Optimization of nitrogenated amorphous carbon films deposited by dual ion beam sputtering

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1 Materials Science and Engineering B64 (1999) Optimization of nitrogenated amorphous carbon films deposited by dual ion beam sputtering L.K. Cheah *, X. Shi, E. Liu, B.K. Tay, J.R. Shi Ion Beam Processing Laboratory, School of Electrical and Electronic Engineering, Nanyang Technological Uni ersity, Block S1, Singapore , Singapore Received 26 January 1999; received in revised form 5 May 1999; accepted 12 May 1999 Abstract Optimization of deposition conditions for nitrogenated amorphous carbon (a-c:n) films prepared by a dual ion beam sputtering (DIBS) technique is reported. A ripple structure was observed on the surface of a-c:n films, which was believed to be corresponding to the off-normal incidence bombardment by N ions during deposition. Infrared spectra indicated that the nitrogen atoms incorporated were bonded as C N, C N and C N in the carbon network. The relative intensity of D and G bands obtained by fitting the Raman spectra showed that the sp 3 content in the a-c:n films increased as the Ar ion energy was increased and the sp 3 content was the highest with 100 ev N ion bombardment. The maximum micro-hardness achieved was about 25 GPa for the 200 nm thick a-c:n films deposited under the optimized conditions. The compressive stress ranged from 1 to 3 GPa. The optical band gap determined by spectral ellipsometry ranged from 0.6 to 1.0 ev. The refractive index and extinction coefficient at 633 nm wavelength were about 2.14 and 0.22 for the films deposited under the optimized conditions, respectively. Hardness, stress and optical band gap measurements showed a similar trend with the relative intensity of I D /I G Elsevier Science S.A. All rights reserved. Keywords: Dual ion beam sputtering; Nitrogenated amorphous carbon film; Characterization 1. Introduction The synthesis of carbon-based hard coating materials such as amorphous carbon, hydrogenated amorphous carbon and tetrahedral amorphous carbon has become of increasing interest in surface technology [1,2]. The preparation techniques that have been tried so far include high-pressure synthesis, ion beam deposition (IBD), chemical vapor deposition, plasma-enhanced or ion-assisted evaporation, nitrogen implantation into solid carbon and laser ablation techniques [3]. Several reports on the synthesis of carbon nitride by IBD technique have been published, i.e. ion beam sputtering [4 7] and filtered cathodic vacuum arc [8,9] technique. This deposition technique was made by a carbon ion beam in nitrogen ambient or by an additional nitrogen ion source. The carbon species are produced from reactive ion beam sputtering (IBS) [4,5], magnetron * Corresponding author. Tel.: ; fax: address: lkcheah@gintic.gov.sg (L.K. Cheah) sputtering [6,7], and vacuum arc technique [8,9]. In these techniques, the structure of the deposited films was mostly amorphous in nature [3]. The reports on reactive IBS technique for depositing carbon nitride films are limited. The carbon nitride films prepared by Kobayashi et al. [5] from IBS technique with a single nitrogen ion beam (0.6 1 kev) bombarding a graphite target showed an amorphous structure. Dual ion beam sputtering (DIBS) technique was performed by Su et al. [4]. N ion energy of ev and Ar ion energy of 3 kev were used to produce the C N films. The hardness of the C N was reported to increase with the decreasing N ion energy [4]. In this paper, DIBS technique was used to grow the nitrogenated amorphous carbon (a-c:n) films. The nitrogen ion bombardment energy was varied from 60 to 200 ev and the Ar ion energy was varied from 600 to 900 ev. The deposited films were analyzed in terms of surface morphology, structure, mechanical property and optical property. The optimum deposition condition for producing maximum sp 3 content in the films was determined /99/$ - see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S (99)

2 L.K. Cheah et al. / Materials Science and Engineering B64 (1999) Experimental details 2.1. Sample preparation a-c:n films were synthesized by a DIBS technique with two ion beam sources: one is a 5 cm diaphragm hollow cathode ion source (Ion Tech) and the other is a 3 cm diaphragm RF ion source (Ion Tech). A high purity graphite target (99.99%) was sputtered by Ar ions of ev with an ion beam current density of 3mAcm 2 using the hollow cathode ion source. In order to optimize the operation of hollow cathode and RF ion sources, the operation pressure in the vacuum chamber must be less than torr. Therefore, the Ar gas flow rates for the hollow cathode ion source and its neutralizer were fixed to 2.5 and 2.0 cm 3, respectively. The N and Ar gas flow rates for the RF ion source and its neutralizer were fixed to 2.5 and 2.0 cm 3, respectively. The base and operating vacuum pressures were about and torr, respectively. The growing film was concurrently bombarded by a nitrogen ion beam from the RF ion beam source with bombardment ion energy from 50 to 200 ev and beam current density of 2 ma cm 2. Both the incident angle of the ion beam towards the target and the angle to the substrate were set to 45. Two series of films were deposited: the first series was the a-c:n films prepared under different Ar ion energy ranging from 600 to 900eV with nitrogen ion energy fixed at 100 ev. The second series was the a-c:n films prepared either without N ions or with different N ion energy from 50 to 200 ev with Ar ion energy fixed at 900 ev. Thickness of the a-c:n films was about 200 nm. 100 Si wafers were used as the substrate for all the films. Prior to the film deposition, the Si substrates were exposed to the Ar ion at 500 ev and 6 ma cm 2 for 5 min to remove the surface oxides. 3. Results and discussion The AFM images displayed smooth surface with a roughness around nm for all the a-c:n films. The ripple structures were observed for all a-c:n films, as shown in Fig. 1(a). This ripple structure was not found in amorphous carbon films without N ion bombardment (Fig. 1(b)). The ripple structure was about 20 nm in width from the cross section analysis. The lateral size for the ripple structure for different nitrogen ion energy gave approximately the same width. The ripple 2.2. Characterization The film thickness was measured by a surface profiler (Tencor P10). The surface morphology was determined by an atomic force microscopy (AFM) in tapping mode (Dimension 3000, Digital Instruments). The structure of the a-c:n was determined by a micro-raman spectroscopy (Ramascope, Renishaw) operated with nm argon ion laser at 5 mw in the range cm 1 and Fourier transform infrared spectrum (FTIR) (Spectrum 2000, Perkin Elmer) in the range cm 1. The micro-hardness was determined using the nanoindentation technique (Nano Indenter II, Nano Instruments). The surface profiler was also used to determine the internal stress of the film. The optical properties such as the complex refractive indices and optical band gap were measured by a spectral ellipsometer (UNISEL, ISA Jobin Yvon). Fig. 1. (a) Atomic force microscopy (AFM) for nitrogenated amorphous carbon (a-c:n) surface (b) a-c surface.

3 8 L.K. Cheah et al. / Materials Science and Engineering B64 (1999) 6 11 Fig. 2. Fourier transform infrared spectrum (FTIR) spectra for 60, 100 and 200 ev N ion energy. structure is believed to be caused by off-normal incident bombardment by nitrogen ions and this is in agreement with the results obtained by Cheah et al. [9] and Bradley [10], who reported that the off-normal incidence ion bombardment often produces periodic height modulations on solid surfaces. This ripple or terraced topography has been observed on various amorphous solids such as glass, fused silica and nitrogenated tetrahedral amorphous carbon [9,10]. Few microparticles and pin holes were observed with an optical microscope (200 ). Fig. 2 shows the IR spectra of the a-c:n films deposited with different N ion energy. Four absorption bands were observed. Band I at 2200 cm 1 is commonly observed in carbon nitride films and is ascribed to the stretching mode of the C N bonds [5,11 13]. In addition to band I, an absorption band (band II) was observed at 2000 cm 1 which was also identified as C N by Bousetta et al. [14]. Band III sits in the range of cm 1, which corresponds to the Ramanactive G (1550 cm 1 ) and D (1350 cm 1 ) bands of amorphous carbon [5,11 13]. The G-band represents the graphite-like sp 2 carbon bonds and the D band represents the disordered sp 2 domains. The D and G bands are active in the IR spectra because the stretching vibrations of C N single bonds and C N double bonds occur in this region when the graphite-like and disordered sp 2 bonded carbon atoms are replaced by nitrogen atoms [5]. Band IV in 970 cm 1 represents aromatic rings in the amorphous carbon films [2]. The C N stretching mode, G- and D-bands showed in the IR spectra are in good agreement with Kaltofen et al. [6] and Kobayashi et al. [5] from their C N films by sputtering technique and Polo et al. [11] from their laser ablation of graphite in a nitrogen plasma. The a-c:n film bombarded at 50 ev N ion shows only weak C N stretching mode suggesting the nitrogen content of this film is low. The C N stretching band becomes more distinctive with the increasing N bombardment energy suggesting that the nitrogen content of the film increases as the N ion energy is increased. The IR spectra for the a-c:n films deposited with different primary (Ar) ion energy resemble the spectrum from a-c:n film deposited under 100 ev N ion energy (Fig. 2). This suggests that the nitrogen content of the films is only marginally influenced by the primary Ar ion energy. The micro-raman spectroscopy is a non-destructive method for measuring the bonding structure of materials. Since the sp 2 bonding is more sensitive to the nm laser light, the sp 2 contribution is always dominant in a Raman spectrum. Consequently, the Raman spectrum of the a-c:n films is dominated by the G peak at about 1580 cm 1 and the D peak at around 1350 cm 1, both of which are attributed to the sp 2 bonding. The Raman spectra of different Ar ion energies are shown in Fig. 3. There are four characteristic bands in each spectrum. Band III is a broad peak between 1100 and 1700 cm 1 and can be divided in two peaks, which are known to be associated with graphitic sp 2 bonded carbon (G-band) centered at about 1575 cm 1 and disordered carbon (D-band) centered at about 1360 cm 1. Band II is around 2200 cm 1 which is assigned to C N triple bonds [5,11]. Band I centers at 2800 cm 1, which is the second order of the D and G peaks. Band IV is at 750 cm 1, which is believed to be contributed by graphite-like sp 2 domains [16]. Fig. 3. Raman spectra for 600, 750 and 900 ev Ar ion energy.

4 L.K. Cheah et al. / Materials Science and Engineering B64 (1999) Fig. 4. Typical Raman spectrum of the nitrogenated amorphous carbon (a-c:n) film prepared by 900 ev Ar + and 100 ev N +. The dashed lines are Gaussian fitting. The Raman spectrum results from the inelastic scattering of photons. The total intensity of a Raman signal depends on the Raman scattering cross sections, beam geometry, excitation power and detection efficiency. The Raman spectrum from visible light excitation does not give a direct indication on the sp 3 structural composition of DLC materials. The Raman spectra for the film in this study give a broad band overlaid by G and D peaks as shown in Fig. 3. The contribution from the sp 3 component to the Raman spectra is not explicit. However, the sp 3 /sp 2 ratio in the DLC films can be qualitatively compared by taking the relative intensity of D and G peaks (I D /I G ) as shown by Tay et al. [15]. A typical Raman spectrum with its G and D Gaussian peaks fitted for the a-c:n film (900 ev Ar + and 100 ev N + ) is shown in Fig. 4. Fig. 5 shows the relative intensity of I D /I G for the different Ar ion energy and N ion energy. The relative intensity of I D /I G decreases as the Ar ion energy increases. This indicates that the sp 3 Fig. 5. I D /I G ratios for different Ar and N ion energy. Fig. 6. Micro-hardness vs indentation depth. The inset shows the micro-hardness as a function of Ar ion energy when the N ion energy is fixed at 100 ev. content in the films increases as the Ar ion energy increases [15]. Furthermore, the absolute intensity of the G and D peaks decrease as the Ar ion energy increases, as shown in Fig. 3. This indicates that the sp 2 bonded materials reduces as the Ar ion energy increases. The relative intensity of I D /I G decreases from 1.6 to 1.1 as the N ion energy is increased to 100 ev, and increases to 1.4 as the energy is further increased to 200 ev. This indicates that the sp 2 content is lowest at 100 ev N ion energy. The continuous stiffness measuring technique installed in the Nano Indenter II was used to continuously monitor the film stiffness without the need for discrete loading unloading cycle [17]. The hardness of the a-c:n films of 200 nm thickness on the Si substrate was determined as a function of the indentation depth (Figs. 6 and 7). The higher hardness at the lower indentation depth indicates the film hardness. Then hardness gradually decreases to a constant value, which corresponds to the substrate material at the deeper indentation depth. In the inset of Fig. 6, the hardness in the first series increases from 18 to 25 GPa as the Ar ion energy increases from 600 to 900 ev. In the second series, the hardness increases from 16 to 25 GPa as the N ion energy is increased to 100 ev and decreases to 23 GPa as the energy is further increased to 200 ev, as shown in the inset of Fig. 7. The intrinsic stress was determined by the radius of curvature technique, which compares the curvatures of the sample surface before and after the deposition. The stress in the film is determined by Stoney s equation [9]. The intrinsic stress was found to be compressive in all the samples. In the first series, the compressive stress of the a-c:n increases from 1 to 3 GPa as the Ar ion energy increases from 600 to 900 ev. In the second

5 10 L.K. Cheah et al. / Materials Science and Engineering B64 (1999) 6 11 series, the compressive stress ranges from 2 to 3 GPa. The maximum compressive stress was noticed from the film deposited with N ion energy of 100 ev. The spectral ellipsometry measurement needs the parameterization of optical model as a function of photon energy [18]. The model used for the a-c:n films is Forouhi and Bloomer (F.B.) model [18], which is an optical dispersion relation used to describe both the real and the imaginary parts of the dielectric constant of an amorphous semiconductor. The structural model for the a-c:n layer coated on Si wafer can be composed of four layers. The first layer is the surface roughness of about 0.5 nm peak to valley. The second layer is an a-c:n layer described by the F.B. model. The third layer is the film-substrate interface layer, which simulates the mixture of a-c:n and Si. The fourth layer is the Si substrate assuming its thickness to be infinite. Optical modeling was described in details in ref. 18. The refractive index for the a-c:n film ranges from 2.05 to 2.14 at the wavelength of 633 nm. The films are semitransparent and their extinction coefficient is also determined at 633 nm and range from 0.22 to The spectra for the complex refractive index as a function of photon energy for the a-c:n films are shown in Fig. 8. The optical band gap is derived from a Tauc interband dispersion model [18]. A typical Tauc plot for the a-c:n is shown in the inset of Fig. 9. In the first series, the optical band gap increases from 0.74 to 0.98 ev as the Ar ion energy is increased from 600 to 900 ev. In the second series, optical band gap of a-c:n increases from 0.2 to 0.98 ev as the N ion energy is increased to 100 ev and decreases to 0.72 ev as the energy increased to 200 ev. The trend for the optical band gap measurement is similar to those of hardness and intrinsic stress. Fig. 8. Typical complex refractive index for the nitrogenated amorphous carbon (a-c:n) film prepared at 900 ev Ar + and 100 ev N +. The experimental results show that the films are generally amorphous and lack of long range order. It is noted that the lower I D /I G ratio, higher hardness, higher compressive stress and higher optical band gap were obtained under higher primary Ar ion energy (900 ev). This is in good agreement with Kobayashi et al. [5] who observed that high acceleration energy of the primary ions onto the target is important to form a sp 3 -rich amorphous C N network. The effect of N ion bombardment on the a-c:n film surface reflects the fact that the sp 3 structure is metastable with respect to the sp 2 structure. This mechanism suggests that the relatively low energy radiation ( ev) leads to an increase in film density and compressive stress, hence a higher sp 3 /sp 2 ratio indicated by a lower I D /I G ratio. Experimentally, linear relations are found among the I D /I G ratio, stress, hardness and optical band gap. The stress can be a direct indication of the film density. A model based on the stress effect has been proposed by McKenzie et al. [19]. It explains the formation of the Fig. 7. Micro-hardness vs indentation depth. The inset shows the micro-hardness as a function of N ion energy when the Ar ion energy is fixed at 900 ev. Fig. 9. Optical band gap vs different Ar and N ion energy. The inset shows a typical Tauc plot for determining the optical band gap (900 ev Ar + and 100 ev N + ).

6 L.K. Cheah et al. / Materials Science and Engineering B64 (1999) sp 3 structure by the ion-induced compressive stress. When the ion energy is increased, the generated stress first increases, as observed at the N ion energy from 50 to 100 ev, and then decreases after reaching a maximum. The maximum stress demonstrates that the increase of stress by the ion-beam-induced defect formation is counterbalanced by the annealing effect caused by the excess N ion energy. The decrease of stress, hardness and optical band gap and the increase of I D /I G ration for N ion energy above 100 ev indicate that the sp 3 content decreases when the N ion energy is greater than the optimal ion energy. 4. Conclusions The structural, optical and mechanical properties of the a-c:n films deposited with the DIBS technique were carefully characterized. The condition for obtaining the a-c:n film was optimized. The maximum hardness (25 GPa), maximum compressive stress (3 GPa), maximum optical band gap (0.98 ev) were obtained from the a-c:n film deposited at 900 ev Ar ion energy and 100 ev N ion energy. The minimum I D /I G ratio obtained under this condition indicates that the sp 3 content is the highest in the corresponding film. References [1] B.K. Tay, X. Shi, L.K. Cheah, D. Flynn, Thin Solid Films (1997) 199. [2] S.R.P. Silva, G.A.J. Amaratunga, C.P. Constantinau, J. Appl. Phys. 72 (1992) [3] L. Wan, R.F. Egerton, Thin Solid Films 279 (1996) 34. [4] X.W. Su, H.W. Song, F.Z. Cui, W.Z. Li, H.D. Li, Surf. Coat. Technol. 84 (1996) 388. [5] S. Kobayashi, S. Nozaki, H. Morisaki, S. Fukui, S. Masaki, Thin Solid Films (1996) 289. [6] R. Kaltofen, T. Sebald, G. Weise, Thin Solid Films (1997) 118. [7] P.V. Kola, D.C. Cameron, B.J. Meenan, K.A. Pischow, C.A. Anderson, N.M.D. Brown, M.S.J. Hashmi, Surf. Coat. Technol (1995) 696. [8] L.K. Cheah, X. Shi, B.K. Tay, Electron. Lett. 33 (1997) [9] L.K. Cheah, X. Shi, J.R. Shi, E. Liu, S.R.P. Silva, J. Non-Cryst. Solids 242 (1998) 40. [10] R.M. Bradley, in: J.J. Cuomo, S.M. Rossnagel, H.R. Kaufman (Eds.), Handbook of Ion Beam Processing Technology, Principles, Deposition, Film Modification and Synthesis, Noyes, Park Ridge, NJ, 1989, p [11] M.C. Polo, R. Aguiar, P. Serra, L. Cleries, M. Varela, J. Esteve, Appl. Surf. Sci (1996) 870. [12] C.W. Ong, X.-A. Zhao, Y.C. Tsang, C.L. Choy, P.W. Chan, Thin Solid Films 280 (1996) 1. [13] F.-R. Weber, H. Oechsner, Surf. Coat. Technol (1995) 704. [14] A. Bousetta, M. Lu, A. Bensaola, A. Schultz, Appl. Phys. Lett. 65 (1994) 696. [15] B.K. Tay, X. Shi, H.S. Tan, H.S. Yang, Z. Sun, Surf. Coat. Technol. 105 (1998) 155. [16] N. Nakayama, Y. Tsuchiya, S. Tamada, K. Kosuge, S. Nagata, K. Takahiro, S. Yamaguchi, Jpn. J. Appl. Phys. 32 (1993) [17] L.K. Cheah, X. Shi, B.K. Tay, E. Liu, Surf. Coat. Technol. 105 (1998) 91. [18] X. Shi, L.K. Cheah, B.K. Tay, Thin Solids Films 312 (1998) 166. [19] D.R. McKenzie, D. Muller, B.A. Pailthorpe, Z.H. Wang, E. Kravtchinskaia, D. Segal, P.B. Lukins, P.D. Swift, P.J. Martin, G. Amaratunga, P.H. Gaskell, A. Saeed, Diamond Relat. Mater. 1 (1991) 51..