Patent ID: 7050162

Claim:
A method for evaluating a sample with a optical wafer metrology tool employing an incoherent light source, illumination optics that collect a portion of the light from the source and illuminate the sample at a small-spot, the illumination interacting with and reflecting from the sample, collection optics that collect at least a fraction of the reflected illumination, a detection system generating output signals in response to the collected illumination and a processor for recording and analyzing the output signals, said method comprising: limiting the angular range of the illumination striking the sample to an illumination numerical aperture, NA I ; limiting the angular variation of the fraction of the reflected radiation collected by the collection optics to a collection numerical aperture, NA C ; constraining the above described angular limitations to configurations where NA C >NA I , whereby the relative numerical aperture NA R ≡NA I /NA C assumes a value less than 1.