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
http://www.memsguide.com
Reactive Ion Etching (RIE)
DRY plasma based etching
Deep RIE (DRIE):
• Excellent selectivity to mask material (30:1)
• Moderate etch rate (1-10 mm/minute)
• High aspect ratio (10:1), large etch depths possible
Deep Reactive Ion Etching (DRIE)
A side effect of a glow discharge  polymeric species created
Plasma processes:
Deposition of polymeric material from plasma vs. removal of material
Usual etching processes result in a V-shaped profile
Bosch Process Alternate etching (SF6) +Passivation (C4F8)
• Bowing: bottom is wider
• Lag: uneven formation
Gas phase Silicon etching
• Room temperature process
• No surface tension forces
• No charging effects
• Isotropic
XeF2
BrF3
Developed at IBM (1962)
2 XeF2 + Si  2 Xe + SiF4
Cost: $150 to etch 1 g of Si
Developed at Bell labs (1984)
4 BrF3 + 3 Si  2 Br2 + 3 SiF4
$16 for 1 g of Si
Etching rate: 1-10 mm/minute
Micro-molding
C. Keller et al, Solid state sensor & actuator workshop, 1994
-For thick films (> 100 mm)
- HEXSIL/PDMS, compatible with Bio-MEMS
- loss of feature definition after repeated replication
- Thermal and mechanical stability
LIGA
(LIthographie, Galvanoformung, Abformung)
For high aspect ratio structures
• Thick resists (> 1 mm)
• high –energy x-ray lithography ( > 1 GeV)
Millimeter/sub-mm sized objects which require precision
Electromagnetic motor
Mass spectrometer with hyperbolic arms
Technology Comparison
Bulk vs. Surface micromachining vs. LIGA
Capability
Bulk
Surface
LIGA
Max. structural thickness Wafer thickness
< 50 mm
500 mm
Planar geometry
Rectangular
Unrestricted
Unrestricted
Min. planar feature size
2 depth
< 1 mm
< 3 mm
Side-wall features
54.7o slope
Limited by dry etch 0.2 mm
Surface & edge
definitions
Excellent
Adequate
Very good
Material properties
Very well
controlled
Adequate
Well controlled
Integration with
electronics
Demonstrated
Demonstrated
Difficult
Capital Investment
Low
Moderate
High
Published knowledge
Very high
High
Moderate
Materials in MEMS
Mechanical MEMS (for micro-motors etc.)
Si, quartz (SiO2), Si3N4, Ti, Ni, permalloy (NiFe),
polycrystalline Si …
RF-MEMS (for wireless communications):
Compound semiconductors: GaAs, InP, GaN
Si, SiO2 …
Bio-MEMS (micro-electrode arrays, DNA probes)
enzymes, antigen/antibody pairs, DNA,
polyimides, hydrogels, plastics, porous Si, C, AgCl…
METALS
used for wiring (Al, Cu), etch masks (Cr),
structural elements (Al, W)
- excellent electrical conductors
- prone to fatigue
SMA : Shape memory alloys (NiTi: Nitinol)
Reversible temperature induced transformation from a
stiff austenite phase (Y.S.: 550 MPa) to a
ductile martensite (Y. S.: 100 MPa) phase.
- used for thermal actuation
- Can exert stresses of up to 100 MN/m2
- Maximum operating temperature ~ 70 oC
- very slow actuation mechanism
Polymers: poly-norbornene
Magnetic materials
• prevalent: Ni, NiFe (permalloy), Co alloys
- Not as widely used as electrostatic actuation
- Needs thick films (10-20 mm); using electro-deposition
A magnetically actuated cantilever
Si oxidation
Si
SiO2
Glass substrate
Photoresist (PR)
Cr/Au
Pattern PR
Si etch
(KOH)
Pattern &
deposit NiFe
Etch Cr/Au
RIE to release
cantilever
Remove Cr/Au
Etch Glass
New applications demand new materials
Silicon Carbide (SiC): structural & isolating layer
- mechanically robust, E(500 GPa)  higher resonance frequency
- high temperature material (>200 oC)
- difficult to shape (chemically inert)
- used in micro-gas turbines
Diamond: very hard, for electrical isolation
- E: 1035 GPa
- excellent thermal conductor, easy heat dissipation
- difficult to machine, needs oxygen-plasmas
- used in Atomic Force Microscope cantilevers
GaAs/InP: opto-electronics
- good combination of electrical and mechanical properties
- high piezo-electric coefficients
- sophisticated manufacture for GaAs and InP substrates
(Molecular Beam Epitaxy)
Polymers: structurally compliant
- 50 times lower E compared to Si/Silicon nitride
- Can withstand large strains (100%)
- Polyimide: used in force sensor, shear stress
sensor skin
Piezoelectrics: have a mechanical response to an
electric field: ZnO, (Pb,Zr)TiO3
- Large mechanical transduction, force sensors
275
1500
660
400
Thermal
Expansion
Coefficient
(10-6/K)
1.0
3.3
5.4
6.4
4.5
0.8
5.0
12.0
17.3
12.0
Thermal
Conductivity
(W/cm-K)
20
3.5
0.5
3.3
1.78
0.19
1.38
0.97
0.329
0.803
2.3
850
2.3
1.57
2.5
2.7
820
130
0.6
25.0
0.014
2.36
Diamond
SiC
Al2O3
TiC
W
Si3N4
Mo
Steel (max)
Stainless Steel
Fe
Elastic
Modulus
(GPa)
1035
700
530
497
410
385
343
210
200
196
Yield
Strength
(GPa)
53
21
15.4
2.1
4.2
2.1
12.6
Density
(g/cm 3)
3.5
3.2
4
4.9
19.3
3.1
10.3
7.9
7.9
7.8
Si
190
7.0
SiO2
Al
73
70
8.4
0.2
Material
20
4.0
14
Knoop
Hardness
(kg/mm 2)
7000
2480
2100
• Silicon is comparable to steel
2470
485
3486