1. Surface micromachining
How a cantilever is made:
Sacrificial material: Silicon oxide
Structural material: polycrystalline Si (poly-Si)
Isolating material (electrical/thermal): Silicon Nitride

2. Silicon oxide deposition
LTO: Low Temperature Oxidation process
For deposition at lower temperatures, use
Low 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 O PH3  SiO2:P + 5 H2O : 700 oC
PSG: higher etch rate, flows easier (better topography)
SiH4 + O2 oC
Torr

3. Case study: Poly-silicon growth
SiH4
by Low Pressure Chemical Vapor Deposition
T: oC, P: Torr
Effect of temperature
Amorphous  Crystalline: oC
Equi-axed grains: oC
Columnar grains: oC
(110) crystal orientation: 600 – 650 oC
(100) crystal orientation: 650 – 700 oC
Amorphous film
570 oC
Crystalline film
620 oC
Kamins,T Poly-Si for ICs and diplays, 1998

4. Poly-silicon growth Mechanisms of grain growth: Strain induced 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

5. 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
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!

6. Silicon Nitride (for electrical and thermal isolation of devices)
r: 1016 W cm, Ebreakdown: 107 kV/cm
Is also used for encapsulation and packaging
Used as an etch mask, resistant to chemical attack
High mechanical strength ( 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
SiH2Cl2 + NH3
x SiH2Cl2 + y NH3  SixNy + HCl + 3 H2 oC
Torr

7. Depositing materials PVD (Physical vapor deposition)
Sputtering: DC (conducting films: Silicon nitride) RF (Insulating films: Silicon oxide)

8. Depositing materials PVD (Physical vapor deposition)
Evaporation (electron-beam/thermal)
Commercial electron-beam evaporator (ITL, UCSD)

9. Electroplating Issues: Micro-void formation Roughness on top surfaces
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

10. 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
Dropper
Si wafer

11. 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

12. 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
crucial

13. Release of MEMS structures
A difficult step, due to surface tension forces:
Surface Tension forces are greater than gravitational forces
( L) ( L)3

14. 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
Si substrate
Cantilever
35oC,
1100 psi

15. A comparison of conventional vs. supercritical drying