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The effect of process conditions on the properties of bioactive films prepared by magnetron sputtering
Vacuum 83 (2009) 249–256
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Review
The effect of process conditions on the properties of bioactive films prepared by magnetron sputtering J.Z. Shi a, C.Z. Chen a, *, H.J. Yu b, S.J. Zhang c a
School of Materials Science and Engineering, Shandong University, Shandong, Ji’nan 250061, PR China School of Mechanical Engineering, Shandong University, Shandong, Ji’nan 250061, PR China c Mechanical Engineering Department, Ji’nan Vocational College, Shandong, Ji’nan 250103, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2008 Received in revised form 6 May 2008 Accepted 6 May 2008
Radio frequency (RF) magnetron sputtering is a promising deposition technique that can produce dense and well-adhered films. This technique is applied to deposit thin HA films on titanium oral implants, which have exhibited excellent bioactive behavior. In this paper, the influence of key deposition parameters, including discharge power, gas composition, process pressure, base pressure, substrate temperature, bias, target-substrate distance on the properties of bioactive films are reviewed. Besides, other influencing factors such as post-deposition heat treatments and initial target materials are also introduced. At last, the future application of RF magnetron sputtering in biomedicine is presented. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Radio frequency (RF) magnetron sputtering Thin HA films Deposition parameters Influencing factors
1. Introduction Hydroxyapatite (HA), Ca10(PO4)6(OH)2, which is the main chemical constituent of bone, is nowadays considered a useful biocompatible material. Unfortunately, HA is brittle and the prepared bone implants can provide poor mechanical performance. This drawback is overcome by the development of HA coatings on metals. This takes advantage of the bioactive behavior of the HA ceramic and preserves the mechanical characteristics of the metallic substrate. Presently, the commercial method applied for growing HA coatings with interest in bone implantology is plasma spraying [1–3]. However, this method presents a few drawbacks, including thermal decomposition of HA from the high-temperature plasma (up to 30,000 C) and fractures in thick-film coatings of >50 mm in thickness [4–6]. To resolve these problems, other deposition methods such as biomimetic process [7], sol–gel [8,9], dipping [10], electrophoretic deposition [11,12], and radio frequent (RF) magnetron sputtering [13] have been investigated. Although these experimental processes have successfully overcome some of the problems associated with plasma spraying, there are unique limitations with each [2,14–16]. For example, dip methods and
* Corresponding author. School of Materials Science and Engineering, Shandong University, No. 73 Jingshi Road, Shandong, Ji’nan 250061, PR China. Tel.: þ86 531 88395991; fax: þ86 531 88392313. E-mail address: [email protected] (C.Z. Chen). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.05.019
electrophoresis techniques showed weak bonding strength between the coating and substrate [4,17]; the biomimetic coatings made with a supersaturated calcifying solutions are difficult to prepare on an industrial scale [18]; the sol–gel deposited coatings are easy to crack in drying processing [16]. In comparison, RF magnetron sputtering exhibits more advantages for producing bioactive films: (1) the sputtered films have good adherence properties [2,17,19]; (2) the films are more uniform and more dense, even on complex implant designs [20,21]; (3) the substrate temperature is low during deposition [22]; and (4) this technology can be applied for large-scale industrial production [23]. RF magnetron sputtering is an improved ion-sputtering method and has been applied to deposit thin bioactive HA films on titanium (Ti) oral implants [24]. In RF magnetron-sputtering deposition, an RF field generates plasma between a sputtering target and the substrate holder. In magnetron sputtering, a magnetic configuration is present below the target, which traps electrons nearby the target. There, the electrons cause ionizations in the sputtering gas. Part of the ions is accelerated towards the target by the voltage drop, the so-called plasma sheath, which is present between the plasma and the target. The bombarding ions cause the ejection of target species and secondary electrons. The secondary electrons maintain the discharge [22]. The atoms from that target travel to and bond with the substrate, becoming the part of thin films. The structure and properties of magnetron-sputtered films largely depend upon the rate at which energy is delivered to the growing film by concurrent ion bombardment. In other words, film
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properties are governed by internal parameters like ion density and ion current to the substrate. The application of external parameters in a magnetron-sputtering system such as discharge power, gas composition, process pressure, base pressure, gas flow rates, substrate temperature, bias and targets-substrate distance, significantly affects these internal parameters and the resulting film properties [25,26]. By adjusting the deposition parameters, thin, uniform, and dense bioactive films can be obtained. Despite these excellent properties, the as-sputtered films have a low HA crystalline which in turn affects the stability or dissolution behavior of the implants [27–29]. This problem can be solved by post heat treatments, however, which may be result in weakened bond strength due to thermal expansion mismatching between substrate and film. The differences in thermal expansion can be reduced by controlling the film composition, which is greatly influenced by initial target material. Consequently, the properties of the bioactive films are also determined by post-deposition treatments and initial target materials [30]. The relationships between processing conditions and structure, elemental composition and properties of the bioactive films are summarized in this paper. 2. Deposition conditions of magnetron sputtering The sputter deposited films had a uniform and dense structure, especially on complex implant designs, like threaded implants [24]. In cell culture experiments, magnetron-sputtered HA films can indeed stimulate extracellular matrix and induce apatite formation [31]. However, the main limitation of RF sputtering of HA is the possible alteration of the HA structure and composition. The calcium–phosphate (Ca/P) ratios measured for sputtered HA films are often different from the stoichiometric ratio of 1.67. This has been reported to result in the incorporation of Ca or CaO particles and the appearance of other Ca–P-based crystalline phases in the sputtered films. The presence of CaO in HA film reduces dramatically their biocompatibility since CaO causes severe cytotoxicity and may therefore lead to inflammation of the surrounding tissue. The other Ca–P-based phases, such as dicalciumphosphate (DCP), tricalcium phosphate (TCP), and tetracalcium phosphate (TTCP), present biocompatibility level comparable to the one of HA but with lower osteoconductivity [27]. Additionally, biodegradation is generally better withstanded by crystalline and stoichiometric HA than the other Ca–P phases. For these reasons, Ca–P films should have the highest possible crystallinity and a good stoichiometry. Of course, the bioactive films should also have good mechanical properties, such as strong adherence to the substrate, appropriate hardness, and high mechanical wear-resistance [28]. Subsequently, the influence of deposition parameters on the properties of bioactive films will be analyzed, respectively. 2.1. RF power In certain conditions, discharge carrier ionization and ion density will rise with the increase of sputtering power. Therefore, highenergy ions enhance film/substrate adhesion and the density of the film. Conversely, low sputtering power leads to low energy ions, and poor adhesion film. To some extent, higher powers are beneficial to better films, however, too high power have detrimental influence on the quality of films. Over high-energy ions have a great heat effect on basement, which causes films injury [32]. With the increase of sputtering power, sputtering rate of the film will rise [33,34]. Besides, van Dijk et al. [35] reported that the Ca/P ratio also increased with the discharge power at 0.52 Pa of pressure. They explained this on the basis that O atoms were easily pumped away, and that the bonding strength of the P atoms arriving at the growing film layer was therefore weak, so that the P atoms were more easily re-sputtered at higher discharge power
[4,35,36]. However, all the films may be shown having the same crystalline structure as apatite after annealing [37]. Generally speaking, the film is usually sputtered using a lowlevel discharge power, in order to avoid target cracking, ensure a lower cost and reduce a technical risk [1,28,38]. In addition, HA begins to decompose at temperatures above 1200 C [39]. The HA target tends to be decomposed at higher discharge powers, since the kinetic energy of the Ar ions and the resulting target surface temperature increases with discharge power. Ozeki et al. [4] investigated the effects of discharge RF power on the phase composition of the HA film, and found that the HA and the Ca/P ratios were largest, not at 150 W, but at 100 W of discharge power, as shown in Figs. 1 and 2. The results differed with the research of van Dijk et al. [35], because of the influence of a phase change of the HA target. On the other hand, Fig. 2 shows that the Ca/P ratio decreased with increasing Ar pressure, whereas for van Dijk et al. [40], the ratio increased [4]. This will be discussed in the next section. According to Ref. [41], the Ca/P ratio was lower than that of ideal HA at 0.1 Pa of pressure. The deficiency of Ca may be due to a lower sputtering rate of Ca atoms at low gas pressure. As the discharge power increased, the energy of Ar ions in plasma increased. It is suggested that the higher energy of Ar ions positively influenced the sputtering of Ca atoms, and the energy of Ar ions was enough to sputter Ca atoms at high-discharge power. The Ca/P ratio increased as the discharge power was increased and became 1.59 close to that of HA at 400 W (Table 1). As such, the biological property of HA films is greatly dependent on the discharge power during the deposition. 2.2. Gas composition and pressure Before deposition took place, the chamber is always evacuated to a base pressure not greater than 1 103 Pa [42]. After that, highpurity argon (99.999%) or mixture of argon with oxygen is introduced into the chamber by means of a mass flow controller. For all the depositions a constant value of the gas pressure is used [1,43]. The work pressure and gas composition are important parameters, which have a great impact on deposition rate and quality of the films. The effect of argon gas pressure at a low discharge power level (150 W) on the surface properties of a series of co-sputtered Ca–P surfaces was investigated in Ref. [44]. Fig. 3 showed that the deposition rate increases up to an argon gas pressure of 2 Pa and then decreases as the argon gas pressure increases up to 5 Pa. In general, the number of Ar ions increases with Ar pressure and the deposition rate increases. However, there is an upper limit to this, since before arriving at the substrate, the sputtered atoms will suffer more collisions with increasing Ar gas pressure. Consequently, at lower pressures, the deposition rate increased with Ar
Fig. 1. Phase-composition weight ratio in the films vs. discharge power at 1.0 Pa of pressure. b-Tricalcium phosphate (TCP), b-calcium pyrophosphate (PYR) [4].
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Fig. 3. Ca–P film thickness at different argon gas pressures [44].
Fig. 2. Ca/P ratio of the sputtered films with various sputtering conditions as determined by EDS [4].
pressure, and after reaching an upper limit, it then decreased [4,40]. Due to the increasing number of collisions at higher gas pressures, the trajectory of the sputtered calcium (Ca) starts to look like a random walk through the deposition chamber, thereby enlarging the chance of getting lost. Therefore, the amount of Ca decreased with increasing Ar pressure [45]. At low Ar pressure, more than half of the material deposited is re-sputtered from the film, preferentially phosphorus (P). With an increase in Ar pressure re-sputtering of phosphorus in the film decreases [22,46]. Another explanation of the increase of phosphorus in the film is that lighter phosphorusbearing species can reach the film faster than heavier calciumbearing species with the increase of collisions [47]. It can be concluded that the Ca/P ratio decreased with increasing argon gas pressure, as depicted in Fig. 2 and Fig. 4 [4,47]. In contrary, van Dijk et al. [40] investigated the sputtered Ca/P ratio at 0.26–1.5 Pa gas pressure, and found that the ratio increased with pressure. It may be a tendency at very low pressure. In these conditions, phosphorus being lighter than calcium will suffer more from these collisions and hence the number of phosphorus atoms in the film decreases resulting in a calcium rich surface. This phenomenon is true up to a point, thereafter, the Ca/P ratio is seen to decrease with increasing argon gas pressure [4,40,44]. Additionally, it should be mentioned that the influence of gas pressure on deposition rate or Ca/P ratio is different with discharge power [40]. In hydroxyapatite, phosphorus is bound as PO4. The introduction of oxygen in the sputtering gas enhances the formation of PO4 and reduces re-sputtering of phosphorus. Consequently,
a stoichiometric Ca/P ratio can be obtained [36,48,49]. But an increase of the oxygen pressure will lead to a strong reduction of the growth rate of the films [36]. The presence of water in the background gas can result in the presence of OH bonds and more crystalline HA in the sputtered films [40]. In addition, carbonatecontaining HA films prepared by RF-sputtering method using Ar/ CO2 gas were studied. Overgrowing of bone-like apatite layers on the carbonate-containing HA surfaces in the simulated body fluid (SBF) (Fig. 5) implied that the obtained films acquired a sufficient osteoconductivity [50]. It can be concluded that the amount and the composition of the background gas are important for the final film composition and properties [40,51].
2.3. Substrate temperature Ca–P films are usually deposited onto substrates at a temperature below 550 C. When the temperature is increased, the atoms acquire more thermal energy and hence vibrate more vigorously. As the atoms on the surface have fewer neighbors and are more loosely bound than in the bulk, the amplitude of their vibrations is greater. This also implies that the energy of the atoms on the surface is higher than in the bulk. When the temperature is high enough, the surface atoms gain sufficient energy to leave their sites,
Table 1 The stoichiometric composition (Ca/P ratio) calculated from the RBS spectra [41] Discharge power (W) Ca/P ratio
200 1.11
300 1.37
400 1.59
Fig. 4. The effect of the gas pressure on the Ca/P ratio (20 mTorr ¼ 2.67 Pa, 40 mTorr ¼ 5.33 Pa, 60 mTorr ¼ 8.00 Pa, 80 mTorr ¼ 10.67 Pa). The deposition parameters were: input power 700 W; substrate temperature 550 C; zero bias [47].
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Fig. 5. SEM photographs of HA films hydrothermally annealed in Ar/CO2 before (a) and after (b) soaking of SBF. Bars in the photographs indicate 1 mm [50].
migrate over the surface, and eventually rearrange to reduce the overall internal energy. This rearrangement is most efficient at higher surface temperatures, which promote higher atom mobility and more efficient surface diffusion, then resulting in the long range ordering and growth of crystalline structures. At lower temperature, the surface atoms have a lower mobility, which results in a build-up of material in the amorphous phase. Fig. 6 shows SEM micrographs of the HA films, deposited at two different temperatures. It can be seen that the more uniform and lower porosity films can be deposited at elevated substrate temperatures. For the sample shown in Fig. 6(a), many pores are clearly visible. On the other hand, the entire surface of the sample synthesized at a higher temperature is covered quite homogeneously with hydroxyapatite crystallite grains, as shown in Fig. 6(b). Therefore, high-substrate temperatures are required to synthesize highly crystalline HA and quite homogeneous films [47]. On the other side, crystalline HA films are usually obtained by post-deposition heat treatments. Nelea et al. [1] observed that both the films directly deposited at a high temperature (e.g. 550 C) and the ones obtained at room temperature and after that annealed at this temperature mostly contain the HA phase and exhibit good mechanical characteristics. But the thin HA film reported in Ref. [52] had the significant advantage of not requiring any post-deposition heat treatment. They showed that the crystalline phase of the HA film still is clearly detectable even if the deposition temperature is as low as 67 C (Fig. 7), which may be due to the effect of other parameters, such as power density, chamber pressure and bias. In addition, the substrate temperature might influence the preferred plane of
orientation of the film [53,54]. So it can be inferred that the substrate temperature is an important parameter during sputtering. 2.4. Bias and target-to-substrate distance The increasing bias voltage can lead to an increased energy of the bombarding ions, which can be related to the surface roughness [55]. A radio frequency or DC bias may be applied to the substrate to vertically accelerate ions into the bottom of the trenches, thus obtaining good bottom coverage and enables complete fillings [56]. Varying the bias can efficiently control the main crystalline phase, and the preferred orientation [57]. Fig. 8 shows the Ca–P–Ti films XRD patterns of three samples prepared at 0-, 70- and 150-V bias and at 18 mTorr (2.4 Pa). In the case of absence of a bias, it is clear seen that a strong diffraction peak at (130) is present, suggesting that the crystalline growth is along a preferred orientation of (130) plane. With increasing of the bias to 70 V, another strong peak corresponding to (132) plane appears. Further increasing the bias to 150 V, these peaks disappear and a new peak corresponding to the (143) plane is formed [58]. The Ca/P ratio increases with the values of the negative DC bias [47,58]. Fig. 9 shows the Ca/P ratio as a function of bias voltage, for films prepared at working gas pressure 21 mTorr (2.8 Pa) and 700 W RF power. Comparatively, a closer ratio of about 1.7 is obtained without any bias voltage [47]. Of course, the result will be changed with other parameters. According to Ref. [58], the Ca/P ratio is measured to be about 1.45 at working gas pressure 70 mTorr (9.3 Pa) and 550 W RF power without any bias voltage, is lower than the result above. That may be the combined effects of higher pressure and lower power.
Fig. 6. SEM images showing that uniformity and low porosity can be achieved through increasing the temperature: (a) 130 C and (b) 300 C [47].
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Fig. 9. The dependence of the Ca/P ratio on the substrate bias [47].
Fig. 7. Infrared spectra of the HA thin films deposited on titanium-alloy specimens at different substrate temperatures. Ca/P ratios are 1.79, 1.76, and 1.74 for substrate temperatures of 241 C, 196 C, and 67 C, respectively [52].
At last, the target-to-substrate distance is also an important deposition parameter [59]. For better flexibility, the spacing between the substrate holder and target electrode is adjustable [47]. As the distance increases, the differences in contributions from the various portions of the target become smaller and the target behaves more like a point source to positions across the substrate, so the view factor across the substrate becomes more uniform [56]. But increasing the distance may result in lower deposition rates [60]. In magnetron sputtering, coating properties are largely determined by the rate at which energy is delivered to the growing
film through simultaneous ion bombardment. The structure and properties depend on three parameters: the homologous temperature; the ratio of the fluxes of bombarding ions and depositing atoms; and the energy of the bombarding ions [61]. These internal parameters are significantly affected by the application of external parameters. The energy and the number of the bombarding ions can be controlled by the working power and gas pressure; the grain size, lattice constant and compressive stress of the growing film are influenced by substrate bias and deposition temperature [62]; The target-substrate distance has a great influence on the carrier concentration [59]. Therefore, the structure and properties of the bioactive film are affected by all the deposition parameters together. And the correlation between the deposition parameters and the properties of the magnetron-sputtered film is complex. The optimum process variable can be obtained by investigating the working mechanism of magnetron sputtering and by adjusting the parameters during deposition. 3. Other influencing factors 3.1. Annealing
Fig. 8. Influence of bias voltage on crystalline structure [58].
As far as we know, the as-sputtered Ca–P films are amorphous or only weakly crystalline. The dissolution/precipitation behavior of the various Ca–P ceramics is influenced by their chemical structure and crystallinity. This low degree of crystallinity accelerates the dissolution of HA films in a living body, and this high rate of dissolution leads to the disappearance of the films before bone tissue can bond to the film, at an early stage after implantation [63,64]. High-crystal quality films can be obtained by post-deposition treatment. van Dijk et al. [35] studied the effect of different annealing temperatures on the characteristics of thin Ca–P films fabricated by radio frequency magnetron sputtering and demonstrated that 600 C was probably the best annealing temperature to obtain a better characterization of the film. However, conventional heat treatment in an electric furnace leads to films tend to degrade and easily form cracks between the film and the titanium substrate because of differences in thermal expansion and the formation of TiO2. Therefore it is necessary to find a more suitable heat treatment procedure that controls the solubility of the Ca–P films without weakening their adhesion to the underlying Ti-substrate [24,35,64]. Accordingly, rapid heating
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with infrared radiation was introduced as a suitable heat treatment for thin Ca–P films [24,31,34,65–67]. In addition, water is an acceleration factor in the conversion of the amorphous precursor phase to microcrystalline HA during the precipitation of HA [68]. Therefore, HA films could be crystallized at low temperatures using a hydrothermal technique [35,64,69]. Ozeki et al. [70] crystallized the sputtered film in distilled water at varying pH during the hydrothermal treatments, and indicated that the HA crystal size increased with pH but crystal growth rate decreases with pH. And some researches showed that biocompatibility and bioactivity of the sputtered film can be greatly improved on using the hydrothermal treatment [64,71]. However, a post-deposition heat treatment in the presence of water vapor resulted in a significantly rougher surface, as shown in Fig. 10, suggesting the etching effect of water vapor during heat treatment
[72]. The etching effect and thermal expansion attributed to these heat treatments may be result in weakened bond strength between substrate and film.
3.2. Initial target materials An approach to improve the bond strength is the deposition of a composite film, wherein a metallic phase was introduced to serve as either an intermediate layer or a second (continuous or dispersed) phase in the HA matrix [73]. In addition, silicon is usually introduced into Ca–P thin films in order to enhance their potential bioactivity [74]. We can produce Si/HA [75], Ti/HA [73,76] and HA/ Zirconia/Yttria stabilized [77] composite films using different composite targets. The composite films are also obtained using several targets in recent researches [53,78–86]. For example, according to Ref. [86], the magnetron-assisted sputter deposition for the HA was performed in the RF mode (13.56 MHz) and for the Ti in the DC mode. A thin Ti layer (about 0.3 mm) was first deposited as an initial bond coat (pre-coat) onto Ti–6Al–4V substrates and then followed by alternating deposition which Ti gradually decreased and HA simultaneously increased by controlling the input power. The multi-layered composite films had the advantages of high and non-declining adhesion strength and high resistance to SBF attack. The calcium phosphate content of the target also has influence on the phase composition and deposition rate of sputtered films. Ozeki et al. [87] coated calcium phosphate from various calcium phosphate targets (tetracalcium phosphate (TTCP), hydroxyapatite (HA), b-tricalcium phosphate (TCP), b-calcium pyrophosphate (CPP), and b-calcium metaphosphate (CMP) powder targets) using RF magnetron sputtering. The composition of the crystal phase of the coated films was changed, depending on the target materials, and the Ca/P molar ratios of the films varied from 0.74 to 2.54, increasing with the Ca/P molar ratio of the target. The theoretical density of the target materials are tabulated in Table 2, and the order was: HA > b-CPP > b-TCP > TTCP > b-CMP. This order might have an inverse correlation with the order of the deposition rate (Fig. 11). This indicates that the density of the target materials might also influence on the sputtering yield. Target material stoichiometry can have a significant influence on the characteristics of sputter deposited films. Because of high crystallinity, the use of compacted HA powder targets [38,44,74] is desirable compare to the plasma sprayed HA targets applied previously [24,35,37,40,43,63,88–90]. In regard of degradation of the HA powder targets, the effective lifespan and the effects of their multi-cycle usage on the composition and structure of the resultant films are of considerable importance. A concentric ring or ‘annular racetrack’ is showed clearly after the target undergone several sequential deposition cycles. Its presence is due to the use of a magnetron sputter source [91]. Therefore, the surface of the HA target needs to be replaced with a new one at each stage of the sputtering process after deposition [4]. The substrate quality is another important factor in attaining good performance of the films. Pre-coating treatments, grit- and glass-bead blasting affect the final residual film stress. This will have a good effect on the film adhesion to the substrate [31]. Additionally, the position of the substrates relative to the sputter target also has a considerable influence on the adhesion of sputtered films [45,92]. With a stationary substrate holder, this position
Table 2 The theoretical density of the target materials [87] Fig. 10. Representative surface profile morphology of sputtered CaP films on glass substrate. (a) As-sputtered surface; (b) 350 C heat treatment for 1 h in absence of water vapor; (c) 350 C heat treatment for 1 h in the presence of water vapor [72].
Theoretical density (g/cm2)
TTCP
HA
b-TCP
b-CCP
b-CMP
3.05
3.16
3.07
3.13
2.87
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Fig. 11. The deposition rate of films sputtered from various targets [87].
is fixed while with a rotating substrate holder, the position of the substrate changes continuously during deposition. The difference in the adhesion strength among the samples deposited on rotatory and stationary holders is obvious. Furthermore, lower interface strength was found deposited on the rotating substrate holder [49]. 4. Conclusion RF magnetron sputtering is a promising technique for forming bioactive films (HA and its composite films). The adhesion of the film to its substrate and the growth of crystalline structures of the films are predominant factors in determining the performance and reliability of dental and orthopedic implant applications. The structure and properties of films are particularly depended on the process conditions, which include deposition parameters, heat treatments and initial target materials. Understanding their relationships is important in optimizing the process parameters to obtain high-quality bioactive films. With the improvement of these conditions, the application of RF magnetron sputtering in biomedicine will be more promising. Acknowledgments This work was financially supported by Department of Science and Technology of Jinan, Shandong Province, P. R. China (Grant No. 051070). Reference [1] Nelea V, Morosanu C, Iliescu M, Mihailescu IN. Surf Coat Technol 2003;173: 315. [2] Ong JL, Appleford M, Oh S, Yang Y, Chen W-H, Bumgardner JD, et al. JOM 2006; 58(July):67. [3] Caulier H, van der Waerden JPCM, Paquay YCGJ, Wolke JGC, Kalk W, Naert I, et al. J Biomed Mater Res 1995;29:1061.
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[4] Ozeki K, Yuhta T, Fukui Y, Aoki H. Surf Coat Technol 2002;160:54. [5] Ogiso Makoto, Yamashita Yasuo, Matsumoto Toshio. J Biomed Mater Res 1998; 39:23. [6] Lewis Gladius. Biomed Mater Eng 2000;10:157. [7] Rigo ECS, Boschi AO, Yoshimoto M, Allegrini Jr S, Konig Jr B, Carbonari MJ. Mater Sci Eng C 2004;24:647. [8] Stoch A, Jastrzebski ˛ W, D1ugon´ E, Lejda W, Trybalska B, Stoch GJ, et al. J Mol Struct 2005;744–747:633. [9] Lopatin CM, Pizziconi V, Alford TL, Laursen T. Thin Solid Films 1998;326:227. [10] Li Tuantuan, Lee Junhee, Kobayashi Takayuki, Aoki Hideki. J Mater Sci Mater Med 1996;7:355. [11] Hamagami Jun-ichi, Ato Yuki, Kanamura Kiyoshi. Solid State Ionics 2004;172: 331. [12] Guo Xingyuan, Gough Julie, Xiao Ping. J Biomed Mater Res 2007;80A:24. [13] Jansen JA, Wolke JGC, Swann S, van der Waerden JPCM, de Groot K. Clin Oral Implants Res 1993;4:28. [14] Dorozhkin Sergey V. J Mater Sci 2007;42:1061. [15] Jaffe William L, Scott David F. J Bone Joint Surg 1996;78A:1918. [16] Yang M, Shao ZC, Cui ZX. Non Ferrous Min Metall 2007;23:58 [in Chinese]. [17] Cheng Gary J, Pirzada Daniel, Cai M, Mohanty Pravansu, Bandyopadhyay Amit. Mater Sci Eng C 2005;25:541. [18] Damia Chantal, Sharrock Patrick. Mater Lett 2006;60:3192. [19] Ong JL, Lucas LC. Biomaterials 1994;15:337. [20] van der Wal E, Wolke JGC, Jansen JA, Vredenberg AM. Appl Surf Sci 2005;246: 183. [21] Hulshoff JEG, van Dijk K, van der Waerden JPCM, Wolke JGC, Kalk W, Jansen JA. J Biomed Mater Res 1996;31:329. [22] Feddes B, Wolke JGC, Jansen JA, Vredenberg AM. J Appl Phys 2003;93:662. [23] Qian MG. Surface technology and application handbook. Machine Industry Press; 1998. p. 700[in Chinese]. [24] Yoshinari M, Hayakawa T, Wolke JGC, Nemoto K, Jansen JA. J Biomed Mater Res 1997;37:60. [25] Kelly PJ, Arnell RD. J Vac Sci Technol A Vac Surf Films 1999;17:945. [26] Schulte J, Sobe G, Reiss G. Contrib Plasma Phys 1997;37:483. [27] Snyders Rony, Bousser Etienne, Music Denis, Jensen Jens, Hocquet Ste´phane, Schneider Jochen M. Plasma Process Polym 2008;5:168. [28] Nelea V, Morosanu C, Iliescu M, Mihailescu IN. Appl Surf Sci 2004;228:346. [29] Clemens JAM, Klein CPAT, Vriesde RC, Rozing PM, de Groot K. J Biomed Mater Res 1998;40:341. [30] Boyd AR, Meenan BJ, Leyland NS. Surf Coat Technol 2006;200:6002. [31] Wolke JGC, van der Waerden JPCM, Schaeken HG, Jansen JA. Biomaterials 2003;24:2623. [32] Wenlong Song, Jianxin Deng, Jinlong Zhao. Sciencepaper Online. Available from: http://www.paper.edu.cn [in Chinese]. [33] Kulisch W, Colpo P, Rossi F, Shtansky DV, Levashov EA. Surf Coat Technol 2004; 188–189:714. [34] Yonggang Yan, Wolke JGC, Yubao Li, Jansen JA. J Mater Sci Mater Med 2007;18: 1061. [35] van Dijk K, Schaeken HG, Wolke JGC, Jansen JA. Biomaterials 1996;17:405. [36] van Dijk K, Verhoeven J, Mare´e CHM, Habraken FHPM, Jansen JA. Thin Solid Films 1997;304:191. [37] van Dijk K, Schaeken HG, Wolke JCG, Mare´e CHM, Habraken FHPM, Verhoeven J, et al. J Biomed Mater Res 1995;29:269. [38] Verestiuc Liliana, Morosanu C, Bercu M, Pasuk Iuliana, Mihailescu IN. J Cryst Growth 2004;264:483. [39] Zhou Jiming, Zhang Xingdong, Chen Jiyong, Zeng Shaoxian, De Groot K. J Mater Sci Mater Med 1993;4:83. [40] van Dijk K, Schaeken HG, Mare´e CHM, Verhoeven J, Wolke JGG, Habraken FHPM, et al. Surf Coat Technol 1995;76–77:206. [41] Yamaguchi Tetsuro, Tanaka Yoshikazu, Ide-Ektessabi Ari. Nucl Instrum Meth Phys Res B 2006;249:723. [42] Zhong D, Sutter E, Moore JJ, Mustoe GGW, Levashov EA. Disam J. Thin Solid Films 2001;398–399:320. [43] Ong JL, Raikar GN, Smoot TM. Biomaterials 1997;18:1271. [44] Boyd AR, Duffy H, McCann R, Cairns ML, Meenan BJ. Nucl Instrum Meth Phys Res B 2007;258:421. [45] Feddes B, Vredenberg AM, Wolke JGC, Jansen JA. Surf Coat Technol 2004;185: 346. [46] Feddes B, Wolke JGC, Jansen JA, Vredenberg AM. J Appl Phys 2003;93:9503. [47] Xu Shuyan, Long Jidong, Sim Lina, Diong Cheong Hoong, Ostrikov Kostya (Ken). Plasma Process Polym 2005;2:373. [48] van Dijk K, Mare´e CHM, Verhoeven J, Habraken FHPM, Jansen JA. J Biomed Mater Res 1998;42:266. [49] van Dijk K, Gupta V, Yu AK, Jansen JA. J Biomed Mater Res 1998;41:624. [50] Nakamura Satoshi, Hamagami Jun-Ichi, Yamashita Kimihiro. J Biomed Mater Res B Appl Biomater 2007;80B:102. [51] Hulshoff JEG, van Dijk K, de Ruijter JE, Rietveld FJR, Ginsel LA, Jansen JA. J Biomed Mater Res 1998;40:464. [52] Lo WJ, Grant DM. J Biomed Mater Res 1999;46:408. [53] Ozeki K, Janurudin Juliana M, Aoki H, Fukui Y. Appl Surf Sci 2007;253:3397. [54] Jha GC, Ray SK, Manna I. Thin Solid Films 2008;516:3416. [55] Macedo Francisco, Vaz Filipe, Fernandes Ana C, Rebouta Luı´s, Carvalho Sandra, Junge Klaus H, et al. Plasma Process Polym 2007;4:190. [56] Lu Junqing, Yoon Jae-Hong, Shin Keesam, Park Bong-Gyu, Yang Lin. Thin Solid Films 2006;515:2452. [57] Yang Q, Zhao LR, McKellar R, Patnaik PC. Vacuum 2006;81:101.
256
J.Z. Shi et al. / Vacuum 83 (2009) 249–256
[58] Long JD, Xu S, Cai JW, Jiang N, Lu JH, Ostrikov KN, et al. Mater Sci Eng C 2002; 20:175. [59] Luo Chen, Xiaofang Bi. Vacuum. Available online 4 April 2008. [60] Safi I. Surf Coat Technol 2000;127:203. [61] Arnell RD, Kelly PJ, Bradley JW. Surf Coat Technol 2004;188–189:158. [62] Chang Hui-Wen, Huang Ping-Kang, Yeh Jien-Wei, Davison Andrew, Tsau ChunHuei, Yang Chih-Chao. Surf Coat Technol 2008;202:3360. [63] Wolke JGC, de Groot K, Jansen JA. J Biomed Mater Res 1998;39:524. [64] Ozeki K, Aoki H, Fukui Y. J Biomed Mater Res A 2006;76:605. [65] Yan Yonggang, Wolke JGC, De Ruijter A, Yubao Li, Jansen JA. J Biomed Mater Res A 2006;78:42. [66] Yonggang Yan, Wolke JGC, Yubao Li, Jansen JA. J Biomed Mater Res A 2006;77: 815. [67] Takahashi K, van den Beucken JJJP, Wolke JGC, Hayakawa T, Nishiyama N, Jansen JA. J Biomed Mater Res A 2008;84:682. [68] Ding Shinn-Jyh, Lee Tien-Ling, Chu Yu-Hua. J Mater Sci Lett 2003;22:479. [69] Tao Wan, Hideki Aoki, Ayako Katoh. J Wuhan Univ Technol Mater Sci Ed 2005; 20:260. [70] Ozeki K, Aoki H, Fukui Y. J Mater Sci 2005;40:2837. [71] Ozeki K, Mishima A, Yuhta T, Fukui Y, Aoki H. Biomed Mater Eng 2003;13:451. [72] Yang Yunzhi, Kim Kyo-Han, Ong Joo L. Biomaterials 2005;26:327. [73] Ding Shinn-Jyh, Ju Chien-Ping, Chern Lin Jiin-Huey. J Biomed Mater Res 1999; 44:266. [74] Boyd AR, Duffy H, McCann R, Meenan BJ. Mater Sci Eng C 2008;28:228. [75] Thian Eng San, Huang Jie, Best Serena M, Barber Zoe H, Brooks Roger A, Rushton Neil, et al. Biomaterials 2006;27:2692.
[76] Ding Shinn-Jyh, Ju Chien-Ping, Chern Lin Jiin-Huey. J Biomed Mater Res 1999; 47:551. [77] Zhao YT, Zhang Z, Dai QX, Lin DY, Li SM. Surf Coat Technol 2006;200:5354. [78] Chen W, Liu Y, S Courtney H, Bettenga M, Agrawal CM, Bumgardner JD, et al. Biomaterials 2006;27:5512. [79] Thian ES, Huang J, Best SM, Barber ZH, Bonfield W. Biomaterials 2005;26:2947. [80] Thian ES, Huang J, Best SM, Barber ZH, Bonfield W. Mater Sci Eng C 2007;27: 251. [81] Thian ES, Huang J, Best SM, Barber ZH, Bonfield W. J Biomed Mater Res B Appl Biomater 2006;76B:326. [82] Thian ES, Huang J, Vickers ME, Best SM, Barber ZH, Bonfield W. J Mater Sci 2006;41:709. [83] Porter AE, Rea SM, Galtrey M, Best SM, Barber ZH. J Mater Sci 2004;39:1895. [84] Thian ES, Huang J, Best SM, Barber ZH, Bonfield W. J Mater Sci Mater Med 2005;16:411. [85] Chen Minfang, Liu Debao, You Chen, Yang Xianjin, Cui Zhenduo. Surf Coat Technol 2007;201:5688. [86] Ding Shinn-Jyh. Biomaterials 2003;24:4233. [87] Ozeki K, Fukui Y, Aoki H. Appl Surf Sci 2006;253:5040. [88] Wolke JGC, de Groot K, Jansen JA. J Biomed Mater Res B Appl Biomater 1998; 43:270. [89] Wolke JGC, de Groot K, Jansen JA. J Mater Sci 1998;33:3371. [90] Lu Xiong, Leng Yang. J Biomed Mater Res 2003;66A:677. [91] Boyd A, Akay M, Meenan BJ. Surf Interface Anal 2003;35:188. [92] Wolke JGC, van der Waerden JPCM, de Groot K, Jansen JA. Biomaterials 1997; 18:483.
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Review
The effect of process conditions on the properties of bioactive films prepared by magnetron sputtering J.Z. Shi a, C.Z. Chen a, *, H.J. Yu b, S.J. Zhang c a
School of Materials Science and Engineering, Shandong University, Shandong, Ji’nan 250061, PR China School of Mechanical Engineering, Shandong University, Shandong, Ji’nan 250061, PR China c Mechanical Engineering Department, Ji’nan Vocational College, Shandong, Ji’nan 250103, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2008 Received in revised form 6 May 2008 Accepted 6 May 2008
Radio frequency (RF) magnetron sputtering is a promising deposition technique that can produce dense and well-adhered films. This technique is applied to deposit thin HA films on titanium oral implants, which have exhibited excellent bioactive behavior. In this paper, the influence of key deposition parameters, including discharge power, gas composition, process pressure, base pressure, substrate temperature, bias, target-substrate distance on the properties of bioactive films are reviewed. Besides, other influencing factors such as post-deposition heat treatments and initial target materials are also introduced. At last, the future application of RF magnetron sputtering in biomedicine is presented. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Radio frequency (RF) magnetron sputtering Thin HA films Deposition parameters Influencing factors
1. Introduction Hydroxyapatite (HA), Ca10(PO4)6(OH)2, which is the main chemical constituent of bone, is nowadays considered a useful biocompatible material. Unfortunately, HA is brittle and the prepared bone implants can provide poor mechanical performance. This drawback is overcome by the development of HA coatings on metals. This takes advantage of the bioactive behavior of the HA ceramic and preserves the mechanical characteristics of the metallic substrate. Presently, the commercial method applied for growing HA coatings with interest in bone implantology is plasma spraying [1–3]. However, this method presents a few drawbacks, including thermal decomposition of HA from the high-temperature plasma (up to 30,000 C) and fractures in thick-film coatings of >50 mm in thickness [4–6]. To resolve these problems, other deposition methods such as biomimetic process [7], sol–gel [8,9], dipping [10], electrophoretic deposition [11,12], and radio frequent (RF) magnetron sputtering [13] have been investigated. Although these experimental processes have successfully overcome some of the problems associated with plasma spraying, there are unique limitations with each [2,14–16]. For example, dip methods and
* Corresponding author. School of Materials Science and Engineering, Shandong University, No. 73 Jingshi Road, Shandong, Ji’nan 250061, PR China. Tel.: þ86 531 88395991; fax: þ86 531 88392313. E-mail address: [email protected] (C.Z. Chen). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.05.019
electrophoresis techniques showed weak bonding strength between the coating and substrate [4,17]; the biomimetic coatings made with a supersaturated calcifying solutions are difficult to prepare on an industrial scale [18]; the sol–gel deposited coatings are easy to crack in drying processing [16]. In comparison, RF magnetron sputtering exhibits more advantages for producing bioactive films: (1) the sputtered films have good adherence properties [2,17,19]; (2) the films are more uniform and more dense, even on complex implant designs [20,21]; (3) the substrate temperature is low during deposition [22]; and (4) this technology can be applied for large-scale industrial production [23]. RF magnetron sputtering is an improved ion-sputtering method and has been applied to deposit thin bioactive HA films on titanium (Ti) oral implants [24]. In RF magnetron-sputtering deposition, an RF field generates plasma between a sputtering target and the substrate holder. In magnetron sputtering, a magnetic configuration is present below the target, which traps electrons nearby the target. There, the electrons cause ionizations in the sputtering gas. Part of the ions is accelerated towards the target by the voltage drop, the so-called plasma sheath, which is present between the plasma and the target. The bombarding ions cause the ejection of target species and secondary electrons. The secondary electrons maintain the discharge [22]. The atoms from that target travel to and bond with the substrate, becoming the part of thin films. The structure and properties of magnetron-sputtered films largely depend upon the rate at which energy is delivered to the growing film by concurrent ion bombardment. In other words, film
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properties are governed by internal parameters like ion density and ion current to the substrate. The application of external parameters in a magnetron-sputtering system such as discharge power, gas composition, process pressure, base pressure, gas flow rates, substrate temperature, bias and targets-substrate distance, significantly affects these internal parameters and the resulting film properties [25,26]. By adjusting the deposition parameters, thin, uniform, and dense bioactive films can be obtained. Despite these excellent properties, the as-sputtered films have a low HA crystalline which in turn affects the stability or dissolution behavior of the implants [27–29]. This problem can be solved by post heat treatments, however, which may be result in weakened bond strength due to thermal expansion mismatching between substrate and film. The differences in thermal expansion can be reduced by controlling the film composition, which is greatly influenced by initial target material. Consequently, the properties of the bioactive films are also determined by post-deposition treatments and initial target materials [30]. The relationships between processing conditions and structure, elemental composition and properties of the bioactive films are summarized in this paper. 2. Deposition conditions of magnetron sputtering The sputter deposited films had a uniform and dense structure, especially on complex implant designs, like threaded implants [24]. In cell culture experiments, magnetron-sputtered HA films can indeed stimulate extracellular matrix and induce apatite formation [31]. However, the main limitation of RF sputtering of HA is the possible alteration of the HA structure and composition. The calcium–phosphate (Ca/P) ratios measured for sputtered HA films are often different from the stoichiometric ratio of 1.67. This has been reported to result in the incorporation of Ca or CaO particles and the appearance of other Ca–P-based crystalline phases in the sputtered films. The presence of CaO in HA film reduces dramatically their biocompatibility since CaO causes severe cytotoxicity and may therefore lead to inflammation of the surrounding tissue. The other Ca–P-based phases, such as dicalciumphosphate (DCP), tricalcium phosphate (TCP), and tetracalcium phosphate (TTCP), present biocompatibility level comparable to the one of HA but with lower osteoconductivity [27]. Additionally, biodegradation is generally better withstanded by crystalline and stoichiometric HA than the other Ca–P phases. For these reasons, Ca–P films should have the highest possible crystallinity and a good stoichiometry. Of course, the bioactive films should also have good mechanical properties, such as strong adherence to the substrate, appropriate hardness, and high mechanical wear-resistance [28]. Subsequently, the influence of deposition parameters on the properties of bioactive films will be analyzed, respectively. 2.1. RF power In certain conditions, discharge carrier ionization and ion density will rise with the increase of sputtering power. Therefore, highenergy ions enhance film/substrate adhesion and the density of the film. Conversely, low sputtering power leads to low energy ions, and poor adhesion film. To some extent, higher powers are beneficial to better films, however, too high power have detrimental influence on the quality of films. Over high-energy ions have a great heat effect on basement, which causes films injury [32]. With the increase of sputtering power, sputtering rate of the film will rise [33,34]. Besides, van Dijk et al. [35] reported that the Ca/P ratio also increased with the discharge power at 0.52 Pa of pressure. They explained this on the basis that O atoms were easily pumped away, and that the bonding strength of the P atoms arriving at the growing film layer was therefore weak, so that the P atoms were more easily re-sputtered at higher discharge power
[4,35,36]. However, all the films may be shown having the same crystalline structure as apatite after annealing [37]. Generally speaking, the film is usually sputtered using a lowlevel discharge power, in order to avoid target cracking, ensure a lower cost and reduce a technical risk [1,28,38]. In addition, HA begins to decompose at temperatures above 1200 C [39]. The HA target tends to be decomposed at higher discharge powers, since the kinetic energy of the Ar ions and the resulting target surface temperature increases with discharge power. Ozeki et al. [4] investigated the effects of discharge RF power on the phase composition of the HA film, and found that the HA and the Ca/P ratios were largest, not at 150 W, but at 100 W of discharge power, as shown in Figs. 1 and 2. The results differed with the research of van Dijk et al. [35], because of the influence of a phase change of the HA target. On the other hand, Fig. 2 shows that the Ca/P ratio decreased with increasing Ar pressure, whereas for van Dijk et al. [40], the ratio increased [4]. This will be discussed in the next section. According to Ref. [41], the Ca/P ratio was lower than that of ideal HA at 0.1 Pa of pressure. The deficiency of Ca may be due to a lower sputtering rate of Ca atoms at low gas pressure. As the discharge power increased, the energy of Ar ions in plasma increased. It is suggested that the higher energy of Ar ions positively influenced the sputtering of Ca atoms, and the energy of Ar ions was enough to sputter Ca atoms at high-discharge power. The Ca/P ratio increased as the discharge power was increased and became 1.59 close to that of HA at 400 W (Table 1). As such, the biological property of HA films is greatly dependent on the discharge power during the deposition. 2.2. Gas composition and pressure Before deposition took place, the chamber is always evacuated to a base pressure not greater than 1 103 Pa [42]. After that, highpurity argon (99.999%) or mixture of argon with oxygen is introduced into the chamber by means of a mass flow controller. For all the depositions a constant value of the gas pressure is used [1,43]. The work pressure and gas composition are important parameters, which have a great impact on deposition rate and quality of the films. The effect of argon gas pressure at a low discharge power level (150 W) on the surface properties of a series of co-sputtered Ca–P surfaces was investigated in Ref. [44]. Fig. 3 showed that the deposition rate increases up to an argon gas pressure of 2 Pa and then decreases as the argon gas pressure increases up to 5 Pa. In general, the number of Ar ions increases with Ar pressure and the deposition rate increases. However, there is an upper limit to this, since before arriving at the substrate, the sputtered atoms will suffer more collisions with increasing Ar gas pressure. Consequently, at lower pressures, the deposition rate increased with Ar
Fig. 1. Phase-composition weight ratio in the films vs. discharge power at 1.0 Pa of pressure. b-Tricalcium phosphate (TCP), b-calcium pyrophosphate (PYR) [4].
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Fig. 3. Ca–P film thickness at different argon gas pressures [44].
Fig. 2. Ca/P ratio of the sputtered films with various sputtering conditions as determined by EDS [4].
pressure, and after reaching an upper limit, it then decreased [4,40]. Due to the increasing number of collisions at higher gas pressures, the trajectory of the sputtered calcium (Ca) starts to look like a random walk through the deposition chamber, thereby enlarging the chance of getting lost. Therefore, the amount of Ca decreased with increasing Ar pressure [45]. At low Ar pressure, more than half of the material deposited is re-sputtered from the film, preferentially phosphorus (P). With an increase in Ar pressure re-sputtering of phosphorus in the film decreases [22,46]. Another explanation of the increase of phosphorus in the film is that lighter phosphorusbearing species can reach the film faster than heavier calciumbearing species with the increase of collisions [47]. It can be concluded that the Ca/P ratio decreased with increasing argon gas pressure, as depicted in Fig. 2 and Fig. 4 [4,47]. In contrary, van Dijk et al. [40] investigated the sputtered Ca/P ratio at 0.26–1.5 Pa gas pressure, and found that the ratio increased with pressure. It may be a tendency at very low pressure. In these conditions, phosphorus being lighter than calcium will suffer more from these collisions and hence the number of phosphorus atoms in the film decreases resulting in a calcium rich surface. This phenomenon is true up to a point, thereafter, the Ca/P ratio is seen to decrease with increasing argon gas pressure [4,40,44]. Additionally, it should be mentioned that the influence of gas pressure on deposition rate or Ca/P ratio is different with discharge power [40]. In hydroxyapatite, phosphorus is bound as PO4. The introduction of oxygen in the sputtering gas enhances the formation of PO4 and reduces re-sputtering of phosphorus. Consequently,
a stoichiometric Ca/P ratio can be obtained [36,48,49]. But an increase of the oxygen pressure will lead to a strong reduction of the growth rate of the films [36]. The presence of water in the background gas can result in the presence of OH bonds and more crystalline HA in the sputtered films [40]. In addition, carbonatecontaining HA films prepared by RF-sputtering method using Ar/ CO2 gas were studied. Overgrowing of bone-like apatite layers on the carbonate-containing HA surfaces in the simulated body fluid (SBF) (Fig. 5) implied that the obtained films acquired a sufficient osteoconductivity [50]. It can be concluded that the amount and the composition of the background gas are important for the final film composition and properties [40,51].
2.3. Substrate temperature Ca–P films are usually deposited onto substrates at a temperature below 550 C. When the temperature is increased, the atoms acquire more thermal energy and hence vibrate more vigorously. As the atoms on the surface have fewer neighbors and are more loosely bound than in the bulk, the amplitude of their vibrations is greater. This also implies that the energy of the atoms on the surface is higher than in the bulk. When the temperature is high enough, the surface atoms gain sufficient energy to leave their sites,
Table 1 The stoichiometric composition (Ca/P ratio) calculated from the RBS spectra [41] Discharge power (W) Ca/P ratio
200 1.11
300 1.37
400 1.59
Fig. 4. The effect of the gas pressure on the Ca/P ratio (20 mTorr ¼ 2.67 Pa, 40 mTorr ¼ 5.33 Pa, 60 mTorr ¼ 8.00 Pa, 80 mTorr ¼ 10.67 Pa). The deposition parameters were: input power 700 W; substrate temperature 550 C; zero bias [47].
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Fig. 5. SEM photographs of HA films hydrothermally annealed in Ar/CO2 before (a) and after (b) soaking of SBF. Bars in the photographs indicate 1 mm [50].
migrate over the surface, and eventually rearrange to reduce the overall internal energy. This rearrangement is most efficient at higher surface temperatures, which promote higher atom mobility and more efficient surface diffusion, then resulting in the long range ordering and growth of crystalline structures. At lower temperature, the surface atoms have a lower mobility, which results in a build-up of material in the amorphous phase. Fig. 6 shows SEM micrographs of the HA films, deposited at two different temperatures. It can be seen that the more uniform and lower porosity films can be deposited at elevated substrate temperatures. For the sample shown in Fig. 6(a), many pores are clearly visible. On the other hand, the entire surface of the sample synthesized at a higher temperature is covered quite homogeneously with hydroxyapatite crystallite grains, as shown in Fig. 6(b). Therefore, high-substrate temperatures are required to synthesize highly crystalline HA and quite homogeneous films [47]. On the other side, crystalline HA films are usually obtained by post-deposition heat treatments. Nelea et al. [1] observed that both the films directly deposited at a high temperature (e.g. 550 C) and the ones obtained at room temperature and after that annealed at this temperature mostly contain the HA phase and exhibit good mechanical characteristics. But the thin HA film reported in Ref. [52] had the significant advantage of not requiring any post-deposition heat treatment. They showed that the crystalline phase of the HA film still is clearly detectable even if the deposition temperature is as low as 67 C (Fig. 7), which may be due to the effect of other parameters, such as power density, chamber pressure and bias. In addition, the substrate temperature might influence the preferred plane of
orientation of the film [53,54]. So it can be inferred that the substrate temperature is an important parameter during sputtering. 2.4. Bias and target-to-substrate distance The increasing bias voltage can lead to an increased energy of the bombarding ions, which can be related to the surface roughness [55]. A radio frequency or DC bias may be applied to the substrate to vertically accelerate ions into the bottom of the trenches, thus obtaining good bottom coverage and enables complete fillings [56]. Varying the bias can efficiently control the main crystalline phase, and the preferred orientation [57]. Fig. 8 shows the Ca–P–Ti films XRD patterns of three samples prepared at 0-, 70- and 150-V bias and at 18 mTorr (2.4 Pa). In the case of absence of a bias, it is clear seen that a strong diffraction peak at (130) is present, suggesting that the crystalline growth is along a preferred orientation of (130) plane. With increasing of the bias to 70 V, another strong peak corresponding to (132) plane appears. Further increasing the bias to 150 V, these peaks disappear and a new peak corresponding to the (143) plane is formed [58]. The Ca/P ratio increases with the values of the negative DC bias [47,58]. Fig. 9 shows the Ca/P ratio as a function of bias voltage, for films prepared at working gas pressure 21 mTorr (2.8 Pa) and 700 W RF power. Comparatively, a closer ratio of about 1.7 is obtained without any bias voltage [47]. Of course, the result will be changed with other parameters. According to Ref. [58], the Ca/P ratio is measured to be about 1.45 at working gas pressure 70 mTorr (9.3 Pa) and 550 W RF power without any bias voltage, is lower than the result above. That may be the combined effects of higher pressure and lower power.
Fig. 6. SEM images showing that uniformity and low porosity can be achieved through increasing the temperature: (a) 130 C and (b) 300 C [47].
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Fig. 9. The dependence of the Ca/P ratio on the substrate bias [47].
Fig. 7. Infrared spectra of the HA thin films deposited on titanium-alloy specimens at different substrate temperatures. Ca/P ratios are 1.79, 1.76, and 1.74 for substrate temperatures of 241 C, 196 C, and 67 C, respectively [52].
At last, the target-to-substrate distance is also an important deposition parameter [59]. For better flexibility, the spacing between the substrate holder and target electrode is adjustable [47]. As the distance increases, the differences in contributions from the various portions of the target become smaller and the target behaves more like a point source to positions across the substrate, so the view factor across the substrate becomes more uniform [56]. But increasing the distance may result in lower deposition rates [60]. In magnetron sputtering, coating properties are largely determined by the rate at which energy is delivered to the growing
film through simultaneous ion bombardment. The structure and properties depend on three parameters: the homologous temperature; the ratio of the fluxes of bombarding ions and depositing atoms; and the energy of the bombarding ions [61]. These internal parameters are significantly affected by the application of external parameters. The energy and the number of the bombarding ions can be controlled by the working power and gas pressure; the grain size, lattice constant and compressive stress of the growing film are influenced by substrate bias and deposition temperature [62]; The target-substrate distance has a great influence on the carrier concentration [59]. Therefore, the structure and properties of the bioactive film are affected by all the deposition parameters together. And the correlation between the deposition parameters and the properties of the magnetron-sputtered film is complex. The optimum process variable can be obtained by investigating the working mechanism of magnetron sputtering and by adjusting the parameters during deposition. 3. Other influencing factors 3.1. Annealing
Fig. 8. Influence of bias voltage on crystalline structure [58].
As far as we know, the as-sputtered Ca–P films are amorphous or only weakly crystalline. The dissolution/precipitation behavior of the various Ca–P ceramics is influenced by their chemical structure and crystallinity. This low degree of crystallinity accelerates the dissolution of HA films in a living body, and this high rate of dissolution leads to the disappearance of the films before bone tissue can bond to the film, at an early stage after implantation [63,64]. High-crystal quality films can be obtained by post-deposition treatment. van Dijk et al. [35] studied the effect of different annealing temperatures on the characteristics of thin Ca–P films fabricated by radio frequency magnetron sputtering and demonstrated that 600 C was probably the best annealing temperature to obtain a better characterization of the film. However, conventional heat treatment in an electric furnace leads to films tend to degrade and easily form cracks between the film and the titanium substrate because of differences in thermal expansion and the formation of TiO2. Therefore it is necessary to find a more suitable heat treatment procedure that controls the solubility of the Ca–P films without weakening their adhesion to the underlying Ti-substrate [24,35,64]. Accordingly, rapid heating
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with infrared radiation was introduced as a suitable heat treatment for thin Ca–P films [24,31,34,65–67]. In addition, water is an acceleration factor in the conversion of the amorphous precursor phase to microcrystalline HA during the precipitation of HA [68]. Therefore, HA films could be crystallized at low temperatures using a hydrothermal technique [35,64,69]. Ozeki et al. [70] crystallized the sputtered film in distilled water at varying pH during the hydrothermal treatments, and indicated that the HA crystal size increased with pH but crystal growth rate decreases with pH. And some researches showed that biocompatibility and bioactivity of the sputtered film can be greatly improved on using the hydrothermal treatment [64,71]. However, a post-deposition heat treatment in the presence of water vapor resulted in a significantly rougher surface, as shown in Fig. 10, suggesting the etching effect of water vapor during heat treatment
[72]. The etching effect and thermal expansion attributed to these heat treatments may be result in weakened bond strength between substrate and film.
3.2. Initial target materials An approach to improve the bond strength is the deposition of a composite film, wherein a metallic phase was introduced to serve as either an intermediate layer or a second (continuous or dispersed) phase in the HA matrix [73]. In addition, silicon is usually introduced into Ca–P thin films in order to enhance their potential bioactivity [74]. We can produce Si/HA [75], Ti/HA [73,76] and HA/ Zirconia/Yttria stabilized [77] composite films using different composite targets. The composite films are also obtained using several targets in recent researches [53,78–86]. For example, according to Ref. [86], the magnetron-assisted sputter deposition for the HA was performed in the RF mode (13.56 MHz) and for the Ti in the DC mode. A thin Ti layer (about 0.3 mm) was first deposited as an initial bond coat (pre-coat) onto Ti–6Al–4V substrates and then followed by alternating deposition which Ti gradually decreased and HA simultaneously increased by controlling the input power. The multi-layered composite films had the advantages of high and non-declining adhesion strength and high resistance to SBF attack. The calcium phosphate content of the target also has influence on the phase composition and deposition rate of sputtered films. Ozeki et al. [87] coated calcium phosphate from various calcium phosphate targets (tetracalcium phosphate (TTCP), hydroxyapatite (HA), b-tricalcium phosphate (TCP), b-calcium pyrophosphate (CPP), and b-calcium metaphosphate (CMP) powder targets) using RF magnetron sputtering. The composition of the crystal phase of the coated films was changed, depending on the target materials, and the Ca/P molar ratios of the films varied from 0.74 to 2.54, increasing with the Ca/P molar ratio of the target. The theoretical density of the target materials are tabulated in Table 2, and the order was: HA > b-CPP > b-TCP > TTCP > b-CMP. This order might have an inverse correlation with the order of the deposition rate (Fig. 11). This indicates that the density of the target materials might also influence on the sputtering yield. Target material stoichiometry can have a significant influence on the characteristics of sputter deposited films. Because of high crystallinity, the use of compacted HA powder targets [38,44,74] is desirable compare to the plasma sprayed HA targets applied previously [24,35,37,40,43,63,88–90]. In regard of degradation of the HA powder targets, the effective lifespan and the effects of their multi-cycle usage on the composition and structure of the resultant films are of considerable importance. A concentric ring or ‘annular racetrack’ is showed clearly after the target undergone several sequential deposition cycles. Its presence is due to the use of a magnetron sputter source [91]. Therefore, the surface of the HA target needs to be replaced with a new one at each stage of the sputtering process after deposition [4]. The substrate quality is another important factor in attaining good performance of the films. Pre-coating treatments, grit- and glass-bead blasting affect the final residual film stress. This will have a good effect on the film adhesion to the substrate [31]. Additionally, the position of the substrates relative to the sputter target also has a considerable influence on the adhesion of sputtered films [45,92]. With a stationary substrate holder, this position
Table 2 The theoretical density of the target materials [87] Fig. 10. Representative surface profile morphology of sputtered CaP films on glass substrate. (a) As-sputtered surface; (b) 350 C heat treatment for 1 h in absence of water vapor; (c) 350 C heat treatment for 1 h in the presence of water vapor [72].
Theoretical density (g/cm2)
TTCP
HA
b-TCP
b-CCP
b-CMP
3.05
3.16
3.07
3.13
2.87
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Fig. 11. The deposition rate of films sputtered from various targets [87].
is fixed while with a rotating substrate holder, the position of the substrate changes continuously during deposition. The difference in the adhesion strength among the samples deposited on rotatory and stationary holders is obvious. Furthermore, lower interface strength was found deposited on the rotating substrate holder [49]. 4. Conclusion RF magnetron sputtering is a promising technique for forming bioactive films (HA and its composite films). The adhesion of the film to its substrate and the growth of crystalline structures of the films are predominant factors in determining the performance and reliability of dental and orthopedic implant applications. The structure and properties of films are particularly depended on the process conditions, which include deposition parameters, heat treatments and initial target materials. Understanding their relationships is important in optimizing the process parameters to obtain high-quality bioactive films. With the improvement of these conditions, the application of RF magnetron sputtering in biomedicine will be more promising. Acknowledgments This work was financially supported by Department of Science and Technology of Jinan, Shandong Province, P. R. China (Grant No. 051070). Reference [1] Nelea V, Morosanu C, Iliescu M, Mihailescu IN. Surf Coat Technol 2003;173: 315. [2] Ong JL, Appleford M, Oh S, Yang Y, Chen W-H, Bumgardner JD, et al. JOM 2006; 58(July):67. [3] Caulier H, van der Waerden JPCM, Paquay YCGJ, Wolke JGC, Kalk W, Naert I, et al. J Biomed Mater Res 1995;29:1061.
255
[4] Ozeki K, Yuhta T, Fukui Y, Aoki H. Surf Coat Technol 2002;160:54. [5] Ogiso Makoto, Yamashita Yasuo, Matsumoto Toshio. J Biomed Mater Res 1998; 39:23. [6] Lewis Gladius. Biomed Mater Eng 2000;10:157. [7] Rigo ECS, Boschi AO, Yoshimoto M, Allegrini Jr S, Konig Jr B, Carbonari MJ. Mater Sci Eng C 2004;24:647. [8] Stoch A, Jastrzebski ˛ W, D1ugon´ E, Lejda W, Trybalska B, Stoch GJ, et al. J Mol Struct 2005;744–747:633. [9] Lopatin CM, Pizziconi V, Alford TL, Laursen T. Thin Solid Films 1998;326:227. [10] Li Tuantuan, Lee Junhee, Kobayashi Takayuki, Aoki Hideki. J Mater Sci Mater Med 1996;7:355. [11] Hamagami Jun-ichi, Ato Yuki, Kanamura Kiyoshi. Solid State Ionics 2004;172: 331. [12] Guo Xingyuan, Gough Julie, Xiao Ping. J Biomed Mater Res 2007;80A:24. [13] Jansen JA, Wolke JGC, Swann S, van der Waerden JPCM, de Groot K. Clin Oral Implants Res 1993;4:28. [14] Dorozhkin Sergey V. J Mater Sci 2007;42:1061. [15] Jaffe William L, Scott David F. J Bone Joint Surg 1996;78A:1918. [16] Yang M, Shao ZC, Cui ZX. Non Ferrous Min Metall 2007;23:58 [in Chinese]. [17] Cheng Gary J, Pirzada Daniel, Cai M, Mohanty Pravansu, Bandyopadhyay Amit. Mater Sci Eng C 2005;25:541. [18] Damia Chantal, Sharrock Patrick. Mater Lett 2006;60:3192. [19] Ong JL, Lucas LC. Biomaterials 1994;15:337. [20] van der Wal E, Wolke JGC, Jansen JA, Vredenberg AM. Appl Surf Sci 2005;246: 183. [21] Hulshoff JEG, van Dijk K, van der Waerden JPCM, Wolke JGC, Kalk W, Jansen JA. J Biomed Mater Res 1996;31:329. [22] Feddes B, Wolke JGC, Jansen JA, Vredenberg AM. J Appl Phys 2003;93:662. [23] Qian MG. Surface technology and application handbook. Machine Industry Press; 1998. p. 700[in Chinese]. [24] Yoshinari M, Hayakawa T, Wolke JGC, Nemoto K, Jansen JA. J Biomed Mater Res 1997;37:60. [25] Kelly PJ, Arnell RD. J Vac Sci Technol A Vac Surf Films 1999;17:945. [26] Schulte J, Sobe G, Reiss G. Contrib Plasma Phys 1997;37:483. [27] Snyders Rony, Bousser Etienne, Music Denis, Jensen Jens, Hocquet Ste´phane, Schneider Jochen M. Plasma Process Polym 2008;5:168. [28] Nelea V, Morosanu C, Iliescu M, Mihailescu IN. Appl Surf Sci 2004;228:346. [29] Clemens JAM, Klein CPAT, Vriesde RC, Rozing PM, de Groot K. J Biomed Mater Res 1998;40:341. [30] Boyd AR, Meenan BJ, Leyland NS. Surf Coat Technol 2006;200:6002. [31] Wolke JGC, van der Waerden JPCM, Schaeken HG, Jansen JA. Biomaterials 2003;24:2623. [32] Wenlong Song, Jianxin Deng, Jinlong Zhao. Sciencepaper Online. Available from: http://www.paper.edu.cn [in Chinese]. [33] Kulisch W, Colpo P, Rossi F, Shtansky DV, Levashov EA. Surf Coat Technol 2004; 188–189:714. [34] Yonggang Yan, Wolke JGC, Yubao Li, Jansen JA. J Mater Sci Mater Med 2007;18: 1061. [35] van Dijk K, Schaeken HG, Wolke JGC, Jansen JA. Biomaterials 1996;17:405. [36] van Dijk K, Verhoeven J, Mare´e CHM, Habraken FHPM, Jansen JA. Thin Solid Films 1997;304:191. [37] van Dijk K, Schaeken HG, Wolke JCG, Mare´e CHM, Habraken FHPM, Verhoeven J, et al. J Biomed Mater Res 1995;29:269. [38] Verestiuc Liliana, Morosanu C, Bercu M, Pasuk Iuliana, Mihailescu IN. J Cryst Growth 2004;264:483. [39] Zhou Jiming, Zhang Xingdong, Chen Jiyong, Zeng Shaoxian, De Groot K. J Mater Sci Mater Med 1993;4:83. [40] van Dijk K, Schaeken HG, Mare´e CHM, Verhoeven J, Wolke JGG, Habraken FHPM, et al. Surf Coat Technol 1995;76–77:206. [41] Yamaguchi Tetsuro, Tanaka Yoshikazu, Ide-Ektessabi Ari. Nucl Instrum Meth Phys Res B 2006;249:723. [42] Zhong D, Sutter E, Moore JJ, Mustoe GGW, Levashov EA. Disam J. Thin Solid Films 2001;398–399:320. [43] Ong JL, Raikar GN, Smoot TM. Biomaterials 1997;18:1271. [44] Boyd AR, Duffy H, McCann R, Cairns ML, Meenan BJ. Nucl Instrum Meth Phys Res B 2007;258:421. [45] Feddes B, Vredenberg AM, Wolke JGC, Jansen JA. Surf Coat Technol 2004;185: 346. [46] Feddes B, Wolke JGC, Jansen JA, Vredenberg AM. J Appl Phys 2003;93:9503. [47] Xu Shuyan, Long Jidong, Sim Lina, Diong Cheong Hoong, Ostrikov Kostya (Ken). Plasma Process Polym 2005;2:373. [48] van Dijk K, Mare´e CHM, Verhoeven J, Habraken FHPM, Jansen JA. J Biomed Mater Res 1998;42:266. [49] van Dijk K, Gupta V, Yu AK, Jansen JA. J Biomed Mater Res 1998;41:624. [50] Nakamura Satoshi, Hamagami Jun-Ichi, Yamashita Kimihiro. J Biomed Mater Res B Appl Biomater 2007;80B:102. [51] Hulshoff JEG, van Dijk K, de Ruijter JE, Rietveld FJR, Ginsel LA, Jansen JA. J Biomed Mater Res 1998;40:464. [52] Lo WJ, Grant DM. J Biomed Mater Res 1999;46:408. [53] Ozeki K, Janurudin Juliana M, Aoki H, Fukui Y. Appl Surf Sci 2007;253:3397. [54] Jha GC, Ray SK, Manna I. Thin Solid Films 2008;516:3416. [55] Macedo Francisco, Vaz Filipe, Fernandes Ana C, Rebouta Luı´s, Carvalho Sandra, Junge Klaus H, et al. Plasma Process Polym 2007;4:190. [56] Lu Junqing, Yoon Jae-Hong, Shin Keesam, Park Bong-Gyu, Yang Lin. Thin Solid Films 2006;515:2452. [57] Yang Q, Zhao LR, McKellar R, Patnaik PC. Vacuum 2006;81:101.
256
J.Z. Shi et al. / Vacuum 83 (2009) 249–256
[58] Long JD, Xu S, Cai JW, Jiang N, Lu JH, Ostrikov KN, et al. Mater Sci Eng C 2002; 20:175. [59] Luo Chen, Xiaofang Bi. Vacuum. Available online 4 April 2008. [60] Safi I. Surf Coat Technol 2000;127:203. [61] Arnell RD, Kelly PJ, Bradley JW. Surf Coat Technol 2004;188–189:158. [62] Chang Hui-Wen, Huang Ping-Kang, Yeh Jien-Wei, Davison Andrew, Tsau ChunHuei, Yang Chih-Chao. Surf Coat Technol 2008;202:3360. [63] Wolke JGC, de Groot K, Jansen JA. J Biomed Mater Res 1998;39:524. [64] Ozeki K, Aoki H, Fukui Y. J Biomed Mater Res A 2006;76:605. [65] Yan Yonggang, Wolke JGC, De Ruijter A, Yubao Li, Jansen JA. J Biomed Mater Res A 2006;78:42. [66] Yonggang Yan, Wolke JGC, Yubao Li, Jansen JA. J Biomed Mater Res A 2006;77: 815. [67] Takahashi K, van den Beucken JJJP, Wolke JGC, Hayakawa T, Nishiyama N, Jansen JA. J Biomed Mater Res A 2008;84:682. [68] Ding Shinn-Jyh, Lee Tien-Ling, Chu Yu-Hua. J Mater Sci Lett 2003;22:479. [69] Tao Wan, Hideki Aoki, Ayako Katoh. J Wuhan Univ Technol Mater Sci Ed 2005; 20:260. [70] Ozeki K, Aoki H, Fukui Y. J Mater Sci 2005;40:2837. [71] Ozeki K, Mishima A, Yuhta T, Fukui Y, Aoki H. Biomed Mater Eng 2003;13:451. [72] Yang Yunzhi, Kim Kyo-Han, Ong Joo L. Biomaterials 2005;26:327. [73] Ding Shinn-Jyh, Ju Chien-Ping, Chern Lin Jiin-Huey. J Biomed Mater Res 1999; 44:266. [74] Boyd AR, Duffy H, McCann R, Meenan BJ. Mater Sci Eng C 2008;28:228. [75] Thian Eng San, Huang Jie, Best Serena M, Barber Zoe H, Brooks Roger A, Rushton Neil, et al. Biomaterials 2006;27:2692.
[76] Ding Shinn-Jyh, Ju Chien-Ping, Chern Lin Jiin-Huey. J Biomed Mater Res 1999; 47:551. [77] Zhao YT, Zhang Z, Dai QX, Lin DY, Li SM. Surf Coat Technol 2006;200:5354. [78] Chen W, Liu Y, S Courtney H, Bettenga M, Agrawal CM, Bumgardner JD, et al. Biomaterials 2006;27:5512. [79] Thian ES, Huang J, Best SM, Barber ZH, Bonfield W. Biomaterials 2005;26:2947. [80] Thian ES, Huang J, Best SM, Barber ZH, Bonfield W. Mater Sci Eng C 2007;27: 251. [81] Thian ES, Huang J, Best SM, Barber ZH, Bonfield W. J Biomed Mater Res B Appl Biomater 2006;76B:326. [82] Thian ES, Huang J, Vickers ME, Best SM, Barber ZH, Bonfield W. J Mater Sci 2006;41:709. [83] Porter AE, Rea SM, Galtrey M, Best SM, Barber ZH. J Mater Sci 2004;39:1895. [84] Thian ES, Huang J, Best SM, Barber ZH, Bonfield W. J Mater Sci Mater Med 2005;16:411. [85] Chen Minfang, Liu Debao, You Chen, Yang Xianjin, Cui Zhenduo. Surf Coat Technol 2007;201:5688. [86] Ding Shinn-Jyh. Biomaterials 2003;24:4233. [87] Ozeki K, Fukui Y, Aoki H. Appl Surf Sci 2006;253:5040. [88] Wolke JGC, de Groot K, Jansen JA. J Biomed Mater Res B Appl Biomater 1998; 43:270. [89] Wolke JGC, de Groot K, Jansen JA. J Mater Sci 1998;33:3371. [90] Lu Xiong, Leng Yang. J Biomed Mater Res 2003;66A:677. [91] Boyd A, Akay M, Meenan BJ. Surf Interface Anal 2003;35:188. [92] Wolke JGC, van der Waerden JPCM, de Groot K, Jansen JA. Biomaterials 1997; 18:483.