Composition and thermal stability of chemically formed SiO2 oxides

The properties of the SiO₂ chemical oxide layer fabricated at the interface between the Si substrate and polysilicon overlayer are critical for obtaining desired electrical response in bipolar polysilicon emitter transistors. The required uniformity in fabricating the layer is achieved by using a wet chemical peroxide bath. In this paper the physical thickness and composition of the chemical oxide is explored. Wet chemistry bath parameters were varied including temperature and hydrogen peroxide concentration. The resulting thickness and composition of the chemical oxide layer were measured using X-ray photoelectron spectroscopy and ellipsometry. The stability of the chemical oxide layers was investigated under ambient conditions. Results indicate that a 1 nm thick chemical oxide film increased to 2 nm in thickness over a 24 hour period in ambient air and that the layer was a mix of stoichiometric SiO₂ and intermediary states. During the 24 hour period in ambient air, an increase in carbon contamination was seen. Varying the wet chemistry bath parameters showed that at the highest temperatures, the thickness of the chemical oxide decreased, suggesting that the hydrogen peroxide is rapidly losing potency