A technique of fabricating semiconductor devices so as to avoid mask alignment difficulties when it is required that electrical contact be made subsequent to the formation of extremely small device regions. The diffusion mask for creating the small device regions is constituted of a first film which is subject to ready etching by a first etchant but which is substantially unaffected by a second etchant. A second film covers the opening in the device region formation. This film being subject to etching by the second etchant but being substantially unaffected by the first etchant. As a result, the second film is removed from within the opening without affecting the first film since the first film will resist the second etchant. Thus, the original opening is re-established for the purpose of making contact to the device region, the first film remaining as a passivating film on the substrate.