Method to determine an absolute position of a movable object, interferometer system, projection system and lithograpic apparatus

An interferometer system to determine an absolute position of a first movable object, said interferometer system comprising:

• a first light source arranged to provide a first beam and a second beam with a first light frequency;
• a second light source arranged to provide a further first beam and a further second beam with a second light frequency, wherein the second light source is arranged to provide a tunable second light frequency,
• a first measurement axis for guiding the first beam and the further first beam to a first reflective surface of the first movable object to obtain a first interferometer signal and a further first interferometer signal, respectively,
• a second measurement axis for guiding the second beam and the further second beam to a second reflective surface of a second object to obtain a second interferometer signal and a further second interferometer signal, respectively,
• at least one first detector to detect the first interferometer signal and the further first interferometer signal,
• at least one second detector to detect the second interferometer signal and the further second interferometer signal,
• a processing device, wherein the processing device is arranged to determine on the basis of the first interferometer signal, the further first interferometer signal, the second interferometer signal and the further second interferometer signal a first count offset and/or a further first count offset using a non-linear equation, wherein the non-linear equation is based on the relationship:
• λ
• ls
• ⁢
• 2
• /
• λ
• ls
• ⁢
• 1
• =
• C
• ma
• ⁢
• 1
• ,
• ls
• ⁢
• 1
• /
• C
• ma
• ⁢
• 1
• ,
• ls
• ⁢
• 2
• =
• C
• ma
• ⁢
• 2
• ,
• ls
• ⁢
• 1
• /
• C
• ma
• ⁢
• 2
• ,
• ls
• ⁢
• 2
• ,
• wherein
• Δls2 is the second wavelength of the second light source,
• Δls1 is the first wavelength of the first light source,
• Cma1, ls1 is a total number of counts with respect to the first measurement axis and the first beam,
• Cma1, ls2 is a total number of counts with respect to the first measurement axis and the further first beam,
• Cma2, ls1 is a total number of counts with respect to the second measurement axis and the second beam,
• Cma2, ls2 is a total number of counts with respect to the second measurement axis and the further second beam;
• and
• wherein the processing device is arranged to determine the absolute position of the first movable object on the basis of the first count offset and the first interferometer signal or on the basis of the further first count offset and the further first interferometer signal.