Replacement gate process for semiconductor devices

1. A method of forming a semiconductor device, comprising:

providing a precursor having a substrate and gate stacks over the substrate, wherein each of the gate stacks includes an electrode layer, a first hard mask (HM) layer over the electrode layer, and a second HM layer over the first HM layer;

forming an etch stop layer (ESL) over top and sidewalls of the gate stacks;

thereafter, depositing a dielectric layer over the substrate and the gate stacks and filling spaces between the gate stacks;

performing a first chemical mechanical planarization (CMP) process to partially remove the dielectric layer, wherein the ESL is used as an end point for the first CMP process;

performing an etching process to remove the second HM layer and to partially remove the dielectric layer, thereby exposing the first HM layer; and

performing a second CMP process to at least partially remove the first HM layer.

2. The method of claim 1 , wherein:

the electrode layer comprises polysilicon;

the first HM layer comprises a nitride;

the second HM layer comprises an oxide; and

the dielectric layer comprises another oxide.

3. The method of claim 1 , wherein the etching process includes a first etching followed by a second etching, wherein:

the first etching has a low etch selectivity with respect to materials of the dielectric layer, the second HM layer, and the ESL; and

the second etching selectively removes the dielectric layer and the second HM layer while the first HM layer remains substantially unchanged in the second etching.

4. The method of claim 3 , wherein both the first etching and the second etching are dry etching.

5. The method of claim 3 , wherein the first etching and the second etching are performed in the same process chamber.

6. The method of claim 3 , wherein the first etching is controlled by a timer and the second etching is controlled by detecting an element contained in the first HM layer but not in the second HM layer and the dielectric layer.

7. The method of claim 1 , wherein the second CMP process applies slurry selectively tuned for removing the first HM layer while the dielectric layer remains substantially unchanged.

8. The method of claim 1 , wherein the gate stacks have different heights.

9. The method of claim 1 , wherein the second HM layer of a first gate stack of the gate stacks has a first thickness and the second HM layer of a second gate stack of the gate stacks has a second thickness that is different from the first thickness.

10. A method of forming a semiconductor device, comprising:

providing a precursor having a substrate and gate stacks disposed over the substrate, wherein each of the gate stacks includes an electrode layer, a first hard mask (HM) layer over the electrode layer, and a second HM layer over the first HM layer;

depositing an inter-layer dielectric (ILD) layer over the gate stacks and filling spaces between the gate stacks;

performing a first chemical mechanical planarization (CMP) process to partially remove the ILD layer;

performing a dry etching process to partially remove, the ILD layer and the second HM layer, thereby exposing the first HM layer; and

performing a second CMP process to at least partially remove the first HM layer.

11. The method of claim 10 , wherein:

the first HM layer comprises a nitride;

the second HM layer comprises an oxide; and

the ILD layer comprises another oxide.

12. The method of claim 10 , wherein the dry etching process includes a first etching followed by a second etching, wherein:

the first etching removes the ILD layer and the second HM layer at about the same etching rate; and

the second etching removes the ILD layer and the second HM layer at a higher etching rate than it removes the first HM layer.

13. The method of claim 12 , wherein the first etching is switched to the second etching before the first HM layer is exposed.

14. The method of claim 12 , wherein the first etching and the second etching are performed in the same process chamber.

15. The method of claim 10 , wherein the second CMP process applies slurry that removes the first HM layer at a higher rate than it removes the ILD layer.

16. The method of claim 10 , further comprising:

cutting an electrode layer in one of the gate stacks into multiple segments.

17. A method of forming a semiconductor device, comprising:

providing a precursor having a substrate and gate stacks disposed over the substrate, wherein each of the gate stacks includes a poly layer, a nitride hard mask (HM) layer over the poly layer, and an oxide HM layer over the nitride HM layer;

forming an etch stop layer (ESL) over top and sidewalls of the gate stacks, the ESL comprising a nitride;

depositing an inter-layer dielectric (ILD) layer covering the ESL and the gate stacks and filling spaces between the gate stacks, the ILD layer comprising an oxide;

performing a first chemical mechanical planarization (CMP) process to partially remove the ILD layer until the ESL is exposed;

performing a first etching process to partially remove at least the ESL, the ILD layer, and the oxide HM layer;

after the performing of the first etching process, performing a second etching process selectively tuned to etch the ILD layer and the oxide HM layer while the nitride HM layer remains substantially unchanged, thereby exposing the nitride HM layer; and

performing a second CMP process to selectively remove the nitride HM layer while the ILD layer remains substantially unchanged.

18. The method of claim 17 , wherein the first and second etching processes are dry etching processes and are performed in the same process chamber.

19. The method of claim 17 , wherein the first etching process has low etch selectivity with respect to the ESL, the ILD layer, and the oxide HM layer.

20. The method of claim 17 , further comprising:

cutting a poly layer in one of the gate stacks into multiple poly segments.