Understanding the contributions of F–, HF, and HF2– to the etching of SiO2 and unveiling the reaction kinetics to represent etching behavior of SiO2 up to pH 5

Abstract

Understanding of the silicon dioxide (SiO2) etching mechanism in hydrofluoric acid (HF) solutions is important in semiconductor manufacturing processes. A study on the previous SiO2 etching mechanism claimed that only difluoride (HF2–) participated in SiO2 etching. However, in this study, it was observed that the etching of SiO2 occurred in the HF solution, where difluoride barely existed. Hence, a new SiO2 etching mechanism and reaction rate were proposed. Various concentrations of HF and ammonium fluoride (NH4F) under varying pH conditions were used to etch SiO2, leading to an SiO2 etching rate of 510[F–] + 610[HF] + 2890[HF2–] (Å/min). HF2– was the primary contributor to SiO2 etching; however, unlike the previous SiO2 etching mechanism, the effects of F– and HF on the etching of SiO2 were included in the rate of SiO2 etching. The SiO2 etching rate, considering the reactions between various fluorine species with varying SiO2 surface states, effectively represented the SiO2 etching behavior under various conditions over a broader pH range, up to pH 5. Controlling the ratio of the fluorine species, F–, HF, and HF2–, our etching model could also be extended to the selective etching of SiO2.