Patent Number: 055286542
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a multilayer mirror for soft X-rays, widely used in a scientific instrument or a manufacturing apparatus, such as an optical element of beam line of a synchrotron radiation, an X-ray microscope, and an X-ray exposure apparatus. Particularly, it relates to a multilayer mirror capable of being used even in a case of a normal incidence in which an incident angle is practically perpendicular to the mirror surface. Generally, in almost all materials, refractive indices are substantially equal to one to a light having a shorter wavelength, as for example, the so-called vacuum ultraviolet light, and the absorption coefficients are more than negligible. As a result, a conventional reflection mirror fails to function properly. In recent years, a method for depositing a multilayer film while controlling the thickness of each layer of a large number of thin films of the order of angstrom has been developed. By using that method, multilayer films have been proposed in which each reflecting light from each of a plurality of layer interfaces constructively interfere each other, and they are currently under active research and development. In general, the multilayer mirror consists of a multilayer film in which tens to hundreds of thin-films made of different materials are deposited alternately. In order to obtain a high reflectivity, a combination of materials of the thin films must be selected so that a difference between the refractive indices of the materials adjacent to each other is as large as possible. A well-known rough criterion for selecting the combination of the materials is a material mainly consisting of a heavy element and a material consisting mainly of a light element. Up to the present, the combinations of materials of the thin films consisting mainly of a heavy element such as silver (Ag), nickel (Ni), or cobalt (Co), and materials of a thin film mainly consisting of a light element such as carbon (C) and boron (B) are known in the cases in which the soft X-ray has a wavelength of around 100 .ANG. or less. Co is particularly suitable for the material of the multilayer film for reflecting the light having a wavelength of 100 .ANG. or less because of a large difference in the refractive index between Co and the refractive index of a light element, and its relatively small absorption coefficient. However, as reported by P. Ruterana (J. Appl. Pys., Vol. 68, p. 1033, 1990), when Co is formed into a thin film of 30 .ANG. or less, condensation of the material occurs. Accordingly, a uniform and continuous thin films cannot be obtained. As a result, in a multilayer film with a thickness of 30 .ANG. or less having Co thin film as a layer having smaller refractive index, the Co layers are formed in an island-like condition. This condition causes an increase in the roughness of the layer interfaces of the multilayer film and further increase of scattering of X-rays. Thus, the reflectivity of the multilayer film reduced, to the extent that, in a serious case, almost no reflection occurs. SUMMARY OF THE INVENTION An object of the present invention is to provide a multilayer film for X-rays of short-period, having high reflectivity, by forming a multilayer film consisting of a uniform and continuous thin films while suppressing the condensation of the materials, even in a case of, for example, a thin film of Co having a thickness of 30 .ANG. or less. One aspect of the present invention is for a multilayer film for X-rays comprising a substrate; a multilayer of thin films for X-rays on said substrate, with each such layer comprising first and second types of thin films, with each thin film having a different refractive index; the second thin film being deposited on the first thin film, wherein, one of the first and second types of thin film has a smaller refractive index than the other thin film, and consists of an alloy containing Co and Cr. In another aspect of the invention an optical element for X-rays, comprises a substrate; a multilayer of thin films for X-rays on said substrate, with each such layer comprising first and second types of thin films, with each thin film having a different refractive index; the second thin film being deposited on the first thin film, wherein, one of the first and second types of thin film has a smaller refractive index than the other thin film, and consists of an alloy containing Co and Cr. According to a further aspect of the present invention an X-ray exposure apparatus comprises an optical system including an optical element for exposing an object with X-rays, said optical element comprising a a substrate; a multilayer of thin film for X-rays on said substrate, with each such layer comprising first and second types of thin films, with each thin film having a different refractive index; the second thin film being deposited on the first thin film, wherein, one of the first and second types of thin film has a smaller refractive index than the other thin film, and consists of an alloy containing Co and Cr. According to yet another aspect of the invention a method of manufacturing a micro-device, comprises the steps of providing an X-ray optical system including an optical element; said optical element being formed by forming a multilayer of thin films for X-rays on said substrate, with each such layer comprising first and second types of thin films, with each thin film having a different refractive index; wherein one of the first and second types of thin film having a smaller refractive index than the other thin film consists of an alloy containing Co and Cr. Depositing said second thin film on said first thin film; providing a mask having a pattern and a wafer; exposing the mask and the wafer to transfer the pattern of the mask onto the wafer by using said X-ray optical system. The present invention comprises, in a multilayer film for X-rays in which two types of thin films having different refractive indices and respective selected thicknesses are deposited alternately to form said multilayer film. The thin films having the smaller refractive index consist mainly of an alloy of Co and chromium (Cr), said alloy being expressed by the componential formula Co.sub.x Cr.sub.1-x, where, X is a number from 0.3 to 0.8 indicating the mole fraction of Co in the formula.