James T. Clemens, AT&T (Panel Chair)
Robert W. Hill, Hill Associates (Panel Cochair)
Franco Cerrina, University of Wisconsin
Gene E. Fuller, Texas Instruments
R. Fabian Pease, Stanford University
Henry I. Smith, MIT
The goal of the JTEC report on X-ray lithography, fully funded by the Office of Naval Research, is to provide a detailed appraisal of the technology, personnel commitments, and strategies for implementation in manufacturing of X-ray lithography in Japan.
Integrated circuits (semiconductors) are the key components of modern computers, communication systems, consumer electronics, and the new generations of smart machines and instruments. Microlithography is one of the most critical elements of the semiconductor manufacturing process because it determines the minimum feature size and the functional capabilities of the semiconductor. The quality of the microlithography process is critical in determining the yield and cost of semiconductors and hence the competitiveness of the electronics industry.
At present, all volume semiconductor manufacturing is done with optical UV (ultraviolet) projection lithography. X-ray lithography, however, holds the promise of providing higher yields in manufacturing semiconductors by virtue of enhanced process latitude, process robustness, and resolution.
The major Japanese microelectronics firms have a broad, well-developed strategy for research and development of microlithography technology that includes UV, deep UV, X-ray proximity and projection, and electron-beam lithographies. They are investing in all of these alternatives. All of the manufacturers visited either had in-house X-ray programs, were members of the SORTEC X-ray consortium, or both. Their commitment to X-ray lithography was firm and appeared to be well balanced. In the U.S. there is limited interest from semiconductor manufacturers in X-ray technology, with the exception of AT&T, IBM, and Motorola.
Most funding for X-ray lithography efforts in Japan comes from individual industrial organizations. The Japanese government directly and indirectly has provided seed money to major research and development efforts. The government has funded roughly $70 million of the SORTEC development through MITI, and industry has funded $30 million. Japanese companies are making the major part of the X-ray investment in their own companies.
In the U.S., there has been a significant X-ray lithography program for over ten years at IBM. Motorola has recently joined the effort. Congress has provided money to DARPA for applied research and development on X-ray lithography in all sectors of the technical and industrial community. However, the U.S. industrial community has not been independently preparing itself for insertion of X-ray lithography into manufacturing.
The consensus among Japanese semiconductor manufacturers was that optical lithography would continue to evolve for advanced semiconductor manufacturing until the late 1990s, and that the potential switch to X-ray lithography would probably occur when the minimum critical dimension reached 0.25 micron or less. While their first choice for 256 megabit dynamic random access memory (DRAM) was optical, they were prepared to use X-ray technology for manufacturing. Although they recognized potential of higher yield and lower manufacturing costs with X-ray, manufacturers will not change technology until absolutely necessary. This same viewpoint prevails in the United States and in Europe.
There were many large efforts in Japan to develop synchrotron-based lithography systems because they are bright, collimated sources. Smaller laser and gas plasma sources, while more desirable from a granularity standpoint, were not visible or discussed in detail. X-ray projection projects exist; they were mentioned at several companies but not extensively discussed.
The size, cost, and configurational aspects of synchrotron-based X-ray lithography did not appear to be serious issues in Japan with the DRAM manufacturers. Their view was that if X-ray lithography were used, it would be for large-volume manufacturing, which would require multiple synchrotron facilities. Cost has been a major issue with the U.S. and European manufacturers since their volume semiconductor production has not been DRAM-based, their companies are smaller, and many are not using the leading edge of microlithography technology. The initial investment is beyond the means of most of these manufacturers; only IBM, AT&T, and Motorola have major active internal X-ray programs. Also, in the U.S. several synchrotrons originally developed for other purposes are being used in part for X-ray lithography R&D.
DARPA is administering a program sponsored and financed by Congress that attempts to overcome some of these difficulties by helping to build the infrastructure necessary for X-ray lithography. DARPA is expanding that program to support other lithographic alternatives.
Development of X-ray mask technology, exposure systems, and resists has been pursued vigorously in Japan, as has integration of the total system.
There appeared to be a consensus that materials for X-ray masks were adequate. The Japanese were using silicon nitride membranes with tantalum absorber mask technology licensed from NTT. They were researching silicon carbide membrane and tungsten absorber materials, and planned to research diamond membranes.
The major mask concern was 1X electron-beam mask patterning, specifically errors in feature placement and dimension control. There was no work on mask inspection and repair underway; the Japanese believe these tools will be available from domestic or overseas sources when required.
Several independent efforts were being pursued on exposure system aligners, with critical elements under development. Heterodyne interferometric alignment techniques were favored for alignment; these were more advanced in concept than current U.S. or European projects.
With respect to fundamental understanding of the science of X-ray lithography, the Japanese and the U.S. technical communities were on a par. The trend, however, was for the Japanese to pull ahead of the U.S. due to a higher level of funding and staffing, particularly at the company level.
If X-ray lithography becomes necessary for producing the next generation of semiconductors, Japanese industry will be in an excellent position to maintain or increase its market share in semiconductors and the advanced systems dependent on them.