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Influence of the substrate position angle on the adhesion of ZnO thin films deposited on polyimide foil substrates
Applied Surface Science 257 (2011) 2801–2805
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of the substrate position angle on the adhesion of ZnO thin films deposited on polyimide foil substrates Hu Changji a , Zhu Daoyun b , He Zhenhui a,∗ , Fu Weichun c , Zhong Qi c a State Key Laboratory of Optoelectronic Materials and Technologies, Center for Space Technology, and School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, PR China b Experiment Teaching Department, Guangdong University of Technology, Guangzhou, 510006, PR China c China Academy of Spacecraft Technology, Beijing 100094, PR China
a r t i c l e
i n f o
Article history: Received 3 February 2010 Received in revised form 9 October 2010 Accepted 16 October 2010 Available online 23 October 2010 PACS: 52.77.Dq 61.05.cp
a b s t r a c t ZnO thin films, as polymer protection layers against ultraviolet radiation, were deposited on polyimide foil substrates using cathodic vacuum arc deposition technique. X-ray diffraction results showed that all the samples had (0 0 2) preferred orientation and the FWHM decreased as the position angle decreased. A fragmentation test was employed to investigate the influence of substrate position angle on the adhesion of ZnO thin films. It was found that the intrinsic adhesion between the ZnO film and the polyimide substrate is about 60 MPa at the substrate position angle of 0◦ . When the position angle increases to ±60◦ , the value of intrinsic adhesion decreases to about 30 MPa. © 2010 Elsevier B.V. All rights reserved.
Keywords: ZnO thin film Cathodic vacuum arc deposition Adhesion Fragmentation test
1. Introduction Cathodic vacuum arc deposition (CVAD) is a plasma-based technology for the fabrication of films [1]. CVAD has the advantages of a high ionization rate, high bombardment energy and high deposition rate [2]. Energetic ions can enhance the adhesion between film and substrate [3]. This makes it a promising method to deposit oxide thin film on polyimide foil substrates. In addition, similar to PBII&D (plasma-based ion implantation and deposition), CVAD is also a non-line-of-sight process, suitable for deposit coatings on irregular surface [4]. However, due to the non-homogeneous of plasma density around the cathode in the deposition chamber, the ion flux distribution is non-homogeneous. So the interaction between the ions and the substrate could be different with respect to the angular distribution, and thus the properties of the films. Yukimura et al. studied the effect of plasma density on the deposition with three-dimensional topologies in metal d.c. plasmabased ion implantation. They found that the metallic arc has a strong directivity and the thickness of the deposited layer for the
∗ Corresponding author. Tel.: +86 020 84113398. E-mail address: [email protected] (H. Zhenhui). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.10.064
tilted substrate has a sinusoidal relationship to the tilt angle in the range of 0–90◦ [5]. Zhitomirsky et al. studied the incidence angle effects on the structure and surface properties of vacuum arc deposited TiN coatings. They found that there is little influence of incidence angle on the preferred grain orientation, grain size and microhardness for the coatings deposited on the face (normal to the plasma flux) and the side (parallel to the plasma flux) substrate surface for different substrate bias voltage [6]. These studies were focused on the other properties of the thin film except for adhesion. However, good adhesion is essential for whatever final application of the film [7]. Since reports on the adhesion of oxide thin films prepared by CVAD to the polymer substrate are relatively few, in this paper, by employing the fragmentation test, we studied mainly the influence of substrate position angle on the adhesion of ZnO thin films prepared by CVAD. 2. Experimental The CVAD system employed was described in [8]. A watercooled Zn metal target (65 mm in diameter, 99.99% in purity) was used as a cathode. Supplied by Guangzhou Donghao Electrical Insulation Material Co. Ltd., the polyimide foil substrates, with a thickness of 50 m, were cleaned in isopropyl alcohol for 10 min
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H. Changji et al. / Applied Surface Science 257 (2011) 2801–2805
was calculated by the division of elongation value and origin length which were measured by the electronic digital indicator with precision up to 0.01 mm. The average fragment size was defined as the inverse of the crack density and could be calculated as follows, 1 = CD = (1 + ε) l
Fig. 1. Schematic diagram of the positions of the substrate holder and the substrates orientation to the cathode.
in an ultrasonic bath, then dried with N2 and finally mounted on a semicircular substrate holder, about 24 cm in radius, and with the center of cathode surface as its center (see Fig. 1 for the top view schematic diagram). The holder was made of iron and no bias voltage was applied. After the chamber was evacuated to about 6.0 × 10−3 Pa, pure argon and oxygen gases were introduced in, with an Ar mass flow rate of 10 sccm and an O2 mass flow rate of about 120 sccm. The deposition pressure was set to 0.9 Pa; the arc current was fixed to 50 A and the deposition time was 10 min for all the samples. The substrates were not intentionally heated during the deposition. The substrate temperatures were measured by calibrated homemade film thermocouples (Cu–Fe72.9 wt%–Cr19.1 wt%–Ni8.0 wt%) with the junction size about 1 mm2 and the film thicknesses around 500 nm. The results showed that the deposition temperatures were about 85◦ C, 92◦ C, 93◦ C, 92◦ C and 91◦ C for the position angles of = −60◦ , −30◦ , 0◦ , 30◦ and 60◦ , respectively. The structural properties of the films were evaluated by X-ray diffraction analysis using an X-ray diffractometer (D/MAX 2200 ˚ VPC, RIGAKU, Jan.) with a Cu K␣ X-ray source (wavelength of 1.54 A). The Young’s modulus of the films was calculated following the classical rule of mixtures [9], EZnO/PI =
(EPI dPI + EZnO dZnO ) (dZnO + dPI )
(1)
where EZnO/PI is Young’s modulus of polyimide substrate coated with ZnO film, EPI is the Young’s modulus of polyimide substrate, EZnO is the Young’s modulus of ZnO film, dPI is the thickness of the polyimide substrate and dZnO is the thickness of ZnO film. The Poisson ratio of ZnO films is assumed to be 0.3, referred to [10,11]. The residual (or internal) stress of the ZnO thin films can be obtained by the following equation [12] f =
2 EZnO (1 + ZnO ) 2tan0
k i=1
kW
Ni
(3)
where, l is the average fragment size, CD is the crack density, ε is the strain, k is the number of micrographs, W is the width of micrograph, N is the number of the cracks, respectively. At each strain interval, the number of observed cracks was counted for at least 10 micrographs. The crack density at saturation was obtained when an increase in strain did not lead to further fragmentation of the film. In our cases, the strains were usually larger than 15%.
3. Results The ZnO films are polycrystalline, belonging to the hexagonal wurtzite structure, with strong c-axis orientation (see Fig. 2). The internal stress, Young’s modulus and structure parameters of the ZnO thin films deposited at different substrate position angles are listed in Table 1. The grain size of all the samples is about 30 nm. All the samples are subjected to the compressive stress. In general, Young’s modulus and the internal stress of ZnO films decrease with the increase of substrate position angle. The dependence of the deposition rate of the films on the position angle is approximately mirror-symmetric. At the position angle of 0◦ , the deposition rate is about 0.67 nm/s, the corresponding thickness of film is about 400 nm. When the position angle varies from 0◦ to 60◦ (or 0◦ to −60◦ ), the deposition rate of the films decrease almost linearly to about 0.5 nm/s, and the corresponding thickness of film is about 300 nm(also see Table 2). Furthermore, the deposition rates approximately agree with a cosine distribution (see Fig. 3). According to the Kelly–Tyson model [13], the interfacial shear strength could be calculated by =
2df max (lc ) lc
(4)
where df is the thickness of the film, and max (lc ) is the cohesive strength of the film.
(2)
where f is the residual stress within the ZnO film, EZnO and ZnO are the Young’s modulus and the Poisson ratio of the ZnO film, respectively; 0 = 17.211 is the Bragg angle under stress free condition and 2 is the difference between the measured 2 0 and 2, which was obtained based on the X-ray diffraction (0 0 2) peak of the ZnO films (see Fig. 2). The thicknesses of the films were measured by employing a stylus surface profiler (AMBios, XP-2). A fragmentation test was carried out to evaluate the adhesion of the film. The films were cut with a gauge 50 mm in length and 10 mm in width, then fixed on a home-made tensile device. The tensile device was placed under an Olympus BX51M optical microscope, and the images of the cracks were taken by a CCD camera mounted onto the microscope. The strain applied to the sample,
Fig. 2. The X-ray diffraction patterns of the ZnO thin films deposited on polyimide foil substrates at different substrate position angles.
H. Changji et al. / Applied Surface Science 257 (2011) 2801–2805
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Table 1 The structure parameters, grain size, Young’s modulus and internal stress of the ZnO thin films deposited on polyimide substrate at the different substrate position angles. Substrate position angle (◦ )
2 (◦ )
FWHM (◦ )
Grain size (nm)
Young’s modulus (GPa)
Internal stress ×106 (Pa)
−60 −30 0 30 60
34.268 (±0.007) 34.247 (±0.004) 34.232 (±0.008) 34.267 (±0.006) 34.287 (±0.007)
0.277 (±0.012) 0.261 (±0.007) 0.257 (±0.013) 0.277 (±0.009) 0.288 (±0.010)
32.3 (±1.3) 34.5 (±1.0) 35.2 (±1.7) 32.2 (±1.1) 30.9 (±1.4)
63.8 ± 11.8 66.7 ± 6.1 68.0 ± 7.6 63.1 ± 4.6 48.0 ± 3.5
−209.7 (±5.9) −249.1 (±3.5) −275.7 (±7.2) −208.8 (±5.0) −138.1 (±4.4)
Table 2 Thicknesses, Weibull parameters, cohesive strength and adhesive strength of ZnO films on polyimide substrates. Substrate position angle (◦ ) −60 −30 0 30 60
Thicknesses (nm)
˛
ˇ (GPa)
max (lc ) (GPa)
max * (lc ) (GPa)
(MPa)
* (MPa)
lc (m)
307.0 ± 18.1 354.0 ± 22.2 399.6 ± 31.0 339.3 ± 34.8 290.5 ± 26.4
6.45 ± 0.99 12.20 ± 2.30 11.84 ± 2.78 9.56 ± 1.55 12.80 ± 3.23
0.52 ± 0.02 0.58 ± 0.02 0.66 ± 0.02 0.54 ± 0.02 0.38 ± 0.01
0.38 ± 0.01 0.49 ± 0.02 0.55 ± 0.01 0.44 ± 0.02 0.33 ± 0.01
0.24 ± 0.01 0.33 ± 0.02 0.37 ± 0.01 0.30 ± 0.02 0.23 ± 0.01
49.1 ± 1.7 81.3 ± 2.6 85.2 ± 1.2 72.1 ± 2.7 49.9 ± 1.7
31.7 ± 1.7 53.9 ± 2.6 56.7 ± 1.1 49.4 ± 2.7 35.7 ± 1.7
4.7 ± 0.2 4.3 ± 0.3 5.2 ± 0.2 4.2 ± 0.2 3.8 ± 0.2
The cohesive strength of the film could be determined as follows [14]: max (lc ) = ˇ(
lc ) l0
1 −˛
(1 +
1 ) ˛
(5)
where ˛ is the Weibull modulus, ˇ the Weibull scale factor, lc the smallest fragment that can undergo failure and l0 = 1 mm, respectively. The Weibull parameters ˛ and ˇ could be derived from a linear approximation of the initial part of the fragmentation diagram, where the mean fragment size is reported as a function of applied strain, in logarithmic coordinates. lc is related to the crack density at saturation, CDsat , by lc = 2/(1.337 CDsat ) is a constant applied in our cases which is related to the mean crack spacing [15,16]. max (lc ) is the combination of the intrinsic cohesive strength i,max (lc ) and the partially relaxed internal stress f in the fragment of the length lc : max (lc ) = i,max (lc ) −
1.337 f 2
(6)
The shear stress is the sum of the intrinsic shear stress i and the internal shear stress f . = i + f =
2df 1.337 (i,max (lc ) − f ) 2 lc
(7)
Here, the interfacial shear strength is used to characterize the practical adhesion, is about 85 MPa at the position angle of 0◦ , taking into account of the internal stress, i decreased to about 60 MPa. In addition, decreases as the position angle increases (see Table 2).
4. Discussion The dependence of deposition rate on substrate position angle indicates that the distribution of the plasma emitted from the cathode has directivity, and thus the ion flux. The deposition rate could be approximately fitted by a cosine function which indicates that the ion flux also has a cosine distribution. The cosine distribution was also found in other published papers for carbon and titanium nitride [17,18]. Based on the available data of angular distribution of ion kinetic energies for Mg, Cu and Ti [19,20], we assume that the kinetic energy of Zn ion emitted from the cathode is identical along all directions. Then, the ion flux is the main factor that influences the deposition rate of the film. Based on the XRD data, the ion flux that varied with the substrate position angle has little influence on the preferred grain orientation. However, ion flux has an influence on the microstructure of the films. The decrease of the FWHM as the position angle decreases is generally related to the improvement of crystalline of the films and to the grain growth (see Table 1) [21]. The increase of ion flux, which increases the ion bombardment, resulted in the improvement of the microstructure of the films, and thus the crystalline quality [22,23], can account for the decrease of the FWHM. The dependence of the Young’s modulus of the ZnO films on the position angle may be understood also from the improvement of crystalline which enhance the cohesive strength of the ZnO films. Because the Young’s modulus of material increases with its cohesive strength [24], the improvement of crystalline results in the increase of the Young’s modulus.
Fig. 3. Dependence of deposition rate of ZnO thin film on (a) the substrate position angle and (b) cos().
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H. Changji et al. / Applied Surface Science 257 (2011) 2801–2805
Table 3 Thicknesses, Young’s modulus, Weibull parameters, cohesive strength and adhesive strength of MgO films on polyimide substrates. Substrate position angle (◦ ) −60 −30 0 30 60
Thicknesses (nm)
Young’s modulus (GPa)
˛
ˇ (GPa)
max (lc ) (GPa)
i,max (lc ) (GPa)
(MPa)
(MPa)
Internal stress (MPa)
873.6 ± 11.9 954.3 ± 28.2 969.3 ± 8.1 768.3 ± 23.3 689.3 ± 13.3
79.3 ± 7.3 96.0 ± 16.5 93.4 ± 23.5 91.8 ± 9.2 86.8 ± 16.2
4.66 ± 0.74 5.91 ± 1.51 2.44 ± 0.67 2.56 ± 0.60 9.61 ± 0.44
0.52 ± 0.01 0.77 ± 0.01 1.32 ± 0.10 0.88 ± 0.04 0.50 ± 0.01
0.32 ± 0.01 0.53 ± 0.01 0.56 ± 0.04 0.42 ± 0.02 0.40 ± 0.01
0.21 ± 0.01 0.36 ± 0.01 0.45 ± 0.04 0.31 ± 0.02 0.34 ± 0.01
86.1 ± 2.2 160.3 ± 2.4 181.7 ± 12.9 128.0 ± 6.5 105.9 ± 1.0
55.9 ± 1.4 109.2 ± 1.6 146.6 ± 10.4 94.2 ± 4.8 90.4 ± 2.6
−167.9 ± 18.4 −252.6 ± 10.1 −162.0 ± 12.9 −165.9 ± 9.6 −87.8 ± 5.4
In the fragmentation test, ˛ is the Weibull shape parameter (or Weibull modulus) dominating the distribution of failure stresses—a higher value represents a narrower distribution and a higher reliability[16]. The value of ˛ for = 60◦ is higher than those for other position angles, because the strength distribution of thinner films is narrower [16]. The value of lc also confirmed this. However, this is not for the case of = −60◦ , though the thickness is close to that of = 60◦ , the value of ˛ is larger for = −60◦ , which means that the strength distribution is wider for = −60◦ . The difference may be due to the presence of a magnet in our apparatus, which is near the sample at the position angle of −60◦ . The magnetic field changes the strength distribution of film. Furthermore, compared to those for = 60◦ , though the intrinsic cohesive strength is close, the larger value of lc for = −60◦ indicates the lower interfacial shear stress, and thus the adhesive strength. The substrate position angle has great influence on the cohesive strength and adhesive strength of ZnO films on polyimide substrates. Due to the improvement of crystalline quality, the cohesive strength increases as the substrate position angle decreases, thus the adhesive strength. When the substrate position angle increases to −60◦ and 60◦ , the intrinsic interfacial shear strength decreases to 31.7 MPa and 35.7 MPa, respectively, which is much lower than the bulk shear yield strength of polyimide (about 60 MPa). This is an indication of bad adhesive performance [16,25]. It is obviously that high ion flux is beneficial to the adhesive performance. More ion flux means more ion bombardment occurs at the position angle of 0◦ compared to other position angles, which can enhance the adhesion of the films (see Table 2) mainly due to the interfacial mixing caused by ion bombardment [3]. Furthermore, the increase of the substrate temperature may also contribute to the adhesion. According to the diffusion theory of adhesion, the increase of substrate temperature enhances the mutual diffusion between the film and the substrate, and thus the adhesive property [25]. However, difference of the measured substrate temperature was relatively small at the different position angle, indicating that the substrate temperature is not the main factor determining the adhesion. Furthermore, MgO films were deposited on polyimide substrates for comparative test. The detailed results were shown in Table 3.Due to the energy of Mg ions is higher than that of Zn ions in CVAD [26], the interaction of the Mg ions and polymeric substrates, such as sub-implantation, is also stronger than that of ZnO ions, so the intrinsic adhesive strength of MgO films is also larger than that of ZnO films with the same deposition parameters. Even so, the dependence of adhesive strength of MgO films on the substrate position angle has the same trend as that of ZnO films. 5. Conclusion In this study, having investigated the adhesion of ZnO films at several substrate position angles, we found that, at lower position angle, the deposition rate is higher and the crystalline of the films is better. The ion flux has obvious influ-
ence on the adhesion between polyimide substrates and ZnO films. Acknowledgments This work was supported by the Prophase research of the National Basic Research Program of China under Grant No 2006CB708613 and the Natural Science Foundation of Guangdong Province under Grant No 9151027501000039. References [1] I.G. Brown, Cathodic arc deposition of films, Annu. Rev. Mater. Sci. 28 (1998) 243–269. [2] D.M. Sanders, A. Anders, Review of cathodic arc deposition technology at the start of the new millennium, Surf. Coat. Technol. 133–134 (2000) 78–90. [3] O.R. Monteiro, Thin film synthesis by energetic condensation, Annu. Rev. Mater. Res. 31 (2001) 111–137. [4] J. Pelletier, A. Anders, Plasma-based ion implantation and deposition: a review of physics, technology, and applications, IEEE Trans. Plasma Sci. 33 (2005) 1–72. [5] K. Yukimura, M. Sano, T. Teramoto, T. Maruyama, The effect of plasma density on the deposition and ion implantation to the materials with three-dimensional topologies in metal d.c. plasma-based ion implantation, Surf. Coat. Technol. 131 (2000) 98–101. [6] V.N. Zhitomirsky, I. Grimberg, L. Rapoport, R.L. Boxman, N.A. Travitzky, S. Goldsmith, B.Z. Weiss, Bias voltage and incidence angle effects on the structure and properties of vacuum arc deposited TiN coatings, Surf. Coat. Technol. 133–134 (2000) 114–120. [7] M. Ohring Materials Science of Thin Films: Deposition and Structure (New York: Academic). (2006) 764–872. [8] D. Zhu, C. Zheng, Y. Liu, D. Chen, Z. He, L. Wen, W.Y. Cheung, S.P. Wong, Influence of bias voltage on morphology and structure of MgO thin films prepared by cathodic vacuum arc deposition, Surf. Coat. Technol. 201 (2006) 2387– 2391. [9] Y. Leterrier, Y. Wyser, J.-A.E. Månson, A method to measure the adhesion of thin glass coatings on polymer films, J. Adhes. 44 (1994) 213–227. [10] Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Do˘gan, V. Avrutin, S.-J. Cho, H. Morkoc¸, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (2005) 041301. [11] T.-H. Fang, W.-J. Chang, C.-M. Lin, Nanoindentation characterization of ZnO thin films, Mater. Sci. Eng. A. 452–453 (2007) 715–720. [12] Y.-H. Hsu, J. Lin, W.C. Tang, RF sputtered piezoelectric zinc oxide thin film for transducer applications, J. Mater. Sci.: Mater. Electron. 19 (2008) 653–661. [13] A. Kelly, Tensile properties of fibre-reinforced metals: copper/tungsten and copper/molybdenum, J. Mech. Phys. Solids 13 (1965) 329. [14] G. Rochat, Y. Leterrier, P. Fayet, J.-A.E. Månson, Mechanical analysis of ultrathin oxide coatings on polymer substrates in situ in a scanning electron microscope, Thin Solid Films 437 (2003) 204–210. [15] A.C. Kimber, J.G. Keer, On the theoretical average crack spacing in brittle matrix composites containing continuous aligned fibres, J. Mater. Sci. Lett. 1 (1982) 353–354. [16] D.G. Howells, B.M. Henry, Y. Leterrier, J.-A.E. Månson, J. Madocks, H.E. Assender, Mechanical properties of SiOx gas barrier coatings on polyester films, Surf. Coat. Technol. 202 (2008) 3529–3537. [17] D. Drescher, J. Koskinen, H.-J. Scheibe, A. Mensch, A model for particle growth in arc deposited armophous carbon films, Diamond Relat. Mater. 7 (1998) 1375–1380. [18] H. Fuchs, B. Engers, E. Hettkamp, H. Mecke, J. Schultz, Deposition rate and thickness uniformity of thin films deposited by a pulsed cathodic arc process, Surf. Coat. Technol. 142–144 (2001) 655–660. [19] A. Anders, G.Y. Yushkov, Angularly resolved measurements of ion energy of vacuum arc plasmas, Appl. Phys. Lett. 80 (2002) 2457–2459. [20] L. Yang, J. Zou, Z. Cheng, Ion energy measurements in vacuum arc deposition, IEEE Trans. Plasma Sci. 25 (1997) 700–702. [21] M.M.M. Bileka, D.R. McKenziea, R.N. Tarranta, et al., Plasma-based ion implantation utilising a cathodic arc plasma, Surf. Coat. Technol. 156 (2002) 136– 142.
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[25] H. Changji, H. Zhenhui, F. Weichun, Q. Zhong, Influence of deposition pressure on the adhesion of ZnO thin films deposited by cathodic vacuum arc deposition on polyimide foil substrates, J. Phys. D: Appl. Phys. 42 (2009) 185303. [26] G.Y. Yushkov, A. Anders, M.O. Efim, I.G. Brown, Ion velocities in vacuum arc plasmas, J. Appl. Phys. 88 (2000) 5618–5622.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of the substrate position angle on the adhesion of ZnO thin films deposited on polyimide foil substrates Hu Changji a , Zhu Daoyun b , He Zhenhui a,∗ , Fu Weichun c , Zhong Qi c a State Key Laboratory of Optoelectronic Materials and Technologies, Center for Space Technology, and School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, PR China b Experiment Teaching Department, Guangdong University of Technology, Guangzhou, 510006, PR China c China Academy of Spacecraft Technology, Beijing 100094, PR China
a r t i c l e
i n f o
Article history: Received 3 February 2010 Received in revised form 9 October 2010 Accepted 16 October 2010 Available online 23 October 2010 PACS: 52.77.Dq 61.05.cp
a b s t r a c t ZnO thin films, as polymer protection layers against ultraviolet radiation, were deposited on polyimide foil substrates using cathodic vacuum arc deposition technique. X-ray diffraction results showed that all the samples had (0 0 2) preferred orientation and the FWHM decreased as the position angle decreased. A fragmentation test was employed to investigate the influence of substrate position angle on the adhesion of ZnO thin films. It was found that the intrinsic adhesion between the ZnO film and the polyimide substrate is about 60 MPa at the substrate position angle of 0◦ . When the position angle increases to ±60◦ , the value of intrinsic adhesion decreases to about 30 MPa. © 2010 Elsevier B.V. All rights reserved.
Keywords: ZnO thin film Cathodic vacuum arc deposition Adhesion Fragmentation test
1. Introduction Cathodic vacuum arc deposition (CVAD) is a plasma-based technology for the fabrication of films [1]. CVAD has the advantages of a high ionization rate, high bombardment energy and high deposition rate [2]. Energetic ions can enhance the adhesion between film and substrate [3]. This makes it a promising method to deposit oxide thin film on polyimide foil substrates. In addition, similar to PBII&D (plasma-based ion implantation and deposition), CVAD is also a non-line-of-sight process, suitable for deposit coatings on irregular surface [4]. However, due to the non-homogeneous of plasma density around the cathode in the deposition chamber, the ion flux distribution is non-homogeneous. So the interaction between the ions and the substrate could be different with respect to the angular distribution, and thus the properties of the films. Yukimura et al. studied the effect of plasma density on the deposition with three-dimensional topologies in metal d.c. plasmabased ion implantation. They found that the metallic arc has a strong directivity and the thickness of the deposited layer for the
∗ Corresponding author. Tel.: +86 020 84113398. E-mail address: [email protected] (H. Zhenhui). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.10.064
tilted substrate has a sinusoidal relationship to the tilt angle in the range of 0–90◦ [5]. Zhitomirsky et al. studied the incidence angle effects on the structure and surface properties of vacuum arc deposited TiN coatings. They found that there is little influence of incidence angle on the preferred grain orientation, grain size and microhardness for the coatings deposited on the face (normal to the plasma flux) and the side (parallel to the plasma flux) substrate surface for different substrate bias voltage [6]. These studies were focused on the other properties of the thin film except for adhesion. However, good adhesion is essential for whatever final application of the film [7]. Since reports on the adhesion of oxide thin films prepared by CVAD to the polymer substrate are relatively few, in this paper, by employing the fragmentation test, we studied mainly the influence of substrate position angle on the adhesion of ZnO thin films prepared by CVAD. 2. Experimental The CVAD system employed was described in [8]. A watercooled Zn metal target (65 mm in diameter, 99.99% in purity) was used as a cathode. Supplied by Guangzhou Donghao Electrical Insulation Material Co. Ltd., the polyimide foil substrates, with a thickness of 50 m, were cleaned in isopropyl alcohol for 10 min
2802
H. Changji et al. / Applied Surface Science 257 (2011) 2801–2805
was calculated by the division of elongation value and origin length which were measured by the electronic digital indicator with precision up to 0.01 mm. The average fragment size was defined as the inverse of the crack density and could be calculated as follows, 1 = CD = (1 + ε) l
Fig. 1. Schematic diagram of the positions of the substrate holder and the substrates orientation to the cathode.
in an ultrasonic bath, then dried with N2 and finally mounted on a semicircular substrate holder, about 24 cm in radius, and with the center of cathode surface as its center (see Fig. 1 for the top view schematic diagram). The holder was made of iron and no bias voltage was applied. After the chamber was evacuated to about 6.0 × 10−3 Pa, pure argon and oxygen gases were introduced in, with an Ar mass flow rate of 10 sccm and an O2 mass flow rate of about 120 sccm. The deposition pressure was set to 0.9 Pa; the arc current was fixed to 50 A and the deposition time was 10 min for all the samples. The substrates were not intentionally heated during the deposition. The substrate temperatures were measured by calibrated homemade film thermocouples (Cu–Fe72.9 wt%–Cr19.1 wt%–Ni8.0 wt%) with the junction size about 1 mm2 and the film thicknesses around 500 nm. The results showed that the deposition temperatures were about 85◦ C, 92◦ C, 93◦ C, 92◦ C and 91◦ C for the position angles of = −60◦ , −30◦ , 0◦ , 30◦ and 60◦ , respectively. The structural properties of the films were evaluated by X-ray diffraction analysis using an X-ray diffractometer (D/MAX 2200 ˚ VPC, RIGAKU, Jan.) with a Cu K␣ X-ray source (wavelength of 1.54 A). The Young’s modulus of the films was calculated following the classical rule of mixtures [9], EZnO/PI =
(EPI dPI + EZnO dZnO ) (dZnO + dPI )
(1)
where EZnO/PI is Young’s modulus of polyimide substrate coated with ZnO film, EPI is the Young’s modulus of polyimide substrate, EZnO is the Young’s modulus of ZnO film, dPI is the thickness of the polyimide substrate and dZnO is the thickness of ZnO film. The Poisson ratio of ZnO films is assumed to be 0.3, referred to [10,11]. The residual (or internal) stress of the ZnO thin films can be obtained by the following equation [12] f =
2 EZnO (1 + ZnO ) 2tan0
k i=1
kW
Ni
(3)
where, l is the average fragment size, CD is the crack density, ε is the strain, k is the number of micrographs, W is the width of micrograph, N is the number of the cracks, respectively. At each strain interval, the number of observed cracks was counted for at least 10 micrographs. The crack density at saturation was obtained when an increase in strain did not lead to further fragmentation of the film. In our cases, the strains were usually larger than 15%.
3. Results The ZnO films are polycrystalline, belonging to the hexagonal wurtzite structure, with strong c-axis orientation (see Fig. 2). The internal stress, Young’s modulus and structure parameters of the ZnO thin films deposited at different substrate position angles are listed in Table 1. The grain size of all the samples is about 30 nm. All the samples are subjected to the compressive stress. In general, Young’s modulus and the internal stress of ZnO films decrease with the increase of substrate position angle. The dependence of the deposition rate of the films on the position angle is approximately mirror-symmetric. At the position angle of 0◦ , the deposition rate is about 0.67 nm/s, the corresponding thickness of film is about 400 nm. When the position angle varies from 0◦ to 60◦ (or 0◦ to −60◦ ), the deposition rate of the films decrease almost linearly to about 0.5 nm/s, and the corresponding thickness of film is about 300 nm(also see Table 2). Furthermore, the deposition rates approximately agree with a cosine distribution (see Fig. 3). According to the Kelly–Tyson model [13], the interfacial shear strength could be calculated by =
2df max (lc ) lc
(4)
where df is the thickness of the film, and max (lc ) is the cohesive strength of the film.
(2)
where f is the residual stress within the ZnO film, EZnO and ZnO are the Young’s modulus and the Poisson ratio of the ZnO film, respectively; 0 = 17.211 is the Bragg angle under stress free condition and 2 is the difference between the measured 2 0 and 2, which was obtained based on the X-ray diffraction (0 0 2) peak of the ZnO films (see Fig. 2). The thicknesses of the films were measured by employing a stylus surface profiler (AMBios, XP-2). A fragmentation test was carried out to evaluate the adhesion of the film. The films were cut with a gauge 50 mm in length and 10 mm in width, then fixed on a home-made tensile device. The tensile device was placed under an Olympus BX51M optical microscope, and the images of the cracks were taken by a CCD camera mounted onto the microscope. The strain applied to the sample,
Fig. 2. The X-ray diffraction patterns of the ZnO thin films deposited on polyimide foil substrates at different substrate position angles.
H. Changji et al. / Applied Surface Science 257 (2011) 2801–2805
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Table 1 The structure parameters, grain size, Young’s modulus and internal stress of the ZnO thin films deposited on polyimide substrate at the different substrate position angles. Substrate position angle (◦ )
2 (◦ )
FWHM (◦ )
Grain size (nm)
Young’s modulus (GPa)
Internal stress ×106 (Pa)
−60 −30 0 30 60
34.268 (±0.007) 34.247 (±0.004) 34.232 (±0.008) 34.267 (±0.006) 34.287 (±0.007)
0.277 (±0.012) 0.261 (±0.007) 0.257 (±0.013) 0.277 (±0.009) 0.288 (±0.010)
32.3 (±1.3) 34.5 (±1.0) 35.2 (±1.7) 32.2 (±1.1) 30.9 (±1.4)
63.8 ± 11.8 66.7 ± 6.1 68.0 ± 7.6 63.1 ± 4.6 48.0 ± 3.5
−209.7 (±5.9) −249.1 (±3.5) −275.7 (±7.2) −208.8 (±5.0) −138.1 (±4.4)
Table 2 Thicknesses, Weibull parameters, cohesive strength and adhesive strength of ZnO films on polyimide substrates. Substrate position angle (◦ ) −60 −30 0 30 60
Thicknesses (nm)
˛
ˇ (GPa)
max (lc ) (GPa)
max * (lc ) (GPa)
(MPa)
* (MPa)
lc (m)
307.0 ± 18.1 354.0 ± 22.2 399.6 ± 31.0 339.3 ± 34.8 290.5 ± 26.4
6.45 ± 0.99 12.20 ± 2.30 11.84 ± 2.78 9.56 ± 1.55 12.80 ± 3.23
0.52 ± 0.02 0.58 ± 0.02 0.66 ± 0.02 0.54 ± 0.02 0.38 ± 0.01
0.38 ± 0.01 0.49 ± 0.02 0.55 ± 0.01 0.44 ± 0.02 0.33 ± 0.01
0.24 ± 0.01 0.33 ± 0.02 0.37 ± 0.01 0.30 ± 0.02 0.23 ± 0.01
49.1 ± 1.7 81.3 ± 2.6 85.2 ± 1.2 72.1 ± 2.7 49.9 ± 1.7
31.7 ± 1.7 53.9 ± 2.6 56.7 ± 1.1 49.4 ± 2.7 35.7 ± 1.7
4.7 ± 0.2 4.3 ± 0.3 5.2 ± 0.2 4.2 ± 0.2 3.8 ± 0.2
The cohesive strength of the film could be determined as follows [14]: max (lc ) = ˇ(
lc ) l0
1 −˛
(1 +
1 ) ˛
(5)
where ˛ is the Weibull modulus, ˇ the Weibull scale factor, lc the smallest fragment that can undergo failure and l0 = 1 mm, respectively. The Weibull parameters ˛ and ˇ could be derived from a linear approximation of the initial part of the fragmentation diagram, where the mean fragment size is reported as a function of applied strain, in logarithmic coordinates. lc is related to the crack density at saturation, CDsat , by lc = 2/(1.337 CDsat ) is a constant applied in our cases which is related to the mean crack spacing [15,16]. max (lc ) is the combination of the intrinsic cohesive strength i,max (lc ) and the partially relaxed internal stress f in the fragment of the length lc : max (lc ) = i,max (lc ) −
1.337 f 2
(6)
The shear stress is the sum of the intrinsic shear stress i and the internal shear stress f . = i + f =
2df 1.337 (i,max (lc ) − f ) 2 lc
(7)
Here, the interfacial shear strength is used to characterize the practical adhesion, is about 85 MPa at the position angle of 0◦ , taking into account of the internal stress, i decreased to about 60 MPa. In addition, decreases as the position angle increases (see Table 2).
4. Discussion The dependence of deposition rate on substrate position angle indicates that the distribution of the plasma emitted from the cathode has directivity, and thus the ion flux. The deposition rate could be approximately fitted by a cosine function which indicates that the ion flux also has a cosine distribution. The cosine distribution was also found in other published papers for carbon and titanium nitride [17,18]. Based on the available data of angular distribution of ion kinetic energies for Mg, Cu and Ti [19,20], we assume that the kinetic energy of Zn ion emitted from the cathode is identical along all directions. Then, the ion flux is the main factor that influences the deposition rate of the film. Based on the XRD data, the ion flux that varied with the substrate position angle has little influence on the preferred grain orientation. However, ion flux has an influence on the microstructure of the films. The decrease of the FWHM as the position angle decreases is generally related to the improvement of crystalline of the films and to the grain growth (see Table 1) [21]. The increase of ion flux, which increases the ion bombardment, resulted in the improvement of the microstructure of the films, and thus the crystalline quality [22,23], can account for the decrease of the FWHM. The dependence of the Young’s modulus of the ZnO films on the position angle may be understood also from the improvement of crystalline which enhance the cohesive strength of the ZnO films. Because the Young’s modulus of material increases with its cohesive strength [24], the improvement of crystalline results in the increase of the Young’s modulus.
Fig. 3. Dependence of deposition rate of ZnO thin film on (a) the substrate position angle and (b) cos().
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Table 3 Thicknesses, Young’s modulus, Weibull parameters, cohesive strength and adhesive strength of MgO films on polyimide substrates. Substrate position angle (◦ ) −60 −30 0 30 60
Thicknesses (nm)
Young’s modulus (GPa)
˛
ˇ (GPa)
max (lc ) (GPa)
i,max (lc ) (GPa)
(MPa)
(MPa)
Internal stress (MPa)
873.6 ± 11.9 954.3 ± 28.2 969.3 ± 8.1 768.3 ± 23.3 689.3 ± 13.3
79.3 ± 7.3 96.0 ± 16.5 93.4 ± 23.5 91.8 ± 9.2 86.8 ± 16.2
4.66 ± 0.74 5.91 ± 1.51 2.44 ± 0.67 2.56 ± 0.60 9.61 ± 0.44
0.52 ± 0.01 0.77 ± 0.01 1.32 ± 0.10 0.88 ± 0.04 0.50 ± 0.01
0.32 ± 0.01 0.53 ± 0.01 0.56 ± 0.04 0.42 ± 0.02 0.40 ± 0.01
0.21 ± 0.01 0.36 ± 0.01 0.45 ± 0.04 0.31 ± 0.02 0.34 ± 0.01
86.1 ± 2.2 160.3 ± 2.4 181.7 ± 12.9 128.0 ± 6.5 105.9 ± 1.0
55.9 ± 1.4 109.2 ± 1.6 146.6 ± 10.4 94.2 ± 4.8 90.4 ± 2.6
−167.9 ± 18.4 −252.6 ± 10.1 −162.0 ± 12.9 −165.9 ± 9.6 −87.8 ± 5.4
In the fragmentation test, ˛ is the Weibull shape parameter (or Weibull modulus) dominating the distribution of failure stresses—a higher value represents a narrower distribution and a higher reliability[16]. The value of ˛ for = 60◦ is higher than those for other position angles, because the strength distribution of thinner films is narrower [16]. The value of lc also confirmed this. However, this is not for the case of = −60◦ , though the thickness is close to that of = 60◦ , the value of ˛ is larger for = −60◦ , which means that the strength distribution is wider for = −60◦ . The difference may be due to the presence of a magnet in our apparatus, which is near the sample at the position angle of −60◦ . The magnetic field changes the strength distribution of film. Furthermore, compared to those for = 60◦ , though the intrinsic cohesive strength is close, the larger value of lc for = −60◦ indicates the lower interfacial shear stress, and thus the adhesive strength. The substrate position angle has great influence on the cohesive strength and adhesive strength of ZnO films on polyimide substrates. Due to the improvement of crystalline quality, the cohesive strength increases as the substrate position angle decreases, thus the adhesive strength. When the substrate position angle increases to −60◦ and 60◦ , the intrinsic interfacial shear strength decreases to 31.7 MPa and 35.7 MPa, respectively, which is much lower than the bulk shear yield strength of polyimide (about 60 MPa). This is an indication of bad adhesive performance [16,25]. It is obviously that high ion flux is beneficial to the adhesive performance. More ion flux means more ion bombardment occurs at the position angle of 0◦ compared to other position angles, which can enhance the adhesion of the films (see Table 2) mainly due to the interfacial mixing caused by ion bombardment [3]. Furthermore, the increase of the substrate temperature may also contribute to the adhesion. According to the diffusion theory of adhesion, the increase of substrate temperature enhances the mutual diffusion between the film and the substrate, and thus the adhesive property [25]. However, difference of the measured substrate temperature was relatively small at the different position angle, indicating that the substrate temperature is not the main factor determining the adhesion. Furthermore, MgO films were deposited on polyimide substrates for comparative test. The detailed results were shown in Table 3.Due to the energy of Mg ions is higher than that of Zn ions in CVAD [26], the interaction of the Mg ions and polymeric substrates, such as sub-implantation, is also stronger than that of ZnO ions, so the intrinsic adhesive strength of MgO films is also larger than that of ZnO films with the same deposition parameters. Even so, the dependence of adhesive strength of MgO films on the substrate position angle has the same trend as that of ZnO films. 5. Conclusion In this study, having investigated the adhesion of ZnO films at several substrate position angles, we found that, at lower position angle, the deposition rate is higher and the crystalline of the films is better. The ion flux has obvious influ-
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