Semiconductor structure and method for forming the same
A semiconductor structure includes a metal gate structure and an isolation structure adjacent to the metal gate structure. The isolation structure includes a first dielectric layer, a second dielectric layer over the first dielectric layer and a third dielectric layer over the second dielectric layer. The first dielectric layer includes carbon of a first concentration, the second dielectric layer includes carbon of a second concentration, and the third dielectric layer includes carbon of a third concentration. The third concentration is greater than the second concentration, and the second concentration is greater than the first concentration.
1 . A method for forming a semiconductor structure, comprising:
forming at least a metal gate structure and a first isolation structure over a substrate, wherein the first isolation structure surrounds the metal gate structure;
removing a portion of the metal gate structure to form a trench;
forming a first dielectric layer comprising carbon of a first concentration in the trench;
forming a second dielectric layer comprising carbon of a second concentration over the first dielectric layer;
forming a third dielectric layer comprising carbon of a third concentration over the second dielectric layer; and
removing superfluous portion of the first dielectric layer, the second dielectric layer and the third dielectric layer to form a second isolation structure.
2 . The method of claim 1 , wherein the third concentration is greater than the second concentration, and the second concentration is greater than the first concentration.
3 . The method of claim 1 , further comprising performing a plasma treatment to transform the second dielectric layer and the third dielectric layer into a porous second dielectric layer and a porous third dielectric layer, respectively.
4 . The method of claim 1 , further comprising performing a plasma treatment to remove a portion of the second dielectric layer to form at least a recess in the second isolation structure.
5 . The method of claim 4 , wherein the performing of the plasma treatment transforms the third dielectric layer into a porous third dielectric layer.
6 . The method of claim 4 , further comprising forming a third isolation structure over the second isolation structure to seal the recess to form at least an air gap in the second isolation structure.
7 . A method for forming a semiconductor structure, comprising:
forming at least a metal gate structure over a substrate;
removing a portion of the metal gate structure to form a trench;
forming a first dielectric layer in the trench;
forming a second dielectric layer over the first dielectric layer in the trench;
forming a third dielectric layer over the second dielectric layer and to fill the trench; and
removing superfluous portion of the first dielectric layer, the second dielectric layer and the third dielectric layer to form a first isolation structure,
wherein a first carbon concentration of the first dielectric layer, a second carbon concentration of the second dielectric layer and a third carbon concentration of the third dielectric layer are different from each other.
8 . The method of claim 7 , wherein the third carbon concentration is greater than the second carbon concentration, and the second carbon concentration is greater than the first carbon concentration.
9 . The method of claim 7 , further comprising performing a plasma treatment to transform the third dielectric layer into a porous third dielectric layer.
10 . The method of claim 9 , wherein the plasma treatment further transforms the second dielectric layer into a porous second dielectric layer.
11 . The method of claim 7 , further comprising performing a plasma treatment to remove a portion of the second dielectric layer to form at least a recess in the first isolation structure.
12 . The method of claim 11 , wherein the performing of the plasma treatment transforms the third dielectric layer into a porous third dielectric layer.
13 . The method of claim 11 , further comprising forming a second isolation structure over the first isolation structure to seal the recess to form at least an air gap in the first isolation structure.
14 . The method of claim 7 , further comprising forming a third isolation structure over the substrate, wherein the metal gate structure and the first isolation structure are surrounded by the third isolation structure.
15 . The method of claim 7 , wherein the substrate comprises a fourth isolation structure formed therein, wherein a portion of the metal gate structure is formed over the fourth isolation structure.
16 . A method for forming a semiconductor structure, comprising:
forming at least a metal gate structure a substrate;
removing a portion of the metal gate structure to form a trench;
forming a first dielectric layer comprising carbon of a first concentration in the trench;
forming a second dielectric layer comprising carbon of a second concentration over the first dielectric layer;
forming a third dielectric layer comprising carbon of a third concentration over the second dielectric layer;
performing a plasma treatment to the third dielectric layer to transform the third dielectric layer into a porous third dielectric layer; and
removing superfluous portion of the first dielectric layer, the second dielectric layer and the third dielectric layer to form a first isolation structure.
17 . The method of claim 16 , wherein the third concentration is greater than the second concentration, and the second concentration is greater than the first concentration.
18 . The method of claim 16 , wherein the plasma treatment further transforms the second dielectric layer into a porous second dielectric layer.
19 . The method of claim 16 , wherein the plasma treatment is performed to remove a portion of the second dielectric layer to form at least a recess.
20 . The method of claim 19 , further comprising forming a second isolation structure over the first isolation structure to seal the recess to form at least an air gap in the first isolation structure.