mems devices: how do we make them? a mechanismmaecourses.ucsd.edu/~pbandaru/mae268-sp09/class... ·...
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MEMS devices: How do we make them?
Sandia MEMS
Gear chain Hinge Gear within a gear
A mechanism
Basic MEMS materials Silicon and its derivatives, mostly
• Micro-electronics heritageSi 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
How a cantilever is made:htt
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One can make devices as complex as one wishes
using deposition and micromachining processesh
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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:
Bulk Si Bulk Si
Masking layer
Isotropic 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
Si + 6 HNO3+6 HF H2SiF6 + HNO2 + H2O + H2
Chemical reaction:
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
Different planes of atoms in a Silicon crystal have different
densities of atoms
(111) (100) (110) (111)
XY
Z- Diamond cubic crystal structure
This implies preferential/anisotropic etching is possible
Applications: Anisotropic Etching
fiber
Aligning fibers Inkjet printers
Wet etching: Anisotropic Etching
(100) (100)
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
Bulk Si Bulk Si
Comparison of wet chemical etches
Etchant Typical etching
conditions
Anisotropic
<100>/<111>
etching ratio
Etch rate of
masking layers
EDP 50-115 oC
20-80 m/hr
10-35 SiO2(2 Å/min)
SiN(1 Å/min)
KOH 50-90 oC
10-100 m/hr
100-400 SiO2(2 Å/min)
SiN(1 Å/min)
TMAH 60-90 oC
10-60 m/hr
10-20 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 channelsbased on (110) preferential etching
MEMS Process Sequence
Slide courtesy: Al Pisano
Surface micromachininghtt
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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:
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) BDG m2/ hour
920 0.0049
1000 0.0117
1100 0.027
Wet Oxidation: uses steam
for thicker oxides (100nm – 1.5 mm, lower quality)
Temperature (oC) BDG (mm2/ hour)
920 0.203
1000 0.287
1100 0.510
Higher thicknesses of oxide: CVD or high pressure steam
oxidation
Oxidation of Silicon Silicon Oxide
(Sacrificial material)
MEMS Processing
Silicon oxide deposition
For deposition at lower temperatures, use
Low 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 oC
0.2-0.4 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 oC
Equi-axed grains: 600 oC
Columnar grains: 625 oC
(110) crystal orientation: 600 – 650 oC
(100) crystal orientation: 650 – 700 oC
SiH4
Amorphous film
570 oCCrystalline film
620 oC
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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 propertiesGrain boundaries are a barrier for electrons
e.g. thermal conductivity could be 5-10 times lower (0.2 W/cm-K)
• Optical propertiesRough 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 films
PECVD: 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 oC
0.2-0.5 Torr
Depositing materials
PVD (Physical vapor deposition)
• Sputtering: DC (conducting films: Silicon nitride)
RF (Insulating films: Silicon oxide)
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Depositing materials
PVD (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
Court
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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 psiSi substrate
Cantilever
A comparison of conventional
vs. supercritical drying
Reactive Ion Etching (RIE)
DRY plasma based etching
Deep RIE (DRIE):
• Excellent selectivity to mask material (30:1)
• Moderate etch rate (1-10 m/minute)
• High aspect ratio (10:1), large etch depths possible
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Deep Reactive Ion Etching (DRIE)
Bosch Process Alternate etching (SF6) +Passivation (C4F8)
• Bowing: bottom is wider
• Lag: uneven formation
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
Gas phase Silicon etching
XeF2 BrF3
Developed at IBM (1962) Developed at Bell labs (1984)
2 XeF2 + Si 2 Xe + SiF4 4 BrF3 + 3 Si 2 Br2 + 3 SiF4
Cost: $150 to etch 1 g of Si $16 for 1 g of Si
• Room temperature process
• No surface tension forces
• No charging effects
• Isotropic
Etching rate: 1-10 m/minute
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4-For thick films (> 100 m)
- 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
Mass spectrometer with hyperbolic armsElectromagnetic motor
Capability Bulk Surface LIGA
Max. structural thickness Wafer thickness < 50 m 500 m
Planar geometry Rectangular Unrestricted Unrestricted
Min. planar feature size 2 depth < 1 m < 3 m
Side-wall features 54.7o slope Limited by dry etch 0.2 m
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
Technology Comparison
Bulk vs. Surface micromachining vs. LIGA