PURPOSE: To prevent the diffusion of impurities, particularly Na elements, etc. from glass substrates by providing a thin film composed of heavy metal oxides between the glass substrate and metal film.

CONSTITUTION: Metal, e.g., Cr, is used for target and a Cr2O3 film 2 is provided as an impurity barrier to thicknesses of about 50 to 100 on a glass substrate 1 by reaction sputtering in an atmosphere comprising mixing O2 in inert gas. Next, the supply of O2 is interrupted, and a metal Cr film 4 is formed to a thickness of about 500 by ordinary sputtering and further O2 is introduced again and a Cr2O3 film 4 is formed a thickness of about 300 by reaction sputtering. Thereby, etching at the patterning is evenly progressed and the desired accurate mask patterns are obtainable.