Patent Number: 
Section: description

FIG. 1 shows a grating element 1 with a plurality of individual gratings 9 in a beam path of an illumination system. Individual gratings 9 are arranged one after the other in the beam direction. Light from a light source 3 is gathered by a collecting component, a collector 5. Collector 5 in this example is an ellipsoidal mirror, which produces an image of light source 3. A collimated light bundle with an aperture of around NA=0.1 is deflected in grazing incidence by grating element 1 after collector 5 so that an intermediate image of light source 3 comes to lie in or near a diaphragm plane 200 of a physical diaphragm, i.e., diaphragm 7.3. By use of further physical diaphragms, i.e., diaphragms 7.1 and 7.2, arranged in front of diaphragm 7.3, a part of the unwanted radiation can be filtered out in order to reduce the heat load on diaphragm 7.3. Diaphragm 7.3 in one embodiment can have a circular opening. Diaphragm 7.3 is situated in a focal plane of a desired diffraction order, here, the xe2x88x921 order (reference numeral 16). Further diaphragms 7.1, 7.2 can be additionally cooled, which is not depicted. Grating element 1 can also be cooled, for example, by a cooling on its backside. A backside cooling device 8 is preferably a liquid cooling device with an inlet 10.1 and an outlet 10.2. With grating element 1 and diaphragm 7.3, it is possible to totally block the 0th order, which comprises all wavelengths of light source 3 in an illumination system according to the invention. Furthermore, all higher orders except the xe2x88x921 order are also blocked. Discrete grating periods for an arrangement of individual gratings 9 arranged one after the other according to the invention shall now be given below. For the derivation, we shall resort to reflective imaging optics, wherein the optics will image light from a virtual intermediate image, corresponding to the 0th order, in an actual image, corresponding to the +1 or xe2x88x921 order. A solution is then given by a hyperboloid. A grating element designed in a plane must thus have grating grooves, in an ideal case, that are given by points of intersection of a hyperbolic family of curves with this plane, the family of curves being defined by hyperbolas, which have a light path difference of nxcfx80 for a point-to-point image between a focal point without a mirror and the nth order. The inventors have discovered that, under grazing incidence, this grating element with optical effect can be resolved sufficiently well by an array of individual gratings arranged one after the other or above the other, without the imaging quality of the illumination system being unacceptably impaired. Parameters used for a formal derivation of a grating element according to the invention are given in FIG. 2. Here: xcex1i: is the angle at which the light beam impinges on the grating element, xcex1t: is the angle at which the beam is diffracted by the grating element, and h, hxe2x80x2: are the heights of the image loci. A particular beam, which impinges on grating element 1 at the angle xcex1i, is reflected at angle xcex1t in the 0th diffraction order. The first diffraction order for this beam needs to be far enough away that a separation of the diffraction orders is possible taking into consideration the diameter of the image of light source 3 at the focal point. It is then possible, by an arrangement of diaphragm 7.3 in the plane where the focal point comes to lie, to completely block the 0th diffraction order, which comprises all wavelengths. The beam angle of the first diffraction order relative to the grating surface, i.e., angle xcex1t, must be respectively larger or smaller than angle xcex1i by xcex94xcex1, where:                               Δ          ⁢                      xe2x80x83                    ⁢          α                 greater than                   2          ⁢                      arctan            ⁡                          (                              D                21                            )                                                          (        1        )             wherein: D: is the distance of a desired diffraction order from the 0th diffraction order in a filter plane l: is the distance between a locus of reflection on a mirror with grating and an image point. For a central ray, hereinafter termed the chief ray, let an angle of incidence be xcex1i(0). From this, we can determine the heights h and hxe2x80x2 of image loci. Likewise, z-coordinates of the image loci can be calculated relative to a ray intersection point of the chief ray with the mirror: h=l0 sin xcex1i(0)xe2x80x83xe2x80x83(2)  hxe2x80x2=l0 sin xcex1t(0)=l0 sin(xcex1i(0)+xcex94xcex1)xe2x80x83xe2x80x83(3)  z=l0 cos xcex1i(0)xe2x80x83xe2x80x83(4)  zxe2x80x2=l0 cos xcex1t(0)=l0 cos(xcex1i(0)+xcex94xcex1)dz=zxe2x88x92zxe2x80x2xe2x80x83xe2x80x83(5)  Now, for every other beam, designated by its angle xcex1i, it is possible to determine a length to the 0th order l(xcex1i) and a length to the 1st order lxe2x80x2(xcex1i), as well as a new z-coordinate zxe2x80x2(xcex1i)=z(xcex1i)xe2x88x92dz, wherein hxe2x80x2(xcex1i)=hxe2x80x2=const. From the quantities lxe2x80x2(xcex1i) and zxe2x80x2(xcex1i), a local diffraction angle xcex1t(xcex1i) is determined as:                                           α            t                    ⁡                      (                          α              i                        )                          =                  arccos          ⁡                      (                                                            z                  xe2x80x2                                ⁡                                  (                                      α                    i                                    )                                                                              l                  xe2x80x2                                ⁡                                  (                                      α                    i                                    )                                                      )                                              (        6        )             and there follows for the local grating period P:                     P        =                  λ                                    cos              ⁢                              xe2x80x83                            ⁢                              α                i                                      -                          cos              ⁢                              xe2x80x83                            ⁢                                                α                  i                                ⁡                                  (                                      α                    i                                    )                                                                                        (        7        )             We shall now give two examples of embodiments of grating spectral filters with individual gratings arranged one after the other, with a grating period being different for the individual gratings. An arrangement of the individual gratings in a plane is especially advantageous for cooling the grating, since the grating can be provided with a cooling gradient on its backside, for example, cooling channels. The values for xcex1i, xcex1t, the grating period, a starting value and an ending value along the z-axis, and a Blaze depth of a grating element produced from individual gratings arranged one after the other will be found in Tables 1 and 2. Regarding a definition of the Blaze depth, refer to FIG. 8 in the following description. Table 1 shows an example of an embodiment for 21 linear gratings. Table 2 shows an example of an embodiment for 31 linear gratings. The following parameters are given: The individual gratings are designed as so-called Blaze gratings, i.e., they are optimized for maximum efficiency in the desired diffraction order. This is achieved approximately by a triangular groove profile. An ideal Blaze depth B in a scalar approximation is calculated by                     B        =                                            "LeftBracketingBar"              n              "RightBracketingBar"                        ⁢            λ                                              sin              ⁢                              xe2x80x83                            ⁢                              α                t                                      +                          sin              ⁢                              xe2x80x83                            ⁢                              α                i                                                                        (        8        )             Table 1: 21 grating segments made from individual gratings, which are arranged one after the other in a plane, together yield a spectral filter. Starting and ending positions of the gratings in terms of a point of incidence of a chief ray with a surface in which the gratings lie are given. Table 2 31 grating segments made from individual gratings, which are arranged one after the other in a plane, together yield a spectral filter. Starting and ending positions of the gratings in terms of a point of incidence of a chief ray with a surface in which the gratings lie are given. FIG. 3 shows a grating period of individual gratings as a function of an angle of incidence xcex1i. The points reflect discrete values of the example of the embodiment with 31 individual gratings according to Table 2. FIGS. 4A and 4B show spot diagrams of a point image of the xe2x88x921 diffraction order for a design wavelength of 13.5 nm in a diaphragm plane, FIG. 4A with 21 and FIG. 4B with 31 individual gratings. The discrete nature of the grating element is made evident by a slight wash-out in the y-direction, but this is negligibly small, especially for N greater than 30 gratings with  less than xc2x10.5 mm; the image of the light source is washed out by this amount in the y-direction. The scale indicated in FIGS. 4A and 4B pertains to the scaling in both the x and the y directions. In order to reduce manufacturing expense, in another embodiment of the invention it is proposed to have identically configured grating segments, but to incline these with an angle of tilt so that a desired diffraction order is pointed in a target direction. In this way, in the simplest case, one can put a spectral filter together from an array of identical individual gratings. FIG. 5 shows such a grating element. Grating element 1 comprises a plurality of individual gratings 9 inclined against a plane of incidence E. In order to compute an angle of inclination of an individual grating 9 with a constant grating period, one can use the Laue construction shown in FIGS. 6A and 6B. The reference symbols used hereafter can be found in these drawings. With the known angle xcfx89=180xc2x0xe2x88x92xcex1ixe2x88x92xcex1t="sgr"i+"sgr"txe2x80x83xe2x80x83(9)  it follows for the angle of inclination xcex2: xcex2=xcex1i+"sgr"ixe2x88x9290xc2x0xe2x80x83xe2x80x83(10)  The angles"sgr"i and"sgr"t are associated by the Laue equation to the grating period P in the following manner:                                           sin            ⁢                          xe2x80x83                        ⁢                          σ              i                                -                      sin            ⁢                          xe2x80x83                        ⁢                          σ              t                                      =                  λ          P                                    (        11        )             Solving equation (11) with (9) in terms of"sgr"i yields                               sin          ⁢                      xe2x80x83                    ⁢                      σ            i                          =                                            -              b                        +                                                            b                  2                                -                                  4                  ⁢                  ac                                                                          2            ⁢            a                                              (        12        )             wherein: a=2(1+cos xcfx89)       b    =                  -        2            ⁢              xe2x80x83            ⁢              λ        P            ⁢              (                  1          +                      cos            ⁢                          xe2x80x83                        ⁢            ω                          )                  c    =                            λ          2                          P          2                    -                        sin          2                ⁢        ω             In this manner, the angles of inclination xcex2 of the individual gratings can be calculated. Table 3 contains an of example an embodiment with 40 individual gratings, the following parameters being given: Table 3: Angles of inclination of the grating element with 40 individual gratings. Grating period: 1.5007 xcexcm FIG. 6C shows the inclination angle xcex2 of the individual gratings as a function of the angle of incidence xcex1i. The points reflect the discrete points of the example of embodiment with 40 individual gratings according to Table 3. FIG. 7 shows a spot diagram of a point image of the xe2x88x921 diffraction order in the diaphragm plane. The discrete nature of the grating element is made evident by a slight washout in the y-direction, but this is negligibly small with xe2x89xa6xc2x10.5 mm; the image of the light source is washed out by this amount in the y-direction. The scale indicated in FIG. 7 pertains to the scaling both in the x and the y directions. With the grating spectral filters according to the examples of embodiment in Table 1, Table 2 and Table 3, wavelengths greater than approximately 17 nm can be almost totally filtered out. Wavelengths less than this are only partly filtered. By the invention the heat load on the mirrors of a projection system can be clearly reduced. As an alternative to an arrangement of individual gratings 9 in a plane or tilted next to each other, they can also be arranged one above the other. An arrangement of one above the other yields a grating spectral filter 1, as shown in FIG. 9. The individual gratings of the individual planes are designated 9.1 and 9.2. The same components as in the embodiment according to FIG. 1 are given the same reference numbers. The gratings arranged one above the other can have a different grating period or can be tilted relative to each other. In order to obtain a grating element 1 with optimal diffraction efficiency, each individual grating of the grating element is preferably designed as a Blaze grating. FIG. 8 shows a Blaze grating with approximately triangular groove profile. Reference 11 designates a ray impinging on grating 9, designed as a Blaze grating, with a grating period P; 12 designates a ray reflected on the grating 9 in the 0th order and 16 designates a ray diffracted in the xe2x88x921 order. Since the Blaze depth according to equation (8) is dependent on an angle of incidence of the beams impinging onto the grating 9, in an ideal case, each individual grating 9 of the grating element will have a different Blaze depth B. If one uses grating elements 1 whose local Blaze angle and, thus, grating depth as indicated in equation (8) changes with the position on the grating, one obtains a maximum efficiency according to FIG. 10, since the diffraction efficiency in the xe2x88x921 order xcex7(xe2x88x921) is a function of the Blaze depth. As FIG. 10 shows, the diffraction efficiency xcex7(xe2x88x921) also depends on the materials used. In FIG. 10, reference number 100 designates the diffraction efficiency xcex7(xe2x88x921) for a wavelength of xcex=13.5 nm for ruthenium; reference number 102 is for palladium; reference number 104 is for rhodium; and reference number 106 is for gold. As follows from FIG. 10, a highest efficiency of 0.7 is achieved with ruthenium. A coating of palladium or rhodium, which has better long-term properties, only has an efficiency xcex7(xe2x88x921) of 0.67, which is only around 3% poorer. Gold is conventionally used for synchrotron gratings, but as curve 106 reveals, it has a much poorer efficiency than the mentioned materials at xcex=13.5 nm. To simplify fabrication, all individual gratings can be produced with the same Blaze depth of, for example, 25 nm, and even so a diffraction efficiency xcex7(xe2x88x921) of  greater than 55% or 0.55 is achieved. FIG. 11 shows an EUV projection exposure system with a grating element 1 according to the invention. The EUV projection exposure system comprises a light source 3, and a collecting optical component or so-called collector 5, which is configured as a nested collector according to the German patent application DE-A-10102934, submitted on Jan. 23, 2001 to the German Patent Office for the Applicant, whose disclosure content is also included in its entirety in the present application. Collector 5 images light source 3, which lies in an object plane 202 of an illumination system, in a secondary light source 4 in or near the diaphragm plane of diaphragm 7.3. In this embodiment the illumination system for illuminating an arc shaped field in the field plane 22 comprises light source 3, grating element 1, diaphragms 7.1, 7.2, 7.4, 7.5, 7.6 and 7.7 as well as diaphragm 7.3, and as further optical elements, facetted mirrors 29.1, 29.2, and mirrors 30.1, 30.2 and 32. In the present case, light source 3, also referred to as a primary light source, which can be, for example, a laser plasma source or a plasma discharge source, is arranged in the object plane 202 of the illumination system; an image of light source 3, which is also termed a secondary light source 4, comes to lie in an image plane of the illumination system. Between grating element 1 and diaphragm 7.3 are arranged additional diaphragms 7.1, 7.2, in order to block out the light of unwanted wavelengths, especially wavelengths greater than 30 nm. According to the invention, the focus of the xe2x88x921 order will come to lie in the diaphragm plane of diaphragm 7.3, i.e., light source 3 is imaged by collector 5 and grating element 1, which functions as a grating spectral filter in the xe2x88x921 diffraction order almost stigmatically in the diaphragm plane of diaphragm 7.3. Imaging in all other diffraction orders is not stigmatic. Furthermore, the illumination system of the projection system comprises an optical system 20 to form and illuminate a field plane 22 with an annular field. The optical system 20 comprises, as a mixing unit for homogeneous illumination of the annular field, two facet mirrors 29.1, 29.2, as well as two imaging mirrors 30.1, 30.2 and a field-forming grazing-incidence mirror 32. Additional diaphragms 7.4, 7.5, 7.6, 7.7 are arranged in optical system 20 to suppress stray light. The first facet mirror 29.1, so-called field facet mirror, generates a plurality of secondary light sources in or near a plane of the second facet mirror 29.2, so-called pupil facet mirror. The optical elements 30.2, 30.1, 32 following this images the pupil facet mirror 29.2 in an exit pupil 34 of the illumination system, which comes to lie in an entrance pupil of a projection objective 26. Angles of inclination of the individual facets of the first and second facet mirrors 29.1, 29.2 are designed so that images of the individual field facets of the first facet mirror 29.1 are superimposed in a field plane 22 of the illumination system and thus a largely homogenized illuminating of a pattern-bearing mask, which comes to lie in field plane 22, is achieved. A segment of the annular field is formed by field-forming grazing-incidence mirror 32 operating under grazing incidence. A double-faceted illumination system is disclosed, for example, in the U.S. Pat. No. 6,198,793, and imaging and field-forming components are disclosed in PCT/EP/00/07258. The disclosure contents of these publications are fully incorporated in the present application. The pattern-bearing mask arranged in the field plane 22, also known as a reticle, is imaged by means of projection objective 26 in an image plane 28 of field plane 22. Projection objective 26 is a 6-mirror projection objective, such as is disclosed in the U.S. Application No. 60/255,214, submitted on Dec. 13, 2000 at the US Patent Office for the Applicant, or DE-A-10037870, whose disclosure content is fully incorporated in the present Application. An object being exposed, for example, a wafer, is arranged in image plane 28. The invention indicates for the first time an illumination system with which it is possible to select unwanted wavelengths directly after the primary light source. 1 Grating element 3 Light source 4 Secondary light source 5 Collector 7.1, 7.2, 7.3 7.4, 7.5, 7.6 7.7 Diaphragms of the illumination system 8 Cooling device 9, 9.1, 9.2 Individual gratings 10.1, 10.2 Inlet and outlet of the cooling device 11 Incident beam 12 Beam diffracted in 0th order 16 Beam diffracted in the xe2x88x921 order 20 Optical system 22 Field plane 26 Projection objective 28 Image plane of the field plane 29.1, 29.2 Facet mirrors 30.1, 30.2 Imaging mirrors 32 Field-forming mirror 34 Exit pupil of the illumination system 100, 102, 104, 106 Diffraction efficiency xcex7(xe2x88x921) for various materials 200 diaphragm plane 202 object plane