Lens barrel and projection aligner

Abstract: A lens barrel (4) includes an optical element (2), for example, a binary-type diffractive element having a light-transmissive surface which is deformed by own weight thereof and/or by being supported, and a plurality of protrusive parts (6) which support the optical element. The plurality of protrusive parts are disposed in such a way as to enable the light-transmissive surface of the optical element to be deformed symmetrically with respect to a plane which includes an optical axis.

Description: The present invention relates to an optical system comprising a lens barrel having an optical element which is deformed by its own weight and/or by being held in position and also to a projection aligner having the lens barrel. More particularly, the present invention relates to a lens barrel having, as such an optical element, for example, a binary-type diffractive optical element and to a projection aligner having the lens barrel. Optical systems of the kind having diffractive optical elements have been developed in various manners during recent years. The diffractive optical elements known to be used for the optical systems include, for example, Fresnel zone plates, kinoforms, binary optics and hologram elements. The diffractive optical element is used for converting an incident wavefront into a predetermined wavefront and has features not possessed by a refraction-type lens. For example, the diffractive optical element has dispersion reverse to that of the refraction-type lens and can be formed thin, so that the whole optical system can be compactly constructed. Generally, with the diffractive optical element arranged to be in a binary type shape, the diffractive optical element can be manufactured by using the manufacturing technique for semiconductor devices. With such a manufacturing technique applied, the diffractive optical element can be manufactured without difficulty to have fine pitches. In view of this, researches are being actively conducted for a binary-type diffractive optical element of the blazed shape which approximates to a stepped shape. Meanwhile, various methods have been employed for positioning optical elements, such as a diffractive optical element, a lens, etc., within a lens barrel. The known methods include a lens pressing method, a throw-in method, etc. Fig. 1 shows in outline the structural arrangement of a lens barrel in which optical elements are positioned by the lens pressing method. Referring to Fig. 1, lenses 8 are arranged to constitute a projection optical system. The lens barrel 9 is arranged to hold the lenses 8. Retaining rings 10 are arranged in the lens barrel 9 to fix the lenses 8 to their positions by causing the lenses 8 to abut on the respective lens setting parts "a" of the lens barrel 9. The outside shapes of the lenses 8 are beforehand cut to be coaxial with respect to a lens optical axis La by machining to a predetermined degree of precision, and the outside diameters of them are beforehand measured and determined also to a predetermined degree of precision. The inside diameter of the lens barrel 9 is beforehand cut and determined according to the outside diameters of the lenses 8 measured, in such a way as to have a predetermined clearance between the inside diameter of the lens barrel 9 and the outside diameter of each of the lenses 8 when the lenses 8 are fitted in the lens barrel 9. The lenses 8 are positioned in the direction of the optical axis La by screwing a male screw part formed on the peripheral part of each of the retaining rings 10 into the corresponding one of female screw parts 90 formed in the inner side wall of the lens barrel 9. Each of the retaining rings 10 is thus screwed to cause each of the lenses 8 to abut on the corresponding lens setting part "a", so that the lenses 8 are fixed in position. In the case of the conventional lens barrel shown in Fig. 1, since each of the lenses 8 is pushed against the corresponding lens setting part "a", the surface shape of each of the lenses 8 tends to be deformed according to the shape of the retaining ring 10 and the shape of the lens setting part "a". Such deformation has presented such a problem as to cause the optical characteristics of the lenses 8 to vary. To solve the above problem, it is possible to lessen a pushing force on the lens 8, by sticking the lens 8 to the inner wall of the lens barrel 9 by adhesives without using the retaining ring 10. However, in a case where the direction of the optical axis coincides with the direction of gravitation, the lens 8 might be sometimes deformed by its own weight to some extent and in some directions according to the shape of the lens setting part "a". It is difficult to machine the lens setting part with its flatness kept more accurate than the flatness of the lens surface. It is also difficult to accurately presume the deformation of the lens abutting on the lens setting part beforehand, because the shape of the lens setting part in each of lens barrels differs from that in another lens barrel. Therefore, in the case of an optical system where even a minute amount of deformation is considered to be a serious drawback, the optical performance of the optical system must be evaluated after assembly work of the optical system and the posture or position of each lens must be adjusted according to the result of the evaluation in a prescribed manner to correct various aberrations resulting from the surface deformation. Accordingly, the number of necessary assembly and adjustment processes has been increased by such additional processes that are necessary. Further, in a case where a thin optical element such as a diffractive optical element or the like is to be held by a lens barrel, in particular, the amount of the above-stated deformation becomes too much to obtain a desired optical performance by adjusting the posture or position of the optical element. According to DE-A-1923418, a lens is supported by three contacting points provided on a lens holder and an adhesive is applied between the outer periphery of the lens and the lens holder. Thus, the lens is supported by the adhesive along the whole outer periphery of the lens and not only by three contacting points. Thus, even if the lens would be deformed due to its own weight, such a deformation cannot be rotational asymmetric relative to the optical axis. According to US-A-4043644, a lens or a mirror is deformed by supporting means so as to achieve specific optical characteristics. However, the lens or the mirror is not deformed by its own weight. Optical characteristics achieved by the deformation by the own weight are not corrected. According to US-A-5636000, a deviation of a focal position is corrected by moving a diffraction optical element in the direction of the optical axis. The optical element is not deformed by its own weight. It is an object of the present invention to provide an optical system comprising a lens barrel which has a high optical performance. This object is achieved by an optical system having the features of claim 1, 3 or 4. A projection aligner comprising such an optical system is defined in claim 15. Advantageous further developments are set out in the dependent claims. Further, in the lens barrel, each of the plurality of protrusive parts are arranged to be substantially in point-contact with the optical element. Further, in the lens barrel, positions of the plurality of protrusive parts in a direction of the optical axis are the same. Further, in the lens barrel, the number of the plurality of protrusive parts is two, three, four, five, six, seven, eight or nine. Further, in the lens barrel, a plurality of points of a peripheral part of the optical element are stuck to an inner wall of the lens barrel by an adhesive. Further, in the lens barrel, the optical element is a lens or a mirror. Further, in the lens barrel, the optical element is a diffractive optical element. Further, the lens barrel further comprises a plurality of optical elements each having a light-transmissive surface which is substantially not deformed by own weight thereof and/or by being pressed, the plurality of optical elements including a lens and/or a mirror. Further, in the lens barrel, each of the plurality of protrusive parts is in a semispherical shape or pin-like shape. Further, in the lens barrel, the plurality of protrusive parts are disposed in such a way as to enable the light-transmissive surface to be deformed symmetrically with respect to both a first plane which includes the optical axis and a second plane which includes the optical axis and is perpendicular to the first plane. In accordance with a further aspect of the invention, there is provided a projection aligner, which comprises the lens barrel, the lens barrel having a projecting optical system which includes the optical element, a pattern formed on a mask being projected by the projecting optical system onto a substrate to be exposed. The above object as well as features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings. Fig. 1 is a sectional view showing in outline the arrangement of the conventional lens barrel. Fig. 2 is a sectional view showing in outline the essential parts of a lens barrel according to a first embodiment of the invention. Fig. 3 is a perspective view showing the details of parts at which a diffractive optical element shown in Fig. 2 is positioned. Fig. 4 is a perspective view showing a state in which the diffractive optical element shown in Fig. 2 is deformed by its own weight. Fig. 5 is a diagram for explaining the result of a computing operation by a finite-element method on the deformation of the diffractive optical element shown in Fig. 2 caused by its own weight. Fig. 6 is a perspective view for explaining a case where spherical members for positioning the diffractive optical element shown in Fig. 2 are disposed at three parts. Fig. 7 schematically shows relative positions of two diffractive optical elements shown in Fig. 2. Fig. 8 is a sectional view showing in outline the essential parts of a lens barrel according to a second embodiment of the invention. Fig. 9 schematically shows a diffraction grating pattern of the diffractive optical element shown in Fig. 2 and a focus position of the diffractive optical element obtained in a state of being deformed by its own weight. Fig. 10 schematically shows a diffraction grating pattern of a diffractive optical element shown in Fig. 8 and a focus position of the diffractive optical element obtained in a state of being deformed by its own weight. Figs. 11(A) and 11(B) are diagrams for explaining cases where protrusive members for positioning a diffractive optical element are disposed at three parts and at four parts, respectively. Fig. 12 is a sectional view showing in outline the essential parts of a lens barrel. Fig. 13 is a sectional view showing in outline the essential parts of a projection aligner according to a third embodiment of the invention. Hereinafter, preferred embodiments of the invention will be described in detail with reference to the drawings. Figs. 2 to 7 relate to a first embodiment of the invention. Fig. 2 is a sectional view showing in outline the essential parts of a lens barrel 4 according to the first embodiment of the invention. Referring to Fig. 2, the direction of gravitation coincides with the direction of an optical axis, which is indicated as the direction of -Z. Refraction lenses 1 (1a and 1b) constitute an optical system. Each of diffractive optical elements 2 and 3 is formed by coaxially forming, in concentric circles, a diffraction grating pattern of the blazed shape on the surface of a glass base plate. The diffractive optical elements 2 and 3 are supported by support members 4a and 4b of the lens barrel 4 through spherical protrusive members 6 which will be described later herein. The lens barrel 4 is arranged to house the optical system. In the lens barrel 4, the support members 4a and 4b are arranged to fixedly set the refraction lens 1a, the diffractive optical element 2, the refraction lens 1b and the diffractive optical element 3 in their positions with adhesives 5 at predetermined spacing intervals in the direction of the optical axis. The support members 4a and 4b are secured to the lens barrel 4 with screws (not shown) in such a way as to have the optical axes of the respective optical elements coincide with each other. Fig. 3 shows the details of a structure by which the diffractive optical element 2 is positioned on the inner wall of the lens barrel 4. The other diffractive optical element 3 is also positioned by a structure which is similar to the positioning structure shown in Fig. 3. Referring to Fig. 3, the two spherical protrusive members 6 are arranged, at equal distances from the optical axis, on an axis L2 (first perpendicular axis), which is perpendicular to the optical axis and is included in a plane perpendicular to the optical axis on the support member 4a. The apexes of the two protrusive members 6, which are located at symmetrical positions with respect to a plane XZ, are disposed within one and the same plane which is perpendicular to the optical axis (i.e., in the same position as viewed in the direction of the optical axis). Then, the apexes of the spherical protrusive members 6 are arranged to be substantially in point-contact with the diffractive optical element 2. Incidentally, the spherical protrusive members 6 may be replaced with pin-like protrusive members having minute planes at their end parts. In that instance, the two minute plane parts are located within the above-stated one and the same plane which is perpendicular to the optical axis (i.e., in the same position as viewed in the direction of the optical axis). The diffractive optical element 2 is thus in contact directly with the support member 4a only at the apexes or the minute plane parts of the protrusive members 6. Further, the adhesive 5 is applied to the peripheral part of the diffractive optical element 2 only at areas in the neighborhood of the protrusive members 6. Accordingly, the diffractive optical element 2 never turns around the axis L2, and the peripheral parts of the diffractive optical element 2 other than the areas having the adhesive 5 are left deformable by the own weight of the diffractive optical element 2. In this instance, the facial shapes of light-entrance and light-exit surfaces of the diffractive optical element 2 obtained after deformation are symmetric with respect to the plane XZ, while the facial shapes of light-entrance and light-exit surfaces of the other diffractive optical element 3 obtained after deformation are symmetric with respect to a plane YZ. In the case of the arrangement of the prior art example described in the foregoing, it is hardly possible to predict or presume how a lens abutting on a lens setting part of the support member will be deformed, because the exact shape of the lens setting part of the support member varies with individual manufactured products thereof. According to the holding method of the first embodiment described above, on the other hand, the diffractive optical element 2 is presumable to be deformed by its own weight in a manner approximately symmetric with respect to both the axis L2 and an axis L2' (second perpendicular axis) which is perpendicular to the axis L2, i.e., with respect to both the planes XZ and YZ, as shown in Fig. 4. The amount of deflection (warp) taking place at each point of the diffractive optical element 2 or 3 can be precisely computed at the time of designing the optical system by the finite-element method or the like. Fig. 5 depicts a result of computation by the finite-element method of a deformed state of the diffractive optical element which is used in the first embodiment and which measures 1 mm in thickness. It is also possible to find beforehand how the diffractive optical element will be deformed through experiments and tests without recourse to a computing operation. Such an experimental method also gives an adequate reproducibility of the deformation of the diffractive optical element. In the case of the first embodiment, the posture of the diffractive optical element 2 is arranged to be set by restricting the diffractive optical element 2 from turning around the axis L2, by means of the adhesive 5. However, this arrangement may be changed to set the posture of the diffractive optical element 2 by replacing one of the protrusive members 6 with protrusive members 6' which are disposed at two parts in the neighborhood of the axis L2 on the support member 4a, as shown in Fig. 6. In that instance, the two protrusive members 6' are disposed near at equal distances from the axis L2 in such a way as to allow the diffractive optical element 2 to symmetrically deform with respect to both the axis L2 and the axis L2'. In addition, the apex of the protrusive member 6 and the apexes of the two protrusive members 6' are arranged to be within one and the same plane which is perpendicular to the optical axis (i.e., in the same position as viewed in the direction of the optical axis). Fig. 7 is a diagram for explaining a case where, for the purpose of correcting the computed various aberrations, such as astigmatism, difference between longitudinal and lateral magnifications, etc., resulting from deformation caused by their own weights when the two diffractive optical elements 2 and 3 shown in Fig. 2 are held in the above-stated manner respectively on the support members 4a and 4b, the diffractive optical elements 2 and 3 are set in such a way as to have the axes L2 and L3 which are datum axes of their deformations perpendicular to each other. In this case, the deformation of one of the two diffractive optical elements 2 and 3 is arranged to be a correcting optical element for correcting the deformation of the other of the two diffractive optical elements 2 and 3. It is also possible to use an anamorphic lens which is undeformable or deformable, as one of the two diffractive optical elements. According to the arrangement of the first embodiment described above, the aberrations of the optical system is accurately presumable at the time when the specifications of design of the diffractive optical elements are decided before the optical system is assembled. Therefore, a correcting optical system can be designed to correct the aberrations, so that the number of necessary assembly processes can be lessened without fail. Further, since the deformation of the diffractive optical element is presumable even if the amount of deformation of the diffractive optical element is too much to correct the aberrations, an optical design can be adjusted to change, for example, the thickness, etc., of the diffractive optical element so as to correct the presumed amount of deformation before machining and assembly processes. The first embodiment thus permits an improvement also in yield of products. In the first embodiment described above, the arrangement according to the invention is applied to a case where thin diffractive optical elements which tend to deform due to their own weights and/or due to the state of being held with the support members of the lens barrel. The optical characteristics of them and the whole optical system including them are then affected by their deformations to a relatively great extent. The arrangement according to the invention is of course likewise applicable also to cases where optical elements which tend to deform, such as refractive lenses and reflecting mirrors, are arranged to be held in place. Further, it is conceivable that there are cases where optical elements other than deformable optical elements are not only lenses but also mirrors or mirrors combined with lenses. Therefore, the term "lens barrel" as used in describing the invention in not limited to a lens barrel containing lenses but may be an optical barrel which does not contain any lens therein. Next, a second embodiment of the invention will be described with reference to Figs. 8 to 11. Fig. 8 schematically shows the essential parts of a lens barrel according to the second embodiment of the invention. In Fig. 8, the same reference numerals as those of Fig. 2 which shows the first embodiment denote the same members. The direction of gravitation coincides with the direction of the optical axis of the lens barrel, which is indicated as the direction of -Z in Fig. 8. Referring to Fig. 8, in the second embodiment, there is used a diffractive optical element 7, which has a diffraction grating formed in a blazed pattern on the surface of a glass base plate. Fig. 9 shows a focus position obtained when the diffractive optical element 2 shown in Fig. 2 is in a state of being deformed by its own weight. Broken lines in Fig. 9 show boundaries of a ring-zonal diffraction grating pattern. In the case of the diffractive optical element 2 shown in Fig. 9, the ring-zonal diffraction grating pattern is composed of cyclic zones which are concentric with respect to the optical axis. A spacing distance between one ring-like zone and another in the ring-zonal diffraction grating pattern (hereinafter referred to as the pitch of the diffraction grating pattern) is constant in the direction of circumference. Therefore, in a state in which the diffractive optical element 2 is deformed by its own weight, a focus position P2 within a plane including the optical axis and the axis L2 shown in Fig. 9 does not coincide with a focus position P2' within a plane perpendicular to the plane of the focus position P2. Fig. 10 shows a focus position of the diffractive optical element 7 used in the second embodiment. As shown in Fig. 10, the pitch of the diffraction grating pattern is so designed as to become narrower in the direction of the symmetry axis L7 of deformation and wider in a direction perpendicular thereto, in such a way as to cause a focus position P7 within a plane including the optical axis and the axis L7 to coincide with a focus position within a plane which is perpendicular to the former plane when the diffractive optical element 7 is deformed by its own weight within the lens barrel 4. The first embodiment is arranged to correct the aberrations resulting from the deformation by combining a plurality of diffractive optical elements. In the second embodiment, one the other hand, the diffraction grating pattern is designed to optimize a pitch distribution by varying the diffraction grating pitch obtained before the deformation of the diffractive optical element 7 in such a way as to obtain desired optical characteristics with the diffractive optical element 7 in a deformed state due to its own weight. By virtue of this arrangement, the desired optical characteristics can be obtained after the diffractive optical element 7 is deformed without recourse to any additional correcting optical system. The optical system thus can be compactly arranged. The reduction in number of optical elements of the optical system, which is possible according to the arrangement of the second embodiment, also gives advantages such as improvement in transmission factor, mitigation of flares, etc. The number and positions of the plurality of protrusive members arranged to support the diffractive optical element are not limited to the arrangement of the second embodiment disclosed but may be changed as desired. For example, the number of parts at which the diffractive optical element is to be supported are not limited to two but may be three or four, as shown in Figs. 11(A) or 11(B), or may be changed to five, six, seven, eight or nine, as desired. In each of such modifications of the diffractive optical element, however, it is necessary to produce a multiple ring-zonal diffraction grating in which the pitch of ring zones of the diffraction grating pattern is varied according to the direction and the amount of deformation, as indicated by broken lines in Figs. 11(A) and 11(B). In a case where there is used a binary-type diffractive optical element in which the blazed shape (sectional shape as viewed along a plane including the optical axis) of the ring-zonal diffraction grating pattern approximates to a stepped shape composed of a plurality of steps, the diffractive optical element may be manufactured by a known method wherein the pattern on an original plate is transferred to the glass base plate by means of an exposure apparatus used in the manufacture of semiconductor products. In that instance, it is of course necessary that the pattern on the original plate be so manufactured as to enable the diffraction grating pattern having the pitch varied with the parts of the pattern to be transferred to the glass base plate. Further, the same advantageous effects can be also attained by an exposure transfer process carried out on the pattern of the original plate which is concentric under the non-loaded state, by in-plane deforming at least one of the original plate and the glass base plate. Fig. 12 schematically shows the arrangement of a lens barrel. In Fig. 12, the same reference numerals as those used for the first embodiment indicate the same members. The direction of gravitation coincides with the direction of an optical axis, i.e., the direction of -Z indicated in Fig. 12. The optical-element holding method employed for the first embodiment is applied to an ordinary, spherical lens 8 of a surface shape which is rotationally symmetric around the optical axis. The spherical lens 8, which is supported by spherical members 6 disposed at two parts spaced 180 degrees, is arranged to deform due to its own weight to form an aspherical surface shape which is symmetric with respect to both a plane XZ and a plane YZ. Therefore, it is possible to make an optical design by computing beforehand the aberrations, such as astigmatism and a difference between longitudinal and lateral magnifications, which take place, for example, with the spherical lens 8 held in position. With the optical design made in this manner, an optical performance which is equal to the optical performance of an optical system using an aspherical lens can be attained. Further, the lens 8 (aspherical lens) may be used as a correcting optical system for correcting the aberrations of an assembled optical system. The number of parts where the spherical members 6 are to be disposed is of course not limited to the two parts spaced 180 degrees. These lens holding members 6 may be arranged at three or four parts. Further, the above-stated holding method may be applied to an aspherical lens. Fig. 13 schematically shows the arrangement of essential parts of a projection aligner adapted for the manufacture of semiconductor devices, to which a lens barrel according to the invention is applied, according to a third embodiment thereof. Referring to Fig. 13, a circuit pattern provided at a reticle (first object) R which is illuminated with exposure light from an illumination system ER is projected by means of a projecting optical system TL onto the surface of a wafer (photosensitive substrate) W. The projecting optical system TL includes a lens barrel having an element BO which is deformable as described in each of the first and second embodiments. Then, the various processes, such as developing and etching, are performed on the wafer W which has been exposed with images of the circuit pattern, so that a semiconductor device is manufactured. A lens barrel includes an optical element having a light-transmissive surface which is deformed by own weight thereof and/or by being supported, and a plurality of protrusive parts which support the optical element. The plurality of protrusive parts are disposed in such a way as to enable the light-transmissive surface of the optical element to be deformed symmetrically with respect to a plane which includes an optical axis.

Claims: An optical system comprising: a lens barrel (4) a plurality of optical elements (1,2,3,7,8) a first supporting means (6) for supporting one optical element (2,7,8) out of said plurality of optical elements within the lens barrel (4) in such a manner that an aberration results from a deformation of said one optical element by its own weight in a rotationally asymmetrical manner with respect to an optical axis; and a means for correcting said aberration by the rotationally asymmetrical deformation, wherein said correcting means comprises a second supporting means (6) for supporting one or more optical elements (3) other than the one optical element (2,7,8) out of said plurality of optical elements and said another optical element or said other optical elements the latter being diffraction optical elements is/are deformed by their own weight in a rotationally asymmetrical manner with respect to an optical axis. An optical system according to claim 1, wherein said plurality of optical elements comprise a plurality of lenses (1,8) and a plurality of diffraction optical elements (2,3,7), and said plurality of diffraction optical elements are deformed by their own weight in a rotationally asymmetrical manner with respect to an optical axis. An optical system comprising: a lens barrel (4) a plurality of optical elements (1,2,3,7,8) a first supporting means (6) for supporting one optical element (2,7,8) out of said plurality of optical elements within the lens barrel (4) in such a manner that an aberration results from a deformation of said one optical element by its own weight in a rotationally asymmetrical manner with respect to an optical axis; and a means for correcting said aberration by the rotationally asymmetrical deformation, wherein said correcting means comprises one of said plurality of optical elements, namely, an anamorphic lens (1) having a rotationally asymmetrical shape with respect to said optical axis. Optical system comprising a lens barrel, a diffractive optical element (7) having an optical axis and first supporting means (6) for supporting the diffractive optical element (7) within the lens barrel in such a manner that the optical characteristic of the diffractive optical element (7) changes due to the deformation of the diffractive optical element (7) by its own weight in a rotational asymmetric manner relative to its optical axis, wherein said diffractive optical element is provided with a diffraction grating pitch rotationally asymmetric relative to the optical axis of the diffractive optical element (7) in such a manner that the optical characteristic of the diffractive optical element (7) in the deformed state is rotationally symmetric relative to its optical axis so as to prevent the change of optical characteristic within the optical system. An optical system according to any of claims 1 to 4, wherein said first and/or second supporting means (6) comprise a plurality of protrusive parts which support said optical element (2, 3, 7, 8). An optical system according to claim 5, wherein each of said plurality of protrusive parts is arranged to be substantially in point-contact with said optical element (2, 3, 7, 8). An optical system according to claim 5 or 6, wherein positions of said plurality of protrusive parts in a direction of the optical axis are the same. An optical system according to claim 5 or 7, wherein the number of said plurality of protrusive parts is two, three, four, five, six, seven, eight or nine. An optical system according to any of claims 1 to 8, wherein a plurality of points of a peripheral part of said optical element (2, 3, 7, 8) are stuck to an inner wall of said lens barrel by an adhesive. An optical system according to claim 1 or 2, wherein said optical element is a lens, a mirror or a diffractive optical element. An optical system according to claim 1 or 2, wherein said further optical element is a diffractive optical element. An optical system according to any of claims 1 to 11, further comprising a plurality of optical elements each having a light-transmissive surface which is substantially not deformed by own weight thereof and/or by being pressed, said plurality of optical elements including a lens and/or a mirror. An optical system according to claim 5, wherein each of said plurality of protrusive parts is in a semispherical shape or pin-like shape. An optical system according to any of claims 5 to 8, wherein said plurality of protrusive parts are disposed in such a way as to enable the light-transmissive surface of the optical element to be deformed symmetrically with respect to both a first plane which includes the optical axis and a second plane which includes the optical axis and is perpendicular to the first plane. A projection aligner comprising an optical system according to one of claims 1 to 14, wherein said projection aligner projects and exposes a pattern formed on a mask on a substrate by using said optical system.

Inventors: SUDOH YUJI, SUDOH, YUJI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1999
Main classification: G03F7
IPC codes: G02B7, G02B27, G03F7, G03B21, H01L21