1. Field of the Invention
This invention relates to an optical displacement measurement system for detecting the relative movement, if any, of a movable part of a semiconductor manufacturing apparatus, a machine tool or some other apparatus.
2. Description of Related Art
Optical displacement measurement systems utilizing a diffraction grating to detect the relative movement of a movable part of an apparatus such as a semiconductor manufacturing apparatus or a machine tool are known.
For example, FIGS. 1 and 2 of the accompanying drawings show a known optical displacement measurement system described in Japanese Patent Application Laid-Open No. 60-98302. FIG. 1 is a schematic perspective view of the known optical displacement measurement system 100 and FIG. 2 is a schematic view of the optical displacement measurement system 100 as viewed along arrow N1 in FIG. 1.
This known optical displacement measurement system 100 comprises a diffraction grating 101 adapted to linearly move in directions indicated respectively by arrows X1 and/or X2 in the drawings in response to a movement of the movable part of a machine tool, a coherent light source 102 for emitting a coherent laser beam, a halfmirror 103 for dividing the laser beam emitted from the coherent light source 102 into two beams and causing the two diffracted beams from the diffraction grating 101 to overlap and interfere with each other, a pair of mirrors 104a, 104b for reflecting the respective beams diffracted by the diffraction grating 101 and a photodetector 105 for receiving the two diffracted beams and generating an interference signal.
The laser beam emitted from the coherent light source 102 is split into two beams by the half mirror 103. Then, the two beams are made to strike the diffraction grating 101. The two beams striking the diffraction grating 101 are then diffracted by the diffraction grating 101 and leave the latter as diffracted beams. The two primary diffracted beams diffracted by the diffraction grating 101 are subsequently reflected by the mirrors 104a, 104b respectively. The diffracted beams reflected by the respective mirrors 104a, 104b are made to strike the diffraction grating 101 once again and diffracted by the diffraction grating 101 for another time before being returned to the half mirror 103, reversely following the same light paths. The diffracted beams returned to the half mirror 103 are caused to overlap and interfere with each other before being detected by the photodetector 105.
With the known optical displacement measurement system 100, the diffraction grating 101 moves in directions indicated by arrows X1, X2 respectively. Then, in the optical displacement measurement system 100, the two diffracted beams produced by the diffraction grating 101 show a phase difference as a function of the movement of the diffraction grating 101. Thus, the optical displacement measurement system 101 can determine the displacement of the movable part of the machine tool by detecting the phase difference of the two diffracted beams from the interference signal produced by the photodetector 105.
FIGS. 3 and 4 of the accompanying drawings show another known optical displacement measurement system described in Japanese Patent Application Laid-Open No. 60-98302. FIG. 3 is a schematic perspective view of the known optical displacement measurement system 110 and FIG. 4 is a schematic view of the optical displacement measurement system 110 as viewed along arrow N1 in FIG. 3.
This known optical displacement measurement system 110 comprises a diffraction grating 111 adapted to linearly move in directions indicated respectively by arrows X1 and/or X2 in the drawings in response to a movement of the movable part of a machine tool, a coherent light source 112 for emitting a coherent laser beam, a half mirror 113 for dividing the laser beam emitted from the coherent light source 112 into two beams and causing the two diffracted beams from the diffraction grating 111 to overlap and interfere with each other, a first pair of mirrors 114a, 114b for reflecting the respective beams diffracted by the diffraction grating 101 to a same and identical spot on the diffraction grating 111 and a second pair of mirrors 115a, 115b for reflecting the respective diffracted beams diffracted by the diffraction grating 111 and a photodetector 116 for receiving the two diffracted beams and generating an interference signal.
The laser beam emitted from the coherent light source 112 is split into two beams by the half mirror 113. Then, the two beams are reflected respectively by the first pair of mirrors 114a, 114b and made to strike the diffraction grating 101 at a same and identical spot. The two beams striking the diffraction grating 101 are then diffracted by the diffraction grating 111 and leave the latter as diffracted beams. The two primary diffracted beams diffracted by the diffraction grating 111 are subsequently reflected by the second pair of mirrors 115a, 115b respectively. The diffracted beams reflected by the second pair of mirrors 104a, 104b are made to strike the diffraction grating 101 once again and diffracted by the diffraction grating 111 for another time before being returned to the halfmirror 113, reversely following the same light paths. The diffracted beams returned to the half mirror 113 are caused to overlap and interfere with each other before being detected by the photodetector 116.
With the known optical displacement measurement system 110, the diffraction grating 111 moves in directions indicated by arrows X1, X2 respectively. Then, in the optical displacement measurement system 110, the two diffracted beams produced by the diffraction grating 111 show a phase difference as a function of the movement of the diffraction grating 111. Thus, the optical displacement measurement system 111 can determine the displacement of the movable part of the machine tool by detecting the phase difference of the two diffracted beams from the interference signal produced by the photodetector 116.
Now, with the trend of enhanced high precision of machine tools and industrial robots in recent years, optical displacement measurement systems of the type under consideration are required more often than not to have a position detecting capability with a degree of resolution of tens of several nanometers to several nanometers.
For an optical displacement measurement system to have a high degree of resolution, it is required to detect a large interference signal. Then, the two diffracted beams to be made to interfere with each other have to be overlapped with a very high degree of precision.
However, with either of the above described known optical displacement measurement systems 100, 110, the diffracted beams can become displaced from each other to abruptly dwarf the interference signal and make it impossible to detect the position of the movable part if the diffraction grating 101 or 111, whichever appropriate, is moved in a direction other than the right direction of movement or has undulations. For example, if the diffraction grating 101 or 111 is rotated in the directions of arrows A1 and A2 of B1 and B2 as shown in FIGS. 1 through 4, it is no longer possible to detect the position of the movable part of the machine tool that is under scrutiny.
FIG. 5 of the accompanying drawings shows an optical displacement measurement system 120 obtained by modifing the above described known optical displacement measurement system 100. Referring to FIG. 5, it has a first lens 106 for focussing the laser beams emitted from the coherent light source 102 on the mirrors 104a, 104b and a second lens 107 for focussing the two diffracted beams that have been made to overlap and interfere with each other by the half mirror 103 on the light receiving plane of the photodetector 105.
However, this optical displacement measurement system 120 is also not free from the above pointed out problem that the diffracted beams can become displaced from each other to abruptly dwarf the interference signal and make it impossible to detect the position of the movable part if the diffraction grating 101 is moved in a direction other than the right direction of movement or has undulations.
For instance, if the diffraction grating 101 is angularly moved by about 1/60 of a degree in the directions of arrows A1 and A2 and about 1/6 of a degree in the directions of arrows B1 and B2, the magnitude of the interference signal will change by 20%. If a reflection type diffraction grating is used, the angle of tolerance in the directions of arrow B1 and B2 will be reduced to a fraction of the above cited value to make it further difficult to detect the position of the movable part.
FIG. 6 of the accompanying drawings illustrates a known optical displacement measurement system described in Japanese Patent Application Laid-Open No. 2-167427.
Referring to FIG. 6, the optical displacement measurement system 130 comprises a diffraction grating 131 adapted to linearly move in directions indicated respectively by arrows X1 and/or X2 in the drawings in response to a movement of the movable part of a machine tool, a laser diode 132 for emitting a laser beam, a first half mirror 133 for dividing the laser beam emitted from the laser diode 132, first and second light receiving elements 134, 135 for receiving the two diffracted beams transmitted through the diffraction grating 131, a pair of lenses 136, 137 for focussing the two diffracted beams respectively and a second halfmirror 138 for separating and synthetically combining the two diffracted beams focussed by the pair of lenses 136, 137.
The optical displacement measurement system 130 further comprises a first pair of mirrors 139, 140 for reflecting the laser beams produced by the halfmirror 133 and causing them to strike the diffraction grating 131, a second pair of mirrors 141, 142 for reflecting the laser beams transmitted by the diffraction grating 131 and causing them to strike the half mirror 138, a 1/4 wave plate 143 and a first analyser 144 arranged between the first light receiving element 143 and the half mirror 138 and a second analyser 145 arranged between the second light receiving element 135 and the half mirror 138.
In the optical displacement measurement system 130, the first and second lenses 136, 137 are arranged in such a way that they focus respective beams on the diffraction plane or the refraction plane of the diffraction grating 11. Therefore, the diffracted beams respectively striking the first and second light receiving elements are always held in parallel with each other and the interference signal will fluctuate little if the diffraction grating 131 shows undulations.
However, the proposed optical displacement measurement system 130 only ensures the parallelism of the two diffracted beams. That is, if the diffraction grating 131 is inclined, a uniform interference will be maintained only in the shaded area in FIG. 7 where the two beams are made to overlap with each other. In other words, the two diffracted beams do not interfere with each other in areas other than the area where the two beams are made to overlap with each other so that consequently the obtained interference signal will become dwarfed. Additionally, if the two beams are not strictly parallel relative to each other and involve aberration in any sense of the word, no uniform interference will be ensured even in the area where the two beams are made to overlap with each other.