| CPC H01L 22/12 (2013.01) [G01N 21/9501 (2013.01); G03F 7/70633 (2013.01); G03F 7/70683 (2013.01); G03F 9/7003 (2013.01)] | 20 Claims |

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1. A method of determining a parameter of a patterning process, the method comprising:
obtaining signals representing primarily zeroth order radiation detected by an optical measurement apparatus from illumination radiation redirected by one or more physical instances of a unit cell of a substrate, the substrate illuminated with a beam of illumination radiation such that a beam spot on the substrate is filled with the one or more physical instances of the unit cell, the unit cell having geometric symmetry at a nominal value of the parameter; and
transforming, by a hardware computer system, the signals into a non-nominal value of the parameter of the unit cell from values of an optical characteristic of the detected radiation, the transforming comprising assigning a different weighting to optical characteristic values from pixels of the detected radiation associated with greater sensitivity to the parameter than to optical characteristic values from other pixels of the detected radiation associated with lower sensitivity to the parameter.
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