Patent ID: 8436998

Claim:
A method of measuring the focus of a lithographic projection apparatus, said method comprising: covering a test substrate with photo resist; placing said test substrate on a substrate table of said lithographic apparatus; exposing a plurality of verification fields on said test substrate, each of said verification fields comprising a plurality of verification markers, wherein said verification fields are exposed using a predetermined focus offset in a predetermined focal curve; exposing a plurality of calibration fields on said test substrate, each of said calibration fields comprising a plurality of calibration markers, said calibration fields being exposed with predefined tilt offsets per calibration field; developing said photo resist; measuring an alignment offset for each of said verification markers; measuring an alignment offset for each of said plurality of calibration markers to render calibration data; determining a calibration curve using said calibration data; making a reference focal curve by using a portion of said calibration curve; translating the measured alignment offsets for each of said verification markers into defocus data using the reference focal curve.