Presentation

Indo German Winter Academy 2007
Silicon Based MEMS Devices
and Processing
BY:
TALLA VAMSI
ECE Department
IIT Guwahati
Outline of the talk
MEMS overview
General Overview of MEMS
¾
Concept, History, Why MEMS?, Miniaturization, VLSI vs. MEMS
Materials for MEMS
¾
Materials Overview, Why Silicon?, Silicon Crystal, Materials for MEMS
MEMS fabrication processes
¾
Etching review, Bulk micromachining, Surface micromachining,
HexSiL, LIGA, Bulk vs Surface micromachining, ZPAL
MEMS devices
¾
Sensors, Actuators, SAW devices, Pressure, Temperature and Inertial
sensor, smart sensors, Design Considerations, MEMS Packaging
Conclusion
MEMS overview
Micro Electro Mechanical systems
The integration of mechanical elements, sensors, actuators,
and electronics on a common silicon substrate through
micro fabrication technology.
Sensor: A device which converts a non electrical quantity
into an electrical quantity, e.g. Pressure sensor.
Actuator: A device which gives a mechanical output
corresponding to an electrical input, e.g. Motor.
Smart sensor: A sensor that has a part or its entire
processing element integrated in a single chip.
Dimensional aspect of MEMS
MEMS overview
Reference 4
Dimensions of micro sensors, MEMS and their comparison with some everyday objects.
(Ref 4)
Historical Background
MEMS overview
1500 Early lithographic processes
1900s First silicon diode developed
1940s Radar development during WWII: Development of pure
semiconductors (Si and Ge)
1947 Point contact transistor
1959 Prof. Feynman lecture:
”There is plenty of room at the bottom.”
1960 Invention of MOSFET, Monolithic integration
1964 First batch fabricated MEMS device: Resonant gate
transistor
1970 Development of Microprocessor and Moore’s law
Historical Background(Contd.)
MEMS overview
1970s and 80s
MEMS commercialization begun.
1982 Kurt Peterson work on mechanical properties of Si
1984 Work by Howe and Muller at UCB
1989 Electro statically controlled micro motor
1991 Micro hinges by Pister et al at UCB
1990’s to present:
Tremendous increase in no of devices, technologies and
applications, focus now on micro machines
Why MEMS?
MEMS overview
IC Technology:
Integrated multiple functions
Precision:
Improved performance
Batch fabrication:
Reduced manufacturing cost and time
Miniaturization:
-
Low power consumption
-
Higher speed & less delay
-
Higher resonant frequency
-
Reliability
-
Portability
-
Ruggedness
MEMS overview
Miniaturization
Mass is reduced by a factor of s3.
Mechanical Systems
Fluidic systems
- Mechanical stiffness (factor of s)
- Low reynolds number
- Withstand tremendous acceleration
without breaking
- Laminar flow conditions
Thermal Systems
Chemical systems
- Rate of heat transfer( factor of s2)
- Total no. of molecules to be
detected reduced
- Rapid removal of heat is easy
Application to Telecommunication, Biomedical, Lab on Chip…
MEMS overview
VLSI v/s MEMS
VLSI
MEMS
Focus on electrons
Focus on mass
1. No moving part
Various moving parts:
micro gears, valves, pumps
3. Two dimensional
Three dimensional structures
4. Electrical systems
Electric, mechanical, chemical,
biological systems
5. Si,Ge,metals, dielectrics used
Si,oxides, polymers, plastics used
6. IC isolated from surroundings
Interaction with environment
Outline of the talk
MEMS overview
General Overview of MEMS
¾ Concept, History, Why MEMS, Miniaturization, VLSI vs. MEMS
Materials for MEMS
¾ Materials Overview, Why Silicon?, Silicon Crystal, Materials for MEMS
MEMS fabrication processes
¾ Etching review, Bulk micromachining, Surface micromachining,
HexSiL, LIGA, Bulk vs Surface micromachining, ZPAL
MEMS devices
¾ Sensors, Actuators, SAW devices, Pressure, Temperature and Inertial
sensor, smart sensors, Design Considerations, MEMS Packaging
Conclusion
Material Overview
MEMS Material
Semiconductors
¾ Conductivity between Metals and insulators, e.g. Si,Ge
Metals
¾ High electric and thermal conductivity, e.g. Al,Fe
Polymers
¾ Long chain of molecules, e.g. epoxies, nylon
Ceramics
¾ Inorganic compounds that are bonded together, e.g. alumina salts
Composite materials
¾ Combination of two or more materials from above categories
Why Silicon???
MEMS Material
¾ Well understood and controllable electrical properties
¾ Availability of existing design tools
¾ Economical to produce single crystal substrates
¾ Desirable mechanical Properties:
-
High melting point. (1400 ºC)
Small thermal expansion coefficient, 10 times smaller than Al
High Young’s modulus (same as steel ~ 2 x 105MPa)
Light as aluminum (density ~ 2.3 g/cm3)
No mechanical hysteresis(Good candidate for sensors and actuators)
¾ Crystalline orientation is important in MEMS fabrication processes
¾ Integration with electronics on same substrate
MEMS Material
Silicon Crystal
Structure
Crstal lattice: FCC packing
Zincblende: Tetragonal configuration
Sandia Labs
Miller indices
(h k l):
reciprocals of intercepts with the x,y,z axis
multiplied by common factor to get set of smaller integers
<a b c>: normal to plane (a b c)
Z
Z
Y
X
Y
X
(100)
Z
Y
X
(110)
(111)
www.usna.edu
Materials for MEMS
¾ Single crystal silicon – SCS
– Anisotropic crystal
– Semiconductor, great heat conductor
¾ Polycrystalline silicon – polysilicon
– Mostly isotropic material
– Semiconductor
¾ Silicon dioxide – SiO2
– Excellent thermal and electrical insulator
– Mask in etching of silicon substrates
– Sacrificial layer
¾ Silicon nitride – Si3N4
– Excellent electrical insulator
– Ion-implantation masks and barrier to diffusion.
¾ Aluminum – Al
– Metal – excellent thermal and electrical conductor
MEMS Material
Outline of the talk
MEMS overview
General Overview of MEMS
¾ Concept, History, Why MEMS, Miniaturization, VLSI vs. MEMS
Materials for MEMS
¾ Materials Overview, Why Silicon?, Silicon Crystal, Materials for MEMS
MEMS fabrication processes
¾ Etching review, Bulk micromachining, Surface micromachining,
HexSiL, LIGA, Bulk vs Surface micromachining, ZPAL
MEMS devices
¾ Sensors, Actuators, SAW devices, Pressure, Temperature and Inertial
sensor, smart sensors, Design Considerations, MEMS Packaging
Conclusion
MEMS fabrication processes
Etching review
Chemical process wherein material is removed by chemical reaction between
the etchants and the material to be etched.
Characterization
- Etch rate:
Material thickness etched per unit time
- Etch selectivity: Measure of effectiveness in removing material to
be etched and not affecting other materials present
- Etch uniformity: Topographic impact
Isotropic etching
Anisotropic etching
Etching in all directions at same rate
Etching in one direction
Surface uniformity problems
Etching along crystal planes.
E.g. HNO3, HF, CH3COOH
E.g. KOH
Anisotropic: Different chemical relativities of certain crystal planes of Si
MEMS fabrication processes
MEMS Manufacturing Technology
Bulk Micromachining
¾ Selectively remove significant amounts of silicon from the substrate
¾ Oxidation of Silicon
¾ Products physically removed from substrate
¾ Isotropic and anisotropic
¾ Large structures( both physically and in
terms of mass) can be fabricated from it
¾ 3D shapes like V-grooves, channels, pits
¾ Compatible with CMOS technology
Reference 2
Bulk Micromachining
MEMS fabrication processes
Wet etching
Isotropic wet etching
HNA (HF, HNO3 , CH3COOH)
NO2 is the oxidizing agent.
18HF + 4HNO3 + 3Si
3H2SiF6 + 4NO(g) + 8H2O
Charge transfer driven process
¾ Etch rate: 4-20 μm/min
¾ Etch masks: Si3N4 and SiO2
¾ SiO2 fairly attacked (30- 70 nm/sec)
¾ Agitation speeds up process
¾ Slows down in light doping (1017 cm-3 n/p)
Reference 1
Bulk Micromachining
MEMS fabrication processes
Wet etching
Anisotropic Wet etching
Etchant
Temperature(°C)
Si (100)
Etch rate( μm/hr)
Si(110)
Si(111)
KOH
75
25-42
39-66
0.5
EDP
110
51
57
1.25
NH4OH
75
24
8
1
TMAH: (100) 0.5-1.5 μm/min
KOH etching smoother than EDP and NH4OH
Oxidizers are added to reduce hillocking
- Consume hydrogen gas
Reference 1
Bulk Micromachining
MEMS fabrication processes
Anisotropic Wet etching
Characteristics:
- KOH etching reduced at Boron doped regions (2 x 10-19 cm-3)
- KOH:
Alkali metal ions, not CMOS compatible
- TMAH and EDP CMOS compatible
- TMAH: Conc. etch rate and surface roughness
- TMAH: Etching reduced 10 times at Boron doped regions (10-20 cm-3)
- EDP potentially carcinogenic
- Commonly used etch masks: Si3N4 and SiO2
Bulk Micromachining
MEMS fabrication processes
Etch rate modulation
Doping selective etching (DSE)
- Boron doping reduces etch rate, e.g TMAH, EDP and KOH
- Fabricate released structures
- May introduce considerable mechanical stress
Electrochemical modulation
- p-n junction is formed, etching stops due to rate difference in p
and n type
- pn junction is reverse biased and etching stops at n material
Optical energy
- Photon pumping and laser
Bulk micromachining
MEMS fabrication processes
Surface Plasma-Phase Etching
External energy like RF power forms ions and electrons and drives the
chemical reaction.
¾ Chlorofluorocarbons,SF6 ,Br compounds and oxygen used
¾ Fluorine free radicals to form SiF4
DRIE: Polymer deposition in parallel with etching
¾ Full spectrum of isotropic through anisotropic available
- Variable Anisotropy etch process
¾ Ion bombardment for etch depth control
¾ Easy to start and stop
¾ Few particulates
Bulk micromachining
MEMS fabrication processes
Variable Anisotropy etch process
Combination of Isotropic and
anisotropic etching
Reference 2
Vapor phase Etching
¾ Reactive gases are used
¾ Xenon Difluouride Etching: XeF2: Al, Si, Si3N4, photo resist resistant
¾ Interhalogen Etching: BrF3 and ClF3 with Xenon diluent: Thermal SiO2 mask
¾ Laser driven Vapor phase Etching: Free radical formation by photolysis
¾ Some Plasma phase etching properties without complex and expensive equipment
MEMS fabrication processes
MEMS Manufacturing Technology
Surface Micromachining
Deposited films to make the mechanical parts extending above the silicon
substrate.
¾ A material is deposited on the underlying sacrificial layer
¾ Sacrificial layer is dissolved, freeing the element attached to the
substrate
¾ Uses Poly silicon and silicon dioxide
- Single crystal silicon is brittle
and fractures without yielding
- Poly silicon has a tighter spread
in fracture stress distribution
- Poly silicon proves to be more
controllable
- Poly silicon shows low
coefficients of variation in
manufacturing
Reference 4
MEMS fabrication processes
Surface Micromachining
Polysilicon lateral resonator
1. Substrate Passivation and
interconnect
2. Sacrificial layer deposition and
Patterning
3. Structural Polysilicon deposition,
Doping and Stress anneal
4. Microstructure Release, Rinse
and dry
Reference 2
MEMS fabrication processes
Surface Micromachining
Mechanical Problems
¾ Interfacial Stresses
- Mismatch of thermal coefficients
- Residual Stresses
¾ Adhesion of Layers
- Wafers are rinsed in deionized water and dried
- Surface tension of water in some cases causes structures to adhere
themselves permanently to the wafer
→ Use thick layers
→ Place small bumps at the bottom surface
¾ Peeling Off
- Severing at the interface Surface
MEMS fabrication processes
MEMS Manufacturing Technology
¾ HexSiL - A molten poly silicon process
- Deep silicon mold etch
- Sacrificial layer deposition
- Structural layer deposition
- Chemical mechanical polish
(CMP)
- Release and extract
¾ Advantages
Reference 3
- Appropriate design of surface and mold fill regions
- Results in both rigid and flexible members on the same device
- Composite structures possible if outside layer can withstand the
sacrificial etching process
MEMS fabrication processes
MEMS Manufacturing Technology
LIGA: German acronym for Lithographie, Galvanoformung
and Abformung
- High aspect-ratio structures (upto 1mm)
- Uses deep x-ray lithography (DXRL)
Steps:
Substrate preparation
Irradiation
Development
Optional
Hot embossing
Resist removal
Electroforming
MEMS fabrication processes
MEMS Manufacturing Technology
¾ Metal structure can itself also act as product or serve as mould
¾ The metal structure, including base plate, may be diced into 3-D parts
¾ Finished metal parts can be removed from the base plate (loose parts)
Advantages
- High precision metal structures
- Rugged - retains alignment over life of the product
- Higher performance
SU-8
Cheap alternative to LIGA:
- Uses special epoxy-resin-based optical resist
- Thick layers(>500 μm) and uses common lithography tools
MEMS fabrication processes
Bulk vs. Surface micromachining
Bulk micromachining
Surface micromachining
Well established (since 1960)
Relatively new (since 1980)
Rugged structures that can withstand
vibration and shock
Less-rugged structures with
respect to vibration and shock
Lager die area that makes gives it
high cost
Small die area that makes it
cheaper
Not fully integrated with IC processes
Fits well within IC technology
Limited structural geometry possible
Wider range of structural geometry
Large mass/area
(suitable for accelerometers and
capacitive sensors)
Small mass/area
Well characterized material (i.e. Si)
Some of the material properties
not very well understood
MEMS fabrication processes
Zone-Plate-Array Lithography (ZPAL)
¾ An array of hundreds of micro fabricated diffractive optical elements
(Fresnel-zone-plate lenses) is used, each focusing a beam of light onto the
substrate
¾ Computer-controlled array of micromechanical elements turns the light to
each lens on or off as the stage is scanned under the array, thereby
printing the desired pattern in a dot-matrix fashion
¾ No mask is required, enabling
designers to rapidly change
circuit designs
¾ Developed at UV and DUV
wavelengths
¾ Extendable to X-ray wavelength
Wmin=k * λ /NA
λ = Wavelength
NA = Numerical Aperture
Nanostructures Lab, MIT
MEMS fabrication processes
Zone-Plate-Array Lithography (ZPAL)
Operating Conditions:
- 25 mW GaN laser diode operating
at 400 nm
- ~200 beams, each operating at a
pixel-transfer rate of ~7.5 kHz
- 0.85 numerical aperture (NA) zone
plates, and k1 = 0.3, with a min.
feature size Wmin = 141 nm
(f) microcomb structure for MEMS
(g) curved waveguides
(h) portion of a zone plate
Scanning electron micrographs of patterns
Outline of the talk
MEMS overview
General Overview of MEMS
¾ Concept, History, Why MEMS, Miniaturization, VLSI vs. MEMS
Materials for MEMS
¾ Materials Overview, Why Silicon?, Silicon Crystal, Materials for MEMS
MEMS fabrication processes
¾ Etching review, Bulk micromachining, Surface micromachining,
HexSiL,LIGA, Bulk vs. Surface micromachining, ZPAL
MEMS devices
¾ Sensors, Actuators, SAW devices, Pressure, Temperature and Inertial
sensor, smart sensors, Design Considerations, MEMS Packaging
Conclusion
Sensors
MEMS devices
¾ Pressure sensors
- Deflection in micro membrane
¾ Inertial sensors
- Test mass (use piezoresistor or capacitive positioning system)
¾ Thermal sensors
- Temperature dependence, e.g. Hg
¾ Chemical sensors
- Electrical impedance function of gas composition and concentration
- Conductive polymers,doped polymers
¾ Resonant sensors
- Resonant frequency dependent on geometry
- Mechanical loading, e.g. SAW sensors
Actuators
MEMS devices
¾ Electrostatic actuator
- Force between two plates of capacitor.
¾ Thermal actuator
- Difference in thermal coefficients, bi-morph design
- Force generated due to phase change
- Shape memory alloys-large force for small change
¾ Magnetic actuator
- Coils are used to generate field
¾ Piezoelectric actuator
- Mechanical force on application of electric field
- High frequency response, e.g. PZT and ZnO for stepper motor
MEMS sensors
MEMS devices
Temperature Sensor
¾ Commonly found embedded in devices for signal compensation
- Resistive
• ρ (T) = ρ0 (1 + αT T + βT T2)
• ρ0 = resistivity at standard temp (0 °C )
• αT , βT = Temperature coefficients
• Platinum is used on account of stability over -260 to +1700 °C
- Microthermocouples
• VT = (PB – PA) ΔT
• P = seeback coefficient
• Silicon Aluminum thermocouple:
Output of order of mV per degree
MEMS sensors
MEMS devices
Pressure sensor
¾ Most mature of all micro sensors and widely commercialized
- Silicon diaphragm made using Bulk/surface micromachining
¾ Piezoresistive
- Deflection measured using piezoresistive strain gauges
- Strain gauges are made from doped silicon
- Wheatstone bridge connection
• Vout α ∏ (P – P0)
• ∏ = Relative piezoresisitive
coefficient
Reference 4
MEMS devices
Pressure sensor
¾ Capacitive
- Capacitive bridge is formed
• Vout α ΔC α Δx α (P – P0)
- Accurate positioning of pickup
capacitor is critical
Reference 4
Capacitive
Piezoresistive
Large pieces of Si for Bulk
micromachining
Smaller structure
Less temperature sensitive
Strong temperature dependence
Electronically more complicated
Simple transducer circuit
Needs integrated electronics
No need for integration
MEMS devices
MEMS sensors
Inertial sensor: linear acceleration
¾ Microaccelerometer
- Cantilever principle in which an end mass displaces under
inertial force
Seismic mass movement
Piezoresisitive pickup
Reference 4
- Capacitive polysilicon are more prevalent
SMART sensors
MEMS devices
A sensor that has a part or its entire processing element integrated in a
single chip.
Advantages
¾ Reduces manufacturing cost of the device
¾ Enhances performance
- Less parasitic components
- High reliability
¾ Additional features
- Damping and overload protection
- Compensation for ambient temp
- Self testing for fault diagnosis
¾ Integration of microprocessors, memory
¾ Examples: electronic nose, web camera, silicon retina
Design Issues
MEMS devices
¾ Requirements of customer
¾ Operating conditions
¾ Size and weight limitations
¾ COST
Selection of Materials
¾ Dependent on mechanism
Selection of signal transduction
¾ Input and output
Selection of fabrication process
¾ LIGA is most expensive but high aspect ratio
¾ Bulk in case of extensive 3D structures
www.memx.com
MEMS Packaging
MEMS devices
Mechanical support: Device may function alone or with other device
Physical protection: Protection from environment
Interfacing
¾ Optical:
Passage of beams but protection against particulates
¾ Biomedical:
No harm to living cells
¾ Fluidic:
Laminar flow and proper mixing
¾ Mechanical:
Moving parts
¾ Chemical:
External particles
¾ Electrostatic: External fields
Interconnections:
Communication of signals
Conclusion
¾ Miniaturization: MEMS
NEMS
¾ Properties of thin films different from bulk
¾ Reduction of size and cost
¾ MEMS micromachining
¾ Evolution of new materials and devices
¾ Focus on smart MEMS, Lab on Chip…
¾ Highly competitive and expanding market
¾ New opportunities (MEMS in space)
www.aero.org
References
MEMS devices
¾ Jack W Judy, “Microelectromechanicalsystem (MEMS): fabrication, design and
application," Smart Materials And Structures, IEEE Review Paper, November 2001.
¾ T. A. Kovacs, Nadim I. Maluf, Gregory And Kurt E. Petersen, “Bulk Micromachining of
Silicon,”IEEE Review Paper, August 1998.
¾ James M. Bustillo, Roger T. Howe And Richard S. Muller, “Surface Micromachining
for MicroelectromechanicalSystems,”IEEE Review Paper, August 1998.
¾ Microsensors MEMS and Smart Devices, Julian W. Gardner, Vijay K. Vardan and
Osama O. Awadelkarim, John Wiley & Sons, Ltd., West Sussex, 2001.
Acknowledgements
My sincere thanks to Prof. Kal, for his guidance and the material provided for study.