Mechanical properties of nanostructed diamond-like carbon films synthesized by low energy cluster beam deposition

NanoStructuredMaterials.Vol.4.No.6.pp.759-767,1994 Copyrigla© 1994ElsevierScienceLtd PrintedintheUSA.Allrightsreserved 0965-9773/94$6.00+ .00

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MECHANICAL PROPERTIES OF NANOSTRUCTURED DIAMONDLIKE CARBON FILMS SYNTHESIZED BY LOW ENERGY CLUSTER BEAM DEPOSITION

V. Paillard*, P. M~linon*, J.P. Perez*, V. Dupuis*, A. Perez*, J.L. Loubet**, H, Pascal**, A. Tonck**, M. Fallavier***, *D6partement de Physique des Mat6riaux (U.A.C.N.R.S. 172) Universit6 Claude Bernard - Lyon I, 43 Boulevard du 11 Novembre 1918 69622 Villeurbanne C6dex, France **Laboratoire de Tribologie et Dynamique des Syst~mes Ecole Centrale de Lyon, B.P. 163, 69131 Ecully C6dex, France ***Institut de Physique Nucl6aire de Lyon Universit6 Claude Bernard - Lyon I, 43 Boulevard du 11 Novembre 1918 69622 Villeurbanne C6dex, France

(Accepted June 1994) Abstract---Mechanical properties of carbon-films obtained by low energy neutral cluster beam deposition (LECBD) have been measured using the nanoindentation technique. Three selected size distributions centered around C2o, C6o and C9oo have been deposited on various substrates at room temperature. The films with a nanostructured morphology (grain size around 15 to 25 nm) conserve a memory of the specific character of the free clusters which is sp3for C2o, spe for C9ooand intermediate (sp23)for C6o. The densities of thefilms (0.8 to 1.1 g.cm -3) are lower than most polymers but their hardnesses (3 to 12 GPa ) are comparable to those of many DLC-films. These results are discussed taking into account the particular structures of the free clusters and the nucleation and growth mechanism specific for the LECBD technique, which corresponds nearly to a random stacking of incident clusters. 1. INTRODUCTION Diamond and diamond-like carbon (DLC) films are of great interest for many technological applications because of their physical and chemical properties: high hardness and wear resistance, chemical inertness, infrared Iransparency and low electrical conductivity. Both pure (a-C) or hydrogenated (a-C: H) DLC-films are commercially available, but the highest potential is held by diamond films if they can be produced with uniform structure and low roughness. DLC-films are generally elaborated using a wide variety of techniques (CVD, magnetron sputtering...) where the role of ions and their energy is essential. These films are often considered as partially crystallized with small graphitic domains (sp2-bonded) dispersed in a more or less diamond-like (sp 3hybridized) amorphous matrix (1). 759

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In this paper we report the mechanical properties of nanostructured DLC-films synthesized using a new method: the low energy neutral carbon cluster beam deposition. The low energy neutral cluster beam deposition technique (LECBD) has already been used to grow thin f'dms of various types of materials (covalent, metallic) with specific structures and properties (2, 3). The most important aspect of the technique concerns the growth mechanism which is essentially governed by the cluster size. When the cluster size is high enough (> 5 nm) to limit their diffusion, the substrate coverage takes place by a nearly random stacking of the incident clusters, leading to the formation of nanostructured films (4-5). Moreover, the "soft landing" process characteristic of the LECBD technique prevents the incident cluster fragmentation. This is an interesting effect to conserve the memory of the free clusters, since in the gas phase the clusters exhibit some original structures (crystallographic and electronic) depending on their size (6, 7). Carbon clusters have been extensively studied for many years and particularly since the discovery of fullerenes by Kroto and Smalley (8). It is now established that small carbon clusters containing less than 20 atoms have a linear or cyclic structure (sp or sp 2 hybridization), while all even numbered clusters with 20 or more atoms can take the form of a closed hollow cage known as fullerene (9,10). In this case, the hybridization degree depends on the cluster size and vary from sp 3 in C2o or C24 to sp 2 with increasing size (11), the hybridization of C60 being calculated equal to sp 2"3 (11). Our goal, using the LECBD technique, was to elaborate thin carbon films which conserve some properties of the free clusters. For this purpose we focused our attention on three particular size distributions of carbon clusters, easily produced in our generator and controlled by time of flight mass spectrometry. The first one was centered around the smallest fullerenes C20 - C32, the hybridization of which is nearly sp3. The second distribution was centered around C60 for its mixed state, and the third one centered around C90ocorresponded to sp2-bonded amorphous carbon clusters. In a previous paper (12) we reported the memory effect of the free cluster in our f i l l s deposited at room temperature. In particular, the characteristic Raman spectra exhibited the sp2-character of the C9oo-fills while the complete phonon density of states of fcc-diamond was observed for the C2o-films (12, 13) (sp3-character); the C6o-films being intermediate (12). The specific structures and properties of our fills were also confirmed by several complementary characterization techniques: electron and X-ray diffraction, electron energy loss spectroscopy (EELS) and electrical conductivity measurements (14). In particular, the electrical conductivity decreased from 10-2 (C90o-films) to 10-8 £21.cm"1(C20-films) and the gaps (Eo4) varied from 0.60 (C90o-films) to 1.43 eV (C2o-films), in good agreement with a decrease of the 7r-electron contribution from C90o to C20-films as observed by EELS and XANES (12,15). Finally, taking account of the granular structure of our Cn-films, observed by atomic force microscopy (AFM) or scanning electron microscopy (SEM), associated with the variable character from diamond to graphite, it appeared interesting to follow their mechanical behaviors using the nanoindentation technique. 2. EXPERIMENTAL PROCEDURE

Intense carbon cluster beams in a wide range of size up to - 1000 atoms were produced in a laser vaporization source similar to the one described by Smalley et al. (16). A plasma created by the impact of a Nd: YAG laser beam (wavelength 532 nm) on a graphite rod or disk was thermalized by injection of a high pressure He-pulse (3 to 5 bars during 150 to 300 ~ts). The thermalization authorizes the nucleation of atoms and cluster growth and the following isentropic

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expansion in a good vacuum (< 10-6 mbar) cools and stabilizes the clusters. The lifetime of unstable clusters (with dangling bonds) is greater than the transit time to the substrate where they can be definitely stabilized by gaining energy. This occurs by saturation of the dangling bonds when adjacent clusters nucleate (13). The cluster beam was analyzed in a time of flight mass spectrometer prior to deposition, either using nascent ion clusters or photoionized neutral clusters with an UV-Dye laser pumped by an XeCI Excimer laser. The three selected size distributions centered around C20, C6o and C90o are presented in Figure 1. They were obtained by varying different adjustable parameters of the source: laser fluence, helium flow, delay between laser pulse and gas pulse, geometry of the nozzle. Depositions were performed on various substrates at room temperature: optical grade polished sapphire for indentation tests and AFM observations, beryllium for Rutherford backscattering analysis (RBS) and silicon for hydrogen content and thickness measurements. During deposition, the ionized clusters were deflected in front of the sample holder and only the neutral clusters were deposited in order to prevent any charge effect capable of influencing the nucleation mechanism. Aquartz balance for deposition rate measurements could be positioned in place of the sample. Stable rates from 0.1 to 1 nm/minute were obtained in the range of cluster sizes investigated. Rutherford backscattering analysis (RBS) was performed using 2 MeV alpha-particles produced in a Van de Graaff accelerator. For carbon films deposited on beryllium substrates, the backscattering edge of beryllium appears at a lower energy than carbon, which allows a good sensitivity for the detection of the carbon peak and also other peaks due to impurities present in the films (i.e. oxygen), except hydrogen. In this last case, analysis was performed by nuclear reaction (1H (15N, o~7)12C)using 15N2+-ions accelerated in a 4 MV Van de Graaff. The absolute number of carbon atoms per unit surface deduced from the RBS analysis, associated with thickness measurements performed with a Talystep measurer, allowed the determination of the real film densities. Atomic force microscopy (AFM) observations have been performed in air using a Parks Instruments system. Carbon films deposited on optical grade polished sapphire substrates were used in this case. The topographic images were performed in air with classical silicon nitride pyramidal tips (typical radius of curvature: 50 nm). The contact force was estimated to be 5.10 -8 N and the scanning speed was 1 I.tm/s. Repeated sequences of observations on the same area were systematically performed and the images compared in order to verify if any cluster displacements with the tip or electrical charge effects taken place. The nanoindentation technique was used to determine the mechanical properties of our carbon films (thickness - 200 nm) deposited on optical grade polished sapphire substrates. The device specially developed for sphere-plane interaction studies (17), in the Laboratoire de Tribologie et Dynamique des Syst~mes was used, with a diamond tip instead of the sphere. During the indentation process, the indentation depth, the resulting normal force and the contact stiffness were continuously recorded. The stiffness was obtained using the superimposed vibrating mode (18). Such a process allows mechanical property measurements as a function of the indentation depth with a single test. The resolutions of forces and displacements were great enough (10 -8 N and 10-1° m, respectively), in such a way that the lower limit of available indentation depth was given by the tip defect, surface heterogeneities and mechanical deformation modes associated with such low deformations. The effect of the tip defect was previously studied by indentation experiments on reference layers of homogeneous known materials (low roughness gold sputtered coatings). The involved experiments were made with a trigonal indenter with an angle of 115.2°

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between the edges (Berkovitch tip). Its tip defect corresponded to a material lack of 4 nm height. Taking into account this defect, a plastic penetration depth of 2-3 nm was sufficient to evaluate the mechanical properties of the surfaces if the contact area was precisely known. The contact area was calculated with great care from the penetration depth, taking into account the elastic deformation of the surface and the plastic flow around the indentation print (18). The substrate effects on the Young's modulus was also taken into account by means of a modelization (19).

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Figure 2. AFM images of C20 (a) and C9oo-films (b) on optical grade polished sapphire subslrates. 3. DENSITY MEASUREMENTS The density values obtained with the C20, C60 and C90o-films are 0.8, 0.8 and 1.1 g.cm -3, respectively. They are incredibly low, lower in fact than those of most polymers, despite the low hydrogen contents. Hydrogen concentrations of 7 to 8.5 at.%, 4 to 4.5 at.% and 1.5 to 2 at.% in the C2o, C60 and C9oo-films have been respectively measured. These results corroborate previous infrared absorption measurements (FTIR), showing a larger hydrogenation of the C2o-films

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compared to C6o and C9oo-films (12). They are also in agreement with the presence of dangling bonds in small fullerenes (one per atom in C2o) compared to the larger and unreactive closed shell fullerenes as C6o or C70. One has to mention that the energy loss of nitrogen ions in the Cn-films determined during the nuclear reaction analysis was much lower than in graphite, confirming the highly porous structure of the films. It is interesting to compare the low density of the C20 and C6ofilms (- 0.8 g.cm 3) to that of the most dense phase of pure C60 (fcc crystal) which reaches the value of 1.68 g.cm °3 (20). On the other hand the density of the C9oo-films (-1.1 g.cm "3) is slightly lower than the glassy carbon one (- 1.3 g.cm-3). 4. Cn-FILM MORPHOLOGY AFM images obtained with C20-films (thickness 3.4 nm) and C9oo-films (thickness 2.3 nm) are presented in Figure 2 a, b. The image for the C60-film is similar to the C20-one. The mean grain size is about 15 nm in the C20 and C6o-films and 25 nm in the C90o-film. The mean sizes of supported carbon particles compared to the sizes of incident clusters (- 0.5 nm, 1 nm and 1.4 nm for C20, C60 and C90o, respectively) indicate that a nucleation process occurs on the surface of the substrate. The mean diameter of the grains does not increase significantly in thick films as shown in Figure 3, corresponding to a scanning electron microscopy (SEM) observation of a 200 nm thick C20-film in which the mean size of the grains is of the order of 25 nm. Finally, the carbon layers are granular from the first steps of growth up to the continuous films, as already observed for films of other materials synthesized by low energy cluster beam deposition technique (2-5).

Figure 3. SEM micrograph of a 200 nm thick C20-film on a Be-substrate.

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TABLE 1 Hardnesses (H) and elastic moduli (E) measured with C2o, C6o and C9oo-films. Ranges of values reported in Refs. 19 and 20 for various DLC-samples are also given for comparison. Sample

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5. NANOINDENTATION EXPERIMENTS Characteristic examples of the load-displacement curves measured with 150 nm thick films are presented in Figure 4. Hardnesses (H) and elastic moduli (E) deduced from these measurements are given in Table 1. Comparable values of H and E are encountered in DLC-films having densities in the range 1.5 to 2 g.cm -3 (21, 22), except for the C2o-film the hardness of which is similar to that of a polymer. However, in this last case the elastic modulus value is much higher than the usual values for polymers. It is remarkable that the most diamond-like film (C20-film sp3bonded) is the softer one, while the C90o-film strongly sp2-bonded is quite hard. A qualitative interpretation of this effect could be found from the previous Raman spectroscopy study (12,13,

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In fact, the Raman spectrum of the C20-film is very similar to the phonon density of states of fcc diamond but with two main differences: a red shift of 130 cm 4 and an enhancement of the transverse acoustic branches. Using the formalism developed by Sokel and Harrison (24), we have shown that such phenomena are correlated to a decrease of the material force constants compared to diamond (13,23). Considering the structure of fullerenes of the C20-type, the conservation of a sp3-1attice is not possible at long range because of the symmetry group. The C2o-cluster (Ih symmetry) has a 0 ~ bond angle of 110° (109028' in diamond), which prevents the formation of pure t~-bonds (11). Consequently, the misaligned hybrids induce rc contributions which diminish the bond strength. Furthermore, the samples contained some sp2-bonded defects attributed to very small clusters (less than 20 atoms) present in the mass distributions (Figure 1) which cannot take a cage structure (8, 25). These linear or cyclic clusters can also be responsible for a lower hardness. The presence of these defects is sometimes observed in the Raman spectra in the form of a weak G-band located around 1580 cm 1 near the optical modes of disordered diamond around 1150 cm-1 Qualitative arguments can also be involved to explain the mechanical behaviors of the C6o and C9o0-films. Pure C6o-samples are not harddue to the Van der Waals bond type between clusters (26). This is easily understood having in mind that C6o is an inert particle without any dangling bond (8). In our case, as the samples are elaborated from a size distribution in which many entities may have at least one dangling bond, we can form a film of intrinsically hard and elastic particles as closed shell fullerenes (27) dispersed in a cluster matrix strengthened by some strong a-bonds. The C900-filmproperties could be interpreted in a similar way if we suppose that a sp2-bonded cluster has surface defects in the form of dangling bonds. Therefore, several adjacent clusters could be linked by t~-bonds. In this case, we obtain a kind of DLC-film formed by sp2-hybridized clusters bonded to each others with a-bonds.

6. CONCLUSIONS Considering the granular structure and the very low density of our carbon cluster films, their mechanical properties become exceptional. In fact, we have synthesized non-hydrogenated carbon thin films less dense than most polymers but as hard as many DLC-films, showing the potentialities of the low energy cluster beam deposition technique. It would be interesting in the future to produce these samples after a mass selection of the incident clusters in order to obtain a better specification (sp3 or sp2) of the character of the film. Up to now, the only adjustable parameter to elaborate the carbon films was the incident mean cluster size. Other parameters which influence the nucleation and growth mechanisms such as the substrate temperature, the particle flow, the presence of ion clusters naturally produced in thelaser vaporization source, could be tested to improve the quality of the films.

ACKNOWLEDGEMENTS We are indebted to Mrs. D. Joslin from Oak Ridge National Laboratory - US A, who provided the sapphire substrates used for the nanoindentation tests.

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