Semiconductor device with air gap between gate-all-around transistors and method for forming the same

1. A semiconductor device, comprising:

a first gate stack and a second gate stack disposed over a semiconductor substrate, wherein at least one of the first gate stack and the second gate stack comprises a plurality of gate layers, and wherein the first gate stack and the second gate stack have an air gap therebetween;

a first gate structure and a second gate structure disposed over the first gate stack and the second gate stack, respectively;

a first dielectric layer surrounding lower sidewalls of the first gate structure and lower sidewalls of the second gate structure;

a first landing pad disposed over a top surface and upper sidewalls of the first gate structure;

a first plug disposed over the first landing pad and electrically connected to the first gate structure, wherein a width of the first gate structure is greater than a width of the first plug; and

a second dielectric layer disposed over the first dielectric layer, wherein the first landing pad and the first plug are surrounded by the second dielectric layer;

a first barrier layer covering a bottom surface, the lower sidewalls and the upper sidewalls of the first gate structure, wherein a portion of the first barrier layer is sandwiched between the first landing pad and the first gate structure.

2. The semiconductor device of claim 1 , further comprising:

a second landing pad disposed over a top surface and upper sidewalls of the second gate structure; and

a second plug disposed over the second landing pad and electrically connected to the second gate structure, wherein a distance between the first plug and the second plug is greater than a distance between the first gate stack and the second gate stack.

3. The semiconductor device of claim 1 , wherein the first landing pad is in direct contact with the first dielectric layer.

4. The semiconductor device of claim 1 , further comprising:

a semiconductor material disposed between the first gate stack and the second gate stack, wherein the air gap is enclosed by the semiconductor material.

5. The semiconductor device of claim 1 , wherein the first landing pad is made of copper germanide (Cu 3 Ge).

6. The semiconductor device of claim 1 , wherein the first landing pad comprises:

an inner portion covering the top surface of the first gate structure; and

a plurality of outer portions covering top surfaces and upper sidewalls of the first barrier layer, wherein the inner portion and the outer portions are made of different materials.

7. The semiconductor device of claim 6 , wherein top surfaces of the outer portions of the first landing pad are higher than a top surface of the inner portions of the first landing pad.

8. The semiconductor device of claim 6 , wherein the inner portion of the first landing pad is made of tungsten silicide (WSi), and the outer portions of the first landing pad are made of titanium silicide (Ti Si).

9. A method for forming a semiconductor device, comprising:

forming a first gate stack and a second gate stack over a semiconductor substrate, wherein at least one of the first gate stack and the second gate stack comprises a plurality of gate layers, and wherein the first gate stack and the second gate stack have an air gap therebetween;

forming a first gate structure and a second gate structure over the first gate stack and the second gate stack, respectively;

forming a first dielectric layer surrounding the first gate structure and the second gate structure, wherein a top surface and upper sidewalls of the first gate structure and a top surface and upper sidewalls of the second gate structure are protruded from the first dielectric layer;

performing a heat treatment process to form a first landing pad over the top surface and the upper sidewalls of the first gate structure and a second landing pad over the top surface and the upper sidewalls of the second gate structure;

forming a second dielectric layer covering the first landing pad and the second landing pad;

forming a first plug in the second dielectric layer and over the first landing pad, wherein a width of the first gate structure is greater than a width of the first plug;

forming a sacrificial dielectric layer with a first opening and a second opening over the first gate stack and the second gate stack;

forming a first barrier layer and a second barrier layer lining the first opening and the second opening, respectively;

forming the first gate structure and the second gate structure over the first barrier layer and the second barrier layer, respectively; and

removing the sacrificial dielectric layer after the first gate structure and the second gate structure are formed.

10. The method for forming a semiconductor device of claim 9 , further comprising:

forming a second plug in the second dielectric layer and over the second landing pad, wherein a distance between the first plug and the second plug is greater than a distance between the first gate stack and the second gate stack.

11. The method for forming a semiconductor device of claim 9 , wherein a silicon-containing gas is used during the heat treatment process.

12. The method for forming a semiconductor device of claim 9 , wherein the first landing pad comprises:

a first inner portion covering the top surface of the first gate structure; and

a plurality of outer portions covering top surfaces and upper sidewalls of the first barrier layer, wherein top surfaces of the outer portions are higher than a top surface of the inner portion, and wherein the inner portion and the outer portions are formed simultaneously by the heat treatment process.