Sputtered ZnO film on aluminium foils for flexible ultrasonic transducers

Ultrasonics xxx (2014) xxx–xxx

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Sputtered ZnO film on aluminium foils for flexible ultrasonic transducers X.S. Zhou a,b, C. Zhao b, R. Hou b, J. Zhang c, K.J. Kirk b, D. Hutson b, Y.J. Guo d, P.A. Hu c, S.M. Peng a, X.T. Zu d, Y.Q. Fu b,⇑ a

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China Scottish Universities Physics Alliance (SUPA), University of West of Scotland, Paisley PA1 2BE, Scotland, United Kingdom c Key Lab of Microsystem and Microstructure, Harbin Institute of Technology, Ministry of Education, No. 2 YiKuang Street, Harbin 150080, Heilongjiang, PR China d School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, PR China b

a r t i c l e

i n f o

Article history: Received 9 February 2014 Received in revised form 9 May 2014 Accepted 10 May 2014 Available online xxxx Keywords: ZnO film Annealing Flexible ultrasonic transducer Pulse-echo Ultrasonic wave

a b s t r a c t Nanocrystalline ZnO films with both C-axis vertical grown and inclined angled grown were sputterdeposited onto aluminium foils (50 lm thick) and characterised for using as flexible ultrasonic transducers. As-deposited C-axis grown ZnO films were annealed at different temperatures up to 600 °C to enhance film crystallinity and reduce film stress. The C-axis grown ZnO film on the Al foil were bonded onto steel plates, and the pulse-echo tests verified a good performance (with dominant longitudinal waves) of the ultrasonic transducers made from both as-deposited and post-annealed films. Inclined angled ZnO films on the Al foil glued onto steel plates generated mixed shear and longitudinal waves in the pulse-echo test. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction There has been an increased interest recently in development of ultrasonic and acoustic wave devices using high performance piezoelectric thin film materials, such as ZnO and AlN [1–7]. These films have been deposited on various substrates, including metal, ceramic, silicon, or glass to make ultrasonic devices for non-destructive ultrasonic testing (NDT) and acoustic wave devices [8]. ZnO film has also been deposited on flexible substrates, such as polymer film or thin metallic foils for various applications, including flat-panel display [9,10], solar cell [11–13] and also ultrasonic applications [14–17], because the physical flexibility and low cost of these foil materials enable them to be easily made into flexible ultrasonic transducers or acoustic wave devices. This could solve the common problems for the conventional planar ultrasonic transducers for ultrasonic detection, including complex surface, shape or geometries of the components, or limitation of site accessibility. However, deposition and optimization of ZnO films deposited onto thin metallic foils and acoustic performance of the prepared flexible devices have not been widely investigated. The sputter-deposited ZnO films of a few microns generally have large film stress (mostly intrinsic stress) and many defects, ⇑ Corresponding author. Tel.: +44 0 141 8483563. E-mail address: [email protected] (Y.Q. Fu).

which could have significant influences on the properties of the film, such as structure/mechanical stability, film adhesion, as well as the electrical resistance or ultrasonic performance of the device [18]. Post-annealing of the as-deposited ZnO film was commonly applied to reduce the film stress and enhance the coating performance [19,20]. However, there is no report on the post-annealing of ZnO film on Al foils as well as the ultrasonic performance of the post-annealed ZnO/Al foil flexible ultrasonic devices. It is not clear if the large thermal stress generated during thermal annealing could have significant deterioration of performance of the flexible ultrasonic devices. ZnO films normally grow in a hexagonal or wurtzite type crystalline structure and the (0 0 0 2) orientation generally has the lowest surface free energy [21]. Therefore, in the absence of epitaxy between film and substrate, and without any external ion or plasma source, ZnO films generally grow with a (0 0 0 2) texture on many different substrates. However, for many microfluidic or sensing applications in liquid conditions, deposition of inclined films is preferred in order to allow longitudinal wave mode (L-mode) and shear wave modes (S-mode) to be generated on a single device [22–24]. Common methods used to deposit inclined ZnO films by magnetron sputtering include varying the substrate-tilting angle or the angle between the substrate and target [25–27]. In this paper, both (0 0 0 2) oriented ZnO film and inclined films were deposited on Al foils using DC magnetron sputtering. Some of the films were post-annealed at different temperatures

http://dx.doi.org/10.1016/j.ultras.2014.05.006 0041-624X/Ó 2014 Elsevier B.V. All rights reserved.

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up to 600 °C to promote crystallization and reduce film stress. Ultrasonic pulse-echo tests were performed by bonding the ZnO/ Al foil transducer onto steel substrate to form simple but efficient flexible ultrasonic transducers. 2. Experimental ZnO films were deposited onto commercial aluminium foils (with thicknesses of 50 lm) using DC magnetron sputtering from a Zn target (100  300 mm2) at a power of 400 W, an Ar/O2 flow ratio of 50/50 SCCM (Standard Cubic Centimetre per Minute) without intentional substrate heating. Before deposition, the substrates were cleaned ultrasonically in acetone. The substrate to target distance was 20 mm and gas pressure was 2.5 m Torr. During deposition of C-axis orientated ZnO films, the substrate holder was rotated for a uniform deposition. For the C-axis inclined films, the substrate holder was fixed and tilted at an angle of 60 °C along the incident plasma direction using a plate holder. After deposition, some of the films were annealed at temperatures from 300 to 600 °C for one hour in a furnace in air, in order to promote crystallization, reduce film stress and defects. Morphology and crystallinity of the films were characterised using a scanning electron microscope (SEM, Hitachi S4100), X-ray diffraction (XRD, Siemens D5000 Cu Ka, 40 kV/30 mA) and Raman spectroscopy (Thermo Scientific DXR with laser beam wavelength of 532 nm). The film residual stress was simply estimated and compared using Eq. (1) based on XRD peak shift [28]:

rf ¼

Ef D2h 2mf 2 tan ho

ð1Þ

Ef and vf are the Young’s modulus and Poisson ratio (0.35) of the ZnO films, ho is the Bragg angle of stress-free ZnO (2ho  34.422° from XRD JCPDS file 36-1451), h is the diffraction angle, and D2h = 2h–2ho. In order to evaluate the ultrasonic performance of the as-deposited and annealed ZnO films on the Al foils, pulse-echo experiments were performed using a standard JSR DPR 300 Pulser/ Receiver. As a standard procedure, circular silver paste (Electrodag 1415, Agar Scientific Ltd) of 2 mm in diameter was used as the top electrode on surface of ZnO film. For demonstration of ultrasonic performance of the ZnO coated Al foils, the ZnO film on a 50 l thick Al foil were bonded onto a steel plates of 3 mm thick using a couplant (a sodium silicate based adhesive with a solid content of 42% High Temperature L-7 Binder, Heatflo Sealants Ltd.). A pulse of RF signal of 100 V was applied to the device producing an ultrasonic wave which propagated through Al foils and the bonding layer into the steel substrate. The reflected wave from the back wall of the steel substrate was received using the flexible ZnO film transducer, and the electrical signal from the pulse receiver was recorded using an Agilent 54641A oscilloscope. A Short-Time Fourier Transform (STFT) of the retuned signal was performed in order to analyze the wave frequency. 3. Results and discussions SEM observation of the as-deposited C-axis ZnO film of 2.5 lm on the foils is shown in Fig. 1(a). The ZnO film on the Al foil shows apparent vertical columnar structure. After depositing with the ZnO film, the Al foils of 50 l thick show curved deformation due to film stress. Therefore, the films were annealed in order to relieve the film stress. Fig. 1(b and c) shows the SEM image of the surface and cross-section morphologies of the 400 °C annealed ZnO film on the Al foil, showing a compact columnar structure without apparent cracking or delamination. Post annealing at a high temperature up to 600 °C changes both the columnar microstructure and

surface morphology of thin film with significant grain growth and micro-crack formation as shown in Fig. 1(d) and 1(e) [29,30]. After post annealed at different temperatures up to 600 °C, the ZnO coated Al foils was found to curl up due to generation of thermal stress attributed to the large differences in coefficient of thermal expansion (CTE) of ZnO and Al foils. Due to the flexible nature of the Al foil substrate, the Al foils can be easily re-stretched into flat shape without showing apparent film damage such as cracking or delamination, observed from an optical microscopy. Fig. 1(f) shows cross-section morphologies of the ZnO films deposited using the tilting holder, and the film has a tilted columnar angle of 30 ± 1°, which seems much less than that of the substrate tilting angles of 60°. The columnar inclination angles with inclined-angle deposition are approximately between that predicted by the two models of Nieuwenhuizen and Haanstra [52] and Tait et al. [53]. The mobility of the adatoms during tilted C-axis growth is reduced by the oblique deposition angle, and the film formation is controlled by the effects of shadowing and oblique ion impingement [27]. The inclined film could only result in slightly curling up of the Al foils after film deposition, indicating that the film stress is relatively small. Crystalline structure and film stress were evaluated using the XRD analysis. Fig. 2 shows the XRD spectra of the as-deposited, following various post-annealing treatments and as-deposited inclined ZnO films on 50 lm thick Al foils. Clearly, apart from the peak from the Al foil substrates and very weak ZnO peak observed which are corresponding to ZnO (1 0–1 0) orientation, the ZnO film has a strong texture or preferred (0 0 0 2) orientation. Fig. 3(a) shows the (0 0 2) orientation normalized intensity (normalized to Al (2 0 0) peak intensity) as a function of annealing temperature, the normalized intensity of (0 0 0 2) orientation is greatly improved at 500 °C annealing, as previously reported by Phan and Chung [20]. The texture coefficient (TC) values corresponding to the (0 0 0 2) orientation are estimated following equation [31]:

TC ð%Þ ¼ P

ð0 0 0 2Þ peak intensity peak intensity ðobserved in ZnOÞ

ð2Þ

As seen in Fig. 3(b), the TC values of the (0 0 0 2) orientation of the as-deposited are 97.9%, and those of the annealed ZnO films continuously increased with the annealing temperature. There is a significant peak shift to the lower angle compared with the stress-free peak position from data of ZnO powder diffraction (34.422° based on XRD JCPDS file 36-1451), indicating a large compressive stress existed in the ZnO film. Origins of the large compressive stress are mainly from the atomic-peening effect during sputtering-ion bombardment and defects/impurities/ dislocations inside the films [29,32,33]. Fig. 3(c) shows the X-ray diffraction peak angles of the ZnO films with (0 0 0 2) orientation as a function of annealing temperature. It is observed that the (0 0 0 2) peak shifts towards those of the ZnO powder as the annealing temperature increases, which means an increase in the annealing temperature leads to an enhancement in the residual compressive stress in the ZnO film. The as-grown film has a residual stress of about 1.28 GPa (the negative sign indicates that the films are in a state of compressive stress). The calculated stress results are listed in Table 1. Clearly post-annealing results in a significant decrease in film residual stress, the residual stress decrease to 0.25 GPa after cooling from annealing temperature of 600 °C. During the annealing process, the atoms of ZnO films have energy to arrange again and migrate to relative equilibrium positions, which reduces the lattice strain. On the other hand, the thermal expansion coefficient of ZnO is lower than that of Al. The thermal expansion coefficient of aluminium (aAl) is 23.1  106/°C at room temperature [34], and the elastic modulus of aluminium EAl is 69 GPa. The average

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Fig. 1. SEM images ZnO films on 50 l Al foil substrate, (a) cross-section of columnar structured 2.5micron ZnO films, (b) and (c) cross-section and top view of 400 °C annealed 2.5 l film, (d) and (e) cross-section and top view of 600 °C annealed 2.1 l film, (f) cross-section of inclined ZnO films.

during cooling (which is compressive) can be estimated using equation:

rthermal ¼ EAl  DT  ðaAl  aZnO Þ

Fig. 2. XRD analysis results of ZnO film on Al foils annealed at different temperatures and inclined angle sample. (a) Inclined angle-deposited sample, (b) 600 °C, (c) 500 °C, (d) 300 °C, and (e) as deposited.

thermal expansion coefficients (a0? and a0k ) between 27 °C and 627 °C of ZnO hexagonal crystal are 6.70  106/°C and 3.98  106/°C, respectively [35]. Thermal stress in the ZnO film

ð3Þ

The estimated thermal stress values are 363 MPa, 627 MPa and 759 MPa after cooling from temperatures of 300, 500 and 600 °C. The intrinsic stress in the film has been reduced significantly after annealing at 300 °C, and the intrinsic stress goes into a state of tensile when annealing above 500 °C. The potential oxidation of aluminium foil in the chamber at higher annealing temperatures may be another effect on the intrinsic stress in the film. Further work is being done to investigate this effect. The full width at half maximum (FWHM) value of the (0 0 0 2) peak continuously decreased as the annealing temperature was increased as shown in Fig. 3(d). Clearly, the average crystallite size increases significantly with the increase of the annealing temperature [30]. XRD result of the inclined angled film shows that the FWMH of the peaks are much smaller (see Fig. 3(d)), indicating that the crystal sizes increase. This can be explained by that the crystal becomes inclined on the surface, so that the top surface crystals which the XRD generally measured are relatively larger. With increase of inclined angle, the film stress decreases dramatically and shows a low value as the (0 0 0 2) peak is near to the theoretical value of the bulk material (Fig. 3(c)). It is clear that the inclined angle film has a much lower stress (0.43 GPa) than the vertical

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Fig. 3. (a) Variation of normalized XRD peak intensity, (b) (0 0 2)-texture coefficient (TC) values of ZnO films, (c) XRD 2h angle of ZnO films with (0 0 2) orientation and (d) FWHM of the (0 0 2) peak of the ZnO films as a function of annealing temperature.

Table 1 ZnO film stress results and average values of crystallite size estimated from XRD spectra for the ZnO films on Al foils. Samples

Residual stress (GPa)

Thermal stress after annealing (GPa)

Intrinsic stress (GPa)

As-deposited 300 °C 500 °C 600 °C Inclined films

1.28 0.71 0.33 0.25 0.43

– 0.36 0.63 0.76 –

1.28 0.35 0.30 0.51 0.43

aligned films. During deposition, shadowing effect of the oblique incidence of ion impinging results in columnar structures with loose and more porous structures, thus can easily relax the stress generated during bombardment [36]. Fig. 4 shows the Raman analysis results of the as-deposited and annealed ZnO films. The space group of the hexagonal wurzite ZnO belongs to C 46m with two formula units per primitive cell. According to the group theory, single-crystalline ZnO has eight sets of optical phonon modes at U point of the Brillouin zone, classified as 2B1 modes (silent), A1 + E1 polar modes (Raman and infrared active), and 2E2 nonpolar modes (Raman active) [37]. Moreover the A1 and E1 modes both split into longitudinal optical (LO) and transverse optical (TO) phonons [37]. The low-frequency E2 mode is associated with the vibration of the heavy Zn sublattice, while the high frequency E2 mode involves only the oxygen atoms. [38] There are five peaks at 97, 331, 379, 436, 580 cm1 and a

broad, intense peak at 1158 cm1. The two intense, sharp peaks at 99 and 436 cm1 correspond to E2 (low) and E2 (high) firstorder modes, correspondingly. After annealing, the linewidths of E2 (low) and E2 (high) mode both decrease apparently, from about 16.2 cm1 to about 7.1 cm1 for E2 (low) and from 23.4 cm-1 to 12.3 cm1, indicating the increase in grain size of the annealed ZnO films. While the LO phonon peak at 582 cm1 has a frequency between those of A1(LO) and E1(LO) phonons [39,40]. In our measurement, Raman spectrum was recorded in the backscattering geometry with incident light exactly perpendicular to the surface of ZnO films, which means incident light parallel to the C-axis of (0 0 0 2) oriented ZnO films. In this configuration, only the E2 and A1(LO) modes are allowed for the (0 0 0 2) oriented ZnO grains, and meanwhile the A1(TO), E1(TO) and E1(LO) modes are forbidden for the (0 0 0 2) oriented ZnO grains according to the Raman selection rules. So the 580 cm1 LO mode is assigned to A1(LO) mode [38,41]. However, the intensity of A1(LO) mode at 579 cm1 is anomalously high and asymmetric, and the low frequency side is broader than the high frequency one. The anomalously high intensity and asymmetric broad feature of A1(LO) can be attributed to the formation of oxygen deficiency, interstitial Zn, and free carrier [42–44]. This indicates that a zinc-rich film could be formed during sputtering. Strong coupling of free carrier with A1(LO) mode would cause broadening of linewidth, frequency down-shift, and asymmetric line-shape [45]. Raman spectra of the ZnO films as a function of annealing temperature are shown in Fig. 4. A decrease in the A1(LO) mode

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Fig. 5. Pulse-echo tests from the C-axis orientation ZnO/Al foil ultrasonic devices glued to the steel substrates with ZnO film annealed at different temperatures. Fig. 4. Raman scattering spectra of ZnO films on Al foils annealed at different temperatures. (a) As deposited, (b) 400 °C, (c) 500 °C and (d) 600 °C.

intensity relative to the E2 (high) mode intensity is observed with the annealing temperature up to 500 °C. Also, the linewidths of the A1(LO) mode are significantly reduced after annealing, from 30.4 cm1 of as-deposited ZnO film to 21.3 cm1 at 500 °C. The decrease of the A1(LO) mode implies formation of less defect electronic states within the bandgap, which causes a lower free carrier density. The frequencies of ZnO A1(LO) peaks is blue-shifted by about 7.5 cm1 from its original position after annealing as shown in Fig. 4. This clearly indicates that less intrinsic defects are present in the annealed films [46]. The higher ratio of A1(LO) mode intensity relative to the E2 (high) mode intensity and the more asymmetric the low frequency side of A1(LO) peak are observed for the 600 °C annealed film, which may be contributed from the generation of Raman peak of Al2O3 (578 cm1) [37,47]. At higher temperatures, O molecules in air possess such a high kinetic energy that they can overcome the potential barrier of the ZnO film, forming the Al2O3. The peak at 330 cm1 and the broad peak at about 1158 cm1 might be ascribed to second-order Raman processes, which are E2 (high)–E2 (low) and 2LO respectively [38]. A weak peak at 378 cm1 is assigned to A1(TO) at the annealing temperature 400 °C, which might be caused by the displacement of Zn2+ and O2 ions, parallel to the C-axis [48]. The A1(TO) mode is from the crystals with non-C-axis orientations (i.e., the (1 01 0) oriented ZnO grains), which is consistent with the corresponding XRD pattern as shown in Fig. 2. A decrease in the A1(TO) intensity relative to the E2 (high) intensity is observed with increase of the annealing temperature, which agrees with the increase of (0 0 2)-TC values of ZnO films with the annealing temperature. The as-deposited and post-annealed ZnO films on the 50 lm Al foil were used in the pulse-echo tests to evaluate their ultrasonic performance. Fig. 5 shows results of the pulse-echo test results from the flexible ZnO/Al foil ultrasonic devices glued onto the steel substrate using the couplant, in which L1 and L2, etc., are the first and second longitudinal mode (L-mode) echoes from the back wall of the steel plate, and S1, S2, etc., are the shear (S-mode) echoes. The time gaps between two consecutive echo peaks are measured to be 0.96 ls for the longitudinal wave and 1.90 ls for the transverse wave, corresponding to a longitudinal propagation speed of 6105 m/s and a transverse propagation speed of 3135 m/s. Only the first four L echoes are displayed in the A-scans, corresponding to a propagation distance of 24 mm. For the as-deposite ZnO/Al foil

ultrasonic flexible transducer, the longitudinal wave is dominant, which can be explained from the theoretical analysis [14,49]. The signal from this flexible transducer typically consisted of 8 cycles (pulse duration 0.20 ls) with 12 dB attenuation from the 1st to 4th echo (see Table 2). The STFT spectrograms of the waveforms for the flexible transducers with as-deposited ZnO films which were glued to the steel substrates are shown in Fig. 6(a). The Hanning window with 256 points corresponding to 1.28 ls was used [50,51]. For the ultrasonic peaks of the as-deposited ZnO film on the Al foil, the obtained primary wave frequency is about 23 MHz with a 6 dB bandwidth of 13.2 MHz. The other frequency components of the reflected echoes are about 9, 14, 19 and 28 MHz. As we know, the natural thickness mode resonance of the ZnO film devices generally has a frequency in the order of GHz, therefore, the ZnO flexible devices were operated in a below resonance mode. In this operation regime, the ZnO thin film transducer will replicate the electrical excitation signal as a mechanical strain and the frequency content will then be modified by the electrical properties of the ZnO transducer and measurement system. Fig. 5 also shows results of the pulse-echo tests from the ZnO/Al foil ultrasonic devices glued to the steel substrates after the ZnO films were annealed at different temperatures from 400 to 600 °C. Clearly, all the annealed films show strong pulse-echo signals, and the pulse echo signals of the annealed samples show significant changes as a function of temperature. The signal in the annealed ZnO films at 400 °C decreases to 6 cycles with 17 dB attenuation from the 1st to 4th echos and the pulse duration increases to 0.30 ls. Both the longitudinal and shear wave modes could be identified from the waveform of the ZnO film after 400 °C annealed. The amplitude ratio of the first longitudinal to first shear wave echo (L1/S1) is around 7. The generation of the S-mode is supposed to mainly result from the (1 01 0) oriented grains in 400 °C annealed ZnO films. After annealed at 500 °C, the signal lasts 7 cycles with a pulse duration of 0.33 ls and 18 dB attenuation from the 1st to 4th echo. This is understandable as the vertical columnar structures of the ZnO films have not been changed after post-annealing. Since both the columnar microstructure and surface morphology were changed by post annealing at 600 °C. Both the longitudinal (8 cycles) and shear wave modes can be obtained with L1/S1 = 9. The pulse duration of the longitudinal wave mode increases to 0.38 ls with a 12 dB attenuation from the 1st to 4th echo. The STFT spectrogram of the echo waveforms for the transducers with 400–600 °C annealed ZnO are shown in Fig. 6(b–d). When the ZnO films were annealed at

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Table 2 Performance of the flexible transducers pasted onto steel plate. Samples

First echo L pulse cycles (duration)

RT 400 °C 500 °C 600 °C

8cycles 6cycles 7cycles 8cycles

(0.20 ls) (0.30 ls) (0.33 ls) (0.38 ls)

S/N ratio

Primary frequency (MHz)

6 dB bandwidth (MHz)

16 10 22 9

23 25 31 27

13.2 8.8 6.5 7.0

Signal attenuation from 1st to 4th echo (dB) 12 17 18 12

Fig. 6. STFT spectrogram of the echo waveforms for the transducers with (a) as-deposited, (b) 400 °C annealed, (c) 500 °C annealed and (d) 600 °C annealed ZnO films.

Fig. 7. (a) Pulse echo waveform and (b) STFT spectrogram of the curve for the transducer with inclined ZnO films.

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400 °C, the primary frequency of the L-mode was changed to 25 MHz and multiple components of the frequency were observed at 12, 19, 28, 37 and 43 MHz. The frequency of the shear peak was at about 26 MHz. The primary frequency and the ratio of signal to noise (S/N) of the 500 °C annealed sample increase to about 31 MHz and 22, respectively. After annealed at 600 °C, the primary frequency and S/N ratio decrease to about 27 MHz and 9. In general, the improvement of crystallinity and the elimination of the stress in the C-axis ZnO film would increase the S/N ratio, the primary frequency and the signal attenuation, and also decrease the bandwidth. The dramatic increased crystal grains could scatter the ultrasonic wave to modulate the time–frequency feature of the echoes in the film transducers [27], therefore, with the increase of annealing temperature the pulse duration increases as shown in Table 2. Fig. 7(a) shows the echo wave form of the tilted angled film transducer from the pulse echo testing. For the tilted angled film device, the shear mode is dominant with minor longitudinal wave mode (the intensity ratio L1/S1 is about 0.57). Theoretical analysis has shown that ZnO film with 41° tiled C axis crystalline orientation can excite almost a pure shear wave, and the electromechanical coupling constant of ZnO film with 30° tiled C axis crystalline orientation for the longitudinal (L) and the shear (S1) waves are about to 0.13 and 0.34, respectively [49]. The echo peak frequency of both the shear and longitudinal modes are at 29 MHz. Fig. 7(b) is the STFT spectrum of that wave form. The signal from this transducer typically lasts 8 longitudinal cycles, duration 0.22 ls with an intensity of 6 dB bandwidth of 10.8 MHz, as well as a shear mode with a duration of 0.20 ls, an intensity of 6 dB bandwidth of 8.8 MHz. 4. Conclusions ZnO films were deposited at room temperature on aluminium foils in order to fabricate flexible ultrasonic transducers. The films were annealed at different temperatures to enhance the crystallinity and reduce the film stress. As-deposited C-axis grown ZnO films were annealed at different temperatures up to 600 °C to enhance film crystallinity and reduce film stress. The C-axis grown ZnO film on the Al foil were bonded onto steel plates, and the pulse-echo tests verified a good performance (with dominant longitudinal waves) of the ultrasonic transducers made from both the asdeposited and post-annealed films. Inclined angled ZnO films on the Al foil glued onto steel plates generated mixed shear and longitudinal waves in the pulse-echo test. The flexible transducers demonstrated its potentials to be used for ultrasonic inspections of irregular geometries and limited accessibility. Acknowledgement The authors acknowledge support from the Royal SocietyResearch Grant (RG090609), the Scottish Sensing Systems Centre (S3C), Carnegie Trust Funding, Royal Society of Edinburgh, Royal Academy of Engineering-Research Exchange with China, the National Natural Science Foundation of China (Nos. 11304032, 61171038, 61204124, 61274037, 61172001, 21373068, 61390502) and the National Basic Research Program of China (2013CB632900). References [1] I. Katardjiev, V. Yantchev, Recent developments in thin film electro-acoustic technology for biosensor applications, Vacuum 86 (2012) 520–531. [2] G. Wingqvist, AlN-based sputter-deposited shear mode thin film bulk acoustic resonator (FBAR) for biosensor applications – A review, Surf. Coat. Technol. 205 (2010) 1279–1286.

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[3] Y.Q. Fu, J.K. Luo, X.Y. Du, A.J. Flewitt, Y. Li, G.H. Markx, A.J. Walton, W.I. Milne, Recent developments on ZnO films for acoustic wave based bio-sensing and microfluidic applications: a review, Sens. Actuator. B: Chem. 143 (2010) 606– 619. [4] X.Y. Du, Y.Q. Fu, S.C. Tan, J.K. Luo, A.J. Flewitt, W.I. Milne, D.S. Lee, N.M. Park, J. Park, Y.J. Choi, S.H. Kim, S. Maeng, ZnO film thickness effect on surface acoustic wave modes and acoustic streaming, Appl. Phys. Lett. 93 (2008) 094105. [5] I. Voiculescu, A.N. Nordin, Acoustic wave based MEMS devices for biosensing applications, Biosens. Bioelectron. 33 (2012) 1–9. [6] X.Y. Du, Y.Q. Fu, J.K. Luo, A.J. Flewitt, W.I. Milne, Microfluidic pumps employing surface acoustic waves generated in ZnO thin films, J. Appl. Phys. 105 (2009). [7] Y.Q. Fu, L. Garcia-Gancedo, H.F. Pang, S. Porro, Y.W. Gu, J.K. Luo, X.T. Zu, F. Placido, J.I. Wilson, A.J. Flewitt, W.I. Milne, Microfluidics based on ZnO/ nanocrystalline diamond surface acoustic wave devices, Biomicrofluidics 6 (2012) 24105–2410511. [8] R. Hou, D. Hutson, K.J. Kirk, Y. Qing Fu, AlN thin film transducers for high temperature non-destructive testing applications, J. Appl. Phys. 111 (2012). [9] Z.L. Pei, X.B. Zhang, G.P. Zhang, J. Gong, C. Sun, R.F. Huang, L.S. Wen, Transparent conductive ZnO:Al thin films deposited on flexible substrates prepared by direct current magnetron sputtering, Thin Solid Films 497 (2006) 20–23. [10] E. Fortunato, D. Ginley, H. Hosono, D.C. Paine, Transparent conducting oxides for photovoltaics, MRS Bull. 32 (2007) 242–247. [11] J. Huang, Z. Yin, Q. Zheng, Applications of ZnO in organic and hybrid solar cells, Energy Environ. Sci. 4 (2011) 3861–3877. [12] F. Peng, H. Wang, H. Yu, S. Chen, Preparation of aluminum foil-supported nano-sized ZnO thin films and its photocatalytic degradation to phenol under visible light irradiation, Mater. Res. Bull. 41 (2006) 2123–2129. [13] A. Umar, High-yield synthesis and properties of symmetrical comb-like ZnO nanostructures on aluminum foil substrate, J. Nanosci. Nanotechnol. 10 (2010) 2381–2388. [14] V. Rajendran, M. Koike, K.-Y. Hashimoto, M. Ryamaguchi, Lamb wave devices employing ZnO-film/Al-foil composite structure, Jpn. J. Appl. Phys. 31S1 (1992) 216–218. [15] M. Kobayashi, J. Cheng-Kuei, D. Levesque, Flexible ultrasonic transducers, IEEE T. Ultrason. Ferr. 53 (2006) 1478–1486. [16] K.-Y. Hashimoto, H. Suzuki, M. Yamamoto, M. Yamaguchi, Underwater ultrasonic transducers employing ZnO-film/Al-foil composite structure, in: Ultrasonics Symposium, 1989. Proceedings. IEEE vol. 331, 1989, pp. 339–342. [17] H. Jin, J. Zhou, X. He, W. Wang, H. Guo, S. Dong, D. Wang, Y. Xu, J. Geng, J.K. Luo, W.I. Milne, Flexible surface acoustic wave resonators built on disposable plastic film for electronics and lab-on-a-chip applications, Sci. Rep. 3 (2013) 2140. [18] M. Lematre, G. Feuillard, T. Delaunay, M. Lethiecq, Modeling of ultrasonic wave propagation in integrated piezoelectric structures under residual stress, IEEE Transx. Ultrason. Ferr. 53 (2006) 685–696. [19] S.-Y. Chu, W. Water, J.-T. Liaw, Influence of post deposition annealing on the properties of ZnO films prepared by RF magnetron sputtering, J. Eur. Ceram. Soc. 23 (2003) 1593–1598. [20] D.-T. Phan, G.-S. Chung, The effect of post-annealing on surface acoustic wave devices based on ZnO thin films prepared by magnetron sputtering, Appl. Surf. Sci. 257 (2011) 4339–4343. [21] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.-J. Cho, H. Morkoç, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (2005) 041301. [22] A. Fardeheb-Mammeri, M.B. Assouar, O. Elmazria, J.-J. Fundenberger, B. Benyoucef, Growth and characterization of c-axis inclined AlN films for shear wave devices, Semicond. Sci. Technol. 23 (2008) 095013. [23] J. Weber, W.M. Albers, J. Tuppurainen, M. Link, R. Gabl, W. Wersing, M. Schreiter, Shear mode FBARs as highly sensitive liquid biosensors, Sens. Actuator., A 128 (2006) 84–88. [24] T. Yanagitani, M. Kiuchi, Control of in-plane and out-of-plane texture in shear mode piezoelectric ZnO films by ion-beam irradiation, J. Appl. Phys. 102 (2007) 044115. [25] S. Bensmaine, L. Le Brizoual, O. Elmazria, J.J. Fundenberger, B. Benyoucef, Deposition of ZnO inclined c-axis on silicon diamond by r.f. magnetron sputtering, Phys. Status Solid (a) 204 (2007) 3091–3095. [26] H.F. Pang, G.A. Zhang, Y.L. Tang, Y.Q. Fu, L.P. Wang, X.T. Zu, F. Placido, Substrate-tilt angle effect on structural and optical properties of sputtered ZnO film, Appl. Surf. Sci. 259 (2012) 747–753. [27] H.F. Pang, Y.Q. Fu, R. Hou, K.J. Kirk, D. Hutson, X.T. Zu, F. Placido, Annealing effect on the generation of dual mode acoustic waves in inclined ZnO films, Ultrasonics 53 (2013) 1264–1269. [28] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, London, 1976. [29] V. Gupta, A. Mansingh, Influence of postdeposition annealing on the structural and optical properties of sputtered zinc oxide film, J. Appl. Phys. 80 (1996) 1063–1073. [30] Z.B. Fang, Z.J. Yan, Y.S. Tan, X.Q. Liu, Y.Y. Wang, Influence of post-annealing treatment on the structure properties of ZnO films, Appl. Surf. Sci. 241 (2005) 303–308. [31] J.-B. Lee, M.-H. Lee, C.-K. Park, J.-S. Park, Effects of lattice mismatches in ZnO/ substrate structures on the orientations of ZnO films and characteristics of SAW devices, Thin Solid Films 447–448 (2004) 296–301. [32] J.H. Jou, M.Y. Han, D.J. Cheng, Substrate dependent internal stress in sputtered zinc oxide thin films, J. Appl. Phys. 71 (1992) 4333–4336.

Please cite this article in press as: X.S. Zhou et al., Sputtered ZnO film on aluminium foils for flexible ultrasonic transducers, Ultrasonics (2014), http:// dx.doi.org/10.1016/j.ultras.2014.05.006

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X.S. Zhou et al. / Ultrasonics xxx (2014) xxx–xxx

[33] J.A. Thornton, D.W. Hoffman, Stress-related effects in thin films, Thin Solid Films 171 (1989) 5–31. [34] E.C. Richard, R.L. David, L.T. George, AlP Physics Desk Reference, third ed., Springer-Verlag, New York, 2003. [35] H. Iwanaga, A. Kunishige, S. Takeuchi, Anisotropic thermal expansion in wurtzite-type crystals, J. Mater. Sci. 35 (2000) 2451–2454. [36] T. Karabacak, C.R. Picu, J.J. Senkevich, G.-C. Wang, T.-M. Lu, Stress reduction in tungsten films using nanostructured compliant layers, J. Appl. Phys. 96 (2004) 5740–5746. [37] N. Ashkenov, B.N. Mbenkum, C. Bundesmann, V. Riede, M. Lorenz, D. Spemann, E.M. Kaidashev, A. Kasic, M. Schubert, M. Grundmann, G. Wagner, H. Neumann, V. Darakchieva, H. Arwin, B. Monemar, Infrared dielectric functions and phonon modes of high-quality ZnO films, J. Appl. Phys. 93 (2003) 126–133. [38] R. Cuscó, E. Alarcón-Lladó, J. Ibáñez, L. Artús, J. Jiménez, B. Wang, M.J. Callahan, Temperature dependence of Raman scattering in ZnO, Phys. Rev. B 75 (2007) 165202. [39] K.A. Alim, V.A. Fonoberov, A.A. Balandin, Origin of the optical phonon frequency shifts in ZnO quantum dots, Appl. Phys. Lett. 86 (2005) 053103. [40] K.A. Alim, V.A. Fonoberov, M. Shamsa, A.A. Balandin, Micro-Raman investigation of optical phonons in ZnO nanocrystals, J. Appl. Phys. 97 (2005) 124313. [41] Y. Zhang, H. Jia, R. Wang, C. Chen, X. Luo, D. Yu, C. Lee, Low-temperature growth and Raman scattering study of vertically aligned ZnO nanowires on Si substrate, Appl. Phys. Lett. 83 (2003) 4631. [42] G.J. Exarhos, S.K. Sharma, Influence of processing variables on the structure and properties of ZnO films, Thin Solid Films 270 (1995) 27–32. [43] X.L. Xu, S.P. Lau, J.S. Chen, G.Y. Chen, B.K. Tay, Polycrystalline ZnO thin films on Si (1 0 0) deposited by filtered cathodic vacuum arc, J. Cryst. Growth 223 (2001) 201–205.

[44] X. Wang, Q. Li, Z. Liu, J. Zhang, Z. Liu, R. Wang, Low-temperature growth and properties of ZnO nanowires, Appl. Phys. Lett. 84 (2004) 4941. [45] J.N. Zeng, J.K. Low, Z.M. Ren, T. Liew, Y.F. Lu, Effect of deposition conditions on optical and electrical properties of ZnO films prepared by pulsed laser deposition, Appl. Surf. Sci. 197–198 (2002) 362–367. [46] A. Moustaghfir, E. Tomasella, S. Ben Amor, M. Jacquet, J. Cellier, T. Sauvage, Structural and optical studies of ZnO thin films deposited by r.f. magnetron sputtering: influence of annealing, Surf. Coat. Technol. 174–175 (2003) 193– 196. [47] G.W. Cong, H.Y. Wei, P.F. Zhang, W.Q. Peng, J.J. Wu, X.L. Liu, C.M. Jiao, W.G. Hu, Q.S. Zhu, Z.G. Wang, One-step growth of ZnO from film to vertically wellaligned nanorods and the morphology-dependent Raman scattering, Appl. Phys. Lett. 87 (2005) 231903. [48] M. Zerdali, S. Hamzaoui, F.H. Teherani, D. Rogers, Growth of ZnO thin film on SiO2/Si substrate by pulsed laser deposition and study of their physical properties, Mater. Lett. 60 (2006) 504–508. [49] C.K. Jen, K. Sreenivas, M. Sayer, Ultrasonic transducers for simultaneous generation of longitudinal and shear waves, J. Acoust. Soc. Am. 84 (1988) 26– 29. [50] S.-C. Woo, N.S. Goo, Identification of failure mechanisms in a smart composite actuator with a thin sandwiched PZT plate based on waveform and primary frequency analyses, Smart Mater. Struct. 16 (2007) 1460. [51] R. Piotrkowski, A. Gallego, E. Castro, M.T. García-Hernandez, J.E. Ruzzante, Ti and Cr nitride coating/steel adherence assessed by acoustic emission wavelet analysis, NDT E Int. 38 (2005) 260–267. [52] J.M. Nieuwenhuizen, H.B. Haanstra, Microfractography of thin films, Phil. Tech. Rev. 27 (1966) 87–91. [53] R.N. Tait, T. Smy, M.J. Brett, Modelling and characterization of columnar growth in evaporated films, Thin Solid Films 226 (1993) 196–201.

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