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Effect of sputtering power on surface topography of dc magnetron sputtered Ti thin films observed by AFM
Applied Surface Science 255 (2009) 4673–4679
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of sputtering power on surface topography of dc magnetron sputtered Ti thin films observed by AFM Yongzhong Jin a,*, Wei Wu b, Li Li b, Jian Chen a, Jingyu Zhang a, Youbing Zuo a, Jun Fu a a b
Department of Materials and Chemistry Engineering, Sichuan University of Science and Engineering, 180 Huixing Road, Zigong, Sichuan 643000, China School of Materials Science and Engineering, Xihua University, Chengdu, Sichuan 610039, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 September 2008 Received in revised form 7 December 2008 Accepted 9 December 2008 Available online 24 December 2008
Titanium films were deposited on glass substrates at room temperature by direct current (dc) magnetron sputtering at fixed Ar pressure of 1.7 Pa and sputtering time of 4 min with different sputtering power ranging from 100 to 300 W. Atomic force microscopy (AFM) was used to study topographic characteristics of the films, including crystalline feature, grain size, clustering and roughening. The amorphous-like microstructure feature has been observed at 100–150 W and the transition of crystal microstructure from amorphous-like to crystalline state occurs at 200 W. The increase in grain size of Ti films with the sputtering power (from 200 to 300 W) has been confirmed by AFM characterization. In addition, higher sputtering power (300 W) leads to the transformation of crystal texture from globularlike to hexagonal type. The study has shown that higher sputtering power results in the non-linear increase in deposition rate of Ti films. Good correlativity between the surface roughness parameters including root mean square (RMS) roughness, surface mean height (Ra) and maximum peak to valley height (P–V) for evaluating the lateral feature of the films has been manifested. Surface roughness has an increasing trend at 100–250 W, and then drops up to 300 W. ß 2008 Elsevier B.V. All rights reserved.
PACS: 64.70.Nd 68.37.Ps 81.07.Bc Keywords: Ti films Magnetron sputtering Sputtering power Surface topography Atomic force microscopy
1. Introduction Titanium (Ti), due to high mechanical strength, excellent thermal stability, good corrosion resistance and intrinsic biocompatibility has received a great deal of attention over the last three decades. These features make it very useful, not only for mechanical applications, but also for the aerospace, medical and microelectronics industries [1–3]. For these applications, smooth surface, fine grain and homogenous structure of Ti thin films are of critical importance. Many studies on magnetron sputtered Ti thin films have been reported [4–7]. For example, Jeyachandran et al. [4] investigated the effect of thickness on the electrical, structural, optical and surface properties of sputtered Ti films. Godfroid et al. [5] have studied the influence of sputtering current on the growth modes of sputtered Ti films. Chemical composition and structural properties of Ti thin films sputtered under various cathode power, sputtering pressure and base vacuum conditions have also been investigated [6]. Martin et al. [7] have researched the influence of bias power on
* Corresponding author. Tel: +86 28 66453877; fax: +86 28 66453877. E-mail address: [email protected] (Y. Jin). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.12.029
some properties of Ti coatings. However, these studies do not deal with a systematic investigation on morphological evolution of Ti films on the nanometer scale with respect to the deposition parameters. Undoubtedly, it is very important for the nanometerscale characterization of surface topography of thin films to obtain smooth surface, fine grain and homogenous structure [8]. The goal of this work was to investigate sputtering power affecting surface topography of direct current (dc) magnetron sputtered Ti films on the nanometer scale using atomic force microscopy (AFM). Since the invention of AFM in 1986 [9], it has became a very popular and effective method to measure surface topography of thin films. An important advantage compared with electron beam methods is that AFM can operate in ambient environments. Obviously, AFM technology has widely been used in the measurement of the hyperfine surface topography of thin solid films [10–12]. 2. Experiment Ti films were deposited on glass substrate at room temperature with dc magnetron sputtering using a 100 mm diameter 5 mm thick Ti target of 99.995% purity. The distance between the Ti target and glass substrates was 110 mm. Commercial argon (99.999%
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Fig. 1. AFM images of Ti films sputtered at 100 W: (a) 2D topography (1 mm2), (b) the 2D topographic details (500 nm2) in the black pane region marked in (a) image, (c) the 3D topography of (b) image.
pure) was used as the sputtering gas. Before deposition, the glass substrates were rinsed with acetone, redistilled alcohol, air dried, and installed into sputtering apparatus. The deposition chamber was pumped down to the ultimate vacuum and repeatedly charged with argon and pumped down in order to minimize the residual gas components. The shutter located in between the target and the substrate, was used to control the sputtering time. Before starting the actual experiment, the pre-sputtering for 1 min was done to remove the surface oxide of Ti target. In this way, a group of samples were deposited at fixed Ar pressure of 1.7 Pa and sputtering time of 4 min with different sputtering power ranging from 100 to 300 W. The topography of these films was characterized by a commercially available AFM system (Seiko SPM400) in tapping mode. The Si probe tips (force constant: 3.0 N/m) used for the AFM measurements are integral with the V-shaped cantilevers. In this mode, force on the cantilever is kept constant and hence a constant force is applied on the sample. AFM detection is done by optical beam deflection, involving a position sensitive detector and laser
beam alignment by piezo driven mirror motors. When the Si tip scans across the sample, surface features can cause a deflection in the cantilever, which is detected by the laser detector and feedback to the Z-axis controller, adjusting the Z-position of the Si tip to restore the set point force and deflection. In addition, the specific software (Seiko SPI3800N) was chosen in order to examine the three-dimensional features on all AFM images. It should be noted that three main surface roughness parameters including RMS, Ra and P–V roughness are simultaneously used for lateral characterization of Ti films. 3. Results and discussion AFM images were collected at various magnifications and different fields of view in order to evaluate the surface features of Ti thin films. Five sets of surface topographies of Ti films deposited on glass substrate with different sputtering power ranging from 100 to 300 W are displayed in Figs. 1–5. To analyze the surface topographies of Ti films by Seiko SPI3800N software, it is assumed
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Fig. 2. AFM images of Ti films sputtered at 150 W: (a) 2D topography (1 mm2), (b) the 2D topographic details (500 nm2) in the black pane region marked in (a) image, (c) the 3D topography of (b) image.
that the dark regions seen in the AFM images (i.e. cavities and channels shown in Figs. 1–5) are related to the areas with the height value of zero or near zero along the positive direction of Z axle and the bright regions (i.e. the tops of protruding grains) represent higher areas. Fig. 1 exhibits a wavy topography created by undulating ridges (ribbons), which are bordered by shallow and wide depressions with the depth of 1.5–5 nm and the width of 10–100 nm. The ridges themselves seem to be bundles of small crystallites, which have a size of 20–30 nm. Since obvious crystallization is not observed at lower sputtering power of 100 W, this microstructure feature may be described as an amorphouslike state. Similar micrograph of other solid films was reported by Sivakumar [13]. It is interesting that the AFM topography of the Ti films deposited at 150 W (Fig. 2) exhibits another amorphous-like feature. In Fig. 2, the films are clearly characterized by a hillock-like topography due to circular to elliptical clusters bordered by relatively narrow and deep channels with the width of 10–50 nm and the depth of 5–10 nm. Every cluster exhibits a substructure of small, dispersed hemispheres, namely a certain number of
randomly oriented crystal grains. The crystallite size was measured to be 15–35 nm. However, these amorphous-like topographies disappear as the sputtering power is increased further. As contrasted with the amorphous-like feature at 100 and 150 W, grain features protruding from the film surface occur at higher sputtering power. The images in Fig. 3 show random distribution of long columnarshaped grains separated by grain boundaries and some channels with the extreme depth of 17 nm. It is clearly observed that formation of larger columnar crystallites through the coalescence of adjacent globular grains (30–60 nm) occurs frequently in Fig. 3b. As sputtering power increases (from 200 to 250 W), the grains agglomerate together and form smaller number of bigger globular-like grains with the average diameter of 59 nm (seen as Fig. 4). The film surface shows a bumpy topography created by relatively loosely packed columns with hemispheres as tops. It is obvious that more globular-like grains protrude from the tops of other grains which locate in lower layers. In addition, the extreme depth of channels increases obviously, up to about 23 nm at 250 W,
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Fig. 3. AFM images of Ti films sputtered at 200 W: (a) 2D topography (1 mm2), (b) the 2D topographic details (500 nm2) in the black pane region marked in (a) image, (c) the 3D topography of (b) image.
compared with that in Fig. 3. Accordingly, the images at 300 W (Fig. 5) show homogenous distribution of hexagonal grain structure with relatively flatted tops and the average grain size is about 104 nm, larger than each showed in Figs. 3 and 4. A lot of shallow and wide depressions with the depth of only up to 13 nm, distribute homogeneously on the film surface, without deep channels or voids. According to the above discussion, there are four distinct features during the morphological evolution at the different sputtering power in Figs. 1–5: (1) the feature of the surface topography remaining amorphous-like at lower sputtering power of 100 and 150 W; (2) the transition of grain microstructure from amorphous-like to crystalline state, which has been described at intermediate stage (from 150 to 200 W); (3) larger grain size and less voided boundaries at higher sputtering power, showed as Figs. 4 and 5; (4) the transformation of crystal texture from globular-like to hexagonal type, that occurs at final stage(from 250 to 300 W). These significant changes of surface morphologies are due to the fact that sputtering power helps to increase the atomic
mobility on the growing surface [14,15]. Generally, in dc sputtering systems the growing film is mainly bombarded by sputtered neutral target atoms, by reflected neutral plasma gas atoms, and by sputtering gas ions. When the substrate temperature and incident angle remains the same, the intensity of film bombardment is determined by incident energy which is related closely to the sputtering power [16–18]. An increase in sputtering power results not only in increase of energy of the particles that bombard the growing film, but also in increase in amount of deposition atoms arriving to substrate per time unit, which manifests in the dependence of deposition rate on sputtering power (Fig. 6). Deposition rate is directly related to two parameters, namely film thickness and sputtering time. Table 1 provides all the data of film thickness on the different sputtering power. The method of measuring film thickness by AFM technology was described in detail in Ref. [8]. Fig. 6 shows a clear non-linear increase in deposition rate as the increase of sputtering power. In the stage of 100–200 W, it can be found easily that the rate of increase in the deposition rate of films is relatively slow, even only 24.5 nm/min
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Fig. 4. AFM images of Ti films sputtered at 250 W: (a) 2D topography (1 mm2), (b) the 2D topographic details (500 nm2) in the black pane region marked in (a) image, (c) the 3D topography of (b) image.
at 100 W. This behavior is related to the fact that at lower sputtering power, the lower energy of the sputtered particles results in formation of amorphous-like microstructure (Figs. 1 and 2) and finer globular-like grains (Fig. 3). In contrast, the increase in the deposition rate is much more significant when sputtering power alters from 200 to 250 W. Apparently, more deposition atoms arriving to substrate per time unit contribute to the swift increase of film thickness and therefore a rapidly growing process at the surface takes place. However, it is worth noting that the slightly slow increase of deposition rate occurs at higher sputtering power, 300 W. The tendency suggests that the overhigh sputtering power can lead to the increase in the probability of impact between sputtered neutral target atoms and other moving atoms or ions, so as to make incident angle larger and thus slow down the deposition rate. To study the surface features of Ti films, it is necessary to measure the main surface roughness parameters of these films, namely RMS, Ra and P–V roughness. Generally, the surface roughness at a certain area is determined by the height differences
of all the individual points at this area. RMS roughness is the mean of the root for the deviation from the standard surface to the indicated surface. Ra represents the three-dimensional expansion of the center line mean roughness as defined by JISB0601 so that it is applicable to the measurement surface. In addition, P–V describes the difference between the maximum and minimum values of Z data within the indicated surface or section profile. The values of the RMS, Ra and P–V roughness given in Table 1 and Fig. 7 are obtained from the five images with the same squared size (1 mm2) in Figs. 1a, 2a, 3a, 4a and 5a. Fig. 7 shows that the tendencies of three curves for the increase in sputtering power are similar, but the correlativity between RMS and Ra roughness is better. Therefore, it implies that either of three roughness parameters may be used for the lateral characterization of film surface topography. Now, we take RMS roughness for example to discuss the dependence of the surface roughness parameters on sputtering power. At the lower sputtering power (below 150 W), RMS roughness of Ti films ranges only from 1.53 to 2.20 nm, related to the formation of amorphous-like microstructure (Figs. 1
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Fig. 5. AFM images of Ti films sputtered at 300 W: (a) 2D topography (1 mm2), (b) the 2D topographic details (500 nm2) in the black pane region marked in (a) image, (c) the 3D topography of (b) image.
Fig. 6. Dependence of the deposition rate on the sputtering power.
and 2). The shallower depressions (Fig. 1) or channels (Fig. 2) contribute to the smaller surface roughness and lead to the smoother film surface. As grain features occurring at 200 W (Fig. 3), the formation of the deeper channels results in higher surface roughness of 3.23 nm. With larger protruding grains occurring on the film surface at 250 W, the maximum value of RMS roughness, 4.42 nm is attained. The fatting of hexagonal grains obtained during the transformation of crystal structure at 250–300 W, contributes to the decrease in film RMS roughness (up to 3.03 nm). According to discussion above, it indicates the dependences of RMS, Ra and P–V roughness on the sputtering power are closely associated with crystalline feature, grain size, crystal texture and the distribution of grain boundary defects. Generally, the sputtering power promotes the film crystallinity and the transformation of crystal structure, which is consistent with the growth as demonstrated in AFM topography in Figs. 1–5. In accordance with the model suggested in [19,20], the better crystallinity and less voided boundaries achieved with higher sputtering power (seen as
Y. Jin et al. / Applied Surface Science 255 (2009) 4673–4679
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Table 1 The thickness, deposition rate and surface roughness of Ti films sputtered on the different sputtering power. Sputtering power (W)
Film thickness (nm)
Deposition rate (nm/min)
RMS roughness (nm)
Ra roughness (nm)
P–V roughness (nm)
100 150 200 250 300
97 125 134 213 259
24.25 31.25 33.50 53.25 64.75
1.53 2.20 3.23 4.42 3.03
1.20 1.72 2.54 3.53 2.43
12.12 20.15 29.14 34.47 22.00
(most than 250 W). We conclude that the growth of dc magnetron sputtered Ti films could be optimized in terms of the modification of the surface topography by properly controlling the sputtering power. Acknowledgements This work was funded by the Key Laboratory of Material Corrosion and Protection, Sichuan University of Science and Engineering. References
Fig. 7. Variations of RMS, Ra and P–V roughness with sputtering power as measured from AFM images in Figs. 1a, 2a, 3a, 4a and 5a.
Figs. 4 and 5) lead to better surface mobility due to larger impact energy of the bombarding particles. 4. Conclusion In summary, we have studied the influence of sputtering power on the surface topography of Ti films produced by dc magnetron sputtering. The study has shown that different topographical features can be modified by varying the sputtering power. As the sputtering power increases, the deposition rate of the Ti films is higher and the films exhibits the enhanced grain features with higher crytallinity, including the transition of grain microstructure from amorphous-like to crystalline state and the transformation of crystal texture from globular-like to hexagonal type. Based on good correlativity between RMS, Ra and P–V roughness with increase in sputtering power, all of them may be used to properly characterize the lateral features of Ti films. Generally, higher sputtering power (up to 250 W) can lead to increase higher surface roughness, but the roughness decrease with the increase of sputtering power
[1] J.W. Elmer, T.A. Palmer, J. Wong, J. Appl. Phys. 93 (2003) 1941–1947. [2] M.C.G. Passeggi Jr., L.I. Vergara, S.M. Mendoza, J. Ferro´n, Surf. Sci. 507–510 (2002) 825–831. [3] J.M. Lo´pez, F.J. Gordillo-Va´zquez, M. Ferna´ndez, J.M. Albella, Thin Solid Films 384 (2001) 69–75. [4] Y.L. Jeyachandran, B. Karunagaran, Sa.K. Narayandass, D. Mangalaraj, Mater. Sci. Eng. A 458 (2007) 361–365. [5] T. Godfroid, R. Gouttebaron, J.P. Dauchot, Ph. Lecle‘re, R. Lazzaroni, M. Hecq, Thin Solid Films 437 (2003) 57–62. [6] Y.L. Jeyachandran, B. Karunagaran, Sa.K. Narayandass, D. Mangalaraj, T.E. Jenkins, P.J. Martin, Mater. Sci. Eng. A 431 (2006) 277–284. [7] N. Martin, D. Baretti, C. Rousselot, J.-Y. Rauch, Surf. Coat. Technol. 107 (1998) 172– 182. [8] Y. Jin, W. Wu, D. Liu, J. Chen, Y. Sun, Surf. Rev. Lett. 16 (2007) 1053–1059. [9] G. Binnig, C.F. Quate, C. Gerber, Phys. Rev. Lett. 56 (1986) 930–933. [10] K.-Y. Chan, B.-S. Teo, Microelectron. J. 37 (2006) 1064–1071. [11] S. Christoulakis, M. Suchea, E. Koudoumas, M. Katharakis, N. Katsarakis, G. Kiriakidis, Appl. Surf. Sci. 252 (2006) 5351–5354. [12] A.R. Boyd, B.J. Meenan, N.S. Leyland, Surf. Coat. Technol. 200 (2006) 6002–6013. [13] R. Sivakumar, M. Jayachandran, C. Sanjeeviraja, Mater. Chem. Phys. 87 (2004) 439–445. [14] Y.M. Lu, W.S. Hwang, W.Y. Liu, J.S. Yang, Mater. Chem. Phys. 72 (2001) 269–272. [15] J.L. Andujar, F.J. Pino, M.C. Polo, A. Pinyol, C. Corbella, E. Bertran, Diamond Relat. Mater. 11 (2002) 1005–1009. [16] J. Shin-Pon, W. Cheng-I, C. Jee-Gong, J. Appl. Phys. 89 (2001) 7825–7832. [17] L. Dong, R.W. Smith, D.J. Srolovitz, J. Appl. Phys. 80 (1996) 5682–5690. [18] R.W. Smith, D.J. Srolovitz, J. Appl. Phys. 79 (1996) 1448–1457. [19] D.K. Hwang, K.H. Bang, M.C. Jeong, J.M. Myoung, J. Cryst. Growth 254 (2003) 449– 455. [20] H. Bellakhder, A. Outzourhit, E.L. Ameziane, Thin Solid Films 382 (2001) 30–33.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of sputtering power on surface topography of dc magnetron sputtered Ti thin films observed by AFM Yongzhong Jin a,*, Wei Wu b, Li Li b, Jian Chen a, Jingyu Zhang a, Youbing Zuo a, Jun Fu a a b
Department of Materials and Chemistry Engineering, Sichuan University of Science and Engineering, 180 Huixing Road, Zigong, Sichuan 643000, China School of Materials Science and Engineering, Xihua University, Chengdu, Sichuan 610039, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 September 2008 Received in revised form 7 December 2008 Accepted 9 December 2008 Available online 24 December 2008
Titanium films were deposited on glass substrates at room temperature by direct current (dc) magnetron sputtering at fixed Ar pressure of 1.7 Pa and sputtering time of 4 min with different sputtering power ranging from 100 to 300 W. Atomic force microscopy (AFM) was used to study topographic characteristics of the films, including crystalline feature, grain size, clustering and roughening. The amorphous-like microstructure feature has been observed at 100–150 W and the transition of crystal microstructure from amorphous-like to crystalline state occurs at 200 W. The increase in grain size of Ti films with the sputtering power (from 200 to 300 W) has been confirmed by AFM characterization. In addition, higher sputtering power (300 W) leads to the transformation of crystal texture from globularlike to hexagonal type. The study has shown that higher sputtering power results in the non-linear increase in deposition rate of Ti films. Good correlativity between the surface roughness parameters including root mean square (RMS) roughness, surface mean height (Ra) and maximum peak to valley height (P–V) for evaluating the lateral feature of the films has been manifested. Surface roughness has an increasing trend at 100–250 W, and then drops up to 300 W. ß 2008 Elsevier B.V. All rights reserved.
PACS: 64.70.Nd 68.37.Ps 81.07.Bc Keywords: Ti films Magnetron sputtering Sputtering power Surface topography Atomic force microscopy
1. Introduction Titanium (Ti), due to high mechanical strength, excellent thermal stability, good corrosion resistance and intrinsic biocompatibility has received a great deal of attention over the last three decades. These features make it very useful, not only for mechanical applications, but also for the aerospace, medical and microelectronics industries [1–3]. For these applications, smooth surface, fine grain and homogenous structure of Ti thin films are of critical importance. Many studies on magnetron sputtered Ti thin films have been reported [4–7]. For example, Jeyachandran et al. [4] investigated the effect of thickness on the electrical, structural, optical and surface properties of sputtered Ti films. Godfroid et al. [5] have studied the influence of sputtering current on the growth modes of sputtered Ti films. Chemical composition and structural properties of Ti thin films sputtered under various cathode power, sputtering pressure and base vacuum conditions have also been investigated [6]. Martin et al. [7] have researched the influence of bias power on
* Corresponding author. Tel: +86 28 66453877; fax: +86 28 66453877. E-mail address: [email protected] (Y. Jin). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.12.029
some properties of Ti coatings. However, these studies do not deal with a systematic investigation on morphological evolution of Ti films on the nanometer scale with respect to the deposition parameters. Undoubtedly, it is very important for the nanometerscale characterization of surface topography of thin films to obtain smooth surface, fine grain and homogenous structure [8]. The goal of this work was to investigate sputtering power affecting surface topography of direct current (dc) magnetron sputtered Ti films on the nanometer scale using atomic force microscopy (AFM). Since the invention of AFM in 1986 [9], it has became a very popular and effective method to measure surface topography of thin films. An important advantage compared with electron beam methods is that AFM can operate in ambient environments. Obviously, AFM technology has widely been used in the measurement of the hyperfine surface topography of thin solid films [10–12]. 2. Experiment Ti films were deposited on glass substrate at room temperature with dc magnetron sputtering using a 100 mm diameter 5 mm thick Ti target of 99.995% purity. The distance between the Ti target and glass substrates was 110 mm. Commercial argon (99.999%
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Fig. 1. AFM images of Ti films sputtered at 100 W: (a) 2D topography (1 mm2), (b) the 2D topographic details (500 nm2) in the black pane region marked in (a) image, (c) the 3D topography of (b) image.
pure) was used as the sputtering gas. Before deposition, the glass substrates were rinsed with acetone, redistilled alcohol, air dried, and installed into sputtering apparatus. The deposition chamber was pumped down to the ultimate vacuum and repeatedly charged with argon and pumped down in order to minimize the residual gas components. The shutter located in between the target and the substrate, was used to control the sputtering time. Before starting the actual experiment, the pre-sputtering for 1 min was done to remove the surface oxide of Ti target. In this way, a group of samples were deposited at fixed Ar pressure of 1.7 Pa and sputtering time of 4 min with different sputtering power ranging from 100 to 300 W. The topography of these films was characterized by a commercially available AFM system (Seiko SPM400) in tapping mode. The Si probe tips (force constant: 3.0 N/m) used for the AFM measurements are integral with the V-shaped cantilevers. In this mode, force on the cantilever is kept constant and hence a constant force is applied on the sample. AFM detection is done by optical beam deflection, involving a position sensitive detector and laser
beam alignment by piezo driven mirror motors. When the Si tip scans across the sample, surface features can cause a deflection in the cantilever, which is detected by the laser detector and feedback to the Z-axis controller, adjusting the Z-position of the Si tip to restore the set point force and deflection. In addition, the specific software (Seiko SPI3800N) was chosen in order to examine the three-dimensional features on all AFM images. It should be noted that three main surface roughness parameters including RMS, Ra and P–V roughness are simultaneously used for lateral characterization of Ti films. 3. Results and discussion AFM images were collected at various magnifications and different fields of view in order to evaluate the surface features of Ti thin films. Five sets of surface topographies of Ti films deposited on glass substrate with different sputtering power ranging from 100 to 300 W are displayed in Figs. 1–5. To analyze the surface topographies of Ti films by Seiko SPI3800N software, it is assumed
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Fig. 2. AFM images of Ti films sputtered at 150 W: (a) 2D topography (1 mm2), (b) the 2D topographic details (500 nm2) in the black pane region marked in (a) image, (c) the 3D topography of (b) image.
that the dark regions seen in the AFM images (i.e. cavities and channels shown in Figs. 1–5) are related to the areas with the height value of zero or near zero along the positive direction of Z axle and the bright regions (i.e. the tops of protruding grains) represent higher areas. Fig. 1 exhibits a wavy topography created by undulating ridges (ribbons), which are bordered by shallow and wide depressions with the depth of 1.5–5 nm and the width of 10–100 nm. The ridges themselves seem to be bundles of small crystallites, which have a size of 20–30 nm. Since obvious crystallization is not observed at lower sputtering power of 100 W, this microstructure feature may be described as an amorphouslike state. Similar micrograph of other solid films was reported by Sivakumar [13]. It is interesting that the AFM topography of the Ti films deposited at 150 W (Fig. 2) exhibits another amorphous-like feature. In Fig. 2, the films are clearly characterized by a hillock-like topography due to circular to elliptical clusters bordered by relatively narrow and deep channels with the width of 10–50 nm and the depth of 5–10 nm. Every cluster exhibits a substructure of small, dispersed hemispheres, namely a certain number of
randomly oriented crystal grains. The crystallite size was measured to be 15–35 nm. However, these amorphous-like topographies disappear as the sputtering power is increased further. As contrasted with the amorphous-like feature at 100 and 150 W, grain features protruding from the film surface occur at higher sputtering power. The images in Fig. 3 show random distribution of long columnarshaped grains separated by grain boundaries and some channels with the extreme depth of 17 nm. It is clearly observed that formation of larger columnar crystallites through the coalescence of adjacent globular grains (30–60 nm) occurs frequently in Fig. 3b. As sputtering power increases (from 200 to 250 W), the grains agglomerate together and form smaller number of bigger globular-like grains with the average diameter of 59 nm (seen as Fig. 4). The film surface shows a bumpy topography created by relatively loosely packed columns with hemispheres as tops. It is obvious that more globular-like grains protrude from the tops of other grains which locate in lower layers. In addition, the extreme depth of channels increases obviously, up to about 23 nm at 250 W,
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Fig. 3. AFM images of Ti films sputtered at 200 W: (a) 2D topography (1 mm2), (b) the 2D topographic details (500 nm2) in the black pane region marked in (a) image, (c) the 3D topography of (b) image.
compared with that in Fig. 3. Accordingly, the images at 300 W (Fig. 5) show homogenous distribution of hexagonal grain structure with relatively flatted tops and the average grain size is about 104 nm, larger than each showed in Figs. 3 and 4. A lot of shallow and wide depressions with the depth of only up to 13 nm, distribute homogeneously on the film surface, without deep channels or voids. According to the above discussion, there are four distinct features during the morphological evolution at the different sputtering power in Figs. 1–5: (1) the feature of the surface topography remaining amorphous-like at lower sputtering power of 100 and 150 W; (2) the transition of grain microstructure from amorphous-like to crystalline state, which has been described at intermediate stage (from 150 to 200 W); (3) larger grain size and less voided boundaries at higher sputtering power, showed as Figs. 4 and 5; (4) the transformation of crystal texture from globular-like to hexagonal type, that occurs at final stage(from 250 to 300 W). These significant changes of surface morphologies are due to the fact that sputtering power helps to increase the atomic
mobility on the growing surface [14,15]. Generally, in dc sputtering systems the growing film is mainly bombarded by sputtered neutral target atoms, by reflected neutral plasma gas atoms, and by sputtering gas ions. When the substrate temperature and incident angle remains the same, the intensity of film bombardment is determined by incident energy which is related closely to the sputtering power [16–18]. An increase in sputtering power results not only in increase of energy of the particles that bombard the growing film, but also in increase in amount of deposition atoms arriving to substrate per time unit, which manifests in the dependence of deposition rate on sputtering power (Fig. 6). Deposition rate is directly related to two parameters, namely film thickness and sputtering time. Table 1 provides all the data of film thickness on the different sputtering power. The method of measuring film thickness by AFM technology was described in detail in Ref. [8]. Fig. 6 shows a clear non-linear increase in deposition rate as the increase of sputtering power. In the stage of 100–200 W, it can be found easily that the rate of increase in the deposition rate of films is relatively slow, even only 24.5 nm/min
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Fig. 4. AFM images of Ti films sputtered at 250 W: (a) 2D topography (1 mm2), (b) the 2D topographic details (500 nm2) in the black pane region marked in (a) image, (c) the 3D topography of (b) image.
at 100 W. This behavior is related to the fact that at lower sputtering power, the lower energy of the sputtered particles results in formation of amorphous-like microstructure (Figs. 1 and 2) and finer globular-like grains (Fig. 3). In contrast, the increase in the deposition rate is much more significant when sputtering power alters from 200 to 250 W. Apparently, more deposition atoms arriving to substrate per time unit contribute to the swift increase of film thickness and therefore a rapidly growing process at the surface takes place. However, it is worth noting that the slightly slow increase of deposition rate occurs at higher sputtering power, 300 W. The tendency suggests that the overhigh sputtering power can lead to the increase in the probability of impact between sputtered neutral target atoms and other moving atoms or ions, so as to make incident angle larger and thus slow down the deposition rate. To study the surface features of Ti films, it is necessary to measure the main surface roughness parameters of these films, namely RMS, Ra and P–V roughness. Generally, the surface roughness at a certain area is determined by the height differences
of all the individual points at this area. RMS roughness is the mean of the root for the deviation from the standard surface to the indicated surface. Ra represents the three-dimensional expansion of the center line mean roughness as defined by JISB0601 so that it is applicable to the measurement surface. In addition, P–V describes the difference between the maximum and minimum values of Z data within the indicated surface or section profile. The values of the RMS, Ra and P–V roughness given in Table 1 and Fig. 7 are obtained from the five images with the same squared size (1 mm2) in Figs. 1a, 2a, 3a, 4a and 5a. Fig. 7 shows that the tendencies of three curves for the increase in sputtering power are similar, but the correlativity between RMS and Ra roughness is better. Therefore, it implies that either of three roughness parameters may be used for the lateral characterization of film surface topography. Now, we take RMS roughness for example to discuss the dependence of the surface roughness parameters on sputtering power. At the lower sputtering power (below 150 W), RMS roughness of Ti films ranges only from 1.53 to 2.20 nm, related to the formation of amorphous-like microstructure (Figs. 1
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Fig. 5. AFM images of Ti films sputtered at 300 W: (a) 2D topography (1 mm2), (b) the 2D topographic details (500 nm2) in the black pane region marked in (a) image, (c) the 3D topography of (b) image.
Fig. 6. Dependence of the deposition rate on the sputtering power.
and 2). The shallower depressions (Fig. 1) or channels (Fig. 2) contribute to the smaller surface roughness and lead to the smoother film surface. As grain features occurring at 200 W (Fig. 3), the formation of the deeper channels results in higher surface roughness of 3.23 nm. With larger protruding grains occurring on the film surface at 250 W, the maximum value of RMS roughness, 4.42 nm is attained. The fatting of hexagonal grains obtained during the transformation of crystal structure at 250–300 W, contributes to the decrease in film RMS roughness (up to 3.03 nm). According to discussion above, it indicates the dependences of RMS, Ra and P–V roughness on the sputtering power are closely associated with crystalline feature, grain size, crystal texture and the distribution of grain boundary defects. Generally, the sputtering power promotes the film crystallinity and the transformation of crystal structure, which is consistent with the growth as demonstrated in AFM topography in Figs. 1–5. In accordance with the model suggested in [19,20], the better crystallinity and less voided boundaries achieved with higher sputtering power (seen as
Y. Jin et al. / Applied Surface Science 255 (2009) 4673–4679
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Table 1 The thickness, deposition rate and surface roughness of Ti films sputtered on the different sputtering power. Sputtering power (W)
Film thickness (nm)
Deposition rate (nm/min)
RMS roughness (nm)
Ra roughness (nm)
P–V roughness (nm)
100 150 200 250 300
97 125 134 213 259
24.25 31.25 33.50 53.25 64.75
1.53 2.20 3.23 4.42 3.03
1.20 1.72 2.54 3.53 2.43
12.12 20.15 29.14 34.47 22.00
(most than 250 W). We conclude that the growth of dc magnetron sputtered Ti films could be optimized in terms of the modification of the surface topography by properly controlling the sputtering power. Acknowledgements This work was funded by the Key Laboratory of Material Corrosion and Protection, Sichuan University of Science and Engineering. References
Fig. 7. Variations of RMS, Ra and P–V roughness with sputtering power as measured from AFM images in Figs. 1a, 2a, 3a, 4a and 5a.
Figs. 4 and 5) lead to better surface mobility due to larger impact energy of the bombarding particles. 4. Conclusion In summary, we have studied the influence of sputtering power on the surface topography of Ti films produced by dc magnetron sputtering. The study has shown that different topographical features can be modified by varying the sputtering power. As the sputtering power increases, the deposition rate of the Ti films is higher and the films exhibits the enhanced grain features with higher crytallinity, including the transition of grain microstructure from amorphous-like to crystalline state and the transformation of crystal texture from globular-like to hexagonal type. Based on good correlativity between RMS, Ra and P–V roughness with increase in sputtering power, all of them may be used to properly characterize the lateral features of Ti films. Generally, higher sputtering power (up to 250 W) can lead to increase higher surface roughness, but the roughness decrease with the increase of sputtering power
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