Spontaneous etching of B₂O₃ by HF gas studied using infrared spectroscopy, mass spectrometry, and density functional theory

The spontaneous etching of boron oxide (B₂O₃) by hydrogen fluoride (HF) gas is important during thermal atomic layer etching after BCl₃ converts the surface of various metal oxides to a B₂O₃ layer. In this study, the chemical vapor etching (CVE) of B₂O₃ by HF was experimentally monitored using Fourier transform infrared (FTIR) spectroscopy and quadrupole mass spectrometry (QMS). The spontaneous etching of B₂O₃ by HF gas was also analyzed using density functional theory (DFT). B₂O₃ films were grown using B₂O₃ atomic layer deposition with BCl₃ and H₂O as the reactants at 40 °C. FTIR spectroscopy then observed the CVE of B₂O₃ by HF at 150 °C. B₂O₃ etching was monitored by the loss of absorbance for B–O stretching vibration in B₂O₃ films. FTIR spectroscopy studies also observed B–F stretching vibrations from BF_x species on the B₂O₃ surface after HF exposures. In addition, the QMS analysis was able to identify the etch products during the spontaneous etching of B₂O₃ by HF gas at 150 °C. The QMS studies observed the main volatile etch products as BF₃, BF₂(OH), and H₂O. Additional volatile etch products were also detected including B₃O₃F₃ and other boroxine ring compounds. The DFT predictions were consistent with the spontaneous etching of B₂O₃ by HF gas. DFT confirmed that CVE was likely because the energetics of the spontaneous etching reaction B₂O₃(s) + 6HF(g) → 2BF₃(g) + 3H₂O(g) were more favorable than the self-limiting reaction B₂O₃(s) + 6HF(g) → 2BF₃(s) + 3H₂O(g). The spontaneous etching of B₂O₃ was predicted at temperatures above −163 °C for an HF reactant pressure of 0.2 Torr and BF₃ and H₂O product pressure of 0.01 Torr.