A positioning system, a lithographic apparatus, an absolute position determination method, and a device manufacturing method

A positioning system to determine an absolute position of a moveable target relative to a reference, comprising:

• a measurement system to measure a position of the moveable target;
• an actuator system to move the moveable target;
• and
• a control unit to control the measurement system and the actuator system,
• wherein the measurement system comprises an interferometer system with a first light source configured to emit a first light beam at a fixed frequency, a second light source configured to emit a second light beam and is able to output the second light beam at a first frequency and at a second frequency, and an optics system configured to allow a portion of the first light beam to interact with the moveable target and another portion of the first light beam to travel along a fixed reference path, and to allow a portion of the second light beam to interact with the moveable target and another portion of the second light beam to travel along a fixed reference path, and
• wherein the control unit is configured to determine an absolute position of the moveable target relative to a reference by:
• moving the moveable target;
• driving the second light source such that during one or more portions of the moving of the moveable target the second light beam has the first frequency and during one or more other portions of the moving of the moveable target the second light beam has the second frequency;
• determining at least two first points in time where the second light source has the first frequency;
• determining at least two second points in time where the second light source has the second frequency;
• determining for each of the at least two first points in time a first phase difference between portions of the second light beam as a function of a position of the moveable target determined by a phase difference between portions of the first light beam;
• determining for each of the at least two second points in time a second phase difference between portions of the second light beam as a function of a position of the moveable target determined by a phase difference between portions of the first light beam;
• extrapolating a first curve fitted through the first phase differences and extrapolating a second curve fitted through the second phase differences;
• and
• determining a cross-point of the extrapolated first curve and the extrapolated second curve in order to determine an absolute position of the moveable target relative to the reference.

A method for determining an absolute position of a moveable target relative to a reference using an interferometer system with a first light source configured to emit a light beam at a fixed frequency and a second light source configured to emit a second light beam and is able to output the second light beam at a first frequency and at a second frequency, wherein a portion of the first light beam interacts with the moveable target and another portion of the first light beam travels along a fixed reference path, wherein a portion of the second light beam interacts with the moveable target and another portion of the second light beam travels along a fixed reference path, the method comprising:

• moving the moveable target;
• driving the second light source such that during a portion of step a. the second light beam has the first frequency and during another portion of step a. the second light beam has the second frequency;
• determining at least two first points in time where the second light source has the first frequency;
• determining at least two second points in time where the second light source has the second frequency;
• determining for each of the at least two first points in time a first phase difference between portions of the second light beam as a function of a position of the moveable target determined by a phase difference between portions of the first light beam;
• determining for each of the at least two second points in time a second phase difference between portions of the second light beam as a function of a position of the moveable target determined by a phase difference between portions of the first light beam;
• extrapolating a first curve fitted through the first phase differences and extrapolating a second curve fitted through the second phase differences;
• and
• determining a cross-point of the extrapolated first curve and the extrapolated second curve in order to determine an absolute position of the moveable target relative to the reference.

A heterodyne interferometer system comprising:

• a first light source configured to emit a light beam at a fixed frequency;
• a second light source configured to emit a second light beam and is able to output the second light beam at a first frequency and at a second frequency;
• an optics system configured to allow a portion of the first light beam to interact with the moveable target and another portion of the first light beam to travel along a fixed reference path, and to allow a portion of the second light beam to interact with the moveable target and another portion of the second light beam to travel along a fixed reference path;
• and
• a control unit,
• wherein the control unit is configured to determine an absolute position of the moveable target relative to a reference by:
• driving the second light source such that during one or more portions of a movement of the moveable target the second light beam has the first frequency and during one or more other portions of the movement of the moveable target the second light beam has the second frequency;
• determining at least two first points in time where the second light source has the first frequency;
• determining at least two second points in time where the second light source has the second frequency;
• determining for each of the at least two first points in time a first phase difference between portions of the second light beam as a function of a position of the moveable target determined by a phase difference between portions of the first light beam;
• determining for each of the at least two second points in time a second phase difference between portions of the second light beam as a function of a position of the moveable target determined by a phase difference between portions of the first light beam;
• extrapolating a first curve fitted through the first phase differences and extrapolating a second curve fitted through the second phase differences;
• and
• determining a cross-point of the extrapolated first curve and the extrapolated second curve in order to determine an absolute position of the moveable target relative to the reference.