Method to determine an absolute position of an object, interferometer system, projection system and lithographic apparatus

A method to determine an absolute position of an object using an interferometer system, comprising:

• providing a light beam;
• splitting the light beam into a measurement beam and a reference beam;
• guiding the measurement beam along a measurement path towards a reflective measurement surface on the object;
• guiding the reference beam along a reference path towards a reflective reference surface on a reference object;
• receiving, at a detector, the measurement beam after interaction with the reflective measurement surface and the reference beam after interaction with the reflective reference surface;
• measuring a phase signal based on the received measurement beam and the received reference beam;
• separating a cyclic error phase component from the phase signal;
• and
• determining the absolute position of the object using the cyclic error phase component.

An interferometer system to determine an absolute position of an object, comprising:

• a light source to provide a light beam;
• a measurement light path optically coupled to the light source to receive the measurement beam and comprising a reflective measurement surface arranged on the object and;
• a reference light path optically coupled to the light source to receive the reference beam and comprising a reflective reference surface arranged on a reference object;
• at least one detector associated with the measurement light path and the reference light path to receive the measurement beam after interaction with the reflective measurement surface and the reference beam after interaction with the reflective reference surface;
• a processing unit connected to the detector and configured to:
• receive a phase signal based on the measurement beam and the reference beam from the at least one detector,
• separate a cyclic error phase component from the phase signal, and
• determine the absolute position of the object using the cyclic error phase component.