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In order to retain the simplicity of optical (400 nm) 1:1 shadow projection even in the case of sub-μm structures, the wavelength of the radiation employed has to be reduced drastically. The application of deep-UV radiation (e- 200 nm) only leads to an insignificant improvement in the Fresnel-limited resolution. Soft X-rays as well as high-energy ions are capable of replicating sub-μm features in shadow projection at reasonable proximity distances (>º 30 μm). X-ray lithography, the technique which is further developed at present, utilizes wavelengths between 0.5 and 5 nm, with structural resolution as good as 0.1 μm under certain conditions. The transition to soft X-rays requires the development of new intense sources as well as sensitive and process-stable resists, high-transmissive (optical light, X-rays) masks, and an appropriate alignment system with an accuracy below 0.1 μm. The type of X-ray source used is the most decisive parameter in determining the attainable resolution. (This is the reason why we only deal in this paper with the properties of the different X-ray sources, with strong emphasis on synchrotron sources. The use of X-ray tubes limits the minimum structure size to about 0.5 μm, even in the case of the tri-level technique.) Parallel and high-intensity synchrotron radiation makes the replication of patterns within the fresnel limit possible and provides a higher flexibility in choosing the suitable resist. Nevertheless, the two different approaches complement each other, since the first available X-ray systems will be equipped with low-cost conventional X-ray tubes which can be replaced later by compact synchrotron sources now under development. However, the technological basis, especially the mask technique, remains nearly unchanged. The technique of shadow projection using ions, as described here employs the dechanneling effect of a thin metal layer as well as the difference in energy loss between the random and the channeling directions. There are two advantageous features of ion-beam lithography in comparison to X-ray and E-beam lithography. Inexpensive ion sources with high intensities, in contrast to those for X-rays, are already available and no proximity effect occurs as in the case of electrons. Furthermore, the high energy-deposition density leads to a high sensitivity of resists. Mask problems, however, are more critical and the resolution is not as high as in the case of X-ray lithography.
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