A new isolation method for high packing density MOS devices has been developed. In this method the LOCOS technique is applied to wide isolation regions and the buried-oxide technique is applied to isolation regions less than 2 µm wide. No additional masks are needed in order to form SiO2film in the wide field regions because the photoresist is thicker near steps and inside the narrow trenches. For reducing the hump that appears in subthreshold current characteristics of n-channel MOSFET's, Using buried-oxide isolation, tilt-angle implantation to each of the four sidewalls is performed as a channel stop. The Sidewall channel stop can also control the narrow-channel effect.