Lithographic apparatus, device manufacturing method, and method for determining z-displacement

A method of determining the displacement of an object in an apparatus along the z-direction, said method comprising:

• providing an interferometer system comprising:
• a laser beam generator system configured to generate at least one reference laser beam and at least one measuring laser beam;
• at least one mirror, wherein said at least one mirror includes a measuring mirror with a fixed position relative to said object and having a mirror surface in which a reference line normal to the mirror surface makes a non-zero acute angle with said z-direction, the object having a surface that extends substantially in a plane perpendicular to the z-direction, the surface of the object to be illuminated by a beam of radiation having an optical axis extending along the z-direction;
• an interference signal processing system configured to receive an interference signal produced by said reference laser beam and said measuring laser beam, and to determine said displacement based on a change in said interference signal;
• providing said reference laser beam and said measuring laser beam such that said measuring laser beam is directed towards said measuring mirror in a direction substantially perpendicular to said mirror surface and said measuring laser beam is substantially retro-reflected by said measuring mirror to at least partly overlap said reference beam to generate said interference signal;
• determining a change in said interference signal;
• processing said change in order to determine the displacement of said object along the z-direction;
• and
• supplying or making available the determined displacement for further analysis or processing.

A method of determining the displacement of an object in an apparatus along the z-direction, said object having a surface which extends substantially in a plane perpendicular to said z-direction and which is illuminated by a beam of radiation having an optical axis extending along the z-direction, said method comprising:

• providing an interferometer system, comprising:
• a laser beam generator system arranged and constructed for generating at least one reference laser beam and at least one measuring laser beam;
• a set of at least two mirrors comprising at least one mirror which is a measuring mirror fixedly connected to said object and having a mirror surface in which a reference line normal to the mirror surface makes a non-zero acute angle with said z-direction and at least one separate mirror having a fixed position with respect to said apparatus;
• an interference signal processing system, arranged and constructed for receiving an interference signal produced by said reference beam and said measuring beam and for determining a displacement on the basis of a change in said interference signal;
• providing said reference laser beam and said measuring laser beam, wherein said measuring laser beam has a direction of incidence on the measuring mirror outside a plane in which the z-direction and said normal to said mirror surface lie and is directed towards said measuring mirror such that said measuring laser beam is reflected by said measuring mirror towards said separate mirror in a direction substantially perpendicular to said separate mirror and at least partly overlaps said reference laser beam to provide an interference signal;
• determining a change in said interference signal;
• processing said change in said interference signal in order to determine said displacement in the z-direction;
• and
• supplying or making available the determined displacement for further analysis or processing.

A lithographic apparatus, comprising:

• a projection system configured to provide a beam of radiation for illuminating an object, said beam of radiation having an optical axis in a z-direction;
• a movable holder configured to hold said object;
• and
• an interferometer system configured to determine a displacement of said object along a z-direction, said interferometer system comprising:
• a laser beam generator system configured to generate at least one reference laser beam and at least one measuring laser beam;
• at least one mirror, wherein said at least one mirror includes a measuring mirror with a fixed position relative to said object and having a mirror surface in which a reference line normal to the mirror surface makes a non-zero acute angle with said z-direction;
• and
• an interference signal processing system configured to receive an interference pattern based on said reference laser beam and said measuring laser beam and to determine said displacement based on a change in said interference signal,
• wherein said normal to said mirror surface is substantially parallel to a direction of incidence of said measuring laser beam on said mirror surface.

A lithographic apparatus, comprising:

• a projection system configured to provide a beam of radiation for illuminating an object, said beam of radiation having an optical axis in a z-direction;
• a movable holder configured to hold said object;
• and
• an interferometer system configured to determine a displacement of said object along a z-direction, said interferometer system comprising:
• a laser beam generator system configured to generate at least one reference laser beam and at least one measuring laser beam;
• at least one mirror, wherein said at least one mirror includes a measuring mirror with a fixed position relative to said object and having a mirror surface in which a reference line normal to the mirror surface makes a non-zero acute angle with said z-direction;
• and
• an interference signal processing system configured to receive an interference pattern based on said reference laser beam and said measuring laser beam and to determine said displacement based on a change in said interference signal,
• wherein said measuring laser beam has a direction of incidence on the measuring mirror outside a plane in which the z-direction and said normal to said mirror surface lie, there being provided a separate mirror constructed and arranged for substantially retro-reflecting said measuring laser beam.

A device manufacturing method, comprising:

• providing an object in a movable holder;
• illuminating said object with a beam of radiation having an optical axis in the z-direction;
• moving said object during at least part of the illuminating;
• and
• determining a displacement of said object along the z-direction by:
• providing a reference laser beam;
• providing a measuring laser beam that is directed substantially perpendicularly incident on a measuring mirror with a fixed position relative to said object such that a reference line normal to the measuring mirror makes a non-zero acute angle with said z-direction, said measuring laser beam being substantially retro-reflected and at least partly overlapping said reference beam to provide an interference signal;
• determining a change in said interference signal;
• processing said change in order to determine the displacement of said object along the z-direction;
• and
• supplying or making available the determined displacement for further analysis or processing.

A device manufacturing method, comprising

• providing a projection system configured to provide a beam of radiation for illuminating an object, said beam of radiation having an optical axis in a z-direction;
• providing a movable holder configured to hold said object;
• providing an interferometer system configured to determine a displacement of said object along a z-direction, said interferometer system comprising:
• a laser beam generator system configured to generate at least one reference laser beam and at least one measuring laser beam;
• at least one mirror, wherein said at least one mirror includes a measuring mirror with a fixed position relative to said object and having a mirror surface in which a reference line normal to the mirror surface makes a non-zero acute angle with said z-direction;
• and
• an interference signal processing system configured to receive an interference pattern based on said reference laser beam and said measuring laser beam and to determine said displacement based on a change in said interference signal, wherein said normal to said mirror surface is substantially parallel to a direction of incidence of said measuring laser beam on said mirror surface;
• securing an object in said holder;
• illuminating said object with said beam of radiation;
• moving said object during at least part of said illuminating;
• wherein the z-displacement of said object is determined based on said interferometer system and said interference signal processing system of said lithographic apparatus;
• and
• supplying or making available the determined displacement for further analysis or processing.