Gas-phase HF/vapor etching of thermal silicon dioxide films

The etching of thermally grown SiO₂ films using gas phase mixtures of anhydrous HF and water vapor was investigated with in situ transmission Fourier transform infrared (FTIR) spectroscopy in real time as a function of the HF flow rate. Although the HF flow and hence the partial pressure were varied by a factor of two, the induction times were similar, in the range of 5-7 s. Only the vibration at 3642 cm^-1 indicative of surface silanols or gas phase molecular clusters of HF and water molecules increased during the induction period. After induction all of the films began etching as shown by the increase in the FTIR stretches at 1030 cm^-1 (SiF₄ gas) and at 1728 and 3482 cm^-1 (adsorbed product H₂O) as well as by the decrease in the 1076 cm^-1 peak (SiO₂). The product water peak areas were approximately the same at the three HF flow rates studied. The SiF₄ product detected in the gas phase scaled inversely with etching rate. Both adsorbed O and F were detected by in situ XPS after etching. The results suggest that there may be an optimum surface coverage of F to efficiently etch SiO₂ films at 35°C and 200 Torr.