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5 RESEARCH AND DEVELOPMENT ISSUES AND OPPORTUNITIES Superhard materials are increasingly important industrial materials whose uses are currently enhancing productivity in metalworking, oil production, and mining. Several of these materials are also emerging as key materials for future advanced electronic and optical systems. It is important to point out that in this latter area the United States is not in a leadership position. There are, however, several research and development efforts in this country directed toward fabrication of high-temperature transistors based on diamond and SiC for possible use as sensors. Other efforts address the use of diamond-like films for wear-resistant coatings on magnetic disks and optoelectronic devices and components, for masks for use in fabricating distributed feedback lasers, distributed Bragg reflectors, filters, modulators, and couplers. During the committeets deliberations it was clear that, in the area of diamond and diamond-like film technology, the U.S. efforts have been dependent on following the technologies first demonstrated in Japan and the USSR. For example, Sumitomo is beginning to market large-area thick films of cubic BN for use as heat sinks for solid-state devices, even though its thermal conductivity is approximately one-third that of diamond. As another example, Showa Denko (press release, ShowaDenko, Tokyo, 1988) in collaboration with Nippon Institute of Technology, has developed a h~gh-speed process for the synthesis of- diamond thin films on silicon wafers. In the area of high-pressure synthesis of diamond and cubic BN, the processes for doing this were invented, developed, and commercialized in the United States. These technologies have become mature and subsequently diffused globally. An example of this is the list of countries in which there is commercial synthesis (high-pressure, high-temperature) of diamond and/or cubic BN: Czechoslovakia, France, East and West Germany, Greece, Ireland, Japan, Peoples Republic of China, Poland, Romania, South Africa, Sweden, Soviet Union, and the United States. Unless the United States invests efforts in developing innovative processes and new materials in this area, it faces the prospect of the field of superhard materials and their applications following the unfortunate precedents of the steel and semiconductor device industries. This would further exacerbate the nation's negative trade balance and create yet another import-dependent critical industry. . , Many applications of superhard materials require well-controlled properties. For mechanical applications this necessitates control of hardness, surface roughness, and adhesion to substrates. In thermal applications, control of thermal conductivity is required. Electronic and optoelectronic applications require control of absorption coefficient, carrier concentration, carrier 91
92 lifetime, and defect-state densities. Research is needed first to control the deposition process to tailor these properties and to scale-up the processes, then to understand how the processing parameters affect the material structure and properties, and finally to formulate measurement methods for characterizing the properties of the material for specific applications. Accordingly, this chapter focuses on key research and development issues identified with growth of superhard materials, their characterization, and their application. Although the emphasis in this report is confined to the low-pressure, lower-temperature (LPLT) processes now being developed, the established high-pressure, high-temperature (HPHT) process should now be examined in the light of emerging thin-film processes to see if knowledge of LPLT processes can be applied to the HPHT process. GROWTH PROCESSES The critical R&D issues associated with growth processes, particularly thin film growth, are discussed in this section within two broad categories--chemica1 vapor deposition (CVD) and physical vapor deposition (PVD). For CVD, the chemical aspects of the overall process dominate whereas in PVD, several physical aspects of the process need to be considered. CVO Growth For the case of vapor-deposited diamond films, control of the deposition process requires understanding the chemistry of the diamond formation process. This in turn requires studies directed toward! (a) understanding the nucleation and growth mechanisms as a function of substrate material and substrate surface orientation, (b) knowing the atomic and molecular species present in the gas phase, (c) finding means for doping in a well-controlled manner, (~) understanding how deposition parameters affect film morphology during home- and heteroepitaxial deposition, and (e) determining the chemical and structural perfection (i.e., impurity and dopant distribution and degree of twinning). Understanding these epitaxial issues may lead to the growth of high-quality large area single-crystal films. Other research and development issues include the followings . . ~ · The surface properties of films as they relate to bonding and adhesion to other materials need to be determined. Currently, diamond adheres to diamond or carbides. This work could lead to additional understanding and control of film morphology as well as increasing film adherence to other substrates ant! to growth of the larger-area heteroepitaxial single-crystal films important for electron devices. Grading the layers to arrive at the desired surface or layer so as to minimize strain resulting from substrate and thermal mismatch is another issue within the context of this item. · Chemical purity and obtaining dopant homogeneity of diamond and diamond-like films as a function of growth technique, growth parameters, and doping need to be established and understood. Doping by ion implantation has not resulted in semiconducting films but might be successful for oriented single-crystal films. · Methods of lowering growth temperatures must be determined. For example, low- temperature deposition of ZrB2 and HfB2 films by thermal decomposition of metal borohydrides (MEBH414) and epitaxial growth of ZrN on silicon has recently been achieved.
93 · What are the factors that limit growth rates? Innovative ways of increasing the efficiency of deposition (grams of deposited material per watt-hour) and of coating three- dimensional objects (throwing power) must be sought. PVD Growth The PVD R&D issues such as deposition rate, throwing power, substrate thermal mismatch, deposition efficiency, and gas-phase chemistry studies related to epitaxy and formation of large-area single-crystal films and layered composites are similar to those discussed in the previous section. For PVD growth, the local or substrate-deposit interface temperature generally, should not exceed 500°C. This will require control of vapor chemistry and particle kinetic energies. MATERIALS CHARACTERIZATION There are voids in knowledge of the dielectric, mechanical (including hardness and friction), thermal, and optical properties of diamond and diamond-like films as a function of hydrogen content and alloy additions. In addition, since carbon can form double and triple bonds, it is important to clarify the nature of the bonding and structural network of the amorphous diamond-like hydrogenated films. Crystal or material features on an atomic level, such as atomic location of hydrogen and impurity atoms, crystal defects (stacking faults, etc.), and electron energy levels for impurity atoms need to be characterized and understood. Furthermore, since Raman spectra are frequently obtained for these materials, much work needs to be done to understand features of the spectra. ~ - Surface reconstruction or rearrangement of surface atoms is well known to occur for Spa bonded materials (silicon, for example), and (100~-~110) surface characterization is a research issue that should be examined so as to understand surface properties that are orientation- and composition- (hydrogen, for example) dependent. Determination of electron affinity of diamond and diamond-like materials is a property of Importance from the standpoint of their possible use as cold cathode materials. APPLICATIONS Diamond and diamond-like materials are already being developed for several electronic applications--e."., as films for masks for fabrication of 1.3-,~m distributed feedback lasers (Gozdz et al., 1988) and as coatings for magnetic disks and heads (Seki et al., 1987; Tsai and Bogy, 1987~. - Smooth, defect-free single-crystal diamond films could find application as transistors for high-temperature, high-frequency applications, just as SiC is now being developed for remote elevated-temperature sensor applications. Issues associated with this application include obtaining dopant uniformity and epitaxial high-temperature-stable metal contacts. Perhaps the nitrides of the transition elements, such as ZrN, now being explored as a metallization for silicon and other metallic nitrides, such as TiN, VN, HfN, and TiCN, are possibilities. The possibility of making high-energy lasers, UV detectors, arid optoelectronic devices from diamond needs to be explored. Diamond is an indirect-gap material, and for efficient laser
94 operation direct electronic transitions are necessary. Perhaps by suitable alloying to form donor and acceptor bands within the gap or by forming a strained epitaxial layer that becomes a direct gap material, efficient lasers, very likely optically pumped, might be developed. Perhaps diamond can be used to fabricate photoconducting, PIN, and/or avalanche detectors. Since the conduction band of diamond is above the vacuum level, it may be possible to form high-current cold cathodes with diamond. The use of such cathodes could be very important for future high-frequency development, not only for traveling-wave tubes but also for micrometer vacuum tubes, which could surpass the present state of the art of semiconductor devices even with the addition of diamond devices. Increasing attention is being directed toward the use of diamond and diamond-like coatings as a packaging material (to minimize wear and abrasion and to protect against corrosion) for electronic items (magnetic disks and heads) and as heat sinks for semiconductor lasers used in optical communications. Application of films surely will increase as one learns how to optimize coating thickness for a given substrate (as well as developing nondiamond substrates), how to dope or alloy these films with nonionized impurities to increase microfracture toughness, and how to scale-up the deposition process. The use of diamond and diamond-like coatings for creating an impervious layer on optical fiber is another possible application. Other diverse applications of diamond films are as x-ray lithography masks for manufacture of ultra-high-density integrated circuit chips and in prosthetic devices. BEYOND DIAMOND: OTHER MATERIALS AND ISSUES There should be increased activity concerned with the synthesis of diamond-like and nondiamond-like materials in bulk and thin-film form and with the control of microstructures, such as in composites and in modulated structures for enhanced properties. Emphasis should be placed on spectroscopic studies directed toward understanding the chemistry and physics of plasmas used for plasma-enhanced deposition of films. This information is important for scaling-up plasma deposition processes. The superlattice TiC-TiCN-TiN is now in commercial use (Quinto, 1988) for cutting tools, while the superiattice TiN-VN is receiving attention (Barrett et al., 1988~. Thin-film ternary nitrides (Randhawa et al., 1988; Penttinen et al., 1988) such as (TiZr)N containing TiN and ZrN in the ratio of 25:75 exhibit a microhardness in excess of 3100 kg/mm2 and excellent wear resistance. More work on the synthesis of cubic BN must be undertaken. It has been suggested that the electronic energy gap of this material is greater than that of diamond. The nature of the bonding and arrangement of atoms in a solid determines its mechanical properties, and quantum-chemical calculations should be undertaken to shed light, in parallel with experiment, on the bonding in superhard materials. Theoretical results suggest that C-N bonds within an appropriate host crystal structure might yield harder materials than pure diamond. Boron-oxygen and boron-carbon-nitrogen materials need to be synthesized.
95 REFERENCES Barnett, S. A., L. Hultman, I. E. Sundgren, F. Ronin, and S. Tahoe. 1988. Epitaxial growth of ZrN on Si(100~. Appl. Phys. Lett., Vol. 53, No. 5, p. 400. Gozdz, A. S., P. S. D. Lin, A. Scherer, and S. F. Lee. 1988. Fast direct e-beam lithographic fabrication of first-order gratings for 1.3~m DFB lasers. Electron. Lett., Vol. 24, p. 123. Penttinen, I., I. M. Molarius, A. S. Korhonen, and R. Lappalainen. 1988. Structure and composition of ZrN and (Ti,Al)N coatings. I. Vac. Sci. Technology, Vol. A6, No. 3 (May/]une), p. 2158. Quinto, D. T. 1988. Mechanical properties and structure relationships in hard coatings for cutting tools. I. Vac. Sci. Technology, Vol. A6, No. 3 (May/June), p. 2149. Randhawa, H., P. C. Johnson, and R. Cunninghan. 1988. Deposition and characterization of ternary nitrides. I. Vac. Sci. Technology, Vol. A6, No. 3 (May/3une), p. 2136. Seki, H., G. M. MCClelland, and D. C. Bullock. 1987. Raman spectroscopy of disk coatings in a working magnetic drive. Wear, Vol. 116, p. 381. Tsa~, H., and D. B. Bogy. 1987. Characterization of diamond-like carbon films and their application as overcoats on thin-film media for magnetic recording. ]. Vac. Sci. Tech. Vol. A5, No. 6, p. 3287.