Patent Number: 
Section: description

FIG. 1 shows the layout of a system in which the light sources are coupled together according to the addition method. The light sources 1.1, 1.2 in the present case have a small source diameter, in this case laser plasma sources are further investigated. Regarding the basic layout of EUV-illumination systems, we refer to the applicant""s pending applications EP 99 1 06348.8, submitted on Mar. 2, 1999, entitled xe2x80x9cIllumination system, especially for EUV-lithographyxe2x80x9d, U.S. Ser. No. 09/305,017, submitted on May 4, 1999, entitled xe2x80x9cIllumination system particularly for EUV-lithographyxe2x80x9d, now U.S. Pat. No. 6,198,793 B1, and PCT/EP 99/02999, submitted on May 4, 1999, entitled xe2x80x9cIllumination system, especially for EUV-lithographyxe2x80x9d, whose disclosure contents are incorporated in their entirety in the present application. Each system part 10.1, 10.2 is essentially identical in construction and comprises a light source 1.1, 1.2, a collector mirror 2.1, 2.2, and a field raster element plate 4.1, 4.2. The light of each source is collected by means of the collector mirror assigned to a particular source and transformed into a parallel or convergent light bundle. The field raster elements of the particular field raster element plate 4.1, 4.2 decompose the light bundle and create secondary light sources 6 in the diaphragm plane of the illumination system. These secondary light sources are imaged by the field lens (not shown) or field mirror in the exit pupil of the illumination system, which is the entrance pupil of the objective lens (not shown) The field raster elements of the field raster element plate are arranged on the plate and oriented so that the images of the field raster elements are superimposed in the reticle plane 9. The systems are brought together where the field raster element plates are located. The field raster element plates are located on a pyramid, the number of the sides of the pyramid corresponds to the number of coupled partial systems. The angle of inclination of the pyramid sides is chosen such that the illuminated fields of the partial systems in.the reticle plane 9 are superimposed. The partial systems parts 10.1, 10.2 are arranged such that their partial pupils fill the diaphragm plane of the illumination system optimally. In the embodiment shown in the drawings, the partial systems are oriented such that they possess a common system axis. The angular spacing of the partial system is then 360xc2x0/number of systems. For four partial systems, FIG. 2 shows the illumination of the pyramid, on each of the four lateral surfaces 20.1, 20.2. 20.3, 20.4 of the pyramid one field raster element plate of a partial system in the area of the illuminated surface 22.1, 22.2, 22.3, 22.4 is arranged. The field raster elements are arranged and oriented such that the images of the field raster elements overlap in the reticle plane 9. The angle of inclination of the pyramid surfaces 20.1, 20.2. 20.3, 20.4 is chosen such that the illuminated fields of the partial system superimpose in the reticle plane. The illumination in the diaphragm plane is provided by four circular partial pupils 30.1, 30.2. 30.3, 30.4, as shown in FIG. 3, which in turn are divided into individual secondary light sources 6, corresponding to the number of illuminated field raster elements of the field raster element plates. In FIG. 3, the aperture of the total system is NAObj=0.025 and the aperture of the system parts is NATeilsystem=0.0104. Depending on the number of coupled partial systems, one can imagine the arrangement and symmetries of the partial pupils 30.1, 30.2. 30.3, 30.4, 30.5, 30.6 as shown in FIGS. 4A through 4D with coupling of 3, 4, 5 and 6 sources. The maximum diaphragm diameters of the partial systems are derived from the total aperture NAObj of the objective lens in the diaphragm plane and the number of partial systems or subsystems.       NA    Teilsystem    =            NA      obj              1      +              1                  sin          ⁢                      xe2x80x83                    ⁢                      (                          π              Anzahl                        )                               Whereby: Teilsystem=partial system; Anzahl=number of partial systems When the pupil of each subsystem is filled, the pupil can be illuminated to xcex7% of the maximum.   η  =      Anzahl    ·          1                        (                      1            +                          1                              sin                ⁢                                  xe2x80x83                                ⁢                                  (                                      π                    Anzahl                                    )                                                              )                2             Whereby: Anzahl=number of partial systems The following table gives NAsystem part and the filling factor xcex7 for NAObj=0.025: Hence, the maximum attainable filling factor with the addition method using four subsystems and NAObj=0.025 is achieved with xcex7max≈0.69. As a boundary condition, the overall Etendu of the coupled sources may not exceed the system, Etendu LCill=xcex7maxxc2x7LCObj; thus, we must always have: xcexa3LCixe2x89xa6LCill all sources FIG. 5 shows a second form of embodiment of the invention, in which the light sources 50.1, 50.2 are pinch plasma sources, for example. The source diameter of the pinch plasma sources is not negligible. A partial illumination system with pinch plasma source comprises the light source 50.1, 50.2, a collector mirror 52.1, 52.2, which collects the light and illuminates the field raster element plate 54.1, 54.2. The field raster elements of the plate produce secondary light sources. At the location of the secondary light sources, the pupil raster elements are arranged on a pupil raster element plate. The field raster elements of the field raster element plate are used to shape the field and the pupil raster element of the pupil raster element plate correctly image the field raster element in the reticle plane. Preferably, each field raster element is assigned to a pupil raster element. The light is guided by reflection from the field raster elements of the field raster element plates to the pupil raster element of the pupil raster element plate 56.1, 56.2 and from there to the reticle, or object 58. The systems are brought together at the location of the pupil raster element plates. The pupil raster element plates are located on a pyramid. The number of sides of the pyramid corresponds to the number of coupled subsystems. The angle of inclination of the pyramid sides is chosen such that the illuminated fields of the partial systems or subsystems are brought together in the reticle plane. If the subsystems have a common system axis, then the angular spacing of the system parts is 360xc2x0/number of systems and the pupil raster element plates of the subsystem are preferably arranged on the lateral surfaces of a pyramid, as shown in FIG. 2. The advantage of the addition method of coupling is that identical or similar illumination systems can be coupled together. The raster element plates of the subsystems are separate and can thus be fabricated separately. In the addition method, it should be noted that intensity differences of the individual sources are directly passed on to the illumination of the pupils, and thus the intensity of the partial pupils is dictated by the source power. The intensity distribution in the diaphragm plane becomes independent of the intensities of the individual sources if one mixes the secondary light sources in the pupil plane. This technique is also hereafter designated as the mixing method. Whereas in the addition method the beam bundles of each source only penetrate after passing through the diaphragm plane, in the mixing method the beam bundles penetrate in front of the diaphragm plane and are mixed in the diaphragm plane. The maximum aperture for each subsystem is adapted to the desired angle of filling of the objective aperture. As in the addition method, systems of identical construction can be coupled together for the individual sources. They are uniformly arranged about a common system axis. The systems are coupled together in the plane of the secondary light sources. FIG. 6 shows an illumination system based on the mixing method for coupling of several light sources. The light sources once again are laser plasma sources. The same components as in FIG. 5 are designated with the same reference numbers. In contrast to FIG. 5, for example, there is a single pupil raster element plate 100, which includes a plurality of pyramids. The pupil raster element plate 100 is arranged at the location of the secondary light sources, which are produced by the field raster elements. A secondary light source is located on each flank, or lateral side, of the plurality of pyramids. The schematic representation of FIG. 7 shows a typical arrangement of the field raster elements 110 on the field raster element plate. Each field raster element plate produces a grid of secondary light sources in the diaphragm plane. The distribution of the secondary light sources in the diaphragm plane corresponds to the arrangement of the field raster elements. By shifting the subsystems, as depicted in FIG. 8, the grids of secondary light sources can be brought to be located next to each other, corresponding to the number of subsystems. If four sources are coupled together, the arrangement of secondary light sources 6 shown in the schematic representation of FIG. 8 is obtained. For the correct superimposing of the four subsystems, each set of secondary light sources is located on a mirrored pyramid. The flanks of the pyramid are inclined such that the images of the field raster elements are superimposed in the reticle plane. The schematic representation of FIG. 9 shows a segment of the pupil raster element plate. One clearly recognizes the individual pupil raster elements 104 that are formed by the flanks of an equilateral pyramid 106. If the Etendu (LC) of the individual sources is small, the pupil raster elements can be designed as plane mirrors, i.e., the flanks of the equilateral pyramids 106 are planar. When the source diameter is not negligible, such as with pinch plasma sources, the pupil raster elements 104 must image the field raster elements in the object plane, for example, the reticle plane. In this case, a concave mirror surface 108, as shown in FIG. 10, must be worked into the pyramid flanks. The schematic representation of FIG. 10 shows a system in which several pinch plasma sources are coupled with a pupil raster element plate comprising pupil raster elements with concave surfaces. The same components as in FIG. 6 are given the same reference numbers. The examples shown in FIGS. 5 through 10 are designed for four coupled sources. However, the same method can be used for three, five, six or more sources. The grids should then be shifted such that the secondary light sources are located on the side faces of pyramids. The degree of filling of the pupil is limited similar to the addition method. The advantages of the mixing method are that the individual sources are mixed in the pupil plane. Fluctuations in source intensity are not shown in the pupil as inhomogeneous pupil illumination. Furthermore, the system pupil can be filled more uniformly with secondary light sources. As a third method of coupling several light sources together, the segment method shall be described. The segment method works similar to the addition method. The coupled illumination systems are uniformly distributed about a common system axis. Each system has a corresponding segment to fill the diaphragm plane. Instead of filling this segment with a circle as in the addition method, one can uniformly fill up the segment by orienting the field raster elements on the field raster element plate. FIG. 11 shows the illumination of one of four segments 200 of the system pupil 202, when four sources are coupled together. In segment 200 secondary light sources 6 corresponding to the number of illuminated field raster elements are formed. In order for the individual light bundles to be correctly superimposed in the reticle plane, pupil raster elements must be arranged at the location of the secondary light sources, which deflect the light bundles so that the images of the field raster elements are superimposed in the reticle plane. Depending on the size of the source, the pupil raster elements have planar surfaces for point like sources or concave surfaces for extended sources. Accordingly, field and pupil raster elements are tilted individually and without symmetry. The advantage of the segment method is the optimal filling of the diaphragm plane with secondary light sources 6 by a pairwise tilting of field and pupil raster elements. Although no optical components have been depicted in the preceding examples of embodiments of the illumination systems after the lenses or mirrors with raster elements, it is obvious to the person skilled in the art that field lenses or field mirrors must be provided after the lenses or mirrors with raster elements in order to shape the annular field in the reticle plane and to image the diaphragm plane into the exit pupil of the illumination system, for example. This is shown in FIG. 12. The illumination system of the second embodiment, shown in FIG. 5 was adapted by introducing a field lens 300 between the pupil raster element plates 56.1 and 56.2. The field lens 300 represents an optical unit, which can also comprise two or more mirrors. The field lens 300 images the plurality of secondary light sources formed on the pupil raster element plates 56.1 and 56.2 into the exit pupil 310. In this regard, concerning the basic layout of EUV illumination systems, refer to the applicant""s pending applications EP 99 1 06348.8, submitted on Mar. 2, 1999, entitled xe2x80x9cIllumination system, especially for EUV-lithographyxe2x80x9d, U.S. Ser. No. 09/305,017, submitted on May 4, 1999 entitled xe2x80x9cIllumination system particularly for EUV-lithographyxe2x80x9d, and PCT/EP 99/02999, submitted on May 4, 1999, entitled xe2x80x9cIllumination system, especially for EUV-lithographyxe2x80x9d, whose disclosure contents are incorporated in their entirety in the present application. An EUV-projection exposure system is shown in FIG. 13. The illumination system is already shown in FIG. 12. The reticle 58 is imaged by the projection objective lens 320 onto the wafer 330. The EUV-projection exposure system can be realized as a stepper or scanning system.