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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