Patent Number: 
Section: description

FIG. 1 shows the structure in principle of an optical device according to the invention. In the present case, the optical device comprises a mirror 1 with, for example, a superpolished mirror surface 3 as well as devices 5 for generating oscillations of the mirror surface, for example, interdigital electrodes. The oscillation frequency by which devices 5 stimulate oscillations of mirror surface 3 is selected such that the radiation 7 impinging on mirror surface 3 is diffracted in a predetermined range. In FIG. 1, the zero diffraction order is 9, the first diffraction order is 11 and the xe2x88x921 diffraction order is 13 for the incident radiation 7. Diffraction is characterized by the interference of many rays 7 reflected at a periodic structure, in the present case, surface wave 15. In order to produce a diffraction effect according to the invention, the frequency of surface wave 15 is selected in such a way that the incident light 7 impinges a large number of grid lines, which are built-up by surface wave 15. The number of grid lines, which are impinged by incident light 7, i.e., the maxima of surface wave 15, should amount to more than 100. In FIGS. 2a and 2b, the diffraction at a surface structure varied periodically by surface waves is represented in detail once more for different times t=t0 and t=t1. The same reference numbers were selected in FIGS. 2a and 2b for the same objects as shown in FIG. 1. Reference 7 designates the incident ray on the surface, reference 9, the ray of zero refraction order, in which the angle of reflection is equal to the angle of incidence, i.e., xcex8r=xcex8i, reference 11 indicates the first diffraction order, in which the angle of reflection xcex8r=xcex8i+xcex8b, whereby xcex8b is the angle of the first diffraction order, as a function of the grid constants of the surface wave and 13 is the ray of the xe2x88x921 diffraction order at an angle of xcex8r=xcex8ixe2x88x92xcex8b. If the frequency of the surface wave 15 is varied over time, i.e., the wavelength of the surface wave and thus the grid distance produced by the surface wave, the 1 and xe2x88x921 order will be diffracted into another range of angles. This can be seen from FIG. 2b where the wavelength and thus the grid distance for time t=t1 was increased. The +1 order and xe2x88x921 orders will be diffracted into a smaller range of angles than in the case of a shorter grid distance, i.e., shorter wavelengths, as is indicated in FIG. 2a for time t=t0. As FIGS. 2a and 2b indicate, it is thus possible to achieve a homogeneous illumination averaged over time of a range of angles by means of the device according to the invention, by continuous variation of the frequency of the surface wave. The wavelength of the surface waves utilized for the production of the continuous illumination of a range of angles, for example of xc2x112 mrad, lies in the range of 1 xcexcm to 50 xcexcm, while the amplitude of the surface wave lies in the range of 1 nm to 100 nm. Since in practice, a wave train of the surface wave traverses mirror surface 15 excited by the surface wave, i.e., the active mirror surface, which preferably lies in the range of 10 mm to 100 mm, in approximately 10 xcexcs, approximately 1000 frequencies can be swept through for an exposure time of 1 ms. A variable angle range of the deflection angle of xc2x112 mrad is then produced. The intensity distribution that can be achieved by means of the device according to the invention, in which the frequency of the surface wave is continually varied in order to adjust all angles of diffraction of a monochromatic source of 0 mrad to xc2x112 mrad is shown in FIG. 3. Reference 20 indicates the intensity of the first diffraction order of a grid selected as an example. Reference 22 designates the intensity averaged over time of all diffraction grids which are produced by varying the frequencies. As can be clearly seen, the intensity course plotted over the diffraction angle shows a xe2x80x9chatxe2x80x9d profile, i.e., except for small fluctuations, the intensity is essentially constant over the entire range of angles. In contrast to diffraction, in the case of pure reflection, the relationship: angle of incidence=angle of reflection applies. For the case of reflection, the incident light impinges such that it illuminates less than one wavelength up to a maximum of one wavelength of the grid produced by the surface wave. A constant surface wave in such a case produces only a continuous variation of the angle of reflection. In fact, by this, a predetermined range of angles also can be swept over, but in the case of reflection, the adjustable range of angles is not traversed homogeneously. This in turn has the consequence that a homogeneous illumination cannot be achieved, in contrast to the invention. An embodiment of the invention is shown in FIG. 4 which permits the modification of a beam impinging on mirror surface 30 in two dimensions. This is achieved by surface waves that are introduced on the active surface 32 in parallel or not in parallel at various locations. For inducing such waves, a total of four excitation devices 34, so-called interdigital transformers, i.e., electrodes, are arranged on the mirror surface in this embodiment. In order to increase the active surface of a component, according to the embodiment shown in FIG. 5, the device, presently the mirror surface, can be assembled from various individual components 40.1, 40.2. The substrate 42.1, 42.2, as well as the excitation devices 34.1, 34.2 arranged on the respective substrate for surface waves 44.1, 44.2 can be clearly seen in FIG. 5. The excitation devices 34.1, 34.2, are formed as interdigital electrodes in the present example. By superimposing the surface waves 44.1, 44.2 stimulated by means of the interdigital electrodes, the active region of a component can be significantly increased. A technical realization of an active component can be embodied, for example, as a silicon wafer, having a mirror surface 50 and a rough side 52, as shown in FIG. 6. A PZT film is applied onto rough side 52, for example, of a silicon wafer disk by means of an atomic connection technique, for example, a TiPt connection. Point-like or line-form electrodes 54 are arranged at defined places. Acoustical surface waves can be produced on very thin wafer disks with these electrodes. A one-dimensional or two-dimensional grid is produced by means of acoustical surface waves, which in turn makes possible the diffraction of an incident light beam according to the invention. In order to be able to produce as large amplitudes as possible, the system can be shifted to self-resonance. The frequencies required for the excitation lie in the 100-10,000-MHz range with powers of 1-200 Watts. FIGS. 7 and 8 show the use of an optical component according to the invention in an illumination system for EUV lithography. With respect to the principle construction of EUV illumination systems, reference is made to pending applications EP 99 106348.8, filed on Mar. 2, 1999 with the title xe2x80x9cIllumination system, particularly for EUV lithographyxe2x80x9d; U.S. Ser. No. 09/305,017, filed on May 4, 1999 with the title xe2x80x9cIllumination system, particularly for EUV lithographyxe2x80x9d; as well as PCT/EP 99/02999, filed on May 4, 1999, with the title xe2x80x9cIllumination system, particularly for EUV lithographyxe2x80x9d; as well as the application DE 29915847.0, filed on Sep. 3, 1999 with the title xe2x80x9cControl of the illumination distribution in the exit pupil of an EUV illumination systemxe2x80x9d. An illumination system is shown in schematic representation in FIG. 7, in which the collector mirror 100 broadens the beam 104 coming from the synchrotron radiation source 102, for example, an undulater source. The collector mirror is formed as a mirror with a periodically changing surface that produces diffraction effects according to the invention. Collector mirror 106, which lies in the vicinity of the radiation protection wall 108, can be illuminated by such a mirror in the range of angles 110, for example, from xe2x88x9212.0 mrad less than xcex1 less than 12.0 mrad. Part of an illumination system is shown in FIG. 8. This part of the illumination system serves for deflecting the image of the reticle produced by the light source on the wafer surface 200, as well as to form the annular field. Such an arrangement comprises two field mirrors 202, 204 in the example shown in FIG. 8. The second of the two field mirrors 204 is formed as a mirror with a deformable surface according to the invention. Such a mirror serves for avoiding the individual raster elements to be discrete in the intensity distribution of the reticle plane. The mirror 204 effects a smearing of point-like light beams from secondary light sources, which are formed by individual raster elements, averaged over time, and thus provide for a uniform illumination of the annular field. In the present invention, thus, for the first time, an optical device is shown for uniform illumination of a predetermined range of angles. The optical device is particularly suitable for use in illumination systems for EUV lithography.