Method for calibration of an optical measurement system and optical measurement system

An optical measurement system, comprising:

• a light source to provide a first measurement beam, a first reference beam, a second measurement beam, and a second reference beam,
• a first optical axis optically connected to the light source to receive the first measurement beam and the first reference beam;
• a second optical axis optically connected to the light source to receive the second measurement beam and the second reference beam, wherein the first optical axis and second optical axis have a different optical path length;
• first interferometer optics associated with the first optical axis;
• second interferometer optics associated with the second optical axis;
• a first detector associated with the first optical axis;
• a second detector associated with the second optical axis;
• and
• a processing unit connected to the first detector and the second detector,
• wherein the optical measurement system is arranged to carry out a calibration method, comprising:
• measuring with the first detector a first measurement phase value along the first optical axis using the first measurement beam and the first reference beam;
• measuring with the second detector a second measurement phase value along the second optical axis using the second measurement beam and the second reference beam;
• changing a wavelength of the first measurement beam, the first reference beam, the second measurement beam, and the second reference beam;
• measuring with the first detector a further first measurement phase value along the first optical axis using the first measurement beam with changed wavelength and the first reference beam with changed wavelength;
• measuring with the second detector a further second measurement phase value along the second optical axis using the second measurement beam with changed wavelength and the second reference beam with changed wavelength;
• determining in the processing unit a cyclic error of the optical measurement system on the basis of the first measurement phase value, the second measurement phase value, the further first measurement phase value and the further second measurement phase value;
• and
• storing a corrective value based on the cyclic error.

A lithographic apparatus comprising an optical measurement system comprising:

• a light source to provide a first measurement beam, a first reference beam, a second measurement beam, and a second reference beam;
• a first optical axis optically connected to the light source to receive the first measurement beam and the first reference beam;
• a second optical axis optically connected to the light source to receive the second measurement beam and the second reference beam, wherein the first optical axis and second optical axis have a different optical path length;
• first interferometer optics associated with the first optical axis;
• second interferometer optics associated with the second optical axis;
• a first detector associated with the first optical axis;
• a second detector associated with the second optical axis;
• and
• a processing unit connected to the first detector and the second detector,
• wherein the optical measurement system is configured to carry out a calibration method, comprising:
• measuring with the first detector a first measurement phase value along the first optical axis using the first measurement beam;
• measuring with the second detector a second measurement phase value along the second optical axis using the second measurement beam;
• changing a wavelength of the first measurement beam, the first reference beam, the second measurement beam, and the second reference beam;
• measuring with the first detector a further first measurement phase value along the first optical axis using the first measurement beam with changed wavelength and the first reference beam with changed wavelength;
• measuring with the second detector a further second measurement phase value along the second optical axis using the second measurement beam with changed wavelength and the second reference beam with changed wavelength;
• determining in the processing unit a cyclic error of the optical measurement system on the basis of the first measurement phase value, the second measurement phase value, the further first measurement phase value and the further second measurement phase value;
• and
• storing a corrective value based on the cyclic error.