Patent ID: 7880889

Claim:
A method comprising: using an optical system comprising an objective lens including an object plane and a pupil plane to direct a first beam of radiation onto a reference member, to collect radiation reflected or scattered by the reference member, and to project a second beam of radiation to form an image of the pupil plane of the objective lens in an image plane; relatively moving the reference member and the optical system in a direction substantially perpendicular to the object plane so as to position the reference member at a plurality of positions having different distances from the object plane; capturing scatterometric spectra of the reference member when the reference member is positioned at each of the plurality of positions; storing the scatterometric spectra of the reference member as a plurality of normalization spectra; using the optical system to direct the first beam of radiation onto a target pattern, to collect radiation reflected or scattered by the target pattern, and to project a second beam of radiation to form a second image of the pupil plane of the objective lens in the image plane; capturing scatterometric spectra of the target pattern; determining the distance between the target pattern and the object plane; obtaining a normalization spectrum based on the determined distance between the target pattern and the object plane; normalizing the spectrum of the target pattern using the obtained normalization spectrum to obtain a normalized spectrum; and determining a value related to a parameter of the target pattern from the normalized spectrum by a processing unit.