TWO AXIS PLANE MIRROR INTERFEROMETER

Abstract: An interferometer system includes an integrated structure including a plurality of interferometer optical systems on a unitary glass cube for the optical element. The system includes dual integrated, parallel interferometers to form a two axis system in the unitary element. Each interferometer is a 90 degree version of a straight line interferometer, with a plane mirror interferometer laser beam path for each single interferometer. The glass cube contains a diagonal polarization interface between two glass segments which are bonded together. Cube corners and quarter wave plates are employed. A half wave plate is employed in one design. The system is a prealigned assembly and it measures translation in one version, translation and theta in another version and pitch, theta and translation in another version.

Description: 1 TWO AXIS PLANE MIRROR INTERFEROMETER ' Background of the Invention 1) Field of the Invention This invention is directed to a plane mirror 5 interferometer system which is particularly useful for a drive mechanism for accurately positioning a work stage along two axes with three degrees of freedom. CROSS REFERENCE TO RELATED APPLICATION 10 U.S. application Serial No. entitled "Servo Guided Stage System", filed contemporaneously herewith, by R.A. Kendall and S. Doran and assigned to the assignee of this application, and is incorpo¬ rated herein by reference. 5 2) Related Art Hewlet Packard 5501 Laser Transducer System Manual, pages 2-14 to 2-15 describes a Plane Mirror Interferometer and a Plane Mirror Interferometer Laser Beam Path" which shows how a conventional plane 0 mirror interferometer is designed and operates. In the case of the system shown on page 2-15 in Figure 2-16, the interferometer is of the straight through configuratio . The HP 5527A "Installation and Optical Align- 5 merit" Manual page 6-27 shows an illustration of an "Optical Method for Yaw Measurement" . Such plane mirror interferometer systems use two interferometers, see above 5501 manual, a beam splitter (to supply a beam to both interferometers 0 from a single source.) In this case a beam splitter is located to receive the beam after the measurement of a first y-axis position measurement by the first laser interferometer. Kallmayer et al "X-Y Table" IBM Technical 5 2 Disclosure Bulletin Vol. 30, No. 7 (Dec. 1987), pp. 376-377 shows three rigidly mounted spindle drives with stators 1, 2, and 3 affixed to a rigid support so they do not pivot relative to the table 4, so the flexibility of the drives is limited by their rigid mounting to a restricted range of motions afforded by the guides 10 and 11 in the slots in the table 4. In addition, two of the spindle drives 1 and 2 are parallel to each other. Laser interferometers referred to in that article as 5, 6 and 7 and mirrors 8 and 9 are used to measure displacement. Axiom 2/20 (TM) Compact Interferometer System Reference Manual OMP-0226, (June 1989) by Zygo Corporation, Laurel Brook Road, Middlefield, CT 06455, at pages CIS-7 to CIS-11 describes a "(straight-through) compact interferometer" with "two separate legs. The first leg measures linear dis¬ placement (pure translation) ; it is represented in the figure as solid black lines. The second leg is represented in the figure as dashed lines. The angle is measured by a combination of the first and second leg." At page CIS-2 reference is made to "monolithic construction." The system requires both a reference mirror and a plane (stage mirror) as well as two sets of quarter wave plates on two side of the optical element. Summary of the Invention An interferometer system in accordance with this invention includes an integrated structure including a plurality of interferometer optical systems on a unitary element, the system including integrated, parallel, dual two axis plane mirror interferometers in the unitary element. An preferred interferometer system in accordance with this invention includes an integrated structure 3 including a plurality of interferometer optical systems on a unitary element. The element structure comprises an optically transparent material divided into halves and having a plurality of planar external surfaces and a composed of two bonded elements with a polarizing beam-splitting surface at the junction between the two bonded elements. A plurality of optical elements secured to the structure, the elements combining to form at least a pair of interferometers aligned to be directed in parallel at a single retroflecting mirror on a surface whose linear and angular position is to be monitored, a first side of the structure including secured thereto a beam bender and a beam splitter for receiving an input laser beam to be employed by the interferometers having a first optical output path directed into the structure and a second optical output directed upwardly towards the beam bender positioned to receive a portion of the beam split from the beam splitter and directing the portion towards the polarizing beam splitting surface, first, second, third and fourth parallel retroreflectors with doubly reflecting surfaces for reflecting a beam parallel to its original path, the first and second retroreflectors being secured to the opposite side of the structure from the first side of the structure with the second one thereof being aligned with the optical output path of the beam splitter, and the first one thereof being aligned with the optical output path of the beam bender, and the polarizing beam splitting surface being located between the first side and the second side whereby beams reflect¬ ed from the beam splitting surface are directed through a third side of the structure with a first and a second quarter wave element secured to the 4 third side in alignment with the deflection of beams from the bender and the splitter reflected from the splitting surface respectively to the mirror, a fourth surface of the structure located opposite from the third surface, to the third retroreflector and the fourth retroreflector, the third and fourth retroreflectors having their axes aligned with the axes of respective beams reflected from the beam splitting surface (130) and returning back up through the beam splitting surface (130 and passing therethrough to pass through the structure (129* *) to reach the third and fourth retroreflectors respec¬ tively, for reflection back down to the mirror respectively for reflection therefrom respectively back to the beam splitting surface along parallel paths, for reflection back up Brief Description of the Drawings FIG. 1 is a perspective view of an x,y, theta stage plate and a plurality of linear drives includ- ing drivebars driven by friction drive units and a control system for positioning the stage plate. FIG. 2 shows the electrical schematic diagram of the control system for the drives employed to posi¬ tion the stage of FIG. 1. FIG. 3 shows the schematic diagram of a prior art straight line interferometer, which is a plane mirror interferometer laser beam path for a single interferometer with a quartz cube with a diagonal polarization interface between two quartz segments which are bonded together. FIG. 4 shows a prior art plane mirror interferometer which is a modification of the interferometer of FIG. 3, where the cube corners are on the top and the right of the diagonal polarization 5 cube, and the quarter wave plate is below the cube and the plane reflector is below it. This is the 90 degree version of the straight line interferometer shown in FIG. 3. FIG. 5 shows a prior art dual interferometer arrangement with a pair of interferometers used to measure positions. FIG. 6 shows an integrated structure incorporat¬ ing, parallel, dual or two axis plane mirror interferometers in accordance with this invention. FIG. 7A-7C show bottom, front and right side views of a pair of interferometers of the kind shown in FIG. 1 and 2 for the Y and Theta motion of the stage. FIG. 8A-8C show modifications of the interferometers of FIGS. 7A-7C. FIG. 9A-9C show further modifications of the interferometers of FIGS. 7A-7C. Description of the Preferred Embodiment Referring to FIG. 1, a base 10 supports a stage plate 11, adapted for carrying a work piece 8 slideably supported for moving on the upper surface of the base 10. The plate moves along the rectilin¬ ear x and y axes which rest substantially parallel with the flat surface of base 10. Base 10 is prefer¬ ably a very flat, massive stable table composed of a material such as granite, ceramic or steel with a highly polished, extremely flat planar upper surface 9 which carries the X-Y stage plate 11, with plate 11 supporting on its upper surface a work piece 8. The lower surface of stage plate 11 is slideably support¬ ed on the upper surface 9 of base 10 by low friction supports such as feet 12 composed of a low friction polymeric material such as PTFE (polytetrafluoroethylene) or, alternatively, the feet 6 12 can be replaced by equivalent support bearings such as air bearings or roller bearings. To the X-Y stage plate 11 there are pivotably secured three linear, friction drive units XI, Yl and Y2. The drive unit XI includes a drive motor Ml which is located along one side of base 10 and the Yl and Y2 include drive motors M2 and M3 which are located along the adjacent side of base 10 spaced well apart from each other to provide three degrees of freedom to the stage plate 11. The transmission of power for drive unit XI on one side of the Xl-drivebar 16 (composed of ceramic or steel materi¬ al) is provided by two pinch rollers 13 and 14 and a capstan 15 driven by motor Ml . The XI drive capstan 15 is located on the opposite side of Xl-drivebar 16 from pinch rollers 13 and 14 so that a friction drive is provided by the capstan 15 and the two rollers 13 and 14. Preload roller 20 presses down on the top of Xl-drivebar 16. Pinch rollers 13 and 14 as well as drive capstan 15 and roller 20 are mounted on car¬ riage 18 to rotate about pivot PI along the axis of the shaft of the motor of drive unit XI to permit pivoting of drivebar 16. The rollers 13 and 14 on one side of the capstan 15 on the other side exert opposing forces which act together to provide friction drive engagement of the capstan 15 with drivebar 16 for reciprocating it longitudinally as capstan 15 turns while concomitantly permitting the drivebar 16 and the carriage 18 carried on the shaft of capstan 15 and rollers 13 and 14 to pivot about the axis PI of the shaft of the capstan 15, thereby permitting rotation of the drivebar 16 on carriage 18 and the shaft of capstan 15. Drivebar 16 is offset at its inner end 17 where it connects to a linkage including pin 19 secured to stage plate 11 to secure the drivebar 16 to stage plate 11. The position of the stage plate 11 is measured by a laser interferometer system with a pair of bars 50 and 51 secured to two orthogonal sides of plate 11 opposite from the drive units XI, Yl and Y2. Each of bars 50 and 51 has a mirrored surface 73 and 52 respectively for measuring the X axis and Y axis displacements of stage plate 11. Laser beam 76 is provided to interferometer 79 which produces beams 77, 78, 80 and 81 which emanate from interferometer 79 towards mirror 52 and which are reflected back from mirror 52 to the interferometers 79 which produces output beams 82 and 83 which pass to receiv¬ ers 84 and 85 from the interferometers 79. Receivers 84 and 85 as well as receiver 45 are optical-to-electrical transducers for converting the laser signal to electronic signals. The receivers 84, 85 and 45 include a lens which focuses the laser beam onto an active chip of a silicon photodiode. Each receiver (which can be a commercially available product such as the Hewlett Packard 10780A receiver) , includes a photodetector, an amplifier and level translator, a line driver, a level sensor (comparator) and local voltage regulators. The receivers 84, 85 and 45 convert the Doppler-shifted laser light into electrical signals that can be processed by the electronic system. This is done to determine the position of mirror 52 along the Y axis, measured from the opposite side of the plate 11 from the Yl and Y2 drive units. Beam 75 passes through interferometer 110 producing beams 111 and 112 which measure the X axis position of a point on mirror 73 by reflection of beams 111 and 112 from mirror 73 to interferometer 110 which produces an output beam 46 directed at receiver 45. 8 Theta Angle Measurement Apparatus With respect to measurement of Theta motion by the laser interferometer system, within a small range of angles sufficient for applications such as VLSI semiconductor chip manufacture, the interferometer apertures are wide enough to receive the beams as the angles of reflection change. For greater angles of rotation, the system can employ a system of markings upon the drivebars which will give absolute positions of the drivebars with respect to their linear drives. X, Y, Theta, Three-Linear-Drivebar System Xl-drivebar 16 reciprocates in general in parallel with the X axis as indicated in FIG. 1, with rotation about PI axis away from parallel with the x axis to afford enhanced flexibility of being able to provide positioning of pin 19 and stage plate 11 anywhere within predetermined boundaries of base 10. The plate 11 can be rotated through an angle theta (as shown in frequency application Ser. No. of Doran and Kendall for "Servo Guided Stage System" (FI988-021) with respect to the X and Y axes using the three drive assemblies of FIG. 1 in cooperation, where the displacement of drivebars 26 and 36 is unequal. FIG. 2 shows the electrical schematic diagram of the control system for the stage 11. Three identical velocity servos shown in FIG. 2 are used to move the three capstan drive units Ml, M2, and M3. When the servo control electronics 86 receives a new destination from a host computer 105 on lines 103, a series of velocity values are sent to the velocity servos of FIG. 2 to cause the stage 11 to move to the desired new destination. The closed loop position servo loop gain vs frequency and the maximum values of stage velocity, acceleration and rate of change of acceleration are controlled by stored parameters and software in the servo control elec¬ tronics 86. The velocity servos can be operated with the position servo loop opened during gaging and initial¬ ization operations when the laser beams are not activated. Also, the analog joy stick can be used for manual stage control with the yaw servo holding the yaw of the stage near zero. Fine actuators such as piezoelectric transducers can be added to each of the three drivebars 16, 26 and 36 when higher positional and angular accuracy is required (less than 1 micrometer and 10 microradians. This provides a coarse and a fine servo combination where the fine servo can have a higher gain bandwidth because it is driving only the mass of the stage and the payload. The mass of the drivebar and the inertia of the motors Ml, M2 and M3 are outside of the fine position servo loop. An X-Y joystick 106 also provides input to the control electronics 86 for manual control of the position of the x-y-theta stage 11. The Laser Position Transducer and Servo Control electronics 86 receives the X position signals from the output of the XI axis receiver 45 through cable 104. Control electronics 86 also receives the output of Y-axis receiver 85 through cable 116. Electronics 86 also receives the output of theta receiver 84 and through cable 115. Interferometer 110 employs a pair of light beams directed to target mirror 73 as shown in greater detail in FIG. 1. For the Theta and Y axis measurements by receivers 84 and 85, a pair of interferometers are housed in an integrated structure 79 in accordance with this invention as shown in FIG. 1 and FIG. 7A-7C. Laser beams 82 and 83 pass to the 10 Theta receiver 84 and Y-axis receiver 85 respectively from the interferometers in structure 79. The interferometers operate with the target mirror 52 as explained in greater detail elsewhere in connection with FIGS. 1 and 7A-7C where the pair of light beams for each are shown, whereas in FIG. 2 they are shown as single beams 200, 201 respectively for convenience of illustration. The electronics 86 have Xl-error output 87 to the positive input of summing circuit 88 which provides an output to XI-drive amplifier 90 which energizes motor Ml which is shown in FIG. 1. The tachometer feedback 91 is shown on line 91 which connects to the negative input of summing circuit 88 to provide negative feedback. The electronics 86 also have a Y Error output 92 to the positive input of summing circuit 99 and summing circuit 94. Summing circuit 99 provides and output 100 to Y + Theta drive amplifier 101 which energizes motor M3 which is shown in Fig. 1. The tachometer feedback on line 103 is connected to the negative input of summing circuit 99 to provide negative feedbac . The electronics 86 further have a Theta- error output 93 to the negative input of summing circuit 94 and the positive input of summing circuit 99. Summing circuit 94 provides an output 95 to Y - Theta drive amplifier 96 which energizes motor M2 which is shown in FIG. 1. The tachometer feedback 98 is shown on line 98 which connects to the negative input of summing circuit 94 to provide negative feedback. PLANE MIRROR INTERFEROMETER FIG. 3 shows the schematic diagram of a prior art plane mirror interferometer laser beam path for a single interferometer with a glass cube with a 11 diagonal polarization interface between two glass segments which are bonded together as is well known by those skilled in the art of polarizer cubes. The cube also includes cube corners on the top and bottom which retroreflect light passing to them on parallel paths to the incoming beam. The beam with horizontal and vertical polarization components fl and f2 enters the interferometer at the same laser frequency with fl and f2 beams being split by the diagonal polariza- tion plane with fl passing straight through the cube and f2 reflecting from the diagonal polarization plane of the polarizer cube up into the upper cube corner and back down and out along the lower path to the left labeled f2 and fl +/-2 delta f. The fl wave passes through the quarter wave plate and is shifted 1/4 wavelength and is reflected back with a frequency shift of (+/-delta f) through the quarter wave plate on the same path whereupon it has been shifted by 1/2 wavelength (at frequency +/-delta f) and thus it is reflected down by the diagonal polarization interface in the cube to the cube corner on the bottom and back to the left and up to the polarization interface where it is reflected to the lower track to the right again (by the plane reflector) through the quarter wave plate again to add another quarter wave of phase shift on the lower track, still at frequency fl +/-delta f where it is now to the plane reflector again where it is reflected back again, now at the doppler shifted frequency fl +/- 2 delta f, through the quarter wave plate again to provide the fourth quarter wavelength of phase shift, and the light at frequency fl +/- 2 delta f passes along through the diagonal polarization interface in the cube to the left with the f2 wave after having passed through the quarter wave plate four times in all. 12 FIG. 4 shows a prior art plane mirror interferometer which * is a modification of the interferometer of FIG. 3, where the cube corners are on the top and the right of the diagonal polarization cube, and the quarter wave plate is below the cube and the plane reflector is below it. This is the 90 degree version of the straight line interferometer shown in FIG. 3. Its operation is analogous with f2 reflecting exactly the same way, and with the paths of fl being modified by the relocation of the ele¬ ments as indicated by the arrows. PARALLEL DUAL PLANE MIRROR INTERFEROMETERS FIG. 5 shows a prior art dual interferometer arrangement with a pair of interferometers used to measure positions. In principle what is involved is that there are two interferometers having optically transparent cubic elements 229 and 329 mounted on the base 10, by screws 202 and 203 on the one hand and by screws 302 and 303 on the other hand. In addition beam splitter 120 and beam bender 121 are also mounted on base 10 by screws as shown. The problem with such an arrangement is that the system must be aligned, with elements, 120, 121, 229 and 329 being perfectly aligned which is far more difficult that it would seem on first reflection. Slippage of the orientation during use and other such problems lead to inaccuracies. Optically transparent cubic ele¬ ments 229 and 329 are composed of two bonded glass elements, each with a polarizing beam-splitting surface (as is well understood by those skilled in the art, but not shown) joining the two halves of the cubic elements 229 and 329. Beam splitter 120 is secured to the base 10 to the left of cubic element 229, just below a beam bender 121. A laser beam 76 with combined vertical 79 13 and horizontal polarization components fl, f2, is supplied to the 50% beam splitter 120 which splits the beam in half to form beams 176 and 191 with components fl and f2 present in both 176 and 191. For convenience of explanation, hereforth beam path 176 will be comprised of f1 ' and f2' while beam path 191 will be comprised of fl and f2. A portion 191 (fl and f2) of the beam for use with one axis of the x y table is reflected up to the 90-degree beam bender 121 from which it is directed along beam path 192 into the interior of transparent element 329. As the fl, f2 beam 192 from bender 121 encounters beam splitting surface within element 329, the vertically polarized portion f2 continues as a straightahead (horizontally travelling beam) to a cube corner 124. In cube corner 124, the vertically polarized beam f2 (travelling horizontally in direction) is reflected at right angles across cube corner 124 and is re¬ flected back along a path from the opposite side of cube corner 124 through the element 329 and beam f2 passes (horizontally in direction) straight out to path 83 out of element 329, as one component of the output beam 83 to receiver 85. At the same time as the above vertically polarized beam f2 of beam 192 from bender 121 passes through the polarizing beam-splitting surface of element 329, e.g. the horizontally polarized portion fl of the beam from bender 121 is reflected down (along beam path 78) through a 1/4 wave plate 123. Plate 123 is secured to the lower surface of element 329. The fl beam 78 passes further down along beam path 78 until it reaches the mirror 52, and the fl +/- delta f beam 78 is reflected back up by mirror 52 along the same beam path 78 through 1/4 wave plate 123 and the polarizing beam splitting surface (not shown) to cube corner 127 14 where it is doubly reflected (in cube corner 127) over to the right and back down along beam path 77 (parallel to beam path 78) through surface 130 and 1/4 wave plate 123 to an adjacent location on the mirror 52 until it is reflected back up a second time through 1/4 wave plate 123 at the frequency fl +/- 2 delta f to the same beam splitting surface (130) where it is reflected because of its four 1/4 wave¬ length changes in polarization caused by two round trip passes through plate 123 to pass out along beam path 83, also, with the two beam paths leading from beam path 83 to a receiver, such as receiver 85 shown in FIG. 1. The two beams (fl and f2) passing along beam path 83 one beam from cube corner 124 and a second beam from surface 130 interfere optically with each other as a function of the position of the mirror 52 relative to the position of interferometer element 329, which is relatively fixed in position. The other half of beam path 76 from beam split- ter 120 for measuring the position of the second axis passes directly along beam path 176 to beam polariz¬ ing beam-splitting surface in cube 229. There the vertically polarized component of the beam, f2' is transmitted to cube corner 125. In cube corner 125, that vertically polarized component f2' of beam 176 is reflected up and over back through the polarizing surface out along beam path 82 to the detector 84 in FIGS. 1 and 14. The remaining, horizontally polar¬ ized component of the beam, fl'passing along path 176 for the second axis from beam splitter 120 is re¬ flected by the beam-splitting surface in cube 229 down through 1/4 wave plate 122 along beam path 81. The beam fl' passes along path 81 to mirror 52 and is reflected therefrom back as a beam fl* +/-delta f up beam path 81 through 1/4 wave plate 122 again and 15 through the polarizing surface 130 as a result of the two trips through 1/4 wave plate 122. Beam fl' +/-delta f passes through to cube corner 126 where it is doubly reflected over and down along beam path 80 through 1/4 wave plate 122 again, passing further along beam path 80 (parallel to beam path 81) to mirror 52 and is reflected back up at doppler fre¬ quency f1 ' +/- 2 delta f back up along beam path 80 to the polarizing beam-splitting surface in cube 229 where as a result of the effects of 1/4 wave plate 122 it in reflected out along beam oath 82 to the detector 84 with the beam f2 and latter beam f1 ' • + ■ /- 2 delta f interfering with each other to indicate the position of the mirror 52 relative to the two tandem interferometers. INTEGRATED PARALLEL DUAL PLANE MIRROR INTERFEROMETERS FIG. 6 shows an integrated structure incorporat¬ ing an embodiment of parallel, dual or two axis plane mirror interferometers in accordance with this invention. In this case the structure of the optical elements (cubes) of the interferometers in FIG. 5 have been merged together and the elements cooperat¬ ing with the unitary or integrated cube have been merged together into close proximity and intimate mechanical contact. Like numbers are used for like elements which are analogous to those shown in FIG. 5 to clarify the similarities and differences to FIG. 5. A pair of interferometers are housed in an integrated structure 79 incorporating the elements of the interferometers in accordance with this invention as shown in FIG. 1 and FIG. 7A-7C. Optical elements of the interferometers of FIGS. 1 and FIGS. 7A-C are bonded onto an optically transparent cubic element 129' '. Element 129' * is composed of two bonded glass elements with a polarizing beam-splitting surface 130 16 joining the two halves of the cubic element 129' '. In other words element 129' * carries bonded thereon the components of the interferometers in an integral structure. The various optical elements are bonded 5 to the element 129''cube by means of uv cured optical cement or the equivalent. A beam splitter 120 is bonded to the exterior of element 129' ' just below a beam bender 121. A laser beam 76 with combined vertical and horizontal polar- 10 ization components f1, f2 is supplied to the 50% beam splitter 120 which splits the beam in half to form beam paths 176 and 191 each containing components fl and f2. For convenience of explanation, henceforth beam path 176 will be comprised of f1 ' and f2' while 15 beam path 191 will be comprised of fl and f2. The fl, f2 portion 191 of the beam for use with one axis of the x y table is reflected up to the 90-degree beam bender 121 from which it is directed along beam path 192 into the interior of transparent element 20 129' '. As the f1 , f2 ~ beam 192 from bender 121 encounters beam splitting surface 130 of element 129'', the f2 vertically polarized portion continues as f2 beam 193 to a retroreflector, cube corner 124. The (horizontally moving) f2 beam 193 is reflected up 25 and over and back at right angles across cube corner 124 and is reflected back along path 183 from the opposite side of cube corner 124 through surface 130 and passes straight along path 83 out of element 129'', as the f2 component of the output beam 83 to 30 receiver 85. At the same time as the above vertical¬ ly polarized f2 portion (beam 193) of beam 192 from bender 121 passes through the polarizing beam-splitting surface 130 of element 129*', e.g. the horizontally polarized fl portion of the beam from 35 bender 121 is reflected down (along beam path 78) 17 through a 1/4 wave plate 123. Plate 123 is bonded to the lower surface of element 129' *. The beam 78 passes further down along beam path 78 until it reaches the mirror 52 (shown in FIGS. 1 and 14) , and the beam 78 is reflected back up by mirror 52 at frequency fl +/- delta f along the same beam path 78 through 1/4 wave plate 123 and surface 130 to cube corner 127 where it is doubly reflected (in cube corner 127) over to the right and back down along beam path 77 (parallel to beam path 78) through surface 130 and 1/4 wave plate 123 as beam fl +/- delta f to an adjacent location on the mirror 52 until it is reflected back up as beam fl +/- 2delta f a second time through 1/4 wave plate 123 to beam splitting surface 130 where it is reflected because of its four 1/4 wavelength changes in polarization caused by two passes through plate 123 to pass out along beam path 83, also, with the two beams f2 and fl +/- 2delta f leading to beam path 83. Those two beams passing along beam path 83 one beam from cube corner 124 and a second beam from surface 130 inter¬ fere optically with each other as a function of the position of the stage 11 relative to the position of interferometer element 79, which is relatively fixed in position. The other half of beam path 76 from beam split¬ ter 120 for measuring the position of the second axis passes components fl'and f2' directly along beam path 176 to beam polarizing beam-splitting surface 130. There the vertically polarized f2' portion of the beam is transmitted to cube corner 125. In cube corner 125, that vertically polarized portion of the beam 176 is reflected up and over back through surface 130 out along beam path 82 to the detector 84 in FIGS. 1 and 14. The remaining, horizontally 18 polarized portion of the beam passing along path 176 for the second axis from beam splitter 120 is re¬ flected by the beam-splitting surface 130 down through 1/4 wave plate 122 along beam path 81. The beam passes along path 81 to mirror 52 and is re¬ flected therefrom back up beam path 81 through 1/4 wave plate 122 again and through surface 130 as a result of the two trips through 1/4 wave plate 122. That beam passes through to cube corner 126 where it is doubly reflected over and down along beam path 80 through 1/4 wave plate 122 again, passing further along beam path 80 (parallel to beam path 81) to mirror 52 and beam fl'+/- 2delta f passes back up along beam path 80 to the polarizing beam-splitting surface 130 where as a result of the effects of 1/4 wave plate 122 it is reflected out along beam path 82 to the detector 84 with the latter beam interfering with the beam which took the other beam route from cube corner 125 to beam path 82 through cube 126. Interferometers FIG. 7A-7C show bottom, front and right side views of a unified housing for the optical elements of a pair of interferometers of the kind shown in FIG. 1 and 14 for the Y and Theta motion of the stage. Referring to FIGS. 7A-7C, a pair of interferometers are housed in an integrated structure 79 incorporating the elements of the interferometers in accordance with this invention as shown in FIG. 1 and FIG. 7A-7C. Optical elements of the interferometers of FIGS. 1 and 7A-C are bonded onto an optically transparent cubic element 129''. Element 129' * is composed of two bonded glass ele¬ ments with a polarizing beam-splitting surface 130 joining the two halves of the cubic element 129'*. In other words element 129'' carries bonded thereon the components of the interferometers in an integral structure. The various optical elements are bonded to the element 129 ' *cube by means of uv cured optical cement or the equivalent. A beam splitter 120 is bonded to the exterior of element 129' ' just below a beam bender 121. A laser beam 76 with combined vertical and horizontal polar¬ ization components is supplied to the 50% beam splitter 120 which splits the beam in half. A portion 191 of the beam for use with one axis of the x y table is reflected up to the 90-degree beam bender 121 from which it is directed along beam path 192 into the interior of transparent element 129''. As the beam 192 from bender 121 encounters beam splitting surface 130 of element 129'', the vertical¬ ly polarized portion continues as beam 193 to a retroreflector, cube corner 124. The horizontal beam 193 is reflected at right angles across cube corner 124 and is reflected back along path 183 from the opposite side of cube corner 124 through surface 130 and passes straight along path 183 out of element 129'*, as one component of the output beam 83 to receiver 85. At the same time as the above vertical- ly polarized portion (beam 193) of beam 192 from bender 121 passes through the polarizing beam-splitting surface 130 of element 129'', e.g. the horizontally polarized portion of the beam from bender 121 is reflected down (along beam path 78) through a 1/4 wave plate 123. Plate 123 is bonded to the lower surface of element 129' '. The beam 78 passes further down along beam path 78 until it reaches the mirror 52 (shown in FIGS. 1 and 14) , and the beam 78 is reflected back up by mirror 52 along the same beam path 78 through 1/4 wave plate 123 and 20 surface 130 to cube corner 127 where it is doubly reflected (in cube corner 127) over to the right and back down along beam path 77 (parallel to beam path 78) through surface 130 and 1/4 wave plate 123 to an 5 adjacent location on the mirror 52 until it is reflected back up a second time through 1/4 wave plate 23 to beam splitting surface 130 where it is reflected because of its four 1/4 wavelength changes in polarization caused by two passes through plate 10 123 to pass out along beam path 83, also, with the two beam paths leading to beam path 83. The two beams passing along beam path 83 one beam from cube corner 124 and a second beam from surface 130 inter¬ fere optically with each other as a function of the 15 position of the stage 11 relative to the position of interferometer element 79, which is relatively fixed in position. The other half of beam path 76 from beam split¬ ter 120 for measuring the position of the second axis 20 passes directly along beam path 176 to beam polariz¬ ing beam-splitting surface 130. There the vertically polarized portion of the beam is transmitted to cube corner 125. In cube corner 125, that vertically polarized portion of the beam 176 is reflected up and 25 over back through surface 130 out along beam path 82 to the detector 84 in FIGS. 1 and 14. The remaining, horizontally polarized portion of the beam passing along path 176 for the second axis from beam splitter 120 is reflected by the beam-splitting surface 130 30 down through 1/4 wave plate 122 along beam path 81. The beam passes along path 81 to mirror 52 and is reflected therefrom back up beam path 81 through 1/4 wave plate 122 again and through surface 130 as a result of the two trips through 1/4 wave plate 122. 35 That beam passes through to cube corner 126 where it 21 is doubly reflected over and down along beam path 80 through 1/4 wave plate 122 again, passing further along beam path 80 (parallel to beam path 81) to mirror 52 and back up along beam path 80 to the polarizing beam-splitting surface 130 where as a result of the effects of 1/4 wave plate 122 it is reflected out along beam path 82 to the detector 84 with the latter beam interfering with the beam which took the other beam route from cube corner 125 to beam path 82 through cube 126. FIG. 8A-8C show modifications of the interferometers of FIGS. 7A-7C. In general the operation is the same except that the 50% beam splitter has been moved up to the output of path 82. The beam on path 82 contains the recombined vertical and horizontally polarized components. The beam 191 reflected upwardly passes through half wave plate 128 to rotate the combined beams 90 degrees and therefrom into the 90 degree beam bender 121 which sends the beam 191 through the second integrated interferometer. If beam fl was the reference compo¬ nent in beam 76, it will be the measurement component in beam 191, and vice versa. This has the effect of reversing the sign of the detected motion thereby allowing the second axis to be insensitive to trans¬ lation since as the first axis increments by +df the second axis will decrement by -df giving a net of 0. This assumes that the mirror moves exactly the same amount for both axes. If it does not then the net will not = 0 and therefore the stage must have rotated to produce the different paths. Therefore, we have an optical signal for yaw. FIG. 9A-9C show further modifications of the interferometers of FIGS. 7A-7C. In this case, a 33% beam splitter 140 precedes the 50% beam splitter 120. There are additional cube corners 154 and 155 and an additional 90 degree beam bender 150, as well as quarter wave plate 162 which all combine to measure pitch as well as position and yaw, measured by the prior systems. Industrial Applicability This x-y positioning system is suitable for use by manufacturers and users of E-beam systems or similar kinds of systems in the semiconductor anu- facturing industry. This x-y table positioning system is designed for use with an E-Beam system employed for exposure of lithographic masks for use for semiconductor manufacturing. These drive tables provide improved manufacturing tolerances well below those possible or required in the past.

Claims: 23 What is claimed is: 1. An interferometer system including an integrated structure including a plurality of interferometer optical systems on a unitary element said system including integrated, parallel, dual two axis plane mirror interferometers in said unitary element. 2. An interferometer system including an integrated structure including a plurality of interferometer optical systems on a unitary element (129'') said element structure (129 *') comprising optically transparent material divided into halves and having a plurality of planar external surfaces and a composed of two bonded elements with a polarizing beam-splitting surface (130) at the junction between said two bonded ele¬ ments, a plurality of optical elements secured to said structure, said elements combining to form at least a pair of interferometers aligned to be directed in parallel at a single retroflecting mirror on a surface whose linear and angular position is to be monitored, a first side of said structure including secured thereto a beam bender (121) and a beam splitter (120) for receiving an input laser beam to be employed by said interferometers having a first optical output path directed into said structure (129**) and a second optical output directed 24 upwardly towards said beam bender (121) posi¬ tioned to receive a portion of the beam split from said beam splitter and directing said portion towards said polarizing beam splitting surface (130) , first, second, third and fourth parallel retroreflectors (124, 125 and 126, 127) with doubly reflecting surfaces for reflecting a beam parallel to its original path, said first and second retroreflectors (124, 125) being secured to the opposite side of said structure from said first side of said structure (129*') with said second one (125) thereof being aligned with said optical output path of said beam splitter (120) , and said first one (124) thereof being aligned with the optical output path of said beam bender (121) , and said polarizing beam splitting surface (130) being located between said first side and said second side whereby beams reflect¬ ed from said beam splitting surface (130) are directed through a third side of said structure (129'*) with a first and a second quarter wave element secured to said third side in alignment with the deflection of beams from said bender (121) and said splitter (120) reflected from said splitting surface (130) respectively to said mirror, a fourth surface of said structure (129'') located opposite from said third surface, to said third retroreflector (126) and said fourth retroreflector (127), said third and fourth retroreflectors (126, 127) having their axes aligned with the axes of respective beams 25 reflected from said beam splitting surface (130) and returning back up through said beam split¬ ting surface (130 and passing therethrough to pass through said structure (129'') to reach said third and fourth retroreflectors (126 and 127) respectively, for reflection back down to said mirror respectively for reflection there¬ from respectively back to said beam splitting surface (130) along parallel paths, for reflec¬ tion back up to said beam splitting surface for reflection laterally through said first side of said structure (129* *) on the same beam path as the reflections from said first and second retroreflectors respectively. An interferometer system in accordance with claim 1 including said polarizing beam-splitting surface (130) of said structure (129 '') being positioned in a plane oriented diagonally with respect to the beams from said beam splitter (120) and said beam bender (121), through the polarizing beam-splitting surface (130) of structure (129''), wherein the horizontally polarized portion of the beam being reflected down through a 1/4 wave plate (123) along a beam path (78) to said mirror (52) , and it is re¬ flected back to retroreflector (127) where it is doubly reflected in retroreflector (127) over and down along beam path (77) (parallel to beam path 78) to the mirror (52) until it is reflect¬ ed back up to beam splitting surface (130) where it is reflected because of its polarization caused by two passes through plate (123) to pass out along beam path (83) , also, with the two beam paths leading to beam path (83) interfering 26 with each other as a function of the distance of movement of the stage (11) , the other half of beam path (76) from beam splitter (120) for the second axis passes directly to beam polarizing beam-splitting surface (130) where the vertically polarized portion of the beam is partially transmitted to retroreflector (125) ; that vertically polarized portion of the beam is reflected back from retroreflector (125 out along beam path 82 to the detector 84. the remaining, horizontally polarized portion of beam for the second axis from beam splitter (120) is partially reflected by the beam-splitting surface (130) down along beam path (81) to mirror (52) and back up beam path (81) and through to retroreflector (126) where it is doubly reflected over and down along beam path (80) to mirror (52) and back up along beam path (80) to the polarizing beam-splitting surface (130) where it is reflected out along beam path (82) to the detector (84) with the latter waves interfering with the waves which took the other beam route to beam path (82) through retroreflector (126). 4. An integrated structure including a plurality of optical elements mounted on a solid optical element (129'') a) said optical element carrying a plurality of optical elements secured to the surfaces thereof, a mirror, said elements combining 27 to form at least a pair of interferometers directed in parallel at said mirror, said mirror being secured to a surface whose linear and angular position is to be monitored, b) said support structure housing a 50% beam splitter (120) for splitting a single input beam into two beams, said support structure being secured to a first side of said cube housing, c) a 90-degree beam bender (121) juxtaposed with said 50% beam splitter having its input aligned to receive a beam from said 50% beam splitter, d) two pairs of parallel retroreflectors (124, 125 and 126, 127) with doubly reflecting surfaces for reflecting a beam parallel to its original path one pair (124, 125) being located opposite from the side of said structure (129'') housing said beam bender (121) and the other pair being located on the adjacent side of said structure (129 1 '), said retroreflectors being on the back and top sides of said structure (129*') opposite from the side housing said beam bender and positioned to receive the output beam from said beam bender, e) a polarizing beam-splitting surface (130) of said structure (129'*) positioned at a diagonal to the beams from said 50% beam splitter (120) and said beam bender (121) , 28 through the polarizing beam-splitting surface (130) ' of structure (129*'), wherein the horizontally polarized portion of the beam being reflected down through a 1/4 wave plate (123) along a beam path (78) to the mirror (52) , and it is reflected back to retroreflector (127) where it is doubly reflected in retroreflector (127) over and down along beam path (77) (parallel to beam path 78) to the mirror (52) until it is reflected back up to beam splitting surface (130) where it is reflected because of its polarization caused by two passes through plate (123) to pass out along beam path (83) r also, with the two beam paths leading to beam path (83) interfering with each other as a function of the distance of movement of the stage (11) , f) The other half of beam path (76) from beam splitter (120) for the second axis passes directly to beam polarizing beam-splitting surface (130) where the vertically polar¬ ized portion of the beam is partially transmitted to retroreflector (125) . That vertically polarized portion of the beam is reflected back from retroreflector (125 out along beam path 82 to the detector 84. g) The remaining, horizontally polarized portion of beam for the second axis from beam splitter (120) is partially reflected by the beam-splitting surface (130) down along beam path (81) to mirror (52) and back up beam path (81) and through to 29 retroreflector (126) where it is doubly reflected over and down along beam path (80) to mirror (52) and back up along beam path (80) to the polarizing beam-splitting surface (130) where it is reflected out along beam path (82) to the detector (84) with the latter waves interfering with the waves which took the other beam route to beam path (82) through retroreflector (126) .

Inventors: KENDALL RODNEY ARTHUR, KENDALL, RODNEY, ARTHUR
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G01B9
IPC codes: G01B9