Method and system for measuring pattern placement error on a wafer

1. A method for measuring pattern placement error (PPE) on a wafer, comprising:

receiving a photomask pattern;

adding one or more unit cell patterns to the photomask pattern, each of the unit cell patterns including at least one reference design pattern and at least one PPE check design pattern;

fabricating a photomask from the photomask pattern with the one or more unit cell patterns added thereto;

patterning the wafer using the fabricated photomask;

acquiring a microscope image of the patterned wafer; and

calculating pattern placement error from the microscope image of the patterned wafer, by measuring a first displacement between the at least one reference design pattern on the patterned wafer and a corresponding structure on the photomask, measuring a second displacement between the at least one PPE check design pattern on the patterned wafer and a corresponding structure on the photomask, and determining a difference between the first displacement and the second displacement,

wherein the unit cell pattern is a test pattern and the reference design pattern is configured to cause less pattern shift than the PPE check design pattern.

2. The method of claim 1 , further comprising:

performing computational lithography simulation on the photomask pattern; and

comparing results of the computational lithography simulation with the measured pattern placement error to determine an efficacy of the computational lithography simulation.

3. The method of claim 1 , wherein the one or more unit cell patterns are added to the photomask pattern either at periodic intervals or at predetermined positions.

4. The method of claim 2 , wherein each of the unit cell patterns includes a plurality of parallel lines.

5. The method of claim 4 , wherein the plurality of parallel lines includes a first set of parallel lines that are part of the reference design pattern and a second set of parallel lines that are part of the PPE check design pattern.

6. The method of claim 5 , wherein the first set of parallel lines are more densely spaced than the second set of parallel lines.

7. The method of claim 5 , wherein the first set of parallel lines are spaced apart from the second set of parallel lines.

8. The method of claim 5 , wherein the first set of parallel lines are in contact with the second set of parallel lines.

9. The method of claim 5 , wherein the first set of parallel lines are disposed on opposite sides of the second set of parallel lines.

10. The method of claim 5 , wherein the first set of parallel lines are spaced apart at equal intervals and the second set of parallel lines are progressively spaced farther apart from each other.

11. The method of claim 5 , wherein the plurality of parallel lines includes horizontal lines and vertical lines.

12. The method of claim 5 , wherein each of the unit cell patterns includes an arrangement of the first set of parallel lines and the second set of parallel lines, a copy of the arrangement rotated 90°, a copy of the arrangement rotated 180°, and a copy of the arrangement rotated 270°.

13. A method for fabricating a semiconductor wafer, comprising:

fabricating a photomask which includes a photomask circuit pattern disposed therein, and one or more unit cell patterns disposed therein, each of the unit cell patterns including at least one reference design pattern including a first set of parallel lines and at least one PPE check design pattern including a second set of parallel lines that are more sparsely spaced than the first set of parallel lines; and

patterning the semiconductor wafer using the fabricated photomask,

acquiring a microscope image of the patterned semiconductor wafer; and

calculating pattern placement error from the microscope image of the patterned semiconductor wafer, by measuring a first displacement between the at least one reference design pattern on the patterned wafer and a corresponding structure on the photomask, measuring a second displacement between the at least one PPE check design pattern on the patterned wafer and a corresponding structure on the photomask, and determining a difference between the first displacement and the second displacement,

wherein each of the unit cell patterns includes an arrangement of the first set of parallel lines and the second set of parallel lines, a copy of the arrangement rotated 90°, a copy of the arrangement rotated 180°, and a copy of the arrangement rotated 270°.