Patent ID: 9678443
Date: 2017-06-13
CPC Classifications: G01B,G03F,H01J

Claim:
1. Method for measuring a relative displacement between a first mirror and a second mirror in a lithography system, wherein said first mirror is connected to an exposure tool of said system and said second mirror is connected to a target to be exposed by said system, wherein said first mirror is moveable relative to said second mirror, wherein said method is performed using a differential interferometer module which comprises: a beam source ( wherein said unitary beam splitter ( a unitary beam combiner ( three beam receivers ( wherein said method comprises the steps of a) generating three coherent beams, and directing said three coherent beams to the unitary beam splitter, b) splitting said three coherent beams up into three measurement beam and associated reference beam pairs, wherein said three coherent beams are split using the unitary beam splitter, wherein said generating is performed before said splitting, c) after said splitting in step b): d) combining said three reflected measurement beams with their three associated reflected reference beams to provide three combined beams, each combined beam being a combination of one reflected measurement beam and one reflected reference beam, wherein the unitary beam combiner is used for combining said three reflected measurement beams with their three associated reflected reference beams to provide three combined beams, e) projecting each one of said three combined beams onto a corresponding one beam receiver, each receiver adapted for converting a combined beam into a signal representative of a change in position and/or orientation of the first minor relative to the second minor.