Thin Solid Films 420–421 (2002) 166–171 0040-6090/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved. PII: S0040-6090 Ž02 .00804-0 Growth and characteristics of carbon films with nano-sized metal particles Wan-yu Wu, Jyh-ming Ting* Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan, ROC Abstract Metal containing carbon thin films were prepared using a reactive sputter deposition technique. A number of growth parameters, including argon to methane ratio, working pressure, electrode distance, and d.c. power were examined. Metallic elements in the resulting films are uniformly-distributed nanoparticles with various sizes, depending on the metal type and growth parameters. Correlation was made among the argonymethane ratio, the deposition rate and the composition. Crystallinity and microstructure of the resulting films were analyzed using transmission electron microscopy and Raman microscopy. � 2002 Elsevier Science B.V. All rights reserved. Keywords: Diamond-like carbon; Sputter deposition; Nano-particle 1. Introduction Diamond-like carbon (DLC) thin film was first obtained using an ion beam deposition technique w1x. It is known that DLC films exhibit excellent tribological properties especially against steels w2,3x. Numerous works on the preparations, and the characterization of chemical w4,5x, optical w6x, electrical w7–9x, and mechan- ical properties w10,11x have been reported. However, high internal film stresses and insufficient adhesion of DLC films to the substrates have also been found to limit their practical applications. Therefore, metallic elements were introduced into DLC films to reduce the stresses and enhance the adhesion. These films have been since then called metal-containing DLC (Me- DLC), in which metallic particles are embedded in a DLC matrix w12,13x. A number of metals have been used in the Me-DLC thin films. They include Au, Cr, Cu, Fe, Nb, Ni, Pt, Si, Sn, Ta, Ti and W. Studies were performed to investigate the nature of the metallic particles and the particles size distribution in Me-DLC films w14,15x. It was found that the mechanical, tribo- logical, and electrical properties are all strongly influ- enced by the size and density of metallic particles in the films. For example, electrical conductivity may be increased by 12 orders of magnitude through varying the metal content of the film w16,17x. Hardness values, *Corresponding author. Tel.yfax: q886-6-238-5613. E-mail address:
[email protected] (J.-m. Ting). ranging from 5 to 20 GPa, and Young’s modulus, from 50 to 200 GPa, have been reported for different Me- DLC thin films w18–22x. Recent researches suggest several new applications for Me-DLC films w23–25x. For examples, Me-DLC can be used as microelectrodes in micro-electrochemical analysis or as emitters in advanced field emission devices. However, reports also indicate the need of correlation among the growth parameter, film composi- tion, and film structure w26,27x. For this purpose, we have investigated three types of Me-DLC, namely, Ni- DLC, Pt-DLC, and Cu-DLC. A reactive sputter deposi- tion technique was used for the growth of the Me-DLC thin films. It is noted nickel forms carbides with carbon, copper and carbon are immiscible, and platinum is a noble metal. 2. Experimental Metal containing DLC thin films were deposited using a d.c. magnetron sputter deposition technique. The 3-in. metal targets were nickel (99.99%), platinum (99.99%) and copper (99.95%). Single crystal wafers of (1 0 0) Si were used as the substrates. The sputter deposition system was evacuated to a pressure lower than 6.67=10 Pa prior to the deposition and then filledy3 with argonymethane gas mixture to a desired deposition pressure of either 6.67 or 1.33 Pa. The argonymethane ratios were 1y1, 2y1, 3y1, 4y1, 5y1, 6y1 and 7y1. The 167W.-y. Wu, J.-m. Ting / Thin Solid Films 420 –421 (2002) 166–171 Fig. 1. Low angle X-ray spectra of Cu-DLC obtained under various AryCH ratios. The intensity is in an arbitrary unit.4 d.c. powers were 100 and 200 W. The electrode distances were 40 and 70 mm. The deposition time was kept at 30 min. The substrates were not heated during the deposition under all the conditions. However, the sub- strate temperature was determined to be near but less than 100 8C due to bombardment of the energetic particles during the deposition. Compositions of the resulting Me-DLC thin films were examined using energy dispersive spectrometry. Surface morphologies and cross-section views of Me-DLC thin films were examined using scanning electron microscopy (SEM). Deposition rates of Me-DLC films were determined from the thickness measured on the SEM cross-sectional images. Crystallinity and microstructure of the Me-DLC thin films were investigated using low-angle X-ray diffractometry, transmission electron microscopy (TEM), and micro-Raman microscopy. 3. Results and discussion A serious of Me-DLC specimens, consisting of a thin film carbon with metallic nanoparticle inclusions, was obtained. It was found that the carbon exhibits an amorphous structure while the metallic nanoparticles are either amorphous polycrystalline. However, in Ni-DLC thin films, a minor amount of polycrystalline nickel carbide was also found. In all the cases, the X-ray spectra vary with the AryCH ratio in a manner similar4 to that shown in Fig. 1 for Cu-DLC. At the lowest ratio of AryCH only an amorphous structure is seen. As the4 ratio increases the feature of amorphous structure dimin- ishes while crystalline peaks of metal appears. This trend continues such that only crystalline peaks of metal show in the X-ray spectra when the AryCH ratio4 reaches a threshold value. The threshold values for Ni-, Cu-, and Pt-DLC are 5y1, 4y1 and 2y1, respectively. As the ratio increases further the X-ray characteristic peaks remain (Fig. 1) but with greater intensities. It is noted that the formation of crystalline Ni accompanies a small amount of nickel carbide. As discussed later, a higher AryCH ratio leads to a higher metallic compo-4 sition or a lower carbon composition. It is therefore believed that a sufficient amount of metallic constituent is required to allow the metal to form crystalline struc- ture. The AryCH ratio also affects the surface mor-4 phology of the Me-DLC thin films. It was observed that a granular structure develops as the AryCH ratio4 increase for both Ni-DLC and Cu-DLC thin films. It seems therefore to indicate that the development of such a granular structure is related to the increase in the metallic composition in the films. It is also found that the granular structure is more pronounced in the case of Cu-DLC. On the other hand, the surface appears featu- reless for all the Pt-DLC thin films. Fig. 2a shows the deposition rates of various Cu-DLC thin films as a function of AryCH ratio. It is obvious4 that the deposition rate increases with the AryCH ratio.4 The three curves shown in Fig. 2a also compare depo- sition rates for films obtained under different pressures and electrode distances. It appears that a lower pressure (top curve vs. middle curve) or a shorter electrode distance (middle curve vs. bottom curve) gives a higher deposition rate. It was also found that the atomic concentration of Cu increases in a manner similar to that of the deposition rate (Fig. 2b). When the Ary CH ratio increases, more copper atoms are sputtered4 due to the increased sputtering gas Ar, and less carbon atoms are produced from the dissociation of the reduced methane. Since the sputter deposition rate of pure Cu is higher than that of reactive sputter deposited DLC, the deposition rate increases with the AryCH ratio. How-4 ever, the thermalization distance of carbon is longer than that of copper (Table 1) w28,29x. This would favor the deposition of carbon but not copper. As a result, under lower AryCH ratios, the increase in the copper4 concentration in the film, and therefore the deposition rate, is not significant. When there is a sufficient amount of copper in the discharges, i.e. under a high enough AryCH ratio, to overcome the loss during the thermal-4 ization, the increase in the copper concentration in the film, and therefore the deposition rate, becomes signifi- cant as shown in Fig. 2. Consequently, for an element exhibiting a thermali- zation distance that is closer to that of carbon than Cu, one would expect the increase in the deposition become 168 W.-y. Wu, J.-m. Ting / Thin Solid Films 420 –421 (2002) 166–171 Fig. 2. (a) Deposition rates and (b) atomic compositions of Cu-DLC thin films. Fig. 3. Deposition rate and Pt concentration of Pt-DLC thin film as a function of AryCH ratio.4 Table 1 Normalized values of thermalization distance for carbon, copper, nick- el, and platinum w28,29x Element C Cu Ni Pt Thermalization distance 1.00 0.39 0.37 0.52 Fig. 4. (a) Deposition rates and (b) atomic compositions of Ni-DLC thin films. significant at a lower AryCH ratio. This is shown by4 the use of Pt as the target material (Table 1). The deposition rate of Pt-DLC thin film increases very fast in the beginning and rises to a maximum when the Ary CH ratio is only at a value of 2, as shown in Fig. 3.4 Beyond this ratio the rate levels off. As expected, the behavior of Pt concentration as a function of AryCH4 resembles that of the deposition rate (Fig. 3b). The Pt concentration levels off at approximately 90%, repre- senting the saturation of Pt concentration in the film. A similar correlation among deposition rate, film composition, and two of the deposition parameters, i.e. pressure and electrode distance, was also found for Ni- DLC. Fig. 4a shows the deposition rates of various Ni- DLC thin films as a function of AryCH ratio. It is4 obvious that the deposition rate increases with the Ary CH ratio. The three curves shown in Fig. 4a also4 compare deposition rates for films obtained under dif- ferent d.c. powers and pressures. It is seen that a higher 169W.-y. Wu, J.-m. Ting / Thin Solid Films 420 –421 (2002) 166–171 Fig. 5. Metal concentration as a function of deposition rate for all three types of Me-DLC thin films. All the thin films were prepared under the same conditions of a power of 100 W, a pressure of 1.33 Pa, an electrode distance 40 mm, and a sputtering time of 30 min. Fig. 6. Bright field images and diffraction patterns of (a) Ni-DLC; (b) Cu-DLC; and (c) Pt-DLC thin films. power (top curve vs. bottom curve) or a lower pressure (middle curve vs. bottom curve) gives a higher deposi- tion rate. The order of deposition rate is then Groups F)B)E (Fig. 4a). Likewise, for the nickel concentra- tion, a higher power (middle curve vs. bottom curve) or a lower pressure (top curve vs. bottom curve) also gives a higher Ni concentration. However, the order of nickel concentration becomes Groups B)F)E (Fig. 4b). It is clear that, unlike Cu-DLC film, the order of deposition rate does not dictate the order of nickel concentration. It is believed that the previous argument holds except an additional factor has to be included into the argument. As the d.c. power is doubled, more methane is ionized such that more carbon presents in the discharge. Furthermore, it is noted that carbon has a higher thermalization distance than Ni (Table 1). There- fore, the contribution of carbon to the deposition rate becomes more important. This explains why Group B has a higher Ni concentration but a lower deposition rate than Group F. 170 W.-y. Wu, J.-m. Ting / Thin Solid Films 420 –421 (2002) 166–171 Fig. 7. (a) Raman spectra of a Cu-DLC specimen with obtained. (b) The I yI ratio and the FWHM as a function of the copperD G concentration. From the above discussion, it is concluded that irrespective of the metal used, the metal concentration and the deposition rate are in general found to have a strong correlation. A monotonic relation as shown in Fig. 5 can therefore be obtained. Fig. 5 shows the metal concentration as a function of the deposition rate for all three types of Me-DLC thin films, which were all prepared under the same conditions. Specimens were also examined using TEM and select- ed area or micro-diffraction technique. The bright field images of all three types of Me-DLC obtained under 100 W and 1.33 Pa are given in Fig. 6. The particles within the carbon matrices are in general spherical nano- sized particles and distribute uniformly through the carbon matrices. The sizes of all three types of metallic nanoparticles also appear to be quite uniform in Cu- DLC and Pt-DLC. Nanoparticles of Cu and Pt are smaller than 5 nm. On the other hand, the average nanoparticle sizes in the Ni-DLC thin films range from 5 to 15 nm. They increase with the Ni concentrations in the films. Furthermore, the size uniformity decreases with decreasing Ni concentration or increasing deposi- tion pressure. As in the XRD analysis discussed above, diffraction patterns from the TEM analysis also indicate an amorphous nature of small nanoparticles, while a polycrystalline nature of large nanoparticles. Also, car- bide of nickel tends to be found in extremely small nanoparticles. Finally the Raman spectra were investigated for Cu- DLC specimens. Both the D-band and G-band signatures were detected as shown in Fig. 7a for a 41.6 at.% Cu- DLC thin film. The I yI ratio and the full width atD G half maximum (FWHM) were examined as a function of the Cu concentration, as shown in Fig. 7b. It is seen that the I yI ratio increases with Cu concentration,D G indicating a smaller carbon cluster size at a higher Cu concentration. This would have given a larger FWHM. However, the FWHM decreases with the Cu concentra- tion. This suggests that through the addition of copper the film stresses are greatly released. The stress relieve is strong enough to overcome the effect of reducing carbon cluster size on the FWHM. 4. Conclusions Me-DLC (Ni, Cu, Pt) thin films were obtained using a d.c. sputter deposition technique under various depo- sition conditions. The crystallinity gradually appears with the AryCH ratio such that an amorphous structure4 occurs at the lowest AryCH ratio, while crystalline4 structure starts to appear at a threshold AryCH ratio.4 The deposition rate was found to increase with the Ary CH ratio. Irrespective of the metal used, the metal4 concentration and the deposition rate are found to have a monotonic relation. The metallic elements exist in the resulting films as uniformly-distributed nanoparticles with various sizes, depending on the metal type and growth parameters. Both the Cu and Pt nanoparticles were found to be less than 5 nm, while Ni from 5 to 15 nm. Raman spectra were also obtained for Cu-DLC thin films. The I yI ratio was found to increase with CuD G concentration, while the FWHM decreases with the Cu concentration, indicating a great deal of stress release due to the addition of Cu into DLC thin films. Acknowledgments This work was supported by National Science Council in Taiwan under Grant no. NSC-90-2218-E-006-050. References w1x S. Aisenbergerg, R. Chabot, J. Appl. Phys. 42 (1971) 2953. w2x K. Enke, Thin Solid Films 80 (1981) 227. w3x T. Mori, Y. Namba, J. Appl. Phys. 55 (1984) 3276. w4x B. Dischler, A. Bubenzer, P. Koisl, J. Appl. Phys. Lett. 42 (1983) 636. w5x C. Wild, P. Koidi, Appl. Phys. Lett. 51 (1987) 2950. w6x J. Robertson, Adv. Phys. 35 (1986) 317. 171W.-y. Wu, J.-m. Ting / Thin Solid Films 420 –421 (2002) 166–171 w7x A.A. Khan, J.A. Woollam, IEEE Electron Dev. Lett. 4 (1983) 146. w8x A.A. Khan, J.A. Woollam, Y. 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