Patent Document

BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to an optical subsystem, in particular for a projection exposure system, wherein a light bundle passes through the subsystem and the optical subsystem has at least one optical element, on which the rays of the light bundle impinge on a first used area. Projection exposure systems for microlithography, in particular for wavelengths of ≦193 nm have become known from a plurality of applications. Relative to catadioptric systems, reference is made to DE-A-100 20 592; relative to refractive systems, reference is made to DE 198 55 157 and DE-A-199 05 203, the disclosure content of which is incorporated in its entirety in the present application. The field of the invention includes the field of projection exposure systems, in particular, those that operate with EUV radiation.  
         [0003]     2. Description of the Related Art  
         [0004]     At the present time, wavelengths in the range of 11-14 nm, in particular 13.5 nm, are discussed as wavelengths for EUV lithography, with a numerical aperture of 0.2-0.3. The image quality in EUV lithography is determined, on the one hand, by the projection objective, and on the other hand, by the illumination system. The illumination system will make available an illumination that is as uniform as possible in the field plane, in which the pattern-bearing mask, the so-called reticle, is disposed. The projection objective images the field plane in an image plane, the so-called wafer plane, in which a light-sensitive object is disposed. Projection exposure systems for EUV lithography are designed with reflective optical elements. The shape of the field in the image plane of an EUV projection exposure system is typically that of an annular field with a high aspect ratio of 2 mm (width)×22-26 mm (arc length). Projection systems are usually operated in scanning mode. With respect to EUV projection exposure systems, reference is made to the following publications: W. Ulrich, S. Beiersdörfer, H. J. Mann, “Trends in Optical Design of Projection Lenses for UV- and EUV-Lithography” in Soft-X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Eds.), Proceedings of SPIE, Vol. 4146 (2000), pages 13-24 and M. Antoni, W. Singer, J. Schultz, J. Wangler. I. Escudero-Sanz, B. Kruizinga, “Illumination Optics Design for EUV-Lithography” in Soft X Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Eds.), Proceedings of SPIE, Vol. 4146 (2000), pages 25-34.  
         [0005]     The disclosure content of the above publications is incorporated in its entirety in the present application.  
         [0006]     In projection exposure systems, which operate at wavelengths of ≦193 nm, in particular, in the range of ≦157 nm, particularly in the EUV range with wavelengths of &lt;30 nm, the problem arises that radiation in the EUV- or VUV and DUV range, respectively, lead to a contamination and/or disruption of the optical surface of the components, which are also denoted optical elements.  
         [0007]     The first and last optical surfaces, for example, of refractive systems are particularly at risk of contamination, since these are found in the direct vicinity, e.g., of a source, of a mask, or of a wafer to be exposed. Contaminants are introduced into the optical system by these surfaces. It is thus usual to protect these surfaces on either end, e.g., by a “skin”, which comprises thin foils. Such foils, however, lead to absorption and, in addition, introduce aberrations into the optical system.  
         [0008]     The high-energy radiation of the light sources of ≦193 nm leads to the fact that, for example, the residual oxygen components are converted into ozone by the radiation, which in turn attacks and can disrupt the surfaces of the optical elements and their coatings.  
         [0009]     In addition, contaminations can be formed on the optical surface due to concentrations of residual gas, such as hydrocarbons from the atmosphere surrounding the optical surface, e.g., due to crystal formation or layers, e.g, of carbon or carbon compounds. Such contamination leads to a reduction in reflection in the case of reflective components and to a reduction in transmission in the case of transmissive components. The contamination may thus depend on the illumination intensity. In the case of EUV lithography, one may use sources, which emit a broadband spectrum. Even after a spectral filtering, e.g., with a grating spectral filter or a zirconium foil, a broad spectrum of high-energy radiation is present. Particularly high is the load in the first optical element up to the first multilayer mirror in an EUV system, since except for the radiation at, e.g., 13.5 nm, the broadband radiation of the source is present and thus the radiation load is maximal. In an EUV projection exposure system, the reflection is particularly reduced due to contamination in the case of the first optical element up to and including the first normal-incidence mirror in an illumination system for a projection exposure system. A normal-incidence mirror in this application is to be understood as a mirror on which the rays of the incident light bundle strike at an angle of &lt;70° relative to the surface normal line.  
         [0010]     The reflection loss on the first normal-incidence mirror is greatest in an illumination system of an EUV projection exposure system for this reason, since this mirror receives the highest power density of the light source, but essentially reflects only selectively at 13.5 nm, on account of the multiple layers. All other radiation which is emitted by the EUV source is thus converted into absorption power. Carbon or carbon compounds are again removed by regular cleaning of the mirror, for example, by means of admixtures of argon and oxygen under an RF plasma.  
         [0011]     Relative to the cleaning of contaminated optics, reference is made to the following publication:  
         [0012]     T. Eggenstein, F. Senf, T. Zeschke, W. Gudat, Cleaning of contaminated XUV-optics at Bessy II α , Nuclear Instruments and Methods in Physics Research A 467-468 (2001) p. 325-328,  
         [0013]     the disclosure content of which is incorporated in its entirety in the present application.  
         [0014]     Such cleaning of the mirror is, of course, necessary at short time intervals. The useful operating time of the machine is thus very sharply reduced in this way. It may be necessary, for example, to repeatedly clean the first normal-incidence mirror in an EUV projection exposure system as often as approximately every 20 hours of operation. This cleaning lasts, for example, for approximately 2 hours, i.e., 10% of the use time.  
       SUMMARY OF THE INVENTION  
       [0015]     The object of the invention is thus to offer an optical subsystem of a projection exposure system that is characterized in that the use times are increased in comparison to the devices known from the prior art. According to the invention, this is achieved by the fact that at least one optical element of the optical subsystem has a surface which is at least twice as large as the dimensions of the first used area on this optical element. By this measure, it can be achieved that an optical area on the mirror or a transmissive optical element can be regularly removed from the beam path in the optical subsystem and is cleaned, while a cleaned used area remains in the beam path. In this manner, it is possible to clean the contaminated optical elements regularly without interrupting the operating time of the machine.  
         [0016]     It is particularly advantageous if the first used area and the used area(s) of the optical element have an identical optical effect. This is achieved in that the optical element is symmetric to a point of rotation, symmetric to an axis of rotation, or has several used areas with an identical optical effect, which are disposed along a translation axis, which is also denoted a displacement axis.  
         [0017]     If the optical element has a point of rotation, then the optical element can be rotated around the point of rotation, in order to bring the different used areas with identical optical effect into the beam path.  
         [0018]     As an alternative to a configuration symmetric to a point of rotation, the optical element can also be configured symmetric to an axis. The optical element then comprises an axis of rotation.  
         [0019]     Such an optical element can be rotated around the axis of rotation in order to bring it from a first position into another position.  
         [0020]     In another configuration of the invention, the optical element has a translation invariancy. In the case of such optical elements, the different optically identical regions can be brought into the beam path by displacement along an axis of translation, which is also denoted a displacement axis.  
         [0021]     In a particularly advantageous embodiment, it is provided that the optical element is a reflective optical element, for example, a planar mirror, a spherical mirror, a grid, or an optical element with raster elements, wherein the raster elements are comprised of identical mirrors, in general a mirror with a behavior that is rotation- or translation-invariant. In another embodiment, it is provided that the optical element is a transmissive optical element, for example, a filter element or a refractive optical element. Refractive optical elements can be, for example, a planar plate, a lens, or an optical element with raster elements, wherein the raster elements are comprised, e.g. of lenses, a beam splitter, or, in general, a refractive element with a rotation- or translation-invariant behavior.  
         [0022]     In particular, in optical subsystems for EUV lithography, the optical element can be a reflective optical element, for example, a mirror with multiple layers, on which the rays of a light bundle impinge at angles α&lt;70° in the used area. It would also be possible to use here a grazing-incidence mirror, on which the rays of the light bundle impinge at an angle α&gt;70° relative to the surface normal line.  
         [0023]     In a particular embodiment of the invention, the optical subsystem comprises a cleaning chamber. The cleaning chamber has an atmosphere separated, e.g., by vacuum technology, from the chamber, for example, the vacuum chamber of the rest of the optical subsystem. In the cleaning chamber that is separate from the remainder of the optical subsystem, a specific gas concentration can be introduced for purposes of cleaning, e.g., preferably an oxygen concentration and/or an argon concentration, or a flow of gas, as well as other means for cleaning, such as, for example, a UV light source, an RF antenna for generating a high-frequency plasma, electrodes for applying fields, or mechanical cleaning means. The mirror surface not in use is cleaned in one or another of the ways described above in the cleaning chamber.  
         [0024]     In EUV lithographic systems, in addition to the formation of a separate vacuum chamber for cleaning purposes, it may also be provided that the optical element to be cleaned, in particular the optical mirror to be cleaned, is itself separated relative to the vacuum chamber of the rest of the system. An arrangement of the optical element to be cleaned in a separate vacuum chamber has the advantage that the mirror can be continually cleaned during operation by mixing in a specific oxygen concentration, and a full cleaning will be necessary only after longer operating times. Due to the arrangement in a separate vacuum chamber, the remainder of the system is protected, for example, from possible harmful effects of the cleaning. The optical subsystem is preferably an illumination system of a projection exposure system, but it can also be the projection objective itself, in which one or more optical elements according to the invention are formed, so that they allow a simple cleaning without interrupting operation.  
         [0025]     In addition to the optical subsystem, the invention also provides a projection exposure system for EUV lithography with such an optical subsystem as well as a method for the production of microelectronic components. The invention will be described below on the basis of the figures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  shows a projection exposure system according to the invention, wherein the illumination system is designed as the first optical subsystem according to the invention  
         [0027]      FIG. 2  shows an optical element according to the invention with a first half being used and a second half introduced into a cleaning position  
         [0028]      FIG. 3  shows a mirror or refractive lens consisting of a concentric meniscus and a curvature midpoint  
         [0029]      FIG. 4  shows a transmissive planar plate with first and second halves  
         [0030]      FIGS. 5, 6  show a transmissive plano-convex lens with first and second halves. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0031]      FIG. 1  shows an EUV projection exposure system with an optical element  1  according to the invention. The optical element  1  is a normal-incidence mirror, onto which rays  2  of a light bundle  4  from light source  3  to field plane  22  impinge at angles α&lt;70° to the surface normal line in a first used area  12 .  
         [0032]     The dimensions of the optical element  1 , here the normal incidence mirror, as can be seen from  FIG. 1 , are essentially larger than the first used area  12  of the mirror. The second used area  14  of the mirror is transported into a cleaning chamber  16  and is cleaned presently and not used.  
         [0033]     Transport into the cleaning chamber  16  is conducted in the present case by turning around the axis of rotation  18 . Other possibilities are also conceivable for transport into the cleaning chamber without departing from the basic concept of the invention. Such possibilities include the lateral transport of planar optical elements or the rolling, e.g., of spherical mirrors or concentric menisci.  
         [0034]     As is clearly seen in  FIG. 1 , the cleaning chamber  16  is largely separated by vacuum technology from the vacuum chamber of the remainder of the illumination system. The cleaning chamber comprises an inlet line  15  for supplying a cleaning gas for cleaning the contaminated surface as well as an outlet channel  17 . The cleaning gas preferably involves oxygen and/or argon. Further, additional and/or alternative means for cleaning, such as, for example, a UV light source, an RF antenna for generation of a high-frequency plasma or electrodes for applying an electrical voltage can be found in cleaning chamber  16 . These additional or alternative means are not shown in  FIG. 1 . If all contaminations are removed from the mirror surface of the second used area  14  of the optical element, then, by rotation around the axis of rotation  18 , the mirror can again be brought into a position in which the cleaned surface is used and the now contaminated surface in the first used area will be cleaned.  
         [0035]     The EUV projection exposure system comprises in addition a light source  3 , a collecting optical element, a so-called collector  5 , which is formed as a nested collector. The collector  5  images the light source  3  lying in the object plane of the illumination system via an optional planar mirror  9 , which additionally bends the beam path into a secondary light source  5 . 1  in or in the vicinity of a diaphragm plane  7 . 1 .  
         [0036]     In the embodiment shown the light source  3 , which can be, for example, a laser-plasma source or a plasma discharge source, is disposed in the object plane of the illumination system of the projection exposure system.  
         [0037]     The planar mirror  9  can alternatively be designed, e.g., as a grating spectral filter. The grating spectral filter together with the physical diaphragm  7 . 1  blocks the light of undesired wavelengths, in particular wavelengths longer than 30 nm. For example, the focal point of the −1 order comes to lie in the plane of the diaphragm  7 . 1 , i.e., the light source  3  is imaged by collector  5  and grating spectral filter  9  in the −1 diffraction order nearly stigmatically in the plane of diaphragm  7 . 1 . The imaging in all other diffraction orders is not stigmatic. The use of a grating spectral filter is shown in the present embodiment, and this is of advantage, since radiation in the wavelength region of &gt;30 nm can be filtered thereby, but it is in no way absolutely necessary. Thus an arrangement would also be conceivable, in which the grating spectral filter  9  is designed only as a planar mirror or is completely absent. It can also be clearly recognized that the entire vacuum chamber  20  of the illumination system is subdivided into three individual chambers. The light source, the collector and the grating spectral filter are disposed in a first vacuum chamber  21 . 1 . Only the used area  12  that is in use of the optical element  1  is disposed in the second vacuum chamber  21 . 2 . The optical elements of the illumination system that serve for the shaping and illumination of the field in the field plane  22  with an annular field are disposed in the third vacuum chamber  21 . 3 . The separation of the individual vacuum chambers  21 . 1 ,  21 . 2 ,  21 . 3  is undertaken each time by means of differential pumping segments at the two intermediate images  5 . 1  and  5 . 2  of the light source  3 . Such a separation of the illumination system into three vacuum chambers has the advantage that, by a separation of the optical element  1  to be cleaned into a separate vacuum chamber  21 . 2  and by adding, for example, a mixture of an oxygen concentration and an argon concentration in the vacuum chamber  21 . 2 , a continual cleaning of the used area  12  in use of the first mirror  1  can be achieved during operation. In this manner, the service life can be clearly prolonged. Such a subdivision of the vacuum chamber of the illumination system is advantageous, but in no way necessary for the invention, however.  
         [0038]     In addition, the illumination system of the projection system comprises an optical system for shaping and illuminating the field plane  22  with an annular field. As a mixing unit for the homogeneous illumination of the field, the optical system comprises two facet mirrors  29 . 1 ,  29 . 2  as well as two imaging mirrors  30 . 1 ,  30 . 2  and one field-forming, grazing-incidence mirror  32 .  
         [0039]     The first facet mirror  29 . 1 , the so-called field facet mirror, produces a plurality of secondary light sources in or in the vicinity of the plane of the second facet mirror  29 . 2 , the so-called pupil facet mirror. The following imaging optics images the pupil facet mirror  29 . 2  in the exit pupil of the illumination system, which comes to lie in the entrance pupil of the projection objective  26 . The angles of inclination of the individual facets of the first and second facet mirrors  29 . 1 ,  29 . 2  are designed in such a way that the images of the individual field facets of the first facet mirror  29 . 1  are superimposed in the field plane  22  of the illumination system and thus a largely homogenized illumination of the pattern-bearing mask, which comes to lie in this field plane  22  is made possible. The segment of the annular field is formed via the field-forming, grazing-incidence mirror  32  operating under grazing incidence.  
         [0040]     A double-faceted illumination system is disclosed, for example in U.S. Pat. No. 6,198,793, imaging and field-forming components are disclosed in PCT Publication No. WO 01/09681 and U.S. Pat. No. 6,840,640. The disclosure content of these documents is incorporated in its entirety in the present application.  
         [0041]     The pattern-bearing mask, which is also denoted the reticle and is disposed in the field plane  22 , is imaged by means of a projection objective  26  in the image plane  28  of the field plane  22 . The projection objective  26  is a 6-mirror projection objective, such as disclosed, for example, in US Patent Application No. 2002-0056815, the disclosure content of which is incorporated in its entirety in the present application. The object to be exposed, for example, a wafer, is disposed in image plane  28 .  
         [0042]     Although the embodiment shows as the optical element a normal-incidence mirror in an illumination system by way of example, the invention is in no way limited thereto. Of course, any other optical element in the illumination system or even in the projection objective can be cleaned in the manner according to the invention, by selecting the mirrors essentially larger than the used areas and moving the areas not in use into cleaning chambers for cleaning. The invention can also be transferred to purely refractive systems, for example, projection exposure systems for 157 nm.  
         [0043]     Examples are illustrated in FIGS.  2  to  6 , which can also be applied to reflective optical elements according to the invention.  
         [0044]      FIG. 2  shows an example of embodiment of a lens  100 , which is part of a refractively designed optical subsystem according to the invention in section. The lens  100  is subdivided into a first lens half  102  and a second lens half  103 . The first lens half  102  is essentially larger than the second lens half  103 . The first lens half  102  is designed so it can rotate around an axis of rotation  106 . By rotating around this axis of rotation  106 , the first used area  108 . 1  of the first lens half  102  can be moved out of the cleaning position into the position of use and the second used area  108 . 2  can be moved out of the position of use and into the cleaning position and vice versa. In the area of use, the rays of the light bundle  104 , which pass through the optical subsystem, impinge on the refractive optical element, here the lens. The division of the lens into first and second lens halves is made by a wedge along the separation line  110 . Due to the wedge-shaped separation, the axis of rotation  106  corresponds to the first lens half  102  with wedge over the wedge angle, and not with the [principal] optical axis HA of lens  100 , which is comprised of first and second lens halves  102 ,  103 . The axis of rotation  106  passes through the first lens half advantageously above the area of use of the first lens half.  
         [0045]     In the cleaning position, the used area that is to be cleaned and that is part of the first lens half  102  is found in a cleaning device, which is not shown and which can be formed, for example, as a cleaning chamber. The contaminants that arise due to the loading of the lens in this used area, such as, e.g., crystal formation, are removed by means of the introduction of reactive gases and/or irradiation or by other means such as mechanical cleaning, in the cleaning device. If need be, a new coating can also be applied or a desired coating can be reactivated or repaired.  
         [0046]     The first lens half  102  is held in a rotatable first holder  112 . The rotatable first holder  112  is introduced in a guide  114 . The second lens half  104  is held in a rigid second holder  116 . A person skilled in the art can transfer the teaching given for refractive systems to reflective systems without inventive step, and vice versa, can transfer the teaching for reflective systems to refractive systems. Although not explicitly described for each individual case, these teachings are included in the scope of protection of the application.  
         [0047]      FIG. 3  shows as a further example of an embodiment of the invention including an optical element that has a center of rotation. Such a component is given, for example, by a concentric meniscus  150 . The concentric meniscus  150  comprises two used areas: a first used area  154  and a second used area  156 . The optical axis of the area in use each time, that is, the area which is brought into the beam path, here the first used area  156 , is denoted HA. The concentric meniscus is constructed symmetrically to a common midpoint of curvature, the so-called point of rotation  160 . The respective used area can be brought into the beam path, and/or can be taken out of the beam path and brought into the cleaning chamber, by rotating around this point of rotation  160 .  
         [0048]      FIG. 4  shows an embodiment including a transmissive planar plate  200 , which can be rotated into a cleaning device and rotated out from it. The planar plate  200  comprises two used areas, a first used area  208 . 1  and a second used area  208 . 2 . In the present  FIG. 4 , the first used area  208 . 1  is found in the cleaning device and the second used area  208 . 2  is found in the beam path of the rays of the light bundle  204 , i.e., in the area of use of the planar plate. The axis of rotation  206  of the planar plate  200  is displaced parallel opposite the optical axis HA. The planar plate is held in a rotatable holder, which runs in a guide  210 .  
         [0049]      FIGS. 5 and 6  show an example of a transmissive plano-convex lens  300  with a first used area  308 . 1  and a second used area  308 . 2 , another possibility for introducing the respective used areas into a cleaning device. This arrangement of several used areas operating in an optically identical manner is then a particularly advantageous embodiment of the invention, if the optical surfaces themselves do not possess a geometric shape invariant to rotation or translation. The arrangement of several optical elements next to one another along a displacement axis  306  makes it possible to achieve a translation invariancy for the optical element, independent of whether the optical surfaces of the optical element itself possess a rotation- or translation-invariant geometric shape. The transport according to  FIGS. 5 and 6  is thus not necessarily achieved by a rotation around an axis of rotation, but, for example, also by a displacement along a displacement axis  306 . In  FIG. 5 , the first used area  308 . 1  of the plano-convex lens  300  is brought into the optical area of use, i.e., the area, in which the rays of the light bundle impinge. The first used area  308 . 1  is centered relative to the optical axis HA. The second used area  308 . 2  is found in the cleaning position in a first cleaning device  303 . 1 . Now, if the first used area  308 . 1  is to be cleaned, then the plano-convex lens  300  is moved along the displacement axis  306 . This is shown in  FIG. 6 . The displacement axis  306  stands perpendicular to the optical axis HA. As can be recognized in  FIGS. 5 and 6 , a second cleaning device  303 . 2  is necessary for cleaning the first used area  308 . 1 . When the first used area  308 . 1  is cleaned in the second cleaning device  303 . 2 , the second used area  308 . 2  of the plano-convex lens is found in the area of use, i.e., in the area in which the rays of a light bundle impinge onto the optical element.  
         [0050]     With the invention, for the first time, an optical subsystem is provided for a projection exposure system, with which it is possible to clean optical elements without shutting down the unit.

Technology Category: 3