Patent ID: 7247825

Claim:
A method for scanning a wafer using an optical imaging system and a scanning stage, comprising the steps of: positioning the wafer with a constant thickness on the scanning stage, the wafer having three-dimensional features within a focusing depth of the imaging system; calibrating the scanning stage by obtaining and storing height values Z at different calibration positions X, Y of the scanning stage, and thereby generating a height profile of the scanning stage; scanning the wafer, and thereby determining a reference height Z ref of the wafer at the beginning of a specimen scan, traveling to wafer points X p , Y p using the scanning stage, setting, while traveling to a respective wafer point X p , Y p , a wafer height position Z p pertinent to the respective wafer point X p , Y p , the wafer specimen height position Z p being determined from the reference height Z ref and the height profile of the scanning stage, and acquiring an image and/or performing a measurement at the respective wafer point X P , Y p .