An ultrafine trench isolation, by which a narrow channel metal oxide semiconductor field effect transistor (MOSFET) can be isolated with excellent electrical properties, was formed using a new fabrication process. A void-free shape was realized by two-step filling, and the trench edge shapes were drastically improved by oxide spacer formation. Down to the ultranarrow field space of 0.13 mu m, the isolation breakdown voltage and the parasitic MOSFETs threshold voltage were maintained as high as 7.7 V and more than 6 V. The reduction in the effective channel width for the MOSFET, furthermore, was estimated to be as small as 0.05 mu m, while it was shown that kink-free MOSFET subthreshold characteristics were kept even without a sidewall implantation to suppress the concentration of the electric field at the channel edges.