Improvement in gate breakdown voltage for SOS devices

It was attempted to improve low gate breakdown voltage for an air-isolated SOS devices, employing ion-beam etching technology for silicon-island formation. Gate breakdown origin was clarified with SEM observation. As a result, it has been found that 1) low gate breakdown voltage for air-isolated SOS devices is caused by thinner gate oxide at the bottom of a steep silicon-island edge, and 2) ion-beam etching with the ion-beam incident angle over 40° allows the silicon island to have a sidewall angle under 35°, thus resulting in uniform gate oxide thickness. Consequently, the gate breakdown voltage for the air-isolated SOS devices can be improved to as high as that for dielectric isolated SOS devices.