KSEF RDE: Laser Spectroscopic Studies of Intermediates in the Production of Silica and Silicon Thin Films

Silanes (SiX4) and siloxanes (X3Si-O-SiX3) are important precursor compounds in the industrial production of silicon thin films for semiconductors and high purity silica for the optical fibers used in telecommunications. In the process of precursor degradation, either by high temperature reactions or in plasmas, it has been shown theoretically that a large variety of reactive intermediates are produced, although few of these have been indentified chemically or spectroscopically. In particular, one class of such species, the siloxy free radicals (X3SiO), are unknown, despite their direct relevance to several industrial processes. We propose to detect and characterize the H3SiO, F3SiO and Cl3SiO free radicals in the gas phase for the first time. They will be generated by electric discharge bombardment of precursor mixtures of SiX4 and oxygen in high pressure argon, cooled to low temperatures (5-10 K) in a supersonic expansion, and studied by laser induced fluorescence techniques. An alternative precursor set would be the simple siloxanes (X3Si-O-SiX3 ; X = H, F or Cl), which are commercially available or readily synthesized compounds. In preliminary experiments with SiCl4/O2 mixtures, we have found a spectrum in the visible near 500 nm that we attribute to the previously unknown Cl3SiO species. The same spectrum is obtained using a Cl3Si-O-SiCl3 precursor and the spectral features agree very well with the ground and excited state vibrational frequencies we predict from our own high level ab initio calculations. We propose to use laser spectroscopy and computational methods to completely characterize Cl3SiO and find the other unknown radicals. The results of this work will be directly applicable to the study of these intermediates in chemical vapor deposition or high temperature reactions, as the LIF and emission spectra provide a convenient and sensitive handle for the detection and quantification of the radicals in industrial environments.