Properties of carbon ion deposited tetrahedral amorphous carbon

Properties of carbon ion deposited tetrahedral amorphous carbon films as a function of ion energy Shi Xu, B. K. Tay, H. S. Tan, Li Zhong, Y. Q. Tu, S. R. P. Silva, and W. I. Milne Citation: Journal of Applied Physics 79, 7234 (1996); doi: 10.1063/1.361440 View online: http://dx.doi.org/10.1063/1.361440 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/79/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Highly tetrahedral amorphous carbon films with low stress Appl. Phys. Lett. 69, 2344 (1996); 10.1063/1.117519 Nanocrystallites in tetrahedral amorphous carbon films Appl. Phys. Lett. 69, 491 (1996); 10.1063/1.117763 Hydrogen‐free amorphous carbon films approaching diamond prepared by magnetron sputtering Appl. Phys. Lett. 69, 158 (1996); 10.1063/1.116906 Influence of ion energy flux on structure and optical properties of a‐C:H thin films J. Appl. Phys. 79, 7676 (1996); 10.1063/1.362432 Raman scattering, laser annealing and pressure‐optical studies of ion beam deposited amorphous carbon films AIP Conf. Proc. 120, 465 (1984); 10.1063/1.34715

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Properties of carbon ion deposited tetrahedral amorphous carbon films as a function of ion energy Shi Xu,a) B. K. Tay, H. S. Tan, Li Zhong, and Y. Q. Tu School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 2263, Singapore

S. R. P. Silva and W. I. Milne Engineering Department, Trumpington Street, Cambridge University, Cambridge CB2 1PZ, United Kingdom

~Received 22 August 1995; accepted for publication 23 January 1996! Ion energy, controlled by the substrate bias, is an important parameter in determining properties of films deposited by the filtered cathodic vacuum arc technique. The substrate bias determines the ion energy distribution of the growth species. The ion energy is varied, while keeping the other deposition conditions constant, in order to study the effect of ion energy on the film properties. The films were characterized by their optical and mechanical parameters using an ellipsometer, surface profilometer, optical spectrometer, and nanoindenter. Electron energy-loss spectroscopy and Raman spectroscopy were used for structural analysis of the films. © 1996 American Institute of Physics. @S0021-8979~96!01309-X#

I. INTRODUCTION

II. EXPERIMENTAL DETAILS

The filtered cathodic vacuum arc ~FCVA! technique has been studied extensively since it was reported to be an efficient method of producing high-quality hard coatings free of macroparticles at room temperature.1–4 Tetrahedral amorphous carbon ~ta-C! film is one of the more interesting materials deposited by this technique. It has been shown by using electron energy-loss spectroscopy ~EELS!5,6 that as much as 80% of the carbon atoms in the ta-C films produced by the FCVA technique form an amorphous tetrahedral ~sp 3! structure. The high s p 3 content in the ta-C films results in unique properties that range from extreme hardness, chemical inertness, and high electrical resistivity to high thermal conductivity. As a result of these favorable properties the ta-C films could be useful for electronic, optical, and mechanical applications.7–9 In the FCVA system the ion energy is controlled by changing the substrate bias which determines the properties of the ta-C films. The ta-C film properties have been correlated to the carbon ion energy by other researchers.4–8,10,11 Different optimum ion energies for maximum s p 3 content and density were reported. Experimental results obtained are not directly comparable since only the substrate bias is analyzed without considering the ion energy distribution. In this article, properties of the ta-C films are extracted from closely controlled experiments and the relation between the substrate bias and the ion energy is carefully established. The sp 3 content, plasmon energy, hardness, compressive stress, optical constants, and optical band gap of the ta-C films are examined as a function of carbon ion energy. The results are discussed in terms of the growth model for ta-C films proposed by Robertson.12,13

Figure 1 shows the schematic diagram of the FCVA deposition system. Carbon ions are produced in a vacuum arc discharge between the cathode and the grounded anode. The cathode is a 60-mm-diam, 99.999% pure graphite disk mounted onto a water-cooled stainless-steel block. The arc is ignited by bringing the striker to the cathode surface and is afterwards self-sustaining for a few minutes unless the power is cutoff. A radial electric field is introduced in our system via the torus duct wall bias and this, coupled with the curvilinear axial magnetic field on a curved toroidal duct, forms the crossed electric–magnetic-field filtering assembly. This assembly effectively filters out the unwanted macroparticles and neutral atoms due to the nonline-of-sight substrate leaving only the plasma which is steered by the field, through the duct, to the substrate in the deposition chamber. A copper coil trap is also installed into the duct to enhance the macroparticle filtering efficiency. To increase the effective area of deposition, magnetic coils and driving circuitry are placed around the exit of the plasma duct to scan the plasma beam on the substrate plane. In our 150-mm-diam deposition chamber, an area of 70370 mm2 can be deposited with a uniformity of 61%. The system allows the deposition of several samples at various substrate dc bias voltages in a single run. The substrate was negatively biased from 0 to 2120 V in this set of experiments.

a!

Electronic mail: [email protected]

A. Sample preparation

The system base pressure was below 2.031024 Pa but rose to 1.531023 Pa during deposition due to outgasing of the cathode. The arc current was set to 60 A, the toroidal magnetic field fixed at 40 mT, and the duct electric bias at 15 V. All depositions were carried out with the substrate held at room temperature. The substrates used were ^100& n-type silicon wafers of average thickness 0.28 mm and Vitreosil quartz of average thickness 0.49 mm. Both the silicon and quartz surfaces were precleaned using detergent and de-

7234 J. Appl. Phys. 79 (9), 1 May 1996 0021-8979/96/79(9)/7234/7/$10.00 © 1996 American Institute of Physics [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.227.184.224 On: Mon, 23 Feb 2015 09:54:30

FIG. 1. Schematic diagram of the FCVA system.

ionized water in an ultrasonic bath. Buffered hydrofluoric acid was also used during the cleaning process to remove the native oxide layer on the silicon water surface. A series of ta-C films was grown under a variety of substrate bias voltages. The deposited film thickness was about 60 nm. Under the conditions described, the deposition rate at the substrate was estimated to be 0.5 nm/s over an area of 25 cm2. B. Control of ion energy via substrate bias

The plasma beam potential in our system was obtained with a Langmuir probe from the voltage measured corresponding to zero ion current. For the deposition conditions described above the plasma potential was found to be 213 V. If an electrically floating substrate is placed in the plasma beam, the substrate will have the same potential as the plasma beam, and the ions reaching the substrate will possess their original ion energy. However, if the substrate is dc biased the ions will then be accelerated or decelerated within the Debye shielding length when they reach the substrate. The original ion energy in the plasma was measured by the Faraday cup. The result is shown in Fig. 2. It is seen that the peak of the ion energy is about 28 eV and the full width at half maximum is about 18 eV. This is in agreement with measurements by Martin et al.3 The actual ion energy reaching the substrate can be calculated by

E i 5e ~ V p 2V b ! 1E o ,

~1!

where E i is the ion energy reaching the substrate, e is the electron charge, V p is the plasma potential ~213 V!, V b is the substrate bias, and E o is the original ion energy in the plasma ~28 eV!. Equation ~1! is valid since over 95% of carbon ions in the plasma are C 1 in our system. Both V p and E o are deposition condition and machine dependent. The depositions were made with ion energies varying from 15 to 135 eV.

C. Film characterization

The properties of the deposited films were characterized in the following manner. The refractive index and thickness of the films were measured by using ellipsometry at a wavelength of 632.8 nm. A Dektak 3030 surface profilometer was used to confirm the film thickness and determine the film stress. The optical properties of the films were assessed over the range 250–900 nm on a Perkin Elmer Lambda 16 UV/ VIS spectrometer. The surface roughness of films was examined using a Park Scientific atomic force microscope ~AFM!. The plastic and elastic properties of the films were investigated using a nanoindenter. Electron energy-loss spectroscopy ~EELS! was performed on a Philips CM30 transmission electron microscope ~TEM! fitted with a Gatan 666 spectrometer operated at 100 keV with a collection angle of 12

J. Appl. Phys., Vol. 79, No. 9, 1 May 1996 Xu et al. 7235 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.227.184.224 On: Mon, 23 Feb 2015 09:54:30

FIG. 4. Graph of plasmon energy as a function of impinging carbon ion energy. FIG. 2. Ion energy distribution of filtered beam at 40 mT magnetic filter field.

mrad in order to determine the plasmon energy and the fraction of s p 3 bonding in the film. Raman spectroscopy was also used to assess the ta-C film structure.

and surface defects were observed. The ion energy has little effect on the surface morphology of the ta-C film.

B. EELS and Raman measurement III. EXPERIMENTAL RESULTS A. Film surface roughness

The AFM surface topography indicates that the film surface is very smooth and results in a root mean square ~rms! surface roughness, over an area of 131 mm2 for ta-C films deposited on silicon of about 0.4 nm. Few macroparticles

FIG. 3. Graph of s p 3 fraction as a function of impinging carbon ion energy.

The fraction of sp 3 bonding in the film was determined from the 285–310 eV region of the carbon K-edge EELS spectra.14 Each carbon K-edge EELS spectrum consists of a peak at 285 eV due to the excitation from 1s to empty p* states of sp 2 sites, followed by a step at around 289 eV due to transitions from 1s level to empty s* states of both sp 2 and sp 3 sites. The fraction of sp 2 sites in any sample may be evaluated by normalizing the area of its 285 eV peak to the area of a large section of the spectrum and then comparing to the value obtained for graphitized carbon which is 100% sp 2.6 Figure 3 shows the variation of sp 3 content for different ion energies. It is seen that the sp 3 content rises from 79% at ion energy 15 eV to a maximum of 87.5% at ion energy 75 eV, then decreases to about 82% when the ion energy increases to 135 eV. The plasmon energy of each film was measured from low-energy EELS. The peak of the spectrum corresponds to the plasma resonance frequency vp . For diamond, the plasmon peak energy E p 5h v p is 33 eV,15 while that of graphite is 27 eV. The plasmon peak position, therefore, can be used to obtain the valence electron density and is a good indicator as to the fraction of sp 3 bonding in the film. Figure 4 shows the plasmon peaks of various samples deposited under different ion energy. The plasmon peak increases from 30.2 eV to a maximum of 31.2 eV when the ion energy increases from 15 to 75 eV. After that, the plasmon energy decreases from the maximum point to 30.5 eV when the ion energy further increases to 135 eV. The trend correlates well with the fraction of sp 3 carbon in the films. The resonant plasmon frequency v p is given by the expression16

7236 J. Appl. Phys., Vol. 79, No. 9, 1 May 1996 Xu et al. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.227.184.224 On: Mon, 23 Feb 2015 09:54:30

TABLE I. EELS characterization of ta-C films.

Sample no.

Ion energy ~eV!

s p 3 fraction ~%!

Plasmon energy ~eV!

Film density ~kg/m3!

1 2 3 4 5 6 7

15 35 55 75 95 115 135

79.1 84.1 86.5 87.5 86.6 85.3 81.9

30.2 30.4 30.8 31.2 30.9 30.7 30.5

3.263103 3.283103 3.333103 3.373103 3.343103 3.323103 3.293103

v 2p 5

n ee 2 , e 0m

~2!

where n e is the local valence electron density, and e and m are the charge and effective mass of electron respectively. The EELS data allow further characterization of the film in terms of the valence electron density and hence the film density. The results are tabulated in Table I, where the density has been normalized to that of graphite. It shows that the density of all samples is close to that of diamond ~3.523103 kg/m3!. The Raman scattering spectrum of the films has been measured and exhibits a broad Raman intensity distribution in the range 1400–1700 cm21 centered at 1550 cm21 which agrees with results reported by other researchers.17,18 It confirms that the films are amorphous. C. Film stress measurement

The stress of the deposited film is always found to be compressive due to the generation of s p 3 bonds. The stress has been determined by the radius of curvature technique which compares the curvatures of the bare silicon substrates and substrates covered with a thin film. The stress s s in a thin film of thickness t c is given by Stoney’s equation

s s5

S

D

t 2s 1 1 Es 2 , 6 ~ 12 n s ! t c R R 0

~3!

where E s , n s , and t s are the Young’s modulus, Poisson ratio, and thickness of the substrate. R and R 0 are the radii of curvature of the film–substrate composite and bare substrate, respectively. Figure 5 shows the dependence of compressive stress on ion energy. For each film examined by the profilometer, several separate areas of the film were measured and the variation of the results gives the deviation. It is observed from the figure that the stress increases from about 2.4 GPa to a maximum value of about 10 GPa when ion energy increases from 15 to about 90 eV then the stress decrease to about 9 GPa when the ion energy increases further to 155 eV. These stresses are comparable to those measured by McKenzie and co-workers5 and Fallon et al.6 Robertson12,13 proposed that the incident flux becomes implanted at subsurface positions when the ion energy exceeds a threshold value needed to penetrate the surface layer. This gives rise to a local quenching which results in an increase in the local stress and gives rise to an increase in the sp 3 bonding and hence the film density. The penetration

FIG. 5. Graph of compressive stress as a function of impinging carbon ion energy.

probability increases rapidly with ion energy above the threshold, but any excess ion energy will be evolved as heat. This local heating then allows a relaxation of the density increment according to the thermal spike model of Seitz and Koehler.19 Thus, these two processes compete to give a net increase in strain e or density increment Dr/r as

e'

Dr f 5 , r 110.016p ~ E/E 0 ! 5/3

~4!

where E is the ion energy, E 0 is the activation energy of the relaxation process, and p is a material diffusion parameter of order unity from the thermal spike model. f was taken as the forward sputtering fraction. A fit of Eq. ~4! to the compressive stress data is seen to follow the data well giving an optimum energy at about 90 eV and is shown in Fig. 5. In our fitting, f is taken as 1.4E 1/2, p'0.1, and E 0 about 3 eV, which are approximately the same as those used by other researchers.4,5,13 D. Plastic and elastic properties

The hardness of the 60 nm ta-C films deposited on silicon substrates was assessed from the loading-unloading curves measured by a nanoindenter. The nanoindenter technique uses an indentation depth below 10 nm to assess film characteristics. The hardness, calculated from the plastic indentation depth of the indentation curve, is high in the surface layer and decreases as the indenter tip approaches the softer silicon substrates. For an indentation depth of about 10% of the film thickness, as shown in Fig. 6, hardness values increase from 36 GPa at ion energy 15 eV to 42 GPa at 75 eV and decreases to 34 GPa at 135 eV. To eliminate the effect of the substrate, a thicker film ~;100 nm! was deposited at an ion energy of 75 eV and its hardness was found to increase from 42 to 59 GPa when the film thickness increased from 60 to 100 nm. The Young’s modulus of the


FIG. 6. Graph of film hardness as a function of impinging carbon ion energy.

film was also extracted from the measured data, as shown in Fig. 7. Young’s modulus values ranging from 295 to 345 GPa were obtained for the various conditions with the maximum value of 345 GPa at ion energy about 75 eV. The Young’s modulus increases from 345 to 507 GPa when the film thickness increased from 60 to 100 nm at the deposition condition of 75 eV ion energy. The data show a good correlation with the hardness results. The mechanical properties of our ta-C films are compared to those reported by Anders et al.20 Their hardness and Young’s modulus values are similar to those of our ta-C films with thickness of 60 nm. However, for our ta-C film with thickness of 100 nm deposited at ion energy 75 eV, FIG. 8. ~a! Graph of refractive index n as a function of wavelength and ~b! absorption coefficient k as a function of wavelength.

higher hardness value ~59 GPa! and Young’s modulus value ~507 GPa! are obtained compared to their reported hardness of 40 GPa and Young’s modulus of 340 GPa. The reason is due to the higher sp 3 contents in our ta-C films, which can be seen from the respective film densities. Our ta-C film at ion energy 75 eV has a density of 3.373103 kg/m3 while their ta-C film has a density of 3.03103 kg/m3. E. Optical measurement

FIG. 7. Graph of Young’s modulus as a function of impinging carbon ion energy.

The refractive index and the absorption coefficient of the films at 633 nm were found to be about 2.5 and 0.04, respectively, compared to the values of 2.49 and 0.71 found by Martin.21 There was little change of refractive index n ~about 2.5! with ion energy. Films produced from the cathodic arc are highly transparent in the long-wavelength region ~from 650 nm up to 25 mm! without any absorption peaks caused by the C—H bond due to the absence of H during deposition. The film trans-


mittance and reflectance was measured and the data converted to give the optical constants n and k. Figure 8 shows the variation of n and k of a ta-C film deposited with a substrate bias of 260 V. The optical band gap varies from about 1.7 to 2.1 eV with the maximum value at a substrate bias of 260 V. IV. DISCUSSION

The properties of ta-C films are predominantly determined by the carbon ion energy. However, the true ion energy reaching the substrate is determined by both the system parameters and the external dc substrate bias, as shown in Eq. ~1!. The system parameters include the plasma potential and the original ion energy of the plasma beam. In order to obtain the true carbon ion energy the system parameters must be measured carefully under all deposition conditions used. In the past, the influence of these system-dependent parameters has not been fully assessed. This partially leads to conflicting optimal ion energy results reported by various researchers using different systems, being 20–40 eV according to McKenzie et al.,5,7 100–150 eV according to Ishikawa et al.10 and Koskinen,11 and 140 eV according to Fallon et al.6 and Veerasamy.17 In this work, the original ion energy arriving at the substrate is carefully determined by means of a Faraday cup which was calibrated using a rf ion-beam source with adjustable known energies. The plasma potential was also determined by an ion current collector probe. The present work has focused on the study of ta-C film properties in terms of ion energy. It was found that the sp 3 fraction, plasmon energy, compressive stress, hardness, Young’s modulus, and energy band gap exhibit a similar dependence on the ion energy, reaching a maximum at around 75–85 eV. The high concentration of the tetrahedral ~sp 3! bonding gives the films their diamondlike properties. The ion peening model employed by Robertson explains the variation in the above results which is caused by an optimum balance between the penetration efficiency of the ions and the graphization initialized by input of too much excess energy. Graphitic properties were observed for ion energy lower than 20 eV where ions fail to penetrate and just stick to the surface to form s p 2 bonded structure, and also at ion energies higher than 100 eV the excess energy relaxes the metastable sp 3 bonding to stable graphitic s p 2 bonding resulting in less diamondlike properties in the film. This is observed in our films when the ion energy is increased above 85 eV. Another important parameter of the deposition process that has not been sufficiently covered by researchers in the past is the effect of deposition rate. The deposition rate in our work was 0.5 nm/s compared to about 0.1 nm/s reported by Koskinen11 and 3 nm/s reported by Veerasamy et al.8 The effect of deposition rate on the properties of the ta-C film is currently under investigation. It is believed that a lower deposition rate is more conducive to the formation of sp 3 bonded C. The ion flux therefore is an important parameter that needs to be carefully studied. It is also of interest to note that the refractive index in the ta-C films studied here is insensitive to change in ion energy and hence the stress. It was reported by McKenzie

et al.22 that the refractive index of the film increases with stress due to elimination of voids. A proportional relationship between the refractive index and stress has been reported for a a-C:H films.23 The nonvariance of the refractive index studied in these films is probably related to the high density and sp 3 content experimentally observed for all our ta-C films in the energy range 15–135 eV. The high density seen throughout the ion energy range studied here suggests that the ta-C films deposited have highly compact structures with little or very few voids present in the films. V. CONCLUSION

The ion energy and plasma potential, which are both deposition condition and machine dependent, have been carefully determined. The structural, optical, and mechanical properties of the ta-C films as a function of the carbon ion energy were studied, and an optimum energy range of 75–85 eV corresponding to a maximum sp 3 fraction was observed. EELS measurement showed that for an ion energy of about 75 eV, an sp 3 fraction of 87.5% was achieved which corresponds to a density of 3.373103 kg/m3, close to that of crystalline diamond. The optical band gap of the film is in the range 1.7–2.1 eV. Hardness and Young’s modulus values for ta-C films ~60 nm thickness! varying from 34 to 42 GPa and 295 to 345 GPa were observed, respectively. A thicker ta-C film ~;100 nm! deposited at an ion energy of 75 eV shows higher hardness and Young’s modulus values of 59 and 507 GPa, respectively. A good fit for the stress versus energy data was obtained using Robertson’s model for the growth of ta-C films. ACKNOWLEDGMENTS

The authors wish to acknowledge the help of Nano Instruments for carrying out the mechanical property measurement of the films and the Nanyang Technological University for providing a research grant ~NTU-CUED collaboration CUED-03! for this work. 1

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