PURPOSE: To convert an Al thin film to an Al-Si alloy and to form the thin film having excellent reliability and good quality by using an Si-contg. gas, etc., as a gaseous raw material at the time of forming the Al thin film on an Si substrate by sputtering.

CONSTITUTION: Gaseous Ar 8 is introduced into a vacuum chamber 1 having an Si substrate S and a metallic target 4 consisting of Al, etc., and microwaves are introduced therein from a microwave generating source 6 by a waveguide tube 5. The electric discharge by electron cyclotron resonance is executed by the magnetic field of an excitation coil 7 and the gaseous Ar is ionized to a positive charge to sputter the Al target 4 connected to the negative potential of a power source 10, by which the thin film of the Al is formed on the substrate S. The gaseous raw material, such as SiH4, is introduced from a gas introducing system 9 into the chamber to form the Al thin film to the thin film consisting of the Al-Si alloy having the good reliability and step coverage by the reaction with the Al on the substrate S. The compsn. pattern in the thickness direction of the film to be formed is freely controlled by adjusting the amt. of the gaseous raw material 9 to be introduced.