Electron beam lithography for advanced LSI fabrication

The current technology used in IC (Integrated Circuit) and LSI (Large Scale Integration) fabrication is based on optical lithography or the art of photocopying. The light wave length sets an ultimate and absolute limit on the resolution of the optical method at about one micron (JLm=10-6 meter) and the current LSI technology has almost reached this limit. In microscopy, a drastic improvement of resolution was achieved in the 1930's by switching from glass (light) optics to electron optics. The similar is taking place in lithography for LSI fabrication. X-ray lithography and electron beam lithography are the two promising methods for improving the resolution by at least one order of magnitude over optical lithography. The characteristic features of these three lithographic methods are summarized in Table I. In Table I, "Mask in Contact" means that a mask, with a master pattern inscribed on it, is placed in close proximity to the target (silicon wafer covered with photo-resist) so as to selectively expose the target to the incident beam. The finite and rather short life of the masks in contact is the major disadvantage of this scheme. In Table I, "Mask Projection" means that an image forming projection system (actually a lens) is placed between the mask and the target so that the mask does not have to be in contact with the target. Since there is no material suitable for making an X-ray projection lens, this scheme is not applicable to X-ray. In Table I, "Direct Pattern Generation" means a feature in which the incident beam is maneuvered so as to generate the pattern directly on the target without using a mask. While this feature can be implemented by suitably deflecting an electron beam at high speed without any fundamental difficulties, this is not the case for light and X-rays. Note that the sub-micron pattern generation feature is also needed for making the masks themselves. Therefore, "electron beam pattern generation" is the key and the very heart of