Abstract: An apparatus includes an interferometer having a polarizing beam splitter to split an input beam into a measurement beam and a reference beam, the measurement beam contacting a measurement object, the reference beam contacting a reference object. The interferometer includes at least one thin retarder on a surface of the interferometer.

Abstract: Apparatus and methods for adjusting the polarizations of the components of an input beam to reduce leakage caused by imperfections in an interferometer are disclosed. The apparatus includes an interferometer positioned to receive an input beam that includes two components having different frequencies and different polarizations, the interferometer including a polarizing beam splitter positioned to spatially separate the input beam into two intermediate beams, the first intermediate beam corresponding to the first component and the second intermediate beam corresponding to the second component, where imperfections in the interferometer cause leakage of one of the components into the intermediate beam corresponding to the other of the components. The apparatus further includes a polarization compensator that includes at least one retardation plate positioned to adjust the polarizations of the components of the input beam to reduce the leakage caused by the imperfections in the interferometer.

Abstract: A two-corner retroreflector that includes a common face positioned to receive two beams and two corners each positioned to receive one of the beams and retro-reflect it back through the common face.

Abstract: In general, in one aspect, the invention features apparatus that include an interferometer including a main assembly having two laterally displaced polarizing beam splitter interfaces, wherein the interfaces are positioned to receive two spatially separated beams and direct them each to make at least two passes to one or more remote objects. Each interface reflects and transmits each beam at least once, and the interferometer is configured to combine the two beams after they make the at least two passes to the one or more remote objects to produce an output beam including information about an optical path length difference between paths traveled by the two beams.

Abstract: In general, in one aspect, the invention features an interferometer assembly for use in a lithography tool used for fabricating integrated circuits on a wafer, wherein the lithography tool includes a support structure and a stage for positioning the wafer relative to the support structure, the interferometer assembly including an interferometer configured to direct a measurement beam between the stage and the support structure and combine the measurement beam with another beam to form an output beam which includes a phase related to a position of the stage relative to the support structure, wherein the interferometer is mechanically secured to the lithography tool through an interferometer surface selected to cause the phase of the output beam to be insensitive to thermal changes of the interferometer over a range of temperatures.

Abstract: An interferometer that includes a first beam-splitting surface positioned to separate an input beam into a first beam and a second beam. A first set of optics is positioned to receive the first beam, direct it to contact a first reflector multiple times and produce a first intermediate beam. The first intermediate beam follows a nominal output path when the first reflector has a first alignment normal to the first beam prior to reflection by the first reflector. The first set of optics includes a second beam-splitting surface, a third beam-splitting surface, and a first fold optic that are positioned to reduce displacement of the first intermediate beam from the nominal output path when the first reflector has an alignment different from the first alignment.

Abstract: An interferometer for receiving a measurement beam from a target location on a stage of a semiconductor lithography machine and a reference beam from a reference location separated from the target location by a separation distance. The interferometer has a reference path to be traversed by the reference beam within the interferometer and a measurement path to be traversed by the measurement beam within the interferometer. Both the measurement path and the reference path are at least as long as the separation distance between the reference location and the target location.

Abstract: A multi-axis interferometer includes a multiplexer portion and a beam splitter portion integrated into the same optically transmissive monolith. The multiplexer portion is configured to split an input beam into a corresponding plurality of intermediate beams, each of which is directed toward the beam splitter portion through a corresponding output port of the multiplexer portion. The beam splitter portion is configured to separate the intermediate beam into a measurement component and a reference component.

Abstract: A multi-axis interferometer includes a mounting block with first and second polarizing beam-splitter cubes contacting first and second faces of the mounting block. A beam-distribution system contacts a third face of the mounting block. The beam-distribution system is thus placed in optical communication with the first polarizing beam-splitter cube and the second polarizing beam-splitter cube.

Abstract: A retarder includes a birefringent film embedded in an adhesive between two plates. The adhesive is index matched to the birefringent film.

Abstract: A monolithic corrector plate for an interferometer, includes a substrate having an input face for intercepting a first beam emitted by the interferometer, and an output face opposite the input face. An aperture integral with the substrate is configured to transform a first beam intercepted by the input face into a second beam emerging from the output face.

Abstract: An interferometer that includes a first beam-splitting surface positioned to separate an input beam into a first beam and a second beam. A first set of optics is positioned to receive the first beam, direct it to contact a first reflector multiple times and produce a first intermediate beam. The first intermediate beam follows a nominal output path when the first reflector has a first alignment normal to the first beam prior to reflection by the first reflector. The first set of optics includes a second beam-splitting surface, a third beam-splitting surface, and a first fold optic that are positioned to reduce displacement of the first intermediate beam from the nominal output path when the first reflector has an alignment different from the first alignment.

Abstract: An interferometer for receiving a measurement beam from a target location on a stage of a semiconductor lithography machine and a reference beam from a reference location separated from the target location by a separation distance. The interferometer has a reference path to be traversed by the reference beam within the interferometer and a measurement path to be traversed by the measurement beam within the interferometer. Both the measurement path and the reference path are at least as long as the separation distance between the reference location and the target location.

Abstract: A multi-axis interferometer includes a mounting block with first and second polarizing beam-splitter cubes contacting first and second faces of the mounting block. A beam-distribution system contacts a third face of the mounting block. The beam-distribution system is thus placed in optical communication with the first polarizing beam-splitter cube and the second polarizing beam-splitter cube.

Abstract: A retarder includes a birefringent film embedded in an adhesive between two plates, The adhesive is index matched to the birefringent film.

Abstract: A monolithic corrector plate for an interferometer, includes a substrate having an input face for intercepting a first beam emitted by the interferometer, and an output face opposite the input face. An aperture integral with the substrate is configured to transform a first beam intercepted by the input face into a second beam emerging from the output face.