Abstract: A heterodyne optical interferometer incorporates error correction elements to correct a cyclic error that may be present in an interferometric measurement. The cyclic error can be caused by various factors such as an imperfect polarization relationship between two wavelength components, deficiencies in optical propagation paths (such as light leakage), imperfect optical coatings, and/or imperfect components. The cyclic error, which typically manifests itself as erroneous displacement information characterized by a low velocity sinusoidal frequency component, can be reduced or eliminated by using birefringent optical elements and other optical elements to alter certain characteristics of one or both wavelength components and reduce light leakage components in one or more light propagation paths in the heterodyne optical interferometer.

Abstract: A heterodyne optical interferometer incorporates error correction elements to correct a cyclic error that may be present in an interferometric measurement. The cyclic error can be caused by various factors such as an imperfect polarization relationship between two wavelength components, deficiencies in optical propagation paths (such as light leakage), imperfect optical coatings, and/or imperfect components. The cyclic error, which typically manifests itself as erroneous displacement information characterized by a low velocity sinusoidal frequency component, can be reduced or eliminated by using birefringent optical elements and other optical elements to alter certain characteristics of one or both wavelength components and reduce light leakage components in one or more light propagation paths in the heterodyne optical interferometer.

Abstract: Generally, in accordance with the various illustrative embodiments disclosed herein, a heterodyne optical interferometer incorporates error correction elements to correct a cyclic error that may be present in an interferometric measurement. The cyclic error can be caused by various factors such as an imperfect polarization relationship between two wavelength components, deficiencies in optical propagation paths (such as light leakage), imperfect optical coatings, and/or imperfect components. The cyclic error, which typically manifests itself as erroneous displacement information characterized by a low velocity sinusoidal frequency component, can be reduced or eliminated by using birefringent optical elements and other optical elements to alter certain characteristics of one or both wavelength components and reduce light leakage components in one or more light propagation paths in the heterodyne optical interferometer.

Abstract: A measurement displacement system and method are described. The measurement displacement system comprises a sensor head configured to transmit input optical beams and to receive measurement beams. The system comprises a transmission grating configured to diffract the input optical beams into sub-beams comprising more than one diffraction order. The transmission grating is adapted move in a direction. The measurement displacement system comprises a reflective element configured to diffract the sub-beams from the transmission grating and to return the sub-beams to the transmission grating. The reflective element is substantially stationary relative to the sensor head and the transmission grating selectively recombines the sub-beams to form the measurement beams and returns the measurement beams to the sensor head.

Abstract: A measurement displacement system and method are described. The measurement displacement system comprises a sensor head configured to transmit input optical beams and to receive measurement beams. The system comprises a transmission grating configured to diffract the input optical beams into sub-beams comprising more than one diffraction order. The transmission grating is adapted move in a direction. The measurement displacement system comprises a reflective element configured to diffract the sub-beams from the transmission grating and to return the sub-beams to the transmission grating. The reflective element is substantially stationary relative to the sensor head and the transmission grating selectively recombines the sub-beams to form the measurement beams and returns the measurement beams to the sensor head.

Abstract: A sensor head for use with a measurement grating is described. The sensor head comprises: a splitter grating configured to split a light beam into first and second measurement beams; a first retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating; and a second retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating. In one embodiment the second measurement beam is diffracted by the measurement grating to form first and second sub-beams and one of the first and second sub-beams comprises a zeroth order diffraction component and a first order diffraction component. In another embodiment, the first and second sub-beams each comprise a zeroth order diffraction component and a first order diffraction component.

Abstract: A displacement measurement apparatus includes a light source, a splitter grating, a measurement grating, and first a second detector arrays. The splitter grating splits a light beam into first and second measurement channels that each illuminates the measurement grating. The first and second measurement channels split into 0th and 1st order diffraction products at the measurement grating in a first pass and recombine at the measurement grating in a second pass before being measured at the first and second detector arrays.

Abstract: A sensor head for use with a measurement grating is described. The sensor head comprises: a splitter grating configured to split a light beam into first and second measurement beams; a first retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating; and a second retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating. In one embodiment the second measurement beam is diffracted by the measurement grating to form first and second sub-beams and one of the first and second sub-beams comprises a zeroth order diffraction component and a first order diffraction component. In another embodiment, the first and second sub-beams each comprise a zeroth order diffraction component and a first order diffraction component.

Abstract: A method for non-linearity compensating interferometer position data generated from a measurement signal includes generating a first set of non-linearity parameters based on received digital position values. The method includes sensing whether a low velocity condition exists. A first one of the non-linearity parameters is updated based on an estimated magnitude of the measurement signal if the low velocity condition exists. At least one digital position value is compensated based on the updated non-linearity parameter if the low velocity condition exists.

Abstract: A displacement measurement apparatus includes a light source, a splitter grating, a measurement grating, and first a second detector arrays. The splitter grating splits a light beam into first and second measurement channels that each illuminates the measurement grating. The first and second measurement channels split into 0th and 1st order diffraction products at the measurement grating in a first pass and recombine at the measurement grating in a second pass before being measured at the first and second detector arrays.

Abstract: An interferometer can achieve a high dynamic range for measurements along vertical and horizontal directions using a first measurement channel providing a high dynamic range measurement of a path including components respectively parallel and perpendicular to the optics-object separation and a second measurement channel providing a high dynamic range measurement with just a perpendicular component. Further, using the same techniques at multiple locations around permits a high dynamic range for measurements of the degrees of freedom of an object.

Abstract: An interferometer provides a large dynamic range for perpendicular displacement measurements. In operation, a measurement reflector on an object reflects a measurement beam to an overlying Porro prism, and a reference reflector on the object returns a reference beam to the interferometer. A second Porro prism in the interferometer can return the reference beam for a second pass to the reference reflector, while the measurement beam complete only one pass. Reductions in beam walk-off result from retroreflections in the Porro prisms and the matching effects that some object rotations have on measurement and reference beams. Perpendicular motion of the object relative to the first Porro prism causes a Doppler shift only in the measurement beam. Accordingly, a beat frequency found when combining the measurement and reference beams can indicate a residual Doppler shift associated with the motion in the perpendicular direction.

Abstract: An interferometer is provided that minimizes the introduction of non-linear errors into displacement measurements. In one embodiment, non-linear errors are reduced by isolating reference and measurement beams over most of their respective optical paths leading to the detector, and by employing a separate amplitude-splitting non-polarizing optical beam splitter for each beam input into the interferometer. Additionally, the interferometer is scalable to an arbitrary number of optical axes or inputs.

Abstract: A system and method for acquiring position information of a movable apparatus relevant to a specific axis is disclosed. In one embodiment, an interferometer generates first and second beams and various beam-steering members are located to define beam path segments for the two beams, but no beam path segment varies in length in unity with displacements of the movable apparatus along the specific axis. In another or the same embodiment, each beam path segment in which the first beam either impinges or has been reflected from the movable apparatus is symmetrical to a corresponding beam path segment of the second beam. The movable apparatus may be a wafer stage in which the “specific axis” is the exposure axis of a projection lens, but with all optical members which cooperate with the stage being located beyond the ranges of the wafer stage in directions perpendicular to the lithographic exposure axis.