Method of aligning a pair of complementary diffraction patterns and associated metrology method and apparatus

Disclosed is a method of aligning a pair of complementary diffraction patterns comprising a first complementary diffraction pattern and a second complementary diffraction pattern. The pair of complementary diffraction patterns are obtained from performance of a metrology process on a structure formed by a lithographic process. The method comprises performing at least a fine alignment stage to align the pair of complementary diffraction patterns, and the alignment stage comprises interpolating measured values of the first complementary diffraction pattern over at least a portion of the detectorarea; and minimizing a residual between measured values in the second complementary diffraction pattern and corresponding interpolated values from the interpolation of the first complementary diffraction pattern, by one or both of translation and rotation of the second complementary diffraction pattern. Also disclosed is a method of measuring a parameter of interest of a structure by using the aligning method, and an ass