Gas Phase Etching of Silicon Dioxide Films

Abstract

The gas phase etching of thermal silicon dioxide films was investigated with in situ Fourier Transformed Infrared Spectroscopy (FTIR) and ex situ X-ray Photoelectron Spectroscopy (XPS). The initiation process, the bulk etching of the oxide, and the termination mechanism were characterized as a function of reactant concentration, temperature, and pressure. The experiments were carried out in a custom made vessel with a gas panel and a data acquisition and control system (DA&C) capable of lowering flow and pressure disturbances originated by reactant introduction. The FTIR technique used to monitor the reaction in real time allowed distinguishing reactions that initiated in a gas/solid regime from reactions that started in a gas/liquid/solid regime. This study was focused on the gas/solid initiation process in order to expand the general assumption in published works that a condensed layer is previously required to initiate and sustain the reaction. It was found in this investigation that, depending on the experimental parameters, the water layer is not always a requisite for the initiation of the reaction but a consequence of the etching process. The FTIR data also showed the role in the initiation process of gas phase heterogeneous associated species, specifically (HF)H₂O and (HF)₂H₂O. After the initiation period, the experimental conditions determined the amount of water present on the surface of the sample, which in turn determined the local environment of the reaction and by extension the etching species. Reactions developing in a gas/solid regime were found to be slow, with etching rates of less than 1 °A/sec. Contrarily, reactions taking place in a gas/liquid/solid regime reached etching rates of 100 °A/sec, a maximum value determined by transport limitations. The condensed layer was found to be especially sensitive to temperature since a variation of 15 ° C changed the local environment from gas/liquid/solid to gas/solid. Finally, it was corroborated through the XPS analysis that the removal process in the gas phase leaves the silicon surfaces with high fluorine and oxygen concentrations in the form of SiFₓ and SiOH.