Abstract:
A double mirror objective lens system uses a three optical surface refractor incorporating a convex mirror into a right surface thereof that reflects incident light to a concave mirror, which reflects the light back through the refractor and on toward a focal point of the system. This arrangement yields better resolution images with low spherical aberration, minimal chromatic aberration, and long working distance. A variation of the invention includes another refractor, a right surface of which carries the concave mirror to form a Mangin mirror. This variation on of the invention has even less aberration over increased wavelength range due to better corrected chromatic aberration.

Description:
RELATED PATENTS 
     This application claims benefit of the filing date of Provisional U.S. Patent Application No. 60/156,501, filed Sep. 28, 1999, and hereby incorporates the disclosure thereof by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to objective lenses used in a variety of applications, including microlithography, photo-ablabon, ultraviolet (UV) inspection, and mastering of optical data storage media, such as CD and DVD. 
     BACKGROUND OF THE INVENTION 
     Prior art objective lenses suffer from problems that render them inferior for our purposes. One type of prior art objective lens suitable for many applications is the double mirror objective lens, examples of which are shown in FIGS. 1A,  2 A, and  3 A, which are shown approximately to scale relative to one another. While relatively easy and inexpensive to manufacture, and while enjoying little, if any, chromatic aberration, double mirror objectives using two spherical mirrors must generally adhere to the Schwarzschild ratio, a ratio of the radii of curvature of the lenses equal to a value of about 2.6, to reduce aberrations. Adherence to the Schwarzschild ratio limits the amount by which obscuration can be reduced without undesirably increasing aberrations. In the example shown in FIG. 1A, the double mirror objective  1  includes a light path  2  along which light travels to strike a convex mirror  3 , which reflects the light onto a concave mirror  4 . In the objective lens of FIG. 1A, the light path  2  includes a hole in the concave mirror  4 , but the light can be transmitted to the convex mirror  3  in other ways. The concave mirror  4  then reflects the light toward the convex mirror  3  and toward a focal point  5  of the objective  1 , which is typically behind the convex mirror  3 . This type of double mirror objective forms images with little or no chromatic aberrations, but suffers a large percentage of obscuration by the convex mirror  3  and has a relatively small maximum numerical aperture. When made with a relatively large working distance, the spherical aberrations throughout the image field are unacceptably large as shown in FIGS. 1B-1F. FIG. 2A shows an objective  1 ′ of the type shown in FIG. 1A, but made in relatively compact form to reduce spherical aberrations. While the spherical aberrations of the objective  1 ′ of FIG. 2A are significantly improved as seen in FIGS. 2B-2F, the objective  1 ′ has a much smaller working distance, still has a large obscuration, and still has a relatively small maximum numerical aperture. In FIGS. 2B-2F, traces for three wavelengths, λ 1 , λ 2 , λ 3 , have been included to illustrate the lack of significant chromatic aberrations introduced by this type of objective. In the particular example, λ 1 =248.6 nm, λ 2 =248.4 nm, and λ 3 =248.2 nm. 
     In applications requiring lower obscurations, the double mirror objective can be forced to have a smaller obscuration as shown in FIG.  3 A. Here, the objective  1 ″ includes a convex mirror  3 ″ that is much smaller in relation to the light cone reflected from the concave mirror  4 ″, yielding a much lower obscuration. However, the reduced obscuration comes at the cost of high spherical aberrations, as seen in FIGS. 3B-3F. While these aberrations can be reduced using large departure aspheric mirror surfaces, such large departure surfaces are more difficult and costly to manufacture. As in FIGS. 2B-2F, traces for three wavelengths, λ 1 , λ 2 , λ 3 , have been included in FIGS. 3B-3F to illustrate the lack of significant chromatic aberrations introduced by this type of objective. In the particular example, λ 1 =248.6 nm, λ 2 =248.4 nm, and λ 3 =248.2 nm. 
     Adding a refractor to the double mirror objective overcomes the problem of spherical aberration in relatively low obscurabon double mirror objectives while avoiding the use of large departure aspheric surfaces. However, introducing such a refractor into the objective yields a catadioptric lens system, which suffers from chromatic aberrations. The chromatic aberrations limit the wavelength range or bandwidth with which a given prior art catadioptric objective can reasonably be used, making such systems impractical for many applications. In addition, prior art catadioptric objectives have problems related to placement of the convex mirror that render the objectives insufficient for our needs. 
     A particularly good prior art catadioptric objective includes the convex mirror placed in front of a two optical surface refractor. See, for example, U.S. Pat. No. 2,520,635 to Grey. While this arrangement significantly reduces spherical aberration and suffers from a lower chromatic aberration than many other catadioptric objectives, it still suffers from an unacceptably large obscuration. Further, the numerical aperture of this type of objective is still limited in its range. In addition, the convex mirror must be glued or otherwise attached to the refractor, which can cause other problems. In particular, glue can foul the refractor and, in some cases, can break down if exposed to UV rays. 
     Another good prior art catadioptric objective, an optical disc reader objective disclosed in U.S. Pat. No. 4,835,380 to Opheij et al., uses a concave Mangin mirror with a convex mirror positioned at the center of the Mangin mirror&#39;s front surface. This objective enjoys the reduced spherical aberrations of the catadioptric system, but can only be used with a narrow bandwidth of light. Here again, the convex mirror must be glued to or otherwise attached to or supported against the Mangin mirror&#39;s front surface. While glues and adhesives work well for the wavelengths used in optical disc readers, many other applications, such as microlithography, use wavelengths that break down most adhesives in short order. In addition, adhesives can alter the light to some degree. Thus, gluing a convex mirror on the front of a Mangin mirror is not a viable solution in applications using wavelengths of light unfriendly to adhesives or that require a minimum of image degradation. 
     Thus, prior art double mirror objectives can enjoy reduced chromatic and spherical aberration when sized in accordance with the Schwarzschild ratio, but suffer from high obscurations and short working distances. Prior art catadioptric objectives enjoy lower obscurations and low spherical aberrations, but suffer from narrow usable bandwidths because of relatively high chromatic errors. Further, no prior art system has adequately low obscuration, adequately low spherical aberration, and a suitable working distance for many applications, let alone the additional feature of low chromatic aberration. All of these prior art objectives further suffer from relatively low numerical apertures. 
     SUMMARY OF THE INVENTION 
     We achieve superior performance in a catadioptric objective by moving the convex mirror to an opposite side of the refractor from the concave mirror, preferably without the use of adhesives or other supports. In the preferred embodiments, we use a multifunction component with a refractor portion and a reflector portion, the reflector portion being the convex mirror and being formed in a surface of the refractor portion. We prefer to grind a depression into a back or right surface of the refractor portion so that the periphery of the reflector portion is surrounded by the back or right refractive interface of the refractor portion. We then polish the depression and coat it with a coating that renders the depression reflective into the refractive material of the refractor portion and toward the concave reflector. In this manner, we produce a multifunction component that retracts, reflects, and again refracts light in one part of a light path through the objective, then refracts the light twice in another part of the light path. For example, in an objective where the light enters the system and crosses the front or left refractive interface of the refractor portion, the light is refracted as it crosses the refractive interface, travels through the refractor portion to reflect from the convex mirror, travels across the front or left refractive interface of the refractor portion where it is again refracted. The light then travels to the concave mirror, where it reflects back toward the multifunction component, at which point the light is refracted twice as it crosses the left and right refractive interfaces of the outer portion of the refractor portion. This preferred embodiment yields an objective with very little spherical aberration over a relatively narrow bandwidth. In a variation of this preferred embodiment, we form the concave mirror as a Mangin mirror and orient the Mangin mirror so that the surface not carrying the reflective coating is an additional refractive interface in the objective. This additional refractive interface corrects chromatic errors, yielding an objective with low spherical and chromatic aberrations that is useful over a relatively wide bandwidth. 
     A major benefit of the invention is that it can be used with other optics on either end of the objective or even inserted between the concave mirror and the multifunction component. When used with other components in this manner, the concave mirror and multifunction component can greatly reduce spherical and chromatic errors for particular applications, such as those discussed above. Thus, the invenbon can be applied in a great many ways and with a great many variations. 
     In a variation of the exemplary embodiment, we form the concave mirror as a Mangin mirror, effectively creating a second multifunction component. The left surface of the Mangin mirror preferably carries the reflective coating so that the right surface of the Mangin mirror is an additional refractive interface. This additional refractive interface corrects chromatic errors introduced by the concave mirror, then reflects from the concave mirror to the refractor so that the light refracts through the first and second surfaces of the refractor as it travels to a second end of the system. Such an arrangement yields an objective that suffers from little spherical aberration, enjoys low obscuration, has a higher numerical aperture, and has a relatively large working distance, overcoming most of the deficiencies of prior art objectives. 
     We have also found that forming the concave mirror as a Mangin mirror not only further reduces the spherical aberration caused by the concave mirror, but also dramatically reduces the chromatic aberration of the system. Thus, this form of our inventive catadioptric objective overcomes even more of the deficiencies of the prior art and is suitable for a wide variety of applications. 
     In our preferred embodiment, we grind and polish a depression in the right surface of the refractor, coating the depression with a reflective coating, if necessary, to form the convex mirror. This overcomes the problems in the prior art associated with supporting the convex mirror, reduces obscuration by moving the convex mirror back, and allows greater correction for spherical aberration since light incident on the convex mirror is refracted on its way to the mirror and on its way out of the refractor. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic representation of a prior art double spherical mirror objective lens system in cross-section with a relatively high working distance, but high obscuration and spherical aberration. 
     FIGS. 1B-1F illustrate the tangential and sagittal optical path difference through the objective of FIG. 1A at various points in the image field at a particular wavelength. 
     FIG. 2A is a schematic representation of a prior art double spherical mirror objective lens system in cross-section with the same obscuration as that shown in FIG. 1 with improved spherical aberration, but with relatively short working distance. 
     FIGS. 2B-2F illustrate the tangential and sagittal optical path difference through the objective of FIG. 1A at various points in the image field for three particular wavelengths. 
     FIG. 3A is a schematic representation of a prior art double spherical mirror objective lens system in cross-section with low obscuration and relatively large working distance, but with high spherical aberration. 
     FIGS. 3B-3F show the tangential and sagittal optical path difference through the objective of FIG. 1A at various points in the image field for three particular wavelengths. 
     FIG. 4A is a schematic representation of an objective lens system according to the invention in cross-section. 
     FIG. 4B is a cross sectional close-up schematic representation of the refractor of the invention of FIG.  4 A. 
     FIG. 4C is a view of the invention as shown in FIGS. 4A and 4B taken along line  4 C- 4 C in FIG.  4 B. 
     FIGS. 4D-4H illustrate the tangential and sagittal optical path difference through the objective of FIG. 4A at various points in the image field for three particular wavelengths. 
     FIG. 5A is a schematic representation of an objective lens system according to the invention in cross-section in which another refractor carries the concave mirror. 
     FIG. 5B is a cross sectional close-up schematic representation of the refractor of the invention of FIG.  5 A. 
     FIG. 5C is a view of the invention as shown in FIGS. 5A and 5B taken along line  5 C- 5 C in FIG.  5 B. 
     FIGS. 5D-5H illustrate the tangential and sagittal optical path difference through the objective of FIG. 5A at various points in the image field for three particular wavelengths. 
     FIG. 6 is an illustrative schematic in cross-section of an apparatus using the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     In this description, and in FIGS. 4A-5H, we provide an exemplary embodiment of the invention in its simplest form, with some variation, as an infinite conjugate objective with a numerical aperture of about 0.6, a wavelength range or bandwidth of from about 248.2 nm to 248.6 nm, silicon dioxide on the refractive surfaces, and, unless otherwise noted, 10% obscuration. However, these examples do not limit the invention in any way. The inventive objective could also be a finite conjugate objective, and other applications, wavelength ranges, numerical apertures, materials, and obscurations can be used as desired and as appropriate without departing from the spirit of the invention. Further, additional components can be added to the system without departing from the spirit of the invention so long as the multifunction component is present and receives and corrects light passing through the system. For example, additional components could be interposed between the concave reflector and the main refractor, after the main refractor, and/or before the concave reflector. Thus, our invention can comprise a core of more complex optical systems. The illustrative schematic of FIG. 6 provides an example of an apparatus in which additional components are used with the invention. Also, for simplicity we describe the light passing through the objective as traveling in one direction. Those skilled in the art will realize that the light can travel either way in the invention, even at the same time, as indicated by the double arrowheads on the traced rays in the FIGS. 
     With particular reference to FIGS. 4A-4C, our inventive objective lens system  10  includes a multifunction component  20  with three or more optical surfaces  211 ,  221 ,  222 . A left side  21  of the multifunction component  20  carries a first refractive interface  211 , while a right side  22  of the multifunction component  20  carries a second refractive surface  221  and a convex reflective surface  222 . As seen in FIG. 4A, a depression in the right side  22  of the multifunction component  20  carries the convex reflective surface  222 . We prefer to form the depression by grinding and polishing, but other methods of manufacture capable of producing the depression to required tolerances can be used. A coating applied to the surface of the depression renders the surface reflective, thus forming the convex reflective surface  222 . 
     A light path  30  through the objective  10  allows light to pass through the objective  10  and through the refractor  20 , as seen in FIG.  4 A. In the exemplary embodiments, the light path  30  includes an opening  31  in a concave reflective surface  40  which allows light to enter or leave the objective  10 , depending upon the particular application. The opening  31  can be a through hole bored through the material of the concave reflective surface  40 , or can simply be a region of the reflective surface  40  where a coating rendering the reflective surface  40  reflective is thin. enough that light can pass through the region to a desired degree. Of course, the most transmissive coating thickness in the region would be a thickness of zero, and this would be the desired thickness for most applications. If the base material carrying the concave reflective surface  40  does not transmit the desired wavelength of light, then the opening  31  must be formed by boring a hole. In some instances, boring a hole to form the opening  31  would be preferred even if the base material carrying the concave reflective surface  40  can transmit the desired wavelength of light. Other light paths  30  can be used, such as light paths including a beam splitter or prism placed between the multifunction component  20  and the concave reflector  40 . 
     While we generally prefer to use spherical surfaces for the mirrors and refractive surfaces, aspheric surfaces can also be used to further enhance correction of aberrations. In addition, the left and/or right surfaces of the multifunction component can be piano, as seen in the example of the invention shown in FIG.  6 . Where the multifunction component  20  has one or both of these surfaces plano, the mirror  222  is still convex and is preferably formed in a depression in the right surface  22  of the multifunction component  20 . 
     The multifunction component  20  is preferably sized and oriented so that light passing through the opening  31  in the concave reflective surface  40  falls on the convex reflective surface  222 , which reflects the light toward the concave reflective surface  40 . The concave reflective surface  40  reflects the light toward the focal point  50  of the system  10 . On its way to the focal point  50 , the light must pass through the multifunction component  20 , which corrects for spherical aberrations. Thus, the multifunction component  20  effectively handles light passing through the objective  10  in two stages, taking the place of two or more components: in one stage, the multifunction component  20  refracts, reflects, and refracts; in another stage, the multifunction component  20  refracts, then refracts again. 
     We have discovered that our multifunction component  20  dramatically reduces spherical aberration in a catadioptric objective  10 , as illustrated in FIGS. 4D-4H. As in FIGS. 2B-2F and  3 B- 3 F, traces for three wavelengths, λ 1 , λ 2 , λ 3 , have been included FIGS. 4D-4H to compare the chromatic aberrations introduced by this type of objective. In the particular example, λ 1 =248.6 nm, λ 2 =248.4 nm, and λ 3 =248.2 nm. In the exemplary embodiment, as discussed above, this multifunction component  20  includes a convex mirror  222  of different radius formed on its right surface  22  and, in a simple catadioptric objective, is arranged so that light headed from a left end of the objective  10  passes through the refractor portion to strike the convex mirror, and reflects from the convex mirror back through the refractor portion and on to the concave mirror in the first stage. The refraction across the left refractive interface  211  of the multifunction component  20  as the light travels between the convex and concave mirrors  222 ,  40  induces increased divergence in the light that significantly increases the aperture. While this introduces chromatic aberrations rendering this simple embodiment useful over a relatively narrow bandwidth, the increase in aperture greatly enhances performance of the objective over that narrow bandwidth. As the light returns through the refractor portion, the refractions across the outer portions of the left and right refractive interfaces  211 ,  221  adjust spherical aberrations and provide enough degrees of freedom that the spherical aberrations introduced by the concave mirror  40  can be corrected. In comparison to the double mirror objectives of FIGS. 1A-3F, our invention has much lower spherical aberration for a given obscurabon. In addition, we have found that we can achieve numerical apertures as high as 0.85 with this form of our invention without introducing significant spherical aberration. 
     In a variation of our invention, shown in FIGS. 5A-5C, a left side  42  of another multifunction component  41  can carry the concave reflective surface  40 ′. This allows for dramatically reducing chromatic aberrations, as seen in FIGS. 5D-5H, so that the system is usable with a broader range of wavelengths, depending on the dispersions of the refractors. As in FIGS. 2B-2F,  3 B- 3 F, FIGS. 4D-4H, traces for three wavelengths, λ 1 , λ 2 , λ 3 , have been included FIGS. 5D-5H to compare the chromatic aberrations introduced by this type of objective. In the particular example, λ 1 =248.6 nm, λ 2 =248.4 nm, and λ 3 =248.2 nm. In this variation, a right surface  43  of the additional multifunction component  41  includes a third refractive surface  431 , which provides the chromatic aberration correction and some spherical aberration correction. The opening  31 ′, if used, can again be a thinning of the reflective coating applied to the left surface of the additional multifunction component  41 , or can be a through hole bored through the entire additional multifunction component  41 . Other light paths  30 ′ can be used, such as light paths including a beam splitter or prism placed between the multifunction components  20 ′,  41 . 
     In our preferred implementation of the invention, the reflective surfaces are mirrors or Mangin mirrors, the multifunction component, which we prefer to form as a three optical surface refractor, is made from a solid material, and the space between the concave mirror and the three optical surface refractor is filled with a gas, such as nitrogen. In a more general sense, however, the refracting surfaces are refractive interfaces between the material of the refractor and a material engaging or surrounding the refractor. Thus, the three optical surface refractor could be made from any material so long as it had an index of refraction higher than that of the material immediately between the three optical surface refractor and the concave mirror. Thus, if there were a way to maintain a suitable liquid, for example, in the desired shape, the three optical surface refractor could be made from the liquid. This is true of the refractive material carrying the concave mirror in the variation of the invention shown in FIGS. 5A-5C as well. 
     Describing our invention in another way, we form the convex mirror  222  by coating a depression in the right side of a special lens  20  with a reflective coating, enabling more correction of light in the system by the lens  20  than that achieved by prior art objectives. The convex mirror  222  is a reflector portion of the lens  20 , the remainder of the lens  20  being a refractor portion. The refractor portion has left and right refractive interfaces  211 ,  221 . As light travels along the light path  30 , it enters the refractor portion of the lens  20  by crossing the left refractive interface  211  and travels to the reflector portion of the lens  20 . The convex mirror  222  reflects the light back into the refractor portion so that it exits the lens  20  by crossing the left refractive interface  211  a second time. This second crossing of the left refractive interface  211  introduces spherical aberration of the opposite sign of that introduced by the concave reflector  40  toward which the light travels from the second crossing of the left refractive interface  211 . When the concave reflector  40  reflects the light back toward the lens  20 , it introduces its spherical aberration so that the total amount of spherical aberration is relatively small. The outer portion of the refractor portion of the lens  20  then provides additional correction for the spherical aberration remaining in the light as the light crosses the left refractive interface of the lens  20  for a third time and the right refractive interfaces of the lens  20  for the first and last time, yielding the very low amounts of spherical aberration seen in FIGS. 4D-4H and  5 D- 5 H. In the variation of the invention in which the concave reflector  40  is the left surface  42  of a Mangin mirror  41 , the refractive material of the Mangin corrects chromatic aberrations in the light. 
     The light path  30  can also be described as crossing the left refractive interface of the lens  20 , passing through the refractor portion of the lens  20  to reflect from the convex reflective surface of the convex mirror  222 , passing through the refractor portion of the lens  20  again, crossing the left refractive interface  222  again, traveling to and reflecting from the concave reflective surface of the concave mirror  40 , traveling back toward the lens  20 , crossing the left refractive interface  222  for a third time, passing through the refractor portion of the lens  20 , and exiting the lens  20  through the right refractive interface  221  of the lens  20 . The light can also travel in the opposite direction along the path as mention above. Where a Mangin mirror is used for the concave mirror  40 ′, the light path includes crossing the right refractive interface  431  of the Mangin mirror twice: once as the light path enters the Mangin mirror, and once as the light path leaves the Mangin mirror. 
     FIGS. 6 shows an example of our invention in use with other optics that condition and/or correct the light passing through our inventive objective. This is but one example of many possible applications of our invention that do not depart from the spirit of the invention. In the example shown in FIG. 6, our invention achieves a numerical aperture of 0.8, a field of view of 40 microns, a bandwidth of 120 pm, and a working distance of 5 mm, at wavelengths of 193 or 157.63 nm with a plate thickness of 2 mm and an EPD of 3 mm using SiO 2  or CaF 2  as the refractive material.