Laser-induced Deposition of Silicon Films and CARS Diagnostics CO₂ Laser Assisted CVD Processes

Abstract

The present paper is concerned with cw CO₂ laser assisted Chemical Vapor Deposition (CVD)of silane molecules (SiH₄). The observed dependence of deposition rates on laser wavelengths indicates that gas phase reactions are involved in deposition processes. We applied a powerful new diagnostic means, Coherent Anti-Stokes Raman Spectroscopy (CARS), in order to detect species involved in the reactions. Temperature was deduced from CARS spectra of N₂ mixed with SiH₄ gas. The CO₂ laser beams were found to raise the temperature of the gas containin SiH₄ when they were tuned to absorption lines of SiH₄. CARS spectra of the heated Sill, gas were characterized by a complicated profile with a broad shoulder and double peaks. We interpreted the shape of the spectra as arising from reaction products, probably Si₂H₆ and hot bands of SiH₄. Applying this interpretation to observed results, we can judge how Siff, molecules are disturbed differently according to laser wavelength and a distance from a substrate. Near the substrate the spectra are characterized by the presence of the broad extra peak of hot SiH₄ and Si₂H₆ in addition to the main peak of SiH₄. As the distance from the substrate increase, the extra peak gets close to the main peak and the widths decrease. This suggests that the effect of substrate on heating the molecules in the gas phase is very efficient when the cw CO₂ laser beam is normal to the substrate.