A Nitric Oxide Process for the Deposition of Silica Films

Abstract

A new process for vapor depositing thin films of silica on a heated substrate is described. The reagents are hydrogen (≥95%), nitric oxide (1–4%), and silicon tetrachloride or tetrabromide (0.1–1%). The chloride system is useful at 950°–1200°C and the bromide at 775°–950°C, so that the latter may serve to coat Ge as well as Si. The over‐all reaction is believed to be and is probably mostly homogeneous. For the more commonly used film thicknesses the nitric oxide process is three to five times more rapid than either the steam or carbon dioxide reactions. Rate curves are presented for various temperatures and concentrations. The apparent temperature coefficient of deposition rate decreases markedly in the upper part of the temperature range for both the Cl and Br systems. Improved film thickness uniformity may be obtained as a practical result. The films produced are transparent, vitreous, smooth, and tightly adherent. Physical tests indicate they are amorphous silica with less short‐range order than in thermal oxides. Films from the chloride system regularly produce Si surface charges of in MOS capacitor tests. Breakdown voltages equal those of steam oxides. Limited bias‐aging experiments have shown shifts similar to those of steam oxides. The films are satisfactory masks for boron and phosphorus diffusions; Kodak Photoresist adheres very well.