Patent ID: 7084979

Claim:
An apparatus to measure height of a feature on a surface of a substrate, comprising: a stage to support a substrate; a polarized light source to generate a polarized light beam; first optical components aligned to receive the polarized light beam and to propagate the polarized light beam in a first plane and incident on a first position on the substrate at a first incidence angle and to reflect the polarized light beam off the first position on the substrate at a first reflection angle; a polarization state retarder aligned to receive the polarized light beam from the first optical components; second optical components aligned to receive the polarized light beam from the polarization state retarder and propagate the polarized light beam in a second plane and incident on the first position on the substrate at the first incidence angle and to reflect the polarized light beam off the first position on the substrate at a second reflection angle, said second plane being substantially orthogonal to the first plane; a position sensitive detector aligned to detect the polarized light beam from the second optical components incident on at least one of a plurality of light sensitive locations, said position sensitive detector comprising: a plurality of detector elements to convert the polarized light beam into a respective plurality of intensity signals, wherein the plurality of detector elements are arranged with at least a first detector element positioned on a first side of a first separation line and with at least a second detector element positioned on a second side of the first separation line, said first separation line oriented substantially congruent to one of the first plane and the second plane; and a processor in communication with the position sensitive detector to receive a plurality of intensity signals from the respective plurality of detector elements and to determine a height of the first position based on a difference signal said difference signal between at least a first intensity signal from the first detector element and a second intensity signal from the second detector element, wherein the difference signal results from a position of the polarized light beam incident on the position sensitive detector, and induced by the first reflection angle in the first plane and the second reflection angle in the second plane.