIE)

Other MEMS fabrication techniques

- Micro-molding


Other materials in MEMS

- SiC, diamond, piezo-electrics,

magnetic materials, shape memory alloys …

MEMS foundry processes

- How to make a micro-motor


Surface micromachining

Carving 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

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



Si substrate

Release of MEMS structures

  • To overcome this problem:
  • Use of alcohols/ethers, which sublimate, at release step
  • Surface texturing
  • Supercritical CO2 drying: avoids the liquid phase


1100 psi


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 mm/minute)
  • High aspect ratio (10:1), large etch depths possible

Deep Reactive Ion Etching (DRIE)

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

Bosch ProcessAlternate etching (SF6) +Passivation (C4F8)

  • Bowing: bottom is wider
  • Lag: uneven formation

Gas phase Silicon etching

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

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

Etching rate: 1-10 mm/minute



  • For thick films (> 100 mm)
  • HEXSIL/PDMS, compatible with Bio-MEMS

C. Keller et al, Solid state sensor & actuator workshop, 1994

- loss of feature definition after repeated replication

- Thermal and mechanical stability



(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 arms

Electromagnetic motor


Technology Comparison

Bulk vs. Surface micromachining vs. LIGA


Materials in MEMS

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 …

Bio-MEMS (micro-electrode arrays, DNA probes)

enzymes, antigen/antibody pairs, DNA,

polyimides, hydrogels, plastics, porous Si, C, AgCl…



used 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


Photoresist (PR)


Glass substrate

Si oxidation



Pattern PR

Si etch


Etch Cr/Au

Pattern &

deposit NiFe

Etch Glass

RIE to release


Remove Cr/Au

Magnetic materials

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

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