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Surface micromachininght
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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
Kam
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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
Cou
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Judy
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