Abstract: An SiO.sub.2 passivation film is formed on a surface of a substrate made of a plastic material by plasma chemical vapor deposition (CVD) process in which organic oxysilane is used as a raw gas. Instead of a reactive gas having an ashing effect, Ar, He or NH.sub.3 is used as a reactive gas which serves as an auxiliary for decomposing the raw gas at a temperature not greater than a temperature at which the substrate is thermally deformed (i.e., about 250.degree. C.). The ashing of the substrate by oxygen or hydrogen radicals is thus prevented.

Abstract: A passivation film is formed by plasma CVD process in which organic oxysilane is used as a raw gas. When an SiO.sub.2 film as the passivation film is formed on a surface of a substrate, Ar, He or NH.sub.3 gas is used as a reactive gas which serves as an auxiliary for decomposing the raw gas. Ashing of the substrate by oxygen or hydrogen radicals is thus prevented. Fluorine group gas of CF.sub.4 or NF.sub.3 may be added to the reactive gas. The SiO.sub.2 film as a passivation film as described above may be formed first as an initial passivation film and then another passivation film may be formed on top of the initial passivation film by using a reactive gas having an ashing effect such as O.sub.2, N.sub.2 O, O.sub.3 and H.sub.2.