Patent ID: 7595471

Claim:
An apparatus for automatically focusing a workpiece in Z-axis, the workpiece being positioned for optical metrology of structures on the workpiece, the apparatus comprising: an auto focusing subsystem comprising: a light source generating a focus illumination beam directed to a workpiece, the focus illumination beam generating a focus detection beam; a focus detector having an array of sensors, the array of sensors having a pitch, each sensor of the array of sensors having a sensor identification (ID), the focus detector configured to measure the focus detection beam, generate a corresponding focus signal, and transmit both the focus signal and associated sensor ID for each sensor in the array of sensors; a processor generating a best focus instruction based on the transmitted focus signal and associated sensor ID for each sensor in the array of sensors; and a motion control subsystem configured to position the workpiece on a best focus location on the Z-axis using the best focus instruction from the processor.