Two-dimensional diffraction grating

1. A two-dimensional diffraction grating for a phase-stepping measurement system for determining an aberration map for a projection system, the diffraction grating comprising:

a substrate provided with a square array of through-apertures having a checkerboard pattern, the substrate comprising:

a support layer; and

a radiation absorbing layer,

wherein the diffraction grating is transmissive,

wherein the radiation absorbing layer is separate from the support layer,

wherein the square array of through-apertures extend through both the support layer and the radiation absorbing layer,

wherein the square array of through-apertures are non-regular octagonal and the square array of through-apertures are formed from a square having truncated corners, such that a connecting portion is formed between each two adjacent apertures, and

wherein the connecting portion has at least two parallel sides.

2. The two-dimensional diffraction grating of claim 1 , wherein a geometry of the two-dimensional diffraction grating is arranged to result in a grating efficiency map that reduces a number of contributions above a threshold to a harmonic of a phase stepping signal in response to the two-dimensional diffraction grating being used with a first patterned region that comprises a one-dimensional diffraction grating with a 50% duty cycle or a two-dimensional checkerboard diffraction grating with a 50% duty cycle.

3. The two-dimensional diffraction grating of claim 1 , wherein:

a geometry of the two-dimensional diffraction grating is arranged to result in a grating efficiency map that suppresses grating efficiencies of the (n, m) th diffraction orders, where either n or m is a non-zero even number;

the square array of through-apertures are square apertures having a length that is half the distance between centers of adjacent through-apertures; and

the sides of the square apertures are parallel to the axes of the square array of through-apertures.

4. The two-dimensional diffraction grating of claim 1 , wherein a geometry of the two-dimensional diffraction grating is arranged to result in a grating efficiency map that suppresses grating efficiencies of the (n, m) th diffraction orders wherein n±m is an even number except the (0, 0) th diffraction order.

5. The two-dimensional diffraction grating of claim 4 , wherein the square is orientated at 45° to the axes of the square array of through-apertures and has a diagonal dimension that matches a distance between the centers of adjacent through-apertures, and

wherein the connecting portion of the substrate between each pair of adjacent through apertures is generally rectangular.

6. The two-dimensional diffraction grating of claim 5 , wherein a width of the generally rectangular connecting portion of the substrate between each pair of adjacent through apertures is approximately 10% of the distance between centers of adjacent through-apertures.

7. The two-dimensional diffraction grating of claim 1 , wherein a geometry of the two-dimensional diffraction grating is arranged to result in a grating efficiency map that suppresses a grating efficiency of one or more diffraction orders, the one or more diffraction orders being the (n, m) th diffraction orders wherein n±m is an even number.

8. The two-dimensional diffraction grating of claim 7 , wherein the geometry of the two-dimensional diffraction grating is arranged to suppress (±2, 0) and (0,±2) diffraction orders.

9. The two-dimensional diffraction grating of claim 7 , wherein the geometry of the two-dimensional diffraction grating is arranged to suppress (±1,±1) diffraction orders.

10. The two-dimensional diffraction grating of claim 1 , wherein a width of the connecting portion is from about 5% to about 15% of a distance between centers of two adjacent apertures.

11. A method of manufacturing a two-dimensional diffraction grating for a phase-stepping measurement system for determining an aberration map for a projection system, the method comprising:

selecting a geometry for the two-dimensional diffraction grating, the geometry having at least one parameter; and

selecting values for the at least one parameter that result in a grating efficiency map for the two-dimensional diffraction grating so as to control contributions to a harmonic of a phase stepping signal,

wherein the two-dimensional diffraction grating is transmissive,

wherein the two-dimensional diffraction grating comprises a substrate provided with a square array of through-apertures having a checkerboard pattern,

wherein the substrate comprises a support layer and a radiation absorbing layer,

wherein the support layer is separate from the radiation absorbing layer,

wherein the square array of through-apertures extend through both the support layer and the radiation absorbing layer,

wherein the square array of through-apertures are non-regular octagonal and the square array of through-apertures are formed from a square having truncated corners, such that a connecting portion is formed between each two adjacent apertures, and

wherein the connecting portion has at least two parallel sides.

12. The method of claim 11 , wherein the selection of values for the least one parameter that result in the grating efficiency map for the two-dimensional diffraction grating so as to control the contributions to a harmonic of a phase stepping signal results from the two-dimensional diffraction grating being used with a first patterned region that comprises a one-dimensional diffraction grating with a 50% duty cycle or a two-dimensional checkerboard diffraction grating with a 50% duty cycle.

13. The method of claim 11 , wherein:

the selecting values for the least one parameter comprises selecting values for the least one parameter that minimizes a grating efficiency of one or more diffraction orders, the one or more diffraction orders being the (n, m) th diffraction orders, wherein n±m is an even number, and

the step of selecting values for the least one parameter involves selecting values for the least one parameter that minimizes a grating efficiency of (±2, 0) and (0,±2) diffraction orders.

14. The method of claim 11 , wherein the selecting values for the least one parameter comprises selecting values for the least one parameter that minimizes a grating efficiency of (±1, ±1) diffraction orders.

15. The method of claim 11 , further comprising:

fabricating the two-dimensional diffraction grating using the selected values.

16. A two-dimensional diffraction grating, comprising:

a geometry configured to be selected to have at least one parameter, such that the least one parameter is configured to result in a grating efficiency map for the two-dimensional diffraction grating so as to control contributions to a harmonic of a phase stepping signal,

wherein the two-dimensional diffraction grating is transmissive,

wherein the two-dimensional diffraction grating comprises a substrate provided with a square array of through-apertures,

wherein the substrate comprises a support layer and a radiation absorbing layer,

wherein the support layer is separate from the radiation absorbing layer,

wherein the square array of through-apertures extend through both the support layer and the radiation absorbing layer, and the square array of through-apertures are non-regular octagonal, and

wherein each through-aperture is formed from a square having a diagonal dimension that matches a distance between centers of two adjacent apertures.