Characterization of residues formed by anhydrous hydrogen fluoride etching of doped oxides

The byproducts in residue layers produced after etching doped oxide films with anhydrous hydrogen fluoride (HF) in a commercial gas phase wafer processing tool operated at atmospheric pressure and 55 °C have been characterized using transmission Fourier transform infrared (FTIR) spectroscopy. Approximately 4000 Å was etched from the borophosphosilicate glass (BPSG), borosilicate glass (BSG), and phosphosilicate glass (PSG) films, creating a condensed layer on the oxide surfaces during etching. The primary etching products in the condensed layer on BPSG were found to be a mixture of boric acid B(OH)3, phosphoric acid H3PO4, and water. The etching products in the residue on PSG were H3PO4 and water. The reaction of boric oxide (B2O3) crystallites with water to produce B(OH)3 is thermodynamically favorable and should react further to boron trifluoride BF3 in the presence of HF. The formation of B(OH)3 rather than BF3 in the etching residue on BPSG indicates that a kinetic limitation exists due to either the relatively low HF exposure used or the chemistry within the mixed acid film. The etching product in the residue on BSG, which was exposed to a higher HF flux, was primarily boron trifluoride dihydrate BF3⋅2H2O. The condensed layer supports the etching of the Si–O matrix by concentrating the HF and water reactants close to the surface, which explains the enhancement in the etching rate when a condensed layer is formed. The etching products identified by FTIR could also play a direct role in the etching reaction since hydroxyl groups on the acids can activate Si–O bonds similar to water and the Lewis acid BF3 can attach to Si–O via the pair of electrons on the O atom.