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Structure and properties of copper nitride films formed by reactive magnetron sputtering
Vacuum 66 (2002) 59–64
Structure and properties of copper nitride films formed by reactive magnetron sputtering J.F. Pierson* Centre de Recherche sur les Ecoulements les Surfaces et les Transferts,(UMR CNRS 6000), Universit!e de Franche-Comt!e, # Universitaire, BP 71427, 25211 Montb!eliard Cedex, France Pole Received 29 June 2001; accepted 17 October 2001
Abstract Copper nitride (Cu3N) coatings are deposited on glass and steel substrates by RF magnetron sputtering of a copper target in various Ar–N2 reactive mixtures. The films are characterised by X-ray diffraction, scanning electron microscopy, Vickers microhardness, four-point probe method and UV-visible spectrometry. Nanocrystals of Cu3N are formed as soon as nitrogen is introduced in the deposition chamber. Determination of the Cu3N lattice constant shows that below a critical nitrogen flow rate the films are substoichiometric, and that they are overstoichiometric above this critical flow rate. The nonstoichiometry, the mean crystal size, the direction of preferred orientation and the surface morphology of the films have been correlated. Except for the film deposited with a nitrogen flow rate of 1 sccm, the film hardness seems to be independent of this experimental parameter. Finally, the electrical resistivity at room temperature and the optical band gap of Cu3N films have been determined versus the nitrogen flow rate. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Reactive sputtering; Cu3N; Structure; Hardness; Electrical resistivity
1. Introduction Copper nitride has a cubic anti-ReO3 type structure (space group: Pm3 m, lattice constant: 0.3815 nm). In this structure, nitrogen atoms are positioned at the corners of the cell and copper atoms are positioned at the centre of the cube edges. In the past years, interest in copper nitride films is growing because of potential applications of this material for write-once optical recording *Corresponding author. Tel.: +33-3-81-99-46-72; fax: +333-81-99-46-73. E-mail address: [email protected] (J.F. Pierson).
media [1–3] and for generating microscopic copper lines by maskless laser writing [4]. Indeed, this nitride is thermally unstable and decomposes into copper and nitrogen. From various authors, the thermal decomposition has been reported in the temperature range 100–4701C [1,2,4–7]. Although copper nitride has important applications, few papers are available in the literature concerning the properties of Cu3N films. In this report, copper nitride coatings are formed by reactive RF magnetron sputtering. The structure, hardness, electrical resistivity and optical band gap of the films are investigated as a function of the nitrogen flow rate (QðN2 Þ) introduced in the reactor.
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 4 2 5 - 0
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2. Experimental Copper nitride films are deposited by RF magnetron sputtering of a copper target (200-mm diameter) in various Ar-N2 reactive mixtures using an Alcatel SCM 650 sputtering system. The 0.1 m3 stainless steel chamber is evacuated to a pressure of 104 Pa before admitting pure argon and nitrogen. Prior to the deposition, the substrates (microscope slides and 32CrMoV13 steel) are in situ cleaned with argon sputter etching for 15 min. During the last 5 min, the target is simultaneously pre-sputtered in an Ar–N2 mixture, a shield being interposed between the target and the substrates. In this study, the argon flow rate is kept constant at 25 sccm and the nitrogen flow rate is varied from 0 to 10 sccm. Thus, the total pressure ranges between 0.5 and 0.7 Pa.The RF power applied to the target is fixed at 600 W (B1.9 W cm2). A target-substrate distance of 60 mm is used throughout the study. The thickness of the film is measured using a tactile profilometer and the growth rate is calculated from the film thickness obtained for a given deposition time. Films of approximately 2.5 mm thickness deposited on glass substrates are examined by X-ray diffraction (XRD) using Cu Ka radiation. The mean crystal size is determined from the full-width at halfmaximum of X-ray diffraction line using the Debye–Scherrer formula and Warren’s correction to account for the instrumental broadening. The morphology of copper nitride films is studied by scanning electron microscopy (SEM). Microhardness measurements are performed on 12-mm thick films deposited on steel substrates by Vickers indentation under a load of 0.05 N. This load is compatible with the coating thickness (i.e. the indentation depth is always lower than 1.2 mm). The electrical resistivity at room temperature of 2.5-mm thick films deposited on glass substrates is deduced from sheet resistance measurements using the fourpoint probe method. The transmission spectra of 100-nm thick films are recorded over the region 200–1100 nm using a UV-visible spectrophotometer.
Fig. 1. Effect of nitrogen flow rate on X-ray diffractograms of copper nitride films deposited on a glass substrate. The nitrogen flow rate is varied from 0 to 10 sccm.
3. Results and discussion 3.1. Structure Fig. 1 shows the effect of the nitrogen flow rate on the X-ray diffraction pattern of copper nitride films deposited on glass substrates. When 1 sccm of nitrogen is introduced in the reactor, two major diffraction peaks are located close to 41.81 and 48.71 (i.e. between the theoretical position of (1 1 1) Cu3N and Cu peaks and between (2 0 0) Cu3N and Cu peaks, respectively) (Fig. 1). Since the (1 0 0) diffraction peak of Cu3N is unambiguously observed at approximately 23.71, these two majors peaks have been assigned to the copper nitride phase. Furthermore, the (1 1 1) and (2 0 0) Cu3N diffraction peaks exhibit a shoulder at higher angle position that can be assigned to copper. Thus, the films formed with QðN2 Þ ¼ 1 sccm are composed of a mixture of a nanocrystalline copper nitride phase and a weakly crystallised copper phase. The
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Fig. 2. Evolution of the lattice constant of Cu3N coatings with QðN2 Þ:
increase of the nitrogen flow rate leads to the disappearance of the copper phase and only copper nitride is detected by X-ray diffraction (Fig. 1). In Fig. 1, it appears clearly that the diffraction peaks of Cu3N shift to lower angle position when QðN2 Þ increases. Thus, the lattice constant of Cu3N changes with the nitrogen flow rate (Fig. 2). In this figure, the lattice constant has been estimated from the position of the (1 0 0) diffraction peak. The Cu3N lattice constant is lower than the theoretical value (i.e. 0.3815 nm) [8] for QðN2 Þo4 sccm and higher than this value for QðN2 Þ > 4 sccm. Thus for low nitrogen flow rate, copper nitride films are substoichiometric whereas for high nitrogen flow rate, they are overstoichiometric in terms of nitrogen content. The transition from substoichiometric to overstoichiometric occurs at a critical value of nitrogen flow rate (QðN2 Þc ¼ 4 sccm) where the films are nearly stoichiometric (Fig. 2). This kind of transition versus the nitrogen partial pressure has been also reported by other authors [5,7,9]. The lattice constant becomes nearly unchanged when the nitrogen flow rate exceeds 7 sccm in the explored deposition conditions. The critical threshold of nitrogen flow rate (QðN2 Þc ¼ 4 sccm) is found also for the film texture (Fig. 1), for the film morphology (Fig. 3) and for the crystal size evolution (Fig. 4). Cu3N films sputtered at QðN2 Þ > 4 sccm exhibit a strong preferred orientation in the [1 0 0] direction. This texture is the most frequently encountered for
Fig. 3. SEM micrograph of the surface of films deposited with (a) QðN2 Þ ¼ 4 sccm and with (b) QðN2 Þ ¼ 9 sccm.
Fig. 4. Influence of nitrogen flow rate on the mean crystal size of Cu3N.
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reactively sputtered copper nitride films [9–13]. According to Kim et al. [14], the film texture changes at low nitrogen flow rate (Fig. 1). For example, Cu3N films sputtered at QðN2 Þ ¼ 3 or 4 sccm show a preferred orientation in the [1 1 1] direction. The surface morphology of Cu3N films, prepared at two different nitrogen flow rates, is shown in Fig. 3. The film deposited at QðN2 Þ ¼ 4 sccm shows a highly facetted morphology (Fig. 3a) probably due to its preferred orientation in the [1 1 1] direction. This facetted morphology is also observed for films deposited at 2 or 3 sccm of nitrogen. The size of the apparent grains increases with the nitrogen flow rate. When QðN2 Þ > 4 sccm, the surface morphology changes markedly (Fig. 3b). All films with a [1 0 0] preferred orientation show a nodular-like morphology. Thus, the film surface morphology can be correlated to the film texture: films exhibiting the [1 1 1] preferred orientation have a facetted morphology and those exhibiting the [1 0 0] preferred orientation have a nodular-like one. Fig. 4 shows the evolution of the mean crystal size calculated from the Cu3N (1 0 0) diffraction peak versus QðN2 Þ using the modified Scherrer formula. This indicates that, for all nitrogen flow rates, the size of grains ranges from 9 to 16 nm and the copper nitride films are of nanocrystalline size. For substoichiometric films, the crystal size decreases when QðN2 Þ increases. The minimal crystal size is obtained for QðN2 Þ ¼ 4 sccm. For films overstoichiometric in nitrogen, the mean crystal size first increases with QðN2 Þ and for QðN2 Þ > 6 sccm saturates at about 15 nm. The mean crystal size well correlates with the lattice constant for overstoichiometric copper nitride films (Figs. 2 and 4). 3.2. Film properties 3.2.1. Hardness Fig. 5 shows the evolution of the film microhardness versus the nitrogen flow rate. The microhardness of copper films is 1.7 GPa. Introduction of nitrogen in the reactive mixture induces an abrupt increase of the film hardness. The film deposited at QðN2 Þ ¼ 1 sccm has a hardness of about 4.25 GPa. The hardness of films deposited
Fig. 5. Vickers microhardness of copper nitride coatings versus QðN2 Þ:
for QðN2 Þ > 1 sccm does not depend on QðN2 Þ and saturates at approximately 3.7 GPa. This means that the stoichiometry of copper nitride films does not affect strongly their hardness under the deposition conditions used in our experiments. The higher hardness of the film deposited with 1 sccm N2 can be attributed to its composite structure: nanocrystallised Cu3N and weakly crystallised copper (Fig. 1) [15]. Indeed in this kind of structure, the microhardness is greater than that of the harder phase of the composite. Microindentation with high-applied loads (up to 20 N) have been performed to estimate the coating brittleness. Whatever the applied load, nonreactively sputtered copper coatings exhibit a ductile behaviour: no radial or tangential cracks have been observed near the indentation imprint. A SEM micrograph of a indentation imprint with an applied load of 3-N is presented in Fig. 6a for a Cu3N film. With this applied load, the Cu3N film is plastically deformed without cracking. However, with an applied load of 20 N, radial cracks are observed near the indentation imprint (Fig. 6b). Since the length of these cracks is smaller than the imprint diagonal, it can be concluded that Cu3N coatings do not present marked brittle behaviour. 3.2.2. Electrical properties The effect of the nitrogen flow rate on the electrical resistivity at room temperature of Cu3N coatings is presented in Fig. 7. Nonreactively
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Fig. 7. Variation of the electrical resistivity at room temperature of Cu3N coatings versus the nitrogen flow rate.
Fig. 6. SEM micrograph of a Vickers indentation on a 12-mm thick Cu3N film (a) with an applied load of 3 N and (b) with an applied load of 20 N.
sputtered copper films have an electrical resistivity of approximately 2.170.1 mO cm. This value is higher than the resistivity of bulk copper (1.67 mO cm) and of normal copper thin film (1.75 to 2 mO cm) [16]. The electrical resistivity of copper nitride films increases with the nitrogen flow rate from 98 to 34700 mO cm. Depending on the deposition conditions, the resistivity of Cu3N films mentioned in the literature varies from 20 to 2 109 mO cm [5,10,11]. Moreover, Terada et al. have produced insulator heteroepitaxial copper nitride films [17]. This means that the films formed in this study are less resistant than those previously synthesised by others and show similar behaviour to those reported by Maruyama and Morishita [9]. Maruyama and Morishita propose that copper
atoms are inserted in the body centre site of the Cu3N structure. Such an inserted copper atom acts as an electron donor and decreases the electrical resistivity. In their paper, they report that conductive Cu3N films always present a lattice constant higher than a critical value of 0.3868 nm and the lattice constant of insulating films is lower than this critical value [9]. From the X-ray diffraction results presented in the present study, the lattice constant of our films is lower than 0.384 nm for all QðN2 Þ values (Fig. 2). Here, the lattice constant variation versus the nitrogen flow rate can be attributed to a nitrogen nonstoichiometry and the nonstoichiometry may lower the electrical resistivity of our Cu3N films. 3.2.3. Optical properties Optical band gap (Eg ) determination is based on UV-visible transmission measurements considering copper nitride as an indirect semiconductor. Thus, Eg is determined using the plot ðahnÞ1=2 versus hn by extrapolating the full line to the abscissa (a denotes the absorption coefficient and hn the photon energy). For films deposited at a nitrogen flow rate lower than 4 sccm, Eg cannot be determined probably due to a metallic behaviour of these films. On the other hand for films deposited at QðN2 Þ higher or equal to 4 sccm, the optical band gap increases with increasing QðN2 Þ from 0.25 to 0.83 eV. These values are low compared to those published in the literature
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where 1.2oEg o1.9 eV [5,9,14,18–19]. The low optical band gap values in this study may be explained by the low electrical resistivity of copper nitride films. This is in agreement with Maruyama and Morishita who observed that the optical band gap decreases along with the resistivity [9]. Copper nitride films deposited by Nosaka et al. present a reddish dark brown colour [11]. However in this study, films formed with QðN2 Þ > 4 sccm exhibit a metallic silver colour as in Ref. [20]. This difference in the film colour can be attributed to the lower electrical resistivity and optical band gap of our Cu3N films.
4. Conclusion This paper is devoted to a study of copper nitride films deposited by reactive RF magnetron sputtering of a copper target in various Ar–N2 mixtures. The structure, hardness, electrical resistivity and optical band gap of these films have been investigated. Under the conditions used in this study, copper nitride is formed as soon as nitrogen is introduced in the deposition reactor. At low nitrogen flow rate, the Cu3N films are substoichiometric whereas at high QðN2 Þ; they are overstoichiometric in nitrogen. The transition from substoichiometric to overstoichiometric occurs at a gas flow rate QðN2 Þc of 4 sccm. Substoichiometric films grow with a preferred orientation in the [1 1 1] direction and have a facetted surface morphology. On the contrary, overstoichiometric films grow with a strong preferred orientation in the [1 0 0] direction and have a nodular surface morphology. Whatever the nitrogen flow rate, copper nitride films are of nanocrystalline form with a mean crystal size ranging between 9 and 16 nm. Incorporation of nitrogen in copper-based coatings leads to an abrupt increase of the microhardness. Due to its composite structure, the highest hardness is obtained for the film deposited with QðN2 Þ ¼ 1 sccm. Higher values of nitrogen flow rates do not affect the Cu3N hardness which remains approximately constant at 3.7 GPa. Microindentations with high-applied load show that the copper nitride films are not brittle. Electrical
resistivity at room temperature and optical band gap of Cu3N films increase with the nitrogen flow rate.
Acknowledgements The Centre de Transfert Industriel en Traitement de Surfaces (CTITS) is acknowledged for its technical assistance.
References [1] Asano M, Umeda K, Tasaki A. Jpn J Appl Phys 1990;29:1985–6. [2] Maruyama T, Morishita T. Appl Phys Lett 1996;69:890–1. [3] Cremer R, Witthaut M, Neuschutz . D, Trappe C, Laurenzis M, Winkler O, Kurz H. Mikrochim Acta 2000;133:299–302. [4] Maya L. Mat Res Soc Symp Proc 1993;282:203–8. [5] Wang DY, Nakamine N, Hayashi Y. J Vac Sci Technol A 1998;16:2084–92. [6] Liu ZQ, Wang WJ, Wang TM, Chao S, Zheng SK. Thin Solid Films 1998;325:55–9. [7] Nosaka T, Yoshitake M, Okamoto A, Ogawa S, Nakayama Y. Appl Surf Sci 2001;169/170:358–61. [8] JCPDS Powder Diffraction File 2-1156, Int. Centre for Diffraction Data, Park Lane, Swartmore PA, USA. [9] Maruyama T, Morishita T. J Appl Phys 1995;78:4104–7. [10] Maya L. J Vac Sci Technol A 1993;11:604–8. [11] Nosaka T, Yoshitake M, Okamoto A, Ogawa S, Nakayama Y. Thin Solid Films 1999;348:8–13. [12] Soukup L, Sicha M, Fendrych F, Jastrabik L, Hubicka Z, Chvostova D, Sichova H, Valvoda V, Tarasenko A, Studnicka V, Wagner T, Novak M. Surf Coat Technol 1999;116/119:321–6. [13] Chwa SO, Kim KH. J Mater Sci Lett 1998;17:1835–8. [14] Kim KJ, Kim JH, Kang JH. J Cryst Growth 2001;222:767–72. [15] Musil J, Karvankova P, Kasl J. Surf Coat Technol 2001;139:101–9. [16] Shi JR, Lau SP, Sun Z, Shi X, Tay BK, Tan HS. Surf Coat Technol 2001;138:250–5. [17] Terada S, Tanaka H, Kubota K. J Cryst Growth 1989;94:567–8. [18] Fendrych F, Soukup L, Jastabik L, Sicha M, Hubicka Z, Chvostova D, Tarasenko A, Studnicka V, Wagner T. Diam Rel Mater 1999;8:1715–9. [19] Ghosh S, Singh F, Choudhary D, Avasthi DK, Ganesan V, Shah P, Gupta A. Surf Coat Technol 2001;142/ 144:1034–9. [20] Miyano R, Kimura K, Izumi K, Takikawa H, Sakakibara T. Vacuum 2000;59:159–67.
Structure and properties of copper nitride films formed by reactive magnetron sputtering J.F. Pierson* Centre de Recherche sur les Ecoulements les Surfaces et les Transferts,(UMR CNRS 6000), Universit!e de Franche-Comt!e, # Universitaire, BP 71427, 25211 Montb!eliard Cedex, France Pole Received 29 June 2001; accepted 17 October 2001
Abstract Copper nitride (Cu3N) coatings are deposited on glass and steel substrates by RF magnetron sputtering of a copper target in various Ar–N2 reactive mixtures. The films are characterised by X-ray diffraction, scanning electron microscopy, Vickers microhardness, four-point probe method and UV-visible spectrometry. Nanocrystals of Cu3N are formed as soon as nitrogen is introduced in the deposition chamber. Determination of the Cu3N lattice constant shows that below a critical nitrogen flow rate the films are substoichiometric, and that they are overstoichiometric above this critical flow rate. The nonstoichiometry, the mean crystal size, the direction of preferred orientation and the surface morphology of the films have been correlated. Except for the film deposited with a nitrogen flow rate of 1 sccm, the film hardness seems to be independent of this experimental parameter. Finally, the electrical resistivity at room temperature and the optical band gap of Cu3N films have been determined versus the nitrogen flow rate. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Reactive sputtering; Cu3N; Structure; Hardness; Electrical resistivity
1. Introduction Copper nitride has a cubic anti-ReO3 type structure (space group: Pm3 m, lattice constant: 0.3815 nm). In this structure, nitrogen atoms are positioned at the corners of the cell and copper atoms are positioned at the centre of the cube edges. In the past years, interest in copper nitride films is growing because of potential applications of this material for write-once optical recording *Corresponding author. Tel.: +33-3-81-99-46-72; fax: +333-81-99-46-73. E-mail address: [email protected] (J.F. Pierson).
media [1–3] and for generating microscopic copper lines by maskless laser writing [4]. Indeed, this nitride is thermally unstable and decomposes into copper and nitrogen. From various authors, the thermal decomposition has been reported in the temperature range 100–4701C [1,2,4–7]. Although copper nitride has important applications, few papers are available in the literature concerning the properties of Cu3N films. In this report, copper nitride coatings are formed by reactive RF magnetron sputtering. The structure, hardness, electrical resistivity and optical band gap of the films are investigated as a function of the nitrogen flow rate (QðN2 Þ) introduced in the reactor.
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 4 2 5 - 0
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2. Experimental Copper nitride films are deposited by RF magnetron sputtering of a copper target (200-mm diameter) in various Ar-N2 reactive mixtures using an Alcatel SCM 650 sputtering system. The 0.1 m3 stainless steel chamber is evacuated to a pressure of 104 Pa before admitting pure argon and nitrogen. Prior to the deposition, the substrates (microscope slides and 32CrMoV13 steel) are in situ cleaned with argon sputter etching for 15 min. During the last 5 min, the target is simultaneously pre-sputtered in an Ar–N2 mixture, a shield being interposed between the target and the substrates. In this study, the argon flow rate is kept constant at 25 sccm and the nitrogen flow rate is varied from 0 to 10 sccm. Thus, the total pressure ranges between 0.5 and 0.7 Pa.The RF power applied to the target is fixed at 600 W (B1.9 W cm2). A target-substrate distance of 60 mm is used throughout the study. The thickness of the film is measured using a tactile profilometer and the growth rate is calculated from the film thickness obtained for a given deposition time. Films of approximately 2.5 mm thickness deposited on glass substrates are examined by X-ray diffraction (XRD) using Cu Ka radiation. The mean crystal size is determined from the full-width at halfmaximum of X-ray diffraction line using the Debye–Scherrer formula and Warren’s correction to account for the instrumental broadening. The morphology of copper nitride films is studied by scanning electron microscopy (SEM). Microhardness measurements are performed on 12-mm thick films deposited on steel substrates by Vickers indentation under a load of 0.05 N. This load is compatible with the coating thickness (i.e. the indentation depth is always lower than 1.2 mm). The electrical resistivity at room temperature of 2.5-mm thick films deposited on glass substrates is deduced from sheet resistance measurements using the fourpoint probe method. The transmission spectra of 100-nm thick films are recorded over the region 200–1100 nm using a UV-visible spectrophotometer.
Fig. 1. Effect of nitrogen flow rate on X-ray diffractograms of copper nitride films deposited on a glass substrate. The nitrogen flow rate is varied from 0 to 10 sccm.
3. Results and discussion 3.1. Structure Fig. 1 shows the effect of the nitrogen flow rate on the X-ray diffraction pattern of copper nitride films deposited on glass substrates. When 1 sccm of nitrogen is introduced in the reactor, two major diffraction peaks are located close to 41.81 and 48.71 (i.e. between the theoretical position of (1 1 1) Cu3N and Cu peaks and between (2 0 0) Cu3N and Cu peaks, respectively) (Fig. 1). Since the (1 0 0) diffraction peak of Cu3N is unambiguously observed at approximately 23.71, these two majors peaks have been assigned to the copper nitride phase. Furthermore, the (1 1 1) and (2 0 0) Cu3N diffraction peaks exhibit a shoulder at higher angle position that can be assigned to copper. Thus, the films formed with QðN2 Þ ¼ 1 sccm are composed of a mixture of a nanocrystalline copper nitride phase and a weakly crystallised copper phase. The
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Fig. 2. Evolution of the lattice constant of Cu3N coatings with QðN2 Þ:
increase of the nitrogen flow rate leads to the disappearance of the copper phase and only copper nitride is detected by X-ray diffraction (Fig. 1). In Fig. 1, it appears clearly that the diffraction peaks of Cu3N shift to lower angle position when QðN2 Þ increases. Thus, the lattice constant of Cu3N changes with the nitrogen flow rate (Fig. 2). In this figure, the lattice constant has been estimated from the position of the (1 0 0) diffraction peak. The Cu3N lattice constant is lower than the theoretical value (i.e. 0.3815 nm) [8] for QðN2 Þo4 sccm and higher than this value for QðN2 Þ > 4 sccm. Thus for low nitrogen flow rate, copper nitride films are substoichiometric whereas for high nitrogen flow rate, they are overstoichiometric in terms of nitrogen content. The transition from substoichiometric to overstoichiometric occurs at a critical value of nitrogen flow rate (QðN2 Þc ¼ 4 sccm) where the films are nearly stoichiometric (Fig. 2). This kind of transition versus the nitrogen partial pressure has been also reported by other authors [5,7,9]. The lattice constant becomes nearly unchanged when the nitrogen flow rate exceeds 7 sccm in the explored deposition conditions. The critical threshold of nitrogen flow rate (QðN2 Þc ¼ 4 sccm) is found also for the film texture (Fig. 1), for the film morphology (Fig. 3) and for the crystal size evolution (Fig. 4). Cu3N films sputtered at QðN2 Þ > 4 sccm exhibit a strong preferred orientation in the [1 0 0] direction. This texture is the most frequently encountered for
Fig. 3. SEM micrograph of the surface of films deposited with (a) QðN2 Þ ¼ 4 sccm and with (b) QðN2 Þ ¼ 9 sccm.
Fig. 4. Influence of nitrogen flow rate on the mean crystal size of Cu3N.
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reactively sputtered copper nitride films [9–13]. According to Kim et al. [14], the film texture changes at low nitrogen flow rate (Fig. 1). For example, Cu3N films sputtered at QðN2 Þ ¼ 3 or 4 sccm show a preferred orientation in the [1 1 1] direction. The surface morphology of Cu3N films, prepared at two different nitrogen flow rates, is shown in Fig. 3. The film deposited at QðN2 Þ ¼ 4 sccm shows a highly facetted morphology (Fig. 3a) probably due to its preferred orientation in the [1 1 1] direction. This facetted morphology is also observed for films deposited at 2 or 3 sccm of nitrogen. The size of the apparent grains increases with the nitrogen flow rate. When QðN2 Þ > 4 sccm, the surface morphology changes markedly (Fig. 3b). All films with a [1 0 0] preferred orientation show a nodular-like morphology. Thus, the film surface morphology can be correlated to the film texture: films exhibiting the [1 1 1] preferred orientation have a facetted morphology and those exhibiting the [1 0 0] preferred orientation have a nodular-like one. Fig. 4 shows the evolution of the mean crystal size calculated from the Cu3N (1 0 0) diffraction peak versus QðN2 Þ using the modified Scherrer formula. This indicates that, for all nitrogen flow rates, the size of grains ranges from 9 to 16 nm and the copper nitride films are of nanocrystalline size. For substoichiometric films, the crystal size decreases when QðN2 Þ increases. The minimal crystal size is obtained for QðN2 Þ ¼ 4 sccm. For films overstoichiometric in nitrogen, the mean crystal size first increases with QðN2 Þ and for QðN2 Þ > 6 sccm saturates at about 15 nm. The mean crystal size well correlates with the lattice constant for overstoichiometric copper nitride films (Figs. 2 and 4). 3.2. Film properties 3.2.1. Hardness Fig. 5 shows the evolution of the film microhardness versus the nitrogen flow rate. The microhardness of copper films is 1.7 GPa. Introduction of nitrogen in the reactive mixture induces an abrupt increase of the film hardness. The film deposited at QðN2 Þ ¼ 1 sccm has a hardness of about 4.25 GPa. The hardness of films deposited
Fig. 5. Vickers microhardness of copper nitride coatings versus QðN2 Þ:
for QðN2 Þ > 1 sccm does not depend on QðN2 Þ and saturates at approximately 3.7 GPa. This means that the stoichiometry of copper nitride films does not affect strongly their hardness under the deposition conditions used in our experiments. The higher hardness of the film deposited with 1 sccm N2 can be attributed to its composite structure: nanocrystallised Cu3N and weakly crystallised copper (Fig. 1) [15]. Indeed in this kind of structure, the microhardness is greater than that of the harder phase of the composite. Microindentation with high-applied loads (up to 20 N) have been performed to estimate the coating brittleness. Whatever the applied load, nonreactively sputtered copper coatings exhibit a ductile behaviour: no radial or tangential cracks have been observed near the indentation imprint. A SEM micrograph of a indentation imprint with an applied load of 3-N is presented in Fig. 6a for a Cu3N film. With this applied load, the Cu3N film is plastically deformed without cracking. However, with an applied load of 20 N, radial cracks are observed near the indentation imprint (Fig. 6b). Since the length of these cracks is smaller than the imprint diagonal, it can be concluded that Cu3N coatings do not present marked brittle behaviour. 3.2.2. Electrical properties The effect of the nitrogen flow rate on the electrical resistivity at room temperature of Cu3N coatings is presented in Fig. 7. Nonreactively
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Fig. 7. Variation of the electrical resistivity at room temperature of Cu3N coatings versus the nitrogen flow rate.
Fig. 6. SEM micrograph of a Vickers indentation on a 12-mm thick Cu3N film (a) with an applied load of 3 N and (b) with an applied load of 20 N.
sputtered copper films have an electrical resistivity of approximately 2.170.1 mO cm. This value is higher than the resistivity of bulk copper (1.67 mO cm) and of normal copper thin film (1.75 to 2 mO cm) [16]. The electrical resistivity of copper nitride films increases with the nitrogen flow rate from 98 to 34700 mO cm. Depending on the deposition conditions, the resistivity of Cu3N films mentioned in the literature varies from 20 to 2 109 mO cm [5,10,11]. Moreover, Terada et al. have produced insulator heteroepitaxial copper nitride films [17]. This means that the films formed in this study are less resistant than those previously synthesised by others and show similar behaviour to those reported by Maruyama and Morishita [9]. Maruyama and Morishita propose that copper
atoms are inserted in the body centre site of the Cu3N structure. Such an inserted copper atom acts as an electron donor and decreases the electrical resistivity. In their paper, they report that conductive Cu3N films always present a lattice constant higher than a critical value of 0.3868 nm and the lattice constant of insulating films is lower than this critical value [9]. From the X-ray diffraction results presented in the present study, the lattice constant of our films is lower than 0.384 nm for all QðN2 Þ values (Fig. 2). Here, the lattice constant variation versus the nitrogen flow rate can be attributed to a nitrogen nonstoichiometry and the nonstoichiometry may lower the electrical resistivity of our Cu3N films. 3.2.3. Optical properties Optical band gap (Eg ) determination is based on UV-visible transmission measurements considering copper nitride as an indirect semiconductor. Thus, Eg is determined using the plot ðahnÞ1=2 versus hn by extrapolating the full line to the abscissa (a denotes the absorption coefficient and hn the photon energy). For films deposited at a nitrogen flow rate lower than 4 sccm, Eg cannot be determined probably due to a metallic behaviour of these films. On the other hand for films deposited at QðN2 Þ higher or equal to 4 sccm, the optical band gap increases with increasing QðN2 Þ from 0.25 to 0.83 eV. These values are low compared to those published in the literature
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where 1.2oEg o1.9 eV [5,9,14,18–19]. The low optical band gap values in this study may be explained by the low electrical resistivity of copper nitride films. This is in agreement with Maruyama and Morishita who observed that the optical band gap decreases along with the resistivity [9]. Copper nitride films deposited by Nosaka et al. present a reddish dark brown colour [11]. However in this study, films formed with QðN2 Þ > 4 sccm exhibit a metallic silver colour as in Ref. [20]. This difference in the film colour can be attributed to the lower electrical resistivity and optical band gap of our Cu3N films.
4. Conclusion This paper is devoted to a study of copper nitride films deposited by reactive RF magnetron sputtering of a copper target in various Ar–N2 mixtures. The structure, hardness, electrical resistivity and optical band gap of these films have been investigated. Under the conditions used in this study, copper nitride is formed as soon as nitrogen is introduced in the deposition reactor. At low nitrogen flow rate, the Cu3N films are substoichiometric whereas at high QðN2 Þ; they are overstoichiometric in nitrogen. The transition from substoichiometric to overstoichiometric occurs at a gas flow rate QðN2 Þc of 4 sccm. Substoichiometric films grow with a preferred orientation in the [1 1 1] direction and have a facetted surface morphology. On the contrary, overstoichiometric films grow with a strong preferred orientation in the [1 0 0] direction and have a nodular surface morphology. Whatever the nitrogen flow rate, copper nitride films are of nanocrystalline form with a mean crystal size ranging between 9 and 16 nm. Incorporation of nitrogen in copper-based coatings leads to an abrupt increase of the microhardness. Due to its composite structure, the highest hardness is obtained for the film deposited with QðN2 Þ ¼ 1 sccm. Higher values of nitrogen flow rates do not affect the Cu3N hardness which remains approximately constant at 3.7 GPa. Microindentations with high-applied load show that the copper nitride films are not brittle. Electrical
resistivity at room temperature and optical band gap of Cu3N films increase with the nitrogen flow rate.
Acknowledgements The Centre de Transfert Industriel en Traitement de Surfaces (CTITS) is acknowledged for its technical assistance.
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