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. Schematic diagrams SUBSTRATE of such systems are shown in Figs. 1 HEATER and 2. Typical ranges of growth condi- tions are shown in Table I. Most of the experiments which led to the condi- THERMOCOUPLE- tions in Table I were performed with methane as a hydrocarbon source, but other hydrocarbon sources have yielded similar results. Typical growth rates range from 0.1 to 10 pm/h. The crystalline diamond coatings grown from low-pressure gases are different from the "diamond-like car- bon" (DLC) films first reported by Aisenberg and Chab~t.'~The term DLC is often used to indicate a variety of noncrystalline carbon materials, ranging from amorphous to microcrys- talline and typically containing from a few to about 50 at.% hydrogen. An ex- cellent review of the extensive litera- ture on noncrystalline carbon coatings TO PRESSURE, GAGE has been given by Angus, Koidl, and --+ TO PUMP Domitz.' A working definition of "crystalline Table I. Growth Conditions of Vapor-Deposited Diamonds diamond material" was recently sug- Typical Best gested by Messier et a/.'5 in a paper conditions crystals discussing the differences between crystalline diamond and DLC coat- Substrate ings. The present review concerns temperature ("C) 700-1 000 950-1 050 crystalline diamond produced by va- Total pressure (kPa) 0.013-12 5.3-1 2 por deposition techniques and defined CH4 (in H2) (mol%) 0.1-5.0 0.1-1 .o by the following properties: (i) a crys- Total flow (sccm)* 20-200 50-1 00 talline morphology visually discernible Filament by microscopy, (ii) a single-phase temperature ("C) 1800-2500 2000-2500 crystalline structure identifiable by X-ray and/or electron diffraction, and "Standard cubic centimeter per minute. (iii) a Raman spectrum typical for crys- 174 Journal of the American Ceramic Society-Spear Vol. 12, No. 2 on silicon substrates. The peak at in fact, a large percentage of the 519 wave number (cm-') is from the film may be noncrystalline carbon that silicon substrate, whereas the peak at codeposited with the highly crystal- 1332 +I wave number is characteris- line grains. tic of diamond. The broad peaks oc- Hydrogen impurity contents from a curring in the 1500 to 1600 wave few percent to greater than 30 at.% are number range are characteristic of common in the vapor-deposited DLC disordered sp'-type carbons in the films. These films are often grown from deposit. The Raman spectra labeled pure hydrocarbons in low-pressure (A) is a typical pattern for a highly per- plasma systems. The crystalline dia- fect diamond film. The pattern labeled mond films, on the other hand, are (C) is typical for an imperfect film of typically grown from hydrogen-1 mol% crystalline diamond with appreciable methane mixtures and usually contain amounts of sp2 carbon present as only small amounts of hydrogen im- defects or in a separate phase. The purity. Table II lists the average hydro- pattern labeled (9) is for a film of inter- gen contentz8of four films grown at mediate perfection. Noncrystalline car- The Pennsylvania State University in bon such as DLC would not show the the microwave-plasma-assisted sys- 1332 wave number peak. tem shown schematically in Fig. 1. Knight and Whitez6 have written a Only the film grown from a gas mixture comprehensive paper and review on containing a large amount of methane the Raman spectroscopy of vapor- (5 mol%) exhibited an appreciable deposited diamond films and of other amount of hydrogen (2500 ppm, forms of carbons. In addition to the atomic). In general, the experimental various vapor-deposited films, they information indicates that greater have included spectra for natural dia- amounts of hydrogen in the feed gas mond, a General Electric HPHT dia- lead to less hydrogen impurity in the mond, lonsdaleite, carbonado (natural diamond deposit. polycrystalline diamond), three forms Discussions later in this paper on of graphite (polycrystalline, highly ori- mechanisms of growth include hy- ented pyrolytic, and a natural graphite potheses on the major roles of atomic crystal), glassy carbon, diamond-like hydrogen in the precursor gas. One carbon, coke, and charcoal. Both the role of the atomic species is to ab- peak position and the width provide stract hydrogen from C-H bonds on information on the type of carbon the growing surface of the diamond deposited. deposits. An argument can be devel- Tsai and Bogyz7have written a de- oped that hydrogen bonded to the WAVENUMBERS (cm ') tailed review on the characterization carbons on the growing surface can- of diamond-like carbon films, and they not be removed efficiently if the con- Fig. 3. Raman spectra for three typical include a section on Raman spectros- centration of atomic hydrogen in the diamond samples vapor deposited on single- copy. They also include other char- system is too small. crystal silicon: (A) highly perfect diamond acterization methods and property The diamond films that are currently film, (B) film of intermediate perfection, and being grown by vapor deposition (C) film containing appreciable amounts of measurements that can be used for sp2 carbon. Note the intensity scale characterizing DLC thin films. evolve from small crystals, like those change in each of the three spectra. The X-ray and/or electron diffraction can shown in the scanning electron micro- 519 cm-' line is from the silicon substrate, be used to identify the crystalline car- scope images in Fig. 4. When these the 1332 cm-' line is characteristic of dia- bon phases of graphite, diamond, crystallites grow together, dense dia- mond, the broad peak in the 1500 to 1600 lonsdaleite, and the various carbynes, mond films such as the one shown in cm-' range is characteristic of sp'-type but not coexisting amorphous carbon Fig. 5 can be prepared. The nucleation disordered carbon. phases, even when they dominate the of diamond crystallites on substrate deposits. A sharp diffraction pattern surfaces is not well understood. It is may sometimes be interpreted as known that when the surface of the "proof" that a high-quality crystalline substrate is scratched with submicrom- diamond film has been grown, when eter diamond paste, nucleation occurs along the scratch. A high density of scratches helps to insure a high nucle- Table II. Hydrogen Content and Growth Parameters for Four Diamond Films* ation density. This is illustrated in Fig. 6, Sample which shows the deposition of diamond ParameterbroDertv DD12 DDl7 DD18 #66 crystallites on a portion of a single- ~ ~ crystal silicon wafer. Yugo et a/.*' and Substrate p-Sic p-Sic p-Sic Si Chang et a/.30 have reported the ef- T ("C) 975 976 975 990 fects of various substrate treatments P (kPa (torr)) 10 (80) 10 (80) 10 (80) 10 (80) on the diamond nucleation density. Thickness (pm) 3.9 14.4 18.0 6.5 The energy and mechanistic differ- Growth rate (pm/h) 0.19 0.74 1.6 0.72 ences between growing "perfect" and CH4 (in H2) (Yo) 0.37 1.78 5.0 1.o defective crystals is apparently quite Average H-impu ri t y small. As a result, twinned and defec- tive diamond crystals are often de- (ppm atomic) 630 350 2500 435 posited, as is shown in the examples 'Grown by microwave-plasma-assisted vapor deposition at The Pennsylvania State University, in Fig. 7. Other examples can be found University Park. PA 'Analysis performed by W Lanford.z8 among the crystals shown previously in Fig. 4. February 1989 Diamond-Ceramic Coating of the Future 175

IV. Chemistry of the Chemical oxygen (as in an oxygen-acetylene Vapor Deposition Diamond flame). Growth Processes (vi) Plasma species. A number of ongoing spectroscopic studies of acti- A generally accepted understand- vated gas environments under the ing of chemical mechanisms which conditions of diamond film growth indi- lead to low-pressure growth of either cate the predominance of acetylene diamond-like carbon or crystalline and methyl-radical growth species. diamond is still lacking, but kinetic (vii) Formation of graphite usually data on the processes have been re- accompanies the growth of diamond. ported. Russian scientists Deryagin, A major problem in growing high-qual- Fedoseev, Spitsyn, and their co- ity diamond films, including single- wOrkers7.t2-15.31-33 developed an exten- crystal films, is the codeposition of sive experimental base of chemical graphitic carbon. and kinetic information and proposed (viii) Effect of temperature. The tem- global kinetic theories for diamond perature dependence of the rate of growth based on nucleation theory, diamond growth exhibits a maximum; Langmuir adsorption-desorption ki- that is, it initially increases with tem- netics, and equilibrium. Reviews in perature and then decreases. This be- English by Fedoseev et a/.'5 and re- havior is a manifestation of the cently by Badzian and DeVrie~~~give competition between the growth of excellent summaries of this Russian diamond and graphite. work. Chauhan, Angus, and Gardner" (ix) Surface treatment. Various reported detailed kinetic data on dia- substrate surface treatments such as mond deposition from thermally scratching, seeding, and etching af- activated methane and methane- fect primarily the induction time hydrogen mixtures and included pre- (nucleation rate), but not the rate of liminary results from the use of ethylene subsequent growth.36 as a hydrocarbon source. Some specu- (x) Crystallite morphology. {lll} lations on nucleation and growth and (100) surfaces dominate. Cubo- Fig. 4. Scanning electron microscope mechanisms were also given. octahedral crystals composed of both images of microwave-plasma vapor- (7) Summary of Experimental of these surfaces are common. Twinning deposited diamond crystals. Observations frequently occurs on {11 I} surfaces. Among a continuous "flood" of pa- Although mechanistic reasons which pers on low-pressure chemical vapor favor the growth of crystalline diamond deposition (CVD) growth of diamond rather than graphite in low-pressure films, the following facts appear to be CVD processes are not well established, prominent:35 proposed mechanisms must be con- (i) Activation of the gas is required. sistent with these ten reported experi- Some form of gas activation is abso- mental observations. lutely necessary for achieving appre- (2) Role of Atomic Hydrogen ciable CVD diamond growth rates. A major breakthrough in developing (ii) Independence on the method both the science and the technology of activation. Good quality diamond of low-pressure diamond growth oc- films have been produced under a va- curred when the Russian scientists riety of different methods: microwave-, experimentally determined the impor- rf-, UV-, laser-, and hot-filament- tance of atomic hydrogen for enhanc- activated gas mixtures. The method of ing the rates of diamond growth and activation influences the ease with reducing or eliminating codeposition which diamond is grown; however, an of graphite. They found that the addi- explanation for this influence has not tion of excess hydrogen to the hy- been established. drocarbon precursor gas led to less (iii) Independence of starting ma- graphite codeposition, just as Chauhan terial. The chemical nature of the hydro- et a/." had determined, but the Rus- carbon precursor does not appear to sian scientists also discovered that be critical. Crystalline diamond has "activating" the gas prior to deposition been grown using aliphatic and aro- increased the diamond growth rates matic hydrocarbons as well as alco- from A/h to prn/h. Two mechanisms hols and ketones. were used to activate the gas: an elec- (iv) Hydrogen is required for effi- tric discharge in the system and a hot cient growth. The Russians proposed tungsten filament over which the gas that it is atomic hydrogen that has to flows before encountering the lower- be present in the gas phase in super- temperature deposition region. equilibrium concentration. Activated Deryagin and Fedoseev'2 proposed Fig. 5. Scanning electron microscope im- hydrogen etches graphite at a much that a superequilibrium concentration ages of the surface and a cross section of higher rate than it does diamond. of atomic hydrogen at the growth sur- a diamond film vapor-deposited on a sili- (v) Effect of oxygen. Small amounts face is responsible for the major re- con substrate. of O2 added to the precursor gas ac- duction in graphite codeposition. They celerate the growth rate of diamond argued that atomic hydrogen behaves films. Diamond can also be grown like a "solvent" for graphite. Their stud- from hydrogen-hydrocarbon gas mix- ies of the relative etching rates of dia- tures which contain larger amounts of mond and graphite showed that the 176 Journal of the American Ceramic Society-Spear Vol. 12. No. 2

removal of graphite by activated hy- porting this is the observation that drogen was orders of magnitude approximately the same growth condi- faster than that for diamond. As a part tions (temperature, pressure, concen- of their diamond growth studies, An- trations of precursors) are needed for gus et al.' had previously determined crystalline diamond growth, regard- that molecular hydrogen would ther- less of the method of activation. The mally etch graphite at a rate almost method of activation influences the 500 times faster than it would etch rate of diamond growth, but not the diamond, but this graphite removal general structure of the deposited process was performed in a separate crystallites. This also supports the cycle from the diamond growth conclusion that the same general process. growth species are produced by all Setaka3' has recently reported etch- activation methods that produce crys- ing rates of graphite, glassy carbon, talline diamond. and diamond in a hydrogen plasma (4) Diagnostics of Activated Gas under typical activated growth condi- tions for diamond to be 0.13, 0.11, and Recent in situ measurements of 0.006 mg/(cm'. h), respectively. Saito species concentrations in activated et a/.38also showed much greater methane-hydrogen gas mixtures etching rates for graphite than for dia- above diamond growth surfaces have mond when subjecting the materials to been made. Although a number of microwave plasmas of either hydrogen species have been detected, indica- or hydrogen-I .6 mol% water mixtures. tions are that the primary species are Oda et a/.39studied the role of H methyl radicals and acetylene. Such atoms in the deposition of amorphous diagnostic measurements are needed silicon and related alloy phases. The as a function of growth conditions and H atoms generated by a microwave activation method, and the results then plasma remote from the deposition re- need to be correlated with growth gion enhanced both the growth rates rates, crystallite morphology, and the and quality of the films. They noted relative amounts of codeposited dia- that impurities such as C and 0 were mond and non-diamond carbon. efficiently removed by active H-atom These data will be a critical portion of reactions and that dangling bonds on the basic information needed for build- the surface of epitaxial films were ef- ing a mechanistic understanding of fectively passivated by H atoms. Both the vapor deposition of crystalline dia- of the observations in this study help mond films. to support hypotheses on the role of Matsumoto et a/.,44 45 Saito et a/.,46 H atoms in diamond deposition. Mitsuda et a/.,47 and Hartnett4' have all performed emission spectroscopic (3) Role of Hydrocarbon Precursor analyses of microwave-plasma- Another experimental observation of activated hydrogen-hydrocarbon gas the Russian scientists was that the na- mixtures above substrate growth sur- ture of the precursor hydrocarbon gas faces. Harnett has reviewed these pre- had little effect on the deposition be- vious studies in his thesis. Only a havior. Sato et a/.40have grown dia- limited number of species of interest mond from gaseous mixtures of various are detected by this technique: H2, atomic H, Cp, and CH. Fig. 6. Scanning electron microscope im- hydrocarbons and hydrogen by ages showing the effect on the nucleation plasma-assisted deposition and found Matsum~to~~examined a hydrogen of vapor-deposited diamond of scratching similar results. Both saturated and un- plasma with no hydrocarbon in the a silicon substrate with submicrometer dia- saturated hydrocarbons were used, gas and no substrate. As expected, mond grit. The diamond films are a result of and similar growth features were only the molecular and atomic hydro- the growing together of the crystallites. noted for all the hydrocarbons when gen emissions were observed. Upon comparisons were made as a function placing a graphite substrate in the hy- of the C/H ratio in the input gas. The drogen plasma, CH, C,, and H were density of nucleation and the growth all observed. Mass spectroscopy rates were found to be essentially the measurements of the plasma showed same as those observed with the more C2H2to be the main reaction product commonly used methane in previous of the chemical etching of the carbon. studies. Attempts have been made by Mit- This relative independence of dia- suda et a/.47to correlate the emission mond growth on the nature of the input spectroscopy intensity ratio of CH-H hydrocarbon species is consistent radicals to diamond formation and C2 with the fact that most hydrocarbon radical concentrations to graphite for- sources tend to chemically transform mation. However, other species, such to common product species (such as as CH3 radicals and CZH2,which can- acetylene, one of the most stable of not be detected by the emission spec- such gaseous products) under harsh troscopy techniques, are probably the environments such as those found in main precursors to diamond growth. high-temperature pyr~lysis,~'combus- Therefore, correlations between CH ti or^,^' plasmas,43and the other typical and C, emission spectroscopy inten- methods used for activating precursor sities and the relative concentrations gases in diamond deposition. Sup- of CH3 and C2H2in the activated gas February 1989 Diamond-Ceramic Coating of the Future 177 are needed. grow crystalline diam~nd.~~-~'Experi- In addition to these emission spec- mental detail is currently quite sparse troscopy measurements, two studies on these latter studies. have been reported on other spectro- Kawato and K~ndo~~examined gas scopic measurements of hydrocarbon mixtures of CH,-H, and CH4-H2-02 species present in typical gas mix- in growing CVD diamond by the heat- tures activated in filament-assisted ed-filament activation technique. They diamond growth systems. Both sets of examined the exhaust gas by gas results indicated acetylene and methyl chromatography and found the major radicals are the most abundant vapor content of this gas to be methane species near the growth surface. (CH,), ethylene (C,H4), acetylene IR diode laser absorption spec- (C,H,), hydrogen (H,), and carbon troscopy was employed as an in situ monoxide (CO). Although the relative method to examine gas-phase species concentrations of these species were present during filament-assisted de- not given, their analyses concentrated position of diamond films by Celii et a/.49 on the CH, and C2H2species. The data From a reactant gas mixture of 0.5 mol% show that the additions of 0.4 mol% 0, methane in hydrogen, acetylene (C,H,), to hydrogen containing from 1.6 mol% methyl radical (CH,), and ethylene to 4.0 mol% CH, cause the growth (CZH4) were detected above the rate of diamond to increase over the growing surface, while ethane (C2H6), oxygen-free system, particularly at various C3H, hydrocarbons, and higher CH4 percentages. The addition methylene (CH,) radical were below of oxygen also decreased the quantity their sensitivity levels. These authors of non-diamond carbon as determined noted that their findings were consis- by the decrease in the -1550 cm-' tent with the Frenklach-Spear mecha- Raman peak. At the same time, the nistic growth model5' for propagating addition of oxygen caused a larger (111) planes through the addition of decrease in the CPHPthan in the CH4 acetylene to activated surface sites. concentration in the exhaust gas. The Mass spectral data were obtained authors reached the following con- by Harris et a/.5' in a hot-filament- clusions from these results: "(1) The assisted diamond growth system as a addition of oxygen reduces the concen- function of filament to substrate dis- tration of acetylene, which is probably tance. They used sampling techniques produced by the pyrolysis of methane which were shown to be effective or hydrogenation of non-diamond car- in sampling flame^.^' The 2.6-kPa bon. (2) The deposition of graphitic or (20-torr) input gas in their diamond amorphous carbon is suppressed by a growth system was hydrogen contain- reduction of the acetylene concentra- ing about 0.3 mol% methane and tion or the oxidation of non-diamond 0.1 mol% Ne for calibration purposes. carbon, so that the quality of the de- The tungsten carbide filament tem- posited diamond is improved. (3) With perature in their system was estimated the addition of oxygen, the growth rate to be 2600 K. They stated that their re- of diamond increases, and the total sults were consistent with those of pressure for diamond synthesis can Celii et a/.49In addition to the mass possibly be extended." spectral sampling of the gas, they also Saito et a/.38 used water-vapor addi- performed detailed chemical kinetic tions to their CH4-H2 precursor gases calculations using a model developed in their microwave-plasma-assisted previously in studies of Their synthesis of crystalline diamond. The initial analysis suggested that dia- growth rates of 1 to 5 pm/h repre- mond growth came mainly from acety- sented an increase of several times lene and/or methyl-radical precursors, those obtained without the water addi- but that contributions from methane tions. Concentrations of input gases and ethylene could not be ruled out. varied from 1 mol% to 10 mol% for CH, and 0 mol% to 7 mol% for HzO, (5) Effects of Oxygen on the Diamond with the rest of the gas being hydro- Deposition Process gen. Small additions of water to the Hirose and Terasawa5, reported on input gases containing 3 mol% and the growth of crystalline diamond films 10 mol% CH, caused large increases from a number of organic compounds, in growth rate, but a maximum in several of which contained oxygen. growth rate was achieved when the However, they did not discuss the role H20/CH4ratio reached between one- of oxygen in the synthesis process. A third and one-half. The additions of number of investigators have since water also resulted in greater crys- studied the influence of oxygen addi- tallinity in the films, and less code- tions in various forms on the growth of posited non-diamond carbon. The diamond: oxygen additions by Kawato authors also examined the relative and K~ndo~~and Chang et a/.,30water etching rates of diamond and graph- additions by Saito et a/.,38and CO and ite in hydrogen and in hydrogen- oxygen additions by Mucha et a/.56 1.6 mol% water mixtures under the More recently, lo5 Pa (1 atm) oxygen- same plasma conditions as used in

acetylene flames have been used to arowth.~ Diamond showed little etching- 178 Journal of the American Ceramic Society-Spear Vol. 72, No. 2

for both mixtures, but the etching Haubner and Lux6' observed mor- rate of graphite was about twice as phological results that are not consis- great when the water was added to tent with the above observations. They the hydrogen. report that in their microwave-plasma- Zaitsev et a/.6ameasured the effect assisted deposition system, cubic of water vapor on the dissociation of crystals with (100) surfaces dominated hydrogen in a glow discharge system at low CH, concentrations (0.3%), low and found a maximum in the hydro- substrate temperatures (600"C), and gen-atom concentration as more and medium plasma intensity. At higher more water was added to the 34-Pa plasma intensities, but the same con- (0.26-torr) gas. The hydrogen-atom centration and substrate temperature, concentration increased by about a cubo-octahedra ((100) and (111) sur- factor of 5 when about 0.1% water was faces) were observed. At 750" to added, then decreased with higher 800°C and medium to high plasma in- percentages of water. The gas in this tensities, octahedral crystals with (1 11) system contained no hydrocarbons, surfaces dominated. Low plasma in- the pressure was lower, and the means tensities led to spherulitic crystallites of gas activation was different than at both lower and higher tempera- that in the diamond growth system tures. (Note: the authors (private com- used by Saito et but the fact that munication) estimate that the true Zaitsev et a/.found the hydrogen-atom substrate temperatures were about concentration to be enhanced by 200°C higher than the above uncor- water vapor in the activated gas is rected measured values they iist in consistent with the hypothesized im- their paper.) portant role of hydrogen atoms in the (7) Substrates for Deposition vapor growth of diamond. Diamond has been vapor deposited (6) Deposit Morphologies on a wide variety of substrate mate- The morphology of vapor-deposited rials, though the dominant substrate diamond crystallites is dominated by has been single-crystal silicon. Exam- cubic (100) and octahedral (111) sur- ples of substrate materials used are faces and (111) twin planes (stack- Si, Ta, Mo, W, SIC, WC, and dia- Fig. 7. Examples of highly twinned and de- ing faults of the {lll} planes). mond;62Cu, Au, Si, Mo,W, and dia- fective vapor-deposited diamond crystals. Cubo-octahedra exhibiting both (100) mond;I3 and diamond, graphite, Si, and {Ill) surfaces are common. Sic, SiO,, and Ni.67Studies on coating Matsumoto and Matsui" utilized various oxide, carbide, and nitride electron microscopy techniques to ex- cutting-tool materials are also under- amine the structures of diamond crystal- way around the world. The diamond lites grown in a hot-filament-assisted nucleation rates and adhesions vary CVD system. The {IOO} and (111) sur- with the tendency to form intermediate faces dominated the crystallites. carbide layers such as Sic or MoZC, The typical crystal habits were cubo- but detailed systematic studies are yet octahedra and singly and multiply to be reported. twinned particles. Examples of these common defective crystals can be V. Energetics of Gas-Solid Growth TEMPERATURE ('C) seen in Fig. 7. Interface Kobashi and ~o-workers~~-~~found The pressure versus temperature that (111) faces dominated their 4-kPa phase diagram for carbon given in 80- 80 (30-torr) microwave-plasma-assisted Fig. 8 clearly shows that graphite DIAMOND N, CVD crystals for substrate tempera- is the stable form of carbon under the

- 60- - tures of about 800°C and methane conditions used in vapor-depositing Fe %j 60 5 am concentrations of <0.4 mol%. When crystalline diamond. Why is it then 0 a - -x the CH4 concentration was between possible to grow diamond at less than w U 0.4% and 1.2 mol%, (100) surfaces atmospheric pressure in the tempera- Ll3 40- 40 II) dominated, but at higher concentra- ture range of 750" to 1100°C? w GRAPHITE w UR tions the de osits were structureless. Although an established mecha- R P 20- 20 Badzian' reports that, at tempera- nistic answer is not yet available for tures of 900°C and lower, (111) faces this question, one answer that is con-

VAPOR-DEPOSITED DIAMOND dominate the crystallite morphology, sistent with reported experimental i 0- 0 and, at 1000°C and higher, {loo} faces facts can be given."869The heart of b 1000 2000 3 i are predominant. At low CH4 concen- the hypothesis on "metastable" dia- TEMPERATURE (K) trations, {lll} faces are predominant, mond growth rests on the fact that and, at high concentrations, (100) the diamond growth process occurs at Fig. 8. Pressure versus temperature phase faces are predominant. This observa- the gas-solid interface in the carbon- diagram for carbon The lines labeled with tion is consistent with those of Kobashi hydrogen system. The vapor-growth metallic elements denote the HPHT condi- and co-workers, and the earlier work process does not involve just elemen- tions utilized for dlarnond growth using reported by Spitsyn et a/.l3 The latter tal carbon, the one component which metallic solvents authors observed octahedral crystals is represented on the phase diagram, with (1 11) faces with growth tem- but it also involves hydrogen. A dia- peratures of 800°C and regular cubo- mond carbon surface saturated with octahedra with {lll} and {loo} faces sp3 C-H bonds is more stable than a at 1000°C. carbon surface free of hydrogen. February 1989 Diamond-Ceramic Coating of the Future 179

Once a surface carbon is covered by another diamond growth layer, then that covered carbon possessing four sp3 C-C bonds is metastable with re- spect to a graphitic carbon. Thus, an upper temperature limit for the vapor growth of diamond is determined by the kinetics of the diamond-to-graphite solid-state transformation (and how these kinetics are influenced by struc- tural imperfections). The close relationship among the dia- mond, lonsdaleite, and graphite crys- tal structures is described in Appendix A. A diamond structure with puckered (111) planes stacked in their ABCA.. . sequence is shown above the stack- ing of the hexagonal planes of graph- ite in Fig. 9. The diamond (111) planes are shaded to emphasize the reiation- ship of the diamond structure to that of graphite. Hydrogen atoms are shown as satisfying the "dangling sp3 bonds" of the carbons on the top diamond DIAMOND plane. Lander and Morrison7' were the first to hypothesize that hydrogen can stabilize a diamond surface by form- ing sp3 C-H bonds with the surface carbons. Without the hydrogens main- taining the sp3 character of these surface carbons, it is easy to imagine the (111) diamond planes collapsing into the more stable planar graphite structure during the growth process. In fact, in the absence of hydrogen, it is well-known that the surface atoms on cleaned bulk diamond crystals will reconstruct from their bulk-related surface sites at about 900" to 1000°C.65~7'(')~7'~73However, in hydro- gen, the reconstruction reverses as dangling surface sp3 bonds become satisfied by C-H b~nding.~' The question then arises as to why earlier thermal CVD studies utilizing hydrogen-methane mixtures for epitax- ial growth on diamond surfaces were of very limited success. The pressure- temperature-composition conditions used by these researchers were quite similar to those used in later success- ful activated vapor growth of crystal- line diamond, but growth rates were in GRAPHITE the range of angstroms per hour, and, without periodically etching away the C C Fig. 9. Schematic diagrams showing the codeposited graphitic carbon, dia- c\ / c\ / similarities in the crystal structures of dia- mond growth would cease.' C=C + H . + H-C-C- \ . mond and graphite The hydrogen atoms The net saturation of a C=C double c' 'c C' C bonded to the surface carbons depict bond with hydrogen has a their role in stabiliztng the diamond sur- face structure C C c\ / c\ / C C C=C + H-H ---z H-CX-H / c\ / /\ H-C-C- . + H-H + H-C-C-H + H . c' C cc \ /\ ' c/c cc favorable negative enthalpy change where a hydrogen radical attacks the (AH" (reaction) = -126 kJ). However, C=C double bond to produce a car- an activation energy to produce either bon radical, which then reacts with a a carbon or a hydrogen radical will be hydrogen molecule to complete the required to get the net reaction to pro- saturation and regenerate a hydro- ceed at a significant rate. Likely gen radical This is in agreement with mechanistic radical reactions are the fact that only when gas-activated 180 Journal of the American Ceramic Society-Spear Vol. 12, No. 2 vapor deposition methods were first negligible partial pressures at about employed in the 1970s did the growth 2000°C and lower. Also, at 2O0O0C, a rates of crystalline diamond become typical temperature for the filament in large enough to be of technological the hot-filament-activated systems, interest. the quantity of atomic hydrogen in As was presented in Section IV, equilibrium with about 1.3 kPa atomic hydrogen etches graphitic car- (10 torr) of molecular hydrogen is bon at a much higher rate than it about 7 at.%, an appreciable amount etches sp3 diamond carbon. Thus, the to interact at the 800" to 1000°C sub- source of hydrogen atoms can serve strate temperature. the dual role of hindering graphite growth as well as etching away any VI. Mechanisms of Growth that does nucleate on the growing dia- In spite of a rapidly growing number mond surface. of publications on the subject of The thermodynamics of the deposi- low-pressure deposition of diamond, tion process may place a lower limit only a few concentrate on mecha- on the deposition temperature for nisms of growth. The most extensive given total pressures and gas concen- set of chemical information is from the trations. Without some type of surface Russian school of Deryagin, Fe- activation, such as bombardment, the doseev, Spitsyn, and co-workers and surface reactions for deposition may from work in the United States by An- approach their equilibrium limits. The gus and co-workers (see Section IV). fact that faceted diamond crystals are The Russian scientists argue, based produced during deposition is an indi- on their experimental studies, that the cation that surface mobilities are large growth of diamond is controlled by ki- enough for surface reactions to reach netic factors. For example, Varnin equilibrium. Lander and Morrison" et a/.33 wrote that, "if the condensate indicate the mobility of carbon on a as a whole under the regular condi- (111) diamond surface is appreciable tions of crystallization mostly reveals at 1000°C. Which reactions reach their properties close to those of graphite, equilibrium limits and which ones are then this is a consequence of not the kinetically limited is still an open ques- energetic advantage of graphite tion, but it is still of value to consider (which, by the way, is quite insignifi- the thermodynamic limits for the depo- cant), but the kinetic preferences in sition process. the growth of the graphite structure." Two plots of the output of equilib- Thus, the Russians proposed that the rium calculations are shown in Fig. 10 formation of diamond competes kineti- to illustrate how the deposition limits cally with the formation of graphite. depend on experimental parame- Although the Russian school devel- ter~.~~,~'Such calculations have also oped kinetic arguments using macro- been made by Bichler et a/.74and scopic concepts, such as classical Sommer et a/.75 The fraction of carbon nucleation theory and Langrnuir ad- deposited from methane-hydrogen sorption-desorption on the surface, 200 400 600 800 1000 (6) TEMPERATURE ('C) mixtures is plotted versus temperature they did not make specific sugges- for several total pressures and compo- tions regarding the elementary chemi- sitions. The following important obser- cal reactions and/or species involved Fig. 10. Equilibrium plots of the fraction in either gas-phase or surface pro- carbon deposited from methane-hydrogen vations can be made: mixtures as a function of temperature: (i) The fraction of carbon de- cesses, except for very brief state- (A) constant total pressure, varying CH4 posited changes from practically zero ments that (i) "graphite grows via content in reactant gas and (B) constant at lower temperatures to close to methyl radicals, while diamond, at CH, content, varying total pressure. 100% over 200°C. least partially, via metastable radicals (ii) High pressures and/or low CH5,"'4 (ii) "complexes" of hydrocar- methane concentrations increase the bons form on the diamond surface;,' lower temperature limit required to ob- and (iii) homogeneously formed "nu- tain any deposit. clei and clusters of the new phase" Thus, thermodynamic consider- are incorporated into the lattice of the ations set a lower temperature limit ~rystal.~' on diamond growth of about 400" to 600°C, depending on specific (1) Chemical Mechanisms of pressure-composition conditions, un- Surface Reactions

I es s " noneq u i I i b r i u m " bomba r d ment Tsuda et a/.76-78 conducted quan- techniques are used. These tech- tum chemical computations in order to niques always produce some DLC (or determine the lowest energy path for a similar highly defective form of car- proposed mechanism of diamond bon) along with crystalline or micro- growth on (111) surfaces. They initially crystalline diamond,', Even small assumed76that only C'H1-3 radicals quantities of a defective form of car- and ions can be the growth species in bon could render the deposited films CH4-H2 plasmas and reported the fol- useless for electronic applications. lowing two-step reaction sequence. In Not shown on the above plots is the the first step, the {Ill} plane of the fact that the elemental gaseous car- diamond surface is covered by the bon species of C, C,, C,, etc. have methyl groups via either methylene in- February 1989 Diamond-Ceramic Coating of the Future 181 sertion or hydrogen abstraction fol- mol) if computed by the method used lowed by methyl radical addition. In by Huang et a/.))and 17 to 25 kJ/mol the second step, following the attack (4 to 6 kcal/mol) energy barriers for of a methyl cation and the loss of three two of their subsequent steps. (ii) Cer- H2 molecules, three neighboring tain multistep bond-breaking, atom- methyl groups on the {lll} plane are transfer, bond-forming processes bound together to form the diamond proposed in the Frenklach and Spear structure. In the subsequent publica- mechanism were calculated to be en- they extended their analysis ergetically more favorable if they oc- and concluded that the epitaxial curred as one simultaneous step growth of a diamond film IS sustained, rather than as sequential steps. provided that the surface maintains a Although the mechanism is de- positive charge and that there is a scribed in detail by Frenklach and supply of methyl radicals. This mecha- Spear” for the addition of acetylene nism does not explain the critical ef- molecules, it is possible for other fect of hydrogen atoms on the growth carbon-hydrogen species to enter and relies on maintaining a positively into the reaction sequences. The charged surface or a precursor of mechanism is in general agreement CH3+cations, whose abundance in a with the macroscopic views of the plasma is q~estionable~~and whose Russian researchers and is consistent abundance in an ion-free environment, with experimental observations out- as is found in the hot-filament method lined in Section IV, such as the impor- of diamond growth, is tance of atomic hydrogen and the A paper by Frenklach and Spear” independence of the diamond growth proposes an alternative mechanism on the chemical nature of the input hy- for the growth of (111) diamond sur- drocarbon species. faces. It basically consists of two alter- (2) Steric Aspects of Gas-Solid nating steps. The first step is the Growth Reactions surface activation by H-atom removal of a surface-bonded hydrogen. Relationships of the atomic structure of (lll), {loo), and (110) diamond C C surfaces with the growth behavior of \ \ these surfaces and the nature of the H. + C-H+H2+ C. / / C,H, and C2H, species which arrive C C at the surface have been discussed In the second step, this surface- by Spear and FrenklachE3and Angus activated carbon radical then acts as and Haymam6 Attempts are made to a site for adding more carbons to the rationally correlate experimental structure by reacting with acetylene growth behavior with atomistic hy- (or other carbon-hydrogen species in potheses on nucleation and growth the gaslplasma). mechanisms and to provide guidelines for designing experiments to test mod- c\ C\v els that provide recipes for tailor mak- C . + Ha-H + C-Cd? ing the type of deposited diamond / C c‘ required for a particular application, whether it is abrasive grit or single- The additional radical reactions for the crystal films. Although current informa- mechanism of propagating a growth tion available for such correlations is step on the (1 11) plane of diamond are sparse, it is instructive to illustrate the given in the paper by Frenklach and preliminary mechanistic conclusions Spear.” The propagation results in the that can be reached from steric con- addition of two acetylene molecules siderations of the grow interface. for one hydrogen abstraction step, Faceted diamond crystallites are with the resulting regeneration of the dominated by (111) and (100) sur- Fig. 11. (A) Four unit cells of diamond de- faces. Very rarely are (110) surfaces picting the zigzag chainlike nature of the hydrogen atom which was consumed carbon atoms in the (100) and (110) planes in forming the activated surface site. observed. Dangling sp3 bonds exist of the structure These parallel chains in the Huang, Frenklach, and MaroncelIia2 on all three surfaces; one for each (100) planes extend in the (110) direction of recently tested the Frenklach and Spear surface carbon on the {Ill) and the structure (B) Every other zigzag car- mechanism using quantum chemical (110) faces, and two for each carbon bon chain has carbon atoms on the surface computation techniques similar to on the (100) face. Figure 9, used in of the unit cell (at the “0” position) separated those used by Tsuda et a/.76-78 The fol- Section V to illustrate an important role by carbons which lie 114 toward the back lowing relevant results emanated from of hydrogen in the diamond growth surface (C) the other chains, which also the work of Huang et a/.: (i) Once the process, shows a {I 1l} surface with are in the (110) direction, have carbons at C-H bonds satisfying the dangling positions of 1/2 and 3/4 toward the back of surface is activated by the 73 kJ/ the cell mol (17.4 kcal/mol) H-atom removal of bonds of the surface carbons. a surface-bonded hydrogen, all other Figure 11 depicts the zigzag chain- subsequent mechanistic steps occur like nature of the carbon atoms in a with a decrease in energy; i.e., no acti- (100) diamond surface. The small fig- vation energy is needed for any of ure of four unit cells shows these these subsequent steps. Tsuda et a/. parallel surface carbon chains in the compute a 129 kJ/mol (30.6 kcal/mol) (110) direction. Every other carbon lies initiation step (130 kJ/mol (31.0 kcal/ on the front surface, while the alternat- 182 Journal of the American Ceramic Society-Spear Vol. 12, No. 2 ing carbons are a of the unit cell to- surfaces. ward the back surface. The surface (3) For { 110) Surfaces: A carbons in the chain are bonded to (i) growth of these faces occurs only two other carbon atoms, leaving rapidly with either C1Hl_3or C2H,-4 two unsatisfied sp3 bonds, unless a species (Because the nature of restructuring of the surface occurs or the precursor is not important, these other atoms, such as hydrogen, form surfaces have fast growth rates and surface bonds. are rarely exhibited by diamond The (110) surface is composed of crystallites.); the same zigzag chains found in the (ii) C-H bonding can satisfy all (100) planes, but they lie on their side of the dangling sp3 bonds of the sur- in the (110) face so that all carbon face carbons-steric hinderances do atoms composing a chain lie on the not occur; surface of this face. Each of these sur- (iii) nucleation of new planes is not face carbons is bonded to two other required for (1 10) surfaces (Addition of surface carbons plus a third carbon carbon species to these planes pro- GRAPHITE which lies below the surface, leaving duces new active growth sites.); only one unsatisfied sp3 bond. (iv) stacking faults and twin planes Steric analyses of molecular species cannot be easily introduced during approaching each of the above three growth; and surfaces lead to the following prelimi- (v) fast growth of (1 10) surfaces re- nary conclusions concerning diamond sults in the formation of (111) surfaces. growth at the gas-solid interface: More quantitative experimental in- (1) For (171) Surfaces: formation is needed to test these (i) growth of a nucleated plane conclusions and to develop a better across the surface will occur rapidly understanding of the growth pro- with C2Hl+, species (CPH1,C2H2, C2H3, cesses, but the conclusions are con- CzHd; sistent with experimental observations (ii) ClH1-3 species can also contrib- on diamond growth summarized in ute to the growth of these planes; other parts of this review. (iii) C-H bonds can satisfy all of the dangling sp3 bonds of the surface VII. Future Directions carbons; The general properties and potential LONSDALEITE (iv) continuation of growth on a applications of diamond in advanced (111) surface requires nucleation of a materials technology are outlined in new (111) plane upon the completion Appendix B. These provide the driving of each growing plane; force for the rapidly developing dia- A (v) stacking faults and twin planes mond coating science and technology can easily be introduced during the which has been summarized in this re- nucleation stage of each new (1 11) view. Applications for these coatings C plane (Once a plane is nucleated and include protection against wear and growth is occurring, the formation of a chemical attack, electrically insulating planar defect is unlikely.); and heat sinks, durable lenses for use in (vi) conditions leading to low rates both the visible and IR regions, and B of nucleation of new (111) planes re- high-temperature and high-power sult in a morphology dominated by electronic devices. Development of (1 11) surfaces. coating techniques for growing clean A (2) For (100) Surfaces: diamond films at temperatures low (if growth of these faces occurs enough to coat plastics would result in most rapidly with CIHl or ClH2 spe- "crash-proof'' optical and magnetic cies (The attachment of C,H, species disks for information storage. DIAMOND to this surface requires the formation The major research and develop- of two sp3 bonds (carbon bonds to ment problems which are slowing the surface carbons which are in two dif- commercialization of products in- ferent chains).); corporating diamond films are not Fig. Al. Schematic drawings of the atomic arrangements in hexagonal graphite, hexa- (ii) C2H, species are not preferred, trivial. The generic problem areas are gonal lonsdaleite, and cubic diamond. but can add in combination with CIHl- as follows: Note the shaded hexagonal rings of car- 2 species; (i) lowering substrate temperatures, bons in these three structures: planar (iii) C-H bonding cannot satisfy all (ii) controlling nucleation rates, (graphite), boat-form (lonsdaleite), and f the dangling sp3 bonds of the sur- (iii) increasing growth rates, chair-form (diamond). face carbons because of steric hin- (iv) better adhesion to a variety of derances (One C-H bond sterically substrate materials, hinders the formation of a C-H bond (v) eliminating graphite codeposi- on the carbon in an adjacent chain.); tion, (iv) nucleation of new planes is not (vi) controlling defect densities, required for (100) surfaces (Addition of (vii) uniform thickness on irregular carbon species to these planes pro- shapes, duces new active growth sites.); and (viii) better electrical contacts, and (v) stacking faults and twin planes (ix) heteroepitaxial growth of single- cannot be easily introduced during crystal films. growth; and (vi) fast growth of (100) A recent Popular Science article" surfaces results in the formation of (1 11) quotes Thomas Anthony, a General February 1989 Diamond-Ceramic Coating of the Future 183

Electric Company scientist spearhead- lonsdaleite form strong, uniform three- ing their low-pressure diamond re- dimensional frameworks. search as saying that they have “found The unit cells of cubic diamond and proprietary ways to get relatively uni- hexagonal lonsdaleite are shown in form coatings on irregular objects,” Fig. A2, and Table Al shows their crys- and that their coating process pro- tallographic data. These structures duces films “thicker than a millimeter.” may at first appear quite different, but If they have indeed solved the film uni- both are made completely of sp3-type formity and growth-rate problems, then tetrahedrally bonded carbon atoms. they have overcome two major hurdles The structures are related to each for getting vapor-grown diamonds to other in a similar manner as sphalerite the marketplace. and wurzite. Correlations of hypothesized nu- The {ill} planes of diamond are DIAMOND cleation and growth mechanisms with composed of puckered hexagonal rings diamond growth rates, crystallite mor- of carbon atoms which have the chair phologies, defect types and concen- form as indicated in Fig. Al. The (001) trations, and plasma chemistry would planes in lonsdaleite are identical to t‘ be of enormous benefit in eliminating these diamond {Ill} planes. The dif- the above problem areas as well as in ference in the two structures is in the providing efficient guidelines for ad- stacking of these planes as is shown vancing the technology needed to fully in Fig. A1 and is similar to the differ- utilize the unique properties of dia- ence between cubic-close-packed mond. If methods can be developed and hexagonal-close-packed metal for producing sinterable diamond pow- structures. Figure A3 shows the differ- der, bulk diamond ceramics can be ences between the two structures in considered for a host of new applica- three ways. The differences in second- -Y tions making use of unique combina- nearest-neighbor coordination as X- tions of its properties. Diamond could viewed down a C-C bond lying in the then become not only the ceramic coat- stacking direction is shown in LONSDALEITE ing of the future, but could also play a Fig. A3(A). In diamond, the three car- major role in bulk form as an advanced bons at one end of the bond are stag- engineering material. gered with respect to the three Fig. A2. Schematic diagrams of unit cells of diamond and lonsdaleite. Table Al gives APPENDIX A carbons at the other end of the bond. In lonsdaleite, these same carbons their crystal structure data. Phases and Crystal Structures eclipse each other. The slightly of Carbon higher energy of these eclipsed lons- Diamond and graphite are both daleite carbons causes its structure to pure carbon with well-known crystal- be slightly less stable than the dia- line forms, but with quite different mond structure. properties. Other crystalline forms of pure carbon which are not as common Table Al. Crystal Structure Data for Diamond and Lonsdaleite include lonsdaleite (sometimes called hexagonal diamond because its struc- Property Diamond* Lonsdaleite’ ture and properties are similar to those Symmetry Cubic Hexagonal of diamond) and the carbynes (cross- Space group Fd 3m ?63/mmc linked linear carbon polytypes of Atoms per unit cell 8 4 which at least six forms have been Positions of atoms (OOO), (; ;O), (0 ;;I, (OOO), (OOt), rep~rted).’~ (1 11) (1 1 I) (LO+,2 2 (I444 1 l), (2444 3 L), 332’ 338 Each of these phases of carbon is 133 313 formed from carbon hybridized in an (z 4 71, (2 z a) sp3,sp2, and/or sp state. The ideal Cell constant (298 K) (nm (A) 0.356683(1)-0.356725(3) a = 0.252 (2.52) structures of diamond and lonsdaleite (3.56683(1)-3.56725(3)) C = 0.412 (4.12) are formed completely from tetrahe- Theoretical density (298 K) drally bonded sp3 carbons. Graphite (g/cm3) 3.51525 3.52 is formed completely from trigonally Carbon-carbon bond distance bonded sp’ hybridized carbons. The (nm (A)) 0.154450(5) (1.54450(5)) 0.154 (1 54) carbynes are formed primarily from linearly bonded sp hybridized car- *Data from Field.” ‘Data from Hanneman et a/.‘11 bons, but the cross linking which is critical to their structure requires sp‘ The puckered hexagonal rings are and/or sp3 carbons. present in the chair form throughout The atomic arrangements in graph- the diamond structure, but not ite, lonsdaleite, and diamond are throughout the lonsdaleite structure. depicted in Fig. A1 , which clearly indi- As can be seen in Figs. A3(B) and (C), cates the similarities and differences the hexagonal rings which lie in the expected in the properties of these stacking direction of lonsdaleite are in materials. The graphitic sp2 bonding is the boat form. The first carbons which similar to that in benzene, and it cre- begin nucleating a new layer in lons- ates strongly bonded two-dimensional daleite and in diamond are shown in planes, but weak bonding between Fig. A3(B), and the basic structural the planes. On the other hand, the four unit of these two structures (which is equivalent sp3 bonds in diamond and also the nucleation kernel) is shown in 184 Journal of the American Ceramic Society-Spear Vol. 12, No. 2 Fig. A3(C). Twin planes (stacking oxidants, on (111) faces at 380°C and faults similar to those found in silicon 181 h.84Sodium nitrate is known to at- carbide) could nucleate and form on tack diamond at 430°C. The dissolu- diamond (111) planes. tion of diamond (and carbon in APPENDIX B general) in metals has been examined in detail because of the use of metal Properties and Applications solvents/catalysts in high-pressure/ of Diamond high-temperature diamond (7) General Properties and Op,CO, CO,, H2, HpO, and C12 at Applications high-temperature have been used to Table BI lists some of the properties etch diamonds. In oxygen, appre- of diamond which can lead to advanced ciable oxidation will begin at about applications in areas ranging from cut- 600°C. Diamonds will burn in an oxy- ting and grinding to high-power, high- gen jet at 720°C and in air at 850°C. speed electrooptic devices. Although Under low vacuum, residual oxygen certain properties of diamond such as causes a dense black film of graphite its room-temperature hardness and to form on a diamond surface, but it thermal conductivity are the best of can be easily removed by boiling in any known material, its unique combi- HCIO,. Aqua regia will also remove nation of properties is what has caught the black surface film.84These films, the attention of many industries. which can form above 600°C, are not true graphitization of the diamond; i.e.,

DIAMOND LONSDALEITE they are not a result of a solid-state transformation of diamond to graphite, but are almost certainly a result of a T? mineralization process through a CO- Con transport mechanism. If diamond is heated in a clean, inert environment, the onset of graphitization begins at about 1500”C, and the rate increases rapidly until 2100”C, where a 0.1 carat octahedra is totally converted to graphite in less than 3 The {OOI} planes of graphite form parallel to the (1 11) planes of diamond.84 Heating diamond to 1500°C pro- duces only very slight surface graphi- tization due to small quantities of oxygen which remain in the heated system even under a vacuum of lo-* to Pa to lo-@atm). Slightly higher pressures result in a thicker film, but if the residual oxygen pres- sure becomes too high, the surface film is removed by oxidation.84 Efremow et developed an ion- beam-assisted etching technology for use in fabricating diamond-thin-film devices. Nitrogen dioxide is physically adsorbed on the diamond surface where it is bombarded with an ion Fig. A3. Comparisons of structural as- pects of diamond and lonsdaleite. (A) View Sumitomo Electric and Sony are retail- beam and is activated so that oxida- down C-C bond in stacking direction of ing the first product involving a dia- tion of the diamond occurs. This pro- planes shown in Fig. A1 ; diamond shows a mond coating: their APM-66ES speak- cess produces etching rates of 200 staggered configuration of the C-C second- ers contain diamond-clad tweeters. nm/min (2000 A/min) as compared nearest-neighbor bonds (chair-form con- The following paragraphs outline with reactive-ion etching with oxygen figuration of puckered hexagonal rings), more details of the chemical, thermal, yhich has a rate of 20 nm/min (200 and lonsdaleite shows an eclipsed con- mechanical, electronic, and optical A/min). figuration (boat-form configuration of hex- agonal puckered rings). (8)Initial carbon properties of diamond. However, many (3) Thermal Properties configuration as a new plane is nucleating. of the applications mentioned in these Table Bll lists a number of the ther- (C) Basic structural unit for each of the discussions critically depend on com- mal properties, including thermody- structures. These are also the nucleation binations of these properties. Badzian namic properties, of diamond and the kernels for new planes. et provide an extensive listing of graphite to diamond tran~ition.”(~)The many possible applications of diamond. maximum thermal conductivity occurs (2) Chemical Properties at about 80 K. At room temperature, Diamonds are resistant to all acids, the conductivity of diamond is about even at high temperatures. However, 4 times greater than the value tor they can be etched by fluxes of caus- p-silicon carbide, 15 times greater tic alkalis, various oxysalts, and than that for silicon, and 5 times metals. Etch figures were formed with greater than that for copper. NaCIO, and KOCL,, both very strong Small amounts of boron impurity, February 1989 Diamond-Ceramic Coating of the Future 185 such as used for semiconductor dop- ing, have little effect on the thermal Table BI Properties and Applications of Diamond Coatings* conductivity of diamond. However, ni- Properties Applications trogen impurities markedly effect its conductivity." The tendency of nitro- Hardest known material Coatings for cutting tools gen to cluster in diamond may cause Low coefficient of friction this effect. The natural type Ila dia- High thermal conductivity Abrasive coatings mond contains very little nitrogen im- (highest known at room temperature) Coatings for bearings purity whereas type la contains up to Low thermal expansion about 0.1 at.% nitrogen. The thermal Heat resistive Heat sinks for electronic devices conductivity of type Ila is about 3 Acid resistive times that oft pe la.84 Radiation resistive High-power microwave devices Ono et a/.'Y made thermal conduc- (to X-ray, ultraviolet, y-ray) tivity measurements at 100" and 130°C Radio-frequency electronic devices on 7- to 30-pm-thick vapor-deposited Electrical insulator diamond films. They used reference High band gap semiconductor High-speed electronic devices films of oxygen-free copper and pure (either p- or n-doped) silver and obtained average values of Low dielectric constant Sensors for severe environments 4.5 and 4.8 W/(cm. K), respectively. High hole mobility Window and lens materials The maximum value for a vapor- Visible and infrared transparent deposited diamond film was 10.0 W/ Large refractive index Electrooptic devices (cm at these temperatures. They K) *Expanded version of table given by Nishimura et a/ 62 found a strong correlation between the diamond conductivity values and the largest thermal conductivity of any ma- methane content in the input gas for terial. These properties and its low co- film deposition. The Raman spectra of efficient of thermal expansion greatly the films, which provide a measure of reduce the problems of heating during the non-diamond carbon codeposited cutting and grinding and, therefore, with the crystalline material, also possible problems with thermal shock. showed a similar strong correlation Cleavage occurs primarily along the with methane concentration. The films {lll} planes of diamond although the exhibiting the highest thermal conduc- energy differences for several planes tivity and the cleanest Raman spectra are quite For example, the (least amount of non-diamond carbon) cleavage energies in J/mm' are 10.6 were grown with methane concentra- for {Ill}, 11.7 for {332}, 12.2 for {221}, tions of 0.1 mol% in hydrogen. The and 12.6 for (331) planes. Cleavage conductivity dropped from 10 to 4 W/ cracks can travel at velocities of thou- (cm K) as the methane concentra- sands of meters per second. Theoreti- tion in the deposition gas increased cal strengths of diamond and various from 0.1 to 0.5 mol%. The values other materials are compared by dropped below 1 W/(cm. K) for meth- The best value for the tensile ane contents of 2 mol% and higher. strength of diamond is 300 kg/mm'. The amount of non-diamond carbon in Above 1800"C, dislocations become the films, as measured qualitatively by the Raman spectra, increased with in- Table Bll. Thermal Properties of Diamond* creasing methane concentration in the Property Value reactant gas. The coefficient of thermal expansion Thermal conductivity (W/(cm K)) of diamond is quite small. It is 0.8 x Typical values at 293 K K-' at room temperature, and then Type la 6-1 0 increases to 4.8 x K-' at about Type Ila 20-21 1200 K. Gruneisen's law is obeyed Thermal expansion (linear) (x1 0-6 K-') between 420 and 1200 K.7' 193 K 0 4(0 1) (4) Mechanical Properties 293 K 0 8(0 1) Specific values for the mechanical 400-1200 K 1 5-4 8 properties of diamond, as given by Debye temperature, 8, at 7 > 600 K (K) 1880 Field," are listed in Table BIII. Dia- Molar volume, V& (cm3/mol) 3417 mond is perfect for many abrasives Bulk modulus, B (Pa) 44 x 10" and wear-resistant applications, ex- Cp - Cv at 7 > 1100 K (J/(mol K)) 42 x 10-~T cept for its high-temperature chemical Cp at 1800 K 24 7 interactions with ferrous alloys.7i(b) Cp at 3000 K 26 3 Cubic boron nitride is a good substi- tute for diamond in these cases." Dia- Transition reaction Graphite e diamond mond is often used with well-defined AH& (J/mol) 1872 * 75 cutting edges, as in turning or milling; AS& (J/(mol K)) -3 22+ as bonded grit in grinding, sawing, ACp above 1100 K (J/(mol K)) O* drilling, and dressing; and as loose Equilibrium pressure at 2000 K (Pa) 64 x 10' abrasive grit in lapping and polishing. Volume change at 2000 K transition (cm3/mol) 14 At near room temperature, diamond is 'Data from Berman ''Id) +Data from DeVries +Thus AH; and AS; for the transition are constant the hardest known material, has a low above 1100 K coefficient of friction, and has the 186 Journal of the American Ceramic Society-Spear Vol. 12, No. 2

alternating silicon and carbon atoms. Table Blll. Mechanical ProDerties of Diamond* Two figures of merit for semiconduc- ProDertv Value tors were devised prior to the process- ing developments which have resulted Hardness Indentation hardness typically in practical considerations of diamond taken as 10000 kg/mm2 for use as a semiconductor.86The Friction Coefficient of friction in air is -0.1; Johnson figure of merit, which com- approaches unity under vacuum pares the product (power) x (fre- Elastic moduli (x10” N/m’) quency squared) x (impedance), is L11 10.8 suitable for semiconductor applica- b2 1.25 tions in power amplifiers at frequen- c44 5.77 cies greater than the UHF spectrum. Pressure coefficient, dC/dP Relevant semiconductor properties are breakdown field and saturated ve- Cl, 6.0(0.7) 3.1(0.7) locity. The values given in Table BIV c12 show that the Johnson figure of merit 0.3) c44 3.0( for diamond is about 8000 times larger Temperature coefficient, than that for silicon and about 8 times (l/C)(dC/dT) (x10-5) larger than that for p-silicon carbide. c11 - 1.4(0.2) The Keyes figure of merit, also -5.7(1.5) shown in Table BIV, is relevant for high- c44 - 1.25(0.1) density integrated circuits. Important Young’s modulus, E (N/m’)+ 10.5 x 10” semiconductor properties are dielec- Poisson’s ratio, tric constant, thermal conductivity, and saturated velocity. Diamond has a v21 = ClZ/(CI, + ?12)* 0.104 Bulk modulus, K = z(Cll + 2CI2) Keyes figure of merit about 30 times f x 10” N/m‘I 4.42 larger than that for silicon and about 6 times larger than that for p-silicon *Data were taken from Field More details on the mechanical properties of diamond aregiven in carbide. this reference ‘The anisotropy IS 1 21 (= 2C,,/(C,, - C,J) for diamond The condition for isotropy is 1 .OO, so Young’s modulus does not vary greatly with orientation *The value of u varies between 0 1 and Linear arrays of diamond diodes 0 29, with an average u of -0 2. have advantages over their silicon relatively mobile, and it is possible to counterparts for the detection of radia- produce appreciable plastic deforma- tion in the far-ultraviolet and X-ray tion. At room temperature, diamond behaves as an elastic brittle solid. regions because diamond does not respond to electromagnetic signals in However, there is still a question the visible and infrared (IR) regions of whether plastic deformation can the spectrum. Therefore, the back- occur around an indenter at room temperature.”(g) ground current of the diamond diode arrays is much lower.86 The coefficient of friction (p)of dia- mond on diamond is about 0.1 in air, The radiation hardness of semicon- and about 1 under vacuum. If the sur- ducting diamond has not yet been faces are cleaned under vacuum by verified, except that its neutron hard- heating or bombardment, the high ness is unmatched.86 However, be- value of unity is approached. Contin- cause the carbon atoms in diamond ual sliding on a dirty surface under exhibit much stronger bond energies vacuum will result in continual increases than any other semiconducting mate- in p until a value of unity is approached. rial in use, diamond is expected to be The high friction leads to marked dis- much more immune to radiation dam- integration of the diamond surfa~e.~~(~)age than an other semiconductor. Berman7’Y d, has discussed the use Friction depends on the crystallo- graphic surface of the diamond. The of diamond for heat sinks, using an ex- (111) surface shows an isotropic p of ample of bonding a natural type Ila 0.05 in all directions in air, whereas the diamond to a semiconductor micro- (100) cube face gives values of 0.05 wave generator. At temperatures at along the (011) direction, and 0.1 to which such microwave generators op- 0.15 along the (010) direction. The di- erate, the thermal conductivity of type rection of polishing the surface before Ila diamond (-20 W/(cm. K) at 298 K) the test can also effect the coefficient is 4 to 5 times larger than that for of friction value. The addition of a Iu- copper. The efficiency of the cooling bricant has little effect on the value. of such a device can be doubled if the Other than for clean surfaces under generator is bonded to such a dia- vacuum, the coefficient of friction rarely mond. For a given working tempera- exceeds a maximum of 0.2. ture, the microwave power can be more than doubled. The thermal re- (5) Electronic and Optical Properties sistance of the device itself and the Table BIV shows a comparison of bonding layers prevent any greater properties of semiconducting diamond, improvements. The direct deposition p-silicon carbide, and silicon. The of a well-adhering diamond coating to atoms in these systems all exhibit sp3 such a device could help to eliminate bonding with cubic unit cells. Silicon the bonding resistance. has the diamond structure, and p-silicon Fujimori et a/. synthesized boron- 3‘w’*bj carbide is the diamond structure with doped diamond films on both silicon February 1989 Diumond-Ceramic Coating of the Future 187 and diamond substrates using Table BIV comparison of Semiconducting Properties microwave-plasma-assisted CVD methods and characterized the con- ProDerties* Diamond B-Silicon carbide Silicon ductivity of these films. For boron dop- Lattice constant (nm (A)) 0 3567 (3 567) 0 4358 (4 358) 0 5430 (5 430) ing, they used diborane in B2H,/CH4 Thermal expansion (x1 O-6/oC) 1 1 47 26 ratios of 0.001 to 0.02, and obtained Density (g/cm3) 3 515 3 216 2 328 respective resistivities of 1 to 0-cm. The boron concentration in the Melting point (“C) 4000t 2540r 1420 film was on the order of lozoatoms/ Band gap (eV) 5 45 30 11 cm3. The activation energy for con- Saturated electron velocity duction in a film produced from a re- ( x 1 o7 cm/s) 27 25 10 actant gas with a diborane/methane Carrier mobility (cm’/(V s)) ratio of 0.001was 0.013 eV. Electron 2200 400 1500 Prinsgofabricated the first bipolar Hole 1600 50 600 transistor using natural p-type dia- Breakdown (x I O5 V/cm) 100 40 3 mond and ion implantation to form Dielectric constant 55 97 11 8 n-type regions. This device did not Resistivity (0 cm)§ 10‘3 150 I 03 exhibit current gain, but demonstrated Thermal conductivity device potential. Geis et a/.’’ later re- ported point contact transistors (W/(cm K)) 20 5 15 formed on synthetic p-type diamond. Absorption edge (pm) 02 04 14 These devices exhibited power gain Refractive index 2 42 2 65 35 even at 510°C, which was the highest Hardness (kg/mm’) 10 000 3500 1000 temperature reported for any transis- Johnson figure of merit” tor. Geis and co-workers summarized ( x 1oZ3 (w n)/s2) 73 856 10 240 90 possible applications of diamond Keyes figure of merit** device^.^' Gildenblatt et a/.93 studied (~10’w/(cm s “C)) 444 90 3 13 8 the electrical characteristics of Schot- tkv diodes fabricated from microwave- *Typical room temperature values for these properties were obtained primarily from Field ’ Nishimura et a/ ,62 and Yoder 86 ‘Triple point of carbon but conversion of d amond to graph te under plasma-assisted CVD polycrystalline vacuum becomes rapid at about 2000°C *Two reported values given in !he JANAF Tables’’o for the diamond films. peritectic decomposition of p SIC to SI(/) and graphite are 2540” and 2830°C Decomposition under Diamond is excellent as a window or vacuum to graphite and vapor will occur a! lower temperatures §Values vary widely with impurity and defect levels “Johnson figure of merit is primarily for comparing power amplifiers at frequencies above lens material, or as a protective coat- the UHF spectrum Relevant factors are breakdown field and saturated velocity **Keyes figure of merit ing for these materials because of its is primarily for comparing high density integrated circuits Relevant factors are dielectric constant ther transparency to both visible and IR ra- ma1 conductivity and saturated velocity diation, its resistance to abrasion, its resistance to chemical attack, its resist- sorb these results interest in the low- ance to radiation damage, and its abil- pressure growth of diamond began a ity to efficiently conduct heat. A natural rapid acceleration, and new modifica- diamond window was chosen for tions on vapor growth methods began transmissivity over a large IR spectrum to be developed on Venus probes.86Transmission In a book published in Russian, losses in diamond are typically as Deryagin and co-workers” outlined much as 4 orders of magnitude lower the following three methods of obtain- than in competing materials in the IR ing hydrogen-atom superequilibrium spectrum. in a diamond vapor growth system (I) catalytic, such as heated Pt for dis- APPENDIX C sociating H2 (ti) an electric discharge, Development of Vapor and (111) a heated tungsten filament Growth Methods located just in front of the substrate Early attempts to vapor deposit dia- Because this book was not readily mond on diamond seed crystals by available outside the Soviet Union thermal pyrolysis methods resulted in and Eastern Europe and because low crystalline diamond growth rates of the general “lack of belief” by the of angstroms per hour, and the code- scientific community these tech- position of non-diamond amorphous niques were not quickly pursued in and graphitic carbon^.^^',^^^" Both sci- other laboratories entific and commercial interest was In 1981, in the Journal of Crystal minimal. However, discoveries by Growth, Spitsyn, Bouilov, and Deryagin13 Deryagin and co-workers in the 1970s wrote a paper in English on the growth markedly increased deposition rates of crystalline diamond that showed and reduced the codeposition of non- photographs of deposits that exhibited diamond carbon. These discoveries il- many faceted crystals with dimensions lustrated the importance of (i) using up to about 30 pm They also showed excess hydrogen in the deposition gas an electron diffraction pattern charac- and (ii) activating the gas before depo- teristic of crystalline cubic diamond sition to produce superequilibrium and listed a number of physical prop- concentrations of atomic hydrogen. erty measurements on grown films (The important role of atomic hydro- which were Characteristic of diamond gen in the diamond growth process Growth rates were on the order of is discussed in Sections IV and VI.) micrometers per hour One sentence As the rest of the world began to ab- in this paper mentions an “electric dis- 188 Journal of the American Ceramic Society-Spear Vol. 12, No. 2

charge" in the system, bui no further The above methods typically utilize details were given However, the im- total pressures of about lo4 Pa (0.1 atm) portance of having atomic hydrogen or less, pressures at which "cold plas- present during growth was empha- mas" operate. Therefore, it was a ma- sized as a means of suppressing the jor breakthrough when Matsumoto and crystallization of graphite his c01Ieagues'~~grew diamond parti- Immediately following the Spitsyn cles and films on molybdenum sub- paper, Mania et a/ 95 published a pa- strates using an rf induction thermal per on the synthesis of diamond utiliz- plasma with a 105-Pa (1-atm) argon- ing an ac discharge in a low-pressure hydrogen-methane mixture. Their flowing-gas system Varnin et a/ 96 growth rates were on the order of later reported more details of the Rus- 60 pm/h (1 pm/min), much higher sian dc electric discharge system men- than the typical growth rates of a few tioned earlier '' l3 A hollow-cathode micrometers per hour and less. The plasma-assisted CVD method which molybdenum substrate was placed on combines thermal and plasma disso- a water-cooled holder, and held at ciation of a methane-hydrogen mix- 700" to 1200°C. The rf frequency used ture for diamond growth was was 4 MHz, and the total plate power developed by Singh eta/ 97 They dou- was 60 kW. The total gas flow rate was bled the growth rates that they had about 72 L/min through the approxi- been obtainin from their heated fila- mately 50-mm diameter tube, with the ment system 9P hydrogen flow rate making up 12 L/min, Following the paper in Englieh by and the methane flow rate was main- Spitsyn l3 many papers were pub- tained between 0.1 and 1.2 L/min. lished in Japan, primarily from the The hot plasma makes it difficult to National Institute for Research on Inor- maintain the low substrate tempera- ganic Materials (NIRIM) on tech- ture and to obtain uniform coating niques for growing diamond from thicknesses across the substrate. activated gases composed of hydro- Other novel methods developed in- carbons and excess h drogen In clude diamond growth at rates up to 1982 Matsumoto et a/ used the 80 pm/h by the application of plasma technique mentioned by Deryagin jets at high temperatures and flow et a/ '' of activating the precursor de- rates.'" Diamond crystals and films position gases with a filament heated have also been grown using 105-Pa to approximately 2000°C about 1 cm (1-atm) oxygen-acetylene flame^.^'-^' from the 1000°C growth surface This The homogeneous crystallization of hot-filament method (HFCVD) has diamond from a vapor utilizing laser since been studied and used with preheating or other gas activation great success by many researchers methods was reported in the Soviet around the world Union in Refs. 14, 15, 107, and in refer- Matsumotog9 reported on diamond ences cited in these papers, but no growth in a radio-frequency (rf) glow further reports on the development of discharge system (rf-PACVD) This these techniques have been found. method has also been combined with The nucleation of diamond powder a heated filament'" to improve on the from methane in an rf plasma was re- diamond growth rates However Mat- ported by Mitura,'" but the evidence sumoto reported that the combined that diamond was actually produced is method "does not produce a clear not very strong. crystal habit of diamond " Sawabe and The growth of single-crystal films of Inuzuka'" later developed an electron- diamond is critical to many electronic assisted method (EACVD) for growing and optical applications, but is a feat diamond with growth rates of 3 to that has not been achieved except for 5 pm/h, as opposed to many of the homoepitaxial growth on diamond earlier rates of about 1 pm/h and less substrates. Spitsyn et a/.l3 reported The successful use of a microwave- the growth of single-crystal films on plasma assisted method (MPACVD) to {I lo} diamond surfaces. Matsumoto activate the gas for growing crystalline et showed a beautiful SEM photo- diamond was reported by Kamo and graph of an approximately 10-pm thick co-workers lo' Kawarada et a/ lo3 de- single-crystal diamond film grown on veloped a magneto-microwave sys- a {lll} diamond surface. Fujimora tem so that large areas could be et a/.89have repeated that growth on coated On a historical note, in 1976, {I 1 I} diamond surfaces in preparing Kn~x''~reported the use of a micro- doped single-crystal samples for elec- wave plasma to activate hydrocarbon- trical measurements. Fujim~ra'~~later hydrogen mixtures in an attempt to reported that the epitaxial growth of produce hard coatings for IR window diamond on (100) diamond surfaces materials He obtained electron dif- gave better quality films than those fraction patterns indicating diamond grown on (1 1l} diamond surfaces. had been produced but this work was Acknowledgments: The author not pursued because of skepticism thanks Andrzej and Teresa Badzian for and the termination of the research providing the SEM diamond photo- contract graphs, Diane Knight for the Raman February 1989 Diamond-Ceramic Coating of the Future 189 spectra, and Michael Frenklach for J Appl Phys, 42 [7] 2953-58 (1971) many stimulating discussions con- 25R Messier, A Badzian, T Badzian. K. E cerning diamond growth mechanisms. Spear, P Bachmann, and R Roy, "From Diamond- Like Carbon to Diamond Coatings," Thin Solid References Films, 153, 1-9 (1987) 'F P Bundy, H T Hall, H M. Strong, and R J *6D S Knight and W. B. 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