Download MEMS Processing

MEMS devices: How do we make them?
A mechanism
Gear chain
Sandia MEMS
Hinge
Gear within a gear
Basic MEMS materials
Silicon and its derivatives, mostly
• Micro-electronics heritage
Si is a good semiconductor, properties can be tuned
Si oxide is very robust
Si nitride is a good electrical insulator
Substrate Cost
Metallization
Machinability
Silicon
High
Good
Very good
Plastic
Low
Poor
Fair
Ceramic Medium Fair
Poor
Glass
Low
Good
Poor
Surface micromachining
http://www.darpa.mil/mto/mems
How a cantilever is made:
http://mems.sandia.gov/
One can make devices as complex as one wishes
using deposition and micromachining processes
Any MEMS device is made from the processes
of deposition and removal of material
e.g. a state-of-the art MEMS electric motor
www.cronos.com
The History of MEMS
Y.C.Tai, Caltech
Bulk micromachining
• Wet Chemical etching:
Masking layer
Bulk Si
Isotropic
Bulk Si
Anisotropic
Bulk micromachining
• Dry etching
Ions: Reactive ion etching (RIE), focused ion beams (FIB)
Laser drilling: using high powered lasers (CO2/YAG)
Electron-beam machining: sequential slow
Wet Etching: Isotropic
• atomic layer by atomic layer removal possible
Isotropic etching: Hydrofluoric + nitric + acetic acids (HNA)
Bulk Si
Chemical reaction:
Si + 6 HNO3+6 HF H2SiF6 + HNO2 + H2O + H2
Principle:
HNO3 (Nitric acid) oxidizes Si  SiOx
HF (Hydrofluoric Acid) dissolves SiOx
Acetic acid/water is a diluent
Anisotropic etching, due to the Silicon crystal structure
- Diamond cubic crystal structure
Z
Y
X
Different planes of atoms in a Silicon crystal have different
densities of atoms
(111)
(100)
(110)
(111)
This implies preferential/anisotropic etching is possible
Applications: Anisotropic Etching
Aligning fibers
fiber
Inkjet printers
Wet etching: Anisotropic Etching
(100)
Bulk Si
(100)
Bulk Si
Chemical recipes:
EDP (Ethylene diamine, pyrocatechol, water)
[NH2(CH2)2NH2, C6H4(OH)2]
- low SiO2 etch rate, - carcinogenic
KOH (Potassium hydroxide),
- high <110> / <111> and <100>/ <111> selectivity ( ~ 500)
- high SiO2 etching
TMAH (Tetra-methyl Ammonium Hydroxide: (CH3)4NOH)
- Low SiO2 and SixNy etch rate
- smaller <100> / <111> selectivity
Comparison of wet chemical etches
Etchant
Typical etching
conditions
Anisotropic Etch rate of
<100>/<111> masking layers
etching ratio
50-115 oC
10-35
20-80 mm/hr
KOH 50-90 oC
100-400
10-100 mm/hr
TMAH 60-90 oC
10-20
10-60 mm/hr
EDP
SiO2(2 Å/min)
SiN(1 Å/min)
SiO2(2 Å/min)
SiN(1 Å/min)
SiO2(2 Å/min)
SiN(1 Å/min)
Reference: “Etch rates for Micromachining Processing”
- K. R. Williams, IEEE Journal of MEMS, vol. 5, page 256, 1996.
Sensors based on (100) preferential etching
Honeywell sensor
Micro-fluidic channels
based on (110) preferential etching
MEMS Process Sequence
Slide courtesy: Al Pisano
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
MEMS Processing
Oxidation of Silicon  Silicon Oxide
(Sacrificial material)
Dry Oxidation: flowing pure oxygen over Si @ 850 – 1100 oC
(thin oxides 1- 100 nm, high quality of oxide)
Uses the Deal-Grove Model: xoxide = (BDGt)1/2
Temperature (oC)
920
1000
1100
BDG (mm2/ hour)
0.0049
0.0117
0.027
MEMS Processing
Oxidation of Silicon  Silicon Oxide
(Sacrificial material)
Wet Oxidation: uses steam
for thicker oxides (100nm – 1.5 mm, lower quality)
Temperature (oC)
920
1000
1100
BDG (mm2/ hour)
0.203
0.287
0.510
Higher thicknesses of oxide: CVD or high pressure steam
oxidation
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,
1100 psi
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