Semiconductor structure and method for forming the same

A method for forming a semiconductor structure, comprising:

• providing a substrate which comprises a predetermined low-potential region and a predetermined high-potential region;
• forming a first gate structure in the predetermined low-potential region and on the substrate;
• forming a second gate structure in the predetermined high-potential region and on the substrate;
• sequentially forming a first dielectric layer and a second dielectric layer on the substrate covering the first gate structure and the second gate structure, wherein a material of the second dielectric layer is different from a material of the first dielectric layer;
• forming a portion of a third dielectric layer along sidewalls of the second gate structure and on the second dielectric layer, wherein a material of the third dielectric layer is different from the material of the second dielectric layer;
• and
• etching the first dielectric layer and the second dielectric layer with the portion of the third dielectric layer as an etching hard mask to form a first composite spacer covering sidewalls of the first gate structure, and a second composite spacer covering the sidewalls of the second gate structure, wherein
• the first composite spacer is formed of a portion of the first dielectric layer and a portion of the second dielectric layer, wherein the portion of the second dielectric layer has a tapered shape;
• the second composite spacer is formed of another portion of the first dielectric layer, another portion of the second dielectric layer and the portion of the third dielectric layer;
• and a width of the first composite spacer is less than a width of the second composite spacer.