Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 3M

SiO2 is deposited below 5000 C by reacting silane and oxygen ia an PCVD, LPCVD or PECVD reactor.

Due to low temp compared to thermally ground oxide, this is known as low-temp oxide.

The optional addition of phosphine or diborane dopes the silicon oxide with phosphorous or boron respectively.

Films doped with phosphorous and boron are known as borophosphosilicate glass (BPSG) and those doped with phosphorous are refered to as phospho silicate glass(PSG).

When annealed at temp near 10000 C, both PSG and BPSG soften and flow to conform with the underlying surface topography and to improve step coverage.

LTO films are used for passivation coatings over aluminum, but the deposition temp. must remain below about 4000 C to prevent degradation of the metal.

SiO2 can also be deposited at temp between 6500 and 750 0 C in a LPCVD reactor by the pyrolysis of tetra ethoxysilane [Si(OC2H4)4] also known as TEOS.

Oxides deposited by TEOS exhibit excellent uniformity and step coverage, but the high temp. process provides their use over Aluminum.

A third but less common method to deposit silicon dioxide involves reacting dichlorosilane (SiCl2H2) with nitrous oxide (N2O) in LPCVD reactor at temp. near 9000 C , film properties and uniformity are excellent, but its use is limited to deposited insulating layer over polysilicon.

In LPCVD deposition rates for SiO2 increases with temp, A typical LTO deposition rate at low pressure is 25 nm/min at 4000 C , rising to 150 nm/min at atmospheric pressure and 4500 C the deposition rate using TEOS varies from 5 nm/min at 6500 C up to 50 nm/min at 750 0C.

Deposited silicon dioxide films are amorphous with a structure similar to fused silica, Heat treatment (annealing) at high temp (600-10000 C) result in the outgassing of hydrogen incorporate in the film and a slight increase in the amorphous structure. This process is called densification.

SiO2 deposition by CVD is very useful as a dielectric insulator between layers of metal or as a sacrificial layer. In surface micromachining .

But its electrical properties are inferior to those of thermally grown SiO2eg. Dielecric strength of CVD SiO2 can be half that of thermally grown SiO2 . That’s why gate insulator for CMOS are made of thermaay grown oxides.

SiO2 by CVD are under compressive stress (100-300 MPa) and stress can not be controlled except when PECVD is used.