Method of forming isolation layer in semiconductor device

The invention discloses a method of forming an isolation layer in a semiconductor device. The method includes providing a semiconductor substrate having a trench formed therein; forming a first insulating layer in the trench; and forming a densified second insulating layer on the first insulating layer. In the above method, a void is not generated in the isolation layer so a bending phenomenon of an active region can be reduced or prevented to improve an electrical characteristic of the semiconductor.