Patent Number: 047013915
Section: summary

The invention relates to a mask for the formation of patterns in lacquer layers by means of X-ray lithography comprising a diaphragm highly transparent to X-ray radiation and a layer present on one of the major surfaces of the diaphragm for absorbing the X-ray radiation and structured in accordance with the pattern to be produced. The article by E. Spiller and R. Feder "X-ray Lithography" in "Topics in Current Physics: X-ray Optics", Springer, N.Y., 1977, p. 35-92 discloses what is the important part played by X-ray lithography in the manufacture of, for example, semiconductor systems or magnetic cylindrical domain devices because especially in these manufacturing processes there is a growing interest in increasingly smaller structures in the .mu.m and nm range. For all high resolution methods, accurate and very precisely defined masks are required. The suitability of X-ray lithography becomes manifest by the minimum strip width of the structures that can be attained with it: photolithography .apprxeq.2 to 3 .mu.m, electron beam lithography 0.05 to 0.1 .mu.m, X-ray lithography&lt;150 nm. The use of X-rays for the exposure of a lacquer to be structured involves the advantage that disturbing diffraction phenomena in the projection of the mask structures onto the lacquer layer are reduced. The exposure to X-rays requires special irradiation masks for structure production in the lacquer. Such a mask consists of the pattern of the structure to be manufactured consisting of a material strongly absorbing X-rays (designated hereinafter as absorber or absorber pattern) with the absorber pattern being applied to a thin diaphragm of a material as highly transparent as possible to X-rays. For the absorber materials can be used having an atomic weight as high as possible; for example, gold, molybdenum and tungsten have proven satisfactory in practice. As material for the diaphragm practically all those materials can be used which are sufficiently transparent to X-rays. Since in the X-ray range, however, no material is fully transparent and no material is fully opaque, the diaphragm should be comparatively thin in order that a higher possible transparency to X-rays is guaranteed. However, the absorber pattern should be comparatively thick in order that a highest possible absorption of X-rays is ensured. For the manufacture of the diaphragm a series of materials have been examined already. A great variety of difficulties were found to occur. In the indicated part of the article by Spiller and Feder, a composition of materials used hitherto for the manufacture of diaphragms for X-ray lithography has been mentioned. A distinction can be made between inorganic materials, such as Si, SiC, Si.sub.x N.sub.y, BN, Al.sub.2 O.sub.3, Be, Ti and organic materials, such as, for example, polyimide, polycarbonate, polyterephthalate, polyethylene. It has been found that masks comprising diaphragms of organic materials have a high transparency to X-ray radiation and a comparatively great breaking strength, but these materials have the disadvantage that their accuracy is already adversely affected under normal environmental conditions. They age strongly under the influence of X-ray radiation and have a tendency to warp at a higher air humidity. The masks comprising diaphragms of the inorganic materials do not change under environmental conditions and retain their accuracy to size, but, except for beryllium and titanium, they are very liable to break so that they are not particularly suitable for mass production. Titanium has favourable properties as diaphragm material. Such diaphragms have a comparatively great breaking strength and retain their accuracy to size also with temperature and air humidity fluctuations. In this case, however, there is the disadvantage that the production cost for titanium is extremely high. Apart from the expensive starting materials, the required investments for the manufacture of titanium are so high that titanium is a comparatively costly material. Because of its toxic effect, Be can be processed only with great care so that the use of produced masks comprising beryllium diaphragms does not seem advisable. SUMMARY OF THE INVENTION The invention has for its object to provide a mask for X-ray lithography in which advantages of the organic and inorganic materials for diaphragms are united, but in which the disadvantages of these materials are avoided. The mask consequently has a high transparency to X-ray radiation, is not liable to break, retains its accuracy to size under normal operating conditions (such as under the influence of X-rays and increased air humidity), and moreover is advantageous because of the material cost price. According to the invention, this is achieved in that the diaphragm consists of magnesium. According to an advantageous further embodiment of the invention, the diaphragm is constituted by a multi-layer structure in a manner such that a magnesium layer is applied to a substrate which consists of a material highly transparent to X-ray radiation. This involves the advantage that diaphragms of comparatively large dimensions and extremely transparent to X-ray radiation can be manufactured. DETAILED DESCRIPTION OF THE INVENTION A method of manufacturing the mask according to the invention is characterized in that a magnesium layer is applied to an auxiliary carrier, which either is left in the mask structure or is subsequently removed, in that the diaphragm thus formed is stretched and secured on a supporting frame and in that then a layer absorbing X-rays is applied to one of the major surfaces of the diaphragm which is structured in accordance with the desired mask pattern. According to advantageous embodiments of the invention, the auxiliary carrier is constituted by a foil of a synthetic material, more particularly of polyimide or polycarbonate. This involves the advantage that the auxiliary carrier can be retained as a part of the mask because both synthetic materials are highly transparent to X-rays and are particularly suitable for use as a diaphragm material in conjunction with the magnesium layer compensating for their unfavourable properties for diaphragm application. A further advantage is that with such a composite mask comparatively large diaphragms can be manufactured, which nevertheless operate with a very high accuracy to size, even under unfavourable environmental conditions. The further advantages obtained by the use of the invention especially consist in that a mask for X-ray lithography particularly suitable for mass production is provided, in which the favourable properties of the inorganic materials known for such diaphragms are united with the favourable properties of the organic materials known for the same purpose while avoiding the disadvantages of these materials. The cost price of these masks is very favourably influenced by the use of magnesium for the diaphragm in masks for X-ray lithography. The cost price of magnesium is about one tenth of the price of titanium. A further advantage is that magnesium has an extremely high transparency to X-ray radiation just in the range between 1 and 4 nm. Because of the lacquer sensitivity generally increasing with an increasing wave-length as well as because of the only just permissible diffraction phenomena, especially this wave-length range is of particular importance for X-ray lithography. A magnesium diaphragm is particularly suitable for X-ray apparatus working with a copper target. In such apparatus, CuK.alpha. radiation of a wave-length .lambda.=1.34 nm is produced. However, a mask comprising a magnesium diaphragm would also be very suitable for exposure apparatus working with synchrotron radiation .