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A low temperature in-situ crystalline TiNi shape memory thin film deposited by magnetron sputtering
Surface & Coatings Technology 284 (2015) 90–93
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
A low temperature in-situ crystalline TiNi shape memory thin film deposited by magnetron sputtering Hikmet Cicek a,⁎, Ihsan Efeoglu a, Yaşar Totik a, Kadri Vefa Ezirmik b, Ersin Arslan b a b
Atatürk University, Faculty of Engineering, Mechanical Engineering, Turkey Atatürk University, Faculty of Engineering, Metallurgy and Materials Engineering, Turkey
a r t i c l e
i n f o
Article history: Received 1 April 2015 Revised 14 August 2015 Accepted in revised form 17 August 2015 Available online 4 September 2015 Keywords: TiNi Crystallization Magnetron sputtering
a b s t r a c t TiNi films deposited by magnetron sputtering usually have amorphous structure and must be annealed at high temperature to obtain crystallization. We have synthesized an in-situ fully crystalline TiNi shape memory thin film at low temperature by dc magnetron sputtering. Application of pulsed direct current to the substrate is effective to obtain a crystalline TiNi film. Nine different conditions were used for deposition in silicon wafer and thin copper plate substrates. Structural properties and phase transformation temperatures of the TiNi films were investigated. To examine the structural properties of the films, XRD, SEM and EDS techniques were used. Austenitic and martensitic phase transformation temperatures were observed via DSC (differential scanning calorimeter) tests. TiNi (110) B2 austenite peaks were observed in the Run7 film. The crystalline Run7 TiNi film showed single-stage phase transformation (B19 to B2 on heating and B2 to B19 on cooling). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Shape memory thin films generally show high recovery force and narrow transformation temperature hysteresis hence they have a wide range of application areas especially in microelectromechanic systems (MEMSs), such as micro-actuators, micro-relays, and micropumps. Moreover, biomedical applications such as stents have been made with shape memory thin films [1–4]. The most studied and used shape memory films for these applications are TiNi-based films. Several methods have been used to produce these films. Magnetron sputtering is the most suitable method due to lower contamination and high purity films and adjustment of the atomic ratio, growing structure and film thicknesses by controlling the deposition parameters [5–9]. The basis of the shape memory is the transformation of phases [10]. The phases consist of martensite (B19) at low temperatures and austenite (B2)/ the main phase at high temperatures [11]. A TiNi film requires crystalline structures to show the shape memory effect. However, TiNi films deposited on unheated substrates are generally amorphous so the shape memory effect (SME) is not observed. To obtain crystalline structure, heat treatment needs to be performed on the films [12]. In-situ or ex-situ heat treatment is carried out at about 500 °C–700 °C for 1 to 4 h in a high vacuum, such as 10−5 Pa, in order to avoid the formation of oxides [13–15]. In recent studies, efforts have been exerted to achieve directly in-situ crystalline form shape memory films to avoid the drawbacks of heat treatment [16–20]. For this purpose, substrate temperatures were raised above 450 °C–500 °C ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (H. Cicek).
http://dx.doi.org/10.1016/j.surfcoat.2015.08.068 0257-8972/© 2015 Elsevier B.V. All rights reserved.
at the deposition process to obtain crystalline structure. At this time, materials having a low melting point, such as polymers, cannot be used as substrates. To overcome these limitations, crystalline TiNi films should be obtained at lower temperatures. Certain researchers used simultaneous irradiation of Ar ions during sputter-deposition [21] and in-situ annealing [22], and applied high bias voltage as 1600 V to the substrates [19] to obtain low temperature crystalline TiNi films. In this work, we synthesized an in-situ fully crystalline TiNi shape memory thin film at a lower temperature than results found in the literature by using conventional magnetron sputtering on copper plates and silicon wafers and applying pulsed-dc to the substrates without using any auxiliary method. 2. Experimental details In order to obtain in-situ crystallized TiNi shape memory thin films, the magnetron sputtering system was used (Fig. 1). TiNi films were deposited on thin copper plates for MEMS applications with 25 × 40 × 0.1 mm dimensions and silicon wafers for the evaluation of structural properties. Copper plates were polished to a roughness value of Ra ≈ 0.05 μm using SiC emery paper with up to 1200-mesh grit and then 0.05 μm grain size α-alumina was applied. After the mechanical polishment, copper plates were ultrasonically cleaned in ethanol bath via Buehler Ultramet 2005 Cleaner. Finally, copper substrates were etched for 10 s with 5% nital solution etchant. The TiNi film deposition process was performed by the unbalanced magnetron sputtering system produced by Teer Coatings Ltd. A TiNi target (Ti 50 at.%) and a Ti target were used for deposition to obtain equiatomic crystalline TiNi films. The coating processes were performed
H. Cicek et al. / Surface & Coatings Technology 284 (2015) 90–93
91
The microstructure, elemental analyses and thickness of TiNi films were analysed with a FEI Quanta FEG-450 SEM system. The film thickness was measured by SEM cross-section images taken after the silicon wafers were cleaved. XRD analyses of the TiNi films were deposited on the copper plates and silicon wafers and were performed using a Rigaku 2200 Dmax diffractometer with CuKα (λ: 1.5405 Å) radiation source at room temperature. Measurement values were obtained at the 2θ: 10–90° scan range, 0.1 step and 2°/minute scan speed. XRD results were evaluated by comparing them with JCPDS (Joint Committee on Powder Diffraction Standards). DSC analyses were performed on the free-standing TiNi films via the Perkin Elmer Diamond DSC tester. Nitrogen gas (purity of 99.999) was used with 45 mL/min flow rate. Analyses were performed between −100 °C and 500 °C with a heating/cooling rate of 5 °C/min. 3. Results and discussion
Fig. 1. Magnetron sputtering system configuration for TiNi films.
under argon gas atmosphere (purity of 99.999). Copper and silicon substrates were fixed on a rotatable holder. During the film deposition, substrates were rotated around its centre axis with a speed of 10 rpm to achieve uniform films. The distance between the substrates and targets was set as 90 mm. Before running the deposition process, the chamber pressure was decreased below 1.0 × 10−5 Pa and then the copper plates and silicon wafers were subjected to ion sputtering for 15 min (using bias voltage of 800 V) to remove possible contaminants. Ti target current was kept constant at 1 A. TiNi target current was changed as 3 A, 4 A and 5 A. The working pressure in the chamber was set to three levels as 0.27 Pa, 0.33 Pa and 0.4 Pa. To obtain crystalline films, pulse voltage was applied to the substrates as − 70 V–150 V and − 250 V. Other pulse parameters were kept constant as T: 2 μs and F: 200 kHz. The film deposition process continued for 90 min. Pulse characterization of the applied power was unipolar and duty time is 2 μs. Nine different deposition processes (R1, R2, R3, R4, R5, R6, R7, R8 and R9) were conducted according to the taguchi L9 orthogonal array. Deposition parameters and final temperatures of the plasma are given in Table 1.
XRD patterns of the TiNi films deposited on silicon substrates under 9 different deposition conditions are given in Fig. 2. XRD patterns of copper samples are also given in Fig. 3. Diffraction patterns of deposited R1, R2, R3, R4, R5, R6, R8 and R9 TiNi films showed a wide hump based on the peak position range of 40° to 45° demonstrating the classic amorphous structure. However, the R7 film deposited at high substrate pulse voltage (−250 V), high TiNi target current (5 A) and low working pressure (0.27 Pa) has a very high and sharp peak at the 42.4° peak position demonstrating a fully crystalline structure (Fig. 2). This very high and sharp peak is also observed in TiNi films deposited on cupper substrates (Fig. 3). The peak situated at 2Θ = 42.4° implies (110) lattice orientation of austenite B2 structure of TiNi thin films [6,23]. The high TiNi target current supplied high energetic atoms into the plasma [24,25]. High substrate pulse voltage gained deposited atoms on the substrate with higher mobility [26]. In addition, low working pressure caused the increased mean free path of Ti and Ni atoms in the plasma and herewith the kinetic energy losses due to collision were reduced [27]. Finally, these advantages to form crystalline structure were obtained at the R7 film and a fully crystalline TiNi film was achieved. Additionally, the final temperature of the plasma for R7 deposition was 290 °C. This temperature value is significantly below the annealing temperatures of 500 °C or 600 °C to obtain crystalline TiNi films [28,29]. Thereupon, crystalline TiNi films can be grown especially on polymer based and other lower melting temperature materials by magnetron sputtering needing neither post-annealing nor any auxiliary method. The microstructural characterization and thickness of the TiNi shape memory thin films deposited on silicon wafer substrates by dc magnetron sputtering were determined by SEM. The SEM micrographs of the deposited R7 film (crystalline) and R8 film (example for non-
Table 1 Deposition parameters of TiNi films. Constant parameters Ti target current (A) Frequency (kHz) Duty time (μs)
1 200 2
Various parameters Films
Substrate pulse voltage (−V)
Working pressure (Pa)
TiNi target current (A)
Final temperature of the plasma (°C)
R1 R2 R3 R4 R5 R6 R7 R8 R9
70 70 70 150 150 150 250 250 250
0.27 0.33 0.4 0.27 0.33 0.4 0.27 0.33 0.4
3 4 5 4 5 3 5 3 4
155 163 170 180 203 170 290 175 185
Fig. 2. XRD patterns of TiNi films deposited on silicon substrates.
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H. Cicek et al. / Surface & Coatings Technology 284 (2015) 90–93
Fig. 3. XRD pattern of R7 fully crystalline TiNi films deposited on copper substrates.
crystalline films) are given in Fig. 4. According to cross-section images, TiNi films had a very dense and uniform microstructure for all films. On the other hand, the crystalline R7 film was grown with bamboo like packed columnar structure but other films were semi- or noncolumnar. The difference of crystalline columns and amorphous structures can be observed on the SEM images of the surface. The surface appearance of the R7 film is small-grained and lumpy, but R8's is large-grained and wavy. It seems that high sputtering current, low
Fig. 4. Surface and cross-section images of TiNi films (R7 and R8).
working pressure and high substrate pulse voltage made the atoms proceed in the correct position to generate crystal lattices. On the other hand, when we look at the fracture types of the films, it seems that the R7 film showed a brittle fracture mode and the R8 film showed a ductile fracture mode after the substrates cleaved. The ductile fracture is also an evidence of amorphous structure. Additionally, a brittle fracture is usually seen in crystalline structures [22]. Zhang and Xie [30] also determined that the structure of TiNi films changed from non-columnar to columnar during the annealing process to be crystalline. Plasma temperature is important for obtaining crystalline TiNi films. Some researchers heat up the substrate or plasma above 500 °C for in-situ annealing [22]. In the present study, we did not use any insitu heating process. The final plasma temperature of R7 deposition was 290 °C. It was observed that the negative voltage applied to the substrate has the primary effect on the plasma temperature. When the substrate voltage increased from −70 V to −250 V, plasma temperature also increased from 163 °C to 217 °C as average values. Raising the target current has the secondary effect on the plasma temperature; increasing the target current will cause increased plasma temperature as expected. The thickness of the films was measured via cross-section SEM images. The highest film thickness was 2.3 μm obtained from the R3 deposition condition (5 A target current, − 70 V pulse voltage, 0.4 Pa working pressure) and the lowest was 1.7 μm obtained from the R8 deposition condition (3 A target current, − 250 V pulse voltage, 0.33 Pa working pressure). It was expected for the same group of working pressure when the TiNi target increasing current also increased the thickness of the film (Fig. 5). Additionally, working pressure generally increased the thickness of the films. Using an equiatomic target (Ti: 50, Ni: 50) for the deposition of the TiNi film, atomic percentages of nickel were generally higher than titanium's. This was caused firstly by the sputtering yield of Ni atoms (about 1.28) that is greater from the titanium atoms (about 0.58). Secondly, the chemical composition of sputtering deposited films is greatly affected by mass transfer phenomena or alias thermalisation phenomena [30]. If the thermalisation distance of the sputtered atoms was greater than the distance between the target and the substrate, atoms could readily accumulate on the surface of the substrate. Thermalisation distance is calculated as D = n · λ where n is the number of collisions of gas atoms with the sputtered atoms and λ is the mean free path in plasma [31]. Accordingly, thermalisation distance is reduced with increasing working pressure. Ting and Chen calculated these values for Ti and Ni atoms by the magnetron sputtering method [32]. They found that the thermalisation distance of nickel atoms is higher than titanium atoms deposited at lower working pressure from 1.33 Pa (DNi N DTi). The working pressure in our study was 0.27 Pa, 0.33 Pa and 0.4 Pa; therefore the Ni atom ratio would be higher in
Fig. 5. Thickness of the deposited TiNi films.
H. Cicek et al. / Surface & Coatings Technology 284 (2015) 90–93
4. Conclusion
Table 2 EDS results of deposited TiNi films. EDS results of TiNi films (at.%)
Ti Ni
93
R1
R2
R3
R4
R5
R6
R7
R8
R9
57.5 42.5
56.3 43.7
57.4 42.6
56.8 43.2
55.9 44.1
58.8 41.2
50.1 49.9
52.7 47.3
50.3 49.7
all films. For these reasons, a Ti target was placed against the TiNi (50% Ti) target to achieve the equiatomic TiNi films. The results of EDS analysis were given in Table 2. It was observed that when the current of the TiNi target was increased from 3 A to 5 A, the Ni ratio in the films increased and nearly equalled to Ti in the R7 film. This is because of the fact that the sputtering yield of the Ni atoms (about 1.28) is greater from the titanium atoms (about 0.58) and it increases the current of the TiNi target so the Ni ratio in the plasma increased. Additionally, when the substrate pulse voltage increased to −250 V, the Ni ratios of the films (R7, R8 and R9) had the highest values compared to other films. In the R9 film, although Ti and Ni atom ratios were close to each other, crystalline structure could not occur. It seems that high working pressure and relatively low plasma temperature could not cause the formation of the crystalline film for R9. In order to determine the phase transformation temperatures, a differential scanning calorimeter was used. According to the DSC results (Fig. 6), the R7 film showed significant thermal transformation temperature during the heating and cooling cycle. However, other films could not show any transformation point as expected depending on the results of XRD. For the R7 film, two transition points were detected at about 24 °C corresponding to martensitic (B19) transformation during cooling and at 2 °C corresponding to austenite (B2) transformation during heating. The phase transformations occurred in very narrow areas. At the martensite transformation during cooling, the martensite start temperature (Ms) was 25 °C and finish temperature (Mf) was 20 °C. At the austenite transformation during heating, the austenite start temperature (As) was −1 °C and finish temperature (Af) was 5 °C. This film showed single-stage phase transformation (B19 to B2 on heating and B2 to B19 on cooling) and hysteresis between austenite and martensite transformations was about 22 °C [6,33,34]. Some researchers have applied bias voltage to the substrates to obtain crystalline TiNi films. DSC results also showed that especially applying pulse dc to the substrate (R2 film) was effective to obtain as-deposited crystalline TiNi films. Detailed studies in this regard must be performed to achieve asdeposited low temperature crystalline TiNi films.
Fig. 6. DSC result of crystalline R7 TiNi film.
The present study investigated the ways to get in-situ low temperature crystalline TiNi films as deposited. The results of the study are given below: • The B2 (110) peak was detected in the R7 film deposited at high substrate pulse voltage (−250 V), high TiNi target current (5 A) and low working pressure (0.27 Pa). • The final plasma temperature at R7 deposition was 290 °C. • The maximum film thickness was 2.3 μm obtained in the R3 film and the lowest was 1.7 μm obtained in the R8 film. • The crystalline R7 film was grown with bamboo like packed columnar structure, but other films were grown as generally semi- or noncolumnar. • The nearly equiatomic R7 TiNi film (50.1% Ti) showed single-stage (B19 to B2 at 2 °C and B2 to B19 at 24 °C) phase transformation.
References [1] N. Kaur, N. Choudhary, R.N. Goyal, S. Viladkar, I. Matai, P. Gopinath, S. Chockalingam, D. Kaur, J. Nanoparticle Res. 15 (2013). [2] L. Sun, W.M. Huang, Z. Ding, Y. Zhao, C.C. Wang, H. Purnawali, C. Tang, Mater. Des. 33 (2012) 577–640. [3] Y.Q. Fu, J.K. Luo, A.J. Flewitt, S.E. Ong, S. Zhang, H.J. Du, W.I. Milne, Smart Mater. Struct. 16 (2007) 2651–2657. [4] S. Konishi, T. Kobayashi, H. Maeda, S. Asajima, M. Makikawa, Sensors Actuators B Chem. 83 (2002) 60–66. [5] Y.Q. Fu, H.J. Du, S. Zhang, Surf. Coat. Technol. 167 (2003) 129–136. [6] S.W. Kim, Y.M. Jeon, C.H. Park, J.H. Kim, D.H. Kim, J.T. Yeom, J. Alloys Compd. 580 (2013) 5–9. [7] H.J. Zhang, C.J. Qiu, Mater. Trans. 47 (2006) 532–535. [8] P.J. Kelly, C.F. Beevers, P.S. Henderson, R.D. Arnell, J.W. Bradley, H. Backer, Surf. Coat. Technol. 174 (2003) 795–800. [9] K. Hori, T. Namazu, S. Inoue, Thin Solid Films 518 (2010) S26–S28. [10] A. Shelyakov, N. Sitnikov, S. Saakyan, A. Menushenkov, R. Rizakhanov, A. Korneev, Mater. Sci. Forum 738-739 (2013) 352–356. [11] A. Fabregat-Sanjuan, F. Ferrando, C. Urbina, S. De la Flor, Mater. Sci. Forum 738-739 (2013) 367–371. [12] Y.Q. Fu, H.J. Du, S. Zhang, Thin Solid Films 444 (2003) 85–90. [13] C. Borgia, S. Olliges, M. Dietiker, G. Pigozzi, R. Spolenak, Thin Solid Films 518 (2010) 1897–1913. [14] A. Shelyakov, N. Sitnikov, N. Resnina, A. Glezer, A. Korneev, Esomat 2009 — 8th European Symposium on Martensitic Transformations, 2009. [15] Y. Kishi, N. Ikenaga, N. Sakudo, Z. Yajima, J. Alloys Compd. 577 (2013) S210–S214. [16] N. Ikenaga, Y. Kishi, Z. Yajima, N. Sakudo, Esomat 2009 — 8th European Symposium on Martensitic Transformations, 2009. [17] A. Isalgue, V. Torra, F.C. Lovey, J.L. Pelegrina, Esomat 2009 — 8th European Symposium on Martensitic Transformations, 2009. [18] A. Isalgue, V. Torra, J.L. Seguin, M. Bendahan, J.M. Amigo, V. Esteve-Cano, Mater. Sci. Eng., A Struct. 273 (1999) 717–721. [19] J.L. Seguin, M. Bendahan, A. Isalgue, V. Esteve-Cano, H. Carchano, V. Torra, Sensors Actuators A Phys. 74 (1999) 65–69. [20] P.D. Fallon, A.P. Gerratt, B.P. Kierstead, R.D. White, NSTI Nanotech, Tech. Proc., 3 2008, pp. 470–473. [21] N. Ikenaga, Y. Kishi, Z. Yajima, N. Sakudo, J. Alloys Compd. 577 (2013) S237–S240. [22] W. Tillmann, S. Momeni, Sensors Actuators A Phys. 221 (2015) 9–14. [23] H. Cho, H.Y. Kim, S. Miyazaki, Sci. Technol. Adv. Mater. 6 (2005) 678–683. [24] D.M. Mattox, Handbook of Physical Vapor Deposition (PVD) Processing, Elsevier, 2010. [25] P. Chen, J.M. Ting, Thin Solid Films 398 (2001) 597–601. [26] S. Benhenda, J.M. Guglielmacci, M. Gillet, L. Hultman, J.E. Sundgren, Appl. Surf. Sci. 40 (1989) 121–128. [27] D.M. Mattox, Handbook of Physical Vapor Deposition (PVD) Processing, Noyes Publications, New Jersey, U.S.A., 1998 [28] P. Surbled, C. Clerc, B. Le Pioufle, M. Ataka, H. Fujita, Thin Solid Films 401 (2001) 52–59. [29] Y. Motemani, M.J. Tan, T.J. White, W.M. Huang, Mater. Des. 32 (2011) 688–695. [30] L. Zhang, C.Y. Xie, J.S. Wu, J. Alloys Compd. 427 (2007) 238–243. [31] K. Tominaga, S. Iwamura, I. Fujita, Y. Shintani, O. Tada, Jpn. J. Appl. Phys. 1 (21) (1982) 999–1002. [32] J.M. Ting, P. Chen, J. Vac. Sci. Technol. A 19 (2001) 2382–2387. [33] N. Resnina, S. Belyaev, J. Alloys Compd. 577 (2013) S159–S163. [34] Z.Y. Gao, H.Z. Wang, Y.Y. Zhu, X.L. Meng, W. Cai, Thin Solid Films 584 (2015) 369–371.
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
A low temperature in-situ crystalline TiNi shape memory thin film deposited by magnetron sputtering Hikmet Cicek a,⁎, Ihsan Efeoglu a, Yaşar Totik a, Kadri Vefa Ezirmik b, Ersin Arslan b a b
Atatürk University, Faculty of Engineering, Mechanical Engineering, Turkey Atatürk University, Faculty of Engineering, Metallurgy and Materials Engineering, Turkey
a r t i c l e
i n f o
Article history: Received 1 April 2015 Revised 14 August 2015 Accepted in revised form 17 August 2015 Available online 4 September 2015 Keywords: TiNi Crystallization Magnetron sputtering
a b s t r a c t TiNi films deposited by magnetron sputtering usually have amorphous structure and must be annealed at high temperature to obtain crystallization. We have synthesized an in-situ fully crystalline TiNi shape memory thin film at low temperature by dc magnetron sputtering. Application of pulsed direct current to the substrate is effective to obtain a crystalline TiNi film. Nine different conditions were used for deposition in silicon wafer and thin copper plate substrates. Structural properties and phase transformation temperatures of the TiNi films were investigated. To examine the structural properties of the films, XRD, SEM and EDS techniques were used. Austenitic and martensitic phase transformation temperatures were observed via DSC (differential scanning calorimeter) tests. TiNi (110) B2 austenite peaks were observed in the Run7 film. The crystalline Run7 TiNi film showed single-stage phase transformation (B19 to B2 on heating and B2 to B19 on cooling). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Shape memory thin films generally show high recovery force and narrow transformation temperature hysteresis hence they have a wide range of application areas especially in microelectromechanic systems (MEMSs), such as micro-actuators, micro-relays, and micropumps. Moreover, biomedical applications such as stents have been made with shape memory thin films [1–4]. The most studied and used shape memory films for these applications are TiNi-based films. Several methods have been used to produce these films. Magnetron sputtering is the most suitable method due to lower contamination and high purity films and adjustment of the atomic ratio, growing structure and film thicknesses by controlling the deposition parameters [5–9]. The basis of the shape memory is the transformation of phases [10]. The phases consist of martensite (B19) at low temperatures and austenite (B2)/ the main phase at high temperatures [11]. A TiNi film requires crystalline structures to show the shape memory effect. However, TiNi films deposited on unheated substrates are generally amorphous so the shape memory effect (SME) is not observed. To obtain crystalline structure, heat treatment needs to be performed on the films [12]. In-situ or ex-situ heat treatment is carried out at about 500 °C–700 °C for 1 to 4 h in a high vacuum, such as 10−5 Pa, in order to avoid the formation of oxides [13–15]. In recent studies, efforts have been exerted to achieve directly in-situ crystalline form shape memory films to avoid the drawbacks of heat treatment [16–20]. For this purpose, substrate temperatures were raised above 450 °C–500 °C ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (H. Cicek).
http://dx.doi.org/10.1016/j.surfcoat.2015.08.068 0257-8972/© 2015 Elsevier B.V. All rights reserved.
at the deposition process to obtain crystalline structure. At this time, materials having a low melting point, such as polymers, cannot be used as substrates. To overcome these limitations, crystalline TiNi films should be obtained at lower temperatures. Certain researchers used simultaneous irradiation of Ar ions during sputter-deposition [21] and in-situ annealing [22], and applied high bias voltage as 1600 V to the substrates [19] to obtain low temperature crystalline TiNi films. In this work, we synthesized an in-situ fully crystalline TiNi shape memory thin film at a lower temperature than results found in the literature by using conventional magnetron sputtering on copper plates and silicon wafers and applying pulsed-dc to the substrates without using any auxiliary method. 2. Experimental details In order to obtain in-situ crystallized TiNi shape memory thin films, the magnetron sputtering system was used (Fig. 1). TiNi films were deposited on thin copper plates for MEMS applications with 25 × 40 × 0.1 mm dimensions and silicon wafers for the evaluation of structural properties. Copper plates were polished to a roughness value of Ra ≈ 0.05 μm using SiC emery paper with up to 1200-mesh grit and then 0.05 μm grain size α-alumina was applied. After the mechanical polishment, copper plates were ultrasonically cleaned in ethanol bath via Buehler Ultramet 2005 Cleaner. Finally, copper substrates were etched for 10 s with 5% nital solution etchant. The TiNi film deposition process was performed by the unbalanced magnetron sputtering system produced by Teer Coatings Ltd. A TiNi target (Ti 50 at.%) and a Ti target were used for deposition to obtain equiatomic crystalline TiNi films. The coating processes were performed
H. Cicek et al. / Surface & Coatings Technology 284 (2015) 90–93
91
The microstructure, elemental analyses and thickness of TiNi films were analysed with a FEI Quanta FEG-450 SEM system. The film thickness was measured by SEM cross-section images taken after the silicon wafers were cleaved. XRD analyses of the TiNi films were deposited on the copper plates and silicon wafers and were performed using a Rigaku 2200 Dmax diffractometer with CuKα (λ: 1.5405 Å) radiation source at room temperature. Measurement values were obtained at the 2θ: 10–90° scan range, 0.1 step and 2°/minute scan speed. XRD results were evaluated by comparing them with JCPDS (Joint Committee on Powder Diffraction Standards). DSC analyses were performed on the free-standing TiNi films via the Perkin Elmer Diamond DSC tester. Nitrogen gas (purity of 99.999) was used with 45 mL/min flow rate. Analyses were performed between −100 °C and 500 °C with a heating/cooling rate of 5 °C/min. 3. Results and discussion
Fig. 1. Magnetron sputtering system configuration for TiNi films.
under argon gas atmosphere (purity of 99.999). Copper and silicon substrates were fixed on a rotatable holder. During the film deposition, substrates were rotated around its centre axis with a speed of 10 rpm to achieve uniform films. The distance between the substrates and targets was set as 90 mm. Before running the deposition process, the chamber pressure was decreased below 1.0 × 10−5 Pa and then the copper plates and silicon wafers were subjected to ion sputtering for 15 min (using bias voltage of 800 V) to remove possible contaminants. Ti target current was kept constant at 1 A. TiNi target current was changed as 3 A, 4 A and 5 A. The working pressure in the chamber was set to three levels as 0.27 Pa, 0.33 Pa and 0.4 Pa. To obtain crystalline films, pulse voltage was applied to the substrates as − 70 V–150 V and − 250 V. Other pulse parameters were kept constant as T: 2 μs and F: 200 kHz. The film deposition process continued for 90 min. Pulse characterization of the applied power was unipolar and duty time is 2 μs. Nine different deposition processes (R1, R2, R3, R4, R5, R6, R7, R8 and R9) were conducted according to the taguchi L9 orthogonal array. Deposition parameters and final temperatures of the plasma are given in Table 1.
XRD patterns of the TiNi films deposited on silicon substrates under 9 different deposition conditions are given in Fig. 2. XRD patterns of copper samples are also given in Fig. 3. Diffraction patterns of deposited R1, R2, R3, R4, R5, R6, R8 and R9 TiNi films showed a wide hump based on the peak position range of 40° to 45° demonstrating the classic amorphous structure. However, the R7 film deposited at high substrate pulse voltage (−250 V), high TiNi target current (5 A) and low working pressure (0.27 Pa) has a very high and sharp peak at the 42.4° peak position demonstrating a fully crystalline structure (Fig. 2). This very high and sharp peak is also observed in TiNi films deposited on cupper substrates (Fig. 3). The peak situated at 2Θ = 42.4° implies (110) lattice orientation of austenite B2 structure of TiNi thin films [6,23]. The high TiNi target current supplied high energetic atoms into the plasma [24,25]. High substrate pulse voltage gained deposited atoms on the substrate with higher mobility [26]. In addition, low working pressure caused the increased mean free path of Ti and Ni atoms in the plasma and herewith the kinetic energy losses due to collision were reduced [27]. Finally, these advantages to form crystalline structure were obtained at the R7 film and a fully crystalline TiNi film was achieved. Additionally, the final temperature of the plasma for R7 deposition was 290 °C. This temperature value is significantly below the annealing temperatures of 500 °C or 600 °C to obtain crystalline TiNi films [28,29]. Thereupon, crystalline TiNi films can be grown especially on polymer based and other lower melting temperature materials by magnetron sputtering needing neither post-annealing nor any auxiliary method. The microstructural characterization and thickness of the TiNi shape memory thin films deposited on silicon wafer substrates by dc magnetron sputtering were determined by SEM. The SEM micrographs of the deposited R7 film (crystalline) and R8 film (example for non-
Table 1 Deposition parameters of TiNi films. Constant parameters Ti target current (A) Frequency (kHz) Duty time (μs)
1 200 2
Various parameters Films
Substrate pulse voltage (−V)
Working pressure (Pa)
TiNi target current (A)
Final temperature of the plasma (°C)
R1 R2 R3 R4 R5 R6 R7 R8 R9
70 70 70 150 150 150 250 250 250
0.27 0.33 0.4 0.27 0.33 0.4 0.27 0.33 0.4
3 4 5 4 5 3 5 3 4
155 163 170 180 203 170 290 175 185
Fig. 2. XRD patterns of TiNi films deposited on silicon substrates.
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H. Cicek et al. / Surface & Coatings Technology 284 (2015) 90–93
Fig. 3. XRD pattern of R7 fully crystalline TiNi films deposited on copper substrates.
crystalline films) are given in Fig. 4. According to cross-section images, TiNi films had a very dense and uniform microstructure for all films. On the other hand, the crystalline R7 film was grown with bamboo like packed columnar structure but other films were semi- or noncolumnar. The difference of crystalline columns and amorphous structures can be observed on the SEM images of the surface. The surface appearance of the R7 film is small-grained and lumpy, but R8's is large-grained and wavy. It seems that high sputtering current, low
Fig. 4. Surface and cross-section images of TiNi films (R7 and R8).
working pressure and high substrate pulse voltage made the atoms proceed in the correct position to generate crystal lattices. On the other hand, when we look at the fracture types of the films, it seems that the R7 film showed a brittle fracture mode and the R8 film showed a ductile fracture mode after the substrates cleaved. The ductile fracture is also an evidence of amorphous structure. Additionally, a brittle fracture is usually seen in crystalline structures [22]. Zhang and Xie [30] also determined that the structure of TiNi films changed from non-columnar to columnar during the annealing process to be crystalline. Plasma temperature is important for obtaining crystalline TiNi films. Some researchers heat up the substrate or plasma above 500 °C for in-situ annealing [22]. In the present study, we did not use any insitu heating process. The final plasma temperature of R7 deposition was 290 °C. It was observed that the negative voltage applied to the substrate has the primary effect on the plasma temperature. When the substrate voltage increased from −70 V to −250 V, plasma temperature also increased from 163 °C to 217 °C as average values. Raising the target current has the secondary effect on the plasma temperature; increasing the target current will cause increased plasma temperature as expected. The thickness of the films was measured via cross-section SEM images. The highest film thickness was 2.3 μm obtained from the R3 deposition condition (5 A target current, − 70 V pulse voltage, 0.4 Pa working pressure) and the lowest was 1.7 μm obtained from the R8 deposition condition (3 A target current, − 250 V pulse voltage, 0.33 Pa working pressure). It was expected for the same group of working pressure when the TiNi target increasing current also increased the thickness of the film (Fig. 5). Additionally, working pressure generally increased the thickness of the films. Using an equiatomic target (Ti: 50, Ni: 50) for the deposition of the TiNi film, atomic percentages of nickel were generally higher than titanium's. This was caused firstly by the sputtering yield of Ni atoms (about 1.28) that is greater from the titanium atoms (about 0.58). Secondly, the chemical composition of sputtering deposited films is greatly affected by mass transfer phenomena or alias thermalisation phenomena [30]. If the thermalisation distance of the sputtered atoms was greater than the distance between the target and the substrate, atoms could readily accumulate on the surface of the substrate. Thermalisation distance is calculated as D = n · λ where n is the number of collisions of gas atoms with the sputtered atoms and λ is the mean free path in plasma [31]. Accordingly, thermalisation distance is reduced with increasing working pressure. Ting and Chen calculated these values for Ti and Ni atoms by the magnetron sputtering method [32]. They found that the thermalisation distance of nickel atoms is higher than titanium atoms deposited at lower working pressure from 1.33 Pa (DNi N DTi). The working pressure in our study was 0.27 Pa, 0.33 Pa and 0.4 Pa; therefore the Ni atom ratio would be higher in
Fig. 5. Thickness of the deposited TiNi films.
H. Cicek et al. / Surface & Coatings Technology 284 (2015) 90–93
4. Conclusion
Table 2 EDS results of deposited TiNi films. EDS results of TiNi films (at.%)
Ti Ni
93
R1
R2
R3
R4
R5
R6
R7
R8
R9
57.5 42.5
56.3 43.7
57.4 42.6
56.8 43.2
55.9 44.1
58.8 41.2
50.1 49.9
52.7 47.3
50.3 49.7
all films. For these reasons, a Ti target was placed against the TiNi (50% Ti) target to achieve the equiatomic TiNi films. The results of EDS analysis were given in Table 2. It was observed that when the current of the TiNi target was increased from 3 A to 5 A, the Ni ratio in the films increased and nearly equalled to Ti in the R7 film. This is because of the fact that the sputtering yield of the Ni atoms (about 1.28) is greater from the titanium atoms (about 0.58) and it increases the current of the TiNi target so the Ni ratio in the plasma increased. Additionally, when the substrate pulse voltage increased to −250 V, the Ni ratios of the films (R7, R8 and R9) had the highest values compared to other films. In the R9 film, although Ti and Ni atom ratios were close to each other, crystalline structure could not occur. It seems that high working pressure and relatively low plasma temperature could not cause the formation of the crystalline film for R9. In order to determine the phase transformation temperatures, a differential scanning calorimeter was used. According to the DSC results (Fig. 6), the R7 film showed significant thermal transformation temperature during the heating and cooling cycle. However, other films could not show any transformation point as expected depending on the results of XRD. For the R7 film, two transition points were detected at about 24 °C corresponding to martensitic (B19) transformation during cooling and at 2 °C corresponding to austenite (B2) transformation during heating. The phase transformations occurred in very narrow areas. At the martensite transformation during cooling, the martensite start temperature (Ms) was 25 °C and finish temperature (Mf) was 20 °C. At the austenite transformation during heating, the austenite start temperature (As) was −1 °C and finish temperature (Af) was 5 °C. This film showed single-stage phase transformation (B19 to B2 on heating and B2 to B19 on cooling) and hysteresis between austenite and martensite transformations was about 22 °C [6,33,34]. Some researchers have applied bias voltage to the substrates to obtain crystalline TiNi films. DSC results also showed that especially applying pulse dc to the substrate (R2 film) was effective to obtain as-deposited crystalline TiNi films. Detailed studies in this regard must be performed to achieve asdeposited low temperature crystalline TiNi films.
Fig. 6. DSC result of crystalline R7 TiNi film.
The present study investigated the ways to get in-situ low temperature crystalline TiNi films as deposited. The results of the study are given below: • The B2 (110) peak was detected in the R7 film deposited at high substrate pulse voltage (−250 V), high TiNi target current (5 A) and low working pressure (0.27 Pa). • The final plasma temperature at R7 deposition was 290 °C. • The maximum film thickness was 2.3 μm obtained in the R3 film and the lowest was 1.7 μm obtained in the R8 film. • The crystalline R7 film was grown with bamboo like packed columnar structure, but other films were grown as generally semi- or noncolumnar. • The nearly equiatomic R7 TiNi film (50.1% Ti) showed single-stage (B19 to B2 at 2 °C and B2 to B19 at 24 °C) phase transformation.
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