a method for measuring a characteristic of a substrate , including directing an incident beam at an inspection grid of points on the substrate , receiving the reflected beam with a position sensitive detector , measuring the displacement of the reflected beam from its expected location , compiling a database of the displacement measurements , examining the database for effects of a pattern induced anomaly in the displacement measurements , producing an adjusted database , and deriving the characteristic of the substrate from the adjusted database . thus , pattern induced errors from the displacement measurements are corrected . in this manner , problems with interpreting the reflection angles of a beam in substrate stress analysis equipment are overcome where distortions in the reflection angles are caused by deposition patterns on the substrates .