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7 Conclusions and Recommendations There are great opportunities for wide bandgap semiconductors to improve the performance of many nonelectronic technologies. Major benefits to system architecture would result if cooling systems for components could be eliminated without compromising system performance (e.g., power, efficiency, speed). The existence of commercially available high-temperature semiconductor devices would provide significant benefits in such areas as: · sensors and controls for automobiles and aircraft; · high-power switching devices for the electric power industry, electric vehicles, etc.; and · control electronics for the nuclear power industry. With the possible exception of LEDs, however, present commercial demand for wide bandgap semiconductor materials is limited. While there are few pressing applications that cannot be achieved without wide bandgap materials, the vast array of applications, and hence the value, will only be realized once these materials have evolved to such an extent that off-the-shelf devices are available. This chapter is divided into two sections. The first section presents general conclusions and recommendations about future research priorities to accelerate the acceptance of high-temperature semiconductor materials. This section discusses the temperature ranges for the different materials to be used, the competitiveness of U.S. research versus foreign competition, the systems in which high-temperature electronic materials should initially be introduced, and the government/industry/un~versity collaborations required to forward the development of high-temperature semiconductor materials. The second section discusses the barriers to the successful 65 development, manufacture, packaging, and integration of wide bandgap materials into existing systems and presents the key research and development priorities to overcome these barriers. GENERAL CONCLUSIONS AND RECOMMENDATIONS Temperature Ranges Silicon and silicon-on-insulator (SOI) electronics may be sufficient for some applications for temperatures up to 300 °C. Such applications include digital logic, some memory technologies, and some aerated analog and power applications. Silicon-based technology will not be sufficient for many applications operating in the 200- 300 °C range, however, such as power-conditioning devices in higher-temperature control systems. These devices will have to be produced from another material system. Devices based on SiC are well positioned to meet this need, particularly e-channel enhancemer~t-mode MOSFETs. However, significant technological barriers, such asmicropipes, oxide qualify, contacts, metallization, packaging, and reliability evaluation still need to be further addressed. As a result of fundamental limitations, silicon-based technologies will not be useful at temperatures above 300 °C. Other materials must be used for these temperature ranges, but the choices are somewhat less clear. Technology based on GaAs might be used for systems operating up to 400 °C. Just working at elevated temperatures is not the only concern, however. It is also essential that the devices reliably function over a wide range from very cold (i.e., -20 °C) to very hot (i.e., 400 °C). Based on the evidence presented in this report,
Materials for High-Temperature Semiconductor Devices devices based on e-type SiC are the only type that currently appear to meet the temperature-range and reliability requirements, but additional development is needed. Eventually, high-temperature electronic technology could be developed for reliable operation even for temperatures above 600 °C. U.S. Competitiveness As described in the Preface, considerable international resources are currently being devoted to developing electronic technologies either tailored for or supportive of high-temperature operation. The United States is focusing most of its efforts on high-temperature applications and currently has a slight lead in SiC research. Europe appears to be increasing its effort in wide bandgap materials, especially for power electronics. This research area is synergistic with high-temperature applications because the generation of internal heat is a limiting factor in power devices and can be mitigated by larger bandgap and higher thermal conductivity materials. The dedication of European resources to this area is seen in the founding of the collaborative organization HITEN, which was established in 1992 to coordinate nascent European efforts in high-temperature electronics. Japan is emphasizing the use of wide bandgap materials for opto-electronics and leads in the use of nitrides for light sources. Japan is also becoming interested in power and high-temperature applications. Unfortunately, the closed nature of Japanese industry made it difficult for the committee to determine the true level of interest in wide bandgap materials research. The increased interest in high-power, high-temperature applications is evident in Japan's annual domestic SiC conference, however. The Third Domestic (Japan) SiC Conference convened in Osaka on October 27-28, 1994, with approximately 160 experts in attendance. Contrary to Japan's previous two conferences, there was a greater emphasis at the Osaka conference on high-power, high- temperature applications than on LEDs. The Commonwealth of Independent States had a number of major programs in SiC development, but the current iFinanciall difficulties of most of the Commonwealth's institutions are preventing many laboratories from continuing their research. There is a wealth of expertise and information available for leveraging by other countries, however. For instance, the 66 European Community is planning on supporting a SiC growth effort in St. Petersburg (Y.M. Tairov and V.E. Chelnekov, personal communication, 1994~. The committee believes that the U.S. wide bandgap materials research community is currently very competitive in the international research community. To remain competitive in the international research community, the committee recommends that demonstration technologies be pursued to motivate further research and increase interest in high-temperature semiconductor applications. Demonstration Technologies To increase interest and motivate further research in wide bandgap materials, a realistic, inspiring application focus must be found that can make system designers aware of the benefits of high-temperature electronics. A wide bandgap transistor that operates at 150 °C will not drive the technology because it will be in direct competition with the more economically efficient silicon technologies. The demonstration technologies must be system circuits (i.e., not an individual device) that can be inserted into essentially nonelectronic systems (e.g., turbine engine, nuclear reactor, chemical refinery, or metallurgical mill) with the goal of measurably increasing system performance. As discussed in Chapter 1, the committee believes that there eventually will be a niche market for semiconductors with temperature capabilities higher than that of silicon, and that this market will be sufficiently large to justify the cost of development. However, this belief is tempered by the recognition that because such electronics will be used in new ways there is little immediate demand. The market will grow only in synergy with the availability of components. This suggests that development of high-temperature electronics not be undertaken in isolation. Instead, such development can and should be leveraged from development of other technologies with more immediate applications, thus reducing the costs and the risks of both. Three suitable application areas are high-power electronics, nuclear reactor electronics, and opto-electronics. Power switching devices, for example, would be a good demonstration technology for high-temperature semiconductor materials. High-voltage, high-power electronics, while not necessarily used as high-temperature
Conclusions and Recommendations devices, nevertheless need wide bandgap semiconductors because of their superior breakdown voltages and high thermal conductivities. There is already considerable research being pursued in this area because (1) improved high-power switching devices could save an estimated $6 billion in the cost of construction of additional transmission lines; and (2) the smoother, more efficient use of the transmission system would reduce the need for new generating capacity, which the Electric Power Research Institute estimates would be a savings of $50 billion in North America alone over the next 25 years (Spitznagel, 1994~. The pursuit of demonstration technologies would not only increase interest in wide bandgap materials, it would also provide significant testbeds for the application of the technology and enhance our understanding of the generic technologies recluired to further high-temperature device operation (e.g., materials etching and implantation; degradation modes of metallic gates, contacts, and interconnects at high temperatures; packaging behavior at high temperatures; and accelerated-testing and reliability- testing methodologies to ensure proper functioning). The ability to grow a reasonably defect-free material is not the only requirement for the realization of a successful technology. The development of demonstration technologies would also help identify other factors that must be resolved for high-temperature electronics to be incorporated into existing systems. Funding Strategy The need for new development funds for demonstration technologies and future wide bandgap materials is not necessary in the comunittee's opinion. Government funding currently exists for long-range research in wide bandgap materials, although additional funding would certainly allow more options to be evaluated within a shorter period of time. Industry has also demonstrated a willingness to commercialize new developments if the projected payback to their investments can occur within the short term (NRC, 1993~. The committee believes that the high-temperature research community should leverage the research funding for wide bandgap materials that Is currently being provided by the high-power and optics markets, where no viable alternatives to wide bandgap materials currently exist. 67 Building on the funding for other areas dependent on wide bandgap materials reduces the need for potential users of high-temperature devices to fund the required materials development exclusively and, thus, may render it cost effective. The committee recommends the following strategy for the development of wide bandgap materials: develop precompetitive alliances and integrated programs (national laboratories, universities, and industries) for coordinating research, technical skills, and capabilities to expedite research in the most efficient manner; direct research at a technology demonstrator that has definite applications (i.e., is a product) and addresses the usually neglected areas of packaging, assembly, testing, and reliability (e.g., high-power switches; integrated motor control; power phase shifter); concurrently develop materials, design, testing, and packaging; and build and test the demonstration component on a cost-share basis that encourages teaming, ensures adequate funds, and requires periodic deliveries. The committee believes that the founding of a newsletter that provides a summary of published worldwide developments in high-temperature semiconductor research would assist the establishment, development, am maintenance of (1) a fundamental long-term materials effort, (2) an infrastructure within the industry, (3) a group to monitor interrzational development, arid (4) a U.S. information group for highlighting advances. MATERIALS-SPECIFIC CONCLUSIONS AND RECOMMENDATIONS The first three parts of this section concentrate on the major wide bandgap materials discussed in this report: SiC, nitrides, and diamond. The final part of this section concerns the generic problems in packaging that will affect the production of all high-temperature electronic O c .evlces.
Silicon Carbide SiC is an indirect bandgap semiconductor and has enjoyed the longest history and greatest development with regard to both materials growth and device realization. As such, SiC is currently the most advanced of the wide bandgap semiconductor materials and in the best position for near-term commercial application. Its main application will be in high-power, high-temperature, high-frequency, and high-radiation environments. It will not be suitable for blue lasers or ultraviolet light emitters, however, except as a potential substrate material. The specific technical issues for SiC that require further research are summarized in the box, Technical Issues for SiC. The three key research efforts for the development of commercially viable SiC devices are Wafer production: The 1- and 2-inch SiC wafers now in production are rapidly approaching device quality where they might be used for commercial production of devices and circuits with acceptable yield. It could be argued that such small wafers are entirely sufficient for what will be a relatively small market (compared with silicon) with a very high-price premium, and therefore an early investment in larger wafers is not justified. However, the entire commercial infrastructure for electronics manufacture is based on a wafer size of at least 3 inches, and Technical Issues for Nitrides Substrate development Nitride substrates for CVD homo-epitaxy High thermal conductivity, quasi-lattice matching substrates (bow high electrical conductivity and semi-insulating) Further improvement of crystal perfection and doping (CVD grown) Reduce defect density and background impurities Better control of n- and pipe doping New technologies for epitaxial growth Improve surface morphology Improve processing (CVD growth) Ohmic contacts Low contact resistance High-temperature contacts High-temperature packaging Improve understanding of basic properties and knowledge of design parameters Materials for High-Temperature Semiconductor Devices Technical Issues for SiC Purger improvement of crystal perfection (boule and CVD growth) Eliminate micropipes Reduce defect density Reduce background impurities Improve surface morphology Further improvement of doping (boule and CVD growth) New n- and p-type dopants New mid-gap impurities for semi-insulating substrates Introduce rare earth elements in growth Improve processing (boule and CVD growth) Improve oxides/passivation Find alternative insulators (nitrides) Reduce contact resistivin,r for pipe material Develop high-temperature n- and p-type contacts High-temperature packaging Improve understanding of basic properties and knowledge of design parameters preferably 4 inches, as a minimum. Reconstructing a small-wafer infrastructure that became obsolete over 30 years ago will be both an expense and an obstacle to the introduction of commercial SiC electronics. The committee believes that the development of larger SiC wafers is viewed as the more cost-effective approach to commercial development. Film growth: Chemical vapor deposition, molecular-beam epitaxy, and other film-growth technologies and chemistries require refinement to produce epitaxial films with n- and p-type doping ranges from 10~3 to 102° cm~3 for nitrogen, aluminum, boron, gallium, transition metals, and rare earth elements. Manufacturing processes: Lower-cost device- production methods are required to make the manufacture of SiC devices more competitive with the silicon technologies. . Nitrides Interest in the direct bandgap nitride materials (i.e., GaN, A1N, AlGaN, and InGaN) has dramatically increased recently because of their optical properties. The materials show great promise and are likely to dominate the visible and ultraviolet opto-electronics market. Nichia's recent bright blue LEDs have already stimulated increased industrial effort (e.g., Hewlett Packard, Spectra 68
Conclusions and Recommendations Technical Issues for Diamond Improvement of grown, crystal perfection, and growth Reduce and control impurities of bulk synthetic diamonds Produce large-area (hetero-epitaxy) single-crystal films of diamond on nondiamond substrates at reasonable cost Synthetically produce larger bulk diamond at reasonable cost Improve n- and p-type doping Improve implantation for doping Improve processing Ohmic contacts Low contact resistance High-temperature contacts Hydrogen passivation Improved understanding Dopant diffusion Knowledge of design parameters Diode Laboratories, Xerox PARC) in materials growth, contact metallurgy and reliability, and device reliability and testing, although the materials have defect densities of greater than 10~°/cm2 and the mechanism of photo emission is currently unknown. Heterojunctions in the nitrides also hold promise for higher-speed devices compared with SiC. Their applicability for power development and nlgn-~requency devices is unproven at this time, and the technologies for wafer production, doping, and etching are currently less developed than SiC and require more longer-term research before they will be competitive with other electronic materials. However, as development of photonic applications for wide bandgap materials progresses, the opto-electronic market may provide an effective way to leverage the development of these materials for high-temperature device applications. The specific technical issues for nitrides research is summarized in the box, Technical Issues for Nitrides. The committee identified the following three research efforts as being key to the development of nitride devices: · Compatible substrates: Better-matched substrates are required for nitride wafer production to be commercially tenable. · Wafer production: Growth of quasi-crystalline films of GaN, AlGaN, and A1N should be pursued on substrates such as SiC to gain thermal advantages. · Doping: Methods for both n- and p-type doping of Group III nitrides are required. 69 Diamond Diamond is a well-understood material, but its use for active electronic device applications is not feasible at this time because of the difficulties associated with its economical growth and doping. While diamond transistors have been designed, fabricated, and tested, their performance is also orders of magnitude less than that which is expected from the electrical properties intrinsic to diamond. The poor performance is thought to result from excessive nitrogen impurities and from as yet not fully explained surface-depletion effects. The current prognosis for diamond is primarily as a protective coating, a thermal management film, and a material for electron-emitting cathodes. The specific technical issues for diamond research are summarized in the box, Technical Issues for Diamond. Packaging Much more research is required in the area of high- temperature packaging. For high-temperature electronics to be commercially viable and provide true performance advantages, interconnection and packaging technologies are required that can reliably operate at temperatures up to 600 °Cfor 1(74 hours. To attain these goals, innovative packaging techniques will be required. The specific technical issues for packaging research are summarized in the box, Technical Issues for Packaging. The three key research efforts for the development of high-temperature packages are · Metallization: Contacts are required in the 10-6 to 10-7 Q/cm2 range that have lon~-term durability at temperatures up to 600 °C. Greater understanding Is needed of the long-term effects Technical Issues for Packaging Improve reliability of high-temperature contacts Improve metallization Improve device development tools Improve process-control tools Improve polishing, cutting, mounting, and etching mends Develop reliability and aging tests Develop computer-aided design tools
Materials for High-Temperature Semiconductor Devices of high temperatures on contact and interconnect metallurgy, degradation and failure modes, reliability, and interfaces. Device reliability and aging testing: Existing methods of accelerated, environmental life testing of packages must be adapted for high 70 temperature applications to ensure the accurate assessment of device reliability and aging. · Computer-aided design tools: Computer-aided design tools are required that incorporate electrical and mechanical simulation of high- temperature electronic systems.
