Fin field-effect transistor and method of forming the same

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

forming a first fin and a second fin over a substrate;

forming a first dummy gate structure that straddles the first fin and the second fin, wherein the first dummy gate structure comprises a first dummy gate dielectric and a first dummy gate disposed over the first dummy gate dielectric;

replacing a portion of the first dummy gate with a gate isolation structure, wherein the portion of the first dummy gate is disposed over the second fin;

removing the first dummy gate;

removing a first portion of the first dummy gate dielectric around the first fin, while leaving a second portion of the first dummy gate dielectric around the second fin intact; and

forming a gate feature straddling the first fin and the second fin, wherein the gate isolation structure intersects the gate feature.

2. The method of claim 1 , wherein the first fin and the second fin extend along the same direction and are adjacent to each other in a first area of the substrate.

3. The method of claim 2 , wherein the first fin is configured as a first active channel in the first area and the second fin is configured as a first dummy channel in the first area.

4. The method of claim 3 , further comprising:

forming a third fin, configured as a second active channel, in a second area of the substrate;

forming a second dummy gate structure that straddles the third fin, wherein the second dummy gate structure comprises a second dummy gate dielectric and a second dummy gate disposed over the second dummy gate dielectric;

forming a fourth fin in the first area of the substrate, wherein the fourth fin is configured as a second dummy channel and wherein the first dummy gate structure also straddles the fourth fin; and

when removing the first portion of the first dummy gate dielectric, removing a third portion of the first dummy gate dielectric around the fourth fin while leaving the second dummy gate dielectric around the third fin intact;

wherein the gate feature also straddles the fourth fin, with no gate isolation structure disposed over the fourth fin.

5. The method of claim 4 , wherein a first transistor, having the first active channel, includes a first gate dielectric with a first thickness, and a second transistor, having the second active channel, includes a second gate dielectric with a second thickness, the second thickness being greater than the first thickness.

6. The method of claim 4 , wherein a first density of transistors formed in the first area is greater than a second density of transistors formed in the second area.

7. The method of claim 1 , wherein the first portion of the first dummy gate dielectric is disposed over a top surface of the first fin and extends along sidewalls of the first fin, and wherein the second portion of the first dummy gate dielectric is disposed between a top surface of the second fin and the gate isolation structure and extends along sidewalls of the second fin.

8. The method of claim 1 , wherein the step of removing a first portion of the first dummy gate dielectric around the first fin comprises forming a mask overlaying the entire second fin.

9. The method of claim 1 , wherein the step of removing a first portion of the first dummy gate dielectric around the first fin further comprises forming a mask overlaying the gate isolation structure.

10. The method of claim 1 , wherein the gate feature includes a high-k dielectric layer and at least one work function layer.