A method for forming a dielectric layer free of voids is disclosed. First, a substrate, a first stressed layer including a recess, a second stressed layer disposed on the first stressed layer and covering the recess and a patterned photoresist embedded in the recess are provided. Second, a first etching step is performed to totally remove the photoresist so that the remaining second stressed layer forms at least one protrusion adjacent to the recess. Then, a trimming photoresist is formed without exposure to fill the recess and to cover the protrusion. Later, a trimming etching step is performed to eliminate the protrusion and to collaterally remove the trimming photoresist.