Patent ID: 8441639

Claim:
A metrology system, comprising: a light source configured to produce a diffraction-limited light beam; an apodizer configured to shape the light beam in an entrance pupil of illumination optics in such a way that an irradiance farther than 1.5 microns from the center of an illumination spot in a wafer plane is less than 10 −6 of the peak irradiance of the center of the spot; optical elements configured to direct the diffraction-limited light beam from the apodizer to the illumination spot on a grating target on a wafer and to collect scattered light from the grating target; a field stop configured to reject a portion of the collected scattered light; a detector configured to detect the scattered light that passes through the field stop and to generate output responsive to the detected scattered light such that the grating target is measured by the metrology system using scatterometry; and a computer system configured to determine a characteristic of the grating target using the output.