Deposition and properties of diamond thin films

Materials Science and Engineering, A140 ( 1991 ) 741-746

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Deposition and properties of diamond thin films Claus-Peter Klages Fraun hoJer-lnstimt fi~rSchicht- und Oberfliichentechnik, Vogt-Kdlln-Strasse 30. W-2000 Hamburg 54 (F.R. G.)

(Received February 25, 1991)

Abstract Owing to a combination of several excellent intrinsic properties, diamond is the ideal material for a wide range of technical applications, from cutting tool coatings to high power, high temperature semiconductor devices. More than 20 years after Eversole's demonstration of the feasibility of diamond growth under conditions of thermodynamic metastability, intensive world-wide research activities emerged, aiming at the development and improvement of processes for the deposition of diamond thin films. This paper gives a short account of the important deposition methods known at present and discusses, as examples, several film properties and applications.

1. Introduction The lattice of a diamond crystal contains an extraordinarily high number density of tightly bound atoms, probably the highest density of all known materials at atmospheric pressure [1]. Because of this, it is not surprising that diamond also exhibits a number of record physical properties [2] such as a Young's modulus of 1050 GPa, a Knoop hardness of 9 0 0 0 - 1 0 000 HK, a thermal conductivity of 2 0 0 0 - 2 1 0 0 W m-1 K-1 (type II at 293 K), an optical transparency of 2 2 5 nm-2.5 /~m and above 6 / z m (type IIa) and several figures of merit related to high temperature, high power, high frequency semiconductor applications, to mention the most important. These properties, aside from its attraction as a gem, make diamond one of the highest-valued materials. The possibility of synthesizing diamond under a low pressure from the gas phase as a metastable carbon allotrope, discovered by Eversole [3], has--more than 20 years later--led to intensive world-wide research activities on the development and improvement of diamond deposition processes and the technological exploitation of diamond thin films. The present paper gives a survey of the most important deposition methods and discusses, as *Paper presented at the 2nd International Conference on Plasma Surface Engineering, Garmisch-Partenkirchen, September 10-14, 1991). 0921-5093/91/$3.50

examples, several properties of polycrystalline diamond films and their technological potential. Emphasis is placed on the reasons and consequences of the marked granularity of diamond films. Besides information from the literature, several results of the work at the FraunhoferInstitut fiir Schicht- und Oberfl~ichentechnik are included.

2. Deposition methods All reliable methods at present known to yield diamond films continuously and on a wide range of substrates are chemical vapour deposition (CVD) processes, i.e. the diamond formation is the result of chemical reactions taking place at the gas-solid interface. In contrast with most other CVD processes, however, some kind of preactivation of the gas phase is necessary in order to end up with the metastable carbon allotrope diamond. Without this pre-activation, graphite (or no deposit at all) is generally formed as a result of ordinary pyrolysis. The activation can be done by combustion of an oxygen-hydrocarbon mixture (combustion CVD), a hot surface with T> 2000 °C in the vicinity of the substrate [hotfilament CVD (HFCVD) or thermal CVDI or a gas discharge sustained by d.c., r.f. or microwave power (plasma CVD). © Elsevier Sequoia/Printed in The Netherlands

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The basic set-up for HPCVD and microwave plasma CVD (MPCVD), process parameters and some film properties have been described previously by another research group [4, 5]. In both processes, mixtures of about 1% methane and hydrogen at pressures between 10 and 100 mbar were used to deposit polycrystalline diamond films at substrate temperatures between 800 and 1000 °C with growth rates of the order of 1 /~m h-l.

Subsequently several other deposition methods were invented, mostly using electrical discharges to activate the gas phase at pressures between 0.1 and 1000 mbar. Considerably increased growth rates were achieved by coupling d.c., r.f. or microwave power to plasma jets at pressures of several hundred millibars. The maximum growth rate so far, 1 mm h- 1, was observed during experiments with a d.c. plasma jet at about 250 mbar and 10 kW electrical power, but the coated area was limited to less than 2 cm 2 [6]. A survey of the most important diamondforming CVD processes published recently by Bachmann and Lydtin [7] shows for nearly all processes known at present a mutual exclusion of high growth rates and large coatable areas. High rate processes with linear growth rates around 100/~m or even more are in general limited to 2 or 3 cm:, at most. The process that one should choose for deposition depends on the specific application of a diamond coating. For example, for cutting tool inserts coated in a batch process with diamond films of 5 ktm thickness, there is hardly any economical benefit in increasing the growth rate above 2 /~m h -~. An increased reactor capacity, on the contrary, decreases the production costs per insert markedly [8]. That is why the H F C V D method is well suited to this application. It combines a simple set-up and reasonable growth rates of several micrometres per hour (from methane-hydrogen mixtures, several tens of micrometres per hour can be achieved, if suitable oxygen-containing gases are used [9, 10]) with promising scale-up potentialities. At our Institute, 2 in silicon and tungsten substrates have been coated with polycrystalline diamond films, using a mechanically and geometrically stable arrangement of four straight tantalum wires 2 mm in diameter [11]. Numerous experiments with a total turn-on time in excess of 150 h could be run without replacing filaments. Experiments showed that filament-induced periodic film thickness non-uniformities were neglig-

ible, if the filament-to-filament distance did not exceed the filament-to-substrate distance substantially, in agreement with results of finite difference calculations of thermal and concentration fields. However, it turned out in these experiments that blowing the source gas from a narrow nozzle directly onto the substrate considerably increased the local growth rate. An example is shown in Fig. 1, representing the spatial diamond growth rate distributions on silicon substrates due to methane(0.5%)-hydrogen jets blown from a nozzle with a 0.5 mm or a 2.0 mm diameter, at 10 mm distance from the substrate surface. These results verify the existence of a depletion layer with a reduced concentration of growth species near the substrate under free convection conditions. The film growth rate is probably too small to cause a depletion of the total carbon content in the gas phase. A more likely explanation is ( 1 ) the rapid conversion of methane to acetylene near the hot filaments similar to what has been observed by Harris et al. [12] and, probably even more effectively, due to the four-filament arrangement used and (2) the low efficiency of acetylene as a diamond-forming species. The latter hypothesis was tested in an experiment with a 0.5% acetylene-hydrogen mixture blown directly onto the substrate. The resulting growth rate distribution is shown in Fig. 2. Instead of the maximum found with a methane-hydrogen mixture under otherwise unchanged conditions, there is now a slight depression in the growth rate distribution where

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the gas jet impinges on the substrate. This result indicates that, under conditions of HFCVD diamond growth, acetylene presumably does not play an important role in the growth process. In fact, recent theoretical considerations [13] and flow tube experiments [14] show that the methyl radical CH3, generated by the reaction between methane and atomic hydrogen, is the dominant growth species. The presence of a superequilibrium concentration of atomic hydrogen around hot filaments has recently been demonstrated by two-proton laser-induced fluorescence experiments [ 15].

3. Film properties and technological potential The growth of diamond films on a wide range of substrates (the exceptions are diamond itself, cubic BN and possibly SiC) is a typical example of Volmer-Weber-type growth [16], characterized by small adhesive forces between substrate and deposit and a large work required to form a nucleus. Lowering of the nucleation barrier by inhomogeneities on the substrate surface plays an important role. A special substrate pre-treatment is generally required in order to increase the density of diamond nuclei. Scratching with diamond powder is the easiest way to cover the substrate surface with submicron-sized diamond particles acting as nuclei for subsequent epitaxial growth of CVD diamond. Nevertheless diamond films are commonly relatively coarse grained, with lateral grain dimensions of the order of 1 ,am or even larger, depending on the substrate pre-treatment and the film thickness.

The low adhesion between the deposit and substrate and the resulting granularity of diamond films have several important implications for film properties and applications. The following section is intended to discuss, for example, the consequences for cutting tool coatings, optical windows and heat conducting layers. 3.1. Cutting tool coatings Because of the excellent mechanical stability of the diamond lattice, diamond thin films exhibit very low abrasive wear, as demonstrated by a comparison with metal-containing and metal-free diamond-like films (Table 1). In spite of a considerable effort spent by several (mainly Japanese) companies, CVD-diamond-coated tools have not yet attained a market share worth mentioning, because of--above all--the limited success of attempts to achieve a sufficient filmsubstrate adherence and because of the still relatively high costs of diamond CVD. Coating of tungsten carbide cutting tools or sintered polycrystalline diamond products is rendered difficult by a cobalt content of several per cent, favouring the deposition of graphite instead of diamond. Several means of substrate pre-treatment were tested at the Toshiba Tungaloy Co., including grinding, etching (to remove cobalt from the surface layer), heat treatment also to remove cobalt from the surface layer and decarburization, resulting in an improved but still insufficient service life during A1-Si alloy cutting [17, 18]. An imaginable solution t o the adhesion problem might be the introduction of some kind of intermediate layer between the refractory metal carbide and diamond. Wong et al. [l 9] were able to enhance the adhesion on a molybdenum sub-

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strate by an intermediate evaporated silicon film with a thickness of several nanometres, supposedly converted to molybdenum-silicide and SiC under the conditions of diamond deposition, thereby providing strong atomic bonding to the diamond film. Aside from the direct CVD coating of cutting tools, low pressure deposition of diamond can be used to produce free-standing films, which are then glued to the tool. This approach was proposed by Asahi Diamond Industrial Corporation and can be used to circumvent several problems inherent in the direct coating process [20]. It is, however, limited to tools with simple cutting face geometries. 3.2. Optical and X-ray windows Diamond has several intrinsic physical properties (transparency, mechanical and chemical stability, good thermal conductivity and a low expansion coefficient) which make it a nearly ideal window material for electromagnetic radiation covering several decades of wavelengths. CVD diamond thin films have been proposed and are being developed as wear-resistant chemically inert coatings on optical devices, as antireflection layers and as high power optic material [21]. Films 0.4/am thick supported on a silicon grid are already being commercialized as X-ray windows (Crystallume, U.S.A.). A challenging application lies in the field of synchrotron-based X-ray lithography. The membrane used to carry the stepper mask must, aside from transparency for X-ray radiation, provide at least 50% optical transparency and a minimal scattering in the visible region in order to allow mask alignment with a precision of about 50 rim. In addition, it must have a high modulus of elasticity in order to reduce distortions induced by the mask, a low thermal expansion coefficient, low film stress and extreme flatness. The SiC/W mask technology is now well advanced [22] but, judging by its intrinsic properties, diamond is the ideal material for this application. Because of their coarse-grained morphology, CVD diamond films grown on diamond-powderscratched silicon substrates do not generally meet the severe requirements quoted above. Much smoother surface morphologies and strongly reduced optical scattering can be achieved by an ultrasound pre-treatment of the wafers in an

aqueous suspension of diamond powder, followed by a suitable growth process. This is demonstrated in Fig. 3, a photograph of a membrane of 5.5 y m thickness prepared from a diamond film grown by an MPCVD process from an H 2 - C O - C H 4 mixture at a 917 °C substrate temperature [23] on silicon. Using an optical microscope, even micron-sized features can be resolved through the membrane, placed at a distance of a few tens of microns from the object. In spite of the fact that the average thermal expansion coefficient of diamond is smaller than that of silicon, films of 1-6/am thickness grown between 900 and 940 °C were in a tensile intrinsic stress state at room temperature, irrespective of the carbon source used in the study quoted above. Growth temperatures between 800 and 820 °C, on the contrary, yielded films with compressive stress. By optical in situ measurements of the wafer's radius curvature R, the development of tensile stress components at the growth temperature was traced in a number of growth experiments. A representative selection of results is shown in Fig. 4. At 800 °C (full circles) a tensile growth stress component appears, which after cooling to room temperature (open circle) is overcompensated by the compressive "thermal" component due to different expansion coefficients. At 919 °C much larger tensile stresses are introduced during growth (full squares) which can

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50% higher thermal conductivities can be obtained, if the ~3C "impurity" content is reduced from 1.1% (natural content) to 0.1% [27]. Using the expressions for phonon-phonon, boundary and isotope scattering published by Nepsha et al. [28], the effect of a reduced 13C concentration on the thermal conductivity of a diamond film with 1 #m crystallites can be estimated. The theoretical result is of the same order of magnitude (35% increase at room temperature for 0.1% 13C) as for 1 mm single crystals (25% increase).

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be compensated no longer by thermal stress after cooling (open square). A more detailed discussion of these results will be published elsewhere [23]. For the moment, these examples are meant to demonstrate the capabilities of tailoring diamond film properties to a specific application.

3.3. Heat-conducting layers Nowadays several ten thousand (natural or high-pressure-grown) single-crystal diamond chips per month are already used as heat "sinks" for discrete high power electronic devices such as medium wave oscillators or solid-state fightemitting diodes. CVD diamond technology offers the opportunity for new ways [24] of using the excellent thermal conductivity of this material in heat-spreading -- and simultaneously electrically insulating -- layers. A coarse-grained film structure, being a disadvantage in many other applications, is beneficial as far as thermal conductivity is concerned. Boundary scattering of phonons reduces the room temperature conductivity markedly for crystallite dimensions below 100 ~m [25]. In films with 1 ktm crystallite dimensions, about 1000 W m ~ K-] has been achieved at room temperature, 50% of the value of natural type II crystals, while in finer-grained films the thermal conductivity is more strongly reduced [26]. Furthermore recent experiments proved that, in single crystals of a few cubic millimetres, about

Nearly three decades after the publication of Eversole's pioneering investigations, the low pressure deposition of diamond is now developing into a mature technology with an increasing number of applications shown to be feasible or commercialized already. Nevertheless several open questions have still to be answered and several problems must be overcome. At present the most important targets of future application-oriented work are (1) lowering the costs of low pressure diamond synthesis, e.g. by an increase in the coatable area or of the deposition rate, depending on the specific application, (2) achievement of better film-substrate adherence, especially for cutting tool applications, (3) deposition of fine-grained films with low scattering and simultaneously low absorption for optical applications and (4) heteroepitaxial deposition on low cost substrates for electronic applications.

Acknowledgments The author is indebted to Mr. R. Six for the diamond film preparation and to Mr. H. H/ibsch for abrasive wear measurements. The work was partly done at Philips Forschungslaboratorium Hamburg, funded by the Bundesministerium fiir Forschung und Technologie under Grant 13N5607.

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17 S. Takatsu, Proc. Int. Congr. for Advanced Coating Technologies, Wiesbaden, 1989. Demat Exposition Managing, Frankfurt, 1989, p. 457. 18 K. Shibuki, M. Yagi, K. Saijo and S. Takatsu, Surf. Coat. Technol., 36 (1988) 295. 19 M. S. Wong, R. Meilunas, T. P. Ong and R. P. H. Chang, AppL Phys. Lett., 54 (1989) 2006 20 t3. Lux and R, Haubner, Refract. Met. Hard Mater., 8 (3) (1989) 158. 21 S. Singer, Proc. 3rd Int. Congr. on Optical Science and Engineering, The Hague, 1990, Vol. 1275, to be published. 22 H. Liithje, M. Harms, B. Matthiessen and A. Bruns, Proc. 1989 Int. MicroProcess Conf., 1989, Jpn. J. Appl. Phys., 28(1 l)(1989) 2342. 23 L. Sch~iferand C.-P. Klages, Diamond Films '90, Proc, 1st Eur. Conf. on Diamond and Diamond-Like Carbon Coatings, Crans-Montana, September 17-19, 1990, Surf. Coat. Technol., 47( 1991 ) 13-21. 24 K. V. Ravi and M. I. Landstrass, Proc. 1st Int. Symp. on Diamond and Diamond-Like Films, Los Angeles, CA, 1989, Proc. Vol. 89-12, Electrochemical Society, Pennington, NJ, 1989, p. 24. 25 D. T. Morelli, C. P. Beetz and T. A. Perry, J. Appl. Phys., 64 (1988) 3063. 26 A. Ono, T. Baba, H. Funamoto and A. Nishikawa, Jpn. J. Appl. Phys., 25 (1986) 1-808. 27 T. R. Anthony, W. E Banholzer, J. F. Fleischer, Lanhua Wei, P. K. Kuo, R. L. Thomas and R. W. Pryor, Phys. Rev. B, 42 (1990) 1104. 28 V. I. Nepsha, N. E Reshetnikov, Yu. A. Kluyev, G. B. Bokii, and Yu. A. Pavlov, Soy. Phys.--Dokl., 30 (1985) 547.