Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 3M

CVD process allows the deposition of poly silicon as a thin film on a Si substrate.

Where film thickness ranges from few tens of nanometer to several µm.

Structures with several layer of poly silicon is also possible.

Poly silicon is deposited by the pyrolysis of silane(SiH4) to silicon and hydrogen in a LPCVD reactor.

Deposition from silane in a low-temp. PECVD reactor is also possible but results in amorphous silicon.

The deposition temp in LPCVD, typically between 5500 and 700 0 C, affects the granular structure of thin film.

Below 6000 C, the thin is completely amorphous and above 6500C, it exhibits a crystalline grain structure. Deposition rates varies from approx. 6nm/min at 620 0 C up to 70 nm/min at 700 0 C

Partial pressure and flow rate of the silane gas also affect the deposition rate.

Poly silicon can be doped during deposition known as in situ doping by introducing dopant source gases(Arsine phosphine for n-type doping and diborane for p-type doping).

Arsine and phosphine greatly decrease the deposition rate (to about 1/3 that of undoped poly silicon), where asdiborane increases it.

The dopant concentration is very high but the field resistivity remains in the range of 1 to 10 m because of the low mobility of electrons and holes.

Intrinsic stresses in Arsine doped poly silicon films can be large (>500 Mpa)on the deposition temp.

There is a stress gradient through the thickness the film, which gives curling of released micromechanical structures.

Annealing at 9000 C or above causes stress relaxation through structural changes in grain boundaries and a reduction is stress to levels < 50 Mpa are acceptable for micro machined structures.