REACTIVE PATHWAYS IN THE CHEMICAL VAPOR-DEPOSITION OF TIN OXIDE-FILMS BY TETRAMETHYLTIN OXIDATION

Chemical pathways for the formation of tin oxide and fluorine-doped tin oxide (FTO) films by atmospheric pressure chemical vapor deposition (CVD) have been investigated. The CVD processes are based on the oxidation of tetramethyltin (TMT) in the temperature range 350°-600°C. FTO films are prepared by adding bromotrifluoromethane (CF3Br) to the reagent gas mixture. The results demonstrate that there are gas-phase reaction intermediates that influence film growth rate. Gaseous reaction by-products were identified and measured by gas chromatography and mass spectrometry (GC-MS). Methane, ethane, ethylene, carbon monoxide, carbon dioxide, and water were found in tin oxide CVD effluent. In addition to the products found in the undoped system, formaldehyde, methyl bromide, and hydrogen fluoride are formed during the FTO process. Small quantities of hexafluoroethane, trifluoromethane, and 1,1-difluoroethene also were detected in FTO CVD effluent. These products may result from a free radical branching chain reaction involving methyl, trifluoromethyl, hydroxyl, and organotin radicals. Volatile radical scavenging agents change the course of reaction. Bromine, t-butyl bromide, and t-butyl fluoride acted as strong inhibitors of film growth. Depending on their concentration, they delayed the onset of film deposition, reduced film growth rate, or stopped film growth entirely. The C2-C4 alkenes acted as selective inhibitors of film growth. They prolonged the later stages of deposition, modified the morphology of film produced in these later stages, and inhibited the formation of powder.