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Influence of bias voltage on the microstructure and physical properties of magnetron sputtered Zr–Si–N nanocomposite thin films
Surface & Coatings Technology 204 (2009) 969–972
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Influence of bias voltage on the microstructure and physical properties of magnetron sputtered Zr–Si–N nanocomposite thin films C.S. Sandu a, N. Cusnir b, D. Oezer b, R. Sanjinés b,⁎, J. Patscheider c a b c
Laboratoire de céramique, IMX, STI, EPFL, CH-1015 Lausanne, Switzerland Institute of Physics of Condensed Matter, EPFL, CH-1015 Lausanne, Switzerland LNMSAMS, EMPA, Uberlandstr. 129, CH-8600 Dübendorf, Switzerland
a r t i c l e
i n f o
Available online 4 July 2009 Keywords: Nanocomposite Zirconium nitride Morphology Thin films
a b s t r a c t We report an investigation concerning the influence of ion bombardment on the nanostructure and physical properties of Zr–Si–N nanocomposite thin films. The films were deposited by reactive magnetron sputtering from individual Zr and Si targets. The Si content was varied by changing the power applied to the Si target. The increase of ion bombardment energy was obtained by applying a negative potential Ub = − 150 V to the substrate. The evolution of the film texture, grain size and lattice constant was mapped out using X-ray diffraction measurements. Zr–Si–N films deposited at a substrate temperature Ts = 510 K with a bias voltage of Ub = − 150 V exhibit less pronounced columnar structure with small crystallites having various orientations. The maximum nanohardness of 39 GPa is reached for the films at about 2.5 at.% Si, 8 nm grain size and 0.3 Si surface coverage. The increased energy of ionic species reaching the substrate when a negative bias voltage is applied seems to have the opposite effect to that of increasing substrate temperature: reduced SiNx coverage on the ZrN nanocrystallites. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Nanocomposite thin films of binary compounds based on 3dtransition metal nitrides (MeN) and amorphous Si3N4 are considered as strategic new materials due to a rich variety of physical and chemical properties [1–5]. In MeN–Si3N4 coatings unusually high hardness values are observed, due to an optimized microstructure in which nanocrystallites of MeN (nc-MeN) are surrounded by thinner amorphous layer of Si3N4 (a-SiNx) [4–14]. Usually these films are deposited by CVD or PVD techniques; among the PVD techniques, magnetron reactive sputtering is often used as a low-temperature film growth technique. The chemical composition and microstructure of the resulting films are influenced by the deposition parameters such as the substrate temperature, the flux and kinetic energy of impinging atomic and ionic species on the surface of the growing film, and the condensation rate. Since phase segregation is a diffusion-controlled mechanism, the deposition temperature is a key parameter for understanding the segregation, atomic diffusion and self structuring mechanisms in these nanocomposite materials [11,13]. Another important parameter
⁎ Corresponding author. Tel.: +41 21 693 4442. E-mail address: rosendo.sanjines@epfl.ch (R. Sanjinés). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.06.042
that plays a role on the film nanostructure is the energy of the impinging species on the substrate (ion bombardment) [14–17]. The role of substrate temperature on the formation of nanostructured Zr– Si–N thin films has been particularly well investigated by the authors [11]. These studies indicate that by increasing the substrate temperature the solubility limit of Si atoms in the lattice of ZrN nanocrystallites decreases whereas the thickness of SiN layer covering ZrN nanocrystallites increases. The values of the pairs {solubility limit in percentage, thickness of SiN layer in monolayer} in the Zr–Si–N films are {5%, 0.2}, {4%,0.5}, {2%, 0.85} and {1%, 1.8} for films deposited at RT, 510, 710 and 910 K, respectively [11,13]. In the present work, we investigated the effects of ion bombardment during the film growth on the film nanostructure and physical properties of the Zr–Si–N system. These results are systematically compared with those obtained on films deposited without bias voltage as reported in [11,13]. Ion bombardment implies multiple atomic collisions, enhancement of the mobility, diffusion, implantation, and/or re-sputtering of atoms which can influence the phase separation and the film morphology. For these studies, a bias voltage of − 150 V and relatively low substrates temperatures (RT and 510 K) were selected. As reported in the literature, at moderate bias voltages the substitutional insertion of Si atoms in the ZrN lattice is hindered and the films exhibit denser morphology and higher hardness [15,17,18]. The ion irradiation induces moderate compressive residual stress; the transferred kinetic energy to impinging atoms is enough to insure high ad-atom mobility on the surface of the growing films [17].
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2. Experimental Zr–Si–N thin films were deposited by magnetron co-sputtering from zirconium (Kurt J. Lesker, 99.99% purity) and silicon (99.995% purity) targets (5 cm diameter) in Ar + N2 atmosphere. A DC magnetron source was used for sputtering of the Zr target and a RF source for the Si target. The residual pressure in the reactor was 10− 6 Pa. During the deposition the total pressure and the nitrogen partial pressure were held constant at 0.44 Pa and 0.04 Pa respectively. A series of Zr–Si–N coatings were deposited by changing the sputtering power applied to the Si target. The power on the Zr target was fixed at 110 W. The power applied to the Si target was varied between 0 and 100 W. Pure ZrN films were grown at bias voltage between 0 and −150 V. Zr–Si–N films were grown at a fixed bias voltage of Ub = − 150 V and at two different substrate temperatures namely 510 K and RT (i.e. on unheated substrates). The substrates were mounted in a rotary sample-holder to ensure homogeneity. The typical deposition rate was 0.5 µm/hour and the film thickness was 0.9–1.1 µm as measured by profilometry. Films were grown on polished Si substrates and polished SiO2 substrates for the electrical measurements. The crystal structure of the films was examined by grazing angle X-ray diffraction analysis (GIXRD) at an incidence angle of 4° (Cu Kα radiation). The microstructure of the films was examined in cross section by TEM on a Philips CM20 at an accelerating voltage of 200 kV. Electrical resistivity was measured by the van der Pauw method in a temperature range between 300 K and 20 K. The mechanical properties were evaluated by nanoindentation (Nano IndenterXP, Nano Instruments) measurements with a Berkovich indenter working on continuous stiffness mode. The chemical composition was measured by electron probe microanalyses (EPMA). 3. Results 3.1. Film morphology In the absence of ion bombardment, pure ZrN films deposited at 510 K exhibit a columnar structure with [111] preferred orientation and average crystallite size of 52 nm. The application of a bias voltage of −50 V modifies the texture; the crystallites in the film exhibit a mixed [200] and [111] preferred orientation. Further increase of the bias voltage up to − 150 V does not change significantly the ratio of the (200) and (111) peak intensities, the main changes observed involve film densification, increase of compressive residual stress up to about 5–6 GPa and the progressive diminution of the crystallite size from 52 nm to about 22 nm. Fig. 1 shows the GIXRD patterns of the ZrSiyNx films deposited with a bias voltage of −150 V at 510 K. The addition of Si modifies the film morphology; as the Si content increases, the intensity of the (111) peak progressively decreases, whilst that of the (200) peak increases. The average size of the crystallites decreases from about 22 nm to 2.5 nm. The change of the texture from [111] to [111] + [200] preferred orientation was confirmed by Bragg Brentano measurements. Similar behavior is observed for films deposited at RT (not show in Fig. 1). It is worth noting that in nanocomposite films, the mean value of the crystallize size D, deduced from GIXRD decreases following the relationship D ∼ 1/CSi as shown in Fig. 2a. This behavior confirms that the amount of the amorphous phase SiNx phase in the film on the surface of the crystallites increases by increasing the surface/volume ratio whilst maintaining a constant thickness of the amorphous layer, as proposed in the model for the formation of nanocomposite films [10,12]. In this model, a decrease in the grain size with increasing Si content in the film is a consequence of the segregation of Si atoms on the surface of ZrN crystallites. The calculation of SiNx layer thickness was possible by some limiting assumptions: a uniform thickness of SiNx layer covering cubic ZrN crystallites. According to our 3-step
Fig. 1. X-ray diffraction patterns in grazing incidence (angle of 4°) of Zr–Si–N thin films for different Si contents: the films have been deposited with a bias voltage of − 150 V and at 510 K.
model for the formation of nanocomposite films [10,12] the Si surface coverage Sicov is given by the relation: Sicov =
D ðCSi − α Þ 3a ðCZr + α Þ
where CSi and CZr are the Si and Zr content respectively, α the solubility limit of Si and a the lattice constant. In a series of Zr–Si–N films, the solubility limit is taken as the value of Si concentration when the crystallite size starts to decrease with increasing Si content. According with the proposed model for the formation of nanocomposite films [10], the segregation of Si atoms towards the grain boundaries determines the decrease of ZrN crystallite size. αÞ 3a The ratio ððCCZrSi − + α Þ as a function of D for the two series of films deposited at 510 K and RT is shown in Fig. 2b. If the Sicov remains constant ðCSi − α Þ 3a should exhibit a linear D ðCZr + α Þ as a function of
then the ratio
relationship. These behaviors are observed in Fig. 2b for both series of Zr– Si–N films. It is found that the Si solubility is α≈0 and the Si coverage is about Sicov =0.31 for films deposited at 510 K. In the case of films deposited at RT the Si solubility is α≈1.7 at.% and Sicov =0.33. Compared with our previous results (films deposited without bias [11]: {5%, 0.2} at RT, {4%,0.5}at 510 K), these results show that the ion bombardment reduces the incorporation of Si into the ZrN lattice. The ion bombardment enhances the thickness of the SiNx coverage layer in the case of the films deposited at RT, whereas in the case of the films deposited at 510 K the SiNx layer thickness decreases. Under ion bombardment the thickness of the SiNx coverage layer is temperature independent.
C.S. Sandu et al. / Surface & Coatings Technology 204 (2009) 969–972
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Fig. 3. Dark-field TEM images in cross section of Zr–Si–N films: at ∼2% Si with − 150 V bias a) and without bias b).
increased hardness cannot be attributed to the residual stress because the samples showing the highest hardness values for the series at 510 K, with and without bias, have similar residual stress. The higher hardness observed in the films deposited under ion bombardment seems to be related to a combined effect of film densification and crystallite size reduction. The thickness of the SiN layer seems not to affect film hardening.
4. Discussion Fig. 2. a) Grain size D vs Si content in Zr–Si–N films for the series deposited at 510 K and αÞ 3a RT on unbiased and biased substrates. b) Ratio ððCCZrSi − + α Þ as a function of D for the two series of films deposited with applied bias voltage at 510 K and RT. CSi and CZr are the Si and Zr content respectively, α the solubility limit of Si, D is the grain size and a the lattice constant.
Fig. 3 shows TEM dark-field images of a Zr–Si–N films with Si content of 2 at.% deposited with − 150 V bias a) and without bias b). The increase of the ion bombardment is accompanied by a significant decrease in the grain size from hundreds of nanometer to ten nanometers. The growth remains columnar and textured in the sample deposited under bias voltage conditions even though the grain size is smaller than 10 nm.
As reported in the literature, the main effect of ion bombardment during the film deposition is to increase the mobility of the atoms at the surface and beneath it due to direct and/or indirect multiple atomic collisions. Light atoms such as Si and N can be forward resputtered from the surface or recoil implanted by atomic collision process. The effective enhancement of the atomic mobility leads to atomic diffusion which in turn promotes the crystallization and surface segregation resulting in phase segregation.
3.2. Mechanical properties The dependence of nanohardness on Si content is similar to that observed in a-SiNx based MeN; a maximum hardness of about 39 GPa is observed at CSi = 2.5 at.% in films deposited at 510 K while in films deposited at RT the maximum hardness of 28 GPa is reached at higher Si content (5 at.%). In order to illustrate the effects of ion bombardment on the film hardening, Fig. 4 shows the nanohardness values as a function of the grain size for samples deposited with and without bias at RT and 510 K. A good correlation between the behaviors of hardness vs grain size is observed, in the case of films growth at 510 K. The highest hardness values observed in films deposited under bias are for crystallite size of about 8 nm and in films deposited without bias are for crystallite size of about 12–14 nm. In the case of RT and Ub = −150 V deposited films, the hardness is also improved compared to those deposited at RT with Ub = 0 though the films exhibit similar crystallite size. The
Fig. 4. Zr–Si–N thin films: nanohardness vs crystallite size for the films deposited with and without bias voltage at substrate temperatures of 510 K and RT.
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Greene et al. [19,20] have reported that TiN films grown under ion irradiation at 573 K a substrate bias −Ub ≥ 120 V results in fully dense films while the dissolution of the columnar structure is initiated at −Ub ≥ 160 V. Concerning the Ti–Si–N system, Faz et al. have reported that the mobility of atoms at the surface of the growing films can be enhanced either by increasing the substrate temperature or by ion bombardment promoting the phase segregation with low Si solubility [15]. Ribeiro et al. have investigated the effects of ion bombardment on the morphology of nanocomposite TiN/a-SiNx thin films. At low ion irradiation energies, Si atoms can be introduced in the TiN lattice mainly as substitution atoms. By increasing the irradiation energy an enhancement of the mobility of the surface species was found leading to effective phase segregation in the Ti–Si–N system. These films exhibit low Si solubility, a dense morphology, and high hardness values [17]. Our results are in good agreement with those reported in the literature. The Zr–Si–N films, grown under ion bombardment (Ub = −150 V) at substrates temperatures of 510 K and at RT, exhibit more compact morphologies than those of films deposited at similar temperatures but without bias voltage. The columnar structure, the average grain size and Si solubility limit are significantly reduced. The unexpected result was the decrease of the thickness of the SiNx layer covering ZrN nanocrystallites in samples exposed to ion bombardment and deposited at 510 K. The increased of energy of ionic species reaching the substrate by using a negative bias voltage seems to have the opposite effect to that observed when substrate temperatures are increased: lower SiNx coverage on the ZrN nanocrystallites. The film morphology significantly affects the electrical properties of these films. Thus, nanocomposite films deposited at RT without and with bias voltage and having similar crystallite size (8 nm) exhibit different resistivity values: 3 times lower in the case of films deposited under bias. The increase of the material density at the grain boundaries, as revealed by electrical measurements (lower resistivity for similar grain size), due to ionic bombardment seems to limit the thickness of the SiNx layer. The smaller crystallites obtained in films deposited at 510 K under bias could also be at the origin of lower thickness of the SiNx layer. Film hardening seems to be directly related to crystallite size and film density and does not depend on the thickness of the SiNx layer. 5. Conclusion The important effects of the ionic bombardment during the growth of ZrN/SiN nanocomposite films are: the decrease of ZrN grain size
and the Si solubility limit and the increase of the film density. The increased kinetic energy of the ad-atom on the film surface as a consequence of the applied bias voltage does not lead to an increase in the thickness of the SiNx layer covering ZrN crystallites. The addition of Si is responsible of the grain size diminution. The thickness of the SiNx layer seems not to affect film hardening. Increased film hardness is related to the higher film density.
Acknowledgments The authors gratefully acknowledge the financial support of the Swiss National Science Foundation and of the EPFL. The authors wish to thank the CIME-EPFL team for their TEM investigation facilities and Dr. A. Karimi for nanoindentation measurements.
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M. Diserens, J. Patscheider, F. Lévy, Surf. Coat. Technol. 108–109 (1998) 241. S. Veprek, J. Vac. Sci. Technol., A, Vac. Surf. Films 17 (1999) 2401. J. Musil, Surf. Coat. Technol. 125 (2000) 322. S. Veprek, Maritza J.G. Veprek-Heijman, Surf. Coat. Technol. 202 (2008) 5063. C.K. Chung, T.S. Chen, J. Lumin. 129 (2009) 370. D. Pilloud, J.F. Pierson, A.P. Marques, A. Cavaleiro, Surf. Coat. Technol. 180–181 (2004) 352. J.H. Park, W.S. Chung, Y.-R. Cho, K.H. Kim, Surf. Coat. Technol. 188–189 (2004) 425. M. Nose, W.A. Chiou, J. Vac. Sci. Technol., A, Vac. Surf. films 20 (3) (2002) 823. R. Sanjinés, M. Benkahoul, C.S. Sandu, F. Lévy, J. Appl. Phys. 98 (2005) 123511. C.S. Sandu, M. Benkahoul, R. Sanjinés, F. Lévy, Surf. Coat. Technol. 201 (2006) 2897. C.S. Sandu, F. Medjani, R. Sanjinés, A. Karimi, F. Lévy, Surf. Coat. Technol. 201 (2006) 4219. C.S. Sandu, R. Sanjinés, M. Benkahoul, F. Medjani, F. Lévy, Surf. Coat. Technol. 201 (2006) 4083. C.S. Sandu, F. Medjani, R. Sanjinés, Rev. Adv. Mater. Sci. 15 (2007) 173. S. Veprek, S. Reiprich, Thin Solid Films, 268 (1995) 64. F. Vaz, L. Rebouta, Ph. Goudeau, T. Girardeau, J. Pacaud, J.P. Riviére, A. Traverse, Surf. Coat. Technol. 146–147 (2001) 274. S.R. Choi, I.W. Park, J.H. Park, K.H. Kim, Surf. Coat. Technol. 179 (2004) 89. E. Ribeiro, L. Rebouta, S. Carvalho, F. Vaz, GG. Fuentes, R. Rodriguez, M. Zazpe, E. Alves, Ph. Goudeau, J.P. Riviere, Surf. Coat. Technol. 188–189 (2004) 351. Z.T. Yang, et al., J. Alloys Compd. (2008). doi:10.1016/j.jallcom.2008.06.003. I. Petrov, L. Hultman, U. Helmersson, J.E. Sundgren, J.E. Greene, Thin Solid Films 169 (1989) 299. L. Hltmann, J.E. Sundgren, J.E. Greene, J. Appl. Phys. 78 (1995) 5395.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Influence of bias voltage on the microstructure and physical properties of magnetron sputtered Zr–Si–N nanocomposite thin films C.S. Sandu a, N. Cusnir b, D. Oezer b, R. Sanjinés b,⁎, J. Patscheider c a b c
Laboratoire de céramique, IMX, STI, EPFL, CH-1015 Lausanne, Switzerland Institute of Physics of Condensed Matter, EPFL, CH-1015 Lausanne, Switzerland LNMSAMS, EMPA, Uberlandstr. 129, CH-8600 Dübendorf, Switzerland
a r t i c l e
i n f o
Available online 4 July 2009 Keywords: Nanocomposite Zirconium nitride Morphology Thin films
a b s t r a c t We report an investigation concerning the influence of ion bombardment on the nanostructure and physical properties of Zr–Si–N nanocomposite thin films. The films were deposited by reactive magnetron sputtering from individual Zr and Si targets. The Si content was varied by changing the power applied to the Si target. The increase of ion bombardment energy was obtained by applying a negative potential Ub = − 150 V to the substrate. The evolution of the film texture, grain size and lattice constant was mapped out using X-ray diffraction measurements. Zr–Si–N films deposited at a substrate temperature Ts = 510 K with a bias voltage of Ub = − 150 V exhibit less pronounced columnar structure with small crystallites having various orientations. The maximum nanohardness of 39 GPa is reached for the films at about 2.5 at.% Si, 8 nm grain size and 0.3 Si surface coverage. The increased energy of ionic species reaching the substrate when a negative bias voltage is applied seems to have the opposite effect to that of increasing substrate temperature: reduced SiNx coverage on the ZrN nanocrystallites. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Nanocomposite thin films of binary compounds based on 3dtransition metal nitrides (MeN) and amorphous Si3N4 are considered as strategic new materials due to a rich variety of physical and chemical properties [1–5]. In MeN–Si3N4 coatings unusually high hardness values are observed, due to an optimized microstructure in which nanocrystallites of MeN (nc-MeN) are surrounded by thinner amorphous layer of Si3N4 (a-SiNx) [4–14]. Usually these films are deposited by CVD or PVD techniques; among the PVD techniques, magnetron reactive sputtering is often used as a low-temperature film growth technique. The chemical composition and microstructure of the resulting films are influenced by the deposition parameters such as the substrate temperature, the flux and kinetic energy of impinging atomic and ionic species on the surface of the growing film, and the condensation rate. Since phase segregation is a diffusion-controlled mechanism, the deposition temperature is a key parameter for understanding the segregation, atomic diffusion and self structuring mechanisms in these nanocomposite materials [11,13]. Another important parameter
⁎ Corresponding author. Tel.: +41 21 693 4442. E-mail address: rosendo.sanjines@epfl.ch (R. Sanjinés). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.06.042
that plays a role on the film nanostructure is the energy of the impinging species on the substrate (ion bombardment) [14–17]. The role of substrate temperature on the formation of nanostructured Zr– Si–N thin films has been particularly well investigated by the authors [11]. These studies indicate that by increasing the substrate temperature the solubility limit of Si atoms in the lattice of ZrN nanocrystallites decreases whereas the thickness of SiN layer covering ZrN nanocrystallites increases. The values of the pairs {solubility limit in percentage, thickness of SiN layer in monolayer} in the Zr–Si–N films are {5%, 0.2}, {4%,0.5}, {2%, 0.85} and {1%, 1.8} for films deposited at RT, 510, 710 and 910 K, respectively [11,13]. In the present work, we investigated the effects of ion bombardment during the film growth on the film nanostructure and physical properties of the Zr–Si–N system. These results are systematically compared with those obtained on films deposited without bias voltage as reported in [11,13]. Ion bombardment implies multiple atomic collisions, enhancement of the mobility, diffusion, implantation, and/or re-sputtering of atoms which can influence the phase separation and the film morphology. For these studies, a bias voltage of − 150 V and relatively low substrates temperatures (RT and 510 K) were selected. As reported in the literature, at moderate bias voltages the substitutional insertion of Si atoms in the ZrN lattice is hindered and the films exhibit denser morphology and higher hardness [15,17,18]. The ion irradiation induces moderate compressive residual stress; the transferred kinetic energy to impinging atoms is enough to insure high ad-atom mobility on the surface of the growing films [17].
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2. Experimental Zr–Si–N thin films were deposited by magnetron co-sputtering from zirconium (Kurt J. Lesker, 99.99% purity) and silicon (99.995% purity) targets (5 cm diameter) in Ar + N2 atmosphere. A DC magnetron source was used for sputtering of the Zr target and a RF source for the Si target. The residual pressure in the reactor was 10− 6 Pa. During the deposition the total pressure and the nitrogen partial pressure were held constant at 0.44 Pa and 0.04 Pa respectively. A series of Zr–Si–N coatings were deposited by changing the sputtering power applied to the Si target. The power on the Zr target was fixed at 110 W. The power applied to the Si target was varied between 0 and 100 W. Pure ZrN films were grown at bias voltage between 0 and −150 V. Zr–Si–N films were grown at a fixed bias voltage of Ub = − 150 V and at two different substrate temperatures namely 510 K and RT (i.e. on unheated substrates). The substrates were mounted in a rotary sample-holder to ensure homogeneity. The typical deposition rate was 0.5 µm/hour and the film thickness was 0.9–1.1 µm as measured by profilometry. Films were grown on polished Si substrates and polished SiO2 substrates for the electrical measurements. The crystal structure of the films was examined by grazing angle X-ray diffraction analysis (GIXRD) at an incidence angle of 4° (Cu Kα radiation). The microstructure of the films was examined in cross section by TEM on a Philips CM20 at an accelerating voltage of 200 kV. Electrical resistivity was measured by the van der Pauw method in a temperature range between 300 K and 20 K. The mechanical properties were evaluated by nanoindentation (Nano IndenterXP, Nano Instruments) measurements with a Berkovich indenter working on continuous stiffness mode. The chemical composition was measured by electron probe microanalyses (EPMA). 3. Results 3.1. Film morphology In the absence of ion bombardment, pure ZrN films deposited at 510 K exhibit a columnar structure with [111] preferred orientation and average crystallite size of 52 nm. The application of a bias voltage of −50 V modifies the texture; the crystallites in the film exhibit a mixed [200] and [111] preferred orientation. Further increase of the bias voltage up to − 150 V does not change significantly the ratio of the (200) and (111) peak intensities, the main changes observed involve film densification, increase of compressive residual stress up to about 5–6 GPa and the progressive diminution of the crystallite size from 52 nm to about 22 nm. Fig. 1 shows the GIXRD patterns of the ZrSiyNx films deposited with a bias voltage of −150 V at 510 K. The addition of Si modifies the film morphology; as the Si content increases, the intensity of the (111) peak progressively decreases, whilst that of the (200) peak increases. The average size of the crystallites decreases from about 22 nm to 2.5 nm. The change of the texture from [111] to [111] + [200] preferred orientation was confirmed by Bragg Brentano measurements. Similar behavior is observed for films deposited at RT (not show in Fig. 1). It is worth noting that in nanocomposite films, the mean value of the crystallize size D, deduced from GIXRD decreases following the relationship D ∼ 1/CSi as shown in Fig. 2a. This behavior confirms that the amount of the amorphous phase SiNx phase in the film on the surface of the crystallites increases by increasing the surface/volume ratio whilst maintaining a constant thickness of the amorphous layer, as proposed in the model for the formation of nanocomposite films [10,12]. In this model, a decrease in the grain size with increasing Si content in the film is a consequence of the segregation of Si atoms on the surface of ZrN crystallites. The calculation of SiNx layer thickness was possible by some limiting assumptions: a uniform thickness of SiNx layer covering cubic ZrN crystallites. According to our 3-step
Fig. 1. X-ray diffraction patterns in grazing incidence (angle of 4°) of Zr–Si–N thin films for different Si contents: the films have been deposited with a bias voltage of − 150 V and at 510 K.
model for the formation of nanocomposite films [10,12] the Si surface coverage Sicov is given by the relation: Sicov =
D ðCSi − α Þ 3a ðCZr + α Þ
where CSi and CZr are the Si and Zr content respectively, α the solubility limit of Si and a the lattice constant. In a series of Zr–Si–N films, the solubility limit is taken as the value of Si concentration when the crystallite size starts to decrease with increasing Si content. According with the proposed model for the formation of nanocomposite films [10], the segregation of Si atoms towards the grain boundaries determines the decrease of ZrN crystallite size. αÞ 3a The ratio ððCCZrSi − + α Þ as a function of D for the two series of films deposited at 510 K and RT is shown in Fig. 2b. If the Sicov remains constant ðCSi − α Þ 3a should exhibit a linear D ðCZr + α Þ as a function of
then the ratio
relationship. These behaviors are observed in Fig. 2b for both series of Zr– Si–N films. It is found that the Si solubility is α≈0 and the Si coverage is about Sicov =0.31 for films deposited at 510 K. In the case of films deposited at RT the Si solubility is α≈1.7 at.% and Sicov =0.33. Compared with our previous results (films deposited without bias [11]: {5%, 0.2} at RT, {4%,0.5}at 510 K), these results show that the ion bombardment reduces the incorporation of Si into the ZrN lattice. The ion bombardment enhances the thickness of the SiNx coverage layer in the case of the films deposited at RT, whereas in the case of the films deposited at 510 K the SiNx layer thickness decreases. Under ion bombardment the thickness of the SiNx coverage layer is temperature independent.
C.S. Sandu et al. / Surface & Coatings Technology 204 (2009) 969–972
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Fig. 3. Dark-field TEM images in cross section of Zr–Si–N films: at ∼2% Si with − 150 V bias a) and without bias b).
increased hardness cannot be attributed to the residual stress because the samples showing the highest hardness values for the series at 510 K, with and without bias, have similar residual stress. The higher hardness observed in the films deposited under ion bombardment seems to be related to a combined effect of film densification and crystallite size reduction. The thickness of the SiN layer seems not to affect film hardening.
4. Discussion Fig. 2. a) Grain size D vs Si content in Zr–Si–N films for the series deposited at 510 K and αÞ 3a RT on unbiased and biased substrates. b) Ratio ððCCZrSi − + α Þ as a function of D for the two series of films deposited with applied bias voltage at 510 K and RT. CSi and CZr are the Si and Zr content respectively, α the solubility limit of Si, D is the grain size and a the lattice constant.
Fig. 3 shows TEM dark-field images of a Zr–Si–N films with Si content of 2 at.% deposited with − 150 V bias a) and without bias b). The increase of the ion bombardment is accompanied by a significant decrease in the grain size from hundreds of nanometer to ten nanometers. The growth remains columnar and textured in the sample deposited under bias voltage conditions even though the grain size is smaller than 10 nm.
As reported in the literature, the main effect of ion bombardment during the film deposition is to increase the mobility of the atoms at the surface and beneath it due to direct and/or indirect multiple atomic collisions. Light atoms such as Si and N can be forward resputtered from the surface or recoil implanted by atomic collision process. The effective enhancement of the atomic mobility leads to atomic diffusion which in turn promotes the crystallization and surface segregation resulting in phase segregation.
3.2. Mechanical properties The dependence of nanohardness on Si content is similar to that observed in a-SiNx based MeN; a maximum hardness of about 39 GPa is observed at CSi = 2.5 at.% in films deposited at 510 K while in films deposited at RT the maximum hardness of 28 GPa is reached at higher Si content (5 at.%). In order to illustrate the effects of ion bombardment on the film hardening, Fig. 4 shows the nanohardness values as a function of the grain size for samples deposited with and without bias at RT and 510 K. A good correlation between the behaviors of hardness vs grain size is observed, in the case of films growth at 510 K. The highest hardness values observed in films deposited under bias are for crystallite size of about 8 nm and in films deposited without bias are for crystallite size of about 12–14 nm. In the case of RT and Ub = −150 V deposited films, the hardness is also improved compared to those deposited at RT with Ub = 0 though the films exhibit similar crystallite size. The
Fig. 4. Zr–Si–N thin films: nanohardness vs crystallite size for the films deposited with and without bias voltage at substrate temperatures of 510 K and RT.
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Greene et al. [19,20] have reported that TiN films grown under ion irradiation at 573 K a substrate bias −Ub ≥ 120 V results in fully dense films while the dissolution of the columnar structure is initiated at −Ub ≥ 160 V. Concerning the Ti–Si–N system, Faz et al. have reported that the mobility of atoms at the surface of the growing films can be enhanced either by increasing the substrate temperature or by ion bombardment promoting the phase segregation with low Si solubility [15]. Ribeiro et al. have investigated the effects of ion bombardment on the morphology of nanocomposite TiN/a-SiNx thin films. At low ion irradiation energies, Si atoms can be introduced in the TiN lattice mainly as substitution atoms. By increasing the irradiation energy an enhancement of the mobility of the surface species was found leading to effective phase segregation in the Ti–Si–N system. These films exhibit low Si solubility, a dense morphology, and high hardness values [17]. Our results are in good agreement with those reported in the literature. The Zr–Si–N films, grown under ion bombardment (Ub = −150 V) at substrates temperatures of 510 K and at RT, exhibit more compact morphologies than those of films deposited at similar temperatures but without bias voltage. The columnar structure, the average grain size and Si solubility limit are significantly reduced. The unexpected result was the decrease of the thickness of the SiNx layer covering ZrN nanocrystallites in samples exposed to ion bombardment and deposited at 510 K. The increased of energy of ionic species reaching the substrate by using a negative bias voltage seems to have the opposite effect to that observed when substrate temperatures are increased: lower SiNx coverage on the ZrN nanocrystallites. The film morphology significantly affects the electrical properties of these films. Thus, nanocomposite films deposited at RT without and with bias voltage and having similar crystallite size (8 nm) exhibit different resistivity values: 3 times lower in the case of films deposited under bias. The increase of the material density at the grain boundaries, as revealed by electrical measurements (lower resistivity for similar grain size), due to ionic bombardment seems to limit the thickness of the SiNx layer. The smaller crystallites obtained in films deposited at 510 K under bias could also be at the origin of lower thickness of the SiNx layer. Film hardening seems to be directly related to crystallite size and film density and does not depend on the thickness of the SiNx layer. 5. Conclusion The important effects of the ionic bombardment during the growth of ZrN/SiN nanocomposite films are: the decrease of ZrN grain size
and the Si solubility limit and the increase of the film density. The increased kinetic energy of the ad-atom on the film surface as a consequence of the applied bias voltage does not lead to an increase in the thickness of the SiNx layer covering ZrN crystallites. The addition of Si is responsible of the grain size diminution. The thickness of the SiNx layer seems not to affect film hardening. Increased film hardness is related to the higher film density.
Acknowledgments The authors gratefully acknowledge the financial support of the Swiss National Science Foundation and of the EPFL. The authors wish to thank the CIME-EPFL team for their TEM investigation facilities and Dr. A. Karimi for nanoindentation measurements.
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