Multi-axis laser interferometer

Abstract

To obtain a high-accuracy multiaxis laser interference length measurer with which alignment work is simple.SOLUTION: Provided is a multiaxis laser interference length measurer, comprising: a polarization beam splitter 21 for splitting the laser beam emitted from a light source 1 into a standard measurement beam 61 and a reference measurement beam 60; a half-mirror 41 for branching the split reference measurement beam 60 into a first reference measurement beam 62, a second reference measurement beam 63, ..., an n-th reference measurement beam; n+1 retroreflectors 51, 52, 53, ..., each reflecting the reference measurement beam 61 and the n reference measurement beams 62, 63, ..., and attached to a measurement unit and an interferometer, etc.; and n optical receivers 11, 12, ..., for photoelectrically converting respective interference signals obtained by branching the reference measurement beam 61 after reflection into n pieces and merging these with the n reference measurement beams 62, 63, ..., respectively.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a multi-axis laser interferometer, and more particularly to a laser interferometer for inspecting the accuracy of a plurality of axes of an NC machine tool or the like. [Background technology]

[0002] Traditionally, laser interferometers have been widely used for measuring the movement amount and controlling the position of ultra-precise stages, taking advantage of their features such as ultra-high accuracy, non-contact measurement, and easy installation. Their range of applications includes ultra-precise measuring instruments such as ultra-precise processing machines, aspheric shape measuring instruments, and ultra-precise 3D measuring instruments, as well as electron beam and laser beam drawing equipment, steppers, bonders, IC handlers, and other semiconductor manufacturing equipment.

[0003] A laser interferometer is basically composed of a laser light transmitting section and an interference light receiving section, a plane mirror that reflects the laser light emitted from the transmitting section, a reflective target such as a retroreflector, and a polarizing beam splitter that is an interferometer installed between them. For example, when measuring the moving distance of an NC machine tool, the reflective target is fixed to the moving table or spindle.

[0004] The operating directions of an NC machine tool are, for example, three axial directions, x, y, and z. A laser head, which serves as a light transmitting section, is fixed in the direction of each axial direction, an interferometer is fixed at the spindle position so as to coincide with the optical axis of the laser head, and a reflective target is fixed on the moving table on the extension of this. In other words, a preparatory work called alignment is required. This alignment work takes time, and becomes more difficult as the number of measurement axes increases. Furthermore, when measuring the angle or length of a certain axis, not only does it take preparation time, but it also takes time to change the equipment for each measurement item, and measurement errors occur due to misalignment of the installation position when the equipment is replaced.

[0005] In addition, the optical fiber coupled laser interferometer combines a laser light source and an interferometer with optical fiber, allowing the interferometer to be installed freely wherever it is desired to measure without having to worry about the optical path of the laser. Measurements can be easily made even if the measurement axis is oblique or the space inside the machine is narrow. In addition to being easy to set up, it is also suitable for reducing the number of steps required for machine tool accuracy inspection.

[0006] In order to have a compact configuration for a multi-axis interferometer with a small number of beam paths, thereby making it compact for a multi-axis interferometer, Patent Document 1 describes using a shared measurement optical path and a shared reference optical path for a first reflection of a shared measurement beam and a shared reference beam from a measurement reflector and a reference reflector, and splitting them into multiple individual beams corresponding to each measurement axis.

[0007] In order to reduce the alignment work between the light source and the interferometer, Patent Document 2 describes a method in which an optical fiber is provided to guide laser light from the spectroscopic mechanism and light source of the multi-axis interferometer to the spectroscopic mechanism, and the emission direction of the multi-axis interferometer is switched to perform length measurements. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2004-239905 A [Patent Document 2] Japanese Patent Application Publication No. 9-196623 Summary of the Invention [Problem to be solved by the invention]

[0009] The device described in Patent Document 1 uses a plane mirror, and if the mirror surface of the plane mirror is not positioned exactly opposite the optical axis of the interferometer, the reflected light is likely to be misaligned, causing an insufficient amount of light received by the length measurement optical system, making installation adjustment difficult. In addition, since it is a double-path length measurement device in which the measurement light travels twice between the interferometer and the measured part, it is susceptible to the effects of atmospheric turbulence, and high accuracy is required for the alignment work.

[0010] Furthermore, even if the alignment work is performed very precisely, if the moving object to be measured performs angular motion such as pitching or yawing, the alignment will shift and the amount of received light will be insufficient, resulting in a decrease in light receiving sensitivity.

[0011] The multi-axis interferometer described in Patent Document 2 requires a multi-axis interferometer with a built-in interferometer body that splits the incident laser light and outputs it in each axial direction as a spectroscopic mechanism of the multi-axis interferometer, and a switching mechanism for sending out interference light of the measurement light and the reference light. The multi-axis interferometer is large because it has a built-in polarizing beam splitter, a pentaprism, and a switching mechanism. And it is difficult to fit it into a small space such as that found in a semiconductor manufacturing facility. It can be costly and difficult to manufacture such large optical quality components.

[0012] The object of the present invention is to solve the problems of the prior art described above, and to provide a highly accurate multi-axis laser interferometer that can be used for a wide range of purposes, such as machine tools of various sizes and moving directions for each model, by reducing the installation space required, and that can be easily aligned. [Means for solving the problem]

[0013] The present invention, which achieves the above object, is configured as follows.

[0014] [1] A multi-axis laser interferometer comprising: a reflective target having a first retroreflector, a second retroreflector adjacent to the first retroreflector in a first direction, and a third retroreflector adjacent to the first retroreflector in a second direction intersecting the first direction; and an optical system having a light source facing the first retroreflector and emitting laser light, a first light receiving unit facing the second retroreflector and performing photoelectric conversion of an interference signal, and a second light receiving unit facing the third retroreflector and performing photoelectric conversion of the interference signal. [2] A multi-axis laser interferometer as described in claim 1, wherein the optical system detects the angle of a first rotation direction rotating on a plane intersecting the second direction by measuring the difference in distance to the first retroreflector and the second retroreflector, and detects the angle of a second rotation direction rotating on a plane intersecting the first direction by measuring the difference in distance to the first retroreflector and the third retroreflector. [3] A multi-axis laser interferometer as described in claim 1 or 2, wherein the first light receiving unit receives a first interference light generated by interference between light reflected by the first retroreflector and light reflected by the second retroreflector, and the second light receiving unit receives a second interference light generated by interference between light reflected by the first retroreflector and light reflected by the third retroreflector.

[0015] Another configuration of the present invention that achieves the above object is as follows.

[0016] [1] A multi-axis laser interferometer comprising: a first polarizing beam splitter that splits a laser beam emitted from a light source into a first laser beam and a second laser beam; a half mirror that splits the first laser beam and the second laser beam into n number of first laser beams and n number of second laser beams, each number being one or more; n+1 retroreflectors that reflect the first laser beam and the n number of second laser beams, respectively; and n photodetectors that photoelectrically convert each of the interference signals obtained by combining the n number of first laser beams split by the half mirror from the reflected first laser beam and the n number of second laser beams after reflection, wherein the multi-axis laser interferometer calculates an optical path difference from the interference fringes obtained by the photoelectric conversion and detects the displacement of the n+1 retroreflectors. [2] The multi-axis laser interferometer described in [1], further comprising n polarizing beam splitters each for combining and interfering with the n first laser beams and the n second laser beams, the n polarizing beam splitters including the first polarizing beam splitter. [3] A displacement detection method comprising: splitting a laser beam emitted from a light source into a first laser beam and a second laser beam; splitting the second laser beam into n number of second laser beams, where n is one or more, reflecting the n number of second laser beams at n number of retroreflectors among n+1 retroreflectors; reflecting the first laser beam at one retroreflector among the n+1 retroreflectors other than the n number of retroreflectors; splitting the reflected first laser beam into n number of first laser beams; photoelectrically converting each of interference signals obtained by combining the n number of first laser beams and the n number of second laser beams after reflection; calculating an optical path difference from the interference fringes obtained by the photoelectric conversion; and detecting displacement of the n+1 retroreflectors based on the optical path difference. [4] A reflection target having a first retroreflector, a second retroreflector adjacent to the first retroreflector in a first direction, and a third retroreflector adjacent to the first retroreflector in a second direction intersecting the first direction; a first laser beam, a second laser beam, and a third laser beam are emitted to the first retroreflector, the second retroreflector, and the third retroreflector, respectively, and the first laser beam reflected by the first retroreflector and the third laser beam are reflected by the second retroreflector. and an optical system into which a second laser beam and the third laser beam reflected by the third retroreflector are incident, wherein the optical system calculates a first optical path difference between the first laser beam and the second laser beam based on a first interference fringe obtained by photoelectrically converting interference between the reflected first laser beam and the reflected second laser beam, and calculates a second optical path difference between the first laser beam and the third laser beam based on a second interference fringe obtained by photoelectrically converting interference between the reflected first laser beam and the reflected third laser beam. [5] A multi-axis laser interferometer as described in [4], wherein the optical system detects an angle of a first rotation direction in which the second direction is used as an axis and the second direction is rotated on a plane perpendicular to the second direction based on the first optical path difference, and detects an angle of a second rotation direction in which the first direction is used as an axis and the second direction is rotated on a plane perpendicular to the first direction based on the second optical path difference.

[0017] Another configuration of the present invention is a multi-axis laser interferometer in which a laser beam emitted from a light source is split by an optical system, and the split beams are reflected by reflection targets and merged to obtain an interference signal, and the interference signal is photoelectrically converted to calculate an optical path difference. The optical system includes a polarizing beam splitter that splits the laser beam emitted from the light source into a reference measurement beam and a reference measurement beam, a half mirror that splits the split reference measurement beam into first, second, ..., nth reference measurement beams, n+1 retroreflectors that are attached to a measurement unit of a movable table as the reflection targets and reflect the reference measurement beam and the n reference measurement beams, respectively, and n photodetectors that split the reflected reference measurement beam into n beams and merge the beams with the n reference measurement beams to photoelectrically convert each of the interference signals obtained.

[0018] In the above, it is preferable to provide n polarizing beam splitters for respectively combining the n reflected standard measurement light beams and the n reference measurement light beams which have been split into n beams.

[0019] Further, the polarizing beam splitter has a first polarizing beam splitter that splits the laser light emitted from the light source into the standard measurement light and the reference measurement light, the half mirror has a first half mirror (41) that splits the reference measurement light into first reference measurement light and second reference measurement light, the retroreflector has a first retroreflector (51) that is attached to the measurement unit of the movable table as the reflection target and reflects the standard measurement light, a second retroreflector (52) that is attached to the measurement unit of the movable table as the reflection target and reflects the first reference measurement light, and a third retroreflector (53) that is attached to the measurement unit of the movable table as the reflection target and reflects the second reference measurement light, and a second half mirror (42) that splits the reference measurement light reflected by the second retroreflector (51), one of the reference measurement light split by the second half mirror (42) and the first reference measurement light reflected by the second retroreflector (52) are incident on the first polarizing beam splitter, the polarizing beam splitter has a second polarizing beam splitter into which the other of the reference measurement light split by the second half mirror (42) and the second reference measurement light reflected by the third retroreflector (53) are incident, and the polarizing beam splitter preferably comprises a first light receiving unit that photoelectrically converts an interference signal between the first reference measurement light and the reference measurement light obtained by the first polarizing beam splitter, and a second light receiving unit that photoelectrically converts an interference signal between the second reference measurement light and the reference measurement light obtained by the second polarizing beam splitter.

[0020] Furthermore, it is preferable that the first reference measurement light reflected by the retroreflector (52) is bent by a mirror (31) and enters the first polarizing beam splitter.

[0021] Furthermore, it is preferable that the second reference measurement light is bent by mirrors (32, 33) and reflected by the retroreflector (53).

[0022] In the above, the optical system includes a first polarizing beam splitter (21) that splits the laser light emitted from the light source into the standard measurement light and the reference measurement light, a half mirror (41, 44) that splits the reference measurement light into a first reference measurement light, a second reference measurement light, and a third reference measurement light, a retroreflector (51) that is attached to the measurement unit of the movable table as the reflection target and reflects the standard measurement light, a retroreflector (52) that is attached to the measurement unit of the movable table as the reflection target and reflects the first reference measurement light, a retroreflector (53) that is attached to the measurement unit of the movable table as the reflection target and reflects the second reference measurement light, a retroreflector (54) that is attached to the measurement unit of the movable table as the reflection target and reflects the third reference measurement light, and a reference measurement light reflected by the retroreflector (51). the first polarizing beam splitter (21) into which the branched reference measurement light and the first reference measurement light reflected by the retroreflector (52) are incident; a second polarizing beam splitter into which the branched reference measurement light and the second reference measurement light reflected by the retroreflector (53) are incident; a third polarizing beam splitter into which the branched reference measurement light and the third reference measurement light reflected by the retroreflector (54) are incident; a first light receiving unit that photoelectrically converts an interference signal between the first reference measurement light and the reference measurement light obtained by the first polarizing beam splitter (21); a second light receiving unit that photoelectrically converts an interference signal between the second reference measurement light and the reference measurement light obtained by the second polarizing beam splitter; and a third light receiving unit that photoelectrically converts an interference signal between the third reference measurement light and the reference measurement light obtained by the third polarizing beam splitter.

[0023] Furthermore, it is desirable that the reference measurement light reflected by the retroreflector (51) is split by half mirrors (42, 43) and enters the first polarizing beam splitter (21), the second polarizing beam splitter (22), and the third polarizing beam splitter (23).

[0024] Furthermore, it is desirable that the first reference measurement light is split by the half mirror (41), the third reference measurement light and the second reference measurement light are split by the half mirror (44), and the second reference measurement light is bent by mirrors (32, 33) and reflected by the retroreflector (53). Furthermore, it is preferable that the reference measurement light is split into the third reference measurement light and the second reference measurement light by the half mirror (44), and the third reference measurement light is reflected by the retroreflector (54). Effect of the Invention

[0025] According to the present invention, in a multi-axis laser interferometer, a laser beam emitted from a light source is split into a standard measurement beam and a reference measurement beam by a polarizing beam splitter. The split reference measurement beam is then split into a first, second, ..., n-th reference measurement beam, and the standard measurement beam and the n reference measurement beams are reflected by n+1 retroreflectors, respectively, and the reflected standard measurement beam is split into n beams, which are then merged with the n reference measurement beams, and the interference signals are photoelectrically converted by n photodetectors. Therefore, this configuration allows for simultaneous measurement of n axes using one light source, one unit, and one standard measurement beam as an axis. Therefore, there is no variation in the position and angle of the measurement axis in the alignment adjustment associated with the setup required when performing each of the n measurements. In addition, there is basically no need to setup the equipment, which reduces the number of work steps.

[0026] Furthermore, the installation space is reduced, and the device is compatible with machine tools of various sizes and moving directions for each model, making it versatile. It is easy to install and adjust, and enables stable measurements that are less susceptible to the effects of atmospheric disturbances. [Brief description of the drawings]

[0027] [Figure 1] Schematic diagram of an optical system for measuring two axes according to an embodiment of the present invention. [Diagram 2] FIG. 1 is a perspective view showing the arrangement of a reflective target and a laser interferometer when measuring yawing and pitching according to an embodiment of the present invention; [Diagram 3] FIG. 1 is a perspective view showing the arrangement of a reflective target and a laser interferometer when measuring yawing and pitching using a conventional single-function laser interferometer. [Figure 4] A top view illustrating the influence of errors due to the measurement axis (movement axis of the movable table 40) and the measurement position (laser optical axis). [Diagram 5] A side view illustrating the influence of errors due to the measurement axis (movement axis of the movable table 40) and the measurement position (laser optical axis). [Figure 6] Schematic diagram of an optical system for measuring three axes according to another embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Fig. 1 is a schematic diagram of an optical system 20 for measuring two axes according to a first embodiment of the present invention. Fig. 2 is a perspective view showing the arrangement of a reflection target and a laser interferometer when measuring yawing and pitching with a multi-axis laser interferometer of the first embodiment as an example of measuring the accuracy of a moving table of a machine tool or the like. Fig. 3 is a perspective view similarly showing the arrangement with a conventional single-function laser interferometer.

[0029] In the single-function laser interferometer, as shown in Fig. 3, retroreflectors 71, 72, 81, and 82, which are reflection targets, are fixed to movable table 40 by a magnetic stand or the like so as not to move as they serve as references during measurement. Polarizing beam splitters 70 and 80, which are interferometers, are fixed to adjustable stages (not shown), and adjustments are made so that retroreflectors 71, 72, 81, and 82 are positioned on the extension of the optical axis of the laser light source, i.e., alignment work is performed, and they are positioned and fixed.

[0030] The retroreflectors 71 and 72 measure the yawing of the movable table 40 (rotation in a horizontal plane around the up-down axis: in the direction of the arrow Y in the figure), and the yawing angle is detected by measuring the distance from the polarizing beam splitter 70 to the retroreflectors 71 and 72. Similarly, the retroreflectors 81 and 82 measure the pitching of the movable table 40 (rotation around the left-right axis: in the direction of the arrow P in the figure), and the pitching angle is detected by measuring the distance from the polarizing beam splitter 80 to the retroreflectors 81 and 82.

[0031] Reference numeral 73 denotes a light source to which an optical fiber that inputs laser light into the optical system of polarizing beam splitter 70 is connected, and reference numeral 74 denotes a light receiving section to which an optical fiber that receives the light is connected. Similarly, reference numeral 83 denotes a light source to which an optical fiber that inputs laser light into the optical system of polarizing beam splitter 80 is connected, and reference numeral 84 denotes a light receiving section to which an optical fiber that receives the light is connected. In the conventional length measuring device using a single-function laser interferometer in Figure 3, light sources 73 and 83 are each required independently.

[0032] In addition, because retroreflectors 71, 72 and retroreflectors 81, 82 must be installed at a distance from each other, it is necessary not only to adjust the optical axis of each but also to ensure that the optical axes are not misaligned with each other. In addition, the error due to the measurement position in angle (yawing and pitching) measurement is small when the measurement object is a rigid body because it changes almost uniformly, but when measuring length at the same time, an error occurs depending on the optical axis position.

[0033] Fig. 2, which is a first embodiment, shows the arrangement of the reflection targets and the optical system 20 when measuring yawing and pitching with a multi-axis laser interferometer, in comparison with the conventional example in Fig. 3. The optical system 20 has three axes connected together. Retroreflectors 51, 52, and 53, which are reflection targets, are integrated and arranged close to each other on the movable table 40, and are fixed by a magnetic stand or the like so as not to move as they serve as a reference during measurement. The optical system 20, which is an interferometer, is fixed to a stage (not shown) whose position can be adjusted, and the retroreflectors 52, 51, and 53 are adjusted so that they are located on the extension of the optical axes of the laser light sources (reference measurement light 61, first reference measurement light 62, and second reference measurement light 63), that is, an alignment operation is performed, and they are positioned and fixed.

[0034] Retroreflectors 51 and 52 measure yawing (rotation in a horizontal plane around the up-down axis: in the direction of arrow Y in the figure) of movable table 40, and the yawing angle is detected by measuring the difference in distance from optical system 20 to retroreflectors 51 and 52. Pitching (rotation around the left-right axis: in the direction of arrow P in the figure) of movable table 40 is measured by retroreflectors 52 and 53, and the pitching angle is detected by measuring the difference in distance from optical system 20 to retroreflectors 52 and 53.

[0035] Reference numeral 1 denotes a light source to which an optical fiber for emitting laser light to an optical system 20 is connected, and reference numerals 11 and 12 denote light receiving sections to which optical fibers for receiving light are connected, with 11 being the first light receiving section and 12 being the second light receiving section. Unlike the conventional length measuring device using a single-function laser interferometer in Fig. 3, only one light source 1 is required. Also, retroreflectors 51, 52, and 53 are integrated.

[0036] Therefore, the retroreflectors 51, 52, and 53 are installed close to each other, and when measuring yawing and pitching, it is sufficient to adjust the combined optical axis. In other words, it is sufficient to perform position adjustment and alignment work for the optical system 20. Furthermore, even when measuring length at the same time, it is possible to reduce error factors due to the position of the optical axis.

[0037] The environment has a large effect on laser interferometers, and it is important to minimize environmental errors. Environmental factors include mechanical vibrations of the measurement object and air turbulence. To prevent mechanical vibrations of the measurement object, the optical system 20 and retroreflectors 51, 52, and 53 must be sufficiently fixed.

[0038] If the retroreflectors 51, 52, and 53 are not fixed properly, the vibrations may be amplified, resulting in poor measurement accuracy. When fixing the retroreflectors 51, 52, and 53, they must be fixed as close as possible to the mounting position, since vibrations will be amplified if the distance from the mounting position is long. Since the retroreflectors 51, 52, and 53 are installed close to each other and the optical axis adjustment only needs to be performed once, the multi-axis laser interferometer in Figure 2 can reduce error factors compared to conventional methods in which each optical axis is adjusted individually.

[0039] When a local temperature difference occurs in the air in the measurement environment, air turbulence occurs, which locally changes the wavelength of the laser light propagating through the air, causing disturbances in the measurement data and deteriorating repeatability.

[0040] As a method for reducing local temperature differences, it is good to take measures to prevent air fluctuations from entering the measurement optical path, for example, to stir the air along the measurement path with a large electric fan, etc. In addition, condensation on optical components can cause lens mold, coating abnormalities, and other problems in the optical system, so temperature and humidity management is important.

[0041] Figure 1 is a schematic diagram showing the details of the optical system 20 of a multi-axis laser interferometer that measures two axes. The basic principle follows that of a Michelson interferometer. A Michelson interferometer uses a laser beam with a known wavelength to split the beam into two paths, which are reflected by a reflective target and then rejoined. An interference signal that generates interference fringes is then created, and the interference signal is photoelectrically converted to calculate the optical path difference based on the number of interference fringes, and the moving distance of the reflective target is calculated.

[0042] The basic configuration is as follows: (1) Laser light output from a light source is split into a standard measurement light and a reference measurement light by a polarizing beam splitter or the like. (2) The split reference measurement light is split into first, second, ..., nth reference measurement light by a half mirror or the like. (3) The standard measurement light and n reference measurement light are each reflected by retroreflectors, which are n+1 reflective targets attached to a measurement unit such as a movable table. (4) The reflected standard measurement light is split into n beams, which are interfered with each reference measurement light, and each interference signal is photoelectrically converted by n photodetectors.

[0043] The number of measurements is equal to the nth order of the reference measurement light, and one retroreflector is required for each reference measurement light. In addition, n polarizing beam splitters are provided to merge the n reflected reference measurement light that has been split into n beams so that they interfere with each other. This configuration allows measurements of n axes to be performed simultaneously with one light source and one unit, with one reference measurement light as the axis.

[0044] Therefore, there is no variation in the position and angle of the measurement axis during alignment adjustment that accompanies the changeover required when performing n measurements. In addition, there is basically no need to change the equipment, which reduces the amount of work required.

[0045] Reference numeral 1 denotes a light source, which uses a laser beam that emits a laser beam of a predetermined wavelength. The laser beam emitted from the light source 1 is split into a standard measurement beam 61 and a reference measurement beam 60 by a first polarizing beam splitter 21. The standard measurement beam 61 is then reflected by a retroreflector 51, which is a reflection target, and split by a half mirror 42. One beam of the standard measurement beam 61 passes through the half mirror 42 and enters the first polarizing beam splitter 21, and the other beam is reflected by the half mirror 42 and enters a second polarizing beam splitter 22 arranged after the half mirror 42.

[0046] The reference measurement light 60 split by the first polarizing beam splitter 21 is split by the half mirror 41 into a first reference measurement light 62 and a second reference measurement light 63. The first reference measurement light 62 is further bent by the mirror 31 and reflected by the retroreflector 52, which is a reflection target fixed to the object to be measured. It is further bent by the mirror 31 and enters the first polarizing beam splitter 21.

[0047] The first reference measurement light 62 interferes with the standard measurement light 61 at the first polarizing beam splitter 21 and enters the first light receiving unit 11. The second reference measurement light 63 is bent by the mirrors 32 and 33, reflected by the retroreflector 53, and enters the second polarizing beam splitter 22. Then, it interferes with the standard measurement light 61 at the second polarizing beam splitter 22 and enters the second light receiving unit 12.

[0048] Each interference fringe, which is the light and dark of the light that has interfered with the first polarizing beam splitter 21 and the second polarizing beam splitter 22, represents the phase difference in the optical path length between the standard measurement light 61 and the first reference measurement light 62, and between the standard measurement light 61 and the second reference measurement light 63. One interference fringe corresponds to the phase difference in the wavelength of the light source. Since the wavelength of the light source is 632.8 nm in the case of a He-Ne laser, the phase of one fringe in the round-trip optical path interferometer is an extremely small length of 0.3 μm, making it possible to measure minute displacements and changes in the retroreflectors 51, 52, and 53.

[0049] As described above, the laser light output from the light source 1 is split by the first polarizing beam splitter 21 into the standard measurement light 61 and the reference measurement light 60. The split reference measurement light 60 is split by the half mirror 41 into the first reference measurement light 62 and the second reference measurement light 63. The standard measurement light 61 and the two reference measurement lights, the first reference measurement light 62 and the second reference measurement light 63, are reflected by three retroreflectors 51, 52, and 53 attached to the measurement unit. The reflected standard measurement light 61 is split into two and made to interfere with each reference measurement light, and the interference signals are observed by two photoreceivers, the first photoreceptor 11 and the second photoreceptor 12.

[0050] Figures 4 and 5 show the effect of errors due to the measurement axis (movement axis of the movable table 40) and measurement position (laser optical axis) when measuring with a laser interferometer as in Figure 2, with Figure 4 being a top view and Figure 5 being a side view. Errors due to the measurement position in angle (yawing and pitching) measurements change almost uniformly when the measurement object is rigid, so they can be said to be negligible. However, when measuring length, errors due to position will cause Abbe errors.

[0051] Since a laser interferometer detects the yawing and pitching angles by measuring the distance to a retroreflector, which is a reflective target, it is preferable as a design principle to arrange the measurement axis and the laser optical axis on the same line in accordance with Abbe's principle.

[0052] Fig. 4 shows the case of yawing measurement, in which the distance and inclination between the laser optical axis, which is the reference measurement light 61, and the measurement axis are factors that cause Abbe error. Fig. 5 shows the case of pitching measurement, in which the distance and inclination between the laser optical axis, which is the second reference measurement light 63, and the measurement axis are factors that cause error. However, according to this embodiment, it is sufficient to perform the optical axis adjustment of multiple axes at once, so that the factors of error can be reduced compared to the conventional method in which each optical axis is adjusted individually.

[0053] 6 is a schematic diagram showing details of an optical system 20 for measuring three axes according to a second embodiment. Reference numeral 1 denotes a light source, which uses a laser beam that emits a laser beam of a predetermined wavelength. The light emitted from the light source 1 is split into a standard measurement beam 61 and a reference measurement beam 60 by a first polarizing beam splitter 21. The standard measurement beam 61 is then reflected by a retroreflector 51, which is a reflection target, and split by half mirrors 42 and 43 to enter the first polarizing beam splitter 21, the second polarizing beam splitter 22, and further the third polarizing beam splitter 23.

[0054] The reference measurement light 60 split by the first polarizing beam splitter 21 is split by the half mirrors 41 and 44 into a first reference measurement light 62, a second reference measurement light 63, and a third reference measurement light 64. The reference measurement light 60 is split by the half mirror 41 into the first reference measurement light 62, and by the half mirror 44 into the second reference measurement light 63 and the third reference measurement light 64. The first reference measurement light 62 is further bent by the mirror 31 and reflected by the retroreflector 52, which is a reflection target fixed to the object to be measured. It is further bent by the mirror 31 and enters the first polarizing beam splitter 21.

[0055] The first reference measurement light 62 interferes with the standard measurement light 61 at the first polarizing beam splitter 21 and enters the first light receiving unit 11. The second reference measurement light 63 is bent by the mirrors 32 and 33, reflected by the retroreflector 53, and enters the second polarizing beam splitter 22. Then, it interferes with the standard measurement light 61 at the second polarizing beam splitter 22 and enters the second light receiving unit 12.

[0056] The third reference measurement light 64 is reflected by the retroreflector 54 and enters the third polarizing beam splitter 23. The reference measurement light 61 and the third reference measurement light 64 interfere with each other at the third polarizing beam splitter 23 and enter the third light receiving unit 13. The interference fringes, which are the light and dark of the light that has interfered at the first polarizing beam splitter 21, the second polarizing beam splitter 22, and the third polarizing beam splitter 23, represent the phase differences in the optical path lengths of the reference measurement light 61 and the first reference measurement light 62, the reference measurement light 61 and the second reference measurement light 63, and the reference measurement light 61 and the third reference measurement light 64. This makes it possible to measure minute displacements and changes in the retroreflectors 51, 52, 53, and 54.

[0057] As described above, the laser light output from the light source 1 is split into the standard measurement light 61 and the reference measurement light 60 by the first polarizing beam splitter 21. The split reference measurement light 60 is split into the first reference measurement light 62, the second reference measurement light 63, and the third reference measurement light 64 by the half mirrors 41 and 44. The standard measurement light 61 and the three reference measurement lights, the first reference measurement light 62, the second reference measurement light 63, and the third reference measurement light 64, are reflected by three retroreflectors 51, 52, and 53 attached to the measurement unit and a retroreflector 54 fixed to the interferometer unit. The reflected standard measurement light 61 is split into three, and is made to interfere with each reference measurement light, and the interference signal is observed by three receivers, the first light receiving unit 11, the second light receiving unit 12, and the third light receiving unit 13.

[0058] According to the second embodiment, it is possible to easily measure angle and length simultaneously. When measuring the angle alone, mechanical errors in the movement of the workpiece directly result in measurement position errors, whereas with this interferometer that can obtain angle and length simultaneously, it is possible to manage the measurement point in submicron units.

[0059] In addition, this configuration allows three measurements to be performed simultaneously using one light source, one unit, and one reference measurement light as the axis, so there is no variation in the position and angle of the measurement axis in the alignment adjustment required when performing each of the three measurements.

[0060] As described above, in the multi-axis laser interferometer, the laser light emitted from the light source is split into a standard measurement light and a reference measurement light by a polarizing beam splitter. The split reference measurement light is further split into first, second, ..., n-th reference measurement light. The standard measurement light and the n reference measurement light beams are each reflected by n+1 retroreflectors, and the reflected standard measurement light is split into n beams, which are each merged with the n reference measurement light beams, and the interference signals are photoelectrically converted by n photodetectors.

[0061] Therefore, it is possible to simultaneously measure n axes with one light source and one unit optical system, with one reference measurement light as the axis. This allows the use of equipment capable of measuring multiple items, which eliminates the need for misalignment of the mounting (measurement) position due to setup changes, and reduces measurement time. In addition, it is possible to save mounting space, make mounting adjustments easier, and enable stable measurements that are less susceptible to the effects of atmospheric turbulence, thereby increasing versatility.

[0062] Furthermore, the change in posture (yawing, pitching) caused by the movement of the stage can be measured as an angle from the change in the optical path length to each retroreflector with a resolution of 0.05 seconds and an accuracy of about ±0.2%. It can also be applied to a system for quickly measuring the angular position of the index table of a machine tool, and errors when installing the system can also be corrected.

[0063] Therefore, since the setting can be completed without precise adjustment, the measurement can be completed in about one-third the time compared to conventional methods. Furthermore, it is possible to handle not only positioning accuracy, but also behavior analysis of stages and spindles, which are required for machine tools, as well as dynamic behavior analysis. [Explanation of symbols]

[0064] 1, 73, 83... light source, 20... optical system, 40... movable table, 70, 80... polarizing beam splitter, 74, 84... light receiving unit 11...First light receiving section, 12...Second light receiving section, 13...Third light receiving section 21...First polarizing beam splitter 22…Second polarizing beam splitter 23…Third polarizing beam splitter 31, 32, 33…Mirror 41, 42, 43, 44…Half mirror 51, 52, 53, 54, 71, 72, 81, 82...Retroreflector 60...Reference measurement light, 61...Reference measurement light 62...First reference measurement light, 63...Second reference measurement light, 64...Third reference measurement light

Claims

1. a reflective target having a first retroreflector, a second retroreflector adjacent to the first retroreflector in a first direction, and a third retroreflector adjacent to the first retroreflector in a second direction intersecting the first direction; an optical system including a light source that emits a laser beam, a first polarizing beam splitter, a second polarizing beam splitter, a first mirror, a second mirror, a third mirror, a first half mirror, a second half mirror, a first light receiver, and a second light receiver; the first polarizing beam splitter splits the laser light emitted from the light source into a first standard measurement light and a first reference measurement light, causes the first standard measurement light to be incident on the first retroreflector, and causes the first reference measurement light to be incident on the first half mirror; the first retroreflector reflects the first reference measurement light to the second half mirror; the second half mirror splits the first reference measurement light into a second reference measurement light and a third reference measurement light, transmits the second reference measurement light and makes it incident on the first polarizing beam splitter, and reflects the third reference measurement light and makes it incident on the second polarizing beam splitter; the first half mirror splits the first reference measurement light into a second reference measurement light and a third reference measurement light, causes the second reference measurement light to be incident on the second retroreflector via the first mirror, and causes the third reference measurement light to be incident on the third retroreflector via the second mirror and the third mirror; the second retroreflector reflects the second reference measurement light and makes the second reference measurement light incident on the first polarizing beam splitter via the first mirror; the third retroreflector reflects the third reference measurement light to make it incident on the second polarizing beam splitter; the first optical receiver photoelectrically converts a first interference signal obtained by interference between the second reference measurement light and the second standard measurement light in the first polarizing beam splitter; the second optical receiver photoelectrically converts a second interference signal obtained by interference between the third reference measurement light and the third standard measurement light in the second polarizing beam splitter; Multi-axis laser interferometer.

2. 2. The multi-axis laser interferometer according to claim 1, wherein the optical system calculates a first optical path difference between the first standard measurement light and the second reference measurement light based on a first interference fringe obtained by photoelectrically converting the first interference signal, detects an angle of a first rotation direction rotating on a plane intersecting the second direction based on the first optical path difference, calculates a second optical path difference between the first standard measurement light and the third reference measurement light based on a second interference fringe obtained by photoelectrically converting the second interference signal, and detects an angle of a second rotation direction rotating on a plane intersecting the first direction based on the second optical path difference.

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