PURPOSE: To improve the accuray of an emitter photoetching process as well as to contribute in the stabilization of characteristics of the titled integrated circuit when it is used as a high frequency integrated circuit and the like by a method wherein an emitter diffusion window and a collector diffusion window are simultaneously formed on a thermal oxide film by performing an etching.

CONSTITUTION: After a thermal oxide film has been formed by performing a process in the same manner as before, a base diffusion window 22 and a collector contact window 23 are formed by performing a photoetching on the thermal oxide film 21. Then, resist is applied on the surface of a wafer, and a patterning is performed on the resist using a mask pattern 24. Subsequently, P type impurities are doped on a base region as shown by the arrow A using said resist as a mask, and a base diffusion layer 26 is formed. Then, the resist is removed completely, a heat treatment is performed in an oxidizing atmosphere, the desired junction depth of the base is obtained, and a thermal oxide film 25 is covered on the base diffusion window 22 and the collector contact window 23. Then, a window is provided on the thermal oxide film 25, and an emitter diffusion window 27 and a collector contact diffusion window 28 are formed. Subsequently, an already known process is performed, and the integrated circuit is completed.