Optical apparatus for the detection of position

An optical apparatus for detecting the position of an object by projecting a light image onto the object through a transparent plate and detecting the reflected light from the object through the transparent plate is disclosed. The projection optical system includes a first objective lens for forming the light image. The optical axis of the projection is disposed inclined to the above-said transparent plate. The detection optical system includes a second objective lens for refocusing the light reflected from the object. The optical axis of the reflection light detection between the second object lens and the object is disposed inclined to the transparent plate. The apparatus further comprises plane parallel optical members mounted obliquely so as to cancel the asymmetric aberrations generated by the above-said transparent plate. One of the aberration-correcting plane parallel optical members is interposed in the projection optical path on the side opposite to the transparent plate relative to the first objective lens. The other one is interposed in the detection optical path on the side opposite to the transparent plate relative to the second objective lens.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to a position detection optical apparatus in 
which a light spot is obliquely projected onto an object surface to be 
examined and the reflected light from the object surface is detected to 
measure the position of the object surface. More particularly, the present 
invention relates to such optical apparatus for the detection of position 
in which the detection is carried out obliquely through a transparent 
plate covering the object surface to be examined. 
2. Related Background Art 
Projection optical systems for projecting a particularly shaped light spot 
onto an object surface and detection optical systems for detecting the 
reflected light from the object surface are all known in the art. 
In this kind of the optical system it is most desirable to use such optical 
elements having an isotropic form relative to the optical axis of the 
optical system, that is, optical elements having a rotation-symmetrical 
configuration relative to the optical axis. By doing so, generally the 
most optimum construction can be obtained in view of the imageforming 
performance of the optical system. When the optical system contains one or 
more plane parallel transparent members such as window glass and prism 
block, the plane parallel member is usually disposed with its light 
incidence and exit surfaces being normal to the optical axis. In other 
words, it is a common practice in the art to arrange such plane parallel 
member in the position in which the normals to the respective planes of 
the member extend in parallel to the optical axis of the optical axis. 
However, there may be such cases where the plane parallel member can not be 
disposed normally to the optical axis but it must be disposed inclined to 
the optical axis owing to some limitations or special conditions and man 
has to carry out the detection of position through the inclined plane 
parallel member. 
For example, the following cases can be mentioned for it: 
Man views obliquely an object surface through a window glass mounted 
extending in parallel to the object surface; 
Man views an object contained in a sealed container obliquely and through a 
window glass; and 
Man carries out the detection of focus to such an object contained in a 
container obliquely and through a window glass. 
In these cases, the plane parallel member such as glass plate existing in 
the optical path is inclined. Therefore, if the light running along the 
optical path in which the plane parallel member is existing is not a 
collimated light beam, then there may be produced aberrations even at the 
object point on axis. The aberrations are asymmetrical to the optical 
axis. In this case, therefore, it is impossible to form a sharp and clear 
image of the object. The generation of such aberrations is more remarkable 
with increasing the inclination and thickness of the plane parallel 
member. 
The reduction of optical performance of the optical system as mentioned 
above may be obviated by disposing the plane parallel member in a 
collimated light beam. However, to form the optical path of a collimated 
light beam, a long optical system is required, which is against the 
realization of the apparatus having a small and compact construction. 
The above-mentioned problem is present also in the manufacture of 
semiconductor devices employing the so-called projection exposure 
apparatus. 
As well known, in the manufacture of semiconductor devices with the 
projection exposure apparatus, there is the case where a wafer is 
contained in an air-tightly closed container filled with a particular gas. 
In this case, exposure to the wafer is carried out vertically from above 
through a glass window of the gas container. Under the condition, the 
focusing and alignment for the wafer must be carried out obliquely through 
the same window glass. 
Many other limitations and conditions are known for this kind of the 
optical system under which we are obliged to dispose a plane parallel 
member (window glass) in a convergent beam or in a divergent beam 
(hereinafter referred to as non-collimated beam inclusively) or we can not 
form the desired collimated beam system. In these cases also the 
image-forming performance of the optical system is inevitably dropped down 
by the generation of asymmetric aberrations and we can not improve the 
accuracy of detection. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to overcome 
the drawbacks of the prior art apparatus as mentioned above. 
More specifically, it is an object of the invention to provide a 
position-detecting optical apparatus in which those asymmetric aberrations 
are corrected which are generated when an image of an object is formed 
through a plane parallel member obliquely disposed to the optical axis. 
It is another object of the invention to provide a position-detecting 
optical apparatus which enables to detect the position of an object 
surface with very highly improved accuracy. 
To attain the above objects, the present invention provides an optical 
apparatus for detecting the position of an object by projecting a light 
image having a determined shape onto the object through a transparent 
plate and detecting, through the transparent plate, the reflected light 
from the object on which the light image has been projected. The apparatus 
comprises a projection optical system and a detection optical system. The 
projection optical system includes a first objective lens for forming the 
light image having a determined shape on the object. The optical axis of 
the projection optical system through which the light image formed by the 
objective lens is projected onto the object surface is disposed inclined 
to the above-said transparent plate. The detection optical system includes 
a second objective lens for refocusing the light reflected from the 
object. The optical axis of the reflection light detecting optical system 
between the second object lens and the object is disposed inclined to the 
transparent plate. The apparatus further comprises plane parallel optical 
members mounted obliquely so as to cancel the asymmetric aberrations 
generated by the above-said transparent plate. To this end, one of the 
aberration-correcting plane parallel optical members is interposed in the 
projection optical path on the side opposite to the transparent plate 
relative to the first objective lens. The other one is interposed in the 
detection optical path on the side opposite to the transparent plate 
relative to the second objective lens. 
In a preferred embodiment of the invention, the transparent plate and the 
plane parallel optical member have the same optical path length. In other 
words, they have substantially the same product of refractive index and 
thickness. Further, the part of the non-collimated beam passing through 
the transparent plate and the part of the non-collimated beam in which the 
plane parallel optical member is interposed, have nearly the same 
numerical aperture. In this embodiment, the aberrations generated by the 
transparent plate can be corrected very well by the plane parallel optical 
members. 
The term "the part of the non-collimated beam in which the plane parallel 
optical member is interposed" as used in the above description means the 
divergent beam incident to the object lens in the projection optical 
system, the convergent beam exiting from the objective lens in the 
detection optical system or a divergent or convergent beam incident to or 
exiting from a relay lens or the like optionally arranged next to the 
objective lens. 
The part of beam passing through the transparent plate and the part of beam 
in which the plane parallel optical member is interposed should have 
substantially the same maximum angle of divergence (corresponding to the 
numerical aperture NA). 
The non-collimated beam part in which the plane parallel optical member is 
interposed may be on the optical axis of the objective lens or on the 
optical axis of a relay lens. More concretely, when the transparent plate 
and the plane parallel optical member are arranged with the objective lens 
therebetween, it is preferable to arrange the two members in parallel to 
each other. In the case where the optical system additionally comprises a 
relay lens and the plane parallel optical member is disposed in the 
convergent beam from the relay lens of the projection optical system or in 
the divergent beam from the relay lens of the detection optical system, it 
is preferable to set the plane parallel optical member in such manner that 
it is inclined in the opposite direction to the transparent plate relative 
to the optical axis. In other words, the transparent plate and the plane 
parallel optical member are preferably arranged symmetrically each other 
relative to the plane normal to the optical axis. 
According to the present invention having the features as described above, 
when an image of light on an object is to be detected through a 
transparent plate inclined to the optical axis, the asymmetric aberration 
of the light image can be corrected very well. 
Therefore, in the apparatus according to the invention, the image-forming 
performance is maintained always good however large the thickness and/or 
the inclination of the slant transparent plate may be. This enables to 
increase up the freedom for optical design and to improve the accuracy for 
the detection of the object position. 
Other and further objects, features and advantages of the present invention 
appear more fully from the following description taken in connection with 
the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an embodiment of the present invention as a position-detection 
optical apparatus adapted for the detection of focus position of a 
projection type exposure apparatus for the manufacture of semiconductor 
devices. 
Designated by 1 is a reticle having a pattern formed thereon. An image of 
the pattern is projected on a wafer 3 through a projection objective lens 
2. The wafer has a coating of sensitizer which is exposed to the projected 
pattern. 
The wafer 3 is in a closed container composed of a box-like main body 31 
and a transparent window 30. The container is filled with a special gas. 
Therefore, the exposure on the wafer 3 is carried out in the special gas 
by projecting the pattern image on the wafer by the projection lens 2 
through the transparent window glass 30. 
In the above-mentioned type of exposure apparatus there is generally 
provided a position detection apparatus for aligning the wafer 3 with the 
projection objective lens 2, especially for positioning the wafer 3 
correctly at the focus position of the lens 2. 
The function of the position-detecting optical apparatus is to attain a 
precise alignment of the wafer 3 with the focus position of the projection 
objective lens 2. The function for alignment must be performed so as not 
to interfere with the exposure on the wafer through the projection lens 2. 
To this end, a beam of light for alignment is projected onto the wafer 
through the window 30 in a direction largely inclined relative to the 
optical axis of the projection lens 2. The alignment beam is focused into 
a light spot having a determined shape on the wafer through the 
position-detection optical apparatus. The surface of the wafer 3 reflects 
the projected light. The detection optical apparatus detects the reflected 
light from the wafer to measure the position of the wafer exactly. 
In the first embodiment shown in FIG. 1, the position-detection optical 
apparatus is formed as a slit image type of position detection unit. It 
comprises a slit image projection system 10 and a slit image detection 
system 20. 
In the slit image projection system 10, a light source 11 which may be, for 
example, a light-emitting diode emits illumination light to illuminate a 
slit plate 12. The light beam passed through the slit plate 12 is once 
focused through a relay lens 13 and then the beam is focused on the wafer 
surface by a first objective lens 14 through the transparent window glass 
30. 
The focused beam on the wafer forms a light spot as an image of the slit. 
Since the slit in the slit plate 12 is so formed that the longitudinal 
direction of the slit corresponds to the direction normal to the incidence 
plane of the incident light to the wafer 3 (the incidence plane is a plane 
parallel with the plane of the drawing), the light spot of the slit image 
formed on the wafer 3 is a spot elongated in the direction normal to the 
plane of the drawing. 
The slit image once focused by the relay lens 13 is refocused on the wafer 
as the elongated light spot by the objective lens 14 with one-to-one 
magnification. On the side of the relay lens 13, a first plane parallel 
plate 41 is interposed in the divergent beam toward the objective lens 14. 
The function of the first plane parallel plate 41 is to correct the 
asymmetirical aberration of which a detailed description will be made 
later. 
The first plane parallel plate 41 has the same thickness and the same 
refractive index as the transparent window glass 30 does. Furthermore, the 
plane parallel plate 41 is disposed parallel to the window glass 30. For 
the reasons as will be described later, those asymmetric aberrations can 
be compensated well by it which may be produced by the transparent window 
glass 30 inclined to the optical axis of the first objective lens 14. 
In the manner described above, a beam of light is projected on the wafer 3 
(an object to be measured) and the wafer reflects the light. The reflected 
light enters the light spot (slit image) detection system 20. 
In the detection system 20, the reflected light is once focused through a 
second objective lens 23 and then it is refocused on a detection slit 22 
through a relay lens 23. Thus, an image of the slit 12 is again formed on 
the detection slit 22. The second objective lens 24 refocuses the light 
spot of the slit image on the wafer with one-to-one magnification. 
The detection slit is mounted for oscillation in the direction normal to 
longitudinal direction of the slit image in a plane normal to the optical 
axis. Disposed behind the detection slit is a photo sensor 21 the output 
of which is modulated in accordance with the oscillation of the detection 
slit 22. The positional relation between lens 2 and wafer 3 can be 
determined by synchronous detection of the output from the sensor 21 with 
the oscillation frequency of the detection slit 22. 
As the transparent window glass 30 is inclinded to the optical axis of the 
second objective lens 24, a second plane parallel plate 42 is provided in 
this detection system 20 to compensate the asymmetric aberration. The 
plane parallel plate 42 is interposed in the convergent light beam from 
the objective lens 21. Again, the plane parallel plate 42 has the same 
thickness and the same refractive index as the window glass 30, and the 
plate 42 is disposed parallel to the window glass 30. 
Since the objective lens 24 is a one-to-one magnification image-forming 
optical system, the divergent beam passing through the window glass 30 and 
the convergent beam passing through the plane parallel plate 42 have the 
same angle of divergence. Consequently, the asymmetric aberrations 
generated by the window glass 30 are cancelled by the second plane 
parallel plate 42. Thus, also in the detection system, the asymmetric 
aberrations are compensated well and the focus detection can be achieved 
at very high degree of accuracy. 
In this embodiment, a slit image is projected on an object. Therefore, the 
ability essential for the optical system of the position-detecting optical 
apparatus is the image-forming ability only in widthwise direction of the 
slit. So long as the slit is disposed on the optical axis of the optical 
system, the off-axial image-forming ability may be left out of 
consideration. 
As will be described later, astigmatic difference is produced by the 
insertion of the plane parallel plate. But, in this embodiment employing 
the slit image projection system, the astigmatic difference constitutes no 
problem. 
It is unnecessary for this position-detecting optical apparatus to include 
a cylindrical lens for correcting the astigmatic difference. However, when 
the position detection is carried out according the point image projection 
method, it is needed to compensate the astigmatic difference by use of a 
cylindrical lens as shown later. 
Further, if it is wished to observe a broader area of the wafer surface 
obliquely without any interference with the optical path of the projection 
objective lens 2, not only the astigmatic aberration but also asymmetric 
field curvature must be compensated by use of a cylindrical lens. 
FIG. 2 shows a second embodiment of the invention. In this second 
embodiment, a pin hole is used instead of the slit plate 12 used in the 
first embodiment, and a point image is projected on an object to be 
measured. 
Like the first embodiment, this second embodiment is composed of a 
projection system and a detection system. Since the two systems are 
arranged substantially symmetrically to each other and have functionally 
the same elements, only the projection system is shown in FIG. 2. As for 
the detection system, only the reference numerals of the elements are 
given in the figure. 
In the second embodiment, a pinhole 12A is illuminated by an illumination 
light source such as a light-emitting diode not shown. A light image 
I.sub.1 of the pinhole is once focused by a relay lens 13 of the 
projection system. A first objective lens 14A refocuses a secondary light 
image I.sub.2 on the object (wafer) 3 as a light spot. The object 3 is 
contained in a closed container having a transparent window glass 30 as 
shown in FIG. 1. The window glass 30 is in the convergent beam between the 
lens 14A and the secondary light image I.sub.2 and inclined to the optical 
axis. The transparent window glass 30 produces asymmetric aberrations. In 
order to compensate the aberrations, a first plane parallel plate 41 is 
interposed in the divergent beam incident to the first objective lens 14A 
composed of two positive lens components a and c and a cylindrical lens b 
between the two positive lens components. As to the cylindrical lens b, a 
further description will be made later. 
The magnification of the first objective lens 14A is one-to-one (by -1). 
The divergent beam in which the first plane parallel plate 41 is 
interposed and the convergent beam in which the window glass 30 is 
interposed have the same angle of divergence. The first plane parallel 
plate 41 and the window glass 30 have the same thickness and the same 
refractive index. Further, they have the same inclination angle to the 
optical axis A and they are parallel to each other. 
The light image (light spot) I.sub.2 is reflected on the object 3. The 
reflected light travels along the optical axis of the detection system in 
the same manner as in the first embodiment previously shown in FIG. 1. 
After passing through the transparent window glass 30 inclined to the 
optical axis, the reflected light enters a second objective lens 24 
containing a cylindrical lens b as shown in FIG. 2. A light image I.sub.3 
conjugated with the above light image I.sub.2 is formed by the objective 
lens 24A through a second plane parallel plate 42 provided to compensate 
aberration. Further, through a relay lens 23, the light image is refocused 
on a photo sensor such as a divided sensor not shown. The position of the 
object 3 is detected from the position at which the light image is formed 
on the sensor. In the detection system also, the second objective lens has 
a magnification of one-to-one. Therefore, the convergent beam in which the 
second plane parallel plate is interposed and the divergent beam in which 
the window glass 30 is interposed have the same angle of divergence. 
Further, the plate 42 and the glass 30 have the same thickness and 
refractive index. They are parallel to each other and have the same 
inclination relative to the optical axis A. 
The optical actions of the transparent window glass and the plane parallel 
plate in the embodiment shown in FIG. 2 will be described hereinafter with 
reference to FIGS. 3A and 3B. 
FIG. 3A illustrates the state of the light beam passing through the 
aberration-compensating plane parallel plate 41(42) and FIG. 3B 
illustrates that of the light beam passing through the transparent window 
glass 30. 
As shown in FIG. 3B, the marginal ray R.sub.1 passing through the lower 
portion of the window glass 30 (solid line) has an angle .alpha..sub.2 to 
the normal N.sub.2 of the glass 30. On the other hand, the marginal ray 
R.sub.2 passing through the upper portion of the window glass 30 (broken 
line) has an angle .beta..sub.2 (&lt;.alpha..sub.2) to the normal N.sub.2 of 
the glass. As a result, as far as the transparent window glass 30 
concerns, the lower marginal ray R.sub.1 has a longer optical path length 
than the upper marginal ray R.sub.2. In the transparent window glass 30, 
therefore, the optical path lengths of the rays are rendered asymmetrical. 
Thereby asymmetric aberration is generated. 
On the contrary, as shown in FIG. 3A, the marginal ray R.sub.1 passing 
through the lower portion of the aberration-compensating plane parallel 
plate 41(42) has an angle .alpha..sub.1 to the normal N.sub.1 of the plate 
41(42). The marginal ray R.sub.2 passing through the upper portion of the 
plane parallel plate 41(42) has an angle .beta..sub.1 (&gt;.alpha..sub.1) to 
the normal N.sub.1 of the plate 41(42). 
Since, as previously noted, the objective lens 14A(24A) is an image-forming 
system of one-to-one magnification, the beams on the both sides of the 
lens have the same angle of divergence: 
EQU .theta..sub.1 =.theta..sub.2 
Further, the transparent window glass 30 and the plane parallel plate 
41(42) are parallel to each other. Consequently, 
EQU .alpha..sub.1 =.beta..sub.2, .beta..sub.1 =.alpha..sub.2 
Generally, the optical path length of a light ray passing through a plane 
parallel plate is determined by insidence angle, thickness and refractive 
index of the plane parallel plate. 
For the purpose of explanation, let the optical path length of the 
above-mentioned lower marginal ray R.sub.1 passing through the plane 
parallel plate 41(42) be l.sub.11 and that of the upper marginal ray 
R.sub.2 be l.sub.12 (&gt;l.sub.11). Similarly, let the lower marginal ray 
R.sub.1 passing through the window glass 30 be l.sub.21 and that of the 
upper marginal ray R.sub.2 be l.sub.22 (&lt;l.sub.21). Then, since the window 
glass 30 and the plane parallel plate 41(42) have the same thickness and 
refractive index, 
EQU l.sub.11 =l.sub.22, l.sub.12 =l.sub.21 
Consequently, 
EQU l.sub.11 +l.sub.21 =l.sub.12 l.sub.22 
Therefore, regarding the rays R.sub.1 and R.sub.2 passing through the lower 
portion and the upper portion of the objective lens 14A(24A) among 
marginal rays from the object point on axis as shown in FIG. 2, their 
composite optical path lengths are equal to each other. The composite 
optical length of the ray R.sub.1 composed of the transparent window glass 
30 and the plane parallel plate 41(42) is equal to the composite optical 
path length of the ray R.sub.2 composed of the glass plate 30 and the 
plane parallel plate 41(42). The two composite optical path lengths are 
symmetrical relative the optical axis. In conclusion, the asymmetric 
aberration generated by the transparent window glass 30 is well 
compensated by the insertion of the plane parallel plate 41(42). 
As will be understood from the embodiments shown in FIGS. 1 and 2, 
according to the present invention, the asymmetric aberration produced by 
the transparent window glass 30 which is a plane parallel plate, disposed 
inclined to the optical axis is well corrected in a simple manner by 
inserting such a plane parallel plate (41, 42) obliquely in the optical 
path which has the same thickness and the same refractive index, that is, 
the same optical path length as the window glass 30. In other words, 
according to the invention, the asymmetric aberration can be corrected by 
cancelling the asymmetricity of the optical path length at the window 30 
with the inverted asymmetricity of the optical path length at the plane 
parallel plate 41(42). 
The feature of the present invention, that is, the correction of asymmetric 
aberration by use of a plane parallel plate may be embodied variously. 
Other embodiments than the above embodiments 1 and 2 are of course 
possible. 
FIG. 4 shows a third embodiment of the invention. 
Referring to FIG. 4 similar to FIG. 2, again asymmetric aberrations are 
produced by the window glass 30 which is a plane parallel plate. To 
compensate the asymmetric aberrations there are provided plane parallel 
plates 41, 42 (only one is shown for the sake of simplicity of the 
drawing). 
The third embodiment is distinguished from the previous embodiments in that 
the aberration-compensating plane parallel plate 41(42) is interposed in 
the light beam between the relay lens 13(23) and the light image I.sub.1 
(I.sub.3). In this embodiment, therefore, the objective lens 14A(24A) is 
sandwiched in between the transparent window glass 30 and the plane 
parallel plate 41(42). The glass plate and the plane parallel plate are 
parallel to each other. With this arrangement, the symmetricity of the 
optical path lengths can be maintained to correct the asymmetric 
aberration. 
In all of the embodiments shown in FIGS. 1, 2 and 4, the projection system 
and the detection system have been shown to have the same optical 
construction and to be symmetrically arranged relative to the object to be 
measured. However, it is to be understood that various modifications are 
possible in the arrangement of the projection system and the detection 
system. For exaxple, the apparatus may be composed of a projection system 
having the construction shown in FIG. 2 and a detection system having the 
construction shown in FIG. 4. Further, one or both of the relay systems 
may be omitted if it is necessary due to the limitation of the available 
space in the apparatus in which the position-detecting unit of the 
invention is to be incorporated. 
As described above, according to the present invention, asymmetric 
aberrations as generated by a plane parallel member obliquely mounted such 
as a transparent glass plate disposed inclined to the optical axis can be 
corrected by providing a aberration-compensating plane parallel plate 
obliquely to the optical axis. However, when a plane parallel plate is 
obliquely interposed in a non-collimated beam (convergent beam or 
divergent beam), there is generated astigmatic difference to the image 
point on axis. The astigmatic difference is a phenomenon where the image 
point in the meridional plane is different from the image point in the 
sagittal plane. Even for the object point on the optical axis, this 
phenomenon of astigmatic difference is produced by the plane parallel 
plate disposed obliquely to the optical axis. (Herein, the meridional 
plane is defined as a plane containing the normal at the intersection of 
the optical axis and the plane parallel plate, and the optical axis). The 
astigmatic difference is caused by the fact that owing to the presence of 
the slant plane parallel plate the image point Im in the meridional plane 
is formed at a position farther distant from the image-forming lens than 
the image point Is in the sagittal plane. 
Let Pm and Ps denote the distances of the image points from the 
image-forming lens respectively and let i.sub.1 denote the angle of 
incidence to the plane parallel plate and i.sub.2 the angle of refraction. 
Then, the astigmatic difference .DELTA.P is represented by the formula: 
##EQU1## 
This astigmatic difference can be corrected by interposing a cylindrical 
lens in the image-forming optical system as previously shown in FIGS. 2 
and 4. More concretely, the astigmatic difference can be corrected by 
bringing the image points in meridional plane and in sagittal plane into 
coincidence according to any of the following methods: 
By interposing in the optical path a negative cylindrical lens which has a 
generating line in the meridional plane and has a diverging action in the 
sagittal plane; or 
By interposing in the optical path a positive cylindrical lens which has a 
generating line in the sagittal plane and has a converging action in the 
meridional plane. 
In the objective lens 14A(24A) show in FIG. 2, the cylindrical lens b has 
the function to correct the astigmatic difference. The cylindrical lens b 
does not have any refractive power in the meridional plane (the plane of 
the drawing) but has a negative refractive power in the sagittal plane 
(the plane normal to the plane of the drawing). By its diverging action, 
the image point in the sagittal plane is brought into coincidence with the 
image point in the meridional plane to correct the astigmatic difference. 
The effect for the correction of asymmetric aberration according to the 
present invention is demonstrated by the spot diagrams of FIGS. 5A, 5B and 
5C. 
To conduct experiments, an image-forming optical system was formed using 
two positive lenses having the same focal length of 100 mm. To obtain a 
collimated light beam between the two positive lenses, they were arranged 
symmetrically in such manner that the object point was at the front focus 
of one of the two positive lenses and an image of the object point was 
formed at the rear focus of the other one. Numerical apertures on the 
object side and on the image side were set to the same value of 0.1. 
Furthermore, on each the positive lens such a non-spherical surface was 
provided which could correct the spherical aberration and satisfy the sine 
condition. 
A transparent glass plate of 8.7 mm thick and 1.5 in refractive index (the 
transparent window glass 30) was disposed obliquely on the object side of 
the image-forming optical system. The angle which the normal of the glass 
plate 30 formed with the optical axis was 30 degrees. FIG. 5A shows the 
spot diagram obtained under this condition. As clearly seen in the 
diagram, the spots are distributed vertical-asymmetrically relative to the 
optical axis. 
An aberration-correcting plane parallel plate 42 was interposed in the 
optical path on the image side of the image-forming optical system (for 
example the second objective lens 24A). The aberration-correcting plane 
parallel plate 42 possessed the same optical characteristics as the window 
glass 30. FIG. 5B shows the spot diagram obtained when the plane parallel 
plate 42 was interposed. 
Comparing the spot diagram of FIG. 5B with that of FIG. 5A, man can 
understood that the asymmetricity in FIG. 5A was improved by the 
interposition of the plane parallel plate 42. 
For the reason previously described, astigmatic difference are produced 
even on the optical axis, not only in the construction without the 
aberration-correcting plane parallel plate (FIG. 5A) but also in the 
construction with the plane parallel plate 41(42) (FIG. 5B). In both of 
FIGS. 5A and 5B, m-image point (meridional image point) was ploted. 
A cylindrical lens b was interposed between the two positive lens 
components a and c of the above image-forming optical system (for example 
the second objective lens 24A). FIG. 5C shows the spot diagram obtained in 
this construction containing the cylindrical lens. FIG. 5C demonstrates 
that image spots laterally distributed as in FIG. 5B can be concentrated 
to one point by the addition of the cylindrical lens and, therefore, the 
astigmatic difference can be corrected well. 
In the above, the spot diagrams have been described in connection with the 
second objective lens 24A of the detection system in which the object 
point is the object to be measured 3 and the image point is I.sub.3. 
However, it is to be understood that the same spot diagrams can be 
obtained even for the first objective lens 14A of the projection system in 
which the image point lies on the surface of the object 3. 
Although the objective lens 14A(24A) shown in FIGS. 2 and 4 contains a 
cylindrical lens, the objective lens 14(24) shown in FIG. 1 does not 
contain such a cylindrical lens. Therefore, in the first embodiment shown 
in FIG. 1, there is generated astigmatic difference at the image point as 
shown in FIG. 5B. 
However, in the first embodiment, the astigmatic difference does not 
disturb the aimed detection of position. The reason for this is that the 
light image formed at the image point is a light image conjugated with the 
slit 12 and elongated in the direction in which the astigmatic difference 
is generated. 
In the position-detecting optical apparatus including the transparent 
window glass 30 and the aberration-correcting plane parallel plates 41, 
42, the optical paths of the optical systems may be deflected various 
directions. Even in such cases, as a matter of course, the positional 
relationship between the window glass and the plane parallel plate is 
optically equivalent to that in the above embodiments considering the 
development of the optical axis of the optical systems. 
In the above, we have discussed the image-forming performance primarily 
about the object point on axis of the optical system containing an 
aberration-correcting plane parallel plate. However, the image-forming 
performance must be considered also about off-axial object point in 
addition to the on-axial object point. 
As previously described, as for the image formation of on-axial object 
point, the asymmetrical aberration is first compensated by the plane 
parallel plate 41(42) and then the astigmatic difference is corrected by 
the cylindrical lens b according to the present invention. 
As for the off-axial object point, there is the possibility that field 
curvature may be produced asymmetrically according as the manner of 
arrangement of the transparent window glass 30 and the 
aberration-compensating plane parallel plate 41(42). In some arrangements, 
the window glass 30 and the plane parallel plate 41(42) may act as if a 
plane parallel plate having a thickness corresponding to the sum of their 
thicknesses were obliquely interposed in the image-forming optical path, 
which may produce curvature of image field asymmetrically. 
As well-known to those skilled in the art, the field curvature as produced 
by a plane parallel plate interposed in a non-collimated light beam is 
attributable to the fact that the principal rays from the on-axial object 
point and from the off-axial object point have different angles of 
incidence to the plane parallel plate and, therefore, they have different 
optical path lengths as illustrated in FIG. 6. 
Referring to FIG. 6, the plane parallel plate P in solid line is disposed 
normally to the optical axis. In this case, a curved image surface Is is 
formed which is convex toward the lens L. On the contrary, the plane 
parallel plate P' in broken line is inclined to the optical axis. In this 
case, the image surface Iu is asymmetrically curved. 
The above phenomenon as illustrated in FIG. 6 may occur also in the 
constructions of the above embodiments for the following reason: 
The principal ray from one off-axial end of the object surface has a 
relatively large angle of incidence not only to the transparent window 
glass 30 but also to the aberration-compensating plate 41(42). Passing 
through the window glass and the plane parallel plate, the principal ray 
has a relatively long object-image optical path length. 
On the contrary, the principal ray from the other off-axial end of the 
object surface enters the window glass 30 and the plane parallel plate 
41(42) at a relatively small incidence angle. Accordingly, the principal 
ray has a relatively short object-image optical path length. 
Thus, the conjugate relation between object and image regarding the 
off-axial rays on one side of the optical axis is asymmetric to that of 
the off-axial rays on the other side of the optical axis. 
However, even if such asymmetric field curvature is produced in the 
embodiments previously shown in FIGS. 1, 2 and 4, it can not have any 
adverse effect on the aimed detection of position. The reason for this is 
that in the embodiments a light image of a slit 12 or a pinhole 12A 
provided on the optical axis of the projection system is projected onto 
the object 3 and the detection system is so formed as to detect the 
reflected light image only on the optical axis of the detection system to 
detect the position of the object surface. In this construction, if the 
light image on the optical axis has asymmetrical field curvature, the 
magnitude of the field curvature is very small and within the depth of 
focus of the projection system and of the detection system. Therefore, it 
can not have any effect on the detection of position. 
In the projection type exposure apparatus as shown in FIG. 1, if the object 
3 is moved together with the transparent window 30 a small distance 
vertically along the optical axis of the projection lens 2, the light 
image projected on the object 3 through the projection optical system will 
shift a little horizontally. Accordingly, the reflected light image also 
shifts in the direction perpendicular to the optical axis in FIG. 1 (in 
the direction of double-arrow indicating the moving direction of the 
detection slit). 
In some known apparatus, detection is made to know the shift of the 
reflected light image from the detection optical axis. From the detected 
shift of the reflected light image, man can measure the magnitude and 
direction of deviation of the object 3 from the reference position. In 
this type of detection apparatus, the detection area of light image is 
relatively broad. Therefore, an adequate correction of the above-mentioned 
asymmetrical field curvature is needed in this case. 
FIG. 7 shows a fourth embodiment of the invention containing mean for 
correcting the asymmetric field curvature. 
Referring to FIG. 7, the detection system of the fourth embodiment 
basically corresponds to that of the third embodiment previously shown in 
FIG. 4. As the projection system of the fourth embodiment it may be of the 
construction shown in FIG. 2 or FIG. 4. 
The detection system of the fourth embodiment contains a field lens 50 
disposed on or near the primary image I.sub.3 of the object to be measure. 
Other parts of the detection system substantially correspond to those of 
the detection system shown in FIG. 4 and need not be further described. 
Like reference numerals to FIG. 4 represent functionally the same members. 
To detect the light image (of an object point on the object 3) there is 
provided a position detection sensor 22B which may be, for example, a 
linear charge coupled device (CCD). The pupil of the objective lens 24A 
and the pupil of the field lens 23 are approximately conjugated by the 
field lens 50. The field lens 50 has the function to correct the 
asymmetric field curvature in addition to its primary function to converge 
the principal rays from off-axial object points thereby preventing the 
decreasing of the light intensity of the edge light image. 
In FIG. 7, the broken lines indicate the running paths of rays from 
off-axial object points (light images) A.sub.1 and A.sub.2 on the object 
3. 
The principal ray S.sub.1 from the off-axial object point A.sub.1 passes 
through the transparent window glass 30 and the object lens 24A and then 
reaches the image point B.sub.1 of the primary image I.sub.3 of the 
object. After being subjected to the converging action of the field lens 
50, the principal ray S.sub.1 is transmitted to the image point C.sub.1 on 
the sensor 22B through the aberration-compensating plane parallel plate 42 
and the relay lens 23. Thereby a secondary image is formed on the sensor 
22B at the image point C.sub.1. 
The principal ray S.sub.2 from another object point A.sub.2 on the opposite 
side edge of the object 3 passes through the transparent window glass 30 
and the object lens 24A and then reaches the image point B.sub.2 of the 
primary image I.sub.3 of the object. Being subjected to the converging 
action of the field lens 50, the ray S.sub.2 is transmitted to the image 
point C.sub.2 on the sensor through the plane parallel plate 42 and the 
relay lens 23. Thereby a secondary image of the object is formed on the 
sensor 24B at the image point C.sub.2. 
The principal ray S.sub.1 from the object point A.sub.1 has a larger angle 
of incidence to the window glass (a plane parallel plate) 30 than the 
principal ray S.sub.2 from the object point A.sub.2 does. On the contrary, 
when the rays enter the aberration-compensating plane parallel plate 42, 
the principal S.sub.2 has a larger incident angle than the principal ray 
S.sub.1 does. Consequently, as will be readily understood, regarding the 
optical path between the object surface 3 and the sensor surface 22B, the 
rays S.sub.1 and S.sub.2 have the same optical path length. Therefore, the 
optical path lengths of the off-axial object points on the object surface 
3 are symmetrical each other relative to the optical axis. As a result, 
the image surface has a shape symmetrical relative to the optical axis, 
although it is curved. 
Such a symmetric field curvature can easily be corrected by an 
image-forming optical system such as the objective lens 24A, the relay 
lens 23 or the like. According to the fourth embodiment, therefore, it is 
possible to form a sharp and clear image over a broad area of the object 
surface by suitably correcting the field curvature by the image-forming 
optical system in combination with the construction shown in FIG. 7. 
In all of the embodiments shown above, the objective lens and the relay 
lens have been disposed eccentrically in accordance with the shift of the 
optical axis caused by the plane parallel plate 41(42) obliquely 
interposed. However, it is not always necessary for the objective lens and 
the relay lens to be eccentrically arranged when they satisfy the sine 
condition. This is because even when the image point is shifted in the 
direction normal to the optical axis by the transparent window glass 30 
and the plane parallel plate 41(42), an image of such object point a 
little shifted from the optical axis can be formed without aberration so 
long as the image-forming lens satisfies the sine condition. But, when the 
optical axis is heavily shifted by the obliquely mounted plane parallel 
plate and probably vignetting of marginal rays may occur, it is advisable 
to arrange the objective lens and the relay lens eccentrically. 
While the aberration-compensating plane parallel plates 41 and 42 in the 
above embodiments have been shown and described to have the same thickness 
and the same refractive index as the transparent window glass 30, it is to 
be understood that it is not always necessary for the plane parallel 
plates to be made of the same material as the material for the transparent 
window glass 30. The thing essential is to make the optical path lengths 
(optical thicknesses) substantially equal to each other. By doing so, the 
asymmetric aberrations can be compensated very well. 
It is most desirable that the non-collimated light beam in which the window 
glass 30 is interposed and the non-collimated light beam in which the 
plane parallel plate 41(42) is interposed should have the same numerical 
aperture. However, it is to be noted that even when they have different 
numerical apertures, the aberration generated by the transparent window 
glass can be corrected remarkedly well as compared with the case where no 
aberration-compensating plane parallel plate is provided. Therefore, the 
image-forming lenses 14, 24, 14A, 24A in the embodiments shown in FIGS. 1 
to 7 are not limited to those lenses of one-to-one magnification only. The 
object of the present invention may be equally attained even when the 
image-forming lens between the transparent window glass and the 
aberration-compensating plane parallel plate is a magnifying or minifying 
image-forming lens. 
Further, it is to be understood that the transparent window obliquely 
disposed between the object and the objective lens may be a transparent 
plate made of other material than glass. For example, it may be a 
transparent plate made of plastics. In addition, the transparent window 
plate is not always necessary to be a plane parallel plate. It may be a 
slightly curved transparent plate. 
FIG. 8 shows a modified embodiment of the invention. 
In this case, a plane parallel plate 3 as the window glass is obliquely 
interposed in the convergent light beam from an objective lens 1. To 
compensate the asymmetric aberration generated by the plane parallel plate 
(window glass) 3, this modified embodiment contains a plane parallel plate 
4 interposed in the divergent light beam incident to a relay lens 2. In 
this modification, therefore, the first and second plane parallel plates 3 
and 4 are arranged between the objective lens 1 and the relay lens 2. The 
object lens 1 forms a primary image I.sub.1 of an object between the two 
plane parallel plates 3 and 4. The two plane parallel plates 3 and 4 are 
arranged symmetrically each other relative to the plane normal to the 
optical axis A of the objective lens 1 so that the symmetricity of optical 
path lengths relative to the optical axis may be maintained and the 
asymmetric aberration may be corrected satisfactorily.