After cleaning a surface of a silicon substrate (1), impurities and
natural oxide film existing on the silicon substrate (1) are removed by
soaking the silicon substrate (1) in a 0.5%-by-volume HF aqueous solution
for 5 minutes. The silicon substrate (1) is rinsed (cleaned) with
ultrapure water for five minutes. Then, the silicon substrate (1) is
soaked for 30 minutes in azeotropic nitric acid heated to an azeotropic
temperature of 120.7.degree. C. In this way, an extremely thin chemical
oxide film (5) is formed on the surface of the silicon substrate (1).
Subsequently, a metal film (6) (aluminum-silicon alloy film) is
deposited, followed by heating in a hydrogen-containing gas at
200.degree. C. for 20 minutes. Through the heat processing in the
hydrogen-containing gas, hydrogen reacts with interface states and defect
states in the chemical oxide film (5), causing disappearance of the
interface states and defect states. As a result, the quality of the film
can be improved. Thus, it is possible to form a high quality (with low
leak current density), extremely thin silicon dioxide film on the silicon
substrate at a low temperature with excellent film-thickness
controllability.