Patent ID: 7136149

Claim:
A lithographic apparatus having an illumination system for providing a radiation beam, a patterning device for imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, and a substrate table constructed to hold a substrate, the lithographic apparatus comprising: a projection system having a plurality of mirrors arranged to project the patterned radiation beam onto a target portion of the substrate, the projection system having a first side that receives the pattern radiation beam and a second side that projects the patterned radiation bean onto the target portion of the substrate; and an autofocus system that is located on the first side of the projection system, comprising: a light source; a position sensitive detector; and a light directing arrangement for coupling a first portion of light from the light source into the projection system, for directing a second portion of light from the light source to the position sensitive detector, and for directing the first portion of light to the position sensitive detector after the first portion of light has traveled through the projection system to the substrate and back through the projection system to the light directing arrangement.