Compact dual pass interferometer for a plane mirror interferometer

An interferometer comprising:

• a terminal configured to receive a polarized input radiation beam from a light source;
• and
• a wire grid polarizer at a focal plane of the input radiation beam and a lens,
• wherein the wire grid polarizer is arranged to reflect a first portion of the input radiation beam to follow a first optical path and is further arranged to transmit a second portion of the input radiation beam to follow a second optical path,
• wherein the first optical path is directed towards the terminal, and the second optical path is directed towards a location on a reflective target surface and back to the wire grid polarizer then again to the location on the reflective target surface and back to the wire grid polarizer,
• wherein a quarter wave plate is disposed between the wire grid polarizer and the reflective target surface, and the second optical path is further directed through the wire grid polarizer to be recombined with the first optical path to provide to a recombined optical path configured to provide an output radiation beam;
• the output radiation beam comprising the first and second portions of the input radiation beam, and
• wherein the terminal is further configured to deliver the output radiation beam to a detector.

A lithographic apparatus comprising:

• an interferometer comprising:
• a terminal configured to receive a polarized input radiation beam from a light source;
• and
• a wire grid polarizer at a focal plane of the input radiation beam and a lens,
• wherein the wire grid polarizer is arranged to reflect a first portion of the input radiation beam to follow a first optical path and is further arranged to transmit a second portion of the input radiation beam to follow a second optical path,
• wherein the first optical path is directed towards the terminal, and the second optical path is directed towards a location on a reflective target surface and back to the wire grid polarizer then again to the location on the reflective target surface and back to the wire grid polarizer,
• wherein a quarter wave plate is disposed between the wire grid polarizer and the reflective target surface, and the second optical path is further directed through the wire grid polarizer to be recombined with the first optical path to provide to a recombined optical path configured to provide an output radiation beam;
• the output radiation beam comprising the first and second portions of the input radiation beam;
• and
• wherein the terminal is further configured to deliver the output radiation beam to a detector.