Photolithography and X-ray Lithography
Very intense technological efforts to increase the integration density of semiconductor devices have been made in the development of new and economical lithography methods for structures below 2 µm. The lithography methods now used for modern fabrication lines are optical projection systems in the wavelength range of 400 to 250 nm, based on high-resolution objective lenses or mirror optics. The resolution limit arising from diffraction is far below 1 µm, and impressive examples of optical sub-µm patterning have already been realized. With increasing resolution, however, equipment costs as well as the complexity of the process technology are growing rapidly, e.g., small step-and-repeat areas, extremely low depth of focus, multilayer techniques, suppression of interference effects, and so on. Therefore, in the practical case of application in circuit fabrication, the resolution limit will be somewhat higher; it seems possible that this will be in the range of 1 µm.
In order to realize a production process to generate sub-µm structures, X-ray lithography is the most promising approach. X-ray lithography at wavelengths between 0.5 and 5 nm is a simple one-to-one shadow-projection process, with structural resolution as good as 0.1 µm under certain conditions. The main factors limiting the resolution are Fresnel diffraction, fast secondary electrons, the relatively-low mask contrast attainable in the soft X-ray range, and — most important — the individual radiation characteristics of the X-ray source in question. For a quantitative comparison of the different X-ray sources which are applicable for lithography, numerical calculations of the generated resist patterns are necessary. The simulation model on which the calculations are based has to take into account the effects mentioned above, depending on the spectral distribution of the individual X-ray source, as well as the spectral absorption of windows, mask substrates, and mask absorbers. Important boundary conditions in this connection derive from the present state of resist technology, especially in regard to sensitivity, and from the necessity of compromising between source radiation power and tolerable mask heating during exposure. Based on these considerations, a comparison between X-ray tubes, storage rings, and plasma sources leads to the conclusion that synchrotron radiation is superior to the others.
KeywordsSynchrotron Radiation Storage Ring Optical Lithography Fresnel Diffraction Proximity Distance
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