Source: http://www.google.com/patents/US7256932?dq=7,468,661
Timestamp: 2014-07-26 07:36:16
Document Index: 506863916

Matched Legal Cases: ['art 120', 'art 110', 'art 130', 'art 330', 'art 210', 'art 230', 'art 430', 'art 550']

Patent US7256932 - Optical system for ultraviolet light - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign in<nobr>Advanced Patent Search</nobr>PatentsAn optical system for ultraviolet light having wavelengths λ≦200 nm, which may be designed in particular as a catadioptric projection objective for microlithography, has a plurality of optical elements including optical elements made of synthetic quartz glass or a fluoride crystal material transparent...http://www.google.com/patents/US7256932?utm_source=gb-gplus-sharePatent US7256932 - Optical system for ultraviolet lightAdvanced Patent SearchPublication numberUS7256932 B2Publication typeGrantApplication numberUS 11/252,598Publication dateAug 14, 2007Filing dateOct 19, 2005Priority dateOct 19, 2004Fee statusPaidAlso published asDE102005045862A1, US20060119750Publication number11252598, 252598, US 7256932 B2, US 7256932B2, US-B2-7256932, US7256932 B2, US7256932B2InventorsAlexander Epple, Toralf Gruner, Wolfgang SingerOriginal AssigneeCarl Zeiss Smt AgExport CitationBiBTeX, EndNote, RefManPatent Citations (12), Non-Patent Citations (2), Referenced by (5), Classifications (17), Legal Events (3) External Links: USPTO, USPTO Assignment, EspacenetOptical system for ultraviolet lightUS 7256932 B2Abstract An optical system for ultraviolet light having wavelengths λ≦200 nm, which may be designed in particular as a catadioptric projection objective for microlithography, has a plurality of optical elements including optical elements made of synthetic quartz glass or a fluoride crystal material transparent to a wavelength λ≦200 nm. At least two of the optical elements are utilized for forming at least one liquid lens group including a first delimiting optical element, a second delimiting optical element, and a liquid lens, which is arranged in an interspace between the first delimiting optical element and the second delimiting optical element and contains a liquid transparent to ultraviolet light having wavelengths λ≦200 nm. This enables effective correction of chromatic aberrations even in the case of systems that are difficult to correct chromatically.
The European patent application EP 1 524 558 A1 with application number 03256499.9 and application date Oct. 15, 2003 (corresponding to US 2005/179877 A) describes a projection system for immersion lithography. In one embodiment, the immersion liquid introduced between the last optical element of the projection objective and the substrate is utilized as a manipulator for shifting/tilting the last optical element in order to produce different focus positions of the projection radiation for so-called �focus drilling�.
SUMMARY OF THE INVENTION It is one object of the invention to provide an optical system for ultraviolet light having wavelengths of λ≦200 nm which, in comparison with conventional optical systems for this wavelength range, has more degrees of freedom for the optical design, and in particular more degrees of freedom for the optical correction. It is another object to provide an optical system that is suitable as a projection objective for a microlithography projection exposure apparatus and which permits high numerical apertures which, with the use of an immersion liquid having a high refractive index, also enable effective numerical apertures NA>1 for image generation.
In one development, the liquid has, at an operating wavelength of the optical system, a dispersion DL greater than the dispersion DS of the highest dispersive solid material used for the optical elements at the operating wavelength, and the interspace has the form of a negative lens. As a result, it is possible with the aid of the liquid to provide a highly dispersive diverging lens which, in interaction with at least one positive lens made of a solid material having less wavelength dependence of its refractive index, enables an effective color correction in particular of the chromatic longitudinal aberration CHL. The term �dispersion� here designates the refractive dispersion dn/dλ describing the wavelength dependence of the refractive index of a transparent material. The dispersion DL may in particular be higher than that of synthetic quartz. In the case of a negative liquid lens, the axial extent of the interspace increases from the optical axis toward the edge, so that the center thickness is less than the edge thickness, a biconcave interspace preferably being formed.
Preferred optical systems are designed as imaging systems for imaging a pattern arranged in a first surface into a second surface, which is optically conjugated with respect to the first surface. In the case of projection objectives for microlithography, a mask (reticle) may be arranged in the first surface (object surface) and a wafer to be exposed may be arranged in the second surface (image surface). The optical system may be designed in such a way that a direct imaging is effected without an intermediate image. It is also possible for one or more intermediate images to be generated between the first and second surfaces, said intermediate images lying in the region of further field surfaces of the system. Pupil surfaces of the imaging lie between the field surfaces. In preferred embodiments, the interspace or the liquid lens is arranged in a region of the optical system that is near the pupil. The liquid lens thus lies at least partly in a pupil surface or in optical proximity thereto. A �region that is near the pupil� in this sense is distinguished in particular by the fact that, in the region which is near the pupil, the marginal ray height of the imaging is larger than the principal ray height. Preferably, the marginal ray height in the region of the liquid lens is at least twice as large as the principal ray height. A negative lens in the region of large marginal ray heights may contribute particularly effectively to the color correction, in particular to the correction of the chromatic longitudinal aberration CHL.
In one development, the liquid used for forming the liquid lens substantially consists of water (H2O). Ultrapure water has very recently turned out to be a candidate for an immersion liquid having sufficient transparency and stability at an operating wavelength of 193 nm. A further advantage afforded by the use of a liquid lens material is that the relevant optical properties, in particular the dispersion and the refractive index, can be altered if appropriate by means of suitable additives. By way of example, by adding additives consisting of sulfates, alkali metals such as e.g. cesium or phosphates to water, it was possible to produce ionized liquids whose refractive index is higher than that of ultrapure water (nH 2 O≈1.43) (cf. Internet publication entitled ��Doped water� could extend 193-nm immersion litho� by D. Lammers, http://www.eetimes.com/semi/news/jan.2004). International patent application WO 2005/050324 also discloses additives suitable for increasing the refractive index of water. Cs2SO4 and H3PO4 in various concentrations are given as examples.
Immersion liquids based on perfluoropolyethers (PFPE) are favored at the present time for 157 nm. One tested immersion liquid has at 157 nm a refractive index nI≈1.37 (cf. article: �Immersion lithography at 157 nm� by M. Switkes and M. Rothschild, J. Vac. Sci. Technol. B19 (6), November/December 2001, page 1 et seq.). From available data, for one immersion liquid that is suitable at 157 nm (trademark Fomblin�), it is possible to estimate a dispersion value DL=0.001186/nm, which is lower than the dispersion of calcium fluoride at this wavelength (DS=0.002259/nm). Therefore, in order to attain a color correction, it is advantageous to configure such liquid lenses as positive lenses, in which case the achromatizing effect may then essentially be provided by surrounding negative lenses made of calcium fluoride.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a lens section through an embodiment of an optical system according to the invention which is designed as a catadioptric projection objective for microlithography at 193 nm;
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is explained below on the basis of exemplary embodiments in which optical imaging systems are optimized in the form of projection objectives for microlithography in accordance with the invention.
The second objective part 120 consists of a first concave mirror 121 with a concave mirror surface pointing toward the object plane 101, and a second concave mirror 122 with a concave mirror surface pointing toward the image plane 102. The aspheric mirror surfaces of the two mirrors are continuous, that is to say that they have no holes or perforations. Each of the mirror surfaces of the concave mirrors defines a curvature surface that is a mathematical surface which extends beyond the edges of the physical mirror surfaces and contains this mirror surface. The first and second mirror surfaces are parts of rotationally symmetrical curvature surfaces with a common axis of symmetry that coincides with the optical axes�arranged coaxially with respect to one another�of the first objective part 110 and of the third objective part 130. Therefore, the entire projection objective 100 is rotationally symmetrical and has a single, straight, unfolded optical axis 105 common to all the refractive and reflective optical components.
Insofar as reference is made to a �marginal ray height� or a �principal ray height� in this application, then these are taken to mean the paraxial marginal ray height and the paraxial principal ray height, even though the paraxial rays do not contribute to the imaging in the case of systems with abaxial object field and image field.
Table 1 summarizes the specification of the design in tabular form. In this case, column 1 specifies the number of a refractive surface or surface distinguished in some other way, column 2 specifies the radius r of the surface (in mm), column 3 specifies the distance d�designated as thickness�between the surface and the subsequent surface (in mm) and column 4 specifies the material of the optical components. Column 5 shows the refractive index of the material, and column 6 specifies the usable free radii or the free semidiameters of the lenses (in mm). The aspheric surfaces are identified by �*� in column 1. Table 1A specifies the corresponding aspheric data, the aspheric surfaces being calculated according to the following specification:
An essential difference with respect to the system from FIG. 2 is that here a lens component 350 is arranged in the refractive third objective part 330 in the region of large marginal ray heights in direct proximity to the pupil surface P3. Said lens component has an object-side, biconcave negative lens 351 made of relatively highly dispersive synthetic quartz glass and a directly succeeding image-side, biconvex positive lens 352 made of�relative thereto�lower-dispersive calcium fluoride. The absolute value of the radius of curvature of the concave exit side of the negative lens 351 is only slightly greater than that of the convex entry side of the positive lens 352, so that the lenses can be brought together in axial proximity at a very small axial distance of approximately 1 mm and the surface distance is less than approximately 4 mm over the entire lens cross section. Whereas all the lenses of the first objective part 210 consist of synthetic quartz glass, positive lenses made of calcium fluoride are provided in the third objective part 230 primarily in proximity to the pupil. The narrow interspace between the lenses 351, 352 is not filled, but in a different embodiment may be filled with a medium n>1 in order to make the system less sensitive toward misalignments.
A non-achromatized system having the basic construction shown has a chromatic longitudinal aberration CHL of approximately 500 nm/pm. In order to improve the color correction, a liquid lens group 450 is provided in the third objective part 430 in the region of large marginal ray heights, said liquid lens group comprising, as delimiting elements, on the object side, a negative meniscus lens 451 that is concave toward the image and, on the image side, a negative meniscus lens 459 that is concave on the object side. During operation of the immersion objective, a liquid lens 455 having positive refractive power is provided in the biconvex interspace 454 delimited by the lenses 451, 459. The liquid is a liquid perfluoropolyether (PFPE), which is also used as an immersion liquid I between objective exit and image plane 402. A liquid known by the brand name Fomblin� is utilized here, which has, at 157 nm, a refractive index n≈1.372 and a dispersion DL≈0.0119/nm. This value is produced as an estimated value from published data. What is important for the function as correction means for the chromatic correction is that the liquid 455 has a lower dispersion than calcium fluoride (DCaF2≈0.002259/nm). On the basis of the principles explained above, within a positive/negative group, the medium having the lower dispersion is used in the positive lens (liquid lens), while the more greatly dispersive medium (calcium fluoride) forms the adjacent negative lenses. The chromatic longitudinal aberration CHL of this exemplary embodiment is approximately 300 nm/pm. This is a significantly improved value compared with an otherwise largely structurally identical, but non-achromatized design having a corresponding value of approximately 500 nm/pm.
The specification of a further exemplary embodiment (not illustrated pictorially) is given in Tables 6, 6A (aspheric data). In this case, the only difference with respect to the embodiment shown in FIG. 5 is that individual positive lenses in the aperture convexity of the third objective part 550 were replaced by lenses made of calcium fluoride and having substantially identical dimensions. The lenses 551, 559 surrounding the liquid lens 555 are still quartz glass lenses, however, in order to avoid the protective layer problem explained above. The positive lenses, which consist of calcium fluoride in the case of the embodiment not shown, are the two positive lenses arranged directly before the liquid lens group 550 and the positive lens arranged directly after the liquid lens group. These lenses are identified by �C� in FIG. 5. By introducing a third optical material in addition to the synthetic quartz glass and the water of the biconcave liquid lens 555, it is possible to provide a fully achromatized design having a chromatic longitudinal aberration CHL=0 nm/pm.
FIG. 6 shows an embodiment of an optical system 600 in the form of a refractive projection objective for immersion lithography. It serves for imaging a pattern of a reticle present in the object plane 601 onto a light-sensitive substrate in the image plane 602 without the generation of an intermediate image with the aid of an immersion liquid I arranged between the exit of the projection objective and the image plane 602. The projection objective is incorporated into a projection exposure apparatus which permits slight manipulations of the axial position of object (mask) and substrate (wafer) (see double arrows). The projection objective itself contains a plurality of manipulators which make it possible to adjust the position of individual lenses and i.e. the axial position thereof and/or the centering thereof and/or the tilting position thereof without demounting the projection objective by activation of the manipulators (see double arrows). A so-called �two-convexity system� is involved, in which a �waist� 620 is formed between an object-side convexity 610 and an image-side convexity 630, the beam bundle diameter having its narrowest constriction in the region of said waist. Two-convexity systems for immersion lithography at 193 nm are known inter alia from the applicant's patent application WO 03/075049 A2.
A special feature of the system consists in the fact that the projection objective contains a liquid lens group 650, which in this example is arranged in the region of the largest beam diameter of the first convexity 610. In general, the liquid lens group can be fitted at any other location of the projection objective. The liquid lens group comprises an image-side, first delimiting element 651, an object-side, second delimiting optical element 659 and a liquid lens 655 arranged between these elements. In this connection, the term �liquid lens� is intended generally to denote a transparent optical element formed by a liquid and having an entry surface and exit surface, the entry surface and/or the exit surface generally being curved, but they may also be essentially planar. A pressure generating device 670 is connected to the interspace connected in liquidtight fashion along its periphery, the liquid lens 655 being situated in said interspace, which pressure generating device is designed for increasing the liquid pressure of the liquid, according to control signals of a control unit, beyond the ambient pressure of the liquid lens group in a targeted manner in order, in this way, to provide a hydraulically actuable manipulator which can be radiated through and by means of which the imaging performance of the projection objective can be varied dynamically within certain limits governed by the construction of the manipulator.
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