The low-pressure, chemical vapor deposition of Si02 layers using CO2 as the oxygen source with applications to CNT-MEMS growth

Deposited silica (SiO$_{2})$ has a number of applications for microfabricated structures, particularly those based on coating carbon nanotube forests. Members of our group have, for example, reported on the fabrication and use of SiO$_{2}$-coated carbon nanotube forests (CNT-MEMS) to prepare liquid chromatography plates of record efficiency. SiO$_{2}$ also has extremely low thermal conductivity and stiff, coated, carbon nanotube forests could be used as thermal barrier layers. We have examined two novel methods for the LPCVD of SiO$_{2}$ and oxygen-rich amorphous silicon. Both methods are based on the hypothesis that carbon dioxide could be used as the source of oxygen in preparing the material. In the case of oxygen-rich amorphous silicon (a-Si:O) we used silane as the silicon source, and the case of SiO$_{2}$ used dichlorosilane. We deposited the a-Si:O material at about 800K while the Si02 from SiH2Cl2 , was deposited at about 1000 K. Depositions were done at low pressure, about 200 millitorr for the a-SI:O and at about 1 to 4 Torr for the SiO$_{2}$. The substrates in all cases were three-inch single-crystal silicon wafers. We subsequently examined the deposited material using variable-angle, spectroscopic ellipsometry (VASE-John A. Woollam M 1000) for of thickness and optical constants and SEM structure and composition. The dichlorosilane deposition of SiO$_{2}$ suffered from vanishingly small deposition rates at very low pressures at 1000 K and the incorporation of ``snow'' into the films in the case of depositions done at higher pressures. We found little evidence of carbon incorporation.