X-Ray Lithography: Recent Progress And Future Developments

Several forms o f lithography, such as x-ray lithography (XRL), projection electron beam lithography (SCALPEL), and projection ion-beam lithography (IPL), are now being developed for sub-0.1 -pm-rule ULSl fabrication. The common feature of these technologies is t ha t they use membraneo r stencil-masks. Among them, the x-ray mask is the simplest and most compact, which are advantages fo r practical use. Recent experimental and simulation data indicates that XRL may allow a practical throughput as high as 50 wafers (48’’ ) per hour. In addition, XRL provides a very large process margin and is insensitive t o sub-0.1-pm dust. These superior characteristics will enable low cost fabrication of ULSl in the future. On the other hand, the pattern-placement accuracy required for x-ray masks used fo r sub-0.1-pm device fabrication is less than f10 nm. The field size required for such device fabrication is expected t o be about 25 X 4 0 mm, as described in the SIA Roadmap.’ Such pattern-placement accuracy corresponds t o mask distortion of less than 0.5 ppm. Among the several key issues concerning x-ray lithography technology, the development of such high precision x-ray masks is the f i rs t priority. For this reason, a great deal of research and development aimed a t both x-ray mask materials and the fabrication processes has been carried out. Owing t o such efforts, very promising x-ray mask materials and mask-fabrication processes, which will make 0.1 -pm-ULSI fabrication a reality, have been developed. In this paper, we will report the recent progress in x-ray mask materials, such as the development of lowand uniform-stress x-ray absorbers (amorphous TaGe’ or TaReGe3), extremely low stress CrN hard-masks4, and high Young’s modulus radiation-resistant membranes. We will also review the progress of key instruments, such as a compact synchrotron with normal-conducting magnet, high-transmission efficiency and uniform beamlines, and high-throughput x-ray steppers. We will then examine the remaining issues, and discuss what sti l l has t o be done t o put x-ray lithography into mass production.