- Email: [email protected]
Ion beam studies of TiNxOy thin films deposited by reactive magnetron sputtering
Surface and Coatings Technology 180 – 181 (2004) 372–376
Ion beam studies of TiNxOy thin films deposited by reactive magnetron sputtering E. Alvesa,b,*, A.R. Ramosa,b, N.P. Barradasa,b, F. Vazc, P. Cerqueirac, L. Reboutac, U. Kreissigd b
a ´ ITN, Departamento de Fısica, E.N.10, 2686-953 Sacavem, Portugal CFN da Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003 Lisbon, Portugal c ´ ´ 4800-058 Guimaraes, Portugal Universidade do Minho, Dep. Fısica, Azurem, d Forschungszentrum Rossendorf e.V., Postfach 510119, 01314 Dresden, Germany
Abstract Titanium oxynitride compounds exhibit interesting properties for applications in fields ranging from protectiveydecorative coatings to solar panels. The properties of TiNx Oy are related to the oxideynitride ratio and can be tailored playing with this ratio. In this work we studied the influence of substrate bias voltage and flow rate of reactive gases (a mixture of N2 and O2) on the properties of TiNxOy films. The films were deposited on steel substrates at a constant temperature of 300 8C by r.f. reactive magnetron sputtering. The depositions were carried out from a pure Ti target. The composition throughout the entire thickness was determined by Rutherford backscattering spectrometry. To obtain information on the profile of light elements (O, N) and detect the presence of hydrogen on the films, heavy ion elastic recoil detection analysis was performed. The results indicate a nearly constant stoichiometry through the entire analysed depth. The colouration varied from the shiny golden for low oxygen contents (characteristic of TiN films) to dark blue for higher oxygen contents. The electrical resistivity of the samples was obtained at room temperature and the values varied from approximately 120 mV cm for a sample with very low oxygen content to values up to 350 mV cm, for the highest oxygen contents. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Optical properties; Sputtering; Ion bombardment; Structural properties; Titanium nitride
1. Introduction The possibility to tailor material properties controlling the growing conditions is one of the major reasons for the great amount of research work in thin film deposition. Among the materials that need to fulfil very restringing requirements are coatings. The coating technology is known to be the best solution to protect surfaces against the aggression of environmental agents. Due to its economic importance, this technology has experienced a large expansion and, today both the experimental conditions and techniques to grow coatings for several industrial applications, e.g. protective, optical and microelectronics are well established w1,2x. Recently, an enormous interest in growing films displaying good mechanical and optoelectronic properties has emerged. The metal-oxynitride (MeNxOy—Me stands for an early transition metal) films belong to this *Corresponding author. Tel.: q351-21-9946086; fax: q351-21991525. E-mail address: [email protected] (E. Alves).
family of materials, which spans a large range of applications. Besides the applications as decorative coatings w3,4x, which give a large added value to the material, there is an enormous interest in this type of films for solar devices w5x. Titanium oxynitride films can be grown with different oxygen contents, covering the pure covalent nitride compound (MeNx) to the ionic oxide (MeOy). The final properties of the films are related to the chemical composition and homogeneity, as well as the crystallographic structure. It is known that the growth conditions are responsible for the film quality. Previous reports reveal the complexity associated with plasma vapor deposition based techniques to grow oxynitride films w6,7x. The difference in reactivity of oxygen (higher) compared to nitrogen (lower) is responsible for the complexity of the process. Some of these difficulties could be overcome using reactive magnetron sputtering with a constant flow mixture of N2qO2, to grow the TiNyOx films. In this work we present results on the influence of
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.131
E. Alves et al. / Surface and Coatings Technology 180 – 181 (2004) 372–376
373
the flow rate and bias voltage on the structural quality and resistivity of the TiNyOx films. We used ion beam techniques to obtain information on the depth composition of the films, which correlated well with their colouration. The final crystalline structure, which is related with the growth parameters, was determined by X-ray diffraction (XRD) and correlated with the electrical behaviour of the films. 2. Experimental details The TiNxOy samples were deposited by reactive r.f. magnetron sputtering from a high purity Ti target (99.731%) onto polished high-speed stainless steel (AISI M2). Prior to all depositions, the substrates were ultrasonically cleaned and sputter etched for 15 min in a 0.4 Pa Ar atmosphere (0.64 Wycm2 r.f. power density). Depositions were carried out in an AryN2qO2 atmosphere in an Alcatel SCM650 apparatus, and the substrates were rotated at 60 mm over the target at a constant speed of 4 rev.ymin. The base pressure in the deposition chamber was approximately 10y4 Pa and rose to values approximately 4=10y1 Pa during depositions. To improve the adhesion of the films to the substrates, a pure titanium adhesion layer was deposited. Substrates were heated to 300 8C and d.c. biased from y50 V up to grounded state. Two sets of samples were prepared: the first group was prepared with varying gas mixture (N2qO2) flux, using constant values of temperature (300 8C) and bias voltage (y50 V). Fluxes varied from 3.3 to 16 sccm. Gas mixture partial pressure ranged from 0.02 to 0.05 Pa. The second group was prepared with varying bias voltage, a fixed temperature of 300 8C and a constant gas flow of 14 sccm. Depositions were carried out with a constant r.f. power of 2.55 Wycm2 (800 W) applied to the Ti target. Both sets of samples were prepared with a constant argon flux of 100 sccm. The atomic composition of the as deposited samples was measured by Rutherford backscattering spectrometry (RBS). RBS studies were performed with a 1-mm diameter collimated beam of 4Heq or 1Hq ions. The backscattered particles were detected at 1408 and close to 1808, with respect to the beam direction using silicon surface barrier detectors with resolutions of 13 and 16 keV, respectively. The results were analysed with the NDF code w8x. Heavy ion elastic recoil detection analysis (ERDA) experiments were done using a 35 MeV 35 Cl7q beam. The recoils and backscattered beam were detected with a Bragg ionisation chamber at 30.768 with the beam, with a 1.2-mm Mylar stopping foil before the detector. To detect H, a Si surface barrier detector at 388 with the beam was used. In this case, a 16-mm Al stopping foil, able to stop all particles except H, was located before the detector.
Fig. 1. (a) RBS spectrum obtained with 2.0 MeV 4Heq beam for a film grown with a constant gas flow (14 sccm) and a bias of 0 V. The continuous line indicates the best fit derived from the NDF simulation using the information obtained with ERDA for the light elements. Measured composition: TiN0.69O1.17 . (b) The same for a film grown with a bias of y30 V and a measured composition of TiN0.78O1.00.
XRD was used for structure characterisation. XRD diagrams were recorded in a SIEMENS D5000 diffractometer (Cu Ka) using the conventional Bragg– Brentano u–2u configuration. The resistivity of the films was measured by the van der Pauw method at room temperature, using silver paint contacts in films especially deposited for this purpose on Si substrates. 3. Results and discussion The composition and homogeneity of the films grown using different conditions was assessed by RBS and ERDA. Fig. 1 shows a typical RBS spectrum for a thick (Fig. 1a) and thin (Fig. 1b) film grown with the same gas mixture flow (14 sccm) and different bias voltage. The peak appearing approximately 220 keV for the thinner film results from the overlap of the signals
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E. Alves et al. / Surface and Coatings Technology 180 – 181 (2004) 372–376
Kinematically, only ions scattered off Ti can reach the detector, and hence the scattered part of the spectrum is solely due to Ti. Energy spectra corresponding to each of the elements are obtained by projecting the corresponding signal in the energy axis. Please note that the Ti spectrum also includes a contribution due to the Ti recoils, which cannot be completely separated from the Cl ions. This is fully taken into account in the analysis. All these data were self-consistently automatically analysed with the IBA DataFurnace code w9x and the results obtained for films displayed in Fig. 1 are shown in Fig. 2a and b, respectively. Using this information we fitted the RBS spectra assuming a constant composition throughout the film. Taking into account the excellent agreement between the experimental and theoretical fit, a homogeneous distribution of Ti, N and O could be determined for the thinner film, Fig. 1a. For the thicker film the 2.0 MeV 4Heq beam was not able to probe the entire film thickness. To overcome this limitation we used a 1.7 MeV proton beam. In this case we were able to reach even the steel substrate and the results found (not shown) for both films reveal a homogeneous composition throughout the entire film thickness. This was the case for all the studied films. The depth scale measured by RBS or ERDA techniques is in units of atomic areal density. If the atomic density is known we can convert directly the experimental values to length units. Film densities are usually difficult to obtain and RBS can provide this information if the thickness of the films is measured with an independent technique. In this case we use the film thickness, obtained by ball crating measurements, to calculate the film densities. The density values and film stoichiometry as function of the applied bias are shown in Fig. 3. The results clearly indicate that the oxygen
Fig. 2. ERDA spectrum for the TiN0.69O1.17 film obtained with 35 MeV 35Cl7q beam (top). The concentration profiles obtained for the reference films discussed in Fig. 1 are shown in (a) and (b). The density used to convert the depth scale from normal RBS units (at.ycm2) to nm was calculated independently as explained in the text.
coming from the film and the Ti buffer layer. The signal coming from the light elements (N and O) is not well resolved in both spectra because of the low scattering cross-section and the overlap with the Ti signal. Due to the difficulty in extracting reliable information on the content of nitrogen and oxygen, we performed elastic recoil measurements (ERDA) with heavy ions on the samples, Fig. 2. The Bragg chamber spectrum of a film TiN0.69O1.17 is shown in Fig. 2 (top). For clarity, pixels with less than three counts are not shown. The N and O signals are very well resolved and background free.
Fig. 3. Composition and density dependence of the films grown with a constant gas flow of 14 sccm as a function of the applied bias (full symbols). Open symbols indicate the values obtained for a film grown with a flow rate of 10 sccm.
E. Alves et al. / Surface and Coatings Technology 180 – 181 (2004) 372–376
content decreases with the applied voltage while titanium and nitrogen increases. This trend suggests a relation between the energy of the impinging ions on the surface and the oxygen incorporation. Furthermore, the high affinity of oxygen with respect to titanium compared to nitrogen must be also considered. Taking into account these two factors we can understand the growth process in the following way. At low bias the energy of the ions is low and the oxygen reactivity is the dominant factor controlling the growth, explaining the higher content in the films grown in these circumstances. For high bias the kinetic energy of the ions is higher, enhancing their mobility. The mobility of the ions allows the formation of the stable stoichiometric phases leading ultimately to phase segregation. In this case the amount of oxygen in the films is reduced. Despite having a good crystalline quality w4x, the growth of films with pure phases using this gas mixture was not yet achieved. For all the growth conditions studied, detailed XRD (not shown) studies always reveal the presence of TiNxOy mixtures with a NaCl-type structure, besides the TiN and TiO2, Ti3O5 oxides w10x. Further increase of the oxygen concentration hinders the crystallization of the films. It was found in previous works that films with oxygen contents above 35 at.% are almost amorphous w4x. Therefore, in order to produce crystalline films we must use growth conditions that reduce the incorporation of oxygen. This is evident in the result shown in Fig. 3 for the film grown with the maximum bias (y50 V) but the lower gas flux, which means less oxygen available. In these conditions the incorporation of oxygen in the films is low (16 at.%) and the dominant phase observed by XRD is the TiN one w4x. It seems evident that the presence of a large range of intermediate phases between pure TiN and TiO is possible. The formation of a TiNxOy metalloid compound results from the replacement of nitrogen by oxygen in the TiN fcc lattice. The existence of a mixture of TiNxOy solid phases was also considered by other authors w11,12x who claim that the high concentration of defects in the TiN phase could favour the incorporation of oxygen in the cation fcc sublattice. Another interesting conclusion that can be drawn from the data shown in Fig. 3 is the decrease of the atomic density with the applied bias. The increase of the crystalline phases leads to less dense films. This result is coherent with the fact that the formation of dense oxide phases is favoured by the high oxygen availability. The electrical resistivity of the films measured at room temperature increases with the oxygen concentration. Resistivity of the films seems to follow the variation of the oxygen content in films, increasing with the increase of the oxygen content in the samples, Fig. 4. Resistivity values increase from approximately 120 mV cm for the sample with the lowest oxygen content
375
Fig. 4. Electrical resistivity of the films as a function of the oxygen concentration.
up to approximately 350 mV cm for samples with the highest oxygen concentrations. This behaviour is somewhat expected due to the transition from the nitride to a progressively more oxide-type coating, as well as with the increasing defect formation due to the increasing oxygen-doped structures. TiN samples prepared with the same bias and temperature conditions of those with oxygen reveal a resistivity value close to 100 mV cm, which is consistent with some published literature in this kind of materials w13x. A more detailed study in the variation of the resistivity is under investigation, mainly in what concerns the samples with the highest oxygen contents. 4. Conclusions Highly homogeneous TiNxOy films were grown by r.f. reactive magnetron sputtering with a reactive gas a mixture of N2qO2. The phase composition can be tailored by the substrate bias controlling the oxygen incorporation. It is mainly the oxygen reactivity and kinetic energy of the impinging ions that determine the film properties. Crystalline quality of the films improves significantly when the oxygen concentration is reduced. The successful and reliable deposition of TiNxOy films using a conventional and not expensive technique is a step forward to expand their technological applications. Acknowledgments The authors gratefully acknowledge the financial support of the FCT institution by the project no. POCTIy CTMy380860y2001 co-financed by European community fund FEDER. We acknowledge the European Union Large Scale Facility grant EUyHPRI-CT1999-00039 for access to the Rossendorf facility.
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E. Alves et al. / Surface and Coatings Technology 180 – 181 (2004) 372–376
References w1x R. Bunshad (Ed.), Deposition Technologies for Films and Coatings, Noyes publications, New Jersey, USA, 1982. w2x M. Ohring, The Materials Science of Thin Films, Academic Press Inc, San Diego, 1992. w3x R. Fraunchy, Surf. Sci. Rep. 38 (2000) 195. w4x F. Vaz, P. Cerqueira, L. Rebouta, S.M.C. Nascimento, E. Alves, ´ Surf. Coat. Technol. 174–175 (2003) Ph. Goudeau, J.P. Riviere, 197. w5x H. Tada, Y. Saito, M. Hirata, M. Hyodo, H. Kawahara, J. Appl. Phys. 73 (1993) 489. w6x A. Von Richthofen, R. Domnick, R. Cremer, D. Neuschutz, Thin Solid Films 317 (1998) 282.
w7x C. Gautier, J. Machet, Surf. Coat. Technol. 94–95 (1997) 422. w8x N.P. Barradas, C. Jeynes, R.P. Webb, Appl. Phys. Lett. 71 (1997) 291–293. w9x C. Jeynes, N.P. Barradas, P.K. Marriott, et al., J. Phys. D: Appl. Phys. 36 (2003) R97–R126. w10x F. Vaz, P. Cerqueira, L. Rebouta, E. Alves, Ph. Goudeau, J.P. Riviere, ´ K. Pischow, J. de Rijk, Thin Solid Films, in press. w11x M. Lazarov, P. Raths, H. Metzger, W. Spirkl, J. Appl. Phys. 77 (1995) 2133. w12x N. Martin, O. Banakh, A.M.E. Santo, S. Springer, R. Sanjines, ´ ´ J. Takadoum, F. Levy, Appl. Surf. Sci. 185 (2001) 123. w13x D. Mao, K. Tao, J. Hopwood, J. Vac. Sci. Technol. A 20 (2) (2002) 379, and references therein.
Ion beam studies of TiNxOy thin films deposited by reactive magnetron sputtering E. Alvesa,b,*, A.R. Ramosa,b, N.P. Barradasa,b, F. Vazc, P. Cerqueirac, L. Reboutac, U. Kreissigd b
a ´ ITN, Departamento de Fısica, E.N.10, 2686-953 Sacavem, Portugal CFN da Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003 Lisbon, Portugal c ´ ´ 4800-058 Guimaraes, Portugal Universidade do Minho, Dep. Fısica, Azurem, d Forschungszentrum Rossendorf e.V., Postfach 510119, 01314 Dresden, Germany
Abstract Titanium oxynitride compounds exhibit interesting properties for applications in fields ranging from protectiveydecorative coatings to solar panels. The properties of TiNx Oy are related to the oxideynitride ratio and can be tailored playing with this ratio. In this work we studied the influence of substrate bias voltage and flow rate of reactive gases (a mixture of N2 and O2) on the properties of TiNxOy films. The films were deposited on steel substrates at a constant temperature of 300 8C by r.f. reactive magnetron sputtering. The depositions were carried out from a pure Ti target. The composition throughout the entire thickness was determined by Rutherford backscattering spectrometry. To obtain information on the profile of light elements (O, N) and detect the presence of hydrogen on the films, heavy ion elastic recoil detection analysis was performed. The results indicate a nearly constant stoichiometry through the entire analysed depth. The colouration varied from the shiny golden for low oxygen contents (characteristic of TiN films) to dark blue for higher oxygen contents. The electrical resistivity of the samples was obtained at room temperature and the values varied from approximately 120 mV cm for a sample with very low oxygen content to values up to 350 mV cm, for the highest oxygen contents. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Optical properties; Sputtering; Ion bombardment; Structural properties; Titanium nitride
1. Introduction The possibility to tailor material properties controlling the growing conditions is one of the major reasons for the great amount of research work in thin film deposition. Among the materials that need to fulfil very restringing requirements are coatings. The coating technology is known to be the best solution to protect surfaces against the aggression of environmental agents. Due to its economic importance, this technology has experienced a large expansion and, today both the experimental conditions and techniques to grow coatings for several industrial applications, e.g. protective, optical and microelectronics are well established w1,2x. Recently, an enormous interest in growing films displaying good mechanical and optoelectronic properties has emerged. The metal-oxynitride (MeNxOy—Me stands for an early transition metal) films belong to this *Corresponding author. Tel.: q351-21-9946086; fax: q351-21991525. E-mail address: [email protected] (E. Alves).
family of materials, which spans a large range of applications. Besides the applications as decorative coatings w3,4x, which give a large added value to the material, there is an enormous interest in this type of films for solar devices w5x. Titanium oxynitride films can be grown with different oxygen contents, covering the pure covalent nitride compound (MeNx) to the ionic oxide (MeOy). The final properties of the films are related to the chemical composition and homogeneity, as well as the crystallographic structure. It is known that the growth conditions are responsible for the film quality. Previous reports reveal the complexity associated with plasma vapor deposition based techniques to grow oxynitride films w6,7x. The difference in reactivity of oxygen (higher) compared to nitrogen (lower) is responsible for the complexity of the process. Some of these difficulties could be overcome using reactive magnetron sputtering with a constant flow mixture of N2qO2, to grow the TiNyOx films. In this work we present results on the influence of
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.131
E. Alves et al. / Surface and Coatings Technology 180 – 181 (2004) 372–376
373
the flow rate and bias voltage on the structural quality and resistivity of the TiNyOx films. We used ion beam techniques to obtain information on the depth composition of the films, which correlated well with their colouration. The final crystalline structure, which is related with the growth parameters, was determined by X-ray diffraction (XRD) and correlated with the electrical behaviour of the films. 2. Experimental details The TiNxOy samples were deposited by reactive r.f. magnetron sputtering from a high purity Ti target (99.731%) onto polished high-speed stainless steel (AISI M2). Prior to all depositions, the substrates were ultrasonically cleaned and sputter etched for 15 min in a 0.4 Pa Ar atmosphere (0.64 Wycm2 r.f. power density). Depositions were carried out in an AryN2qO2 atmosphere in an Alcatel SCM650 apparatus, and the substrates were rotated at 60 mm over the target at a constant speed of 4 rev.ymin. The base pressure in the deposition chamber was approximately 10y4 Pa and rose to values approximately 4=10y1 Pa during depositions. To improve the adhesion of the films to the substrates, a pure titanium adhesion layer was deposited. Substrates were heated to 300 8C and d.c. biased from y50 V up to grounded state. Two sets of samples were prepared: the first group was prepared with varying gas mixture (N2qO2) flux, using constant values of temperature (300 8C) and bias voltage (y50 V). Fluxes varied from 3.3 to 16 sccm. Gas mixture partial pressure ranged from 0.02 to 0.05 Pa. The second group was prepared with varying bias voltage, a fixed temperature of 300 8C and a constant gas flow of 14 sccm. Depositions were carried out with a constant r.f. power of 2.55 Wycm2 (800 W) applied to the Ti target. Both sets of samples were prepared with a constant argon flux of 100 sccm. The atomic composition of the as deposited samples was measured by Rutherford backscattering spectrometry (RBS). RBS studies were performed with a 1-mm diameter collimated beam of 4Heq or 1Hq ions. The backscattered particles were detected at 1408 and close to 1808, with respect to the beam direction using silicon surface barrier detectors with resolutions of 13 and 16 keV, respectively. The results were analysed with the NDF code w8x. Heavy ion elastic recoil detection analysis (ERDA) experiments were done using a 35 MeV 35 Cl7q beam. The recoils and backscattered beam were detected with a Bragg ionisation chamber at 30.768 with the beam, with a 1.2-mm Mylar stopping foil before the detector. To detect H, a Si surface barrier detector at 388 with the beam was used. In this case, a 16-mm Al stopping foil, able to stop all particles except H, was located before the detector.
Fig. 1. (a) RBS spectrum obtained with 2.0 MeV 4Heq beam for a film grown with a constant gas flow (14 sccm) and a bias of 0 V. The continuous line indicates the best fit derived from the NDF simulation using the information obtained with ERDA for the light elements. Measured composition: TiN0.69O1.17 . (b) The same for a film grown with a bias of y30 V and a measured composition of TiN0.78O1.00.
XRD was used for structure characterisation. XRD diagrams were recorded in a SIEMENS D5000 diffractometer (Cu Ka) using the conventional Bragg– Brentano u–2u configuration. The resistivity of the films was measured by the van der Pauw method at room temperature, using silver paint contacts in films especially deposited for this purpose on Si substrates. 3. Results and discussion The composition and homogeneity of the films grown using different conditions was assessed by RBS and ERDA. Fig. 1 shows a typical RBS spectrum for a thick (Fig. 1a) and thin (Fig. 1b) film grown with the same gas mixture flow (14 sccm) and different bias voltage. The peak appearing approximately 220 keV for the thinner film results from the overlap of the signals
374
E. Alves et al. / Surface and Coatings Technology 180 – 181 (2004) 372–376
Kinematically, only ions scattered off Ti can reach the detector, and hence the scattered part of the spectrum is solely due to Ti. Energy spectra corresponding to each of the elements are obtained by projecting the corresponding signal in the energy axis. Please note that the Ti spectrum also includes a contribution due to the Ti recoils, which cannot be completely separated from the Cl ions. This is fully taken into account in the analysis. All these data were self-consistently automatically analysed with the IBA DataFurnace code w9x and the results obtained for films displayed in Fig. 1 are shown in Fig. 2a and b, respectively. Using this information we fitted the RBS spectra assuming a constant composition throughout the film. Taking into account the excellent agreement between the experimental and theoretical fit, a homogeneous distribution of Ti, N and O could be determined for the thinner film, Fig. 1a. For the thicker film the 2.0 MeV 4Heq beam was not able to probe the entire film thickness. To overcome this limitation we used a 1.7 MeV proton beam. In this case we were able to reach even the steel substrate and the results found (not shown) for both films reveal a homogeneous composition throughout the entire film thickness. This was the case for all the studied films. The depth scale measured by RBS or ERDA techniques is in units of atomic areal density. If the atomic density is known we can convert directly the experimental values to length units. Film densities are usually difficult to obtain and RBS can provide this information if the thickness of the films is measured with an independent technique. In this case we use the film thickness, obtained by ball crating measurements, to calculate the film densities. The density values and film stoichiometry as function of the applied bias are shown in Fig. 3. The results clearly indicate that the oxygen
Fig. 2. ERDA spectrum for the TiN0.69O1.17 film obtained with 35 MeV 35Cl7q beam (top). The concentration profiles obtained for the reference films discussed in Fig. 1 are shown in (a) and (b). The density used to convert the depth scale from normal RBS units (at.ycm2) to nm was calculated independently as explained in the text.
coming from the film and the Ti buffer layer. The signal coming from the light elements (N and O) is not well resolved in both spectra because of the low scattering cross-section and the overlap with the Ti signal. Due to the difficulty in extracting reliable information on the content of nitrogen and oxygen, we performed elastic recoil measurements (ERDA) with heavy ions on the samples, Fig. 2. The Bragg chamber spectrum of a film TiN0.69O1.17 is shown in Fig. 2 (top). For clarity, pixels with less than three counts are not shown. The N and O signals are very well resolved and background free.
Fig. 3. Composition and density dependence of the films grown with a constant gas flow of 14 sccm as a function of the applied bias (full symbols). Open symbols indicate the values obtained for a film grown with a flow rate of 10 sccm.
E. Alves et al. / Surface and Coatings Technology 180 – 181 (2004) 372–376
content decreases with the applied voltage while titanium and nitrogen increases. This trend suggests a relation between the energy of the impinging ions on the surface and the oxygen incorporation. Furthermore, the high affinity of oxygen with respect to titanium compared to nitrogen must be also considered. Taking into account these two factors we can understand the growth process in the following way. At low bias the energy of the ions is low and the oxygen reactivity is the dominant factor controlling the growth, explaining the higher content in the films grown in these circumstances. For high bias the kinetic energy of the ions is higher, enhancing their mobility. The mobility of the ions allows the formation of the stable stoichiometric phases leading ultimately to phase segregation. In this case the amount of oxygen in the films is reduced. Despite having a good crystalline quality w4x, the growth of films with pure phases using this gas mixture was not yet achieved. For all the growth conditions studied, detailed XRD (not shown) studies always reveal the presence of TiNxOy mixtures with a NaCl-type structure, besides the TiN and TiO2, Ti3O5 oxides w10x. Further increase of the oxygen concentration hinders the crystallization of the films. It was found in previous works that films with oxygen contents above 35 at.% are almost amorphous w4x. Therefore, in order to produce crystalline films we must use growth conditions that reduce the incorporation of oxygen. This is evident in the result shown in Fig. 3 for the film grown with the maximum bias (y50 V) but the lower gas flux, which means less oxygen available. In these conditions the incorporation of oxygen in the films is low (16 at.%) and the dominant phase observed by XRD is the TiN one w4x. It seems evident that the presence of a large range of intermediate phases between pure TiN and TiO is possible. The formation of a TiNxOy metalloid compound results from the replacement of nitrogen by oxygen in the TiN fcc lattice. The existence of a mixture of TiNxOy solid phases was also considered by other authors w11,12x who claim that the high concentration of defects in the TiN phase could favour the incorporation of oxygen in the cation fcc sublattice. Another interesting conclusion that can be drawn from the data shown in Fig. 3 is the decrease of the atomic density with the applied bias. The increase of the crystalline phases leads to less dense films. This result is coherent with the fact that the formation of dense oxide phases is favoured by the high oxygen availability. The electrical resistivity of the films measured at room temperature increases with the oxygen concentration. Resistivity of the films seems to follow the variation of the oxygen content in films, increasing with the increase of the oxygen content in the samples, Fig. 4. Resistivity values increase from approximately 120 mV cm for the sample with the lowest oxygen content
375
Fig. 4. Electrical resistivity of the films as a function of the oxygen concentration.
up to approximately 350 mV cm for samples with the highest oxygen concentrations. This behaviour is somewhat expected due to the transition from the nitride to a progressively more oxide-type coating, as well as with the increasing defect formation due to the increasing oxygen-doped structures. TiN samples prepared with the same bias and temperature conditions of those with oxygen reveal a resistivity value close to 100 mV cm, which is consistent with some published literature in this kind of materials w13x. A more detailed study in the variation of the resistivity is under investigation, mainly in what concerns the samples with the highest oxygen contents. 4. Conclusions Highly homogeneous TiNxOy films were grown by r.f. reactive magnetron sputtering with a reactive gas a mixture of N2qO2. The phase composition can be tailored by the substrate bias controlling the oxygen incorporation. It is mainly the oxygen reactivity and kinetic energy of the impinging ions that determine the film properties. Crystalline quality of the films improves significantly when the oxygen concentration is reduced. The successful and reliable deposition of TiNxOy films using a conventional and not expensive technique is a step forward to expand their technological applications. Acknowledgments The authors gratefully acknowledge the financial support of the FCT institution by the project no. POCTIy CTMy380860y2001 co-financed by European community fund FEDER. We acknowledge the European Union Large Scale Facility grant EUyHPRI-CT1999-00039 for access to the Rossendorf facility.
376
E. Alves et al. / Surface and Coatings Technology 180 – 181 (2004) 372–376
References w1x R. Bunshad (Ed.), Deposition Technologies for Films and Coatings, Noyes publications, New Jersey, USA, 1982. w2x M. Ohring, The Materials Science of Thin Films, Academic Press Inc, San Diego, 1992. w3x R. Fraunchy, Surf. Sci. Rep. 38 (2000) 195. w4x F. Vaz, P. Cerqueira, L. Rebouta, S.M.C. Nascimento, E. Alves, ´ Surf. Coat. Technol. 174–175 (2003) Ph. Goudeau, J.P. Riviere, 197. w5x H. Tada, Y. Saito, M. Hirata, M. Hyodo, H. Kawahara, J. Appl. Phys. 73 (1993) 489. w6x A. Von Richthofen, R. Domnick, R. Cremer, D. Neuschutz, Thin Solid Films 317 (1998) 282.
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