Transcript Spintronics Integrating magnetic materials with semiconductors

Surface micromachining
http://www.darpa.mil/mto/mems
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)
SiH4 + O2
425-450 oC
0.2-0.4 Torr
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)
Case study: Poly-silicon growth
-
SiH4
by Low Pressure Chemical Vapor Deposition
T: 580-650 oC, P: 0.1-0.4 Torr
Crystalline film
620 oC
Effect of temperature
Amorphous  Crystalline:
Equi-axed grains:
Columnar grains:
(110) crystal orientation:
(100) crystal orientation:
570 oC
600 oC
625 oC
600 – 650 oC
650 – 700 oC
Kamins,T. 1998 Poly-Si for ICs and diplays, 1998
Amorphous film
570 oC
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
• 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
(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 (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
SiH2Cl2 + NH3
Depositing materials
PVD (Physical vapor deposition)
http://web.kth.se/fakulteter/TFY/cmp/research/sputtering/sputtering.html
• Sputtering: DC (conducting films: Silicon nitride)
RF (Insulating films: Silicon oxide)
Depositing materials
PVD (Physical vapor deposition)
• Evaporation (electron-beam/thermal)
Commercial electron-beam evaporator (ITL, UCSD)
Courtesy: Jack Judy
Electroplating
Issues:
e.g. can be used to form porous Silicon, used for
sensors due to the large surface to volume ratio
•Micro-void formation
• Roughness on top surfaces
• Uneven deposition speeds
Used extensively for LIGA processing
Depositing materials –contd.• Spin-on (sol-gel)
Dropper
Si wafer
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
http://www.darpa.mil/mto/mems
Carving of layers put down sequentially on the substrate by
using selective etching of sacrificial thin films to form freestanding/completely released thin-film microstructures
HF can etch Silicon oxide but does not affect Silicon
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 CO2 drying: avoids the liquid phase
35oC,
A comparison of conventional
vs. supercritical drying