MEMS selection of manufacturing process

1. ATHUL B S
M1 AEI
ROLL NO: 03

2.  Selection of manufacturing process
 Bulk micromanufacturing
 Surface micromachining
 LIGA & SLIGA
 Selection of signal transduction
 Materials and Signal transduction techniques

3. Selection of micromanufacturing processes includes 3 types;
1. Bulk Micromanufacturing
2. Surface Micromachining
3. LIGA and SLIGA and other High aspect Ratio Processes

4. • It is an oldest technology and same as sculpture designing.
• It involves mainly etching and least expensive than other.
• The process is suitable for simple geometry.
• Substrates used for this process are ; SiC, GaAs, Quartz etc.
Drawbacks
• It has low aspect ratio ( Dimension of depth to that of plane).
• High material consumption.

5. • In this process building layers of materals over the substrate.
• It requires design and fabrication of complex masks for deposition
and etching processes.
• Etching of sacrificial layer necessary after layer building.
• The process is more expensive than Bulk manufacturing process.

6. Advantages
• Not constrained by the thickness of silicon wafer.
• It provides wide choices of materials for layer building.
• Suitable for complex geometries (Eg: Microvalves and Comb-
driven-actuators).

7. • It is the most expensive micromanufacturing technique of all.
• Both LIGA and SLIGA done deep X-Ray lithography , so it
requires Synchroton radiation facility.
• These processes also require the develpoment of microjunction
molding technology.

8. Advantages
• Provides high aspect ratio(200 in LIGA Process).
• More flexibility in microstructure configuration and geometrty.
• There is no restriction on materials for microstructure including metal in LIGA
process.
• They are best of 3 manufacturing for mass production.

9. • Signal transduction is necessary in both microsensor and
actuators.
• One critical design consideration in signal transduction is signal
mapping.
• Signal mapping selects the optimal locations for transducers that
transmit the signal.
Eg: Pressure sensors and piezoresistors for signal transduction
must be placed in optimal locations.

10. •
•
• Piezoresistors
• Piezoelectric
Capacitance
Resoant vibrator
Microsensors and
Accelerometers
Produced signals:
Mechanical, Thermal, Optical,
Acoustical, Chemical etc.
Transducer
Electrical
Signals
Signal
Processing

11. Piezoelectric
Electrostatic
Electroresistant Heating
Shape Memory Alloy
Micro Actuators
Signal to be converted:
Motion Transducer Electrical Signals
Power Supply

12. • For placing pressurized medium there are 2 options:
I. Place transduction arrangement in top surface, but protect
piezoresistors, wire bond, metal circuitary and film pads from
direct contact with pressurized medium.
II. Place transduction unit at bottom of the die, but here much more
complicated process for implanting piezoresistors and wiring
needs to be arranged.

13. 1. Piezoresistors
Silicon piezoresistors are commonly used in microsensors because of minute
size and high sensitivity of signal transduction.
Drawbacks
• Stringent control of doping process required to achieve good quality.
• Stong temperature dependance of resistivity.
2. Piezoelectric
• PZT( PbTi1-x Zrx O3 )crystals are primarily used for displacement of
transducers and accelerometers.

14. • Barium Titanate(BaTiO3) is to transduce signals from microaccelerometers.
• Quartz used a oscillator in ultrasonic transducers.
• Most piezoelectric materials are brittle.
• Size and machinability are two other problems.
• Suitable for measuring dynamic or impact forces that exists in short period of
time.
• This is because sustained piezoelectric action will cause overheating of
crystals and thus deteriorate their conversion capability.
• PZT has high piezoelectric co-efficient.

15. 3. Capacitance
• This is good for high temeperature applications.
• But, the non-linear input-output relationship between change of gap of
electrodes and output voltage require special compensation to avoid errror in
interpreting output signal.
• Requires larger space in microsystem, because capacitance directly
proportional to overlapped area of plate electrodes.

16. 4. Resonant Vibrator
• It offers higher resolution and accuracy for signal transduction in
micropressure sensors.
• Its application to other devices is limited by complexity of fabrication.
• Required space for vibrating members is also a limitation.
5. Electroresistant Heating
• Used in microactuation, such as microvalves and pumps.
• The requirement for precise control of heating of actuating element with
thermal inertia may affect the timely response of the intended actuation.
• Contact of the heated element with working medium can cause local heat
transfer , which alter flow pattern in microvalves.

17. • Sealing of heat transmitting fluids cause problems in packaging and in
microsytem.
6. Shape Memory Alloy
It is a good actuating material when it is used in conjuction with electroresistant
heating.
Drawbacks
• Limited availability of SMA.
• Deformation of SMA cannot be accurately predicted because of the
sensitivity to temperature.