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Characterisation of titanium nitride thin films prepared using PVD and a plasma immersion ion implantation system
Nuclear Instruments and Methods in Physics Research B 190 (2002) 723–727 www.elsevier.com/locate/nimb
Characterisation of titanium nitride thin films prepared using PVD and a plasma immersion ion implantation system S.H.N. Lim a
a,*
, D.G. McCulloch a, S. Russo a, M.M.M. Bilek b, D.R. McKenzie
b
Department of Applied Physics, RMIT University, G.P.O. Box 2476V, Melbourne 3001, Australia b School of Physics (A28), University of Sydney, NSW 2006, Australia
Abstract We compare two titanium nitride films deposited on silicon using PVD. One was produced with the aid of a pulsed high voltage power supply which is attached to the substrate holder. The stoichiometry and structure of the samples was investigated using Rutherford backscattering spectroscopy and cross-sectional TEM. It was found that while the stoichiometry of the two samples were comparable, there was a difference in the preferred orientation of the films. A change in orientation was observed from (1 1 1) to (2 0 0) as the pulsed high voltage power supply was applied. This change in orientation indicates that the intrinsic stress level in the film has been reduced which may allow the growth of thicker coatings. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films; Titanium nitride; Cathodic arc; Plasma immersion ion implantation
1. Introduction Cathodic arc deposition is a well-established technique for the growth of thin films onto a variety of substrates. For a review of this method, refer to the article by Brown [1]. Recently, work has been presented which combines cathodic arc deposition with a plasma immersion ion implantation (PIII) system [2,3]. The combined system offers several different advantages over conventional methods. The use of PIII may eliminate the need to elevate the substrate temperature in order to produce high quality films by PVD. In this context, high quality is taken to mean dense, stress
*
Corresponding author. E-mail address: [email protected] (S.H.N. Lim).
free and stoichiometric. Such a process would allow the deposition of coatings onto temperature sensitive substrates such as polymers. In addition, lower deposition temperatures will significantly reduce processing times in the coating of tools steels. There is evidence that PIII may reduce the embodied stress in a film, allowing a thicker film to be deposited without delamination [4]. Finally, it may be possible to modify the nature of the film/ substrate interface by using ion implantation to produce a mixing layer which will enhance the adhesion of coatings. In this work, a combined PIII and cathodic arc deposition system is used to produce titanium nitride (TiN) thin film samples. The objective of this work is to determine the effect on the structure of the films resulting from the presence of the PIII. The samples are characterised using Rutherford
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 2 4 5 - 9
724
S.H.N. Lim et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 723–727
backscattering spectroscopy (RBS) and crosssectional TEM (X-TEM). X-TEM analysis gives a detailed insight into the structure of the film and interface region. It also provides information on the crystallographic structure of the film.
2. Experimental A schematic diagram of the deposition system is shown in Fig. 1. The cathodic arc deposition system consists of a cathode, at one end of the chamber, connected using a duct with the substrate holder at the other. The PIII power supply is attached to the substrate holder as indicated by the diagram. The basic functions of the PIII are as follows: A high negative pulse bias is applied to the substrate at a voltage of up to 20 kV while it is immersed in plasma. During each pulse, an electric sheath is formed, which accelerates the plasma ions towards the substrate. The PIII process can achieve implantation into the substrate which causes interface mixing, as well as a modification of the material deposited onto the substrate.
Fig. 1. Schematic diagram of the cathodic arc deposition system, with the PIII attached.
In our experiments two types of plasmas were used. The first type was a plasma containing only ions from the background of N2 gas. This situation resulted in implantation of nitrogen ions into the substrate during the HV pulses. In the second operating mode the substrate was HV pulsed in the background gas with the cathodic arc running. In this case the plasma consisted of a mixture of titanium and nitrogen ions which were implanted into the substrate during the HV pulses. Because a titanium–nitrogen plasma is condensable, unlike the pure nitrogen plasma, a film is formed during the intervals between the application of the HV bias pulses, resulting in a combined deposition and implantation process. Macro-particles are filtered using the 90° curved solenoid duct filter. The walls of the duct are surrounded by a series of coils which when supplied with a current act together as a curved magnetic solenoid filter. The magnetic filter duct has several power sources for controlling the currents in the coils. This gives optimum control of the magnetic field generated [5]. The pulse generator has the capability of producing pulses of up to 30 kV, with durations of 10–60 ls and a repetition rate of up to 1 kHz. Two samples were prepared for this work (called sample A and B). Prior to deposition, the PIII was turned on for approximately 2 min with the silicon substrates in place. This was intended to act as a surface cleaning stage, using nitrogen ions to etch the top surface of the silicon. The TiN film in sample A was grown without the PIII, high voltage pulsing, whereas the TiN film in sample B was grown with PIII high voltage pulsing operating at 20 kV. The pulse duration was 20 ls and the repetition rate was 250 Hz. Preparation of the specimens for TEM involves thinning down using a mechanical polishing method known as tripod polishing, and then further thinned using a GATAN Dual Ion Mill Model 600. The prepared specimen was then analysed using a JEOL 2010 TEM operating at 200 kV. RBS was performed using a 2.0 MeV Heþ beam from a 3MV Van de Graaff accelerator. The backscattered and glancing detectors were set at 169° and 112° respectively. The spectra was analysed using the RBS analytical software called RUMP [6].
S.H.N. Lim et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 723–727
3. Results and discussions The first major difference that could be directly observed in the two samples was the difference in the colour of the films. Sample A, which was the sample without PIII, has the conventional gold colour, which is usually observed in stoichiometric TiN films. On the other hand, sample B, which was deposited using PIII had a distinct purple colour. The RBS spectra of samples A and B are shown in Fig. 2(a) and (b), respectively. The spectra show that the coating contains mainly titanium and nitrogen with a small amounts of oxygen and zinc. The zinc is most likely contamination and its presence may be due to sputtering from the sample holder.
Fig. 2. RBS spectra of (a) sample A using the backscattering detector, and (b) sample B using the glancing detector.
725
RUMP simulations were performed on each spectrum, and the resulting fit is also shown in Fig. 2. The coating was found to consist mainly of titanium and nitrogen with some oxygen present. The simulations shown in this figure are for titanium concentrations of 55% and 58% for samples A and B respectively. This indicates that both films are close to stoichiometric. However, there are considerable levels of uncertainty in the concentration of nitrogen due to the presence of oxygen and the fact that the films may not be uniform in concentration. In addition to this main layer, several thin layers with varying concentrations of titanium, nitrogen, oxygen and silicon were found between the coating and the silicon substrate. This indicates the presence of a mixing region at the substrate coating interface. Another feature in the RBS spectra is a small peak on top of the main titanium signal (indicated with a arrow). This peak was found to originate from a thin (approximately 8 nm) titanium rich buried layer. The ratio of Ti to N of this layer is approximately 3:2. The X-TEM image of sample A is shown in Fig. 3. The main features are (I) the epoxy glue, (II) the TiN film, (III) mixing region, (IV) silicon substrate. The coating thickness was found to be
Fig. 3. X-TEM of sample A, showing the (I) epoxy glue, (II) flim, (III) mixing layer and (IV) Si substrate.
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approximately 90 nm for sample A, and 110 nm for sample B. Enlargements of the X-TEM images showing the film/substrate interface for samples A and B are shown in Figs. 4(a) and 5(a) respectively. Both samples show a similar largely amorphous mixing region, which contains a layer of crystallites approximately 7–8 nm thick. This result is consistent with the RBS simulations, which predicted that there was a buried layer under the thin film. From this, it can be concluded that this
buried layer is rich in titanium, and is most likely a row of titanium crystallites. This row of titanium appears in both of the samples. The process that the two samples have in common was the initial nitriding stage. Therefore this layer probably results from the pre-nitriding stage where the energetic nitrogen ions etch titanium off the wall of the deposition chamber and from the titanium substrate holder. These titanium atoms could then be implanted into the silicon substrate
Fig. 4. (a) X-TEM image of sample A (no PIII), (b) diffraction pattern of the TiN flim. Note that the diffraction pattern is correctly aligned with respect to the image in (a).
Fig. 5. (a) X-TEM image of sample B (with PIII), (b) diffraction pattern of the TiN film. Note that the diffraction pattern is correctly aligned with respect to the image in (a).
S.H.N. Lim et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 723–727
during the high voltage pulsing. The origin of this feature is presently under further investigation. The diffraction patterns for sample A and sample B are shown in Figs. 4(b) and 5(b) respectively. These diffraction pattern photographs are aligned according to the TEM photographs of the corresponding film cross-sections. The strong spots along a line perpendicular to the film indicate that planes with indexes corresponding to the ring containing the spots lie in the plane of the film (i.e. the crystal direction is perpendicular to the plane). Careful inspection of the diffraction pattern reveals that the structures of the two samples are different. Sample A, which did not use the PIII during film deposition shows a strong preferred orientation with (1 1 1) planes parallel to the surface, whereas in sample B, where the PIII was used during film growth, there was preferred orientation of the (2 0 0) planes parallel to the surface. It is well known that TiN films produced using PVD techniques where ions energies are typically in the order of tens to a few hundreds eV, contain crystallites oriented with their (1 1 1) planes parallel to the surface of the film [7]. Pelleg [8] showed that using an energy minimisation model for preferred orientation, there should be a cut-off stress level in TiN, where the (1 1 1) orientation is preferred. At stress levels lower than the cut-off, the (2 0 0) orientation is preferred. If we assume that the total energy of the system comes only from the bulk strain energy and the surface energy, then the change in the orientation could be explained. Preliminary stress measurements support this idea [9]. Therefore, based on energy minimisation the transition from the (1 1 1) orientation to (2 0 0) orientation is most likely due to the presence of the PIII, which provides energy to the ions in the tens of kV range. This results in a local annealing effect, which relieves the intrinsic stress in the film. We believe that it is this reduction in intrinsic stress that leads to the transition from (1 1 1) orientation to (2 0 0) orientation [9].
4. Conclusion In this paper, we have compared TiN films produced on silicon with and without PIII high
727
voltage pulse biasing of the substrate holder in a cathodic arc PVD system. Both coatings were found to have similar stoichiometry, however the colour of the films is found to be different. The RBS results show that the interface region between coating and substrate was complex, with several thin layers. The main difference between the two films was a change in the preferred orientation of the TiN microstructure. This change in the preferred orientation was found to be from (1 1 1) in the case where no PIII was used to (2 0 0) where PIII was in operation. This orientation can be explained by a reduction in the level of intrinsic stress within the coating. This result indicates that combining PIII with a PVD technique can be used to control the orientation of TiN thin films and may also be useful in reducing the intrinsic stress level of coatings.
Acknowledgements We would like to thank, Dr. P. Evans (ANSTO), J. Piggot and R. Tarrant (University of Sydney) and Assoc. Prof. Phil Wilksch (RMIT) for their contributions to this work. We also acknowledge the support of the Australian Research Council (ARC) and AINSE.
References [1] I. Brown, Ann. Rev. Mater. Sci. 28 (1998) 243. [2] A. Anders, Surf. Coat. Technol. 93 (1997) 157. [3] M.M.M. Bilek, D. McKenzie, D.G. McCulloch, H. Zreiqat, J. Appl. Phys. 87 (9) (2000) 4198. [4] M.M.M. Bilek, D.R. McKenzie, R.I. Tarrant, T. Oates, D.G. McCulloch, The 12th International Federation of Heat Treatment and Surface Engineering Congress, 2000. [5] M.M.M. Bilek, J. Appl. Phys. 85 (9) (1999) 6385. [6] L.R Doolittle, M.O. Thompson, RUMP Program, Cornell University Ithaca NY, USA. [7] H.H. Nguyen, D.R. McKenzie, W.D. McFall, Y. Yin, J. Appl. Phys. 80 (11) (1996) 6279. [8] J. Pelleg, L.Z. Zevin, S. Lungo, Thin Solid Films 197 (1991) 117. [9] S.H.N. Lim, M.M.M. Bilek, D.G. McCulloch, D.R. McKenzie, in preparation, 2002.
Characterisation of titanium nitride thin films prepared using PVD and a plasma immersion ion implantation system S.H.N. Lim a
a,*
, D.G. McCulloch a, S. Russo a, M.M.M. Bilek b, D.R. McKenzie
b
Department of Applied Physics, RMIT University, G.P.O. Box 2476V, Melbourne 3001, Australia b School of Physics (A28), University of Sydney, NSW 2006, Australia
Abstract We compare two titanium nitride films deposited on silicon using PVD. One was produced with the aid of a pulsed high voltage power supply which is attached to the substrate holder. The stoichiometry and structure of the samples was investigated using Rutherford backscattering spectroscopy and cross-sectional TEM. It was found that while the stoichiometry of the two samples were comparable, there was a difference in the preferred orientation of the films. A change in orientation was observed from (1 1 1) to (2 0 0) as the pulsed high voltage power supply was applied. This change in orientation indicates that the intrinsic stress level in the film has been reduced which may allow the growth of thicker coatings. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films; Titanium nitride; Cathodic arc; Plasma immersion ion implantation
1. Introduction Cathodic arc deposition is a well-established technique for the growth of thin films onto a variety of substrates. For a review of this method, refer to the article by Brown [1]. Recently, work has been presented which combines cathodic arc deposition with a plasma immersion ion implantation (PIII) system [2,3]. The combined system offers several different advantages over conventional methods. The use of PIII may eliminate the need to elevate the substrate temperature in order to produce high quality films by PVD. In this context, high quality is taken to mean dense, stress
*
Corresponding author. E-mail address: [email protected] (S.H.N. Lim).
free and stoichiometric. Such a process would allow the deposition of coatings onto temperature sensitive substrates such as polymers. In addition, lower deposition temperatures will significantly reduce processing times in the coating of tools steels. There is evidence that PIII may reduce the embodied stress in a film, allowing a thicker film to be deposited without delamination [4]. Finally, it may be possible to modify the nature of the film/ substrate interface by using ion implantation to produce a mixing layer which will enhance the adhesion of coatings. In this work, a combined PIII and cathodic arc deposition system is used to produce titanium nitride (TiN) thin film samples. The objective of this work is to determine the effect on the structure of the films resulting from the presence of the PIII. The samples are characterised using Rutherford
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 2 4 5 - 9
724
S.H.N. Lim et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 723–727
backscattering spectroscopy (RBS) and crosssectional TEM (X-TEM). X-TEM analysis gives a detailed insight into the structure of the film and interface region. It also provides information on the crystallographic structure of the film.
2. Experimental A schematic diagram of the deposition system is shown in Fig. 1. The cathodic arc deposition system consists of a cathode, at one end of the chamber, connected using a duct with the substrate holder at the other. The PIII power supply is attached to the substrate holder as indicated by the diagram. The basic functions of the PIII are as follows: A high negative pulse bias is applied to the substrate at a voltage of up to 20 kV while it is immersed in plasma. During each pulse, an electric sheath is formed, which accelerates the plasma ions towards the substrate. The PIII process can achieve implantation into the substrate which causes interface mixing, as well as a modification of the material deposited onto the substrate.
Fig. 1. Schematic diagram of the cathodic arc deposition system, with the PIII attached.
In our experiments two types of plasmas were used. The first type was a plasma containing only ions from the background of N2 gas. This situation resulted in implantation of nitrogen ions into the substrate during the HV pulses. In the second operating mode the substrate was HV pulsed in the background gas with the cathodic arc running. In this case the plasma consisted of a mixture of titanium and nitrogen ions which were implanted into the substrate during the HV pulses. Because a titanium–nitrogen plasma is condensable, unlike the pure nitrogen plasma, a film is formed during the intervals between the application of the HV bias pulses, resulting in a combined deposition and implantation process. Macro-particles are filtered using the 90° curved solenoid duct filter. The walls of the duct are surrounded by a series of coils which when supplied with a current act together as a curved magnetic solenoid filter. The magnetic filter duct has several power sources for controlling the currents in the coils. This gives optimum control of the magnetic field generated [5]. The pulse generator has the capability of producing pulses of up to 30 kV, with durations of 10–60 ls and a repetition rate of up to 1 kHz. Two samples were prepared for this work (called sample A and B). Prior to deposition, the PIII was turned on for approximately 2 min with the silicon substrates in place. This was intended to act as a surface cleaning stage, using nitrogen ions to etch the top surface of the silicon. The TiN film in sample A was grown without the PIII, high voltage pulsing, whereas the TiN film in sample B was grown with PIII high voltage pulsing operating at 20 kV. The pulse duration was 20 ls and the repetition rate was 250 Hz. Preparation of the specimens for TEM involves thinning down using a mechanical polishing method known as tripod polishing, and then further thinned using a GATAN Dual Ion Mill Model 600. The prepared specimen was then analysed using a JEOL 2010 TEM operating at 200 kV. RBS was performed using a 2.0 MeV Heþ beam from a 3MV Van de Graaff accelerator. The backscattered and glancing detectors were set at 169° and 112° respectively. The spectra was analysed using the RBS analytical software called RUMP [6].
S.H.N. Lim et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 723–727
3. Results and discussions The first major difference that could be directly observed in the two samples was the difference in the colour of the films. Sample A, which was the sample without PIII, has the conventional gold colour, which is usually observed in stoichiometric TiN films. On the other hand, sample B, which was deposited using PIII had a distinct purple colour. The RBS spectra of samples A and B are shown in Fig. 2(a) and (b), respectively. The spectra show that the coating contains mainly titanium and nitrogen with a small amounts of oxygen and zinc. The zinc is most likely contamination and its presence may be due to sputtering from the sample holder.
Fig. 2. RBS spectra of (a) sample A using the backscattering detector, and (b) sample B using the glancing detector.
725
RUMP simulations were performed on each spectrum, and the resulting fit is also shown in Fig. 2. The coating was found to consist mainly of titanium and nitrogen with some oxygen present. The simulations shown in this figure are for titanium concentrations of 55% and 58% for samples A and B respectively. This indicates that both films are close to stoichiometric. However, there are considerable levels of uncertainty in the concentration of nitrogen due to the presence of oxygen and the fact that the films may not be uniform in concentration. In addition to this main layer, several thin layers with varying concentrations of titanium, nitrogen, oxygen and silicon were found between the coating and the silicon substrate. This indicates the presence of a mixing region at the substrate coating interface. Another feature in the RBS spectra is a small peak on top of the main titanium signal (indicated with a arrow). This peak was found to originate from a thin (approximately 8 nm) titanium rich buried layer. The ratio of Ti to N of this layer is approximately 3:2. The X-TEM image of sample A is shown in Fig. 3. The main features are (I) the epoxy glue, (II) the TiN film, (III) mixing region, (IV) silicon substrate. The coating thickness was found to be
Fig. 3. X-TEM of sample A, showing the (I) epoxy glue, (II) flim, (III) mixing layer and (IV) Si substrate.
726
S.H.N. Lim et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 723–727
approximately 90 nm for sample A, and 110 nm for sample B. Enlargements of the X-TEM images showing the film/substrate interface for samples A and B are shown in Figs. 4(a) and 5(a) respectively. Both samples show a similar largely amorphous mixing region, which contains a layer of crystallites approximately 7–8 nm thick. This result is consistent with the RBS simulations, which predicted that there was a buried layer under the thin film. From this, it can be concluded that this
buried layer is rich in titanium, and is most likely a row of titanium crystallites. This row of titanium appears in both of the samples. The process that the two samples have in common was the initial nitriding stage. Therefore this layer probably results from the pre-nitriding stage where the energetic nitrogen ions etch titanium off the wall of the deposition chamber and from the titanium substrate holder. These titanium atoms could then be implanted into the silicon substrate
Fig. 4. (a) X-TEM image of sample A (no PIII), (b) diffraction pattern of the TiN flim. Note that the diffraction pattern is correctly aligned with respect to the image in (a).
Fig. 5. (a) X-TEM image of sample B (with PIII), (b) diffraction pattern of the TiN film. Note that the diffraction pattern is correctly aligned with respect to the image in (a).
S.H.N. Lim et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 723–727
during the high voltage pulsing. The origin of this feature is presently under further investigation. The diffraction patterns for sample A and sample B are shown in Figs. 4(b) and 5(b) respectively. These diffraction pattern photographs are aligned according to the TEM photographs of the corresponding film cross-sections. The strong spots along a line perpendicular to the film indicate that planes with indexes corresponding to the ring containing the spots lie in the plane of the film (i.e. the crystal direction is perpendicular to the plane). Careful inspection of the diffraction pattern reveals that the structures of the two samples are different. Sample A, which did not use the PIII during film deposition shows a strong preferred orientation with (1 1 1) planes parallel to the surface, whereas in sample B, where the PIII was used during film growth, there was preferred orientation of the (2 0 0) planes parallel to the surface. It is well known that TiN films produced using PVD techniques where ions energies are typically in the order of tens to a few hundreds eV, contain crystallites oriented with their (1 1 1) planes parallel to the surface of the film [7]. Pelleg [8] showed that using an energy minimisation model for preferred orientation, there should be a cut-off stress level in TiN, where the (1 1 1) orientation is preferred. At stress levels lower than the cut-off, the (2 0 0) orientation is preferred. If we assume that the total energy of the system comes only from the bulk strain energy and the surface energy, then the change in the orientation could be explained. Preliminary stress measurements support this idea [9]. Therefore, based on energy minimisation the transition from the (1 1 1) orientation to (2 0 0) orientation is most likely due to the presence of the PIII, which provides energy to the ions in the tens of kV range. This results in a local annealing effect, which relieves the intrinsic stress in the film. We believe that it is this reduction in intrinsic stress that leads to the transition from (1 1 1) orientation to (2 0 0) orientation [9].
4. Conclusion In this paper, we have compared TiN films produced on silicon with and without PIII high
727
voltage pulse biasing of the substrate holder in a cathodic arc PVD system. Both coatings were found to have similar stoichiometry, however the colour of the films is found to be different. The RBS results show that the interface region between coating and substrate was complex, with several thin layers. The main difference between the two films was a change in the preferred orientation of the TiN microstructure. This change in the preferred orientation was found to be from (1 1 1) in the case where no PIII was used to (2 0 0) where PIII was in operation. This orientation can be explained by a reduction in the level of intrinsic stress within the coating. This result indicates that combining PIII with a PVD technique can be used to control the orientation of TiN thin films and may also be useful in reducing the intrinsic stress level of coatings.
Acknowledgements We would like to thank, Dr. P. Evans (ANSTO), J. Piggot and R. Tarrant (University of Sydney) and Assoc. Prof. Phil Wilksch (RMIT) for their contributions to this work. We also acknowledge the support of the Australian Research Council (ARC) and AINSE.
References [1] I. Brown, Ann. Rev. Mater. Sci. 28 (1998) 243. [2] A. Anders, Surf. Coat. Technol. 93 (1997) 157. [3] M.M.M. Bilek, D. McKenzie, D.G. McCulloch, H. Zreiqat, J. Appl. Phys. 87 (9) (2000) 4198. [4] M.M.M. Bilek, D.R. McKenzie, R.I. Tarrant, T. Oates, D.G. McCulloch, The 12th International Federation of Heat Treatment and Surface Engineering Congress, 2000. [5] M.M.M. Bilek, J. Appl. Phys. 85 (9) (1999) 6385. [6] L.R Doolittle, M.O. Thompson, RUMP Program, Cornell University Ithaca NY, USA. [7] H.H. Nguyen, D.R. McKenzie, W.D. McFall, Y. Yin, J. Appl. Phys. 80 (11) (1996) 6279. [8] J. Pelleg, L.Z. Zevin, S. Lungo, Thin Solid Films 197 (1991) 117. [9] S.H.N. Lim, M.M.M. Bilek, D.G. McCulloch, D.R. McKenzie, in preparation, 2002.