Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
- 7 - SOLID STATE ELECTRONICS INTRODUCTION Solid state electronics has been the foundation of the technologi- cal revolutions that have taken place since World War II in information processing, computation, communication, control, and other fields. Modern naval defense systems depend heavily on high-performance solid state electronics. It is critically important that naval research policy promote the advancement of research in solid state electronics and sensors. The current program of the ONR in this area is broad, well-directed, and basically sound. However, it is clear that over the next two decades there will be major changes in the structure of microelectronic devices and the systems based on them. Specifically, devices will continue to shrink in size, and the transistor will very likely give way to fundamentally new devices based on quantum mechani- cal effects. Advanced sensor arrays and devices to convert the output of electronic systems to physical actions will lead to systems whose complexity and capabilities are not dreamed of even today. We there- fore urge the ONR to fill the vacancy that currently exists in the solid state electronics area with an outstanding individual who has a broad interest in the area of device research. One of the significant problems in solid state electronics re- search is the rapidly increasing cost of equipment and facilities. Two decades ago research could be done in normal laboratory space with low- cost homemade equipment. Today, research is carried out in cleanrooms with equipment that costs orders of magnitude more. An individual in- vestigator must have access to millions of dollars worth of equipment and facilities in order to participate in solid state electronics re- search. Equipment replacement costs are proportionately high. Future budgets for solid state electronics research should reflect the rapidly increasing cost of equipment purchase, operation, and replacement. NOVEL DEVICES AND SYSTEMS Integrated circuits, whether in Si or III-Vs, are based on tran- sistors interconnected by means of metallic or semiconducting wiring. As devices decrease in size below 1 micron, more and more of the circuit area is taken up by wiring. This "wiring crisis," together with problems that arise as the minimum feature size of transistors approaches 100 run, has led many experts to predict that totally new devices, based on quantum mechanical effects and local coupling (i.e., nonwired), will replace the transistor and wired-up integrated circuit. No one has any idea at present what such devices will look like or how systems based on them will operate. We only anticipate that devices will be small and systems will be quite different from what we have learned to deal with in today's technology. Hence, it is important for the ONR to pursue a basic research program aimed at promoting this envisioned fundamental change in solid state electronics. Most of the
- 8 - ONR research over the last several years on artificially structured materials and quantum-well devices has been focused in this direction. We wish to encourage this trend and suggest some opportunities for high impact. As we approach the turn of the century, the direct link between electrical material properties and electrical performance of the solid state device must be established as an integral part of proposed re- search efforts. The role of defects and their influence on device electrical characteristics must be emphasized to understand reliability and fabrication. MATERIALS The ONR research program in electronics is one of the premier pro- grams in the country on electronic materials. The well-managed inno- vative nature of this program has made it the source of many of the major developments in this field over the years. The current efforts are very much in keeping with this tradition and are required to secure the technology base of U.S. electronics, both private and military. Hence, we feel that the new efforts we are proposing should be addi- tional to those currently funded. In the future, we anticipate that heterostructures between mate- rials of both similar and dissimilar substances are going to be the key to electronics in the 1990s and beyond. Epitaxial layers involving group IVs, as well as III-Vs and II-Vis, will be important. Although the ONR already has very substantial efforts in this area of research, we feel that particular emphasis should be placed on a few key areas where major progress can be obtained because of a confluence of new developments. The arrival of inexpensive, high-performance computing is going to make it possible to use realistic models of materials fabrication in the simulation of device formation. Soon it will be possible to add Cray levels of computing to a crystal puller, MBE machine, MOCVD reac- tor, or reactive ion etcher for roughly 10 percent of the cost of the system. Hence, it will be feasible to use computers in a realistic process control mode with very complex, sophisticated equipment for device growth and processing. The production of new, very complex heteroepitaxial devices will require exceedingly precise control over materials preparation and fabrication. Structures in which uniformity of fabrication over wafers whose characteristic dimensions are inches must be maintained to levels measured in angstroms; this will require control of fabrication to one part in 108. These levels of control are going to require the construction of computer-based models that will allow the development of fabrication engines with a very high level of control. We feel that the ONR should begin to initiate pro- grams aimed at this very exciting and essential direction of develop- ment. The ruggedness and compactness of modern electronics are based on the fabrication of microchips that contain large numbers of electronic elements. To date, these microchips have been based on a single
- 9 - semiconductor material--Si for most chips, and GaAs for a few chips. Yet some of the advanced concepts in electronics require the integra- tion of the high-speed or optoelectronic properties of III-V semicon- ductors with the complex circuits that can be fabricated in Si, coupled with the detector or display properties of II-VI semiconductors. It is possible to conceive of the use of heteroepitaxy to fabricate chips with a number of different semiconductors, each performing the function it does best. We believe that such structures will be very important to the Navy. A major feature of these new heteroepitaxial systems is the existence of large lattice mismatch between the layers in the struc- tures. The development of techniques for controlling the defects that may be introduced by such a procedure will be important to the success- ful development of a heteroepitaxial technology. Hence, we think that the ONR should have a major research program to understand and control the fundamental characteristics of electronic materials that affect device performance. This can be accomplished by focusing more effort on the device behavior of novel structures. MICROFABRICATION Future generations of solid state electronic devices and systems will require advances in microfabrication (lithography, etching, and additive processes) as well as in materials. Especially is this true for quantum-effect devices that involve sub-100-nm linewidth struc- tures. Although electron-beam lithography techniques have demonstrated sub-100-nm patterning, this method is extremely slow and produces coherent structures over very small fields. For example, to cover a 1-cm diameter area with a dense array of 50-nm features requires a writing time in an electron-beam system of the order of one year! Achieving a linewidth control of 10 percent at 50-nm linewidths is highly problematic, yet such control may be required. For 50-nm linewidths, the writing field of electron-beam systems is only about 100 microns in diameter. Larger fields require a step and repeat procedure. Electron-beam lithography is adequate for certain first order research demonstrations but inadequate for Navy needs beyond that point. Metrology in the sub-100-nm domain is crude or nonexistent. Replication techniques such as x-ray lithography, masked-ion-beam lithography, and perhaps x-ray projection are capable of producing sub-l-nm features with short exposure times, but much further research needs to be done to make such techniques useful for both research and the development of the next generation of microelectronic devices and systems. Electron-beam lithography systems are extremely expensive facil- ities, as are the synchrotrons being contemplated for x-ray lithog- raphy. Unless lower cost approaches are developed, research on microelectronic devices and systems will be beyond the means of the vast majority of university researchers. Such research cannot be done by occasional visits to major facilities that have such equipment.
- 10 - Beyond lithography, techniques of etching and additive patterning require extensive research to achieve the precision, accuracy, and linewidth control required by the next generation of microelectronics. Perhaps entirely new approaches to making microstructures need investi- gation. In summary, the ONR should not overlook the opportunity for sup- porting research on the raicrofabrication techniques required to take us into the next generation of microelectronics in the sub-100-nm domain. REFERENCES Leadership in Microstructure Technology. March 3, 1986. Report of the Steering Committee Workshop on "The Future of Microstructure Technology," held at Seabrook Island, South Carolina, October 13-15, 1987, under the sponsorship of the National Science Foundation, Air Force Office of Scientific Research, and Office of Naval Research. Research on Artificially Structured Materials. Solid State Sciences Committee, National Research Council. National Academy Press, Washington, D.C., 1985.
