Abbe arm calibration system for use in lithographic apparatus

Abstract

In a lithographic apparatus, a reference grating 11 mounted on the wafer table WT is illuminated with a measurement beam 20 incident in a direction independent of wafer table tilt. The diffraction orders are detected by detector 30 and used to determine the lateral shift in the wafer table resulting from a non-zero Abbe arm, and hence the Abbe arm, for calibration purposes. The detector 30 may be a detector also used for off-axis alignment of the wafer and wafer table.

1 . A lithographic projection apparatus comprising: 
 an illumination system for supplying a projection beam of radiation;    a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern;    a second object table for holding a substrate having a surface to be exposed, such that, when held on the table, the said surface lies in a reference plane;    a projection system for imaging the patterned beam onto a target portion of the substrate; and    a positioning system for moving said second object table between an exposure position, at which said projection system can image said patterned beam onto said substrate, and a measurement position; characterized by:    a calibration system for measuring lateral displacements of a reference point in a plane of said second object table as a function of tilt, at said measurement position, wherein said calibration system comprises:    a diffraction grating mounted to said second object table;    illuminating means for generating a measurement beam of radiation and directing it to be incident on said diffraction grating so as to be diffracted thereby; and    a detector for detecting the position of said diffraction grating.