Method of measuring a target, metrology apparatus, polarizer assembly

1. A polarizer assembly comprising:

a first polarizing structure comprising a substantially planar form and configured to polarize radiation passing there through, wherein the first polarizing structure is located in an illumination path of a metrology apparatus and in a first pupil plane of an optical system of the metrology apparatus; and

a second polarizing structure comprising a substantially planar form and configured to polarize radiation passing there through, wherein the second polarizing structure is located in a detection path of the metrology apparatus and in a second pupil plane of the optical system of the metrology apparatus,

wherein a first region of the first polarizing structure has an opposite polarization compared to a first region of the second polarizing structure having same position relative to the first region of the first polarizing structure.

2. The polarizer assembly of claim 1 , wherein:

the first polarizing structure comprises a circular polarizing region,

the circular polarizing region of the first polarizing structure comprises the first region and a second region,

the first region of the first polarizing structure is configured to apply a first polarization to radiation passing through the first region of the first polarizing structure comprising a first one of two pairs of diametrically opposite quadrants of the circular polarizing region, and

a second region of the first polarizing structure is configured to apply a second polarization, orthogonal to the first polarization, to radiation passing through the second region of the first polarizing structure comprising a second one of two pairs of diametrically opposite quadrants of the circular polarizing region.

3. The polarizer assembly of claim 2 , wherein the first polarization comprises a polarization in a horizontal direction and the second polarization comprises a polarization in a vertical direction.

4. The polarizer assembly of claim 2 , wherein the first polarization comprises a polarization in a vertical direction and the second polarization comprises a polarization in a horizontal direction.

5. The polarizer assembly of claim 1 , wherein:

the second polarizing structure comprises a circular polarizing region,

the circular polarizing region of the second polarizing structure comprises the first region and a second region,

the first region of the second polarizing structure is configured to apply a first polarization to radiation passing through the first region of the second polarizing structure comprising a first one of two pairs of diametrically opposite quadrants of the circular polarizing region, and

a second region of the second polarizing structure is configured to apply a second polarization, orthogonal to the first polarization, to radiation passing through the second region of the second polarizing structure comprising a second one of two pairs of diametrically opposite quadrants of the circular polarizing region.

6. The polarizer assembly of claim 1 , wherein:

the first polarizing structure comprises a second region, and

the second region of the first polarizing structure has same polarization as the first region of the second polarizing structure having same position relative to the first region of the first polarizing structure.

7. A method comprising:

directing radiation to a first polarizing structure, the first polarizing structure having a substantially planar form located in an illumination path of a metrology apparatus and in a first pupil plane of an optical system of the metrology apparatus; and

directing the radiation, after scattered from a target, to a second polarizing structure, the second polarizing structure having a substantially planar form and located in a detection path of the metrology apparatus and in a second pupil plane of the optical system of the metrology apparatus,

wherein a first region of the first polarizing structure has an opposite polarization compared to a first region of the second polarizing structure having same position relative to the first region of the first polarizing structure.

8. The method of claim 7 , wherein the directing the radiation to the first polarizing structure comprises directing the radiation to the first region of the first polarizing structure.

9. The method of claim 8 , wherein the directing the scattered radiation to the second polarizing structure comprises directing the scattered radiation to the first region of the second polarizing structure.

10. The method of claim 8 , wherein the directing the scattered radiation to the second polarizing structure comprises directing the scattered radiation to a second region of the second polarizing structure, wherein the second region of the second polarizing structure has same polarization compared to the first region of the first polarizing structure.

11. A polarizer assembly configured in a metrology apparatus, the polarizer assembly comprising:

a polarizing structure comprising a substantially planar form and configured to polarize radiation passing there through in a circular polarizing region,

wherein the polarizing structure is located in a pupil plane of an optical system of the metrology apparatus, the metrology apparatus configured to measure an overlay error at an image plane, and

wherein the circular polarizing region is configured to apply a first polarization to radiation passing through a first one of two pairs of diametrically opposite quadrants of the circular polarizing region and to apply a second polarization, orthogonal to the first polarization, to radiation passing through a second one of two pairs of diametrically opposite quadrants of the circular polarizing region.