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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical apparatus for detecting the
position of an object by sensing the light from the object through an
objective lens. More specifically, the present invention relates to an
optical position-detecting apparatus of the type in which a laser beam is
projected onto an object to be examined through an objective lens and the
scattered or diffracted light from the object is received to detect the
position of the object.
2. Related Background Art
In the known projection type exposure apparatus now used in the manufacture
of semiconductor elements such as LSI, an optical alignment system is
needed to align a reticle (or a mask) with a wafer very precisely. An
optical alignment system making use of laser beam has been known and
widely used for this purpose.
In this alignment optical system according to the prior art, the position
of a wafer as an object to be examined is detected in the following
manner:
A laser beam is focused on the wafer. The focused light is reflected,
scattered or diffracted by the surface of the wafer having an alignment
mark preformed. The reflected, scattered or diffracted light is received
through a spatial filter disposed at the pupil of an objective lens or at
a position conjugate with the pupil of the objective. The received light
contains information of the position of the alignment mark. The position
of the alignment mark is detected as a position on a coordinate system
orthogonal to the optical axis of the objective lens to know the exact
position of the wafer.
The function of the spatial filter used in the above apparatus is to cut
off the normal reflection light from the object surface and to transmit
only the scattered light or the diffracted light derived from the
alignment mark. Through the spacial filter it is possible to detect the
alignment mark in a dark view field and, therefore, it is possible to
improve the accuracy in the detection of position.
However, the prior art apparatus involves a problem of noise. A laser beam
usually contains not only parallel rays to the optical axis but also some
oblique rays which have a small angle of divergence relative to the
optical axis (hereinafter, such oblique rays of the laser are referred to
as the divergent rays). This is true also for gas laser which is generally
regarded as a beam of parallel rays. These oblique rays produce a halo,
that is, an area lightly blurred with light surrounding the entrance pupil
of the objective lens. Because of this phenomenon, when the light from the
object is filtered at the entrance pupil or at a conjugated point with it,
the divergent rays form noise which disturbs the correct detection of
object position.
SUMMARY OF THE INVENTION
Accordingly, the principal object of the present invention is to provide an
optical apparatus for the detection of position using a laser beam which
is able to detect the position of an object with high accuracy
irrespective of the existence of those rays which have an angle of
divergence.
According to the present invention, the above object is attained by an
optical apparatus comprising a laser light source; an optical system for
focusing a beam of light from said laser light source onto an object
surface through an objective lens; and a spatial filter disposed at the
entrance pupil of said objective lens with respect to the side of said
laser light source or at a position conjugated with the entrance pupil,
wherein the origin of the divergence of the laser beam having an angle of
divergence emitted from said laser light source is projected on the
entrance pupil of said objective lens by said optical system.
With the construction of the position-detecting optical apparatus according
to the invention, even divergent rays of a laser beam are focused on the
entrance pupil of a laser beam and, therefore, there is produced no halo
of the laser beam on the entrance pupil. This assures good filtering by
the spatial filter at the entrance pupil or at a conjugated point with the
entrance pupil. A substantial improvement in the accuracy of position
detection can be attained by it.
Other objects, features and advantages of the invention will appear more
fully from the following description taken in connection with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically shows a first embodiment of the invention;
FIG. 2A is a plan illustrating the state of a beam on the entrance pupil of
the projection objective lens of the first embodiment;
FIG. 2B is a plan of the spatial space filter of the first embodiment;
FIG. 3A is an illustrative view showing the state of a beam on the entrance
pupil of the projection lens when the position of the light source is
deviated in the direction along the optical axis in FIG. 1;
FIG. 3B is an illustrative view showing the form of the spatial filter when
the position of the light source is deviated in the direction along the
optical axis in FIG. 1;
FIG. 4A is a schematic exploded view of the optical path in cross section
taken in the sagittal direction of a second embodiment in which the
present invention has been applied to an alignment optical system in a
minifying projection type exposure apparatus;
FIG. 4B is a schematic exploded view of the optical path of the second
embodiment in cross section taken in the meridional direction;
FIG. 5 is a plan view showing the form of a beam spot focused on an object
surface in the second embodiment;
FIG. 6 is a plan view showing the state of a beam on the entrance pupil of
the projection objective lens in the second embodiment;
FIG. 7 is a plan view illustrating the forms of beam spot and alignment
mark used in the second embodiment and their positional relationship as an
example;
FIG. 8 is a plan view illustrating the state of a beam on the spatial
filter in the second embodiment;
FIG. 9 shows an output waveform as an example of the output of the
photo-sensor in the second embodiment;
FIG. 10 is an illustrative view showing the state of the beam on the
entrance pupil of the projection objective lens when the position of the
light source is deviated in the direction along the optical axis in FIG.
4A; and
FIG. 11 is an illustrative view showing the state of the beam on the
spatial filter when the position of the light source is deviated in the
direction along the optical axis in FIG. 4A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1 there is shown a first embodiment of the optical
apparatus for the detection of position according to the present
invention. In this embodiment, the present invention has been applied to
the alignment optical system of a minifying projection type exposure
apparatus used for the manufacture of semiconductor elements such as LSI.
In the figure the reference numeral 1 denotes a laser light source which
emits a beam of laser light. The beam is expanded in beam width by a beam
expander 2 at a predetermined magnification and then passes through a beam
splitter 3. After passing through the beam splitter, the beam is once
focused on a reticle R by a condenser lens 4. L.sub.o denotes a
telecentric projection objective lens. W denotes a wafer as the object
whose position is to be detected. The wafer W is on a stage. The reticle R
and the wafer W are so disposed as to hold a conjugated relation
therebetween relative to the objective L.sub.o. The laser beam transmitted
through the reticle R is refocused on the wafer W through the objective
L.sub.o.
The reflected light (including scattered light and diffracted light) from
the wafer W enters again the objective L.sub.o. After passing through the
reticle R and the condenser lens 4, the beam is reflected toward a relay
lens 5 by the beam splitter 3. The relay lens 4 transmits the reflected
beam to a photo-sensor 6 which generates a signal corresponding to the
intensity of the light then received. F.sub.1 is a spatial filter on which
an entrance pupil plane P.sub.1 of the projection objective lens L.sub.o
with respect to the side of the laser light source is projected through
the condenser lens 4 and the relay lens 5. The beam of reflected light
from the wafer W is filtered by this spatial filter F.sub.1.
The laser beam emitted from the laser light source 1 contains not only
those rays running parallel to the optical axis 10 but also such rays
having an angle .epsilon. of divergence as suggested by two-dotted chain
lines in FIG. 1. The origins of divergence angle .epsilon. of these rays
are distributed on a plane P.sub.o normal to the optical axis at the
so-called beam waist. Therefore, the laser radiation beam appears as if it
was emitted from the plane P.sub.o. The rays of the beam may be regarded
as being radiated from the plane P.sub.o. We, therefore, refer to the
plane P.sub.o as virtual radiation plane.
An important feature of the present invention is that the virtual radiation
plane P.sub.o is projected on the entrance pupil plane P.sub.1 of the
projection objective lens L.sub.o through an optical system including the
beam expander 2, the condenser lens 4 etc. In FIG. 1, a beam having an
angle of divergence .epsilon. is emitted from a point A on one end of the
virtual radiation plane P.sub.o and focused at a point a on the entrance
pupil plane P.sub.1 of the objective lens. Similarly, another beam having
a divergent angle .epsilon. is emitted from a point B on the opposite end
of the virtual radiation plane P.sub.o and is focused at a point b on the
entrance pupil plane P.sub.1 of the projection objective lens. Therefore,
as seen from FIG. 2A, all of the radiation rays from the laser light
source 1 pass through the entrance pupil plane P.sub.1 of the lens L.sub.o
exactly within a circular area S. There is no ray blurred out of the
circular area S.
In order to project the virtual radiation plane P.sub.o on the entrance
pupil plane P.sub.1 of the projection lens L.sub.o and to focus the beam
of the divergent angle .epsilon. from the plane P.sub.o on the entrance
pupil plane P.sub.1 of the lens L.sub.o, it is necessary for the beam
expander 2 to be formed as a so-called Kepler type afocal system composed
of two lens groups 21 and 22 as shown in FIG. 1. If a Galileo type afocal
system is used as the beam expander, the projection of the virtual
radiation plane P.sub.o on the entrance pupil plane P.sub.1 can be
performed only by a very complicated construction because it is required,
for example, to add a Kepler type afocal system to the Galilean afocal
system. In this case, it is hardly possible to completely exclude the
blurring of light caused by the divergent angle .epsilon. of the laser
beam.
As previously mentioned, the wafer W has an alignment mark formed on the
surface. The alignment mark is a part projecting from the surface of the
wafer. The rays normally reflected by the wafer W return back into the
circular area S on the entrance pupil plane P.sub.1 of the projection
objective lens L.sub.o. On the contrary, those rays which are scattered by
the edges of the alignment mark and reflected back to the entrance pupil
plane P.sub.1, do not enter the circular area S but enter the area outside
of the circular area S as indicated by dots D in FIG. 2A.
In order to detect these scatter-reflected rays D, there is provided a
spatial filter F.sub.1 at the entrance pupil plane P.sub.1 of the
projection objective lens L.sub.o or at a point P.sub.2 conjugated with
the entrance pupil plane as shown in FIG. 1. FIG. 2B shows the form of the
spatial filter F.sub.1. The spatial filter has a circular screen area of d
in diameter for blocking off the normal reflection light returning back
along the optical axis from the wafer. The scatter-reflected light from
the alignment mark passes through the filter at the area outside of the
screen area. Therefore, the alignment mark can be detected by sensing the
scatter-reflected light by a photo-sensor 6 in a dark viewfield.
In the apparatus according to the invention, as described above, the laser
beam is definitely shaped without any blurring on the entrance pupil plane
P.sub.1 of the projection lens L.sub.o. The area S in which the normal
reflection light from the wafer W returns can be defined clearly in the
entrance pupil plane P.sub.1. This means that the size of the screen area
(the size of the circular area having a diameter d in FIG. 2B) which the
spatial filter F, should have on the optical axis can be reduced to the
minimum and that the largest amount of the scatter-reflected light from
the alignment mark on the wafer can be received for the detection of the
mark. Thus, according to the present invention, it is possible to
remarkably improve the S/N ratio in the detection signal of an alignment
mark on the wafer W.
The advantage of the apparatus of the present invention over the prior art
is obvious from FIGS. 3A and 3B.
In the prior art apparatus, the angle of divergence .epsilon. of the laser
beam has not been taken into consideration. Therefore, when the virtual
radiation plane P.sub.o of the laser light source 1 is at a position
deviated from the conjugated position with the entrance pupil plane, such
rays having a divergent angle .epsilon. of the laser beam blur out into a
circular area Se (indicated by a broken line in FIG. 3A) on the entrance
pupil plane P.sub.1 of the projection objective lens L.sub.o. As readily
seen from FIG. 3B, the spatial filter F.sub.1 is required to have a larger
screen area of de in diameter (de>d) on the optical axis in order to block
off the normal reflection light from the wafter W. Because of the large
size, the screen area of the spatial filter undesirably cuts off some of
the weak scatter-reflected light from the alignment mark which is to be
detected. Consequently, the S/N ratio is low in the prior art apparatus.
A second embodiment of the present invention is shown in FIGS. 4A and 4B in
which like reference characters to FIG. 1 represent functionally the same
members and elements.
Referring to FIG. 4A which is a sectional view taken in the sagittal
direction of the second embodiment, a laser beam from a laser light source
1 is expanded to a determined beam width by a beam expander 2. The beam
expander 2 is of the Kepler type afocal system composed of two positive
lenses 21 and 22.
Herein, for the purpose of explanation, X-Y-Z three-dimensional coordinate
are used. The section in the sagittal direction is on the X-Z plane and
that of the meridional direction is on the Y-Z plane of the
three-dimensional coordinates.
The expanded parallel beam is converged by a cylindrical lens L.sub.c which
has a converging action in the sagittal plane. Further, the beam is
collimated by a first condenser lens 41. After passing through a beam
splitter 3, the collimated beam is focused on a reticle R by a second
condenser lens. The beam focused on the reticle R is in the form of a
light spot elongated in the Y direction. The laser beam once focused on
the reticle R is refocused, as a light spot elongated in the Y direction,
on a wafer W conjugated with the reticle R regarding the projection
objective lens L.sub.o.
FIG. 4B shows a meridional section of the second embodiment. In the
meridional plane, the cylindrical lens L.sub.c has no refractive power.
Therefore, the parallel beam expanded by the Kepler type afocal system
beam expander 2 enters the first condenser lens 41 as it is. After passing
through the beam splitter 3 and the second condenser lens 42, the laser
focuses at the entrance pupil P.sub.1 of the projection lens L.sub.o and
further the principal rays emerge the lens L.sub.o as rays running
parallel with the optical axis of the lens to telecentrically illuminate
the wafer W.
In the second embodiment in which a cylindrical lens L.sub.c is
additionally provided, the laser beam on the wafer W is in the form of a
light spot B.sub.1 elongated in the meridional direction (Y direction) as
shown in FIG. 5. On the other hand, the laser beam on the entrance pupil
P.sub.1 of the projection lens L.sub.o is in the form of a light spot
B.sub.2 elongated in the sagittal direction (X direction) as shown in FIG.
6. Between the cylindrical lens L.sub.c and the first condenser lens 41
there is provided a slit 7 at a point where the laser beam and the optical
axis intersect. The slit 7 is elongated in the sagittal direction (X
direction).
The function of the elongate slit 7 is to screen off the marginal portion
in the meridional direction (Y direction) of the laser beam whose
intensity distribution is a Gaussian distribution, thereby making the
intensity distribution more uniform in the longitudinal direction of the
light spot focused on the object surface.
FIG. 7 shows an embodiment of the alignment mark formed on the wafer W.
In this embodiment, the alignment mark M is composed of square projection
patterns C.sub.1, C.sub.2, C.sub.3. arranged in a straight line in Y
direction. This alignment mark M is scanned relatively by the laser light
spot B.sub.1 linearly elongated in Y direction.
When the light spot B.sub.1 comes into a position to illuminate the
alignment mark M, there is produced scattered light in X direction at
first by the edges a along Y of the patterns C.sub.1, C.sub.2, C.sub.3. .
. The edges b along x of the patterns C.sub.1, C.sub.2, C.sub.3. .
practically function as diffraction grating grooves and, therefore, there
is produced diffracted light in Y direction by these X side edges b of the
patterns.
FIG. 8 shows the form of a spatial filter F.sub.2 disposed conjugated with
the pupil P of the lens L.sub.o. At two off-axial positions which are
substantially symmetric in X direction, the spatial filter has openings
A.sub.11 and A.sub.12. The above-mentioned scattered light in the X
direction produced by the Y side edges a of the alignment mark M (see FIG.
7) can pass through the openings A .sub.11 and A.sub.12 of the spatial
filter F.sub.2.
The edge of the alignment mark and, therefore, the position of the wafer
can be detected by detecting the scattered light in the X direction and
the diffracted light in the Y direction. In this manner one can obtain a
detection signal. FIG. 9 shows a waveform of the output obtained by
receiving the scattered light in X direction passed through the spatial
filter F.sub.2 in the above-described manner. By means of such an output
signal, one can detect the position of the alignment mark M, that is to
say, the position of the wafer.
The first order and second order diffracted light produced by the X side
edges b of the mark patterns C.sub.1, C.sub.2, C.sub.3 . . . is
distributed over the off-axial areas on the entrance pupil plane P.sub.1
of the lens L.sub.o and on a plane conjugated with the entrance pupil. The
distribution of the first order and second order diffracted light in Y
direction is indicated by .+-.D.sub.1 and .+-.D.sub.2 in FIG. 8. The
position of the alignment mark M can be detected also by detecting the
diffracted light in Y direction produced by X side edges b of the
alignment mark independently of the scattered light in X direction
produced by the Y side edges a of the mark. To this end, such a spatial
filter is used which has off-axial openings symmetrically arranged in the
Y direction.
In the alignment optical system using the above-shown alignment mark, high
accuracy in the detection of position is assured only when the angle of
divergence .epsilon. of the laser beam is taken into consideration
according to the feature of the present invention. If the divergent angle
.epsilon. is not taken into consideration for the above embodiment, there
is produced a light-blurring area B.sub.2 e as shown in FIG. 10 at the
entrance pupil P.sub.1 of the projection lens L.sub.o like the
light-blurring area Se previously shown in FIG. 3A.
In FIG. 11, Do denotes the light directly reflected by the alignment mark
M. Doe denotes blurring marginal rays of the normally reflected light. As
shown in FIG. 11, some quantity of the blurring marginal rays Doe mingle
in the openings A.sub.11 and A.sub.12 which have been provided for
exclusively allowing the scattered light in X direction to pass through.
The blurring light Doe passing through the openings A.sub.11 and A.sub.12
is undersirably sensed by the photo-sensor 6 disposed only for receiving
the scattered light in X direction from the edge of the alignment mark M.
Therefore, the output of the photo-sensor 6 contains noise as suggested by
a phantom line in FIG. 9. Since such noise is generated in the apparatus,
the accuracy in the detection of position is decreased as compared with
the above embodiment of the invention.
As described above, in the second embodiment of the alignment optical
system according to the invention, a cylindrical lens L.sub.c has been
used to form a linear light spot B.sub.1 on an object W. Even for this
embodiment, the accuracy of filtering by the spatial filter F.sub.2 can be
improved by projecting the virtual radiation plane R.sub.o of the laser
beam on the entrance pupil plane P.sub.1 of the objective lens L.sub.o in
the plane in which the cylindrical lens L.sub.c has a refractive power, as
indicated by the broken line in FIG. 4A. Even in this case, it is
preferable to use a Kepler type afocal system as the beam expander.
If X-rays are used to carry out the exposure on the wafer, the projection
lens L.sub.o is omitted. Therefore, in this case it is necessary to
provide an objective lens on the laser light source side relative to the
reticle for the purpose of alignment.
As readily understood from the foregoing, the apparatus for the detection
of object position according to the present invention has advantages over
the prior art. It assures better spatial filtering and higher accuracy in
the detection of object position in spite of the existence of a divergent
angle of the laser beam.
Accordingly, the present invention has improved the accuracy of alignment
for the projection type exposure apparatus necessary for the manufacture
of semiconductor elements. It contributes to the production of
semiconductor elements the fineness of which is increasing more and more.
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