Lithographic apparatus and method

摘要

A displacement measurement system comprising at least one retro reflector and a diffraction grating. Said displacement measurement system is constructed and arranged to measure a displacement by providing a first beam of radiation to the measurement system, wherein the diffraction grating is arranged to diffract the first beam of radiation a first time to form diffracted beams. The at least one retro reflector is arranged to subsequently redirect the diffracted beams to diffract a second time on the diffraction grating. The at least one retro reflector is arranged to redirect the diffraction beams to diffract at least a third time on the diffraction grating before the diffracted beams are being recombined to form a second beam. And the displacement system is provided with a sensor configured to receive the second beam and determine the displacement from an intensity of the second beam.

主权项

1. A displacement measurement system, for measuring a displacement of a first beam of radiation, the displacement measurement system comprising:
a two dimensional reflection grating configured to diffract the first beam of radiation a first time to form at least two diffracted beams; at least two retro reflectors configured to redirect the at least two diffracted beams to diffract a second time on the reflection grating; wherein the at least two retro reflectors are configured to redirect the at least two diffracted beams to diffract a third time on the reflection grating before the at least two diffracted beams are being recombined to form the second beam; wherein the at least two retro reflectors are arranged to redirect the at least two diffraction beams to diffract at least a fourth time on the reflection grating before the at least two diffracted beams are recombined to form the second beam, and a sensor configured to receive the second beam and determine the displacement from an intensity of the second beam, wherein reflectors of the at least two retro reflectors are positioned only between the reflection grating and the sensor, and wherein the at least two diffracted beams comprise information about the displacement in a first and a second direction, and wherein the first direction is different from the second direction.