Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 3M

SiN3is common in the semiconductor industry for the passivation of electronic device because it forms an excellent protective barrier against the diffusion of water and Na+ ions.

In MEMS, LPCVD silicon nitride films are effective as masks for the selective etching of silicon in alkaline solutions, such as potassium hydroxide. It can also be used as structural material.

Stoichiometric pressure by reacting silane (SiH4) and ammonia (NH3) , or at low pressure. by reacting dichlorosilane (SiCl2H2) and ammonia.

The deposition temp for either of the method is between 7000 C and 9000 C.

CVD and LPCVD silicon nitride films has large tensile stresses approaching 1000 Mpa. But if LPCVD silicon nitride is deposited at 8000 – 8500 C and is silicon-rich due to greatly increased dichlorosilane flow rate, the stress can be below 100 Mpa, which is acceptable for MEMS application.

For deposition below 4000 C, nonstiometric silicon nitride is obtained by reacting silane with ammonia or nitrogen in a PECVD chamber, hydrogen is also a byproduct of this reaction and is incorporated at high concentration (20-25%) in the film.

The refractive index for stoichiometric LPCVD silicon nitride is 2.01 and ranges between 1.8 and 2.5 for PECVD films.

High value in the range indicates excess of silicon and low value represents an excess of nitrogen.

One of the advantages of PECVD nitride is the ability to control stress during deposition.

Silicon nitride is deposited at a plasma excitation frequency of 13.56 exhibits tensile stress of about 400 Mpa, by alternating frequencies during deposition lower stress films can be obtained.