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Diamond—Ceramic Coating of the Future

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Diamond—Ceramic Coating of the Future J Am. Cerum. SOC., 72 121 171-91 (1989) journals-a- P'mcxy--d*=<*c. M"."--~ Diamond-Ceramic Coating of the Future Karl E. Spear* Ceramic Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 1. Introduction The high-pressure high-temperature THE unique properties of diamond are (HPHT) synthetic diamonds developed yet to be tapped for a large number of by General Electric in the 1950s',' are advanced materials applications. Fas- now commonplace in cutting, grind- cination with this king of all gemstones ing, and polishing, but many potential has turned into excitement recently applications of diamond require thin with the development of techniques for films or coatings which cannot be pro- creating crystalline diamond films and duced from either natural or HPHT coatings using low-pressure gases synthetic diamonds. rather than the high pressures and The diamond coating process which temperatures previously considered has generated the recent excitement essential. These developments herald utilizes temperature and pressure a new era in diamond technology and conditions under which graphite is offer the potential for exploiting the clearly the stable form of carbon. How- unique properties of diamond in appli- ever, kinetic factors allow crystalline cations ranging from coatings for diamond to be produced by the net bearings and cutting tools to free- chemical reaction Karl E. Spear is the chair of the Ceramic standing windows and lens coatings Science and Engineering Program in the for visible and infrared (IR) transmis- CH4(g) -C(diamond) + 2H,(g) Department of Materials Science and En- sion, to thin films for high-temperature, gineering at The Pennsylvania State high-power semiconductor devices. In addition to methane, a wide variety of carbon-containing reactant gases University. Dr. Spear earned a B.S. in Applications requiring advanced mathematics from Baker University in materials can uniquely utilize diamond can be used. The typical process con- sists of a reactant gas at less than at- 1961 and a Ph.D. in chemistry from the because it (i) is the hardest known University of Kansas in 1967. Before join- material; (ii) has the highest room- mospheric pressure and containing >95% hydrogen which is activated by ing the Penn State faculty in 1970, he was temperature thermal conductivity of at Oak Ridge National Laboratory. During any material; (iii) is resistant to heat, passing it through a plasma or past an =2OOO"C filament before contact- 1978-79, Dr. Spear was on sabbatical acids, and radiation; (iv) is a good leave at the United Kingdom Atomic Ener- electrical insulator, but can be doped ing an 800" to 1000°C substrate on which the diamond is deposited. Many gy Research Establishment at Harwell and to produce either p-type or n-type at Oxford University. He has broad re- semiconductors; (v) has a small questions must be answered concern- ing this deceivingly simple-looking search interests in the high-temperature dielectric constant; (vi) has a large thermochemistry and phase behavior of hole mobility; and (vii) is transparent "metastable" process before the po- tential of the new coating technology ceramic and metal systems. Dr. Spear is to visible and IR radiation. (See a member of the American Ceramic So- Appendix for more details on prop- can be achieved. Our understanding of B ciety Basic Science and Nuclear Divisions, erties and applications of diamond.) the basic science must be extended far beyond our present knowledge, a is the secretary-treasurer of the Ceramic challenge currently being met by labo- Educational Council, and is chair of the ratories around the world. Advisory Committee for the Phase The purpose of this paper is to pre- Equilibria Data Center. Supported in part by the U.S. Office of Naval sent a review of diamond-coating sci- Research under Contract Nos. N00014-86-K- ence and technology and to summarize 0283 and N00014-86-K-0443. the properties of diamond and a num- [Key words: diamond, coatings, graphite, ber of applications which can uniquely plasma, chemical vapor deposition.] 'Member, American Ceramic Society utilize these properties. 171 I72 Journal of the American Ceramic Society-Spel; ir Vol. 72, No. 2 II. Historical Perspective on marily in the 1950s in the Soviet Union Vapor-Deposited Diamond and the United States. The major Japanese effort began in the 1970s. A thorough review of previous inves- The first two decades of research on tigations on the "Synthesis of Diamond vapor-deposited diamond in the Soviet Under Metastable Conditions," includ- Union was summarized in a 1975 ar- ing the history of vapor-deposited dia- ticle in Scientific American titled "The mond, was recently written by DeVrie~.~ Synthesis of Diamond at Low Pres- This review and a paper by Badzian sures.'" The reported diamond growth et aL4 also include patent literature on rates were low (angstroms per hour) this topic. An excellent review of "Car- and the simultaneous codeposition of bon Thin Films," which includes non- graphitic carbon was always a prob- crystalline and microcrystalline hard lem. The early research primarily in- carbon films, was jointly written by volved the thermal decomposition of Angus, Koidl, and D~mitz,~and an- hydrocarbons and hydrogen-hydrocar- other very recent review was written bon gas mixtures, with no additional by Angus and Hayman.6 This section activation of the gas. presents only a brief historical per- Similar research was also being spective on the early work on the va- conducted in the United States during por synthesis of crystalline diamond this early time period. In 1958, Ever- sole' filed for a patent on a low-pres- sure vapor synthesis process, but again the growth rates were very low and graphitic carbon was deposited simultaneously. The synthesis process required many cycles of growth fol- lowed by hydrogen etching to remove excessive graphitic deposits. Angus -SILICA TUBE and co-workersg-" continued to pur- FLOW CONTROL - sue these techniques during the 1960s SYSTEM and early 1970s and they obtained re- sults similar to those of Eversole. k-MICROWAVE As in the early 19OOs, most of the scientific community viewed the re- sults of the 1950s through the mid- 1970s with great skepJicism, more of a curiosity than as a technological break-through. However, these results began laying the groundwork for our (2.45 GHz) I scientific understanding of vapor pro- cesses leading to diamond growth. Soon after the Scientific American article,' Deryagin's group reported the use of gas activation techniques which tiI SUBSTRATE resulted in dramatic increases in dia- mond growth rates while eliminating much of the graphite codep~sition.'~-'~ Starting in the early 1980s, Japanese researchers began reporting dramatic SSURE GAGE successes in low-pressure diamond growth using a variety of new gas acti- vation techniques (see Appendix C). MPS A major increase in activity around the world on both the science and the technology of vapor-deposited dia- mond is quite apparent today. This ac- Fig. 1. Schematic diagram of microwave- without attempting to dupllcate these tivity is a result of a combination of plasma-assisted chemical vapor deposition above comprehensive literature re- demands from product designers for (MPACVD) diamond growth system views Indications of the current com- new super materials, decades of un- mercial interest in this process are heralded research on low-pressure also mentioned diamond growth, and the break- The production of synthetic dia- throughs of the 1970s which dramati- monds from low-pressure gases was cally increased diamond growth rates first reported in 1911 by von Bolton while decreasing the codeposition of (from Ref 3) He claimed to have graphite. A stream of popular reports achieved growth on diamond seed on diamond coatingsi6-" is a reflec- crystals from illuminating gas (acety- tion of the strong commercial interest. lene) decomposition at 100°C in the Industries in the United States with presence of mercury vapor However, activities in low-pressure diamond little attention was given to these growth include large companies, such claims Systematic studies of diamond as Air Products, Alcoa, Amp, Arm- vapor deposition techniques began pri- strong, DuPont, Exxon, Ford, General February I989 Diamond-Ceramic Coating of the Future 173 Electric, General Motors, GTE, Her- talline diamond. Of all these characteri- cules, IBM, Kennametal, Martin Mari- zation techniques, the Raman spec- etta, Philips, PPG, Raytheon, Sandvik, trum is the most sensitive for identifying Texas Instruments, and Westinghouse, the presence of crystalline diamond in as well as startup companies, such as mixtures of various forms of carbon. Crystallume (Palo Alto, CA) and Dia- The Raman scattering efficiency for mond Materials Institute (DMI) (State the sp2-bonded graphite is more than College, PA). The interest in Japan has 50 times greater than that for the swelled over the past decade to well sp3-bonded diamond; therefore, small over 40 industrial firms that are ac- amounts of sp2-bonded carbon in tively involved in this area. They in- the diamond deposits can be readily clude Fujitsu, Hitachi, Kobe Steel, detected. Fig. 2. Schematic diagram of heated- Kyocera, Matsushita Electric, Mit- Figure 3 shows three typical Raman filament-assisted chemical vapor deposi- subishi, NEC, Seiko, Showa Denka, spectra for diamond films deposited tion (HFCVD) diamond growth system. Sumitomo Electric, and Toshiba. More than 500 patents have been filed in 7 GAS INLET Japan related to diamond-coating processes." On September 14, 1986, the front page of the Sunday New York Times announced a "New Era of Technology Seen in Diamond Coating Pro~ess."'~ Current activities strongly support the optimism of this headline. SiOp Ill. Growth and Characterization CHAMBER - of Crystalline Diamond In the vapor deposition growth of crystalline diamond, the two most common methods for achieving the re- GAS quired activation of the precursor DIFFUSER gases are the use of a microwave TUNGSTEN plasma and an =2O0O0C heated fila- FILAMENT ment located about 1 cm above the SUBSTRATE growth surface.
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