Interferometer system and lithographic apparatus

An interferometer system for determining a position of a target surface, wherein the interferometer system comprises:

• a first detector configured to receive a first reference beam that is adapted to travel a first reference beam path including a reference beam path, and receive a first measurement beam that is adapted to travel a first measurement beam path including a measurement beam path and a reference axis path determined by a reference surface;
• a second detector configured to receive a second reference beam that is adapted to travel a second reference beam path including the reference beam path, and receive a second measurement beam that is adapted to travel a second measurement beam path including the measurement beam path and a measurement axis path determined by the target surface;
• and
• a delay path controller configured to adapt a delay length of:
• a reference variable delay path which is part of the reference beam path, or a measurement variable delay path which is part of the measurement beam path, or both the reference variable delay path and measurement variable delay path,
• wherein during operational use:
• a reference spectral coherence pulse of the first reference beam and the first measurement beam occurs at the first detector for the delay length that corresponds to a reference coherence arrangement, and
• a measurement spectral coherence pulse of the second reference beam and the second measurement beam occurs at the second detector for the delay length that corresponds to a measurement coherence arrangement;
• and
• a control unit configured to:
• receive a reference coherence signal from the first detector, and a measurement coherence signal from the second detector;
• and
• determine a zero-position of the target surface based:
• on the reference coherence signal and the measurement coherence signal;
• and
• on the reference coherence arrangement and the measurement coherence arrangement, or a delay path difference between the reference coherence arrangement and the measurement coherence arrangement, or the reference coherence arrangement, the measurement coherence arrangement and the delay path difference.