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Peculiar deformation characteristics of turbostratic boron nitride thin film
Thin Solid Films 483 (2005) 218 – 221 www.elsevier.com/locate/tsf
Peculiar deformation characteristics of turbostratic boron nitride thin film Hangsheng Yanga,*, Chihiro Iwamotob, Toyonobu Yoshidaa a
Department of Materials Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan b Engineering Research Institute, School of Engineering, The University of Tokyo, Yayoi 2-11-16, Bunkyo-ku, Tokyo 113-8656, Japan Received 21 April 2004; accepted in revised form 15 December 2004
Abstract The deformation behavior of a textured turbostratic boron nitride thin film of 120 nm in thickness, with its c-axis parallel to the film surface, was investigated under nanoindentation. The load displacement response of the textured turbostratic boron nitride film was revealed to be loading rate dependent, and a creep-like phenomenon was observed under constant load. Moreover, the textured turbostratic boron nitride film was found to be able to recover with hysteresis, even when pressed to the maximum depth of approximately 20% of the film thickness. The apparent hardness and Young’s modulus were estimated to be 1.5 and 30 GPa, respectively. The reversible hysteretic deformation, similar to that of a viscoelastic body, is clearly controlled by the bending of the sp2-bonded turbostratic boron nitride basal planes. D 2004 Elsevier B.V. All rights reserved. Keywords: Boron nitride; Plasma processing and deposition; Elastic properties; Hardness
1. Introduction Turbostratic boron nitride (t-BN) thin films have interesting properties, such as high thermal conductivity, chemical stability at high temperatures, a wide band gap and negative electron affinity [1–4]. In addition to these properties, recently, it was also revealed that the t-BN deposited on the edge of ultrathin Si flake exhibits remarkable flexibility and resiliency, when it comprises of a texture structure with its sp2-bonded basal plane perpendicular to the substrate aligning in the same direction. This textured t-BN was found to bend repeatedly to its minimum radius of curvature of approximately 4 nm. Even after repeated bending to the minimum radius of curvature of approximately 0.3 nm, it underwent no catastrophic failure [5]. To analyze further more details of this unique flexibility of t-BN, in the present study, we prepared t-BN films on Si substrates with the basal plane perpendicular to the substrate
* Corresponding author. Tel./fax: +81 358417099. E-mail address: [email protected] (H. Yang). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.12.034
surface by inductively coupled plasma-enhanced chemical vapor deposition (ICP-CVD). The deformation behavior and mechanical properties of the textured t-BN film were evaluated under nanoindentation.
2. Experimental details Textured t-BN films were deposited on 3030 mm2 mirror polished Si (111) wafers by ICP-CVD. A time dependent biasing technique was employed along with continuous decrease in the substrate bias from 200 to 10 V, under 5 kW plasma power. Details of the experimental apparatus and parameters are described elsewhere [6,7]. Gas flow rates were selected to be 18.0, 0.8, and 8.0 sccm of Ar, N2 and 10% B2H6 (He), respectively. The deposition temperature was approximately 300 8C, which was heated by direct exposure to the plasma. The prepared t-BN films were characterized by a Fourier transformed infrared spectrometer (JASCO FT/IR-700), and a high-resolution transmission electron microscope (HRTEM) (JEM-4010) with the point-to-point resolution of 0.15 nm. The deformation behavior of the t-BN thin films
H. Yang et al. / Thin Solid Films 483 (2005) 218–221
was evaluated by applying nanoindentation (Triboscope, Hysitron) under ambient conditions using a Berkovich type diamond indenter, which was equipped with an atomic force microscope (AFM) (Nanoscope III, Digital Instruments). The displacement resolution, load resolution, displacement noise floor, and load noise floor of the present nanoindentation instrument are 0.02 nm, 1.0 nN, 0.2 nm, and 100 nN, respectively. The thermal drift is estimated to be smaller than 0.05 nm/s. The Berkovich indenter has a half angle of 65.38 and a total included angle of 142.28 with the tip radius less than 200 nm. Surface image collection and indentation were performed with the same intender. The elastic modulus (76.2 GPa) and poison’s ratio (0.14) for a quartz glass (T1030, Toshiba Ceramics) was selected for contact area calibration. All of the load displacement curves were confirmed by three or four indentation tests under same conditions. The structural variation of the t-BN film during indentation was observed by using a triaxial positioning Cu indenter in HRTEM for an as-deposited t-BN film on an ultrathin Si flake [5,8]. The Cu indenter is made to have a curvature radius of approximately 100 nm, and its displacement resolution was estimated to be approximately 0.18 nm. The variation of the lattice image was recorded on videotape using a fiber-optics-coupled TV system with a time resolution of 1/30 s.
3. Results and discussion Fig. 1(a) shows the transmission mode IR spectrum of a typical t-BN film after 5 min deposition. Two peaks near 1380 and 780 cm 1 are attributed to the sp2-bonded t-BN. The absorption band near 1080 cm 1 due to cubic BN (c-BN) is not detected, indicating pure sp2-bonded BN. On the basis of the IR absorption, the thickness of this t-BN film is considered to be approximately 120 nm [6]. Fig. 1(b) is a HRTEM image of the t-BN film close to the Si substrate. The vertical lattice image corresponds to the (0002) basal plane of t-BN. This image reveals that the basal planes of t-BN are perpendicular to the substrate surface even near the substrate. The root mean square roughness of the film surface measured from the AFM images was less than 1.0 nm. Fig. 2(a) shows the nanoindentation load displacement response of the t-BN film. No holding time under the maximum load was applied in the present series of indentations, because the creep-like deformation occurs, we will discuss it in the next paragraph. Under the 23 AN loading force, the maximum load depth increased from 14.4 to 19.6 nm with decreasing the loading rate from 23.4 to 2.3 AN/s. The load displacement curves under the loading rates of 2.3 and 0.73 AN/s are almost completely overlapped, which indicates the existence of a maximum load depth at a certain loading force. Such loading-rate dependence of the maximum depth has been reported for different inorganic materials due to the creep deformation
219
Fig. 1. (a) FTIR spectrum of a t-BN film deposited using time-dependent biasing technique for 5 min deposition. (b) HRTEM image showed that the (0002) basal plane of the t-BN film is perpendicular to the substrate surface even near the substrate.
during indentation [9–11], similar to the loading-rate dependent viscoelasticity of polymers such as polymethyl-methacrylate (PMMA) [12,13]. However, in contrast to the unloading behavior of PMMA and Y2O3 stabilized ZrO2 thermal barrier coatings reported in references that have different final depths according to different maximum load depths [11–13]. Similar final depths of the indentations were observed in the load displacement responses in Fig. 2(a), in spite of there being different maximum depths. Fig. 2(b) shows the apparent hardness and Young’s modulus of the t-BN film, as measured from Fig. 2(a) [14]. The apparent hardness and Young’s modulus decreased with decreasing the loading rate and approached approximately 1.5 and 30 GPa, respectively. These values of our films are much lower than reported value of approximately 12–20 GPa [15]. The difference plausibly attributes that the textured structure of our films is different from those prepared in Ref. [15]. To investigate the peculiar deformation behavior of the t-BN films in detail, nanoindentation under constant load was applied. As shown in Fig. 3, in a 13 s holding duration with an approximately 22 AN force, the indentation depth increases from 15 to 19 nm, similar to the time-dependent deformation (creep) of PMMA [13]. Moreover, the depth increase is saturated under 13 s holding time and the unloading curve after the 13 s holding duration is almost overlapped with the unloading curve (gray curve) under a loading rate of 0.73 AN/s, and
220
H. Yang et al. / Thin Solid Films 483 (2005) 218–221
Fig. 4. Final indentation depths of t-BN and fused silica evaluated immediately by load displacement curves, and then evaluated again by AFM cross-sectional images after 10 min.
Fig. 2. (a) Load-displacement responses of t-BN under loading rates ranging from 0.73 to 23.4 AN/s under a 23 AN loading force. (b) Apparent hardness and Young’s modulus calculated from the load-displacement curves according to Ref. [14].
similar final depths were observed for all three indentations in Fig. 3. Fig. 4 shows the final indentation depths of t-BN film under a series of indentations up to a load force of 300 AN. The final depth was evaluated from load displacement curves immediately after unloading, and then evaluated again by AFM cross-sectional images 10 min after indentation. For comparison, the data obtained from fused silica are also shown in Fig. 4. Similar final depths of fuse silica were obtained by these two measurements. However, for the case of t-BN, the final depths measured after 10 min are much shallower than those measured immediately after indentation, indicating the recovery of tBN after indentation. Moreover, for the case of an indentation load of 50 AN or smaller, resulted in the maximum load depth of less than 27 nm, the AFM image
Fig. 3. Load-displacement response of an indentation program under 11.7 AN/s loading-unloading rate with a 13 s holding under 22.0 AN. For comparison, the load displacement curves under 11.7 and 0.73 AN/s loading-unloading rates, respectively, are also shown.
scan conducted 10 min after the indentation revealed no impression. Fig. 5 shows cross-sectional high-resolution images of the as-deposited t-BN taken (a) before, (b) during, and (c) after the indenter been pressed onto it perpendicular to the film surface. In Fig. 5(b), the t-BN is deformed parallel to the sp2-bonded basal plane of t-BN, as indicated by an arrow. A part of the lattice appears slightly bent, however, the morphology of the t-BN film is almost the same as that of the original t-BN film shown in Fig. 5(a). During the loading cycle, no general deformation accompanied by stacking fault or twin formation, or dislocation movement, was observed. The t-BN after pressing shown in Fig. 5(c) is almost recovered to the original morphology shown in Fig. 5(a), even though it was pressed to approximately 10% of the film thickness. These images suggested that no general plastic deformation mechanism was operative during the indentation. The t-BN film in the present study has a textured structure with its basal plane perpendicular to the film surface, and the hexagonal linked layers are twisted randomly about the [0002] direction. This textured structure gives the tBN film a high in-plane strength, while the out-of-plane interaction involves weak van der Waals force. Therefore, during indentation, the bending of basal planes can occur easily, and thus the excess strain caused by indentation is released [5]. The major reason for the dependence of hardness on loading rate attributes to the effective bending volume of the basal plane of tBN, which varies with the rate of loadings. When indenting at rather fast loading rate, the tBN basal plane tends to bend rather locally, and readily generates additional strain to increase the apparent hardness. In contrast, at slower rate of loading, the basal plane tends to bend over the entire film rather than a limited volume region and imposes less significantly stress on around the indentation, affected directly by the indenter, which eventually increases the maximum depth and decreases an apparent hardness. After indentation, the recovery of the bent basal planes is ready to occur. Accordingly, the final depth evaluated 10 min after indentation is shallower than that measured immediately as shown in Fig. 4. The indentation impression is completely recov-
H. Yang et al. / Thin Solid Films 483 (2005) 218–221
221
Fig. 5. High-resolution images of as-deposited t-BN taken while the indenter pressed it along the z-direction. (a) shows an image of the original t-BN. In (b), tBN under indentation is deformed parallel to the sp2-bonded BN layers indicated by the arrow. (c) shows the t-BN after pressing. The lines in the figure indicate the interface between the tBN surface and Cu indenter.
ered if the maximum load depth is less than 20% of the film thickness. Although, the reason why this recovery showed hysteresis is unclear, the different position shifts of B and N atoms in each BN sheet, leading to electrostatic potential variation between adjacent sheets, are considered to play a role.
ChemistryQ from the Ministry of Education, Culture, Sports, Science and Technology, Japan. One of the authors (H.S. Yang) acknowledges the support of the Japan Society for the Promotion of Science (JSPS) during this work (ID No. P03055).
References 4. Conclusions In summary, we have found that textured t-BN film shows polymer-like time-dependent deformation, and can recover completely with hysteresis, even when pressed to a load depth of approximately 20% of the film thickness. This novel mechanical deformation was considered to be controlled by the bending of the sp2- bonded t-BN basal planes. The mechanical properties of the textured t-BN films found in this study, together with their superior electronic properties and chemical stability at high temperatures, may lead to novel applications of these films.
Acknowledgements This work was financially supported by a Grant-in Aid for Scientific Research (S) (Grant No. 16106009), and partly supported by a Grant for the 21st Century COE Program bHuman Friendly Materials based on
[1] T. Sugino, S. Kawasaki, K. Tanioka, J. Shirafuji, Appl. Phys. Lett. 71 (1997) 2704. [2] K.P. Loh, I. Sakaguchi, M.N. Gamo, S. Tagawa, T. Sugino, T. Ando, Appl. Phys. Lett. 74 (1999) 28. [3] C. Kimura, T. Yamamoto, T. Sugino, Diamond Relat. Mater. 10 (2001) 1404. [4] K. Nose, K. Tachibana, T. Yoshida, Appl. Phys. Lett. 83 (2003) 943. [5] C. Iwamoto, H.S. Yang, S. Watanabe, T. Yoshida, Appl. Phys. Lett. 83 (2003) 4402. [6] H.S. Yang, C. Iwamoto, T. Yoshida, Thin Solid Films 407 (2002) 67. [7] H.S. Yang, C. Iwamoto, T. Yoshida, J. Appl. Phys. 94 (2003) 1248. [8] H.S. Yang, C. Iwamoto, T. Yoshida, J. Appl. Phys. 95 (2004) 2337. [9] T. Chudoba, F. Richter, Surf. Coat. Technol. 148 (2001) 191. [10] M.I. Simoes, J.V. Fernandes, A. Cavaleiro, Philos. Mag., A 82 (2002) 1911. [11] S. Guo, Y. Kagawa, Surf. Coat. Technol. 182 (2004) 92. [12] M.L. Oyen, R.F. Cook, J. Mater. Res. 18 (2003) 139. [13] H. Lu, B. Wang, J. Ma, G. Huang, H. Viswanathan, Mech. TimeDepend. Mater. 7 (2003) 189. [14] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [15] M.C. Polo, E. Martinez, J. Esteve, J.L. Andujar, Diamond Relat. Mater. 8 (1999) 423.
Peculiar deformation characteristics of turbostratic boron nitride thin film Hangsheng Yanga,*, Chihiro Iwamotob, Toyonobu Yoshidaa a
Department of Materials Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan b Engineering Research Institute, School of Engineering, The University of Tokyo, Yayoi 2-11-16, Bunkyo-ku, Tokyo 113-8656, Japan Received 21 April 2004; accepted in revised form 15 December 2004
Abstract The deformation behavior of a textured turbostratic boron nitride thin film of 120 nm in thickness, with its c-axis parallel to the film surface, was investigated under nanoindentation. The load displacement response of the textured turbostratic boron nitride film was revealed to be loading rate dependent, and a creep-like phenomenon was observed under constant load. Moreover, the textured turbostratic boron nitride film was found to be able to recover with hysteresis, even when pressed to the maximum depth of approximately 20% of the film thickness. The apparent hardness and Young’s modulus were estimated to be 1.5 and 30 GPa, respectively. The reversible hysteretic deformation, similar to that of a viscoelastic body, is clearly controlled by the bending of the sp2-bonded turbostratic boron nitride basal planes. D 2004 Elsevier B.V. All rights reserved. Keywords: Boron nitride; Plasma processing and deposition; Elastic properties; Hardness
1. Introduction Turbostratic boron nitride (t-BN) thin films have interesting properties, such as high thermal conductivity, chemical stability at high temperatures, a wide band gap and negative electron affinity [1–4]. In addition to these properties, recently, it was also revealed that the t-BN deposited on the edge of ultrathin Si flake exhibits remarkable flexibility and resiliency, when it comprises of a texture structure with its sp2-bonded basal plane perpendicular to the substrate aligning in the same direction. This textured t-BN was found to bend repeatedly to its minimum radius of curvature of approximately 4 nm. Even after repeated bending to the minimum radius of curvature of approximately 0.3 nm, it underwent no catastrophic failure [5]. To analyze further more details of this unique flexibility of t-BN, in the present study, we prepared t-BN films on Si substrates with the basal plane perpendicular to the substrate
* Corresponding author. Tel./fax: +81 358417099. E-mail address: [email protected] (H. Yang). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.12.034
surface by inductively coupled plasma-enhanced chemical vapor deposition (ICP-CVD). The deformation behavior and mechanical properties of the textured t-BN film were evaluated under nanoindentation.
2. Experimental details Textured t-BN films were deposited on 3030 mm2 mirror polished Si (111) wafers by ICP-CVD. A time dependent biasing technique was employed along with continuous decrease in the substrate bias from 200 to 10 V, under 5 kW plasma power. Details of the experimental apparatus and parameters are described elsewhere [6,7]. Gas flow rates were selected to be 18.0, 0.8, and 8.0 sccm of Ar, N2 and 10% B2H6 (He), respectively. The deposition temperature was approximately 300 8C, which was heated by direct exposure to the plasma. The prepared t-BN films were characterized by a Fourier transformed infrared spectrometer (JASCO FT/IR-700), and a high-resolution transmission electron microscope (HRTEM) (JEM-4010) with the point-to-point resolution of 0.15 nm. The deformation behavior of the t-BN thin films
H. Yang et al. / Thin Solid Films 483 (2005) 218–221
was evaluated by applying nanoindentation (Triboscope, Hysitron) under ambient conditions using a Berkovich type diamond indenter, which was equipped with an atomic force microscope (AFM) (Nanoscope III, Digital Instruments). The displacement resolution, load resolution, displacement noise floor, and load noise floor of the present nanoindentation instrument are 0.02 nm, 1.0 nN, 0.2 nm, and 100 nN, respectively. The thermal drift is estimated to be smaller than 0.05 nm/s. The Berkovich indenter has a half angle of 65.38 and a total included angle of 142.28 with the tip radius less than 200 nm. Surface image collection and indentation were performed with the same intender. The elastic modulus (76.2 GPa) and poison’s ratio (0.14) for a quartz glass (T1030, Toshiba Ceramics) was selected for contact area calibration. All of the load displacement curves were confirmed by three or four indentation tests under same conditions. The structural variation of the t-BN film during indentation was observed by using a triaxial positioning Cu indenter in HRTEM for an as-deposited t-BN film on an ultrathin Si flake [5,8]. The Cu indenter is made to have a curvature radius of approximately 100 nm, and its displacement resolution was estimated to be approximately 0.18 nm. The variation of the lattice image was recorded on videotape using a fiber-optics-coupled TV system with a time resolution of 1/30 s.
3. Results and discussion Fig. 1(a) shows the transmission mode IR spectrum of a typical t-BN film after 5 min deposition. Two peaks near 1380 and 780 cm 1 are attributed to the sp2-bonded t-BN. The absorption band near 1080 cm 1 due to cubic BN (c-BN) is not detected, indicating pure sp2-bonded BN. On the basis of the IR absorption, the thickness of this t-BN film is considered to be approximately 120 nm [6]. Fig. 1(b) is a HRTEM image of the t-BN film close to the Si substrate. The vertical lattice image corresponds to the (0002) basal plane of t-BN. This image reveals that the basal planes of t-BN are perpendicular to the substrate surface even near the substrate. The root mean square roughness of the film surface measured from the AFM images was less than 1.0 nm. Fig. 2(a) shows the nanoindentation load displacement response of the t-BN film. No holding time under the maximum load was applied in the present series of indentations, because the creep-like deformation occurs, we will discuss it in the next paragraph. Under the 23 AN loading force, the maximum load depth increased from 14.4 to 19.6 nm with decreasing the loading rate from 23.4 to 2.3 AN/s. The load displacement curves under the loading rates of 2.3 and 0.73 AN/s are almost completely overlapped, which indicates the existence of a maximum load depth at a certain loading force. Such loading-rate dependence of the maximum depth has been reported for different inorganic materials due to the creep deformation
219
Fig. 1. (a) FTIR spectrum of a t-BN film deposited using time-dependent biasing technique for 5 min deposition. (b) HRTEM image showed that the (0002) basal plane of the t-BN film is perpendicular to the substrate surface even near the substrate.
during indentation [9–11], similar to the loading-rate dependent viscoelasticity of polymers such as polymethyl-methacrylate (PMMA) [12,13]. However, in contrast to the unloading behavior of PMMA and Y2O3 stabilized ZrO2 thermal barrier coatings reported in references that have different final depths according to different maximum load depths [11–13]. Similar final depths of the indentations were observed in the load displacement responses in Fig. 2(a), in spite of there being different maximum depths. Fig. 2(b) shows the apparent hardness and Young’s modulus of the t-BN film, as measured from Fig. 2(a) [14]. The apparent hardness and Young’s modulus decreased with decreasing the loading rate and approached approximately 1.5 and 30 GPa, respectively. These values of our films are much lower than reported value of approximately 12–20 GPa [15]. The difference plausibly attributes that the textured structure of our films is different from those prepared in Ref. [15]. To investigate the peculiar deformation behavior of the t-BN films in detail, nanoindentation under constant load was applied. As shown in Fig. 3, in a 13 s holding duration with an approximately 22 AN force, the indentation depth increases from 15 to 19 nm, similar to the time-dependent deformation (creep) of PMMA [13]. Moreover, the depth increase is saturated under 13 s holding time and the unloading curve after the 13 s holding duration is almost overlapped with the unloading curve (gray curve) under a loading rate of 0.73 AN/s, and
220
H. Yang et al. / Thin Solid Films 483 (2005) 218–221
Fig. 4. Final indentation depths of t-BN and fused silica evaluated immediately by load displacement curves, and then evaluated again by AFM cross-sectional images after 10 min.
Fig. 2. (a) Load-displacement responses of t-BN under loading rates ranging from 0.73 to 23.4 AN/s under a 23 AN loading force. (b) Apparent hardness and Young’s modulus calculated from the load-displacement curves according to Ref. [14].
similar final depths were observed for all three indentations in Fig. 3. Fig. 4 shows the final indentation depths of t-BN film under a series of indentations up to a load force of 300 AN. The final depth was evaluated from load displacement curves immediately after unloading, and then evaluated again by AFM cross-sectional images 10 min after indentation. For comparison, the data obtained from fused silica are also shown in Fig. 4. Similar final depths of fuse silica were obtained by these two measurements. However, for the case of t-BN, the final depths measured after 10 min are much shallower than those measured immediately after indentation, indicating the recovery of tBN after indentation. Moreover, for the case of an indentation load of 50 AN or smaller, resulted in the maximum load depth of less than 27 nm, the AFM image
Fig. 3. Load-displacement response of an indentation program under 11.7 AN/s loading-unloading rate with a 13 s holding under 22.0 AN. For comparison, the load displacement curves under 11.7 and 0.73 AN/s loading-unloading rates, respectively, are also shown.
scan conducted 10 min after the indentation revealed no impression. Fig. 5 shows cross-sectional high-resolution images of the as-deposited t-BN taken (a) before, (b) during, and (c) after the indenter been pressed onto it perpendicular to the film surface. In Fig. 5(b), the t-BN is deformed parallel to the sp2-bonded basal plane of t-BN, as indicated by an arrow. A part of the lattice appears slightly bent, however, the morphology of the t-BN film is almost the same as that of the original t-BN film shown in Fig. 5(a). During the loading cycle, no general deformation accompanied by stacking fault or twin formation, or dislocation movement, was observed. The t-BN after pressing shown in Fig. 5(c) is almost recovered to the original morphology shown in Fig. 5(a), even though it was pressed to approximately 10% of the film thickness. These images suggested that no general plastic deformation mechanism was operative during the indentation. The t-BN film in the present study has a textured structure with its basal plane perpendicular to the film surface, and the hexagonal linked layers are twisted randomly about the [0002] direction. This textured structure gives the tBN film a high in-plane strength, while the out-of-plane interaction involves weak van der Waals force. Therefore, during indentation, the bending of basal planes can occur easily, and thus the excess strain caused by indentation is released [5]. The major reason for the dependence of hardness on loading rate attributes to the effective bending volume of the basal plane of tBN, which varies with the rate of loadings. When indenting at rather fast loading rate, the tBN basal plane tends to bend rather locally, and readily generates additional strain to increase the apparent hardness. In contrast, at slower rate of loading, the basal plane tends to bend over the entire film rather than a limited volume region and imposes less significantly stress on around the indentation, affected directly by the indenter, which eventually increases the maximum depth and decreases an apparent hardness. After indentation, the recovery of the bent basal planes is ready to occur. Accordingly, the final depth evaluated 10 min after indentation is shallower than that measured immediately as shown in Fig. 4. The indentation impression is completely recov-
H. Yang et al. / Thin Solid Films 483 (2005) 218–221
221
Fig. 5. High-resolution images of as-deposited t-BN taken while the indenter pressed it along the z-direction. (a) shows an image of the original t-BN. In (b), tBN under indentation is deformed parallel to the sp2-bonded BN layers indicated by the arrow. (c) shows the t-BN after pressing. The lines in the figure indicate the interface between the tBN surface and Cu indenter.
ered if the maximum load depth is less than 20% of the film thickness. Although, the reason why this recovery showed hysteresis is unclear, the different position shifts of B and N atoms in each BN sheet, leading to electrostatic potential variation between adjacent sheets, are considered to play a role.
ChemistryQ from the Ministry of Education, Culture, Sports, Science and Technology, Japan. One of the authors (H.S. Yang) acknowledges the support of the Japan Society for the Promotion of Science (JSPS) during this work (ID No. P03055).
References 4. Conclusions In summary, we have found that textured t-BN film shows polymer-like time-dependent deformation, and can recover completely with hysteresis, even when pressed to a load depth of approximately 20% of the film thickness. This novel mechanical deformation was considered to be controlled by the bending of the sp2- bonded t-BN basal planes. The mechanical properties of the textured t-BN films found in this study, together with their superior electronic properties and chemical stability at high temperatures, may lead to novel applications of these films.
Acknowledgements This work was financially supported by a Grant-in Aid for Scientific Research (S) (Grant No. 16106009), and partly supported by a Grant for the 21st Century COE Program bHuman Friendly Materials based on
[1] T. Sugino, S. Kawasaki, K. Tanioka, J. Shirafuji, Appl. Phys. Lett. 71 (1997) 2704. [2] K.P. Loh, I. Sakaguchi, M.N. Gamo, S. Tagawa, T. Sugino, T. Ando, Appl. Phys. Lett. 74 (1999) 28. [3] C. Kimura, T. Yamamoto, T. Sugino, Diamond Relat. Mater. 10 (2001) 1404. [4] K. Nose, K. Tachibana, T. Yoshida, Appl. Phys. Lett. 83 (2003) 943. [5] C. Iwamoto, H.S. Yang, S. Watanabe, T. Yoshida, Appl. Phys. Lett. 83 (2003) 4402. [6] H.S. Yang, C. Iwamoto, T. Yoshida, Thin Solid Films 407 (2002) 67. [7] H.S. Yang, C. Iwamoto, T. Yoshida, J. Appl. Phys. 94 (2003) 1248. [8] H.S. Yang, C. Iwamoto, T. Yoshida, J. Appl. Phys. 95 (2004) 2337. [9] T. Chudoba, F. Richter, Surf. Coat. Technol. 148 (2001) 191. [10] M.I. Simoes, J.V. Fernandes, A. Cavaleiro, Philos. Mag., A 82 (2002) 1911. [11] S. Guo, Y. Kagawa, Surf. Coat. Technol. 182 (2004) 92. [12] M.L. Oyen, R.F. Cook, J. Mater. Res. 18 (2003) 139. [13] H. Lu, B. Wang, J. Ma, G. Huang, H. Viswanathan, Mech. TimeDepend. Mater. 7 (2003) 189. [14] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [15] M.C. Polo, E. Martinez, J. Esteve, J.L. Andujar, Diamond Relat. Mater. 8 (1999) 423.