Lithographic apparatus with autofocus system

A method of focusing a beam of radiation onto a substrate in a lithographic apparatus that includes an illumination system for providing the radiation beam, a patterning device for imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, a substrate table constructed to hold the substrate, and a projection system having a plurality of mirrors arranged to project the patterned radiation beam onto a target portion of the substrate, the method comprising:

• providing a beam of light;
• providing a position sensitive detector that receives one or more portions of the beam of light;
• directing a first portion of the beam of light from the light source to the projection system, to enable the first portion of light to travel through the projection system to the substrate and back through the projection system before being projected onto the position sensitive detector;
• directing a second portion of the beam of light from the light source to the position sensitive director;
• and
• adjusting a focus for the beam of radiation on the substrate based on information derived from the position sensitive detector.

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 beam 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 first optical device;
• a second optical device;
• and
• a position sensitive detector;
• wherein the first optical device and the second optical device are arranged to direct a first portion of light from the light source into the projection system 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, and wherein the first optical device is arranged to direct a second portion of light from the light source to the position sensitive detector.