Patent ID: 8643835

Claim:
An optical inspection system for inspecting equivalent substrates at constant layer depth relative to a particular device layer of the substrates, the inspection system comprising: an image sensor having a fixed focal plane, a focus sensor for sensing Z distance in regard to a surface topography of the substrates and outputting the Z distance in a focus data stream, the focus sensor and the image sensor disposed in a known relationship, an XY stage for moving the substrates in an XY plane relative to the image sensor and the focus sensor, a Z motor for moving the substrates in a Z dimension relative to the image sensor and the focus sensor, and a controller for selectively operating the optical inspection system in one of a setup mode and an inspection mode, in the setup mode the controller for, controlling XY movement of a first one of the substrates using the XY stage so as to scan only a first reticle field of the first substrate under the focus sensor, receiving the focus data stream from the focus sensor, concurrently receiving XY data from the XY stage relative to the first reticle field, and storing correlated XYZ data for the first portion of the first substrate in a memory, in the inspection mode the controller for, controlling XY movement of the substrate using the XY stage so as to scan at least one of any other reticle field of the first substrate and any reticle field of any of the other equivalent substrates under the focus sensor and the image sensor, receiving the focus data stream from the focus sensor, concurrently receiving XY data from the XY stage relative to the scanned reticle field, subtracting the Z distance in the memory from the focus data stream of the focus sensor to produce a virtual data stream, where the Z distance from the memory is correlated with the XY data from the stage, and feeding the virtual data stream plus an offset to the Z motor for moving the scanned substrate up and down during the inspection, thereby holding the focal plane at a desired Z distance, regardless of the surface topography of the scanned substrate.