Methods of mitigating crosstalk in metrology images

1 . A method of determining an orthonormalized structure of interest reference image, the orthonormalized structure of interest reference image configured to be applied to a measured image of the orthonormalized structure of interest to correct for effect of crosstalk from at least one nuisance structure, the method comprising:

determining a structure of interest reference image based on knowledge of the structure of interest;

determining at least one nuisance structure reference image based on knowledge of the at least one nuisance structure; and

orthonormalizing the structure of interest reference image to the at least one nuisance structure reference image to obtain the orthonormalized structure of interest reference image.

2 . The method of claim 1 , wherein the determining the structure of interest reference image and the determining the at least one nuisance structure reference image are both based on knowledge of imaging optics used to obtain the measured image.

3 . The method of claim 2 , wherein one or both of the determining the structure of the interest reference image and the determining the at least one nuisance structure reference image comprise applying one or more known point spread functions of the imaging optics to the structure of interest and/or the at least one nuisance structure respectively.

4 . The method of claim 1 , wherein one or both of the determining the structure of interest reference image and the determining the at least one nuisance structure reference image comprise measuring one or both of the structure of interest and/or the at least one nuisance structure in isolation.

5 . The method of claim 1 , wherein the orthonormalizing comprises determining the orthonormalized structure of interest reference image from a product of an inverse matrix and a vector comprising the structure of interest reference image and each at least one nuisance structure reference image, the inverse matrix comprising an outer product of the vector with its complex conjugate.

6 . The method of claim 1 , further comprising:

obtaining a measured image comprising an image of the structure of interest;

determining an overlap between the measured image and the orthonormalized structure of interest reference image; and

determining a measurement parameter for the structure of interest from the overlap.

7 . The method of claim 6 , wherein the determining the overlap comprises determining a dot product of the measured image and the orthonormalized structure of interest reference image.

8 . The method of claim 6 , wherein the measurement parameter comprises amplitude, intensity, or diffraction efficiency.

9 . The method of claim 6 , further comprising:

determining a parameter of interest from the measurement parameter and/or the parameter of interest being overlay or focus.

10 . The method of claim 1 , further comprising:

using substantially coherent measurement radiation to capture the measured image.

11 . The method of claim 10 , further comprising:

determining or estimating a pre-aberration amplitude and phase distribution of the at least one nuisance structure; and

using the pre-aberration amplitude and phase distribution in the determining the at least one nuisance structure reference image.

12 . The method of claim 1 , further comprising:

using incoherent or partially coherent measurement radiation to capture the measured image.

13 . A computer program comprising program instructions operable to perform the method of claim 1 , when run on a suitable apparatus.

14 . A processing arrangement, comprising:

a computer program carrier comprising a computer program comprising program instructions operable to perform the method of claim 1 ; and

a processor operable to run the computer program.

15 . A metrology device, comprising:

imaging optics configured to capture scattered radiation from a structure of interest;

a detector configured to detect the scattered radiation to obtain a measured image of the structure of interest; and

a processing arrangement comprising:

a computer program carrier comprising a computer program comprising program instructions operable to perform the method of claim 1 ; and

a processor operable to run the computer program.