ME 381R Fall 2003
Micro-Nano Scale Thermal-Fluid Science and Technology
Lecture 18:
Introduction to MEMS
Dr. Li Shi
Department of Mechanical Engineering
The University of Texas at Austin
Austin, TX 78712
www.me.utexas.edu/~lishi
lishi@mail.utexas.edu
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Outline
• MEMS: Applications
• Pattern transfer Method
- Lithography
• Subtractive Method
- Wet and Dry Etching
• Additive Method
}
Basic Fabrication
Techniques
- Thin Film Deposition
• Example (Cantilever Beam)
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MEMS Applications
3
http://mems.colorado.edu/c1.res.ppt/ppt/g.tutorial/ppt.htm
MEMS Applications
4
http://mems.colorado.edu/c1.res.ppt/ppt/g.tutorial/ppt.htm
MEMS Applications
http://evlweb.eecs.uic.edu/scharver/asci/mems.html
This image depicts a mirror system
moved by a small rotating gear
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MEMS Fabrication Techniques
•
Pattern Transfer Method :
• Photo Lithography
• E-beam Lithography
• Nano-imprinting Lithography
•
Subtractive Method :
• Wet Etching
• Dry Etching
•
Additive Method :
• Thin Film Deposition
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Photolithography
•
Energy
•
•
Mask
•
•
Change cross linking of polymer chains of a photo-sensitive polymer
called photoresist, and modify its dissociation rate in a developer similar
to that for developing photos
Absorber (Dark Area) & window (Open area)
Resist
•
Transfer image from mask to wafer
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Photolithography
•
Positive Photoresist
- the polymer is weakened
and more soluble in
the developer
•
Negative Photoresist
- the polymer is hardened
and less soluble
http://www.ece.gatech.edu/research/labs/vc/theory/photolith.html
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Exposure
Systems
Photolithography
Z

2bmin  3   g  
2

2bmin 

2 NA
• Contact – not limited by diffraction
• Proximity – 2 ~ 20 m gap, limited by diffraction
• Projection – limited by diffraction
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Electron Beam Lithography
•
•
•
•
Minimum beam size (5nm)
for optical & X-ray masks, and nano-devices
Direct writing on resist-coated substrate
No Mask
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Energy Sources
Sources
Energy
• Energy
E  h 
hc

• Minimum Line Width
R  1.22
f
d
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Comparison
Comparison
Advantage
Optical
Electron Beam
 Low ~High precision
 Fast exposure speed
 Relatively low cost
 No diffraction
 Easy to control
 Available for small
features
 Light diffraction
 Alignment problem
Disadvantage  Debris between mask
and wafer
 Needs vacuum
 High system cost
 Slow
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Nano-imprinting Lithography:
Fused Silica Template
1. Orient substrate and
treated template
Release Layer
2. Dispense drops of low
viscosity UV curable low
viscosity organosilicon
monomer
3. Close gap and illuminate
with UV (Room
Temperature, Low
Pressure)
Planarization Layer
Substrate
Low Viscosity Monomer
UV blanket expose
HIGH resolution, LOW
aspect-ratio relief
4. Separate the template
from the substrate
5. Halogen break-thru etch
followed by oxygen etch
Residual Layer
HIGH resolution, HIGH
aspect-ratio feature
Step and Flash Imprinting Lithography (S-FILTM)
Courtesy: Profs. C. G. Willson & S.V. Sreenivasan, UT Austin
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S-FIL Patterned Nanowires
Sub-40 nm contacts
30 nm dense lines
20 nm isolated lines
Courtesy: Profs. C. G. Willson and S.V. Sreenivasan
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Subtractive Method
Wet and Dry Etching
• Wet Etching
- Chemical Reaction
- Liquid source
• Dry Etching
- Chemical + Physical Reaction
- Gas or Vapor phase source
http://www.ee.ucla.edu/~jjudy/classes/ee150L/lectures/EE150L_Lecture_Dry_Etching_files/frame.htm
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Etching Mechanism
http://www.ee.ucla.edu/~jjudy/classes/ee150L/lectures/EE150L_Lecture_Dry_Etching_files/frame.htm
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Dry Etching
Ion Energy
• Physical Etching (Sputtering)
• Momentum transfer
 bond breakage
• Particle Collisions
• Aniosotropic
•Physical- chemical Etching
• Ion bombardment to make
the surface more reactive
• Anisotropic
• Chemical Etching
• Reactive etchant species
• Isotropic
Pressure
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Silicon Crystal Structure
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Isotropic and Anisotropic in
Wet and Dry Etching
Isotropic
Anisotropic
Wet
For Silicon
Dry
- HNA (HF, HNO3, CH3COOH)
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Comparison
Parameter
Wet Etching
Directionality Only with single
Dry Etching
Cost
crystal materials
Low
With most
materials
High
Selectivity
Can be very good
Poor
Typical Etch
Rate
Control
Fast (1um/min)
Slow (0.1 um/min)
Difficult
Good
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Etching Process
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http://www.saabmicrotech.se/node4084.asp
Additive Method
Thin Film Deposition
• Physical Vapor Deposition (PVD)
•
•
•
Chemical Vapor Deposition (CVD)
•
•
•
Thermal Evaporation
Sputtering
PECVD (Plasma Enhanced)
LPCVD (Low Pressure)
Electroplating
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PVD: Thermal Evaporation
• Thermal Evaporation
• Based on the
sublimation
• Fast
• Low surface damage
• Difficult to control
• Limited materials (metal)
• Low working pressure
to increase mean free path
• Low surface damage
• Faster than sputtering
• Limited material
http://www.icmm.csic.es/fis/english/ evaporacion_resistencia.html
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PVD: Sputtering
• Sputtering
• Based on Ion bombardment
• Unlimited material
• Possible surface damage
• Excellent adhesion
• Expensive
http://web.kth.se/fakulteter/TFY/cmp/research/sputtering/images/sputtering_anim.gif
http://www.ee.ucla.edu/~jjudy/classes/ee150L/lectures/EE150L_Lecture_Dry_Etching_files/frame.htm
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CVD: Plasma Enhanced
• PECVD
• Plasma helps reaction
• Low substrate temperature
• Good step coverage
• Chemical contamination
http://www.seas.ucla.edu/Chang/pictures/ research-pecvd.jpg
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CVD: Low Pressure
• LPCVD
• < 10 Pa
• Excellent purity
• Low stress
• High temperature
• Low deposition rate
http://www.memsnet.org/mems/beginner/ images/cvd_reactor.jpg
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Electroplating
• Various metal (Au, Ni, etc)
• Fast
• > 10 m
• Hydrogen bubble generation
• Difficult for sub-m features
• Needs seed layer
J.W. Judy, "Magnetic microactuators with polysilicon
flexures," Masters Report, Department of EECS,
University of California, Berkeley, August 29, 1994
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Making a Cantilever Beam
(Surface Micromachining)
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