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The effect of annealing treatment on microstructure and properties of TiN films prepared by unbalanced magnetron sputtering
Journal of Alloys and Compounds 496 (2010) 695–698
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The effect of annealing treatment on microstructure and properties of TiN films prepared by unbalanced magnetron sputtering Yingxue Xi a,b,∗ , Huiqing Fan a , Weiguo Liu b a b
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, PR China Shaanxi Province Thin Film Technology and Optical Test Open Key Laboratory, School of Optoelectronic Engineering, Xi’an Technological University, PR China
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 18 February 2010 Accepted 26 February 2010 Available online 4 March 2010 Keywords: Annealing treatment TiN Unbalanced magnetron sputtering Crystal structure
a b s t r a c t TiN thin films were deposited on Si wafers using an unbalanced magnetron (UBM) sputtering technique and then annealed in vacuum at temperatures ranging from 300 to 900 ◦ C. The microstructure and surface morphology of the thin films at various processing parameters were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The variation of the hardness and surface roughness over the course of annealing treatment temperature was investigated. Annealing treatment was found to induce changes in the microstructure, crystallinity, texture, grain size, hardness and roughness of TiN films. The hardness of TiN films decreased after heat treatment in the vacuum, while surface roughness increased significantly. Moreover, the maximum values of hardness and roughness could be observed in films annealed at 450 ◦ C in comparison to those at other annealing temperatures. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium nitrides (TiN) are extensively used in industry as hard protective coatings, as diffusion barriers in semiconductor technology, and also have optical applications in heat mirrors, decorative coatings, etc., due to their high hardness, wear resistance, thermal stability, low resistivity and diffusion barrier properties [1–3]. Various techniques have been employed to deposit TiN thin films. Studies have concentrated on how to obtain the excellent properties of the thin films, especially regarding the effect of the related processing parameters on the improvement of the film structure. The preferred crystallographic orientation strongly influences the resulting physical properties [4,5]. Therefore, it is important to elucidate the factors that modify the structure of the growing films. Annealing treatment has been found to improve the physical properties by reducing residual stresses and defects in the deposited films [6,7]. Recently, the understanding of the effect of annealing treatment on texture development in nitride thin films has generated a great deal of interest among researchers [8–11]. However, a literature review of nitride coatings reveals contradictory results on the change in microstructure after annealing treatment. Some have reported that the microstructure of annealed
∗ Corresponding author at: State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, PR China. Tel.: +86 29 88494463; fax: +86 29 88495414. E-mail addresses: [email protected] (Y. Xi), [email protected] (H. Fan). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.02.176
TiN films is not significantly changed [9]; others have found that the annealing treatment shows a great effect on the grain size and crystallographic orientation in TiN films [10]. In addition, it has been found that under the same heat treatment conditions, the effect of heat treatment on the variation in microstructure is more obvious in thinner films than in the thicker films because the extent of atom rearrangement is wider in the thinner films [11]. This reveals that during the annealing process, the structural evolution is controlled by thermal energy. Hence, the annealing process is intimately associated with the annealing temperatures, which provide thermal energy to facilitate the atom rearrangement diffusion in the films. In this paper, TiN films were deposited on silicon substrates using the unbalanced magnetron (UBM) sputtering method, and the microstructure and the properties of the TiN thin films were changed through treatment. The effect of annealing temperatures on the properties of films such as preferred orientation, hardness and surface roughness were analyzed. 2. Experimental procedure TiN films were deposited using the DC unbalanced magnetron (UBM) sputtering technology. The N-type Si (1 0 0) wafer was used as the substrate material. To ensure a surface finish with suitable quality, the specimens were cleaned before deposition by the standard RCA cleaning technique and then immediately placed into the coating chamber. The distance of target-to-substrate was 15 cm. The deposition process was performed in the UBM (By-700II) system. The sputtering target was pure rectangular titanium (99.995%) with dimensions of 480 mm × 80 mm × 6 mm. Prior to deposition, the chamber was evacuated to a base pressure of 3.0 × 10−3 Pa, and then Ar+ ions were introduced into the chamber for sputter-cleaning the substrate. Typical conditions for sputtering were −1000 V bias, 2.0 × 10−2 Pa pressure and 15 min. After sputter-cleaning, high purity working gas
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Y. Xi et al. / Journal of Alloys and Compounds 496 (2010) 695–698
Fig. 1. SEM surface morphology of TiN films at various temperatures: (a) as-deposited, (b) 300 ◦ C, (c) 450 ◦ C, (d) 700 ◦ C, (e) 900 ◦ C.
and reactive gas were introduced, using the mass flow controller to regulate both gas flows. Argon (99.999% in purity) gas and nitrogen gas (99.99% in purity) flow rates were fixed at 90 and 15 sccm, respectively. The total gas pressure was controlled at 0.3 Pa. The DC power supply of the Ti target was operated at 7 A using the constant-current mode. The additional coiling current was kept at 120 A to give a desired unbalancing effect. During deposition, a negative substrate bias voltage of 50 V was applied and no external heating or cooling of the substrate was used. The thickness of all films was fixed at about 400 nm by controlling the deposition duration. A high-temperature tube furnace equipped with a mechanical pumping system was used to perform the heat treatment. After inserting the grown films, the quartz tube was sealed and pumped down to 0.2 Pa. Post-annealing was then carried out at various temperatures from 300 to 900 ◦ C. The heating rate was 50 ◦ C/s and the annealing duration was 30 min. The phase and crystal structure were identified by XRD (PANalytical X’Pert Pro) using Cu-K␣ radiation. The morphological structure of the films was examined by a field-emission gun scanning electron microscope (JEOL JSM-6700F). The hardness was measured by a Vickers micro-hardness tester using a load of 25 g. To determine the hardness of the films, the Jonsson and Hogmark [12] modeling approach was used based on the understanding that the substrate will influence the measured
hardness. The surface roughness of the films was measured using a Talysurf CCI 2000 non-contact 3D profiler with the root mean square roughness value (Rrms) reported.
3. Results and discussion 3.1. Microstructure and preferred orientation The surface morphology of newly deposited films and films annealed at various temperatures are shown in Fig. 1a–e, respectively. From Fig. 1, it is clear that the grain size gradually increases as the annealing temperature is increased up to 700 ◦ C but clearly decreases when the sample is annealed at 900 ◦ C. This shows that the newly deposited films have irregular non-faceted spherical crystals (Fig. 1a). The non-faceted spherical grains are still present in the films annealed at 300 ◦ C (Fig. 1b), but the crystals in-between have changed to those with a pyramidal structure. In the films
Y. Xi et al. / Journal of Alloys and Compounds 496 (2010) 695–698
Fig. 2. XRD diffraction patterns for TiN films heat treatment at various temperatures (a) as-deposited, (b) 300 ◦ C, (c) 450 ◦ C, (d) 700 ◦ C, (e) 900 ◦ C.
annealed at 450 ◦ C, the morphology alters to one with a strongly faceted pyramidal structure (Fig. 1c). When the temperature is continuously raised to 700 ◦ C, the crystallites are oriented in the (1 1 0) direction and still exhibit strongly faceted structures (Fig. 1d). But the films annealed at 900 ◦ C again exhibit irregularly arranged plates (Fig. 1e). Fig. 2 shows X-ray diffraction patterns for the samples before and after heat treatment at various temperatures ranging from 300 to 900 ◦ C. Four peaks are observed for the newly deposited films corresponding to the diffractions of the planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1). This indicates that TiN films with polycrystalline structure are formed under this deposition condition. At low annealing temperatures (T = 300 ◦ C), the films exhibit a mixture of the structure in a more pronounced (1 1 1) orientation with less (2 0 0) and (2 2 0) orientation and then change to one in the (1 1 1) orientation with an increase in the annealing temperature to 450 ◦ C. However, the most intense reflection of TiN is in the (2 2 0) orientation almost without (1 1 1) reflection peaks of TiN, indicating a strong (2 2 0) texture exists when thin films are annealed at 700 ◦ C. The polycrystalline structure is observed again after annealing at 900 ◦ C. From the diffraction patterns, the texture coefficients of the TiN films as a function of annealing temperature are calculated from their respective XRD peaks using the following formula [13]: Texture coefficiente (T ) =
I(h k l) [I(1 1 1)+I(2 0 0)+I(2 2 0)]
(1)
where (h k l) represents the (1 1 1), (2 0 0) or (2 2 0) orientations. The effect of the annealing temperature on the texture coefficients is shown in Fig. 3. The (1 1 1) texture coefficient is found to increase at a heat treatment temperature up to 450 ◦ C beyond which it marginally decreases with the heat treatment temperature further increasing to 700 ◦ C but slightly increases at 900 ◦ C. The opposite trend is observed for TC200 and TC220; that is, their texture coefficients are relatively small compared to those when the annealing temperature increases up to 450 ◦ C. When the temperature is continuously raised to 700 ◦ C, the texture coefficient of TC220 is markedly increased but that for TC200 is still small. This obviously reveals that the growth velocity of (2 0 0) planes is relatively low under the heating treatment. The textural evolution of TiN thin films is primarily explained based on thermodynamics and kinetics arguments [14–17]. The preferred orientation of TiN film is associated with annealing temperature, which can be explained by the competitive growth mechanism. The heat treatment mainly provides thermal energy to facilitate the atom rearrangement by diffusion in the thin film. In the low temperature range, the input thermal energy is favorable
697
Fig. 3. (1 1 1), (2 2 0) and (2 0 0) texture coefficient of TiN films annealed at various temperatures.
to the growth of the (1 1 1) planes, so the (1 1 1) planes grow much faster than the (2 0 0) planes. When the temperature increases to 700 ◦ C, the (2 2 0) planes also acquire sufficient thermal energy to grow, which might lead to the preferred orientation in which the TiN films change from the (1 1 1) to the (2 2 0) texture. As the temperature further increases, the TiN films are found to be polycrystalline with (1 1 1), (2 0 0), (2 2 0) or (3 1 1) orientations, because the heat treatment provides enough thermal energy for competitive growth on the major low-index planes. A similar investigation for texture changes by Huang et al. [11] showed that there is a decrease in TC111 of annealed TiN films at 700 ◦ C and an increase in TC200, but there is not any orientation switch from the existing (1 1 1) to (2 0 0). Such a difference might be related to the reduction in the residual stress on annealing, because the relaxation of residual stresses of TiN films during annealing is correlated with the relaxation of the strains by crystallization of TiN crystals [17]. However, the details of this are still unclear and further research is required to probe into the causes. 3.2. Hardness Fig. 4 shows the effect of heat treatment on the hardness of the TiN films at annealing temperature ranging from 300 to 900 ◦ C. The results indicate that at 300 ◦ C, the hardness of the film is the lowest and gradually increases with the increase in the negative bias, and
Fig. 4. The variation of the hardness of TiN films for as-deposited, annealing treat at 300, 450, 700 and 900 ◦ C conditions
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Y. Xi et al. / Journal of Alloys and Compounds 496 (2010) 695–698
TiN orientation from (1 1 1) to (2 2 0), the surface roughness of films annealed at 700 ◦ C becomes smaller. Moreover, the surface roughness may be associated with the grain structure in thin films. Previous study [10] reported that grain growth in the films with the nanocrystalline structure is more pronounced, thereby increasing the roughness. Comparing morphological images in Fig. 1d and e, it can be found that the grain size of films heat-treated at 900 ◦ C is smaller than that of films heat-treated at 700 ◦ C. A possible explanation for the reduction of the grain size in the films is that the velocity of grain growth is inhibited by the competitive growth of all major low-index planes at relatively high annealing temperature. Therefore, the decrease in roughness for films annealed at 900 ◦ C can also be attributed to the decrease of grain size. 4. Conclusions Fig. 5. The variation of the surface roughness of TiN films for as-deposited, annealing treat at 300, 450, 700 and 900 ◦ C conditions.
beyond 450 ◦ C, the hardness shows a decrease again. A maximum hardness value of 2415 HV, which is still small in comparison with that for as-deposited films, is observed for the films annealed at 450 ◦ C. A possible explanation for the decrease in hardness of films annealed is that the heat treatment of the films results in a relief of the compressive intrinsic stresses to reduce the resistance to plastic deformation during indentation. On the other hand, when compared to the effect of annealing temperature, it is found that the highest hardness value corresponds to single-crystal TiN films with (1 1 1) orientations annealed at 450 ◦ C and next to the highest is single-crystal TiN films with (2 2 0) orientations annealed at 700 ◦ C. This suggests that preferred orientation may be a major factor that affects the hardness. Ma et al. [18] found that the hardness dependence on texture varies as the grain size changes. Comparing the hardness test on two polycrystalline films annealed at 300 and 900 ◦ C, respectively, the value of higher hardness is observed in the latter, which may be attributed to its larger grain size in comparison to that at 300 ◦ C as revealed by the SEM images shown in Fig. 3b and e. 3.3. Surface roughness Fig. 5 denotes that the change in the roughness values of the TiN films varies with the annealing temperature. The variation trend of roughness is similar to that of the hardness at the annealing temperature. A minimum roughness value of 19 nm is observed for newly deposited films. With an increase in the annealing temperature from 300 to 450 ◦ C, the surface roughness increases from 23 to 69 nm, but a further decrease in the temperature results in the gradual decrease of surface roughness. At the annealing temperature of 900 ◦ C, the roughness decreases to 28 nm. The variation of roughness can be attributed to the changes in surface topography. It can be seen from SEM images (Fig. 1) that there is a change in grain morphology from strongly non-faceted spherical grains to faceted pyramid-like grains, with a change in crystallographic orientation of newly deposited films from (2 0 0) to (1 1 1). Therefore, the surface roughness increases with the annealing temperature increasing up to 450 ◦ C, due to an increase in the average protrusion size and a broadening of their size distribution. However, with a sudden change in crystallographic texture of the
TiN films were prepared by the USBM technique at room temperature and then subjected to annealing treatment at various temperatures from 300 to 900 ◦ C. The influence of the annealing temperature on the crystal structure, surface morphology and mechanical properties was systematically studied. The structure was transformed from irregular facetted structure in newly deposited films to faceted structure in the (1 1 1) preferred orientation in films annealed at temperatures ranging from 300 to 450 ◦ C. This texture gradually diminishes at higher temperatures, while strongly (2 2 0) oriented films of the facetted structure appeared at 700 ◦ C. Irregular faceted crystals without the faceted texture were formed at 900 ◦ C again. Moreover, the grain size in films gradually increase as the annealing temperature is increased up to 700 ◦ C but decreases at 900 ◦ C. These microstructural characteristics are responsible for the changes in mechanical properties associated with annealing. The decrease in hardness of the films and the increase in surface roughness were observed after heat treatment. The films annealed at 450 ◦ C yield higher hardness value and roughness due to highly preferred (1 1 1) pyramidal-like structure. Acknowledgement The authors would like to acknowledge the National Natural Science Foundation of China (60878032) for the financial support to this research. References [1] J.V. Humbeeck, Adv. Eng. Mater. 3 (2001) 837. [2] K. Akira, Surf. Coat. Technol. 132 (2000) 152. [3] T. Bacci, L. Bertamini, F. Ferrari, F.P. Galliano, E. Galvanetto, Mater. Sci. Eng. A 283 (2000) 189. [4] M. Kobayashi, Y. Doi, Thin Solid Films 111 (1984) 259. [5] G. Abadias, Surf. Coat. Technol. 202 (2008) 2223. [6] Z. Cheng, H. Peng, G. Xie, Y. Shi, Surf. Coat. Technol. 138 (2001) 237. [7] K. Xu, J. Chen, R. Gao, J. He, Surf. Coat. Technol. 58 (1993) 37. [8] F.-H. Lu, H.-Y. Chen, Thin Solid Films 388–389 (2001) 368. [9] W.-J. Chou, G.-P. Yu, J.-H. Huang, Surf. Coat. Technol. 168 (2003) 43. [10] P.H. Mayrhofer, F. Kunc, J. Musil, C. Mitterer, Thin Solid Films 415 (2002) 151. [11] J.-H. Huang, K.-J. Yu, P. Sit, G.-P. Yu, Surf. Coat. Technol. 200 (2006) 4291. [12] B. Jonsson, S. Hogmark, Thin Solid Films 114 (1984) 257. [13] V. Chawlaa, R. Jayaganthan, R. Chandra, Mater. Charact. 59 (2008) 1015. [14] J. Pelleg, L.Z. Zevin, S. Lungo, N. Croitoru, Thin Solid Films 197 (1991) 117. [15] T.Q. Li, S. Noda, H. Komiyama, T. Yamamoto, Y. Ikuhara, J. Vac. Sci. Technol. A 21 (2003) 1717. [16] J.E. Greene, J.E. Sundgren, L. Hultman, I. Petrov, D.G. Bergstrom, Appl. Phys. Lett. 67 (1995) 2928. [17] T. Matsue, T. Hanabusa, K. Kusaka, O. Sakata, Vacuum 83 (2009) 585. [18] C.-H. Ma, J.-H. Huang, H. Chena, Surf. Coat. Technol. 200 (2006) 3868.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The effect of annealing treatment on microstructure and properties of TiN films prepared by unbalanced magnetron sputtering Yingxue Xi a,b,∗ , Huiqing Fan a , Weiguo Liu b a b
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, PR China Shaanxi Province Thin Film Technology and Optical Test Open Key Laboratory, School of Optoelectronic Engineering, Xi’an Technological University, PR China
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 18 February 2010 Accepted 26 February 2010 Available online 4 March 2010 Keywords: Annealing treatment TiN Unbalanced magnetron sputtering Crystal structure
a b s t r a c t TiN thin films were deposited on Si wafers using an unbalanced magnetron (UBM) sputtering technique and then annealed in vacuum at temperatures ranging from 300 to 900 ◦ C. The microstructure and surface morphology of the thin films at various processing parameters were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The variation of the hardness and surface roughness over the course of annealing treatment temperature was investigated. Annealing treatment was found to induce changes in the microstructure, crystallinity, texture, grain size, hardness and roughness of TiN films. The hardness of TiN films decreased after heat treatment in the vacuum, while surface roughness increased significantly. Moreover, the maximum values of hardness and roughness could be observed in films annealed at 450 ◦ C in comparison to those at other annealing temperatures. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium nitrides (TiN) are extensively used in industry as hard protective coatings, as diffusion barriers in semiconductor technology, and also have optical applications in heat mirrors, decorative coatings, etc., due to their high hardness, wear resistance, thermal stability, low resistivity and diffusion barrier properties [1–3]. Various techniques have been employed to deposit TiN thin films. Studies have concentrated on how to obtain the excellent properties of the thin films, especially regarding the effect of the related processing parameters on the improvement of the film structure. The preferred crystallographic orientation strongly influences the resulting physical properties [4,5]. Therefore, it is important to elucidate the factors that modify the structure of the growing films. Annealing treatment has been found to improve the physical properties by reducing residual stresses and defects in the deposited films [6,7]. Recently, the understanding of the effect of annealing treatment on texture development in nitride thin films has generated a great deal of interest among researchers [8–11]. However, a literature review of nitride coatings reveals contradictory results on the change in microstructure after annealing treatment. Some have reported that the microstructure of annealed
∗ Corresponding author at: State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, PR China. Tel.: +86 29 88494463; fax: +86 29 88495414. E-mail addresses: [email protected] (Y. Xi), [email protected] (H. Fan). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.02.176
TiN films is not significantly changed [9]; others have found that the annealing treatment shows a great effect on the grain size and crystallographic orientation in TiN films [10]. In addition, it has been found that under the same heat treatment conditions, the effect of heat treatment on the variation in microstructure is more obvious in thinner films than in the thicker films because the extent of atom rearrangement is wider in the thinner films [11]. This reveals that during the annealing process, the structural evolution is controlled by thermal energy. Hence, the annealing process is intimately associated with the annealing temperatures, which provide thermal energy to facilitate the atom rearrangement diffusion in the films. In this paper, TiN films were deposited on silicon substrates using the unbalanced magnetron (UBM) sputtering method, and the microstructure and the properties of the TiN thin films were changed through treatment. The effect of annealing temperatures on the properties of films such as preferred orientation, hardness and surface roughness were analyzed. 2. Experimental procedure TiN films were deposited using the DC unbalanced magnetron (UBM) sputtering technology. The N-type Si (1 0 0) wafer was used as the substrate material. To ensure a surface finish with suitable quality, the specimens were cleaned before deposition by the standard RCA cleaning technique and then immediately placed into the coating chamber. The distance of target-to-substrate was 15 cm. The deposition process was performed in the UBM (By-700II) system. The sputtering target was pure rectangular titanium (99.995%) with dimensions of 480 mm × 80 mm × 6 mm. Prior to deposition, the chamber was evacuated to a base pressure of 3.0 × 10−3 Pa, and then Ar+ ions were introduced into the chamber for sputter-cleaning the substrate. Typical conditions for sputtering were −1000 V bias, 2.0 × 10−2 Pa pressure and 15 min. After sputter-cleaning, high purity working gas
696
Y. Xi et al. / Journal of Alloys and Compounds 496 (2010) 695–698
Fig. 1. SEM surface morphology of TiN films at various temperatures: (a) as-deposited, (b) 300 ◦ C, (c) 450 ◦ C, (d) 700 ◦ C, (e) 900 ◦ C.
and reactive gas were introduced, using the mass flow controller to regulate both gas flows. Argon (99.999% in purity) gas and nitrogen gas (99.99% in purity) flow rates were fixed at 90 and 15 sccm, respectively. The total gas pressure was controlled at 0.3 Pa. The DC power supply of the Ti target was operated at 7 A using the constant-current mode. The additional coiling current was kept at 120 A to give a desired unbalancing effect. During deposition, a negative substrate bias voltage of 50 V was applied and no external heating or cooling of the substrate was used. The thickness of all films was fixed at about 400 nm by controlling the deposition duration. A high-temperature tube furnace equipped with a mechanical pumping system was used to perform the heat treatment. After inserting the grown films, the quartz tube was sealed and pumped down to 0.2 Pa. Post-annealing was then carried out at various temperatures from 300 to 900 ◦ C. The heating rate was 50 ◦ C/s and the annealing duration was 30 min. The phase and crystal structure were identified by XRD (PANalytical X’Pert Pro) using Cu-K␣ radiation. The morphological structure of the films was examined by a field-emission gun scanning electron microscope (JEOL JSM-6700F). The hardness was measured by a Vickers micro-hardness tester using a load of 25 g. To determine the hardness of the films, the Jonsson and Hogmark [12] modeling approach was used based on the understanding that the substrate will influence the measured
hardness. The surface roughness of the films was measured using a Talysurf CCI 2000 non-contact 3D profiler with the root mean square roughness value (Rrms) reported.
3. Results and discussion 3.1. Microstructure and preferred orientation The surface morphology of newly deposited films and films annealed at various temperatures are shown in Fig. 1a–e, respectively. From Fig. 1, it is clear that the grain size gradually increases as the annealing temperature is increased up to 700 ◦ C but clearly decreases when the sample is annealed at 900 ◦ C. This shows that the newly deposited films have irregular non-faceted spherical crystals (Fig. 1a). The non-faceted spherical grains are still present in the films annealed at 300 ◦ C (Fig. 1b), but the crystals in-between have changed to those with a pyramidal structure. In the films
Y. Xi et al. / Journal of Alloys and Compounds 496 (2010) 695–698
Fig. 2. XRD diffraction patterns for TiN films heat treatment at various temperatures (a) as-deposited, (b) 300 ◦ C, (c) 450 ◦ C, (d) 700 ◦ C, (e) 900 ◦ C.
annealed at 450 ◦ C, the morphology alters to one with a strongly faceted pyramidal structure (Fig. 1c). When the temperature is continuously raised to 700 ◦ C, the crystallites are oriented in the (1 1 0) direction and still exhibit strongly faceted structures (Fig. 1d). But the films annealed at 900 ◦ C again exhibit irregularly arranged plates (Fig. 1e). Fig. 2 shows X-ray diffraction patterns for the samples before and after heat treatment at various temperatures ranging from 300 to 900 ◦ C. Four peaks are observed for the newly deposited films corresponding to the diffractions of the planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1). This indicates that TiN films with polycrystalline structure are formed under this deposition condition. At low annealing temperatures (T = 300 ◦ C), the films exhibit a mixture of the structure in a more pronounced (1 1 1) orientation with less (2 0 0) and (2 2 0) orientation and then change to one in the (1 1 1) orientation with an increase in the annealing temperature to 450 ◦ C. However, the most intense reflection of TiN is in the (2 2 0) orientation almost without (1 1 1) reflection peaks of TiN, indicating a strong (2 2 0) texture exists when thin films are annealed at 700 ◦ C. The polycrystalline structure is observed again after annealing at 900 ◦ C. From the diffraction patterns, the texture coefficients of the TiN films as a function of annealing temperature are calculated from their respective XRD peaks using the following formula [13]: Texture coefficiente (T ) =
I(h k l) [I(1 1 1)+I(2 0 0)+I(2 2 0)]
(1)
where (h k l) represents the (1 1 1), (2 0 0) or (2 2 0) orientations. The effect of the annealing temperature on the texture coefficients is shown in Fig. 3. The (1 1 1) texture coefficient is found to increase at a heat treatment temperature up to 450 ◦ C beyond which it marginally decreases with the heat treatment temperature further increasing to 700 ◦ C but slightly increases at 900 ◦ C. The opposite trend is observed for TC200 and TC220; that is, their texture coefficients are relatively small compared to those when the annealing temperature increases up to 450 ◦ C. When the temperature is continuously raised to 700 ◦ C, the texture coefficient of TC220 is markedly increased but that for TC200 is still small. This obviously reveals that the growth velocity of (2 0 0) planes is relatively low under the heating treatment. The textural evolution of TiN thin films is primarily explained based on thermodynamics and kinetics arguments [14–17]. The preferred orientation of TiN film is associated with annealing temperature, which can be explained by the competitive growth mechanism. The heat treatment mainly provides thermal energy to facilitate the atom rearrangement by diffusion in the thin film. In the low temperature range, the input thermal energy is favorable
697
Fig. 3. (1 1 1), (2 2 0) and (2 0 0) texture coefficient of TiN films annealed at various temperatures.
to the growth of the (1 1 1) planes, so the (1 1 1) planes grow much faster than the (2 0 0) planes. When the temperature increases to 700 ◦ C, the (2 2 0) planes also acquire sufficient thermal energy to grow, which might lead to the preferred orientation in which the TiN films change from the (1 1 1) to the (2 2 0) texture. As the temperature further increases, the TiN films are found to be polycrystalline with (1 1 1), (2 0 0), (2 2 0) or (3 1 1) orientations, because the heat treatment provides enough thermal energy for competitive growth on the major low-index planes. A similar investigation for texture changes by Huang et al. [11] showed that there is a decrease in TC111 of annealed TiN films at 700 ◦ C and an increase in TC200, but there is not any orientation switch from the existing (1 1 1) to (2 0 0). Such a difference might be related to the reduction in the residual stress on annealing, because the relaxation of residual stresses of TiN films during annealing is correlated with the relaxation of the strains by crystallization of TiN crystals [17]. However, the details of this are still unclear and further research is required to probe into the causes. 3.2. Hardness Fig. 4 shows the effect of heat treatment on the hardness of the TiN films at annealing temperature ranging from 300 to 900 ◦ C. The results indicate that at 300 ◦ C, the hardness of the film is the lowest and gradually increases with the increase in the negative bias, and
Fig. 4. The variation of the hardness of TiN films for as-deposited, annealing treat at 300, 450, 700 and 900 ◦ C conditions
698
Y. Xi et al. / Journal of Alloys and Compounds 496 (2010) 695–698
TiN orientation from (1 1 1) to (2 2 0), the surface roughness of films annealed at 700 ◦ C becomes smaller. Moreover, the surface roughness may be associated with the grain structure in thin films. Previous study [10] reported that grain growth in the films with the nanocrystalline structure is more pronounced, thereby increasing the roughness. Comparing morphological images in Fig. 1d and e, it can be found that the grain size of films heat-treated at 900 ◦ C is smaller than that of films heat-treated at 700 ◦ C. A possible explanation for the reduction of the grain size in the films is that the velocity of grain growth is inhibited by the competitive growth of all major low-index planes at relatively high annealing temperature. Therefore, the decrease in roughness for films annealed at 900 ◦ C can also be attributed to the decrease of grain size. 4. Conclusions Fig. 5. The variation of the surface roughness of TiN films for as-deposited, annealing treat at 300, 450, 700 and 900 ◦ C conditions.
beyond 450 ◦ C, the hardness shows a decrease again. A maximum hardness value of 2415 HV, which is still small in comparison with that for as-deposited films, is observed for the films annealed at 450 ◦ C. A possible explanation for the decrease in hardness of films annealed is that the heat treatment of the films results in a relief of the compressive intrinsic stresses to reduce the resistance to plastic deformation during indentation. On the other hand, when compared to the effect of annealing temperature, it is found that the highest hardness value corresponds to single-crystal TiN films with (1 1 1) orientations annealed at 450 ◦ C and next to the highest is single-crystal TiN films with (2 2 0) orientations annealed at 700 ◦ C. This suggests that preferred orientation may be a major factor that affects the hardness. Ma et al. [18] found that the hardness dependence on texture varies as the grain size changes. Comparing the hardness test on two polycrystalline films annealed at 300 and 900 ◦ C, respectively, the value of higher hardness is observed in the latter, which may be attributed to its larger grain size in comparison to that at 300 ◦ C as revealed by the SEM images shown in Fig. 3b and e. 3.3. Surface roughness Fig. 5 denotes that the change in the roughness values of the TiN films varies with the annealing temperature. The variation trend of roughness is similar to that of the hardness at the annealing temperature. A minimum roughness value of 19 nm is observed for newly deposited films. With an increase in the annealing temperature from 300 to 450 ◦ C, the surface roughness increases from 23 to 69 nm, but a further decrease in the temperature results in the gradual decrease of surface roughness. At the annealing temperature of 900 ◦ C, the roughness decreases to 28 nm. The variation of roughness can be attributed to the changes in surface topography. It can be seen from SEM images (Fig. 1) that there is a change in grain morphology from strongly non-faceted spherical grains to faceted pyramid-like grains, with a change in crystallographic orientation of newly deposited films from (2 0 0) to (1 1 1). Therefore, the surface roughness increases with the annealing temperature increasing up to 450 ◦ C, due to an increase in the average protrusion size and a broadening of their size distribution. However, with a sudden change in crystallographic texture of the
TiN films were prepared by the USBM technique at room temperature and then subjected to annealing treatment at various temperatures from 300 to 900 ◦ C. The influence of the annealing temperature on the crystal structure, surface morphology and mechanical properties was systematically studied. The structure was transformed from irregular facetted structure in newly deposited films to faceted structure in the (1 1 1) preferred orientation in films annealed at temperatures ranging from 300 to 450 ◦ C. This texture gradually diminishes at higher temperatures, while strongly (2 2 0) oriented films of the facetted structure appeared at 700 ◦ C. Irregular faceted crystals without the faceted texture were formed at 900 ◦ C again. Moreover, the grain size in films gradually increase as the annealing temperature is increased up to 700 ◦ C but decreases at 900 ◦ C. These microstructural characteristics are responsible for the changes in mechanical properties associated with annealing. The decrease in hardness of the films and the increase in surface roughness were observed after heat treatment. The films annealed at 450 ◦ C yield higher hardness value and roughness due to highly preferred (1 1 1) pyramidal-like structure. Acknowledgement The authors would like to acknowledge the National Natural Science Foundation of China (60878032) for the financial support to this research. References [1] J.V. Humbeeck, Adv. Eng. Mater. 3 (2001) 837. [2] K. Akira, Surf. Coat. Technol. 132 (2000) 152. [3] T. Bacci, L. Bertamini, F. Ferrari, F.P. Galliano, E. Galvanetto, Mater. Sci. Eng. A 283 (2000) 189. [4] M. Kobayashi, Y. Doi, Thin Solid Films 111 (1984) 259. [5] G. Abadias, Surf. Coat. Technol. 202 (2008) 2223. [6] Z. Cheng, H. Peng, G. Xie, Y. Shi, Surf. Coat. Technol. 138 (2001) 237. [7] K. Xu, J. Chen, R. Gao, J. He, Surf. Coat. Technol. 58 (1993) 37. [8] F.-H. Lu, H.-Y. Chen, Thin Solid Films 388–389 (2001) 368. [9] W.-J. Chou, G.-P. Yu, J.-H. Huang, Surf. Coat. Technol. 168 (2003) 43. [10] P.H. Mayrhofer, F. Kunc, J. Musil, C. Mitterer, Thin Solid Films 415 (2002) 151. [11] J.-H. Huang, K.-J. Yu, P. Sit, G.-P. Yu, Surf. Coat. Technol. 200 (2006) 4291. [12] B. Jonsson, S. Hogmark, Thin Solid Films 114 (1984) 257. [13] V. Chawlaa, R. Jayaganthan, R. Chandra, Mater. Charact. 59 (2008) 1015. [14] J. Pelleg, L.Z. Zevin, S. Lungo, N. Croitoru, Thin Solid Films 197 (1991) 117. [15] T.Q. Li, S. Noda, H. Komiyama, T. Yamamoto, Y. Ikuhara, J. Vac. Sci. Technol. A 21 (2003) 1717. [16] J.E. Greene, J.E. Sundgren, L. Hultman, I. Petrov, D.G. Bergstrom, Appl. Phys. Lett. 67 (1995) 2928. [17] T. Matsue, T. Hanabusa, K. Kusaka, O. Sakata, Vacuum 83 (2009) 585. [18] C.-H. Ma, J.-H. Huang, H. Chena, Surf. Coat. Technol. 200 (2006) 3868.