METHOD FOR FORMING SEMICONDUCTOR-ON-INSULATOR (SOI) SUBSTRATE

A method for forming an SOI substrate includes following operations. A first semiconductor layer, a second semiconductor layer and a third semiconductor layer are formed over a first substrate. A plurality of trenches and a plurality of recesses are formed in the first semiconductor layer, the second semiconductor layer and the third semiconductor layer. The plurality of trenches extend along a first direction, and the plurality of recesses extend along a second direction different from the first direction. The plurality of trenches and the plurality of recesses are sealed to form a plurality of voids. A device layer is formed over the first substrate. The devices layer is bonded to an insulator layer over a second substrate. The third semiconductor layer, the device layer the insulator layer and the second substrate are separated from the first semiconductor layer and the first substrate. The device layer is exposed.