# US 5,708,505 A · Laser interferometer having a sheath for the laser beam > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,708,505 A - Document ID: US5708505A - Status: Granted - Kind: A - Application number: 08/687,896 - Filed: 1996-07-29 - Granted: 1998-01-13 - Earliest claimed: 1994-12-02 - Assignees: Nikon Corporation - Inventors: Michael Reid Sogard; Martin Edward Lee - Web page: https://patentgrep.com/patent/US5708505A ## CPC - G01B 9/0207: {Error reduction by correction of the measurement signal based on independently determined error sources, e.g. using a reference interferometer} - G01B 9/02021: {contacting different faces of object, e.g. opposite faces} - G01B 9/02027: {Two or more interferometric channels or interferometers} - G01B 9/02052: {Protecting, e.g. shock absorbing, arrangements} - G03F 7/70691: {Handling of masks or workpieces} - G03F 7/70858: {Environment aspects, e.g. pressure of beam-path gas, temperature (pollution aspects G03F7/70916)} - G03F 9/70: {for microlithography (measuring printed patterns for monitoring overlay G03F7/70633 or focus G03F7/70641; projection system adjustment G03F7/70258; position control G03F7/70775)} - G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS - G03F: PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR; (phototypographic composing devices B41B; photosensitive materials or processes for photographic purposes G03C; electrophotography, sensitive layers or processes therefor G03G) ## Abstract An interferometer used to measure distance to an object is provided with a laser sheath. The sheath encloses a substantial part of the measurement beam's path to provide a controlled environment which reduces environmental influences on the measured distance. The sheath is of variable length and responsive to a follower so as to maintain a sheath end nearest the object at a small distance from the object. An environmental controller controls the environment within the sheath. The environment within the sheath can be a vacuum or a suitable gas or gas mixture. A corrector can be used to compensate the interferometer's measured-distance signal for detected environmental characteristics to produce a corrected signal which indicates distance between the interferometer and the reflective surface. The apparatus and methods can be used to measure and control stage position in a projection-type wafer exposure system which is affected by variations in its atmospheric environment. ## Claims ### Claim 1 Apparatus for measuring position of a stage, comprising: a. a stage movable along mutually-orthogonal first and second axes, the stage having a first-axis mirror and a second-axis mirror; b. a first laser interferometer for directing a first-axis measurement beam along a first-axis beam path toward the first-axis mirror and for producing a first measured-distance signal indicative of an optical path length between the first laser interferometer and the first-axis mirror; c. a sheath enclosing the first-axis measurement beam along substantially the entire first-axis beam path wherein the sheath defines an interior volume such that an atmosphere outside the interior volume is prevented from entering the sheath, the sheath including a first-axis sheath portion having an effective length which varies in response to a follower system as the first-axis mirror changes position; d. a second laser interferometer for directing a second-axis measurement beam along a second-axis beam path toward the second-axis mirror and for producing a second measured-distance signal indicative of an optical path length between the second laser interferometer and the second-axis mirror; e. a sheath enclosing the second-axis measurement beam along substantially the entire second-axis beam path wherein the sheath defines an interior volume such that an atmosphere outside the interior volume is prevented from entering the sheath, the sheath including a second-axis sheath portion having an effective length which varies in response to a follower system as the second-axis mirror changes position; f. a first-axis follower system for varying the effective length of the first-axis sheath portion so that substantially the entire first-axis beam path remains enclosed as the first-axis mirror changes position relative to the first laser interferometer, wherein the first-axis follower system comprises a follower which follows movement of the stage while remaining out of contact with the stage, and wherein an end of the first-axis sheath portion nearest the first-axis mirror is mounted for movement with the follower such that said end of the first-axis sheath remains at a substantially constant spacing from the first-axis mirror; and g. a second-axis follower system for varying the effective length of the second-axis sheath portion so that substantially the entire second-axis beam path remains enclosed as the second-axis mirror changes position relative to the second laser interferometer. ### Claim 2 (depends on 1) The apparatus of claim 1, further comprising: h. an environmental control system for controlling an environment within the sheaths. ### Claim 3 (depends on 2) The apparatus of claim 2, further comprising a correction system for detecting characteristics of the second-axis beam path and for compensating the second measured-distance signal in response to the detected characteristics to produce a second corrected signal indicative of actual distance between the second laser interferometer and the second-axis mirror. ### Claim 4 (depends on 1) The apparatus of claim 1, further comprising a correction system for detecting characteristics of the first-axis beam path and for compensating the first measured-distance signal in response to the detected characteristics to produce a first corrected signal indicative of actual distance between the first laser interferometer and the first-axis mirror. ### Claim 5 (depends on 1) The apparatus of claim 1, wherein the second-axis follower system comprises a follower which follows movement of the stage while remaining out of contact with the stage, and wherein an end of the second-axis sheath portion nearest the second-axis mirror is mounted for movement with the follower such that said end of the second-axis sheath remains at a substantially constant spacing from the second-axis mirror. ### Claim 6 (depends on 1) The apparatus of claim 1, further comprising a window covering an end of the first-axis sheath portion nearing the first-axis mirror and sealing the interior volume of the first-axis sheath. ### Claim 7 (depends on 1) The apparatus of claim 1, further comprising a bearing at an end of the first-axis sheath portion nearest the first-axis mirror and sealing the interior volume of the first-axis sheath by bearing against the first-axis mirror. ### Claim 8 Apparatus for measuring position of a stage movable along mutually-orthogonal first and second axes, the stage having a first reflector substantially normal to the first-axis, comprising: a. a first-axis interferometer for directing a first-axis measurement beam along a first beam path parallel to the first-axis toward the first reflector and for producing a measured-distance signal indicative of an optical path length between the first-axis interferometer and the first reflector; b. a first-axis follower system having a first-axis follower which follows movement of the stage along the first-axis while remaining out of contact with the stage; and c. a first-axis sheath enclosing the first-axis measurement beam along a substantial part of the first beam path wherein the sheath defines an interior volume such that an atmosphere outside the sheath is prevented from entering the sheath, the first-axis sheath including a movable sheath portion carried by the follower such that the sheath has an effective length which varies with movement of the stage along the first-axis while remaining out of contact with the stage. ### Claim 9 (depends on 8) The apparatus of claim 8, wherein the first-axis follower maintains an end of the first-axis sheath at a substantially constant spacing from the first reflector. ### Claim 10 (depends on 9) The apparatus of claim 9, wherein the stage has a second reflector substantially normal to the second-axis, further comprising: d. a second-axis interferometer for directing a second-axis measurement beam along a second beam path parallel to the second-axis toward the second reflector and for producing a measured-distance signal indicative of an optical path length between the second-axis interferometer and the second reflector; e. a second-axis follower system having a second-axis follower which follows movement of the stage along the second-axis while remaining out of contact with the stage; and f. a second-axis sheath enclosing the second-axis measurement beam along a substantial part of the second beam path wherein the sheath defines an interior volume such that an atmosphere outside the sheath is prevented from entering the sheath, the second-axis sheath including a movable sheath portion carried by the follower such that the second-axis sheath has an effective length which varies with movement of the stage along the second-axis while remaining out of contact with the stage. ### Claim 11 (depends on 10) The apparatus of claim 10, wherein the second-axis follower maintains an end of the second-axis sheath at a substantially constant spacing from the second reflector. ### Claim 12 (depends on 9) The apparatus of claim 9, further comprising an environmental controller for controlling an environment within the first-axis sheath. ### Claim 13 (depends on 12) The apparatus of claim 12, wherein the environmental controller comprises a vacuum pump for evacuating the first-axis sheath and pressure controller for maintaining pressure within the first-axis sheath within predetermined limits. ### Claim 14 (depends on 9) The apparatus of claim 9, further comprising a corrector for detecting characteristics of the first beam path and for compensating the measured-distance signal in response to the detected characteristics to produce a corrected signal indicative of actual distance between the first-axis interferometer and the first reflector. ### Claim 15 (depends on 14) The apparatus of claim 14, further comprising means for maintaining a substantially constant gas pressure within the first-axis sheath wherein the gas differs from an atmosphere outside the first-axis sheath. ### Claim 16 (depends on 9) The apparatus of claim 9, wherein the first-axis sheath comprises a housing enclosing a portion of the first beam path, the housing having a wall and a beam opening through the wall, and wherein the sheath portion comprises an elongate hollow member extending through the beam opening and supported for movement relative to the wall in a direction generally parallel to the first beam path. ### Claim 17 (depends on 16) The apparatus of claim 16, further comprising an air-guided bearing for supporting the sheath portion in the beam opening. ### Claim 18 (depends on 16) The apparatus of claim 16, wherein an end of the elongate hollow member nearest the first reflector is fitted with a bearing for maintaining the sheath portion at a substantially fixed spacing from the reflector. ### Claim 19 (depends on 18) The apparatus of claim 18, wherein the bearing comprises a tip end having a face resiliently biased toward the reflector and having at least one channel for expelling pressurized gas to maintain separation between the face and the first reflector. ### Claim 20 (depends on 8) The apparatus of claim 8, wherein the first-axis sheath portion comprises a housing enclosing a portion of the first beam path, the housing having a wall and a beam opening through the wall, wherein the first-axis sheath portion comprises an elongate hollow member extending through the beam opening and supported for movement relative to the wall in a direction generally parallel to the first beam path, and wherein the first-axis sheath portion further comprises a window covering an end of the elongate hollow member nearest the first reflector, the window allowing passage of the first-axis measurement beam between the interior volume of the sheath and the first reflector. ### Claim 21 (depends on 20) The apparatus of claim 20, wherein the follower varies the effective length of the sheath portion by displacing the elongate hollow member along the first beam path to maintain the window at a substantially constant distance from the first reflector. ### Claim 22 (depends on 20) The apparatus of claim 20, further comprising a gap sensor for measuring distance between the window and the first reflector and wherein the corrector is responsive to the gap sensor for compensating the measured-distance signal in dependence on measured distance between the window and the reflector. ### Claim 23 (depends on 22) The apparatus of claim 22, further comprising a temperature sensor for detecting atmospheric temperature in the region of a gap between the window and the reflective surface and wherein the corrector is responsive to the temperature sensor for compensating the measured-distance signal in dependence on the detected atmospheric temperature. ### Claim 24 (depends on 20) The apparatus of claim 20, further comprising a pressure sensor for detecting pressure within the interior volume of the sheath and wherein the corrector is responsive to the pressure sensor for compensating the measured-distance signal in dependence on the detected pressure. ### Claim 25 (depends on 20) The apparatus of claim 20, wherein the window has an index of refraction which differs from the index of refraction of the interior volume of the sheath, and wherein the corrector compensates the measured-distance signal for the optical path length of the window. ### Claim 26 (depends on 8) The apparatus of claim 8, wherein the first-axis sheath comprises a housing enclosing a portion of the first beam path, the housing having a wall and a beam opening through the wall, and wherein the sheath portion comprises a telescoping assembly of elongate hollow members, the assembly extending through the beam opening and mounted for movement relative to the wall in a direction generally parallel to the first beam path. ### Claim 27 (depends on 8) The apparatus of claim 8, wherein the sheath comprises a housing enclosing a portion of the beam path, the housing having a wall and a beam opening through the wall, and wherein the sheath portion comprises an elongate hollow member having a bellows of variable length, the sheath portion mounted in the opening for extension and retraction relative to the wall in a direction generally parallel to the beam path. ### Claim 28 (depends on 8) The apparatus of claim 8, wherein the first-axis sheath is filled with a gas differing from an atmosphere outside the first-axis sheath. ### Claim 29 (depends on 28) The apparatus of claim 28, further comprising means for recirculating gas within the first-axis sheath to maintain a substantially uniform gas density within the first-axis sheath. ### Claim 30 (depends on 8) The apparatus of claim 8, wherein the first-axis sheath is filled with helium. ### Claim 31 (depends on 8) The apparatus of claim 8, wherein the first-axis sheath is filled with a gas, and further comprising a wavelength tracker for monitoring optical properties of the gas and for supplying a correction signal to said corrector. ### Claim 32 (depends on 8) The apparatus of claim 8, further comprising a window,covering an end of the first-axis sheath nearest the first reflector and sealing the interior volume of the first-axis sheath. ### Claim 33 (depends on 8) The apparatus of claim 8, further comprising a bearing at an end of the first-axis sheath and sealing the interior volume of the first-axis sheath. ### Claim 34 An exposure apparatus for exposing an image of a reticle onto a substrate mounted on an XY-stage which is movable in a predetermined X-Y plane, the apparatus comprising: (a) a base assembly for supporting the XY-stage thereon to move in the X-Y plane; (b) a frame structure having a rectangular frame assembly located around the base assembly and an X-follower assembly movable in the X direction on the frame assembly so as to maintain a predetermined space from the XY-stage in the X direction when the XY-stage moves in the X direction; (c) a Y-drive actuator partially mounted to the X-follower assembly for generating a force to move the XY-stage in the Y direction relative to the X-follower assembly on the base assembly; (d) an X-axis interferometer system fixedly located with respect to the base assembly for directing an X-axis measurement beam along an X-axis beam path toward an X-axis mirror mounted on said XY-stage and for producing an X-axis signal indicative of the position of the XY-stage; and (e) an X-axis sheath enclosing the X-axis measurement beam along a substantial part of the X-axis beam path, wherein the sheath defines an interior volume such that an atmosphere outside the interior volume is prevented from entering the sheath, the X-axis including a movable sheath portion carried by the X-follower assembly such that the X-axis sheath has an effective length which varies with movement of the XY-stage along the X axis while remaining out of contact with the XY-stage. ### Claim 35 (depends on 34) The exposure apparatus of claim 34, wherein the frame structure further includes a Y-follower assembly movable in the Y direction on the frame assembly so as to maintain a predetermined space from the XY-stage in the Y direction when the XY-stage moves in the Y direction. ### Claim 36 (depends on 35) The exposure apparatus of claim 35, further comprising: (f) an X-drive actuator partially mounted to the Y-follower assembly for generating a force to move the XY-stage in the X direction relative to the Y-follower assembly on the base assembly; (g) a Y-axis interferometer system fixedly located with respect to the base assembly for directing a Y-axis measurement beam along a Y-axis beam path toward a Y-axis mirror mounted on the XY-stage and for producing a Y-axis signal indicative of the position of the XY-stage; and (h) a Y-axis sheath enclosing the Y-axis measurement beam along a substantial part of the Y-axis beam path, wherein the sheath defines an interior volume such that an atmosphere outside the interior volume is prevented from entering the sheath, the Y-axis sheath including a movable sheath portion carried by the Y-follower assembly such that the Y-axis sheath has an effective length which varies with movement of the XY-stage along the Y-axis while remaining out of contact with the XY-stage.