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

Abstract

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 BFx 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.