Method for forming mos device having field shield isolation

Abstract

A first conductor for a field shield and a first insulating film are sequentially formed in a predetermined shape on a major surface of a P-type semiconductor substrate through an insulating film. A third insulating film is formed over the semiconductor substrate so as to cover the first conductor and a second insulating film thereon. The third insulating film is anisotropically etched, so that a sidewall insulating film is formed on sidewalls of the first conductor. Second and third conductors respectively serving as gate electrodes of field effect transistors are formed through a fourth insulating film. n-type impurities are implanted into the major surface of the semiconductor substrate utilizing as masks the first insulating film, the sidewall oxide film, the second conductor and the third conductor and are diffused, to form impurity regions. Since the sidewall oxide film is thick, the impurity regions are not overlapped even by diffusion with a portion where the first conductor is projected on the semiconductor substrate. Thus, a threshold voltage of a field shield transistor comprising the first conductor and the impurity regions on both sides thereof is raised, so that isolation characteristics of the field shield is improved.