Cost-effective x-ray lithography

Abstract

The push towards faster, denser VLSI device structures and eventually to ULSI devices means ever-decreasing design rules for IC manufacturers. In order to define patterns on silicon and gallium arsenide substrates with feature sizes of 0.25 microns, lithography, metallization, and electronic materials processing techniques will be pushed beyond current limitations. Of these technologies, lithography in the sub-0.5 micron region appears to be the main obstacle yet to be overcome. As deep-UV optical systems become more expensive and the useful field sized decrease in the attempt to achieve finer resolutions, the question of whether to switch to an alternate lithographic method becomes imminent. X-ray lithography is the leading candidate. In this paper, the question of whether x-ray lithography is economically superior to optical lithography and the cost-effectiveness of x-ray lithography are addressed. Also, the question of how x-ray lithography can be performed in a production environment is considered. First shown is that more elaborate optical systems are simply not going to match x-ray proximity system in terms of resolution because of the need to use exotic lens materials or complicated and ever finer reflection systems, none of which can correct for diffraction effects, yet must be corrected for every other aberration. The economic superiority of a synchrotron-based x- ray lithography beamline is demonstrated in a production facility using a processing-cost model based on Shinji Okazaki's cost-per-bit model. Considered, as well, is the strong possibility that exists for the use of an optically based production line which would use an anode or plasma x-ray stepper to define only the smallest geometries, such as the gate level on a DRAM chip. It is shown that it is unlikely, even pushing the limits of materials and optics, that deep-UV systems will be able to define patterns below 0.35 microns in a production environment. X-ray lithography systems could define 0.20 micron patterns in a production environment with a yield that promised to be better than that of optical systems.