Two-dimensional diffraction grating

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.

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.