N/P boundary effect reduction for metal gate transistors

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

forming a first dummy gate and a second dummy gate over a substrate, wherein the first and second dummy gates each extend in a first direction;

forming a patterned mask over the first and second dummy gates, the patterned mask exposing a first segment of the first dummy gate and a second segment of the second dummy gate, while covering a third segment of the first dummy gate and a fourth segment of the second dummy gate, wherein the forming the mask is carried out in a manner such that the first and second segments have significantly different lengths, wherein the patterned mask defines an elongate contour that extends in a second direction different from the first direction;

replacing the first segment and the second segment with a first metal gate and a second metal gate, respectively, the first and second metal gates containing a first type metal material; and

replacing the third segment and the fourth segment with a third metal gate and a fourth metal gate, respectively, the third and fourth metal gates containing a second type metal material different from the first type metal material.

2. The method of claim 1 , wherein:

one of the first and second segments is longer than the other; and

a ratio of a longer one of the first and second segments to a shorter one of the first and second segments is greater than 1:1 but less than 1.5:1.

3. The method of claim 1 , wherein the forming the patterned mask is carried out using an optical proximity correction (OPC) technique.

4. The method of claim 1 , wherein:

the first direction is substantially perpendicular to the second direction; and

the first and second segments are confined within the elongate contour.

5. The method of claim 1 , wherein:

an end portion of the contour is wider in the first direction than a rest of the contour; and

the end portion of the contour coincides with an edge of one of the first and second segments.

6. The method of claim 1 , wherein the first and second dummy gates each contain a polysilicon material.

7. The method of claim 1 , wherein:

the first type metal material includes a P-type metal; and

the second type metal material includes an N-type metal.

8. The method of claim 7 , wherein:

the first and second metal gates are formed over an active region;

a first N/P boundary is formed by an interface between the first and third segments;

a second N/P boundary is formed by an interface between the second and fourth segments; and

a first distance from an edge of the active region to the first N/P boundary is less than a second distance from the edge of the active region to the second N/P boundary.

9. A method of fabricating a semiconductor device, comprising:

forming a plurality of dummy gates over a substrate, the dummy gates extending along a first axis;

forming a masking layer over the dummy gates, the masking layer defining an elongate opening that extends along a second axis different from the first axis, wherein the opening exposes a plurality of first portions of the dummy gates and protects a plurality of second portions of the dummy gates, wherein a tip portion of the opening has a width greater than a width of a non-tip portion of the opening, and wherein the forming the masking layer includes performing an optical proximity correction (OPC) process;

replacing the first portions of the dummy gates with a plurality of first metal gates; and

replacing the second portions of the dummy gates with a plurality of second metal gates different from the first metal gates.

10. The method of claim 9 , wherein:

the second axis is approximately orthogonal to the first axis; and

the width of the tip portion is measured along the first axis.

11. The method of claim 9 , wherein the OPC process includes using a serif assistant feature or a hammerhead assistant feature.

12. The method of claim 9 , wherein:

the first metal gates contain P-type work function metal layers; and

the second metal gates contain N-type work function metal layers.

13. The method of claim 12 , wherein:

the first metal gates are formed over an active region for a P-type transistor;

a plurality of N/P boundaries are formed by respective pairs of the first and second metal gates; and

an outer-most N/P boundary is spaced farther apart from the active region than the rest of the N/P boundaries.

14. The method of claim 9 , wherein a ratio between the width of the tip portion of the opening and the width of the non-tip portion of the opening is greater than 1:1 but less than 1.5:1.

15. The method of claim 9 , wherein the dummy gates each contain a polysilicon gate electrode.

16. A method of fabricating a semiconductor device, comprising:

forming a polysilicon gate over a substrate, wherein the polysilicon gate extends in a first direction;

forming a patterned mask over the polysilicon gate, the patterned mask exposing a first segment of the polysilicon gate and covering a second segment of the polysilicon gate, wherein the patterned mask defines an elongate contour that extends in a second direction different from the first direction, and wherein the first segment is confined in the elongate contour;

replacing the first segment with a P-type metal material; and

replacing the second segment with an N-type metal material.

17. The method of claim 16 , wherein the first direction is substantially perpendicular to the second direction.

18. The method of claim 16 , wherein an end portion of the contour is wider in the first direction than a rest of the contour.

19. The method of claim 16 , wherein an end portion of the contour coincides with an edge of the first segment.

20. The method of claim 16 , wherein the forming of the patterned mask is carried out using an optical proximity correction (OPC) technique.