Gas and Adsorbed-Phase UV Photochemistry of Tetramethyltin (TMT) Probed by In-Situ Optical Diagnostics and Surface-Sensitive Techniques

Abstract

Excimer lasers have been widely used in recent years to induce chemical vapor deposition (L-CVD) of thin films from volatile precursors. The purpose of this paper is to correlate the excimer laser induced molecular processes in gas and adsorbed phase with film composition and growth,using ArF laser irradiated TMT (tetramethyltin) as a model system. On-line optical diagnostics were employed to investigate the photolysis of TMT induced at 193 nm (ArF) in the gas and adsorbed phases. Laser-induced fluorescence was used to monitor the production of ground state Sn atoms, whereas excited Sn atoms were observed by detecting their spontaneous emission. The comparison of the corresponding yields as a function of the ArF laser pulse energy evidenced that ground state and emitting Sn atoms are produced through two competing reaction channels, which are both quenched when TMT is irradiated in the presence of oxygen. The photochemistry occurring in the adsorbed phase was monitored by resonant ionization of the Sn containing fragments desorbed by the ArF laser pulse from the cooled surface of a Si wafer (T= 160-180 K) immersed in TMT vapor. A time interval was observed between the beginning of the UV irradiation and the onset of desorption, foliowed by steep rise and saturation. The dependence of the desorption signal on ArF laser fluence and TMT gas pressure permitted an outline of the mechanisms working in the first stages of film nucleation and growth. In situ Auger analysis permitted determination of the composition of the films deposited by ArF laser irradiation of TMT and investigation of the progressive oxidation of the thin films resulting from dilution of TMT in oxygen. It was found that incomplete elimination of organic radicals in the gas- and adsorbed-phase UV photolysis of TMT causes the presence of C in the photodeposited films. 02 is effective in promoting Sn film oxidation and elimination of carbon contamination through a chemical reaction with the ligand-carrying photofragments foliowed by the formation of volatile organic compounds and stable Sn-O bonds.