Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 10M

Selection of materials

• Material Selection is an important task as large no. of materials are available for various application.

• Performance of material depends on

i. Stability

ii. Young’s modulus

iii. Melting point

iv. Thermal expansion coefficient

v. Mechanical Hysteresis

Silicon

Ideal substrate material used in MEMS because of the following reasons:

i. It is mechanically stable & easy to integrate with mechanical parts

ii. It has same thermal expansion coefficient as that of steel (215 GPa) , but it is as light as Al with d = 2.3 g/cm3

iii. Melting Point = 14000C (2*Al) = makes dimensionally stable

iv. Thermal expansion coefficient, 8 times smaller than steel & 10 times Al

v. Naturally ,no mechanical hysteresis therefore , ideal for actuators

Silicon compounds:

• Si dioxide (SiO2)

• Si Carbide (SiC)

• Si Nitride (Si3N4)

1) Silicon dioxide (SiO2)

• Least expensive

• Good thermal & electrical isolation

• Used as masking & sacrificial materials in surface micro-machining technique

• Easy to produce

o Dry heating Si , Si + O2 ---> SiO2

o Wet oxidation Si + 2H2O ---> sio2 + 2H2

2) Silicon Carbide (SiC)

• High Melting Point

• Masking

• Superior dimensional stability

3) Silicon Nitride (Si3N4)

• Ultra strong Resistance to oxidation

• High strength Electrical insulator

• Masking for deep etching

4) GaAs

• Compound semi-conductor with equal no. of Ga & As atoms

• Excellent material for nanolithography photonic devices

• Therefore GaAs compound is not easy to produce

• High electron mobility (7 * Si)

• Good thermal insulator

5) Quartz

• Used in sensor , excellent structural stability

• High Melting Point

• Used as piezoelectric material

• Good insulator

6) Polymer

• Used as masking in photolithographic technique

• Used as organic substrate

• Electric insulator & dielectric substances in micro transistor

• Used as packing material

SU – 8

• Negative , epoxy type based polymer sensitive to UV light

• Used to produce thin films from 1µm-2µm

• Much lower production cost that Si films

• Commercially available in liquid.