Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 10M

Silicon compounds:

1) Silicon dioxide [ $SiO_2$ ]

2) Silicon nitride [ $Si_3N_4$ ]

3) Silicon carbide [ S i C ]

1) Silicon dioxide [ $SiO_2$ ] :

• Various forms of silicon oxides are widely used in micro-machining due to their excellent electrical and thermal insulating properties.

• They are also used as sacrificial layers in surface micro-machining processes because they can be preferentially etched in hydrofluoric acid [ HF] with high selectivity to silicon.

• $SiO_2$ is thermally grown by oxidizing silicon at temperatures above $800^o$ C other forms of oxides and glass are deposited by CVD, spin-on

• Silicon oxides and glass layers are known to soften and flow when subjected to temperatures above $700^o$ C

• A drawback is large intrinsic stresses which are difficult to control. So they can not be used as materials for large suspended beams or membranes.

Characteristics:

1. Density : 2.27g/c $m^3$

2. Resistivity $\geq$ $10^{16}$

3. Melting point : $1700^o$ C

4. Thermal conductivity : 0.014w/c$m^o$c

5. coefficient of thermal expansion: 0.5

6. young's modulus : 73

Uses :

1. It is used as thermal and electric insulator.

2. It is used as mask and sacrificial layer in surface micro machining.

2) Silicon Nitride [$si_3$ $N_4$] :

• Silicon Nitride is also a widely used insulating thin film and is effective as a barrier against mobile, ion diffusion in particular, sodium and potassium ions found in biological environments.

• It's young's modulus is higher than that of silicon and its intrinsic stress can be controlled by the specifics of the deposition process. Silicon nitride is effective masking materials in many alkaline etch solutions.

Characteristics :

1. Density : 3.44g/ $cm^3$

2. TCR : 3.1

3. Thermal conductivity : 29

4. Young's modulus = 310

Uses :

1. Provides excellent barrier to diffusion of water and ions such as sodium.

2. It is used for masking deep etching.

3. It is used in optical wave guides encapsulants to prevent diffusion of water.

4. It is high strength dielectric insulators.

3) Silicon carbide [S i C]

Characteristics :

1. Density : 3.2 g/ $cm^3$

2. TCR : 7508 PPM$/^o$C

3. Thermal conductivity = 3.8

4. Young's modulus : 96

Uses :

1. As it has strong resistance to oxidation even at high temperature it is deposited to protect film from extreme temperature.

2. Used in dry etching.

3. It is used as passivation layer in micro machining.