Method of forming semiconductor device

A method of forming a semiconductor device, comprising:

• providing a target layer;
• forming a stacked structure on the target layer, the stacked structure comprising a first mask layer, a sacrificial layer and a second mask layer stacked on one over another, wherein the first mask layer and the second mask layer comprises the same material composition, and proportions of components are different in the first mask layer and the second mask layer;
• patterning the second mask layer and the sacrificial layer to form a plurality of mandrels;
• forming a plurality of spacer patterns surrounding the mandrels respectively;
• transferring the spacer patterns into the first mask layer, to form a plurality of openings in the first mask layer, wherein the openings are not penetrated through the first mask layer;
• and
• after completely removing the spacer patterns, patterning the target layer through the first mask layer and the openings, to form a plurality target patterns in the target layer.