A method of fabricating a semiconductor device includes forming an ion implanted region on a semiconductor substrate in a cell/core region. The semiconductor substrate is selectively etched to form a recess. The recess exposes a boundary of the ion implanted region. The ion implanted region exposed at the bottom of the recess is removed to form an under-cut space in the semiconductor substrate. An insulating film is formed to form a substrate having a silicon-on-insulator (SOI) structure in the cell/core region. The insulating film fills the under-cut space and the recess.