Verfahren und Sensoreinheit zur Abstandsmessung in einer lithografischen Vorrichtung und lithographische Vorrichtung

The method involves deflecting light (1) of an illumination device around a defined angle by using a diffraction structure (3) located on a photomask (5) or on a substrate (6). An axial focus is produced by using the diffraction structure. The deflection light is reflected at the substrate or at the photomask. An impact point of reflected light is determined on the photomask or on the substrate. The distance between the photomask and substrate is determined from the position of impact point. Independent claims are included for the following: (1) a sensor unit for measuring distance between photomask and substrate in lithographic device; and (2) a lithographic device.