Investigation of SiO₂ plasma enhanced chemical vapor deposition through tetraethoxysilane using attenuated total reflection Fourier transform infrared spectroscopy

In situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was used to study surface processes during plasma enhanced chemical vapor deposition (PECVD) of silicon dioxide through tetraethoxysilane (TEOS) and oxygen. ATR-FTIR studies were conducted on thin (about 50 A) silicon dioxide films deposited on GaAs. This approach allowed us to obtain the infrared spectrum of TEOS adsorbed on Si0₂ in the spectral region 4000–770 cm_-1 and to determine the surface species and their relative surface concentrations as a function of deposition conditions in a helical resonator plasma reactor. Studies were conducted where the SiO₂ surface was exposed to TEOS and 0₂ plasma sequentially and/or simultaneously. Surface processes were studied as a function of exposure to TEOS and substrate temperature. In situ ATR-FTIR studies of adsorption of TEOS on the SiO₂ surface show that TEOS adsorbs chemically and irreversibly onto the SiO₂ surface above 100 °C. SiO₂ growth was found to occur even without an oxygen plasma at temperatures as low as 200 °C, albeit very slowly. Below 100 °C TEOS was also found to adsorb physically: The extent of the physically adsorbed state was found to increase with decreasing temperature. Hence, during PECVD of SiO₂ below 100 °C, physically adsorbed TEOS can be trapped in the growing oxide giving rise to increased ethoxy and OH species which in turn adversely affect the film integrity and quality. The ATR-FTIR studies of the surface exposed to TEOS at various temperatures indicate that the chemical adsorption of TEOS onto SiO₂ proceeds through a precursor-mediated adsorption mechanism where the precursor is a physically adsorbed TEOS molecule. Exposure of adsorbed TEOS to O atoms (oxygen plasma) removed the ethoxy ligands of the surface ethoxysiloxanes, produced surface SiOH species as reaction product, and resulted in deposition of SiO₂. In situ and real time studies of the actual PECVD process revealed that water and SiOH species are formed as intermediate surface reaction products and subsequent reaction and elimination of the silanol species can become the rate limiting step (as far as SiOH incorporation in the film is concerned) if the deposition rate is too high.