Beam-splitting optical system

An optical system for insertion into projection apparatus to permit viewing an image projected on an image plane without affecting its quality, size or position, including a beam-splitting element disposed in the optical path before the image plane which will transmit the radiation to the image plane and reflect a portion of the radiation reflected from the image plane in a direction which permits viewing, optical elements disposed in the optical path to correct for image displacement and aberrations caused by the beam-splitting element, said optical system being of unit power; according to one aspect of the invention the system is afocal so that it will remain at unit power when it is shifted longitudinally and the image position is insensitive to longitudinal displacement; and according to another aspect of the invention each element of the system is afocal so that the system remains at unit power and insensitive to changes in spacing between the elements.

FIELD AND BACKGROUND OF INVENTION 
This invention relates to optical systems and more particularly to an 
optical system for insertion into projection apparatus to permit viewing 
of an image projected on an image plane. The optical system of the 
invention is particularly adapted, among other possible uses, for use as 
an insertion in the optical train used in microprojection apparatus to 
provide access to the image of a mask on a light sensitive wafer in order 
to closely align the image with the pattern on the wafer. It will be 
appreciated that such insertion of an optical system must not affect the 
quality, size, or position of the image. A good example of an optical 
train used in microprojection apparatus is shown and described in the 
Offner U.S. Pat. No. 3,748,015, issued July 24, 1973. 
SUMMARY OF THE INVENTION 
In order to accomplish the desired results, the invention provides a new 
optical system, which is the optical equivalent of a zero thickness 
pellicle beam splitter. Thus, the invention provides an optical system for 
insertion into projection apparatus to permit the viewing of an image 
projected on an image plane without affecting its quality, size or 
position, which comprises, in one form thereof, a beam splitting element 
disposed in the optical path before the image plane so that the beam 
splitting element will transmit the radiation to the image plane and 
reflect a portion of the radiation reflected from the image plane in a 
direction which permits viewing. In addition, means are disposed in the 
optical path to correct for image displacement and aberrations caused by 
the beam splitting element. The optical system of the invention is of unit 
power. According to one aspect of the invention, the optical system is 
afocal so that it will remain at unit power when it is shifted 
longitudinally and the image position is insensitive to longitudinal 
displacement. In one form of the invention the means for correcting for 
image displacement and aberrations comprises first and second miniscus 
lense elements. According to another aspect of the invention, the optical 
system further includes a tiltable plane parallel plate element disposed 
between the meniscus elements for adjusting the position of the image 
laterally. Further, according to another aspect of the invention, each of 
the elements of the system is afocal so that the system remains at unit 
power and insensitive to changes in the spacing between the elements. It 
will be appreciated that the system, according to the invention, is 
corrected for coma, distortion, and lateral color, making the system 
insensitive to small lateral displacements and tilts. 
There has thus been outlined rather broadly the more important features of 
the invention in order that the detailed description thereof that follows 
may be better understood, and in order that the present contribution to 
the art may be better appreciated. There are, of course, additional 
features of the invention that will be described hereinafter and which 
will form the subject of the claims appended hereto. Those skilled in the 
art will appreciate that the conception upon which the disclosure is based 
may readily be utilized as a basis for the designing of other systems for 
carrying out the several purposes of the invention. It is important, 
therefore, that the claims be regarded as including such equivalent 
systems as do not depart from the spirit or scope of the invention. 
Specific embodiments of the invention have been chosen for purposes of 
illustration and description, and are shown in the accompanying drawings 
forming a part of the specification.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a first embodiment of the present invention. Shown in 
solid lines are projected rays 11 which, in microprojection apparatus, 
image a mask on an image plane 13, e.g., a wafer. Without the requirement 
for being able to observe what is being imaged thereon, those rays could 
be projected directly onto the image plane 13. However, as previously 
described, there is a need to observe what is being projected in order to 
closely align the image of the mask with the pattern on the wafer. To 
accomplish this, a beam splitting element which may, for example, be a 
beam splitting cube 15, is inserted in the optical path. The light rays 
pass through this beam splitting cube to the image plane; however, because 
of the insertion of the beam splitting cube 15, a portion of the light 
rays 11 which pass through the beam splitting cube 15 and are reflected 
off the wafer return to the beam splitting cube and are reflected by the 
diagonal surface thereof to a viewing device, as indicated by the rays 17. 
However, the beam splitting cube causes image displacement and 
aberrations. In accordance with the present invention in order to correct 
this, in other words, to insure convergence at the same image point in the 
image plane 13, an afocal unity power optical system 19 comprising two 
meniscus elements 21 and 23, each of which is afocal, is inserted in the 
optical path. That is, the optical system including the beam splitting 
cube is unit power. By making the system afocal insures that it will 
remain at unit power when the conjugates are changed, as happens when it 
is shifted longitudinally. Also, the image position is insensitive to 
longitudinal displacements. It is noted that in some microprojection 
applications it is desirable to swing or pivot as a unit the entire 
optical system, elements 15, 21, 23, into and out of the path of the 
projected rays 11. Because the optical system is afocal and unit power, 
the image position is insensitive to longitudinal displacements, which 
result from the mechanical tolerances required for such movement. 
Further, by making each component of the optical system afocal, the system 
remains afocal even if the separation between the lens elements changes. 
This is a useful, practical feature since the lens separation can be 
adjusted during assembly to put the image at the desired location. 
The dashed lines 25 on FIG. 1 illustrate the manner in which the correction 
takes place. The refraction within the lenses 21 and 23 cause the rays 
entering the beam splitting cube 15 to deviate from the position they 
would normally take were the lenses 21 and 23 not present. This 
compensates for refraction within the beam splitting cube so that, upon 
exiting from the beam splitting cube 15, the rays follow the same path as 
they would have had all the elements of the present invention not been 
inserted. In this manner, the errors introduced by the beam splitting 
cube, which is necessary in order to view the positioning and the 
alignment of the mask image on the wafer, are compensated. 
FIG. 2 illustrates an alternate embodiment of the present invention. In 
this embodiment, all of the same elements are present; however, their 
locations are changed. In this embodiment a beam splitting cube 15a is 
disposed between two meniscus lenses 21 and 23a. The two systems are 
optically equivalent and the same type of corrections as took place in 
FIG. 1 take place in the embodiment of FIG. 2. 
Another embodiment of the invention is shown in FIG. 3. This embodiment is 
similar to the embodiment of FIG. 2 as it includes a beam splitting cube 
15a disposed between the two meniscus lenses 21 and 23a. In addition, the 
embodiment of FIG. 3 includes a tiltable plane parallel plate 27 inserted 
between the meniscus elements 21 and 23a. The insertion of this plate 
provides a very useful, practical feature because it is used to adjust the 
position of the image laterally. This relieves the otherwise very tight 
tolerances on the wedge and squaring on the beam splitting cube and 
meniscus lenses. Thus, the system is corrected for coma distortion and 
lateral color, which makes it insensitive to small lateral displacements 
and tilts. 
Table 1 is an example, indicating the construction data, of the optical 
system of FIG. 3. 
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GLASS AIR 
SURFACE 
RADIUS 
THICKNESS 
GLASS 
INDEX OF REFRACTION 
SEATION 
NUMBER 
(mm) (mm) TYPE .5600 .mu.m 
.5200 .mu.m 
.6000 .mu.m 
(mm) 
__________________________________________________________________________ 
S1 24.48(cx) 
3.93 BK-7 1.5180 
1.5202 
1.5163 
S2 23.14(cc) 28.97 
S3 .infin. (plane) 
2.00 F-2 1.6226 
1.6272 
1.6190 
S4 .infin. (plane) 8.74 
S5 .infin. (plane) 
7.00 F-2 1.6226 
1.6272 
1.6190 
S6 .infin. (plane) 2.26 
S7 14.72(cc) 
2.50 BK-7 1.5180 
1.5202 
1.5163 
S8 15.57(cx) 3.00 
__________________________________________________________________________ 
Spectral Range .5200 .mu.m .ltoreq. .lambda. .ltoreq. .6000 .mu.m 
The distance from surface S1 to the image plane is 58.4 mm and the distance 
from surface S8 to the image plane is 3.0 mm. 
It will thus be seen that the present invention does indeed provide a new 
optical system for insertion into projection apparatus for viewing an 
image projected on an image plane without affecting its quality, size or 
position. Although specific embodiments have been illustrated and 
described, it will be obvious to those skilled in the art that various 
modifications may be made without departing from the spirit and scope of 
the invention, which is to be limited solely by the appended claims.