Physical Vapour Deposition and Bioactivity of Crystalline Titanium Dioxide Thin Films

Transcription

1 100 Trends Biomater. Artif. Organs, Vol 22(2), J. Heinr1ichs, pp T. (2008) Jarmar, U. Wiklund, H. Engqvist Physical Vapour Deposition and Bioactivity of Crystalline Titanium Dioxide Thin Films J. Heinrichs, T. Jarmar, U. Wiklund, H. Engqvist Department of Engineering Sciences The Angstrom Laboratory Uppsala University Sweden Received 14 March 2008; Published online 1 August 2008 Lack of interfacial stability is one of the causes behind the need for revision surgery of metallic implants. Thin films of titanium oxide have recently been proven to be bioactive (in vitro) and non-resorbable. The objective of the paper is to investigate the relationship between deposition parameters of thin crystalline titanium oxide films (using reactive sputtering), their microstructure and the in vitro bioactivity. The relative amounts of anatase and rutile were hard to control via the deposition parameters tested. No direct coupling between the hydroxylapatite layer and the crystalline films from different deposition runs could be derived from this investigation. Society for Biomaterials and Artificial Organs (India), Introduction Titanium is due to its biocompatibility, mechanical, chemical stability and osseointegration widely used for indications such as dental implants, orthopaedics and otology [1]. However, untreated Ti implants are biologically inert with no bone bonding ability. A bioactive implant can result in a faster bone in growth and more importantly a bonding between the implant and the surrounding bone. A material is generally found to be bioactive when promoting the formation of hydroxylapatite on the surface in contact with body fluids [2]. There are several materials that are known to be bioactive but almost all of them are resorbable in vivo [3]. It has been proven that Ti can be made bioactive via surface treatments, e.g. heat treatment [4], sol-gel coatings [5] and physical vapour deposition of titanium oxide [6]. It is the crystalline phases of that have showed bioactive properties, e.g. anatase and rutile [7]. The bioactivity of crystalline is connected to the existence of surface hydroxyl (Ti-OH) groups and the induced negative charges, which, in turn, draw calcium and phosphorus ions from the (simulated) body fluid to the implant surface. With electron beam evaporation of titanium and simultaneous oxygen supply physical vapour deposition can be an efficient technique to obtain crystalline titanium oxide coatings. The deposition of during heat treatment gives a smooth polycrystalline anatase film with large grains and a preferred crystal orientation. Without heat treatment the film has an amorphous feature. A low partial oxygen pressure allows some rutile to be formed together with anatase during deposition [8-17]. When sputtering titanium in a reactive oxygen atmosphere, it is important to adjust the flux of oxygen to achieve stoichiometric. Too high flux results in a low growth rates due to oxidation of the titanium target and too low flux gives an oxygen deficient film [10]. The partial pressure of oxygen influence the phase of the oxide. Thin films appear amorphous when

2 Physical Vapour Deposition and Bioactivity of Crystalline Titanium Dioxide Thin Films 105 analysed with XRD whereas thicker films appear polycrystalline [11]. The distance between target and substrate during the sputtering process is important. A long mean free path impose a risk that small charged clusters nucleate in the gas phase and grow. These clusters do not adapt to the surface structure of the substrate. An amorphous, nanocrystalline film is achieved. A short mean free path and surface charged clusters results in less agglomeration and offer the possibility to get well crystalline anatase. Control of cluster size and density of charge makes it possible to control the microstructure [12]. It is possible to achieve a strongly oriented anatase film at non-heated substrates but high temperature clusters means a greater chance for recrystallisation [13]. Structure-zone models can be used to predict the microstructure of a PVD film. It provides a relationship between the microstructure, argon pressure and the temperature relationship between chamber temperature and melting temperature of the film to be deposited. The microstructure achieved can be divided into growth mechanism zones [6, 8, 9]. The objective of the paper is to investigate the relationship between deposition parameters of titanium oxide using reactive magnetron sputtering and the microstructure of the films and their in vitro bioactivity and to correlate the results with the findings from reference [6]. Also the films in reference [6] were deposited on Si substrates; it is of importance to understand the deposition also on Ti. Experimental A titanium target was cleansed and mounted in a Balzers 640R, PVD chamber above a magnetron to be used in a reactive magnetron sputtering process. To achieve during deposition an oxygen flow was connected with the chamber. Prior deposition titanium substrates were cleansed in alkaline cleaning agent (UPON, ph 11.6) in an ultrasonic bath, heated to 60 ˆC, for five minutes followed by a rinse in deionised water and finally in ultrasonic bath with room tempered ethanol for five minutes. The alkaline solution removed organic compounds and water was used to rinse off the alkaline residues which ethanol not likely can remove. The substrates were carefully dried in compressed air. The titanium dioxide deposition runs were performed using a range of coating conditions. The varied parameters were: oxygen pressure and magnetron power. Temperature, time and argon pressure were kept constant at ~240 ˆC during deposition, 20 minutes and 3x10-3 mbar partial pressure, respectively. The limits of the PVD equipment regarding magnetron effect and oxygen partial pressure were chosen to kw and mbar respectively. Experiments were carried out according to Table 1 where low and high values where combined to cover to full experimental space. Additional tests were added covering the medium settings, see Table 1. Every run included three pieces of 4" silicon wafer and three titanium substrates (grade 2, surface roughness <0.5 mm). The substrates deposited in run 1 were called Ti 1 and Si 1 respectively and so on. The exact deposition parameters were as described in Table 2. No change in surface roughness before and after coating deposition could be detected using profilometry. After completed film deposition a surface film could be confirmed by the change in colour of the substrates. The coatings adhered well to the substrates and did not flake of even when very roughly handled. X-ray diffraction (Siemens D5000 diffractometer with parallel beam geometry) and scanning electron microscopy (LEO 440 with LaB 6 filament) analysis were performed on the films achieved on silicon as well as on the films achieved on titanium. XRD analysis with a grazing incidence angle of 0.5 made it possible to enhance the contribution from the surface layer. To have equal surface treatment before starting the soaking experiments, the substrates and films were cleaned in ultrasonic baths with different solvents. First the samples were placed in acetone for 5 minutes, followed by ethanol for 5 minutes and finally in deionised water for 5 minutes. After cleaning two coated titanium substrates and two coated silicon substrates, from each run, were soaked in Dulbecco s Phosphate Buffered Saline (PBS, 37 ˆC) following the described method in [16]. The surface area to PBS volume in the experiments

3 106 J. Heinrichs, T. Jarmar, U. Wiklund, H. Engqvist Table 1: Experimental Conditions H=High, M=Medium, L=Low Magnetron effect O 2 partial pressure 1 L 1 kw L 1 x10-3 mbar 2 H 2.5 kw L 1 x10-3 mbar 3 H 2.5 kw H 2 x10-3 mbar 4 L 1 kw H 2 x10-3 mbar 5 M 1.5 kw M 1.5 x10-3 mbar was fixated at 1 cm 2 /10ml. The ionic content of the PBS were similar to the more commonly used simulated body fluid. After seven days a set of samples were rinsed with deionised water and dried in air. The other set of samples were rinsed and dried after four weeks of soaking. The PBS solution were changed every seventh day to maintain initial ion concentration in the surroundings of the substrates soaked for four weeks. All substrates were analysed with XRD and SEM. Cross-section analysis of bioactivity tested films (deposition run Si 3 in Table 1) where performed using TEM. FIB was used to fabricate the TEM-samples [17]. The FIB used in this study was the FEI Strata DB235 equipped with an ion gun and an electron gun (Dualbeam) as well as an Omniprobe in situ TEM sample liftout system. All TEM analysis was conducted in a FEI Tecnai F30 ST TEM fitted with a Gatan Imaging Filter and operated at 300 kv. Results All substrates showed as a surface layer but with differing proportions of anatase and rutile, see Fig. 1 and Table 3. The difference in crystalline structure did not show any clear trend but comparatively more rutile were formed on the Ti substrates and more anatase were formed on the Si substrates. The proportion of anatase and rutile reported in Table 3 is a relative intensity from XRD, where the peak with the highest intensity of rutile and anatase respectively is fixed to The surfaces covered with titanium dioxide were studied with SEM as well, see Fig. 2. The microstructure of the surface film could not be observed since the films were too thin to reveal the effect from the substrate but according to theory the microstructure should be columnar with voided open boundaries. The thicknesses of the deposited films were studied with SEM as well. Cross-section analysis confirmed film thicknesses of nm, see Table 3. Table 2: Deposition Program Total pressure [mbar] P Ar [mbar] Temp [?C] Time [min] Arc current [A] Substrate voltage [V] 1 Pump to 2x RT ~ Heat 2.5x x10-3 to Etch 1.5x x Coat Varying with P O2 3.0x Pump to 2x Cool 1x Pump to 2x RT-Room Temperature Table 3: Summary of results of the different PVD substrates Titanium Run Magnetron Anatase Rutile effect Partial pressure of O 2 Oxide film thickness [nm] Anatase 101 Rutile 110 Silicon 1 week [µm] 4 weeks [µm] 1 L L H L H H L H M M

4 Physical Vapour Deposition and Bioactivity of Crystalline Titanium Dioxide Thin Films 107 Anatase-101 Rutile-110 Rutile-101 Anatase-004 Anatase-200 Anatase-101 Rutile-110 Rutile-211 After soaking the substrates in PBS for one week hydroxylapatite () was found on the surface according to XRD, see Fig. 3. All coatings showed XRD peaks from. Corresponding SEM-images from the surfaces showed the characteristic plate-like structure of the, see Fig. 4. After four weeks the peaks were sharper, see Fig. 5, and the appearance of the films in SEM similar as after one week. There is a shoulder between the (112) and the (202) peaks in the XRD spectra. This shoulder is unidentified but could originate from a calcium phosphate hydrate (e.g. octacalciumphosphate, OCP). The peak could also originate from broadening of the (112). The thickness of the layers were measured on areas on the Si substrates where the layer had flaked off during vacuum pumping in the SEM, see Table 3. To obtain a cross-section measurement the samples were tilted in the SEM. Figure 1: XRD of deposited on a) Si and b) Ti substrates. Order from the top and down is Ti Ti Figure 2: Top-view images of the coatings on a) Si and b) Ti substrates (SEM) Figure 3: XRD after PBS soaking for one week on deposited a) Si and b) Ti. Order from the top and down is 1-5

5 108 J. Heinrichs, T. Jarmar, U. Wiklund, H. Engqvist a) b) 10 µm 1 µm c) d) 10 µm 1 µm Figure 4: Top-view images of deposited on titanium oxide on a-b)si and c-d) Ti, from run 4 Table 1(SEM) The /titanium dioxide interface was studied with cross-sectional TEM analysis. The TEM foil was prepared with the FIB and the in situ lift-out technique, see Fig. 6 for preparation images. TEM images are shown in Fig. 7. The hydroxylapatite film was dense close to the substrate/ surface oxide and further away voids and pores can be observed. The nucleated at specific sites/areas on the surface. From the TEM sample it was not possible to obtain information about the exact nucleation sites. Discussion Different amounts of the two crystalline phases of, anatase and rutile, could not be clearly connected to the applied magnetron power or oxygen pressure during sputtering deposition as discussed in [6]. A high magnetron power resulted in a thicker crystalline titanium dioxide film, which means that the low oxygen flux is enough to result in stoichiometric titanium di- oxide even with the highest magnetron power. The difference in anatase and rutile proportion from the same run but at different substrates shows that the substrate plays a role in which crystalline form that should build up the titanium dioxide film. Silicon and titanium seems to advocate anatase and rutile respectively. It is also difficult to tell which crystalline form that results in thicker and denser hydroxyapatite film and there by is the most bioactive substrate. There were large differences in adhesion to titanium and silicon. The flaked off from the silicon substrates immediately after drying and especially when placed in the low pressure in a SEM. The titanium substrate is covered with a film with few cracks and it is fully covered even at low pressures. The similarity in formation at the different substrates shows that it is possible to get a bioactive surface without underlying titanium as long as there is crystalline covering the surface. The relative intensities of the peaks in the XRD spectra is not obvious

6 Physical Vapour Deposition and Bioactivity of Crystalline Titanium Dioxide Thin Films (a) TiO2 (b) (c) Ti-002 Ti Figure 5: XRD after soaking in PBS for four weeks on on a) Si and b) Ti. Order from the top and down is 1-5 and does not follow a clear trend. One parameter not considered in this paper is the possible change in water wetting angle on the different surfaces. There could be a positive influence on the formation as function of coating composition regarding this parameter. This should be considered in a future investigation. Figure 6: a) Image of a flaked hydroxyapatite film on Si substrate soaked for one week in PBS b) Image of in situ lift-out with the Omniprobe micromanipulator needle during FIB preparation of the TEM sample and c) image of the sample mounted on a copper grid before final polishing for electron transparency (SEM) Si 1 µm Si 20nm Figure 7: Bright field TEM cross-section images of the PVD deposited Si 1 (Table 1) soaked for one week in PBS. Layers seen from the top are hydroxyapatite, and silicon

7 110 J. Heinrichs, T. Jarmar, U. Wiklund, H. Engqvist Conclusions It was possible to deposit a bioactive crystalline film with reactive sputtering. The relative amounts of anatase and rutile were hard to predict and the substrate material largely influenced the phase formation. The underlying substrate was less important in means of bioactivity as long as it was covered with crystalline but highly important in means of the adhesion. The thickness of the was not easily coupled to the underlying titanium dioxide film but varies at the substrates from different deposition runs. References 1. Brunette D, Tengvall P, Textor M, Thomsen P, Titanium in medicine, Springer Germany (Ed Brunette D) (2001).d 2. Kokubo T, Design of bioactive bone substitutes based on biomineralization process, Materials Science and Engineering 2005; C 25: Kim H-M, Ceramic bioactivity and related biomimetic strategy, Current Opinion in Solid State and Materials Science 7 (2003) Rossi S, Moritz N, Tirri T, Peltola T, Areva S, Jokinen M, Happonen R.-P, Närhi T, Comparison between solgel-derived anatase- and rutile structured coatings in soft-tissue environment, J Biomedical Materials Research 82A (2007). 5. Li P, De Groot K, Calcium phosphate formation within sol-gel prepared titania in vitro and in vivo, J Biomedical Materials Research 28, 7-15 (1994). 6. Zhou W, Zhong X, Wu X, Yuan L, Shu Q, Xia Y, Ostrikov K, Plasma-controlled nanocrystallinity and phase composition of TiO2: a smart way to enhance biomimetic response, J Biomedical Materials Research 81A, (2007). 7. Lin C-M, Yen S-K, Biomimetic growth of apatite on electrolytic coatings in simulated body fluid, Materials Science and Engineering C 26 (2006) Thornton J A, The microstructure of sputter-deposited coatings, Journal of Vacuum Science & Technology A 4(6) (1986) Schubert U, Hüsing N, Synthesis of inorganic materials CH 2.3, Wiley-VCH Verlag GmbH & Co. KGaA (2 nd edition, Germany 2005) 10. Sankara Subramanian N, Santhi B, Veeraganesh V, Vinoth C, Murugan G, Karthik Subbaraj G, Studies on evaporated titanium oxide thin films for oxygen gas detection, Ionics 6 (2000) Van de Krol R, Goossens A, Structure and properties of anatase thin films made by reactive electron beam evaporation, Journal of Vacuum Science & Technology A 21(1) (2003) Guerin D, Ismat Shah S, Reactive-sputtering of titanium oxide thin films, Journal of Vacuum Science & Technology A 15(3) (1997) Baroch P, Musli J, Vlcek J, Nam K H, Han J G, Reactive magnetron sputtering of TiO x films, Surface & Coatings Technology 193 (2005) Barnes M C, Gerson A R, Kumar S, Hwang N-M, The mechanism of deposition by direct current magnetron reactive sputtering, Thin Solid Films 446 (2004) Barnes M C, Kumar S, Green L, Hwang N-M, Gerson A R, The mechanism of low temperature deposition of crystalline anatase by DC magnetron sputtering, Surface & Coatings Technology 190 (2005) Zhou W, Zhong X, Wu X, Yuan L, Shu Q, Li W, Xia Y, Low temperature deposition of nanocrystalline TiO2 film: enhancement of nanoscrystal formation by energetic particle bombardment., Journal of Physics D: Applied Physics. 40 (2007) Zhou W, Zhong X, Wu X, Yuan L, Zhao Z, Wang H, Xia Y, Feng Y, He J, Chen W, The effect of surface roughness and wettability of nanostructured TiO2 film on TCA-8113 epithelial-like cells. Surf. Coat. Tech. 200 (2006), Kokubo T, Takadama H, How useful is SBF in predicting in vivo bone bioactivity?, Biomaterials 27 (2006) Engqvist H, Botton GA, Couillard M, Mohammadi S, Malmstrom J, Emanuelsson L, Hermansson L, Phaneuf MW, Thomsen P. A novel tool for high-resolution transmission electron microscopy of intact interfaces between bone and metallic implants. J Biomed Mater Res A 2006;78(1):20-4.