Transcript Spintronics Integrating magnetic materials with semiconductors
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
LTO
: Low Temperature Oxidation process For deposition at lower temperatures, use
Low Pressure Chemical Vapor Deposition (LPCVD) SiH 4 + O 2
425-450 o C 0.2-0.4 Torr
SiH 4 + O 2
SiO 2 + 2 H 2
: 450 o C Other advantages: Can dope Silicon oxide to create PSG (phospho-silicate glass)
SiH 4 + 7/2 O 2 + 2 PH 3
SiO 2 :P + 5 H 2 O
: 700 o C PSG: higher etch rate, flows easier (better topography)
Case study: Poly-silicon growth
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by Low Pressure Chemical Vapor Deposition T: 580-650 o C, P: 0.1-0.4 Torr Amorphous film 570 o C Crystalline film 620 o C Effect of temperature Amorphous
Crystalline: 570 o C Equi-axed grains: 600 o C Columnar grains: 625 o C (110) crystal orientation: (100) crystal orientation: 600 – 650 o C 650 – 700 o C SiH 4
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: 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
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Mechanical properties Stress state: Residual compressive stress (500 MPa) - Amorphous/columnar grained structures: Compressive stress - Equiaxed grained structures: Tensile stress
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Thick films have less stress than thinner films
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ANNEALING CAN REDUCE STRESSES BY A FACTOR OF 10-100
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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)
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Optical properties Rough surfaces!
Silicon Nitride
(for electrical and thermal isolation of devices) r: 10 16 W cm, E breakdown : 10 7 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 Si x N y , 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 SiH 2 Cl 2 + y NH 3 Si x N y + HCl + 3 H 2 700 - 900 o C 0.2-0.5 Torr
SiH 2 Cl 2 + NH 3
Depositing materials PVD (Physical vapor deposition)
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Sputtering:
DC (conducting films: Silicon nitride) RF (Insulating films: Silicon oxide)
Depositing materials PVD (Physical vapor deposition)
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Evaporation (electron-beam/thermal)
Commercial electron-beam evaporator (ITL, UCSD)
Electroplating
Issues: •Micro-void formation • Roughness on top surfaces • Uneven deposition speeds
e.g. can be used to form porous Silicon, used for sensors due to the large surface to volume ratio
Used extensively for LIGA processing
Depositing materials –contd.-
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Spin-on (sol-gel)
Si wafer Dropper
e.g. Spin-on-Glass (SOG) used as a sacrificial molding material, processing can be done at low temperatures
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 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
Release of MEMS structures
To overcome this problem: (1) Use of alcohols/ethers, which sublimate, at release step (2) Surface texturing Cantilever Si substrate (3) Supercritical CO 2 drying: avoids the liquid phase 35 o C, 1100 psi