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, 2004, 46, . 4 Precursors for CVD growth of nanocrystalline diamond T. Soga, T. Sharda, T. Jimbo Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Nagoya 466-8555, Japan Research and Development, Seki Technotron Corp., Tokyo 135-0042, Japan E-mail: soga@elcom.nitech.ac.jp Variuos routes to grow nanocrystalline diamond films by chemical vapor deposition technique are reviewed.

Among various routes, NCD films deposited on mirror polished silicon substrates by biased enhanced growth by microwave plasma chemical vapor deposition are described in detail. Qualitative concentration of NCD was assessed by Raman spectroscopy and X-ray diffraction patterns of the films. The hardness of the films approaches to that of natural diamond at optimized conditions while still having low amount of stress (< 1GPa).

1. Introduction been realised in the last few years, new routes have also emerged that mostly aim to grow smooth NCD films. In Conventional polycrystalline diamond films composed this paper, based on the literature, some of the growth of micrometer size of diamond crystal (microcrystalline routes of NCD are briefly discussed followed by description diamond MCD) grown by chemical vapor deposition on some properties of NCD film grown particularly using (CVD) have surface roughness that limits its uses in many biased enhanced growth.

of the potential areas. For example, the high surface roughness is a major problem for machining and wear 2. Nanocrystalline diamond growth applications [1,2]. Two solutions can be suggested to routes overcome the problem of high surface roughness, either post-polishing can be adopted or naturally grown smooth 2.1. Hydrogen def i ci ent gas- phase. One such films can be developed without compromising much with process that has been studied in detales was developed at their useful properties. However, post-polishing is expensive Argonne National Laboratory, USA [11,12]. In this process, and time consuming [35] and it may be considered to carbon dimer (C2) is used as a reactive species in hydrogen be better to concentrate on as-grown smooth and hard deficient (CH4/Ar or C60/Ar) microwave plasma CVD [11].

films [47]. Nanocrystalline diamond (NCD) films, that have C2 is produced by replacing molecular hydrogen by argon superb tribological properties, can be a better alternative and using CH4 or C60 as precursor gases and also by using of post-polished conventional chemical vapor deposited N2/CH4 as the reactant gases in a microwave plasma CVD diamond films. Moreover, smooth NCD films will be used (MPCVD) system. The NCD films grown on diamond as DNA chip, electrochemical electrode, heat sink, SAW seeded substrates by this technique are composed of filter, MEMS, NEMS, etc. Some studies (growth method 3-15 nm diamond crystallites with up to 110% sp2 carbon properties and applications) that have been carried out on residing at the boundaries [10]. Lin et al. [13] studied the NCD in various laboratory all over the world can be found CH4/H2/Ar process in hot filament CVD (HFCVD) system in detail in the most recent review article [8].

and proposed a compositional map, which demarcates A common feature of the majority of the deposition regions for a well-faceted diamond growth, NCD and nontechniques of CVD diamond films is a high concentration diamond deposition. Well-faceted MCD (210 m) grow up of hydrogen gas (H2) used as one of the constituents with to 90% Ar. They reported change in microstructure from some hydrocarbon gas such as CH4. The high concentration MCD to NCD, with a grain size smaller than 50 nm, at of H2 results in the generation of large flux of atomic 95.5% Ar addition.

hydrogen, which is generally believed to play a central The effects of addition of nitrogen to CH4/H2 have also role in the various diamond CVD processes. The growth been reported. Wu et al. [14,15] grew NCD films using of diamond is described to take place mostly via surface N2/CH4/H2 in MPCVD system. NCD films with grain size processes of addition and abstaction of radicals from the of 8 nm embedded in a-C matrix was obtained without gas-phase [9,10]. Before realizing the importance of growing any hydrogen. The diamond crystallite size in their films naturally smooth surface, the aim in the area of CVD increases from 20 to 50 nm while increasing hydrogen from diamond was to maximize the crystalline quality of CVD 5 to 10 sccm, respectively.

diamond. However, diamond grown under non-optimum conditions, such as lower hydrogen concentration or higher Lee et al. [16,17] developed a low temperatures process carbon activity in the plasma, gives films with small grain (350 < T < 500C) for low power but high growth rate size, e. g., several nanometers. (up to 2.5 m/h) NCD film by MPCVD. They used CO-rich In the already existing ways to grow smooth NCD films, CO/H2 mixtures and obtained smooth NCD films consisting a number of techniques and conditions have been employed. of 3040 nm grain size. The temperature at which the As the need of having as-grown smooth diamond films have peak growth rate is obtained in this temperature window Precursors for CVD growth of nanocrystalline diamond decreases with increasing the CO/H2 ratio. Recently, Teii et to 100 nm size micro-polycrystalline diamond particles with al. [18] also reported growth of NCD films of size 20 nm at each of the particle consisting of 1 to 20 nm crystallites low pressure (80 mTorr) at 700C by inductively coupled and associated grain boundaries in aqueous suspension.

plasma employing CO/CH4/H2 and O2/CH4/H2. In this Gohl et al. [26] deposited nanodiamond powder coating by case a positive biasing of 20 V vas applied to the substrate dielectrophoresis technique. They used the nanodiamond to reduce the influence of ion bombardment. The films powder of size 110 nm produced from the shock synthesis consisted of ball shape grains of size 100 nm, which are on a Si tip array, rough Si stumps and flat standard mirrorfurther composed of 20 nm NCD grains. polished Si substrates. Xu et al. [27] deposited nanostructured diamond coating on etched silicon substrates 2.2. Bi as enhanced nucl eat i on/ growt h usi ng by dielectrophoresis method using 5 nm nanodiamond CH4/H2 g a s s y s t e m. In the case of the growth route for powder, produced by explosives, by suspending them in NCD by hydrogen deficient plasmas (section 2.1), mostly ethanol. Maillard-Schaller et al. [28] deposited 45nm the substrates were pretreated externally before deposition size diamond nanoparticles on flat Si(100) substrates by either ultrasonically or mechanically using diamond or other electrophoresis/dielectrophoresis. Hiraki [29] grew the abrasive powders. There is another well-established method diamond films at 200C by nanodiamond seeding.

to nucleate diamond internally in the conventional growth of MCD films called biased enhanced nucleation (BEN) [19] in which the substrates are biased negatively hydrocarbon3. Biased enhanced growth of NCD films rich mixture of hydrocarbon-hydrogen precursor gases.

This method, which results in a high density of diamond In this section, we will discuss the growth of NCD by the nucleation (1010 cm-2 or more), is the first step followed by BEG process in detail. The NCD films were grown by the another step or two steps to grow heteroepitaxial diamond BEG process in a 2.45 GHz Seki Technotron Corporation, films. In order to achieve overgrowth of diamond on these Japan (formerly Applied Science and Technology, USA) nuclei, the BEN process is followed by conventional growth made the MPCVD system. The mirror-polished Si(100) in which the bias is put off and the growth is continued substrates were kept on a Mo holder that rests on a with lower hydrocarbon percentage in the gas-phase. Is was graphite susceptor. No diamond powder of any other observed that in the later stages of growth, when the biasing ex situ treatment was performed prior to the depositions.

is put off and hydrocarbon to hydrogen ratio is reduced, The substrate assembly was immersed in 5% methane only the stable part of the nuclei continue to grow while and hydrogen plasma. In a special arrangment to the remaining get etched off in the process due to increased substrate assembly, a quartz shield was used to cover the concentration of hydrogen in the gas-phase [20].

conducting parts of the subtrate holder assembly (other than To obtain an NCD film, Sharda et al. [2123] suggested the substrate). This assembly enhances the bias current to achieve higher densities of diamond nucleation, similar density when a negative bias is applied to the substrate to the BEN process, and, second, in the later stages to at low microwave powers without affecting the microwave maintain the same high density and to continue their growth plasma. The whole growth was performed for 60 min in a throughout the process. In order to materialize this idea, single stage run without breaking the bias to the substrate, they extended the BEN in an MPCVD system for the whole unlike the conventional two or three stages process for the growth process and termed their process biased enhanced heteroepitaxial growth of diamond [30,31]. Applied biasing growth (BEG) in which they obtained diamond nucleation voltage was varied from 200 to 320 V while keeping other and growth on silicon substrates in a single process. The parameters constant. Films were grown at a pressure of details of the BEG process will be described later.

30 Torr ( 4000 Pa) with a microwave power of 1000 W 2.3. Di amond seedi ng. Yang et al. [24] grew at 600C. The substrate temperature was measured using a transparent diamond films with a crystallite size below thermo-couple at the bachside of the substrate holder.

70 nm in hydrogen and methane microwave plasma CVD Structural characterizations of the films were carried on quartz substrates, ultrasonically pretreated by 0.5 m out using Raman spectroscopy, X-ray diffraction (XRD), diamond powder, for 30 min. Grain size and surface rough- scanning electron microscopy (SEM) and atomic force ness were observed to decrease with increasing methane microscopy (AFM). The laser Raman spectra were obtained concentration. A significant reduction in grain size seems to in the range 10001700 cm-1 with a step of 1 cm-1. An occur at 3% methane, which further reduces at 4% methane Ar+-laser ( = 488 nm) of 200 m diameter spot size was though the nanodiamond concentration in the films also used for recording the spectra. Hardness of the films was goes up. The crystallite size, as estimated by TEM, near measured by nano-indentor (UMIS-2000) using a Berkovich the interface was 30 nm and increases to 65 nm near the diamond pyramid.

growth side. The stress in the films was calculated by measuring In another method, a diamond layer can be coated on the radius of curvature of the substrates before and after substrates by dielectrophoresis or spraying method. Zhu the deposition using a modified Stoneys equation. The et al. [25] attached thin nano-structured diamond films curvature of the films was measured by the Alpha-to silicon substrates by spraying or brushing technique profiliometer. The length of a scanned sample segment using commercially available, produced by explosives, 10 was 5 mm.

, 2004, 46, . 704 T. Soga, T. Sharda, T. Jimbo 4. Properties of NCD films grown by biased enhanced growth A high resolution SEM photograph of the NCD film grown at 200 V is shown in Fig. 1. Careful observation of the micrograph reveals that the film consists of bunches of sharp faceted nanocrystallites of a size less than 30 nm.

The transmission electron microscopic examination of the same film showed nanocrystallites confined in the form of oriented nanodiamond tubes of a size of 1030 nm and a height of a few hundreds of nm. Although, this structure resembles the columnar or dendritic kind of growth, nanocrystals confining in nanodiamond tubes is a unique feature of this new approach and method presented in this article. These nanodiamond tubes, having a high packing density of crystallites, appeared to be nearly parallel to the film growth direction. The corresponding electron diffraction patterns were indexed to diamond with the [111] and [220] diamond textured rings having their texture maxima parallel, respectively, to [111] and [220] Si Figure 2. Typical Raman spectra of NCD film and MCD film.

diffraction spots when superimposed. This indicated that majority of the NCD crystallites are preferentially oriented to the Si substrate. On the other hand, the film grown at 320 V, 5% CH4/H2 and 600C substrate temperature showed a very different microstructure. This film also had a high packing density, however, of randomly oriented diamond crystallites of size 25 nm. The detailed structure of the NCD film will be published elsewhere [32].

Fig. 2 shows the typical Raman spectra of the nanocrystalline films and conventional polycrystalline diamond film.

The most significant feature in the Raman spectrum of the NCD film is the intense peak near 1150 cm-1 without any feature near 1332 cm-1, an unambiguous signature of cubic crystalline diamond. Although the origin of this peak is assigned to the trans-polyacetylene [33], it is widely accepted to be related to NCD [34]. Absence of any peak near 1332 cm-1, in spite of having an intense NCD feature, could be due to high density of defects incorporated in the Figure 3. Raman spectra of NCD films at the bias voltage of 200 (a) and 320 V (b).

films but could also be a sign of uniformly distributed shortrange sp3 crystallites in the films [17,35]. Other significant bands near 1350 and 1580 cm-1 are well-known graphitic D and G bands. The Raman feature near 1500 cm-1 may be related to the disordered sp3 carbon in the films [34].

The intensity of this band increases proportionally with the intensity of the NCD feature in the films as observed by others also [35]. High density of defects and a significant amount of graphitic carbon are, in fact, expected in the growth of uniformly distributed short range nanocrystals of Figure 1. A high resolution SEM micrograph of an NCD sample. diamond because of their large grain boundary area.

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