Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-07-01
2002-06-18
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S329000
Reexamination Certificate
active
06407815
ABSTRACT:
BACKGROUND OF THE INVENTION
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 N
1
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 X
1
and/or X
2
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 half mirror
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
104
a
,
104
b
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
104
a
,
104
b
respectively. The diffracted beams reflected by the respective mirrors
104
a
,
104
b
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 X
1
, X
2
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 N
1
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 X
1
and/or X
2
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
114
a
,
114
b
for reflecting the respective beams diffracted by the diffraction grating
111
to a same and identical spot on the diffraction grating
111
and a second pair of mirrors
115
a
,
115
b
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
114
a
,
114
b
and made to strike the diffraction grating
111
as a same and identical spot. The two beams striking the diffraction grating
111
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
115
a
,
115
b
respectively. The diffracted beams reflected by the second pair of mirrors
104
a
,
104
b
are made to strike the diffraction grating
111
once again and diffracted by the diffraction grating
111
for another time before being returned to the half mirror
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 X
1
, X
2
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 A
1
and A
2
of B
1
and B
2
as shown in
FIGS. 1 through 4
, it is no longer possible to detect the position of the movable part or the machine tool that is under scrutiny.
FIG. 5
of the accompanying drawings shows an optical displacement measurement system
120
obtained by modifying 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
104
a
,
104
b
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
.
Howe
Font Frank G.
Maioli Jay H.
Natividad Phil S.
Sony Precision Technology Inc.
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