Patent ID: 8107073

Claim:
An order sorting filter for an optical metrology system spectrometer, comprising: an optically-transparent substrate; and at least two zones formed on one side of the optically-transparent substrate, wherein each pair of adjacent zones forms a zone joint therebetween, and each zone is configured to receive and transmit an incident light beam to form a transmitted light beam, each zone comprising: a thin-film stack filter; and an underlayer disposed between the optically-transparent substrate and the thin-film stack filter, the underlayer having substantially the same index of refraction as the optically-transparent substrate, wherein the thickness of an underlayer of an at least one zone of an at least one pair of zones adjacent to an at least one zone joint, is configured so that transmitted light beams of a wavelength of a measured diffraction order incident at the at least one zone joint, emerge from the order sorting filter substantially in-phase from the pair of zones adjacent to the at least one zone joint.