Measurement systems, lithography systems, and methods of measuring displacement of a target structure of the lithography systems

Abstract

A measurement system is provided. The measurement system includes an optical energy source configured to provide optical energy. The measurement system also includes a measurement head configured for interferometry using the optical energy provided by the optical energy source. The measurement head includes a polarizing beam splitter, and a rooftop prism in optical communication with the polarizing beam splitter, the rooftop prism being attached to the polarizing beam splitter. The measurement system also includes a retroreflector configured to be attached to a structure to be measured. The measurement system also includes a detector configured to determine displacement using optical energy provided by the measurement head.

Description

MEASUREMENT SYSTEMS, LITHOGRAPHY SYSTEMS, AND METHODS OF MEASURING DISPLACEMENT OF A TARGET STRUCTURE OF THE LITHOGRAPHY SYSTEMSCLAIM OF PRIORITY

[0001] This application claims the benefit of the filing date of U.S. Provisional Patent Application Serial No. 63 / 664,393, filed June 26, 2024 entitled, "MEASUREMENT SYSTEMS, LITHOGRAPHY SYSTEMS, AND METHODS OF MEASURING DISPLACEMENT OF A TARGET STRUCTURE OF THE LITHOGRAPHY SYSTEMS," which application is incorporated by reference herein in its entirety.FIELD

[0002] The invention relates to lithography, and in particular, measurement systems used to measure displacement of target structures of lithography systems.BACKGROUND

[0003] In lithography applications, radiation provided by a radiation source is used to expose fields on a workpiece (e.g., a wafer). Certain conventional lithographic systems include wafer chucks which may rotate. Certain conventional lithography systems are deficient in measuring the displacement of certain lithographic components. For example, the rotations of the wafer chucks (e.g., Rx, Ry and Rz) can be too large for a conventional laser interferometer implementation to measure.

[0004] Thus, it would be desirable to provide improved lithography systems (and related measurement systems and methods) to overcome one or more of the deficiencies of conventional systems.SUMMARY

[0005] According to an exemplary embodiment of the invention, a measurement system is provided. The measurement system includes an optical energy source configured to provide optical energy. The measurement system also includes a measurement head configured for interferometry using the optical energy provided by the optical energy source. The measurement head includes a polarizing beam splitter, and a rooftop prismin optical communication with the polarizing beam splitter, the rooftop prism being attached to the polarizing beam splitter. The measurement system also includes a retroreflector configured to be attached to a structure to be measured. The measurement system also includes a detector configured to determine displacement using optical energy provided from the measurement head.

[0006] According to other embodiments of the invention, the measurement system recited in the immediately preceding paragraph may have any one or more of the following features: the structure to be measured is a support structure; the support structure is a wafer chuck; further including a mirror attached to a reference structure; the reference structure is a granite base; the measurement head further includes a first quarter wave plate and a second quarter wave plate, the first quarter wave plate and the second quarter wave plate being configured to modify a polarity of a beam of light; the first quarter wave plate is configured to modify a polarity of a beam of light of the provided optical energy and the second quarter wave plate is configured to further modify the polarity of the beam of light; further including a second retroreflector configured to be attached to the support structure at a second position, and a third retroreflector configured to be attached to the support structure at a third position, the first retroreflector being configured to be attached to a support structure at a first position, the first retroreflector and the second retroreflector are configured to measure a first horizontal position of the support structure and a rotational position of the support structure, and the third retroreflector is configured to measure a second horizontal position of the support structure; further including a second measurement head; further including a third measurement head; and / or further including a plurality of additional measurement heads, wherein the measurement head and the plurality of additional measurement heads are configured to measure the structure to be measured in six up to degrees of freedom.

[0007] According to another exemplary embodiment of the invention, a lithography system is provided. The lithography system includes a support structure configured to support a substrate. The lithography system also includes a motion system configured to move the support structure. The lithography system also includes an optical energy source configured to provide optical energy. The lithography system also includes a measurement head configured for interferometry using the optical energy provided bythe optical energy source. The measurement head also includes a polarizing beam splitter, and a rooftop prism in optical communication with the polarizing beam splitter, the rooftop prism being attached to the polarizing beam splitter. The lithography system also includes a first retroreflector configured to be attached to a structure to be measured. The lithography system also includes a detector configured to determine displacement using optical energy provided from the measurement head.

[0008] According to other embodiments of the invention, the lithography system recited in the immediately preceding paragraph may have any one or more of the following features: the structure to be measured is the support structure; the support structure is a wafer chuck; further including a mirror attached to a reference structure; the reference structure is a granite base; the measurement head further includes a first quarter wave plate and a second quarter wave plate, the first quarter wave plate and the second quarter wave plate being configured to modify a polarity of a beam of light; the first quarter wave plate is configured to modify a polarity of a beam of light of the provided optical energy, and the second quarter wave plate is configured to further modify the polarity of the beam of light; further including a second retroreflector configured to be attached to the support structure at a second position, and a third retroreflector configured to be attached to the support structure at a third position, the first retroreflector being configured to be attached to the support structure at a first position, the first retroreflector and the second retroreflector being configured to measure a first horizontal position of the support structure and a rotational position of the support structure, the third retroreflector being configured to measure a second horizontal position of the support structure; further including a second measurement head; further including a third measurement head; and / or further including a plurality of additional measurement heads, wherein the measurement head and the plurality of additional measurement heads are configured to measure the structure to be measured in up to six degrees of freedom.

[0009] According to yet another exemplary embodiment of the invention, a method of measuring displacement of a target structure of a lithography system is provided. The method includes: (a) providing a measurement system, the measurement system including an optical energy source configured to provide optical energy, a retroreflector attached to the target structure, a measurement head configured for interferometryusing optical energy, the measurement head including (i) a polarizing beam splitter, and (ii) a rooftop prism in optical communication with the polarizing beam splitter, the rooftop prism being attached to the polarizing beam splitter, the measurement head being optically between a retroreflector and a mirror attached to a reference structure, and a detector configured to determine displacement using optical energy provided by the measurement head; (b) transmitting optical energy to the measurement head from the optical energy source, the optical energy including (i) a first laser beamlet provided to the rooftop prism in connection with a reference path and (ii) a second laser beamlet provided to the polarizing beam splitter in connection with a measurement path; (c) transmitting the second laser beamlet through the polarizing beam splitter, one or more quarter wave plates, the retroreflector, the mirror, and the rooftop prism; (d) combining the first laser beamlet and the second laser beamlet to form interfered optical energy; (e) receiving the interfered optical energy with the detector; and (f) determining displacement of the target structure using the detector.

[0010] The methods of the present invention may also be embodied as an apparatus (e.g., as part of the intelligence of a lithography machine), or as computer program instructions on a computer readable carrier (e.g., a computer readable carrier including a lithography program used in connection with a lithography machine).BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures:

[0012] FIG. 1A is a block diagram top view of a lithography system, including a measurement system, in accordance with an exemplary embodiment of the invention;

[0013] FIG. IB is a block diagram side view of a support structure and related motion systems of the lithography system of FIG. 1A, in accordance with an exemplary embodiment of the invention;

[0014] FIG. 2A is a block diagram top view of another measurement system in accordance with an exemplary embodiment of the invention;

[0015] FIG. 2B is a block diagram side view of certain elements of the measurement system of FIG. 2A in accordance with an exemplary embodiment of the invention; and

[0016] FIG. 3 is a flow diagram of a method of measuring displacement of a target structure of a lithography system in accordance with an exemplary embodiment of the invention.DETAILED DESCRIPTION

[0017] Interferometry measures distance with the help of interfering waves (e.g., laser light, etc.) by comparing a length of a reference path of a reference beam with a length of a measurement path of a measurement beam. In certain embodiments, a measurement system (e.g., an interferometry-based measurement system) may measure the distance between optical elements like retroreflectors, which may be attached to a support structure (e.g., a wafer chuck), and long flat mirrors, which may be attached to a reference structure (e.g., a granite base). A retroreflector returns a beam of light under the same angle at which the beam of light hits the retroreflector. Retroreflectors can be used in methods for measuring displacement using a laser interferometer with large angles. In such methods, the beam of light may be mirrored over a nodal point of the retroreflector. Accordingly, the beam of light from the measurement path shifts with respect to the beam from the reference path when the target retroreflector moves in a direction perpendicular to the direction of measurement. Such a phenomenon is understood in the art to be "beam walk-off." If the measurement beam and the reference beam do not overlap, the light can no longer interfere, impeding continued measurement. Certain measurement systems and lithography systems provided herein comprise a configuration / layout which minimizes beam walk-off, allowing continued measurement in X, Y and Rz directions, while a support structure (e.g., a wafer chuck) is actuated in multiple degrees of freedom (e.g., six degrees of freedom). As used herein, the terms "beam," "beam of light," and "laser beamlet" of optical energy may be used interchangeably, as would be understood by those skilled in the art. It should be understood that various frequencies of optical energy may be used in connection with the various embodiments of the invention.

[0018] In certain embodiments, an interferometry-based measurement system (e.g., a metrology system for a back-end wafer stepper) is provided herein, where thedisplacement of a structure (e.g., a support structure, a wafer chuck, "target structure," etc.) is configured to be measured with respect to a reference structure (e.g., a granite base, a mirror, etc.). As used herein, the term "displacement" of a structure to be measured may be understood as a positional, translational, rotational, and / or angled movement or difference of the structure with respect to a reference (e.g., a reference structure, an expected position, etc.). In certain embodiments, a lithography system includes a support structure (e.g., wafer chuck) which is actuated in six degrees of freedom with respect to the reference structure (e.g., a granite base). The configuration (e.g., layout) of certain embodiments of a measurement system and / or a lithography system enable measuring the support structure (e.g., wafer chuck) in three degrees of freedom (e.g., X, Y, and Rz), while the support structure is actuated over six degrees of freedom (DOF). In certain embodiments, a measurement system and / or a lithography system enable X, Y, Rz metrology of a support structure (e.g., within a measurement range of + / - 15 mrad Rz and + / - 2 mrad Rx, Ry).

[0019] In certain embodiments, a measurement system includes a polarizing beam splitter, two quarter wave (A / 4) plates, a retroreflector, a flat mirror, a rooftop prism, an optical energy source, and a detector. In such embodiments, a reference path of a laser beamlet travels from the optical energy source through the beam splitter, is reflected by the rooftop prism, and passes the beam splitter again towards a detector. The measurement path is reflected by the polarizing beam splitter (e.g., passing through a first quarter wave plate), passes through the retroreflector, travels towards the flat mirror (e.g., passing through a first quarter wave plate and a second quarter wave plate), passes through the rooftop prism, travels toward the flat mirror, passes again through the retroreflector, and is reflected by the beam splitter towards the detector. Such a configuration of a measurement system may be used (e.g., duplicated) multiple times (e.g., twice, thrice, etc.) around a structure to be measured (e.g., a support structure, a wafer chuck, a "target structure", etc.) in a larger measurement system, thereby enabling measurement in X, Y and Rz directions.

[0020] The polarizing beam splitters (which split the optical energy for the reference path and the measurement path) may be placed on bodies actuated in linear directions (e.g., X and Y directions) such that laser beamlets of the split optical energy remain in linewith the retroreflectors over full stroke along the linear direction (e.g., in X and Y directions).

[0021] The paths of measurement may be tangential to a circumference of the structure to be measured (e.g., support structure, a wafer chuck, "target structure," etc.), and the retroreflectors may be placed perpendicular to the radius of the structure to be measured (e.g., support structure, a wafer chuck, "target structure," etc.). Accordingly, small rotations of the structure to be measured (e.g., support structure, a wafer chuck, "target structure," etc.) around a vertical axis (e.g., the Z-axis) may result in a movement of the retroreflectors, which is in line with the measurement path, and may not result in beam walk-off.

[0022] In certain embodiments, as the structure to be measured (e.g., support structure, a wafer chuck, etc.) is actuated along the vertical axis, a measurement laser beamlet translates along the vertical axis. However, since the path of the measurement laser beamlet passes the retroreflector twice, the translation is removed by the second pass through the retroreflector. This is enabled using the rooftop prism for the reference path of a reference laser beamlet, since the rooftop prism does not translate the beam along the vertical axis as the beam passes the rooftop prism. Small rotations of the wafer chuck around Rx and Ry direction will result in either a rotation of the retroreflectors around their nodal point, to which they are insensitive, or a translation in the vertical direction (e.g., Z direction), to which the configuration (e.g., layout) of certain embodiments is made insensitive as described herein.

[0023] Certain embodiments of the invention may be used for a back-end wafer stepper application. Back-end production typically deals with a larger variety of wafers. Therefore, the back-end wafer stepper must be able to handle a large range of: wafer thicknesses, wafer flatnesses, and / or reconstructed wafers, where a rotational correction at every die may be necessary. Certain embodiments provided herein may be able to continue measuring X, Y and Rz, while all six degrees of freedom of a support structure (e.g., a wafer chuck) and / or associated motion systems are actuated. Certain embodiments provided herein may enable X, Y, Rz metrology of the structure to be measured (e.g., a support structure, a wafer chuck, etc.) within metrology ranges + / - 15 mrad Rz and + / - 2 mrad Rx, Ry. Such metrology ranges of certain embodimentsprovided herein are an improvement over typical interferometer implementations in the art (which only provide a metrology range of + / - 1 mrad in Rz, Rx, and / or Ry).

[0024] Certain embodiments provide advantages over related art. For example, in certain embodiments provided herein, the polarizing beam splitters are placed on a moving body (and / or multiple moving bodies). Such embodiments allow for the path of measurement to remain in-line with the retroreflectors, enabling continued measurement over a long X / Y stroke and over the length of associated mirrors. Such embodiments are distinguished from related art where beam splitters are used in a fixed place. Beam splitters used in related art are placed in a fixed location (in contrast to a moving body) because certain conventional interferometers measure the distance from the beam splitter to a target and placing the beam splitter on a moving body may introduce more vibrations into the measurement signal, which is unfavorable for closed loop control. Certain embodiments provided herein are insensitive to the position of the beam splitter because the distance between the target retroreflector and a flat mirror on a fixed location is measured. Certain embodiments provided herein are insensitive to small rotations around Rx and Ry (while continuing measurement over Rz without beam walk-off) because the retroreflectors may be place perpendicular to a radius of the structure to be measured (e.g., support structure 130 of FIG. 1A, a wafer chuck, etc.) with the line of measurement tangential to the structure to be measured (e.g., support structure 130 of FIG. 1A, a wafer chuck, etc.). In certain embodiments provided herein, the beam walk-off over the full stroke of a support structure (e.g., a wafer chuck) is zero. Therefore, the possible range of motion over which measurement can continue is not limited by the size of laser beam used.

[0025] The invention is best understood in connection with the various drawings herein. It should be understood that invention is not limited to merely the illustrated embodiments. Referring now to FIG. 1A, a lithography system 100 is illustrated. It should be understood that certain conventional elements of a lithography system are omitted for simplicity (e.g., a mask / reticle, a projection lens, etc.). Lithography system 100 includes a support structure 130 configured to support a substrate (e.g., a workpiece, a wafer, etc.). Lithography system 100 also includes a motion system 122a (e.g., a Y-stroke motion system) and a motion system 122b (e.g., an X-stroke motion system). It should be understood that one or more of motion systems 122a / 122b canbe configured to displace, rotate, and / or tilt support structure 130. As indicated by the double-solid arrows, motion system 122a is configured to move along the Y-axis and motion system 122b is configured to move along the X-axis. In certain embodiments, support structure 130 can be moved and measured in three degrees of freedom (e.g., X, Y, and Rz) using motion system 122a, motion system 122b, and / or another motion system (e.g., a rotational motional system rotating about the Z-axis). As illustrated in FIG. IB, motion system 122a and / or motion system 122b may be configured to move support structure 130 along the Z-axis (e.g., ± 2 mm) and / or to tilt support structure 130 along one or more tilt axes (e.g., ± 0.2 mrad). In certain embodiments, only one motion system (e.g., motion system 122a) is configured to move along the Z-axis and tilt-axes (e.g., an X-tilt-axis, a Y-tilt-axis, etc.).

[0026] Referring again to FIG. 1A, lithography system 100 also includes a measurement system 102 (e.g., an interferometry-based measurement and positioning system). Measurement system 102 is illustrated including an optical energy source 124 configured to provide optical energy 110. Measurement system 102 is illustrated also including a measurement head 104a (e.g., a beam-splitting assembly), a measurement head 104b, and / or a measurement head 104c. Each of measurement heads 104a I 104b I 104c are configured for interferometry using optical energy 110 provided by optical energy source 124. As illustrated, a beam splitter 126 (e.g., of measurement system 102, of lithography system 100, etc.) may be used to split optical energy from optical energy source 124 (e.g., a common optical energy source) and provide the split optical energy to one or more of measurement heads 104a I 104b I 104c. Beam splitter 126 is illustrated receiving, splitting, and distributing optical energy 110 to each of measurement heads 104a I 104b I 104c. Although beam splitter 126 is illustrated as a single beam splitter, it should be understood that beam splitter 126 may include a plurality of beam splitters (e.g., two beam splitters) sufficient to distribute optical energy 110 to measurement heads 104a I 104b I 104c (and / or any additional measurement heads). Each of measurement heads 104a I 104b / 104c include (i) a polarizing beam splitter (e.g., beam splitter 108 of FIG. 2A), and (ii) a rooftop prism (e.g., rooftop prism 106 of FIG. 2A) in optical communication with the polarizing beam splitter, the rooftop prism being attached (e.g., mechanically coupled) to the polarizing beam splitter. In certain embodiments, the polarizing beam splitters may be placed on one or moremoving bodies (e.g., an actuator, a motion system, etc.), thereby allowing for paths of measurement (e.g., of laser beamlets) to remain in-line with associated retroreflectors (e.g., retroreflector 116a I 116b 1 116c) and enable continued measurement over a long X / Y stroke (e.g., of a motion system) and over the length of associated mirrors (e.g., flat mirror 118a I 118b). Each of measurement heads 104a I 104b I 104c may include a first quarter wave plate (e.g., quarter wave plate 112 of FIG. 2A) and a second quarter wave plate (e.g., quarter wave plate 114 of FIG. 2A).

[0027] Measurement system 102 is illustrated also including a retroreflector 116a (e.g., a corner cube, a corner reflector, etc.), a retroreflector 116b, and a retroreflector 116c. Each of retroreflectors 116a I 116b I 116c are configured to be attached to a structure (e.g., a support structure 130, a wafer chuck, etc.) to be measured. As used herein, a "structure to be measured" may be used interchangeably with a "target structure" and can be understood as an element of lithography system 100 which is targeted by measurement system 102 to be measured (e.g., in position, rotation, and / or angle). In many examples, the "structure to be measured" I "target structure" is a support structure and / or a wafer chuck.

[0028] Each of retroreflectors 116a I 116b I 116c are illustrated attached to support structure 130. In the illustrated embodiment, retroreflectors 116a I 116b I 116c are configured such that three degrees of freedom (e.g., X, Y, and Rz) of support structure 130 can be measured.

[0029] Measurement system 102 is illustrated also including a detector 120a, a detector 120b, and a detector 120c. Each of detectors 120a / 120b / 120c are configured to determine displacement using optical energy 110 provided from one or more of measurement heads 104a / 104b / 104c.

[0030] Measurement system 102 is illustrated also including a flat mirror 118a and a flat mirror 118b. Flat mirrors 118a I 118b are attached to a reference structure 128 (e.g., a granite base) of lithography system 100.

[0031] Although measurement system 102 is illustrated including measurement head 104a, measurement head 104b, measurement head 104c, retroreflector 116a, retroreflector 116b, retroreflector 116c, detector 120a, detector 120b, detector 120c, flat mirror 118a, and flat mirror 118b, the invention is not so limited to such anembodiment. Various embodiments are described in connection with a measurement system 102' of FIG. 2A.

[0032] Referring now to FIG. 2A, measurement system 102' is illustrated. Measurement system 102' is similar to measurement system 102 of FIG. 1A, where like elements have like reference numerals or the same reference numerals. Accordingly, it is understood that the description of such elements in connection with FIG. 1A is applicable here and may be omitted here for conciseness.

[0033] Measurement system 102' is illustrated including measurement head 104a configured for interferometry using optical energy 110 provided by optical energy source 124. Measurement head 104a includes a beam splitter 108 (e.g., a polarizing beam splitter), and a rooftop prism 106 in optical communication with beam splitter 108. Rooftop prism 106 is illustrated attached (e.g., mechanically coupled) to beam splitter 108.

[0034] Optical energy source 124 is illustrated providing optical energy 110 (e.g., a laser, light, etc.) to beam splitter 108 of measurement head 104a. A portion of optical energy 110 may be provided in a plurality of directions. As illustrated, a laser beamlet 110a of optical energy 110 is provided in a first direction (e.g., along the X-axis) along a first path (e.g., a measurement path) and a laser beamlet 110b of optical energy 110 is provided in a second direction (e.g., along the Y-axis) along a second path (e.g., a reference path). As illustrated, laser beamlet 110a is reflected and may be polarized (e.g., vertically polarized) prior to reaching quarter wave plate 112. As laser beamlet 110a passes through quarter wave plate 112 towards retroreflector 116a (e.g., a corner cube), the polarization state of laser beamlet 110a is changed (e.g., to a circularly polarized state). As laser beamlet 110a passes through (or within) retroreflector 116a, laser beamlet 110a is shifted (e.g., along the Y-axis), as illustrated. Laser beamlet 110a is illustrated next passing through quarter wave plate 112, thereby changing the polarization state of laser beamlet 110a (e.g., horizontally polarized). Laser beamlet 110a is illustrated next being transmitted by beam splitter 108 and passing through quarter wave plate 114, thereby changing the polarization state (e.g., to a circularly polarized state). Laser beamlet 110a is illustrated then travelling toward flat mirror 118a and then reflected toward quarter wave plate 114. As laser beamlet 110a passes through quarter wave plate 114, the polarization state of laser beamlet 110a is changed (e.g., toa vertically polarized state). Laser beamlet 110a is illustrated then being reflected by beam splitter 108 towards rooftop prism 106. As laser beamlet 110a passes through (or within) rooftop prism 106, laser beamlet 110a is shifted (e.g., along the X-axis), as illustrated. Laser beamlet 110a is illustrated next being reflected by beam splitter 108 and passing through quarter wave plate 114, thereby changing the polarization state (e.g., to a circularly polarized state). Laser beamlet 110a is illustrated then travelling toward flat mirror 118a and then reflected toward quarter wave plate 114. As laser beamlet 110a passes through quarter wave plate 114, the polarization state of laser beamlet 110a is changed (e.g., to a horizontally polarized state). Laser beamlet 110a is illustrated next being transmitted by beam splitter 108 and passing through quarter wave plate 112, thereby changing the polarization state (e.g., to a circularly polarized state). Laser beamlet 110a is illustrated then travelling toward retroreflector 116a. As laser beamlet 110a passes through (or within) retroreflector 116a, laser beamlet 110a is shifted (e.g., along the Y-axis), as illustrated. Laser beamlet 110a is illustrated next passing through quarter wave plate 112, thereby changing the polarization state of laser beamlet 110a (e.g., vertically polarized). Laser beamlet 110a is illustrated next being reflected by beam splitter 108 towards detector 120a (e.g., after being combined / interfered with laser beamlet 110b).

[0035] As laser beamlet 110a travels along the first path (e.g., a measurement path), laser beamlet 110b concurrently travels along the second path (e.g., a reference path). As illustrated, laser beamlet 110b is provided (e.g., along the Y-axis) toward rooftop prism 106. As laser beamlet 110b passes through (or within) rooftop prism 106, laser beamlet 110a is shifted (e.g., along the X-axis), as illustrated. Laser beamlet 110b is then provided toward detector 120a. It should be understood that laser beamlet 110a and laser beamlet 110b may intersect and / or interfere with each other at certain points along their respective paths. For example, laser beamlet 110a and laser beamlet 110b may intersect and / or interfere while being provided to detector 120a. Superimposing laser beamlet 110a and laser beamlet 110b results in interference (e.g., constructive interference, destructive interference, etc.) which can be used by detector 120a to determine the position (or rotation or angle) of a structure (e.g., support structure 130, a wafer chuck, etc.) to which retroreflector 116a is attached.

[0036] Referring now to FIG. 2B, a side view of rooftop prism 106, retroreflector 116a, and flat mirror 118 (as arranged in measurement system 102' of FIG. 2A) is illustrated. The path of laser beamlet 110a is illustrated, with various elements of measurement system 102' omitted for clarity. FIG. 2B illustrates how beam walk-off is corrected in the Z-direction using the various embodiments of the invention. As illustrated, the measurement path of laser beamlet 110a may be insensitive to vertical translations of retroreflector 116a (and / or the structure to be measured, such as support structure 130, a wafer chuck, a target structure, etc.). In one exemplary application, retroreflector 116a is attached to a wafer chuck. As the wafer chuck is actuated in the vertical Z- direction, laser beamlet 110a (e.g., a measurement beam) translates in the Z direction. However, since laser beamlet 110a passes retroreflector 116a twice, the translation is effectively "removed" by the second pass through retroreflector 116a. Such an effect is enabled using rooftop prism 106 for the reference path (e.g., see laser beamlet 110b of FIG. 2A), since the rooftop prism does not translate beamlets (e.g., laser beamlet 110a, laser beamlet 110b, etc.) in the vertical Z-direction as beamlets pass through rooftop prism 106.

[0037] In certain embodiments, measurement system 102' includes: (i) one or more of measurement head 104a, measurement head 104b, and / or measurement head 104c;(ii) one or more of retroreflector 116a, retroreflector 116b, and / or retroreflector 116c;(iii) one or more of detector 120a, detector 120b, and / or detector 120c; and / or one or more of flat mirror 118a and flat mirror 118b. In one exemplary embodiment, measurement system 102' includes measurement head 104a, retroreflector 116a, detector 120a, and flat mirror 118a; such an embodiment may be used to determine an X-axis displacement (see such elements in FIG. 1A). In another exemplary embodiment, measurement system 102' includes measurement head 104b, retroreflector 116b, detector 120b, and flat mirror 118b; such an embodiment may be used to determine a Y-axis displacement (see such elements in FIG. 1A). In yet another exemplary embodiment, measurement system 102' includes measurement head 104b, measurement head 104c, retroreflector 116b, retroreflector 116c, detector 120b, detector 120c, and flat mirror 118b; such an embodiment may be used to determine an Rz displacement (e.g., rotational displacement). In certain embodiments, measurement system 102' includes a plurality of additional measurement heads, wherein ameasurement head and / or a plurality of additional measurement heads are configured to measure the structure to be measured (e.g., a support structure, a wafer chuck, a target structure, etc.) in multiple degrees of freedom (e.g., one degree of freedom, two degrees of freedom, three degrees of freedom, four degrees of freedom, five degrees of freedom, six degrees of freedom, etc.). It should be understood that multiple measurement systems 102' may be used in connection with shared elements (e.g., optical energy source 124, flat mirror 118b, etc.), thereby forming a larger measurement system (e.g., measurement system 102 of FIG. 1A).

[0038] FIG. 3 is a flow diagram of a method of measuring displacement of a target structure of a lithography system. As is understood by those skilled in the art, certain steps included in the flow diagram may be omitted; certain additional steps may be added; and the order of the steps may be altered from the order illustrated - all within the scope of the invention.

[0039] At Step 300, a measurement system (e.g., measurement system 102 of FIG. 1A, measurement system 102' of FIG. 2A) is provided. The measurement system includes an optical energy source (e.g., optical energy source) configured to provide optical energy (e.g., optical energy 110). The measurement system also includes a retroreflector (e.g., retroreflector 116a I 116b I 116c) attached to a target structure (e.g., support structure 130). The measurement system also includes a measurement head (e.g., measurement head 104a I 104b I 104c) configured for interferometry using optical energy. The measurement head includes (i) a polarizing beam splitter (e.g., beam splitter 108), and (ii) a rooftop prism (e.g., rooftop prism 106) in optical communication with the polarizing beam splitter. The rooftop prism is attached to the polarizing beam splitter. The measurement head is optically between a retroreflector and a mirror (e.g., flat mirror 118a I 118b) attached to a reference structure (e.g., reference structure 128). The measurement system also includes a detector (e.g., detector 120a I 120b I 120c) configured to determine displacement using optical energy provided from the measurement head.

[0040] At Step 302, optical energy is transmitted to the measurement head from the optical energy source (e.g., see provided optical energy 110 in FIG. 1A and FIG. 2A). The optical energy includes (i) a first laser beamlet (e.g., laser beamlet 110b) provided to the rooftop prism in connection with a reference path and (ii) a second laser beamlet(e.g., laser beamlet 110a) provided to the polarizing beam splitter in connection with a measurement path. It should be understood that the first laser beamlet and the second laser beamlet may be split using a polarizing beam splitter (e.g., a beam splitter 108).

[0041] At Step 304, the second laser beamlet is transmitted through the polarizing beam splitter, one or more quarter wave plates (e.g., quarter wave plate 112 I 114), the retroreflector, the mirror, and the rooftop prism. As the second laser beamlet passes through various optical elements (e.g., quarter wave plate 112 / 114, beam splitter 108, etc.) of the measurement system, the polarity of the second laser beamlet may be changed (e.g., see FIG. 2A). At Step 306, the first laser beamlet and the second laser beamlet is combined to form interfered optical energy (see FIG. 2A). At Step 308, the interfered optical energy is received with the detector (e.g., detector 120a I 120b I 120c).

[0042] At Step 310, displacement (and / or rotation) of the target structure is determined using the detector. The determination may be made using a computer configured to implement interferometry-related calculations in view of information related to the provided optical energy (e.g., frequency) and / or information related to the first laser beamlet and the second laser beamlet (e.g., the measurement path, the reference path, geometry of the target structure, geometry / positional information related to the retroreflector, geometry / positional information of the mirror / reference structure, geometry / positional information of the measurement head, etc.).

[0043] Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.

Claims

What is Claimed:

1. A measurement system comprising: an optical energy source configured to provide optical energy; a measurement head configured for interferometry using the optical energy provided by the optical energy source, the measurement head including (i) a polarizing beam splitter, and (ii) a rooftop prism in optical communication with the polarizing beam splitter, the rooftop prism being attached to the polarizing beam splitter; a retroreflector configured to be attached to a structure to be measured; and a detector configured to determine displacement using optical energy provided from the measurement head.

2. The measurement system of claim 1 wherein the structure to be measured is a support structure.

3. The measurement system of claim 2 wherein the support structure is a wafer chuck.

4. The measurement system of any preceding claim further comprising a mirror attached to a reference structure.

5. The measurement system of claim 4 wherein the reference structure is a granite base.

6. The measurement system of any preceding claim wherein the measurement head further comprises a first quarter wave plate and a second quarter wave plate, the first quarter wave plate and the second quarter wave plate being configured to modify a polarity of a beam of light.

7. The measurement system of any preceding claim further comprising a second retroreflector configured to be attached to the support structure at a second position, and a third retroreflector configured to be attached to the support structure at a third position, wherein the first retroreflector configured to be attached to a support structure at a first position, wherein the first retroreflector and the second retroreflector are configured to measure a first horizontal position of the support structure and a rotational position of the support structure, wherein the third retroreflector is configured to measure a second horizontal position of the support structure.

8. The measurement system of any preceding claim further comprising a second measurement head.

9. The measurement system of claim 8 further comprising a third measurement head.

10. The measurement system of any preceding claim further comprising a plurality of additional measurement heads, wherein the measurement head and the plurality of additional measurement heads are configured to measure the structure to be measured in up to six degrees of freedom.

11. A lithography system comprising: a support structure configured to support a substrate; a motion system configured to move the support structure; an optical energy source configured to provide optical energy;a measurement head configured for interferometry using the optical energy provided by the optical energy source, the measurement head including (i) a polarizing beam splitter, and (ii) a rooftop prism in optical communication with the polarizing beam splitter, the rooftop prism being attached to the polarizing beam splitter; a first retroreflector configured to be attached to a structure to be measured; and a detector configured to determine displacement using optical energy provided from the measurement head.

12. The lithography system of claim 11 wherein the structure to be measured is the support structure.

13. The lithography system of claim 12 wherein the support structure is a wafer chuck.

14. The lithography system of any of claims 11 to 13 further comprising a mirror attached to a reference structure.

15. The lithography system of claim 14 wherein the reference structure is a granite base.

16. The lithography system of any of claims 11 to 15 wherein the measurement head further includes a first quarter wave plate and a second quarter wave plate, the first quarter wave plate and the second quarter wave plate being configured to modify a polarity of a beam of light.

17. The lithography system of any of claims 11 to 16 further comprising a second retroreflector configured to be attached to the support structure at a second position, and a third retroreflector configured to be attached to the support structure at a third position,wherein the first retroreflector is configured to be attached to the support structure at a first position, wherein the first retroreflector and the second retroreflector are configured to measure a first horizontal position of the support structure and a rotational position of the support structure, wherein the third retroreflector is configured to measure a second horizontal position of the support structure.

18. The lithography system of any of claims 11 to 17 further comprising a second measurement head.

19. The lithography system of claim 18 further comprising a third measurement head.

20. The lithography system of any of claims 11 to 19 further comprising a plurality of additional measurement heads, wherein the measurement head and the plurality of additional measurement heads are configured to measure the structure to be measured in up to six degrees of freedom.

21. A method of measuring displacement of a target structure of a lithography system, the method comprising:(a) providing a measurement system, the measurement system including an optical energy source configured to provide optical energy, a retroreflector attached to the target structure, a measurement head configured for interferometry using optical energy, the measurement head including (i) a polarizing beam splitter, and (ii) a rooftop prism in optical communication with the polarizing beam splitter, the rooftop prism being attached to the polarizing beam splitter, the measurementhead being optically between a retroreflector and a mirror attached to a reference structure, and a detector configured to determine displacement using optical energy provided by the measurement head;(b) transmitting optical energy to the measurement head from the optical energy source, the optical energy including (i) a first laser beamlet provided to the rooftop prism in connection with a reference path and (ii) a second laser beamlet provided to the polarizing beam splitter in connection with a measurement path;(c) transmitting the second laser beamlet through the polarizing beam splitter, one or more quarter wave plates, the retroreflector, the mirror, and the rooftop prism;(d) combining the first laser beamlet and the second laser beamlet to form interfered optical energy;(e) receiving the interfered optical energy with the detector; and(f) determining displacement of the target structure using the detector.

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