Integration of dense and variable pitch fin structures

1. A method for forming fin structures, comprising:

forming a protective layer over a set of mandrels in a variable fin pitch region;

forming first sidewalls around a set of mandrels in a uniform fin pitch region;

removing the set of mandrels in the uniform fin pitch region;

removing the protective layer;

forming second sidewalls around the first sidewalls in the uniform fin pitch region and the mandrels in the variable fin pitch region;

removing the first sidewalls and the mandrels; and

etching an underlying layer around the second sidewalls.

2. The method of claim 1 , further comprising removing one or more of the first sidewalls in the uniform pitch region before forming the second sidewalls.

3. The method of claim 1 , wherein the underlying layer comprises a hardmask layer.

4. The method of claim 3 , wherein the underlying layer further comprises one or more additional layers to be etched.

5. The method of claim 1 , wherein the second sidewalls are formed from a different material than a material of the first sidewalls.

6. The method of claim 5 , wherein the material of the second sidewalls has etch selectivity with the material of the first sidewalls and the mandrels.

7. The method of claim 1 , wherein the second sidewalls are thinner than the first sidewalls.

8. The method of claim 1 , wherein the second sidewalls in the uniform fin pitch region have a uniform separation between sidewalls and wherein the second sidewalls in the variable fin pitch region have a variable separation between sidewalls.

9. A method for forming fin structures, comprising:

lithographically forming a first set of mandrels in a uniform fin pitch region with uniform width and spacing over a hardmask layer to be patterned;

lithographically forming a second set of mandrels in a variable fin pitch region with variable width and spacing over the hardmask layer to be patterned;

forming a protective layer over a set of mandrels in a variable fin pitch region;

forming first sidewalls around a set of mandrels in a uniform fin pitch region;

removing the set of mandrels in the uniform fin pitch region;

removing the protective layer;

forming second sidewalls around the first sidewalls in the uniform fin pitch region and the mandrels in the variable fin pitch region;

removing the first sidewalls and the mandrels; and

etching an underlying layer around the second sidewalls to form fins, wherein fins in the uniform fin pitch region have a spacing that is less than half of a minimum pitch of a lithography process used to form the mandrels and wherein fins in the variable fin pitch region have a spacing that is greater than half, but less than the minimum pitch of the lithography process.

10. The method of claim 9 , further comprising removing one or more of the first sidewalls in the uniform pitch region before forming the second sidewalls.

11. The method of claim 9 , wherein the underlying layer comprises a hardmask layer.

12. The method of claim 11 , wherein the underlying layer further comprises one or more additional layers to be etched.

13. The method of claim 9 , wherein the second sidewalls are formed from a different material than a material of the first sidewalls.

14. The method of claim 13 , wherein the material of the second sidewalls has etch selectivity with the material of the first sidewalls and the mandrels.

15. The method of claim 9 , wherein the second sidewalls are thinner than the first sidewalls.

16. The method of claim 9 , wherein the second sidewalls in the uniform fin pitch region have a uniform separation between sidewalls and wherein the second sidewalls in the variable fin pitch region have a variable separation between sidewalls.