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Titanium interlayer to improve the adhesion of multilayer amorphous boron carbide coating on silicon substrate
Applied Surface Science 266 (2013) 170–175
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Titanium interlayer to improve the adhesion of multilayer amorphous boron carbide coating on silicon substrate E. Vassallo a,∗ , R. Caniello a , A. Cremona a , D. Dellasega b , E. Miorin c a b c
CNR, Istituto di Fisica del Plasma “P.Caldirola”, Italy Politecnico di Milano, Dipartimento di Energia, Italy CNR, Istituto per l’Energetica e le Interfasi, Italy
a r t i c l e
i n f o
Article history: Received 15 October 2012 Received in revised form 20 November 2012 Accepted 23 November 2012 Available online 30 November 2012 Keywords: Boron carbide Titanium interlayer Plasma sputtering Plasma chemical vapor deposition Internal stress Roughness
a b s t r a c t Boron-based coatings can be very useful in neutrons detection application. Here, stable boron carbon (BC) films in nanometer scale were deposited on silicon substrates with titanium interlayer by RF plasma magnetron sputtering. Ti interlayer is likely to decrease the internal stress at the substrate–film interface, thus smoothing the difference in lattice parameters between the growing film and substrate. The B-C/Ti/Si coatings were characterized using SEM, EDS, AFM and XRD. The enhancement in adhesion of the B-C film to the substrate was analyzed by a scratch tester measurements. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Boron carbide is one of the most relevant material because of its very interesting characteristics such as high hardness, good electronic and tribological properties, chemical and thermal stability [1–3]. At room temperature, boron carbide is the third hardest known material and above 1100 ◦ C is the hardest one [4]. Films of few micron show good performance on cutting tools [3] and can be used as mirrors with high reflectivity in the ultraviolet range [5]. Boron-based coatings can also be very useful in neutrons detection application [6]. Indeed, due to the rapidly dwindling of helium3 availability and to the huge increase in its cost, researchers are working to develop an efficient alternative to helium-based neutron detectors. Boron seems to be the best candidate because of its high absorption neutron cross section and non-toxicity to health. Several techniques such as chemical vapor deposition, plasma enhanced chemical vapor deposition [7], hot filament chemical vapor deposition [8], ion beam assisted evaporation [9], and vacuum arc deposition technology [10] were utilized to synthetize boron carbide films. Magnetron sputtering [11] is one of the most used techniques in thin film deposition on industrial scale due to its application at low temperature and without dangerous gases.
∗ Corresponding author. E-mail address: [email protected] (E. Vassallo). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.123
Coatings produced by this technique often show internal stress induced from the deposition conditions. In order to reduce the stress of the deposited coatings, many suitable deposition recipes have been studied and optimized in terms of deposition parameters [3,12,13], and several methods such as post-process annealing have been explored [14]. In several cases, examples are reported in the literature on the possibility to control thin films stress by using a multilayer structure [15–17] or to increase the adhesion by interposition of a metallic inter-layers between the substrate and the coating [18–21]. Mechanical proprieties, mean life-time and adhesion to the substrate are fundamental requirements to achieve useful coatings in many applicative fields. Furthermore, the use of a specific substrate is often dictated by market requirements and technological solutions. In the case of neutron detector device, a conductive metallic substrate is necessary for charge transport, and a good adhesion of boron to substrate is desirable. For a successful boron carbide application in neutron detectors field, it is necessary to understand the relationship between the performance of boron carbide coatings, in terms of stability in time and adhesion to the substrate, and the substrate morphology. This issue is the main goal of the present work that looks towards the possibility to define a specific deposition recipe by exploring the amorphous boron carbide growth on different substrates. In this study we report on the structure of an amorphous boron carbide (a-B4 C) coating prepared by radio frequency (RF) magnetron sputtering as a function of the titanium interlayer (Ti-i) thickness deposited on (1 0 0)
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silicon substrate at room temperature. We found that a-B4 C films are time unstable if deposited on bare silicon or in presence of Ti-i with thickness lower than 100–150 nm. In some cases the coatings start to delaminate and peel off in a few hours after being taken out of the deposition chamber. As Ti-i thickness further increases, the films show a very good stability and adhesion further up to a 400 nm thickness. Emphasis is given to the relationship between substrate roughness and structure of the sputter-deposited a-B4 Cs. A comparison between the adhesion of a-B4 C coating to the bare silicon and to a 400 nm Ti coated silicon is reported as measured by a CETR UMT-2 scratch tester. 2. Experimental details Amorphous boron carbide films and Ti interlayer were prepared by RF (1356 MHz) magnetron sputtering of a-B4 C and Ti targets, respectively. (1 0 0) silicon substrates were ultrasonically cleaned with acetone and ethanol, and carefully placed on the grounded substrate holder kept at 7 cm distance from the RF powered electrode. The vacuum before deposition was less than 1 × 10−4 Pa and the substrate temperature was monitored by using a k thermocouple placed in contact with the sample. High purity Ar (99.9995%) gas was introduced into the chamber through a mass flow controller and a gate valve was used to adjust the pressure during the process. In the same experimental conditions, three samples of Ti interlayer with thicknesses of 25 nm, 200 nm, and 400 nm were sputter-deposited on Si substrate for 4, 30 and 60 min, respectively, implying a 6.6 nm/min constant deposition rate. A radiofrequency power source, capacitively coupled with the deposition chamber, was fixed at 150 W and Ar pressure at 1 Pa. Amorphous boron carbide was deposited on the three interlayers with a multilayer structure which consists in growing four layers at two different pressures. For the first and third layers, the working pressure was fixed at 2 Pa, and at 0.8 Pa for the second and fourth layers. The morphological properties and physical structure of the films were investigated by Scan Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and X-ray Diffraction (XRD). SEM measurements were performed using a ZEISS Supra System with an accelerating voltage of 15 kV. AFM measurements were made in air by a Nano-RTM AFM System (Pacific Nanotechnology, Santa Clara, CA, USA) operating in close contact mode. Silicon conical tips of 10 nm radius mounted on silicon cantilevers of 125 m length, 42 N/m force constant and 320 kHz resonance frequency were used. Images were processed and analyzed by means of the NanoRule+TM software provided by Pacific Nanotechnology. The structural properties studied by X-ray diffraction measurements were performed with a wide angle Siemens D-500 diffractometer (WAXD) equipped with a Siemens FK 60-10 2000W tube. The radiation was a monochromatized Cu K␣ beam with wavelength = 0.15418 nm. The operating voltage and current were 40 kV and 40 mA, respectively. The data were collected from 10 to 80 2 ◦ at 0.02 2 ◦ intervals by means of a silicon multi-cathode detector Vortex-EX (SII). The substrate temperature can be preset. In the present work, ultra-thin boron carbide films were deposited either at room temperature or at an elevated temperature of 600 K to examine the influence of temperature on the film properties. Scratch test measurements were made in compliance with the European standard UNI EN 1071-3-2005 by using a CETR UMT-2 tester equipped with a Rockwell C standard geometry diamond indenter having an angle of 120 degrees and a spherical tip radius of 200 micron and a 1000× optical microscope. 3. Results and discussion Fig. 1 shows SEM cross-section micrographs of the boron carbide coatings grown with different Ti interlayer thicknesses. The
Fig. 1. SEM cross-section images of the boron carbide coatings prepared with (a) 25 nm, (b) 200 nm, (c) 400 nm of Ti interlayer thickness.
cross sectional view allows to determine the B-C deposited coating thickness, which is about 0.5 micron (obtained by a sequential deposition of four layers), implying a deposition rate of about 0.9 nm/min. At 25 nm Ti-i thickness (Fig. 1a), the coating exhibits both fine columnar structure (layers 1 and 3) and compact structure (layers 2 and 4). The interface separation of the four layers is quite clear. The four layer structure becomes less pronounced as the Ti interlayer thickness increases further up to 200 nm (Fig. 1b); the columnar structure becomes more continuous and the interface separation less visible. As the Ti-i thickness further increases (Fig. 1c), the layered structure completely disappeared and the coating assumes a continuous columnar structure. The coating structure remained columnar as the Ti-i thickness increased above 400 nm. We have observed that the film starts to delaminate and peel off with a Ti-i thickness less than 100–120 nm. The films gradually start to delaminate after being taken out of the coating chamber and brought in atmospheric pressure at room temperature. These stresses can be caused by the large discrepancy between the lattice parameters of the (1 0 0) silicon plane and the structure of the growing coating. Introducing the Ti interlayer is likely to decrease the internal
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stress at the substrate–film interface, thus smoothing the difference in lattice parameters between the thin film and substrate. X-ray diffraction has been used to characterize the structure. The diffraction spectrum (Fig. 2) of the boron carbide deposited on 400 nm Ti interlayer reveals the amorphous character of the coating. This result also applies to the other coatings with different Ti-i thicknesses. The analysis shows, besides the reflection at 69.3◦ 2 corresponding to the (1 0 0) Si substrate, some crystallographic orientations of the hexagonal ␣-titanium phase [22,23]. The main orientation is (0 0 2) at the 2 angle of 38.5◦ ; other three less intense peaks are visible. For clarity the XRD spectrum of a 400 nm Ti interlayer deposited on a Si substrate is also reported. The columnar geometry depends on the substrate topology, because it results from the competition between the growth of the irregularities and the surface atoms diffusion [24,25]. The substrate roughness influences strongly the initial stage of the coating growth [26] and it also plays an important role in the evolution of the physical structure. Surface roughness usually increases during the deposition, and in some cases [27], columnar structures gradually appear in sputtered thick films in correspondence of a certain roughness value.
Fig. 2. XRD spectra of amorphous boron carbide deposited on Ti-interlayer (400 nm) and bare Si.
Fig. 3. AFM images of Ti-i interlayers of thickness (a) 25 nm, (b) 200 nm and (c) 400 nm.
Fig. 4. SEM top-view images of a-B4 C boron carbide coatings prepared with (a) 25 nm, (b) 220 nm, and (c) 400 nm of Ti interlayer thickness.
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Fig. 5. (a) SEM cross-section and (b) AFM image of an a-B4 C four-layer coating deposited on a Si substrate.
As shown in Fig. 3, the roughness of the Ti interlayer increases as a function of the thickness. The surface root mean square roughness (Rrms ), measured with the fixed area of 2 m × 2 m, of B4 C coatings grown with 25, 200 and 400 nm of Ti interlayer was found to be 1, 10.5 and 15.5 nm, respectively. Correspondingly, an increase of the lateral dimension of the a-B4 C columns is observed (Figs. 1 and 4). Furthermore, the interface of each layers becomes less clear until it disappears at a value of about 300 nm of Ti-i thickness. Another interesting feature to note is that above this thickness the samples (Fig. 1c) show dense and continue columns independent of which working pressure has been used. As previously reported in the literature [26], if the surface adatoms diffusion length is longer than the irregularities characteristic length, the roughness of the deposited coating is smoothed out and the coating becomes denser. So, we explain the growth features of our coatings by this notion. Decreasing the working pressure, when the deposition is switched from layer 1 to layer 2 (Fig. 1a), the energy released from the particles at the surface will be higher, consequently, the increased surface adatoms diffusion length will give rise to an a-B4 C denser layer. In order to enhance and understand that aspect, a dedicated experiment, in which a four-layer boron carbide coating was deposited on bare Si substrate in the same condition of the samples discussed above, was performed. The sample was immediately analyzed after the
deposition process in order to prevent cracking and instability. As expected, amorphous boron carbide starts to grow with columns (Fig. 5) much thinner (Rrms ∼ 2.5 nm), than the ones deposited on Si substrate with a 25 nm Ti interlayer. This can be clearly related with the high flatness of Si substrate (Rrms < ∼0.35 nm [28]) and with the poor surface diffusion due to the high working pressure. In this case also, the diffraction spectrum (Fig. 2) of the boron carbide deposited on Si reveals the amorphous character of the coating. With regard to the sample c (Fig. 1), the a-B4 C coating has started to grow with a large basal lateral dimension of the columns and, when the pressure was decreased, the growth was proceeded with the same texture of the previous layer. We estimate that at this value of Ti-i roughness, an equilibrium between surface adatoms diffusion length and roughness length scale of the substrate was achieved. We emphasize that, during the deposition process, the temperature of the substrate was very low (about 320 K), indicating that thermal induced surface diffusivity can be neglected. In order to investigate the thermal effect during film deposition, a heated sample holder equipped with a resistive heating has been used. The sample holder was heated in the range 300–600 K. A four-layer boron carbide coating was deposited on bare Si substrate with about 400 nm Ti-i thickness. Some difference (Fig. 6) was found in the structure for films grown above 500 K. The continuous columnar structure becomes less visible. Evidence of this
Fig. 6. SEM cross-section and top-view images of B4C coatings prepared with a heated sample holder: (a–c) at 300 K and (b–d) at 600 K.
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Fig. 7. Scratch test diagram of a-B4 C deposited on 400 nm Ti-i thickness. Optical micrograph of the track is reported on the bottom.
can be seen in Fig. 6d (marker A) at the interface with titanium interlayer. No significant difference was found in surface morphology (Fig. 6a and b). The morphology was relatively uniform for both films. The grain size slightly increased. The coating thickness was also slightly increased. The deposition rate increased from 1.7 to 1.9 nm/min in the range 300–600 K. Therefore, we assess that the temperature increases the surface adatoms diffusion which gives rise to an a-B4 C denser layer. The coating has also in this case an amorphous structure (diffraction not shown here). In order to investigate the mechanical properties of the coatings, scratch test measurements have been performed on a-B4 C film grown with a 400 nm Ti interlayer. In Fig. 7 the coefficient of friction (COF), the acoustic emission (AE), the friction force (Fx ) and the normal load (Fz ) are reported as a function of time. An optical micrograph of the scratch is also reported. The scratch is performed by Progressive Load Scratch Test (PLST) mode in which the applied normal load increases linearly with time. The slide velocity of the indenter and the applied load were fixed to 9.0 mm/min and 9.0 N/min−1 , respectively, starting from a contact load of 0.4 N. After a short initial transient period, COF and Fx were observed to increase linearly with the load. In this region, as can be observed in Fig. 8a, the tip interacts mainly with punctual defects of the coating. At the point 1 (Fig. 7), a sharp decrease of COF and a smoother change of the slope for the friction force were detected. Looking over the optical micrograph (Fig. 8b), a groove within the scratch track is found, which becomes much more pronounced as the applied load increases. Coating undergoes a plastic deformation without any cracks. In the region from point 2 to point 3, a second transition of the COF and Fx was obtained. Some forward chevron cracks at the scratch track borders appear, as highlighting in Fig. 8c. At the point 4 (Fig. 7), the first interfacial spallation of the coating can be observed (Fig. 8d). The applied load was 5.1 N which represents the applied critical load associated with the a-B4 C coating. Furthermore, tensile type Hertzian cracks and forward chevron cracks are visible on the Ti-i. In the region from point 4 to point 5, the a-B4 C coating completely undergoes interfacial spallation. Around point 5 (Fig. 8e), a failure of both a-B4 C and Ti interlayer occurs at a load of 6.5 N. Scratch test was also performed in the same condition on the sample shown in Fig. 5, where no Ti-i was deposited. A failure of the film occurs at a critical load of 1.4 N. A comparison between the two scratch test clearly shows an increase of the adhesion, that becomes three times stronger in the presence of Ti-i, thus changing from 1.4 N (without Ti-i) to 5.1 N (with Ti-i). The destructive effect generated by the indenter while scratching along the two samples is shown in Fig. 9. Scratch test was also performed on the coating prepared with the sample holder heated to 600 K (Fig. 6b–d). This coating showed
Fig. 8. 400× optical micrographs of scratch test track: (a) interaction tip-punctual defects of the coating, (b) plastic deformation without cracks, (c) forward chevron cracks, (d) local interfacial spallation, (e) failure of B4 C coating and Ti interlayer.
a critical load (6.2 N) slightly higher than those of the roomtemperature prepared coating. The increase in the hardness of B-C films with the temperature was reported in the literature [29]. For our B-C films, however, the higher critical load of the film prepared at elevated temperature might be related with its dense microstructure.
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Fig. 9. Optical micrographs of the scratch track generated from the indenter on a-B4 C deposited on bare and 400 nm Ti coated silicon substrate.
4. Conclusion Boron carbide has been deposited by magnetron sputtering on silicon substrate with different titanium interlayer thickness. Ti thickness above 300 nm leads to the growth of a-B4 C with a dense and continue columnar structure. In this case, no delamination effects were found. Correspondingly, scratch test measurements show that the adhesion of the a-B4C coating becomes three times stronger. We attribute this result to the Ti-i which decreases the internal stress at the substrate–film interface. This result is connected to the enhanced roughness of Ti-i which induces the growth of a dense and continue columnar structure. For films prepared at 600 K, no significant changes in the surface morphology was observed in comparison with that of the room-temperature prepared films. In addition, higher preparation temperatures appeared to enhance film density and result in higher critical load. Acknowledgment The authors gratefully thank Dr. M. Canetti of Institute for Macromolecular Studies (CNR) for assistance in diffraction analysis. References [1] H. Werheit, IEEE International Conference on Thermoelectrics, vol. 15, 2006, pp. 9–163, http://dx.doi.org/10.1109/ICT.2006.331323. [2] E. Pascual, E. Martınez, J. Esteve, A. Lousa, Diamond and Related Materials 8 (1999) 402. [3] T. Hu, L. Steihl, W. Rafaniello, T. Fawcett, D.D. Hawn, J.G. Mashall, S.J. Rozeveld, C.L. Putzig, J. Blackson, W. Cernigiani, M.G. Robinson, Thin Solid Films 332 (1998) 80.
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[4] S. Ulrich, H. Ehrhardt, J. Schwan, et al., Diamond and Related Materials 7 (1998) 838. [5] G.M. Blumenstock, R.A.M. Keski-Kuha, Applied Optics 33 (1994) 5962. [6] D.S. McGregor, R.T. Klann, H.K. Gersch, E. Ariesanti, J.D. Sanders, B. VanDerElzen, IEEE Transactions on Nuclear Science 49 (4) (2002). [7] A. Annen, M. Sab, R. Beckmann, A. von Keudell, W. Jacob, Thin Solid Films 312 (1998) 147. [8] S.V. Deshpande, E. Gulari, S.J. Harris, A.M. Weiner, Applied Physics Letters 65 (1994) 1757. [9] R. Gago, I. Jimenez, J.M. Albella, Thin Solid Films 373 (2000) 277. [10] C.C. Klepper, R.C. Hazelton, E.J. Yadlowsky, E.P. Carlson, M.D. Keitz, J.M. Williams, R.A. Zuhr, D.B. Poker, Journal of Vacuum Science and Technology A 20 (3) (2002) 725. [11] J.S. Chapin, Research and Development 25 (1) (1974) 37. [12] M.-L. Wu, J.D. Kiely, T. Klemmer, Y.-T. Hsia, K. Howard, Thin Solid Films 449 (2004) 120. [13] Z. Han, G. Li, J. Tian, M. Gu, Materials Letters 57 (2002) 899. [14] V. Kulikovsky, V. Vorlicek, R. Ctvrtlik, P. Bohac, L. Jastrabik, H. Lapsanska, Surface and Coatings Technology 205 (2011) 4052. [15] N. Kuratani, O. Imai, A. Ebe, S. Nishiyama, K. Ogata, Surface and Coatings Technology 66 (1994) 310. [16] D.L. Windt, Journal of Vacuum Science and Technology B 17 (1999) 1385. [17] E. Vassallo, R. Caniello, A. Cremona, G. Croci, D. Dellasega, G. Gorini, G. Grosso, M. Passoni, M. Tardocchi, Surface & Coatings Technology (2012), http://dx.doi.org/10.1016/j.surfcoat.2012.11.001. [18] S.W. Russell, S.A. Rafalski, R.L. Spreitzer, J. Li, M. Moinpour, F. Moghadam, T.L. Alford, Thin Solid Films 262 (1995) 154. [19] M.D. Kriese, N.R. Moody, W.W. Gerberich, Acta Materialia 46 (1998) 6623. [20] J.H. Huang, C.-H- Ma, H. Chen, Surface and Coatings Technology 200 (2006) 5937. [21] K.A. Pischow, L. Eriksson, E. Harju, A.S. Korhonen, Surface and Coatings Technology 58 (1993) 163. [22] J.C. Avelar-Batista, A.D. Wilson, A. Davison, A. Matthews, K.S. Fancey, Journal of Vacuum Science & Technology A 21 (2003) 1702. [23] A.Z. Sadek, H. Zheng, K. Latham, W. Wlodarski, K. Kalantar-zadeh, Langmuir 25 (2009) 509–514. [24] A. Mazor, D.J. Srolovitz, P.S. Hagan, B.G. Bukiet, Physical Review Letters 60 (1988) 424. [25] S. Lichter, J. Chen, Physical Review Letters 56 (1986) 1396. [26] P. Bai, J.F. McDonald, T.-M. Lu, M.J. Costa, Journal of Vacuum Science and Technology A 9 (1991) 2113. [27] W. Xu, B. Li, T. Fujimoto, I. Kojima, Surface and Coatings Technology 135 (2001) 274. [28] Z.J. Liu, P.W. Shum, Y.G. Shen, Applied Physics Letters 86 (2005) 251908. [29] S. Aoqui, H. Miyata, T. Ohshima, T. Ikegami, K. Ebihara, Thin Solid Films 407 (2002) 126.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Titanium interlayer to improve the adhesion of multilayer amorphous boron carbide coating on silicon substrate E. Vassallo a,∗ , R. Caniello a , A. Cremona a , D. Dellasega b , E. Miorin c a b c
CNR, Istituto di Fisica del Plasma “P.Caldirola”, Italy Politecnico di Milano, Dipartimento di Energia, Italy CNR, Istituto per l’Energetica e le Interfasi, Italy
a r t i c l e
i n f o
Article history: Received 15 October 2012 Received in revised form 20 November 2012 Accepted 23 November 2012 Available online 30 November 2012 Keywords: Boron carbide Titanium interlayer Plasma sputtering Plasma chemical vapor deposition Internal stress Roughness
a b s t r a c t Boron-based coatings can be very useful in neutrons detection application. Here, stable boron carbon (BC) films in nanometer scale were deposited on silicon substrates with titanium interlayer by RF plasma magnetron sputtering. Ti interlayer is likely to decrease the internal stress at the substrate–film interface, thus smoothing the difference in lattice parameters between the growing film and substrate. The B-C/Ti/Si coatings were characterized using SEM, EDS, AFM and XRD. The enhancement in adhesion of the B-C film to the substrate was analyzed by a scratch tester measurements. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Boron carbide is one of the most relevant material because of its very interesting characteristics such as high hardness, good electronic and tribological properties, chemical and thermal stability [1–3]. At room temperature, boron carbide is the third hardest known material and above 1100 ◦ C is the hardest one [4]. Films of few micron show good performance on cutting tools [3] and can be used as mirrors with high reflectivity in the ultraviolet range [5]. Boron-based coatings can also be very useful in neutrons detection application [6]. Indeed, due to the rapidly dwindling of helium3 availability and to the huge increase in its cost, researchers are working to develop an efficient alternative to helium-based neutron detectors. Boron seems to be the best candidate because of its high absorption neutron cross section and non-toxicity to health. Several techniques such as chemical vapor deposition, plasma enhanced chemical vapor deposition [7], hot filament chemical vapor deposition [8], ion beam assisted evaporation [9], and vacuum arc deposition technology [10] were utilized to synthetize boron carbide films. Magnetron sputtering [11] is one of the most used techniques in thin film deposition on industrial scale due to its application at low temperature and without dangerous gases.
∗ Corresponding author. E-mail address: [email protected] (E. Vassallo). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.123
Coatings produced by this technique often show internal stress induced from the deposition conditions. In order to reduce the stress of the deposited coatings, many suitable deposition recipes have been studied and optimized in terms of deposition parameters [3,12,13], and several methods such as post-process annealing have been explored [14]. In several cases, examples are reported in the literature on the possibility to control thin films stress by using a multilayer structure [15–17] or to increase the adhesion by interposition of a metallic inter-layers between the substrate and the coating [18–21]. Mechanical proprieties, mean life-time and adhesion to the substrate are fundamental requirements to achieve useful coatings in many applicative fields. Furthermore, the use of a specific substrate is often dictated by market requirements and technological solutions. In the case of neutron detector device, a conductive metallic substrate is necessary for charge transport, and a good adhesion of boron to substrate is desirable. For a successful boron carbide application in neutron detectors field, it is necessary to understand the relationship between the performance of boron carbide coatings, in terms of stability in time and adhesion to the substrate, and the substrate morphology. This issue is the main goal of the present work that looks towards the possibility to define a specific deposition recipe by exploring the amorphous boron carbide growth on different substrates. In this study we report on the structure of an amorphous boron carbide (a-B4 C) coating prepared by radio frequency (RF) magnetron sputtering as a function of the titanium interlayer (Ti-i) thickness deposited on (1 0 0)
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silicon substrate at room temperature. We found that a-B4 C films are time unstable if deposited on bare silicon or in presence of Ti-i with thickness lower than 100–150 nm. In some cases the coatings start to delaminate and peel off in a few hours after being taken out of the deposition chamber. As Ti-i thickness further increases, the films show a very good stability and adhesion further up to a 400 nm thickness. Emphasis is given to the relationship between substrate roughness and structure of the sputter-deposited a-B4 Cs. A comparison between the adhesion of a-B4 C coating to the bare silicon and to a 400 nm Ti coated silicon is reported as measured by a CETR UMT-2 scratch tester. 2. Experimental details Amorphous boron carbide films and Ti interlayer were prepared by RF (1356 MHz) magnetron sputtering of a-B4 C and Ti targets, respectively. (1 0 0) silicon substrates were ultrasonically cleaned with acetone and ethanol, and carefully placed on the grounded substrate holder kept at 7 cm distance from the RF powered electrode. The vacuum before deposition was less than 1 × 10−4 Pa and the substrate temperature was monitored by using a k thermocouple placed in contact with the sample. High purity Ar (99.9995%) gas was introduced into the chamber through a mass flow controller and a gate valve was used to adjust the pressure during the process. In the same experimental conditions, three samples of Ti interlayer with thicknesses of 25 nm, 200 nm, and 400 nm were sputter-deposited on Si substrate for 4, 30 and 60 min, respectively, implying a 6.6 nm/min constant deposition rate. A radiofrequency power source, capacitively coupled with the deposition chamber, was fixed at 150 W and Ar pressure at 1 Pa. Amorphous boron carbide was deposited on the three interlayers with a multilayer structure which consists in growing four layers at two different pressures. For the first and third layers, the working pressure was fixed at 2 Pa, and at 0.8 Pa for the second and fourth layers. The morphological properties and physical structure of the films were investigated by Scan Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and X-ray Diffraction (XRD). SEM measurements were performed using a ZEISS Supra System with an accelerating voltage of 15 kV. AFM measurements were made in air by a Nano-RTM AFM System (Pacific Nanotechnology, Santa Clara, CA, USA) operating in close contact mode. Silicon conical tips of 10 nm radius mounted on silicon cantilevers of 125 m length, 42 N/m force constant and 320 kHz resonance frequency were used. Images were processed and analyzed by means of the NanoRule+TM software provided by Pacific Nanotechnology. The structural properties studied by X-ray diffraction measurements were performed with a wide angle Siemens D-500 diffractometer (WAXD) equipped with a Siemens FK 60-10 2000W tube. The radiation was a monochromatized Cu K␣ beam with wavelength = 0.15418 nm. The operating voltage and current were 40 kV and 40 mA, respectively. The data were collected from 10 to 80 2 ◦ at 0.02 2 ◦ intervals by means of a silicon multi-cathode detector Vortex-EX (SII). The substrate temperature can be preset. In the present work, ultra-thin boron carbide films were deposited either at room temperature or at an elevated temperature of 600 K to examine the influence of temperature on the film properties. Scratch test measurements were made in compliance with the European standard UNI EN 1071-3-2005 by using a CETR UMT-2 tester equipped with a Rockwell C standard geometry diamond indenter having an angle of 120 degrees and a spherical tip radius of 200 micron and a 1000× optical microscope. 3. Results and discussion Fig. 1 shows SEM cross-section micrographs of the boron carbide coatings grown with different Ti interlayer thicknesses. The
Fig. 1. SEM cross-section images of the boron carbide coatings prepared with (a) 25 nm, (b) 200 nm, (c) 400 nm of Ti interlayer thickness.
cross sectional view allows to determine the B-C deposited coating thickness, which is about 0.5 micron (obtained by a sequential deposition of four layers), implying a deposition rate of about 0.9 nm/min. At 25 nm Ti-i thickness (Fig. 1a), the coating exhibits both fine columnar structure (layers 1 and 3) and compact structure (layers 2 and 4). The interface separation of the four layers is quite clear. The four layer structure becomes less pronounced as the Ti interlayer thickness increases further up to 200 nm (Fig. 1b); the columnar structure becomes more continuous and the interface separation less visible. As the Ti-i thickness further increases (Fig. 1c), the layered structure completely disappeared and the coating assumes a continuous columnar structure. The coating structure remained columnar as the Ti-i thickness increased above 400 nm. We have observed that the film starts to delaminate and peel off with a Ti-i thickness less than 100–120 nm. The films gradually start to delaminate after being taken out of the coating chamber and brought in atmospheric pressure at room temperature. These stresses can be caused by the large discrepancy between the lattice parameters of the (1 0 0) silicon plane and the structure of the growing coating. Introducing the Ti interlayer is likely to decrease the internal
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stress at the substrate–film interface, thus smoothing the difference in lattice parameters between the thin film and substrate. X-ray diffraction has been used to characterize the structure. The diffraction spectrum (Fig. 2) of the boron carbide deposited on 400 nm Ti interlayer reveals the amorphous character of the coating. This result also applies to the other coatings with different Ti-i thicknesses. The analysis shows, besides the reflection at 69.3◦ 2 corresponding to the (1 0 0) Si substrate, some crystallographic orientations of the hexagonal ␣-titanium phase [22,23]. The main orientation is (0 0 2) at the 2 angle of 38.5◦ ; other three less intense peaks are visible. For clarity the XRD spectrum of a 400 nm Ti interlayer deposited on a Si substrate is also reported. The columnar geometry depends on the substrate topology, because it results from the competition between the growth of the irregularities and the surface atoms diffusion [24,25]. The substrate roughness influences strongly the initial stage of the coating growth [26] and it also plays an important role in the evolution of the physical structure. Surface roughness usually increases during the deposition, and in some cases [27], columnar structures gradually appear in sputtered thick films in correspondence of a certain roughness value.
Fig. 2. XRD spectra of amorphous boron carbide deposited on Ti-interlayer (400 nm) and bare Si.
Fig. 3. AFM images of Ti-i interlayers of thickness (a) 25 nm, (b) 200 nm and (c) 400 nm.
Fig. 4. SEM top-view images of a-B4 C boron carbide coatings prepared with (a) 25 nm, (b) 220 nm, and (c) 400 nm of Ti interlayer thickness.
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Fig. 5. (a) SEM cross-section and (b) AFM image of an a-B4 C four-layer coating deposited on a Si substrate.
As shown in Fig. 3, the roughness of the Ti interlayer increases as a function of the thickness. The surface root mean square roughness (Rrms ), measured with the fixed area of 2 m × 2 m, of B4 C coatings grown with 25, 200 and 400 nm of Ti interlayer was found to be 1, 10.5 and 15.5 nm, respectively. Correspondingly, an increase of the lateral dimension of the a-B4 C columns is observed (Figs. 1 and 4). Furthermore, the interface of each layers becomes less clear until it disappears at a value of about 300 nm of Ti-i thickness. Another interesting feature to note is that above this thickness the samples (Fig. 1c) show dense and continue columns independent of which working pressure has been used. As previously reported in the literature [26], if the surface adatoms diffusion length is longer than the irregularities characteristic length, the roughness of the deposited coating is smoothed out and the coating becomes denser. So, we explain the growth features of our coatings by this notion. Decreasing the working pressure, when the deposition is switched from layer 1 to layer 2 (Fig. 1a), the energy released from the particles at the surface will be higher, consequently, the increased surface adatoms diffusion length will give rise to an a-B4 C denser layer. In order to enhance and understand that aspect, a dedicated experiment, in which a four-layer boron carbide coating was deposited on bare Si substrate in the same condition of the samples discussed above, was performed. The sample was immediately analyzed after the
deposition process in order to prevent cracking and instability. As expected, amorphous boron carbide starts to grow with columns (Fig. 5) much thinner (Rrms ∼ 2.5 nm), than the ones deposited on Si substrate with a 25 nm Ti interlayer. This can be clearly related with the high flatness of Si substrate (Rrms < ∼0.35 nm [28]) and with the poor surface diffusion due to the high working pressure. In this case also, the diffraction spectrum (Fig. 2) of the boron carbide deposited on Si reveals the amorphous character of the coating. With regard to the sample c (Fig. 1), the a-B4 C coating has started to grow with a large basal lateral dimension of the columns and, when the pressure was decreased, the growth was proceeded with the same texture of the previous layer. We estimate that at this value of Ti-i roughness, an equilibrium between surface adatoms diffusion length and roughness length scale of the substrate was achieved. We emphasize that, during the deposition process, the temperature of the substrate was very low (about 320 K), indicating that thermal induced surface diffusivity can be neglected. In order to investigate the thermal effect during film deposition, a heated sample holder equipped with a resistive heating has been used. The sample holder was heated in the range 300–600 K. A four-layer boron carbide coating was deposited on bare Si substrate with about 400 nm Ti-i thickness. Some difference (Fig. 6) was found in the structure for films grown above 500 K. The continuous columnar structure becomes less visible. Evidence of this
Fig. 6. SEM cross-section and top-view images of B4C coatings prepared with a heated sample holder: (a–c) at 300 K and (b–d) at 600 K.
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Fig. 7. Scratch test diagram of a-B4 C deposited on 400 nm Ti-i thickness. Optical micrograph of the track is reported on the bottom.
can be seen in Fig. 6d (marker A) at the interface with titanium interlayer. No significant difference was found in surface morphology (Fig. 6a and b). The morphology was relatively uniform for both films. The grain size slightly increased. The coating thickness was also slightly increased. The deposition rate increased from 1.7 to 1.9 nm/min in the range 300–600 K. Therefore, we assess that the temperature increases the surface adatoms diffusion which gives rise to an a-B4 C denser layer. The coating has also in this case an amorphous structure (diffraction not shown here). In order to investigate the mechanical properties of the coatings, scratch test measurements have been performed on a-B4 C film grown with a 400 nm Ti interlayer. In Fig. 7 the coefficient of friction (COF), the acoustic emission (AE), the friction force (Fx ) and the normal load (Fz ) are reported as a function of time. An optical micrograph of the scratch is also reported. The scratch is performed by Progressive Load Scratch Test (PLST) mode in which the applied normal load increases linearly with time. The slide velocity of the indenter and the applied load were fixed to 9.0 mm/min and 9.0 N/min−1 , respectively, starting from a contact load of 0.4 N. After a short initial transient period, COF and Fx were observed to increase linearly with the load. In this region, as can be observed in Fig. 8a, the tip interacts mainly with punctual defects of the coating. At the point 1 (Fig. 7), a sharp decrease of COF and a smoother change of the slope for the friction force were detected. Looking over the optical micrograph (Fig. 8b), a groove within the scratch track is found, which becomes much more pronounced as the applied load increases. Coating undergoes a plastic deformation without any cracks. In the region from point 2 to point 3, a second transition of the COF and Fx was obtained. Some forward chevron cracks at the scratch track borders appear, as highlighting in Fig. 8c. At the point 4 (Fig. 7), the first interfacial spallation of the coating can be observed (Fig. 8d). The applied load was 5.1 N which represents the applied critical load associated with the a-B4 C coating. Furthermore, tensile type Hertzian cracks and forward chevron cracks are visible on the Ti-i. In the region from point 4 to point 5, the a-B4 C coating completely undergoes interfacial spallation. Around point 5 (Fig. 8e), a failure of both a-B4 C and Ti interlayer occurs at a load of 6.5 N. Scratch test was also performed in the same condition on the sample shown in Fig. 5, where no Ti-i was deposited. A failure of the film occurs at a critical load of 1.4 N. A comparison between the two scratch test clearly shows an increase of the adhesion, that becomes three times stronger in the presence of Ti-i, thus changing from 1.4 N (without Ti-i) to 5.1 N (with Ti-i). The destructive effect generated by the indenter while scratching along the two samples is shown in Fig. 9. Scratch test was also performed on the coating prepared with the sample holder heated to 600 K (Fig. 6b–d). This coating showed
Fig. 8. 400× optical micrographs of scratch test track: (a) interaction tip-punctual defects of the coating, (b) plastic deformation without cracks, (c) forward chevron cracks, (d) local interfacial spallation, (e) failure of B4 C coating and Ti interlayer.
a critical load (6.2 N) slightly higher than those of the roomtemperature prepared coating. The increase in the hardness of B-C films with the temperature was reported in the literature [29]. For our B-C films, however, the higher critical load of the film prepared at elevated temperature might be related with its dense microstructure.
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Fig. 9. Optical micrographs of the scratch track generated from the indenter on a-B4 C deposited on bare and 400 nm Ti coated silicon substrate.
4. Conclusion Boron carbide has been deposited by magnetron sputtering on silicon substrate with different titanium interlayer thickness. Ti thickness above 300 nm leads to the growth of a-B4 C with a dense and continue columnar structure. In this case, no delamination effects were found. Correspondingly, scratch test measurements show that the adhesion of the a-B4C coating becomes three times stronger. We attribute this result to the Ti-i which decreases the internal stress at the substrate–film interface. This result is connected to the enhanced roughness of Ti-i which induces the growth of a dense and continue columnar structure. For films prepared at 600 K, no significant changes in the surface morphology was observed in comparison with that of the room-temperature prepared films. In addition, higher preparation temperatures appeared to enhance film density and result in higher critical load. Acknowledgment The authors gratefully thank Dr. M. Canetti of Institute for Macromolecular Studies (CNR) for assistance in diffraction analysis. References [1] H. Werheit, IEEE International Conference on Thermoelectrics, vol. 15, 2006, pp. 9–163, http://dx.doi.org/10.1109/ICT.2006.331323. [2] E. Pascual, E. Martınez, J. Esteve, A. Lousa, Diamond and Related Materials 8 (1999) 402. [3] T. Hu, L. Steihl, W. Rafaniello, T. Fawcett, D.D. Hawn, J.G. Mashall, S.J. Rozeveld, C.L. Putzig, J. Blackson, W. Cernigiani, M.G. Robinson, Thin Solid Films 332 (1998) 80.
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[4] S. Ulrich, H. Ehrhardt, J. Schwan, et al., Diamond and Related Materials 7 (1998) 838. [5] G.M. Blumenstock, R.A.M. Keski-Kuha, Applied Optics 33 (1994) 5962. [6] D.S. McGregor, R.T. Klann, H.K. Gersch, E. Ariesanti, J.D. Sanders, B. VanDerElzen, IEEE Transactions on Nuclear Science 49 (4) (2002). [7] A. Annen, M. Sab, R. Beckmann, A. von Keudell, W. Jacob, Thin Solid Films 312 (1998) 147. [8] S.V. Deshpande, E. Gulari, S.J. Harris, A.M. Weiner, Applied Physics Letters 65 (1994) 1757. [9] R. Gago, I. Jimenez, J.M. Albella, Thin Solid Films 373 (2000) 277. [10] C.C. Klepper, R.C. Hazelton, E.J. Yadlowsky, E.P. Carlson, M.D. Keitz, J.M. Williams, R.A. Zuhr, D.B. Poker, Journal of Vacuum Science and Technology A 20 (3) (2002) 725. [11] J.S. Chapin, Research and Development 25 (1) (1974) 37. [12] M.-L. Wu, J.D. Kiely, T. Klemmer, Y.-T. Hsia, K. Howard, Thin Solid Films 449 (2004) 120. [13] Z. Han, G. Li, J. Tian, M. Gu, Materials Letters 57 (2002) 899. [14] V. Kulikovsky, V. Vorlicek, R. Ctvrtlik, P. Bohac, L. Jastrabik, H. Lapsanska, Surface and Coatings Technology 205 (2011) 4052. [15] N. Kuratani, O. Imai, A. Ebe, S. Nishiyama, K. Ogata, Surface and Coatings Technology 66 (1994) 310. [16] D.L. Windt, Journal of Vacuum Science and Technology B 17 (1999) 1385. [17] E. Vassallo, R. Caniello, A. Cremona, G. Croci, D. Dellasega, G. Gorini, G. Grosso, M. Passoni, M. Tardocchi, Surface & Coatings Technology (2012), http://dx.doi.org/10.1016/j.surfcoat.2012.11.001. [18] S.W. Russell, S.A. Rafalski, R.L. Spreitzer, J. Li, M. Moinpour, F. Moghadam, T.L. Alford, Thin Solid Films 262 (1995) 154. [19] M.D. Kriese, N.R. Moody, W.W. Gerberich, Acta Materialia 46 (1998) 6623. [20] J.H. Huang, C.-H- Ma, H. Chen, Surface and Coatings Technology 200 (2006) 5937. [21] K.A. Pischow, L. Eriksson, E. Harju, A.S. Korhonen, Surface and Coatings Technology 58 (1993) 163. [22] J.C. Avelar-Batista, A.D. Wilson, A. Davison, A. Matthews, K.S. Fancey, Journal of Vacuum Science & Technology A 21 (2003) 1702. [23] A.Z. Sadek, H. Zheng, K. Latham, W. Wlodarski, K. Kalantar-zadeh, Langmuir 25 (2009) 509–514. [24] A. Mazor, D.J. Srolovitz, P.S. Hagan, B.G. Bukiet, Physical Review Letters 60 (1988) 424. [25] S. Lichter, J. Chen, Physical Review Letters 56 (1986) 1396. [26] P. Bai, J.F. McDonald, T.-M. Lu, M.J. Costa, Journal of Vacuum Science and Technology A 9 (1991) 2113. [27] W. Xu, B. Li, T. Fujimoto, I. Kojima, Surface and Coatings Technology 135 (2001) 274. [28] Z.J. Liu, P.W. Shum, Y.G. Shen, Applied Physics Letters 86 (2005) 251908. [29] S. Aoqui, H. Miyata, T. Ohshima, T. Ikegami, K. Ebihara, Thin Solid Films 407 (2002) 126.