Abstract: A multi-axis plane mirror interferometer uses shared measurement and reference beams that respectively reflect from measurement and reference reflectors before that shared beams are split into individual beams corresponding to the measurement axes of the interferometer. An N-axis interferometer thus requires only N+1 measurement beam paths, one for the shared measurement beam and N for individual measurement beams, to provide for each measurement axis the two reflections that cancel angular misalignment between the measurement and reference reflectors.

Abstract: A method establishing a reference point for a machine tool with X-, Y-, and Z-stages. A YZ-plane is established by reflecting a light beam off an X-stage reflector such that the light beam is sensed at an interference detector. The X-stage is moved while repositioning the head and reflector and maintaining sensing. An optical alignment module (OAM) is mounted optically perpendicular to the beam, with a bending mirror centered at the Z-axis. The Z-axis is established by setting the bending mirror to deflect the beam to the Z-axis, reflecting it off of a Z-stage reflector so it is sensed, and moving the Z-stage while repositioning the OAM relative to the X- and Y-axes to maintain sensing. An XY-plane is established by bending mirror deflecting the beam to the Y-axis, reflecting it off of a Y-stage reflector so it is sensed, and moving the Y-stage while repositioning the Y-reflector relative to the X- and Z-axes to maintain sensing.

Abstract: An interferometry system includes an interferometer to split an input beam into a measurement beam and at least one other beam. The interferometer directs the measurement beam along a measurement path that includes at least two passes to a measurement object, and overlaps the measurement beam with the other beam after the measurement beam completes the at least two passes. The path of the measurement beam is sheared during the first and second passes when the measurement object moves along a direction orthogonal to a portion of the measurement path that contacts the measurement object. The interferometry system includes optics to redirect the measurement beam after the first pass and before the second pass so that shear imparted during the second pass cancels shear imparted during the first pass.

Abstract: A positioning apparatus includes first and second stages, a pair of first mirrors having reflective surfaces disposed on the first stage at an acute angle with a vertical axis, a pair of second mirrors disposed on the second stage and having reflective surfaces orthogonal to the vertical axis, a reference structure functioning as a reference for measuring the vertical position of the second stage, a pair of third mirrors disposed on the reference structure and having reflective surfaces orthogonal to the vertical axis, and a pair of interferometers for measuring the vertical position of the second stage using the first, second, and third mirrors.

Abstract: A lithographic projection apparatus is presented, which includes a housing, which comprises therein a first exposure system that has at least one movable part (e.g. a movable first substrate holder or a movable second substrate holder in a twin stage apparatus). The apparatus also includes a position disturbance correction system for correcting a position of the first substrate holder with respect to the patterned projection beam due to the influence of gas pressure differences or gas movements, caused by movements of the movable part. A related device manufacturing method is also presented in which, during the exposure of a first substrate, the position thereof is corrected by means of position disturbance correction system.

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: A system for measuring a displacement along a first axis includes an apparatus movable at least along a second axis perpendicular to the first axis, a measurement mirror mounted to the apparatus at an angle greater than 0° relative to the first axis, and an interferometer with a beam-splitter. The beam-splitter splits an input beam into a measurement beam and a reference beam, directs the measurement beam in at least two passes to the measurement mirror, and combining the measurement beam after said at least two passes and the reference beam into an output beam. At least exterior to the interferometer, the measurement beam travels in paths that are not parallel to the first axis.

Abstract: An apparatus including: a multi-axis interferometry system configured to receive an input beam and direct at least two beams derived from the input beam to contact different locations on a measurement object to monitor changes in an angular orientation of the measurement object; a beam steering assembly having a beam steering element positioned to direct the input beam into the interferometry system and a positioning system to selectively orient the beam steering element relative to the interferometry system; and a control circuit which during operation orients the beam steering element based on the changes in the angular orientation of the measurement object monitored by the interferometry system.

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.

Abstract: Multidirectional retroreflectors and methods of reflecting light beams from multiple directions are provided. The multidirectional retroreflectors utilize a four-mirror retroreflector with a common virtual reflection point.

Abstract: A microlithography method includes: interferometrically measuring information about a position of a microlithography stage with respect to each of multiple metrology axes during a photolithographic exposure cycle; analyzing the position information to determine correction factors indicative of a local slope on a side of the stage used to reflect an interferometric measurement beam and optical gradients caused by environmental effects produced by the photolithographic exposure cycle; and applying the correction factors to subsequent interferometric measurements of the stage.

Abstract: A measuring system for an apparatus having a bed, a first stage movable in a first direction relative to the bed, and a second stage movable in a second direction at right angles to the first direction, relative to the first stage. A first interferometer measures displacement in the first direction between a first reference reflector and a first target reflector fixed to the first stage. A second interferometer (a) measures displacement in the second direction between a second reference reflector and a second target reflector fixed to the second stage, (b) includes an intermediate reflector fixed to the first stage, (c) produces a beam that extends in the first direction from a position fixed to the bed to the intermediate reflector and in the second direction from the intermediate reflector to the second target reflector and (d) can therefore have a head that is mounted on the bed.

Abstract: The invention features a system and method for reducing the contribution of cyclic errors to an interferometric position measurement of a movable stage. An initial interferometric position measurement of the stage is averaged with at least one additional measurement corresponding to a displacement(s) of the stage from its initial position. The displacements are selected to reduce the overall cyclic error contribution to an average position measurement. As a result, the average position of the stage can be measured more accurately than any of its individual positions. The average position can be used to more accurately determine the average position of an alignment mark on a wafer carried by the stage. Furthermore, the averaging described above can be applied to additional interferometric measurement axes.

Abstract: An interferometric apparatus includes: a polarizing beam-splitting interface positioned to separate an input beam into two orthogonally polarized beams; and interferometer optics positioned to receive a first set of beams derived from one of the orthogonally polarized beams and a second set of beams derived from the other of the orthogonally polarized beams. The interferometer optics are configured to direct each beam in the first set to contact different locations of a measurement object at least once, and subsequently combine each beam in the first set with a corresponding beam from the second set of beams to produce a corresponding output beam comprising information about changes in the position of the measurement object with respect to a different degree of freedom.

Abstract: The invention features a method for determining non-linear cyclic errors in a metrology system that positions a measurement object under servo-control based on an interferometrically derived position signal. The method includes: translating the measurement object under servo-control at a fixed speed; identifying frequencies of any oscillations in the position of measurement object as it is translated at the fixed speed; and determining amplitude and phase coefficients for sinusoidal components at the identified frequencies which when combined with the position signal suppress the oscillations. The invention also features a method for positioning a measurement object under servo-control based on an interferometrically derived position signal indicative of a position for the measurement object.

Abstract: A high speed, wide-range, high-accuracy positioning stage device for eliminating errors due to deterministic elements such as variation of surface machining accuracy of a bar mirror and for preventing error accumulation due to indeterminate elements such as air fluctuations when switching between laser interferometers. When an X1 laser interferometer as a first position measurement device and an X2 laser interferometer as a second position measurement device are switched, at a place at which at least two position measurement devices are activated, when a value is handed over from the X1 laser interferometer, which was activated, to the X2 laser interferometer, which is to be activated, errors due to the effect of X-bar mirror flatness and air fluctuations are found by a correction function for the X-bar mirror and/or a table and the average of the measured values, and the value of the X2 laser interferometer is corrected by a correction device such as a calculation device.

Abstract: A chuck having a high specific stiffness and high thermal conductivity compared to conventional chucks, with an apparatus for measuring thermal expansion in the chuck. High specific stiffness allows for a higher control bandwidth and improved scanning performance. High thermal conductivity enables excellent positioning accuracy because thermal expansion and strain may be accurately measured or predicted, and thus compensated.

Abstract: Interferometer systems are described that comprise optical assemblies to measure multiple degrees of freedom of a stage mirror and the topography of a reflecting surface of a stage mirror represented by datum lines and datum lines with corresponding local rotations of the reflecting surface of the stage mirror about the datum lines with high lateral spatial resolution. The interferometer systems measure slopes of datum lines and local rotations of the reflecting surface about the datum lines using single pass interferometric measurements of angular changes of directions of beams reflected by the reflecting surfaces. Two or more datum lines on a reflecting surface with concomitant measures of local rotations may be used to characterize topographic features, and these may be measured in situ in a lithography tool or measured off line either before installation or after removal from a lithography tool.

Abstract: A first set of interferometric measuring beams is used to determine a location of a patterned surface of a reticle and a reticle focus plane for a reticle that is back clamped to a reticle stage. A second set of interferometric measuring beams is used to determine a map of locations of the reticle stage during scanning in a Y direction. The two sets of interferometric measuring beams are correlated to relate the reticle focal plane to the map of the reticle stage. The information is used to control the reticle stage during exposure of a pattern on the patterned surface of the reticle onto a wafer.

Abstract: Interferometers can be configured to maintain a measurement beam substantially orthogonal to a plane measurement mirror and to minimize the lateral beam shear between the measurement and reference beam components of the output beam to a detector. These interferometers use a dynamic beam steering element to redirect the input beam in response to changes in the orientation of the measurement plane mirror. These interferometers may be further configured to measure displacement and change in orientation of the plane mirror or configured in a combination of single pass interferometers fed by the input beam for measuring displacement and change in orientation of the plane mirror.

Abstract: A method and apparatus used to reduce glitches in measurements made by an accumulation of phase changes over a range greater than 2?. Embodiments of the invention feature the use of a glitch filter that receives a phase input indicating a position value. The glitch filter reduces glitches by using a feedback term that has been reduced modulo 2?. The method includes measuring a plurality of values of a measurement signal, determining a current position value based on each of the plurality of measured values, wherein determining includes determining a delta value based on a current measured value and a previous position value, wherein the delta value is reduced modulo 2? to a range of ±?, and determining the current position value based on the determined delta value, and outputting at least one of the determined current position value and a velocity value.

Abstract: The invention features a method for determining non-linear cyclic errors in a metrology system that positions a measurement object under servo-control based on an interferometrically derived position signal. The method includes: translating the measurement object under servo-control at a fixed speed; identifying frequencies of any oscillations in the position of measurement object as it is translated at the fixed speed; and determining amplitude and phase coefficients for sinusoidal components at the identified frequencies which when combined with the position signal suppress the oscillations. The invention also features a method for positioning a measurement object under servo-control based on an interferometrically derived position signal indicative of a position for the measurement object.

Abstract: Techniques for monitoring the position of a stage in a twin stage lithography tool are disclosed.

Abstract: This optical arm applicable to robots is comprised of: some bent tubular parts (1, 6, 16, 18), a straight tubular length (21) and an outlet part (23), coupled together with the possibility of rotation; also including some means (7) to secure the optical arm to a robot (5). The bent tubular part (1) is mounted with the possibility of rotation on the outlet (2) of a laser generator (3) secured to the third arm (4) of the robot (5). Parts (1, 6, 16 and 18) have a window (28), in each bend, equipped with some inner lugs (28) which a mirror (29) is pressed on to by means of some springs (33) foreseen with a removable part (32).

Abstract: an apparatus including: a support structure; a stage configured to move relative to the support structure; a first reflection surface carried by one of the support structure and the stage; and a first interferometry system. The first interferometry system is configured to direct a first measurement beam to contact the first reflection surface and monitor changes in the position and orientation of the stage relative to the support structure along multiple degrees of freedom using the first measurement beam and no other measurement beam that contacts the first reflection surface. For example, the first measurement beam can define a first measurement axis and the multiple degrees of freedom can include at least two of: distance along the first measurement axis, pitch about the first measurement axis, and yaw about the first measurement axis.

Abstract: A stage device suitable for exposure apparatus used to produce semiconductor devices, is movable in an area wider than the measurement area of interferometer for position measurement, and is capable of measuring the position with high precision. When a movable stage moves from the position where the laser beams from laser interferometers are not applied into the measurement area of the laser interferometer, the position of reference mark is measured by a wafer alignment sensor, and the measurement value measured by the laser interferometer is corrected based on the results of the measurement by the wafer alignment sensor. When another movable stage enters the measurement area of the laser interferometer, the position of the reference mark is similarly measured by a wafer alignment sensor, and the measurement value measured by the laser interferometer is corrected based on the results of the measurement by the wafer alignment sensor.

Abstract: The invention features a system and method for reducing the contribution of cyclic errors to an interferometric position measurement of a movable stage. An initial interferometric position measurement of the stage is averaged with at least one additional measurement corresponding to a displacement(s) of the stage from its initial position. The displacements are selected to reduce the overall cyclic error contribution to an average position measurement. As a result, the average position of the stage can be measured more accurately than any of its individual positions. The average position can be used to more accurately determine the average position of an alignment mark on a wafer carried by the stage. Furthermore, the averaging described above can be applied to additional interferometric measurement axes.

Abstract: Interferometric apparatus and methods by which the local surface characteristics of photolithographic mirrors or the like may be interferometrically measured in-situ to provide correction signals for enhanced distance and angular measurement accuracy. Surface characterizations along one or multiple datum lines in one or more directions may be made by measuring the angular changes in beams reflected off the surfaces during scanning operations to determine local slope and then integrating the slope to arrive at surface topology. The mirrors may be mounted either on the photolithographic stages or off the photolithographic stages on a reference frame.

Abstract: A position measuring system is utilized for measuring a position of a stage positioned on a pedestal. As the stage is moved along a predetermined direction, a controlling device utilizes a first measuring apparatus to measure a moving direction of the stage during the first period. Then, the controlling device uses the moving direction measured by the first measuring apparatus to adjust the moving direction measured by the second measuring apparatus during the overlapping period of the first period and a second period. After adjusting the second measuring apparatus, the controlling device utilizes the second measuring apparatus to measure the moving direction of the stage during the second period.

Abstract: A scan type exposure for transferring a pattern onto a substrate by scan exposure. The apparatus includes a stage for moving the substrate in a Y direction corresponding to a scan direction, and in an X direction intersecting the scan direction, an alignment scope for performing measurement for alignment of the substrate, at a position spaced, in the Y direction, from a position where the exposure of the substrate is to be carried out, an X measuring device for performing yaw measurement of said stage by use of an X reflection surface provided on said stage along the Y direction, a Y measuring device for performing yaw measurement of the stage by use of a Y reflection surface provided on the stage along the X direction, and a controller being operable to select yaw measurement information of the X measuring device for an alignment operation including the alignment measurement using the alignment scope, and being operable to select yaw measurement information of the Y measuring device for the scan exposure.

Abstract: A first set of interferometric measuring beams is used to determine a location of a patterned surface of a reticle and a reticle focus plane for a reticle that is back clamped to a reticle stage. A second set of interferometric measuring beams is used to determine a map of locations of the reticle stage during scanning in a Y direction. The two sets of interferometric measuring beams are correlated to relate the reticle focal plane to the map of the reticle stage. The information is used to control the reticle stage during exposure of a pattern on the patterned surface of the reticle onto a wafer.

Abstract: Methods and system for compensating for effects of changes and variations of gas refractivity in a measurement and/or reference beam path of an interferometer are disclosed.

Abstract: A stage assembly (10) for moving and positioning a device (26) is provided herein. The stage assembly (10) includes a stage base (12), a device table (48), a stage mover assembly (16), a measurement system (20), and a control system (22). The stage mover assembly (16) moves the device table (48) along an X axis, along a Y axis, and about a Z axis relative to the stage base (12). The measurement system (20) includes a first X mirror (25A) for monitoring the position of the device table (48) in an alignment position (31A) and a second X mirror (25B) for monitoring the position of the device table (48) in an operational position (31B). In one embodiment, the stage assembly (10) includes a first fiducial mark (76) and a second fiducial mark (78) that are secured to the device table (48) to determine the relative position of the X mirrors (25A)(25B).

Abstract: In general, in one aspect, the invention features a method, including interferometrically measuring first and second optical path lengths to a measurement object along respective first and second paths, wherein the measurement of the optical path lengths includes directing first and second measurement beams to reflect from the measurement object, measuring propagation directions of the first and second measurement beams, compensating the first measured optical path length for time-varying optical properties of gas in the first path based on the first and second measured optical path lengths and the first and second measured propagation directions.

Abstract: An apparatus includes a multi-axis interferometer for measuring changes in a position of a measurement object. The interferometer is configured to receive a progenitor input beam, direct a first angle-measuring beam derived from the progenitor input beam to make a pass to a first point on the measurement object, direct a second angle-measuring beam derived from the progenitor input beam to make a pass to a second point on the measurement object, and then combine the angle-measuring beams to produce an angle-measuring output beam, wherein each angle-measuring beam makes only a single pass to the measurement object before being combined to form the angle-measuring output beam. The interferometer is further configured to direct another set of beams derived from the progenitor input beam along different paths and then combine them to produce another output beam comprising information about changes in the position of the measurement object.

Abstract: To accurately measure the position and posture of a stage apparatus used in an exposure apparatus or the like, the Z position and displacement of a top stage (27) are measured using as a reference a lens barrel support (35) independently of the top stage in terms of vibrations by an interferometer system which includes a projection optical system (34) for projecting a pattern formed on a master onto a substrate, stages (27, 31, 40) capable of moving with respect to the projection optical system (34) while holding the substrate or master, and a lens barrel support (35) that supports the projection optical system (34), and which uses a Z measuring mirror (30) that is arranged on a Y stage (31) and has a reflecting surface almost parallel to the XY plane, and a Z interferometer (25) arranged on the Y stage (31).

Abstract: A multi-axis interferometer uses a combined beam for a first pass through the interferometer optics. Measurement and reference components of the combined beam that exit the interferometer optics are subject to walk-off that measurement or reference reflector misalignment can cause. A return reflector and non-polarizing beam splitter system split the combined beam into separated input beams for the various axes of the interferometer and return the separated beams for respective second passes through the interferometer optics. Walk-off for the separated beams in the interferometer optics cancels the walk-off for the combined beam to eliminate beam walk-off in separated output beams. Sharing a combined beam for a first pass through the interferometer optics reduces the sizes required for the interferometer optics and reference and measurement mirrors. The multi-axis interferometer may have a single return reflector.

Abstract: Interferometry system including a multiple-pass interferometer having reflectors to reflect at least two beams along multiple passes through the interferometer. The multiple passes include a first set of passes and a second set of passes. The reflectors have first alignments that are normal to the directions of the paths of the beams that are reflected by the reflectors. The two beams provide information about changes in a first location on one of the reflectors after the first set of passes, and provide information about changes in the first location and changes in a second location on the reflector after the second set of passes. The paths of the beams are sheared during the first set of passes and during the second set of passes if at least one of the reflectors has an alignment other than the first alignment.

Abstract: This invention relates to an apparatus and method for positioning dual stages during semiconductor wafer processing. More particularly, the invention facilitates the use of interferometers to determine the positions of both wafer stages at all times during processing. While the movement of a typical twin stage apparatus causes one of the stages to eclipse the other and requires the addition of a significant number of additional interferometers, this invention minimizes the number of interferometers necessary through dimensioning the stages so that one stage never totally eclipses the other.

Abstract: A scale (10) is illuminated by a light source (18). After interaction with an index grating (12), fringes (F) are formed in a Talbot plane and analyzed by an analyzer grating (14). To decrease the sensitivity to changes in the ride height of the index grating above the scale, the light source is restricted to a small but finite size, and positioned so that it subtends a small angle Ø at the analyzer grating (or, if a collimating lens (24) is used, positioned so that it subtends a small angle Ø at the lens). The size of the light source should preferably be smaller than a predetermined value such that the extent of a geometric fringe visibility envelope exceeds the extent of a Talbot fringe visibility envelope.

Abstract: Methods and apparatus for accurately measuring a position of a wafer chuck which holds a wafer are disclosed. According to one aspect of the present invention, a stage apparatus includes a sensing device and a chuck. The chuck substantially directly supports an object, and includes at least one side surface that is a mirrored surface. The sensing device is arranged to cooperate with the mirrored surface to measure a position of the chuck. In one embodiment, the mirrored surface is polished onto the chuck.

Abstract: In general, in one aspect, the invention features a method that includes directing a first measurement beam along a first path contacting a measurement object to determine a first interferometric phase related to a position of a measurement object, directing a second measurement beam along a second path contacting the measurement object to determine an second interferometric phase related to the position of the measurement object, and determining a contribution to the first interferometric phase due to perturbations in refractivity along the first path based on the second interferometric phase.

Abstract: Interferometry system including a multiple-pass interferometer having reflectors to reflect at least two beams along multiple passes through the interferometer, the multiple passes including a first set of passes and a second set of passes, the reflectors having first alignments that are normal to the directions of the paths of the beams that are reflected by the reflectors, the paths of the beams being sheared during the first set of passes and during the second set of passes if at least one of the reflectors has an alignment other than the first alignment, and optics to redirect the beams after the first set of passes and before the second set of passes so that shear imparted during the second set of passes cancels shear imparted during the first set of passes.

Abstract: The invention features a method for determining non-linear cyclic errors in a metrology system that positions a measurement object under servo-control based on an interferometrically derived position signal. The method includes: translating the measurement object under servo-control at a fixed speed; identifying frequencies of any oscillations in the position of measurement object as it is translated at the fixed speed; and determining amplitude and phase coefficients for sinusoidal components at the identified frequencies which when combined with the position signal suppress the oscillations. The invention also features a method for positioning a measurement object under servo-control based on an interferometrically derived position signal indicative of a position for the measurement object.

Abstract: A method for measuring two-dimensional displacement using conjugate optics comprises the steps of emitting an incident beam onto a diffraction element to generate many firstly diffracted beams, selecting two axially symmetric beams of the same order of diffraction from the firstly diffracted beams, introducing corresponding sets of wavefront reconstruction optics to reflect the two selected beams back onto the same incident spot along the same optical paths and to generate many secondly diffracted beams, selecting two axially symmetric pairs of beams of the same order of diffraction from the secondly diffracted beams, forming two interference fringes by superposing the two selected pairs of beams via corresponding mirrors and interferometric optics, and obtaining two linearly independent displacements of the diffraction element relative to the rest of the optics by decoding the two interference fringes.

Abstract: An exposure apparatus which transfers a pattern image formed on a mask on to a substrate through a projection optical system, comprising a substrate table which holds said substrate, a first sensor which measures a gap between said substrate surface held by said substrate table and a control target position, and a second sensor which measures a distance between said projection optical system and said substrate table in an optical axis direction and corrects the control target position of said first sensor.

Abstract: In general, in one aspect, the invention features a method for determining the location of an alignment mark on a stage including measuring a location, x1, of a stage along a first measurement axis using an interferometer, measuring a location, x2, of the stage along a second measurement axis substantially parallel to the first measurement axis, and determining a location of the alignment mark along a third axis substantially parallel to the first measurement axis based on x1, x2, and a correction term, &psgr;3, calculated from predetermined information including information characterizing imperfections in the interferometer.

Abstract: Interferometric apparatus and methods by which the local surface characteristics of photolithographic mirrors or the like may be interferometrically measured in-situ to provide correction signals for enhanced distance and angular measurement accuracy. Surface characterizations along one or multiple datum lines in one or more directions may be made by measuring the angular changes in beams reflected off the surfaces during scanning operations to determine local slope and then integrating the slope to arrive at surface topology. The mirrors may be mounted either on the photolithographic stages or off the photolithographic stages on a reference frame.

Abstract: The invention measures a thickness variation at a high accuracy around a wide range of a thin plate by a comparatively large spot diameter between 0.5 mm and 2 mm. A polarization beam splitter separating a laser beam emitted from a laser generator and transmitting through an isolator into a measurement light and a reference light is provided. A quarter wavelength plate is provided between the polarization beam splitter and a measurement surface, and between the polarization beam splitter and a reference surface. A focusing and reflecting means for focusing and reflecting the measurement light reflected by the measurement surface and reflected by the polarization beam splitter, and the reference light reflecting by the reference surface and transmitting through the polarization beam splitter is provided. A half mirror reflecting the measurement light and the reference light which return from the polarization beam splitter is provided.

Abstract: Interferometric apparatus and methods by which the local surface characteristics of photolithographic mirrors or the like may be interferometrically measured in-situ to provide correction signals for enhanced distance and angular measurement accuracy. Surface characterizations along one or multiple datum lines in one or more directions may be made by measuring the angular changes in beams reflected off the surfaces during scanning operations to determine local slope and then integrating the slope to arrive at surface topology. The mirrors may be mounted either on the photolithographic stages or off the photolithographic stages on a reference frame.