Growth of sputtered AlN thin film on glass in room temperature

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Growth of sputtered AlN thin film on glass in room temperature. C. K. Lee, F. Placido, S. Cochran and K. J. Kirk. School of Information and Communication ...
Growth of sputtered AlN thin film on glass in room temperature C. K. Lee, F. Placido, S. Cochran and K. J. Kirk School of Information and Communication Technologies, University of Paisley, Paisley, PA1 2BE Abstract - Highly (002) oriented AlN thin film is deposited on glass substrate by RF magnetron sputtering method. The X-ray diffraction shows that the AlN thin film has grown in a preferred (002) orientation but other orientation starts to build up as the thickness increases. The surface morphology of the c-axis texture of the AlN thin film is obtained by scanning electron microscopy. The d33 coefficient of the AlN thin film is measured using piezoresponse microscopy and the result obtained is 3.8pm/V. I. INTRODUCTION AlN thin film has been an important piezoelectric material in applications such as surface acoustic wave device (SAW) and Thin Film Resonators (TFRs) [1]. Due to its high surface acoustic and longitudinal velocity, it is suitable for very high frequency application in wireless networks which have moved rapidly up the spectrum from 500MHz to 6GHz. Many researchers have been able to deposit good columnar structure with highly oriented AlN thin film using deposition methods such as chemical vapour deposition [2], reactive sputtering [3-9], pulsed the laser deposition [10-11] and reactive molecular beam deposition [12]. Very high substrate temperature and mixture of argon and nitrogen gases have been used in the deposition process in order to produce highly oriented AlN thin film. We have been able to deposit crystalline, highly c-axis oriented AlN on glass substrate at ambient temperature in pure nitrogen atmosphere. AlN has a closed packed hexagonal structure and it is known that the preferred growth of AlN thin film is the (002) orientation whereby the caxis normal to the substrate. (002) orientation is the fast growing plane, it is shown that other orientation can be observed as the thickness of the film increases.

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II. EXPERIMENTAL AlN thin film is deposited on glass substrate using Cryo Vacuum Chamber (CVC) r.f. magnetron sputtering machine. A target of Al of purity of 99.999% with a diameter of 8 inches (20.32cm) was used. The target to substrate distance is 24cm. The r.f. power is maintained at 800W, nitrogen gas flow at 10sccm and sputtering pressure at 5mTorr. The substrate is cleaned in the ultrasonic bath with Iso propyl alcohol for 1 hour to remove any impurities on the surface and to improve adhesion. The chamber is pumped down to low pressure region around 10-6 Torr using a cryo pump. Nitrogen gas is pumped into the chamber, the target is pre-sputtered at the same deposition conditions for 10 min with the shutter closed to remove any oxides on the surface. The shutter is opened and the Al target is being sputtered in pure nitrogen gas. AlN thin film was deposited on the glass substrate and no substrate heating is used during the process. Different durations of deposition were done and the sample was characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Piezoresponse Microscope (PFM). The film thickness is measured by the SEM (Hitachi S4100). Because the sputtering is done in pure nitrogen gas without the aid of argon gas which has a much larger atomic mass, the deposition is slow and the rate is 0.14µm-1 – 0.35µm-1. The crystallographic structure is determined by using XRD (Siemens D5000).

III. GROWTH ORIENTATION OF ALN THIN FILM Figure 1 shows the thickness of AlN thin film with respect to duration of deposition. The XRD shows highly (002) oriented structure in all the samples and the intensity is as shown in figure 2. The intensity of (002) peak increases with respect to the thickness of the film.

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A cross-section of a 2.1µm thick AlN thin film is investigated using the SEM as shown figure 4. It is shown that the film is made of good columnar structure [14] and at high magnification, we can see spiral layer from the thin film. The surface morphology is made up of needle-shaped structure.

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Time (hrs) Figure 2: Increases of (002) peak intensity with respect to deposition time.

Figure 3 shows the XRD pattern of AlN thin films grown for different lengths of time on glass substrates. A strong (002) peak is observed for all the films but for film deposited at 5 hours and above, we can see other peaks occurring. It is only possible to detect these peaks at log scale. Visible peaks of (100), (101) and (110) are detected from the XRD pattern when the thickness is increased. Although a nickel filter is in place, it is known that quite a significant amount of Kβ passes through the β-filter and we can observed a growing peak corresponding to the Kβ intensity as shown in figure 3. [13].

Figure 4: SEM image of the AlN thin film shows good columnar structure.

V. PROPERTIES OF ALN THIN FILM AlN is deposited on Al layer using the same sputtering conditions and the XRD pattern shows a highly oriented (002) AlN film. Other peaks such as (102) and (004) are detected as shown in figure 5.

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A small contact (1mm diameter) is deposited on the top as electrode. There are no short-circuit between the top and bottom electrode indicating that there were no pinholes. For measurement, a piezoresponse force microscope (PFM) as shown in figure 6 is used. It is based on an Atomic Force Microscopy (AFM) which is used to measure surface roughness in nano scale, modified to make measurement of small displacements and so to determine the d33 coefficient of the material [15].

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Figure 7: Piezoresponse microscopy result shows that the AlN thin film deposited containing other orientations have a relatively good d33 coefficient.

VI. CONCLUSION These results suggest that it is possible to deposit good (002) oriented AlN thin film without substrate heating which it is an advantage whereby AlN can be deposited on lower temperature tolerant material. It is shown from the x-ray diffraction results that as the thickness of AlN thin film increases, other orientations starts to build up which may cause propagation losses for SAW devices. AlN thin film deposited using the same sputtering condition on Al layer, XRD has shown highly (002) oriented AlN film but other peaks have been detected. Using PFM, a relatively good d33 coefficient is measured, showing a piezoelectric effect in an AlN thin film containing other undesired orientations.

ACKNOWLEDGEMENTS Figure 6: Piezoresponse Force Microscope is used to measure the d33 coefficient of the AlN thin film.

X-cut quartz of d11 = 2.3pm/V is used as a reference piezoelectric material for the PFM. Measurements of d33 of our AlN film are shown in figure 7. The d33 obtained is 3.8pm/V and shows that good piezoelectric material has been deposited [16].

The authors wish to thank Queen’s University, Belfast for providing the PFM measurement. C.K. Lee would like to thank the University of Paisley for the studentship and funding for this research. REFERENCES [1] K. M. Lakin, “Thin film resonators and filters”, IEEE Ultrasonics Symp. Proc., pp. 895 – 906 (1999)

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[2] A. J. Shuskus, T. M. Reeder and E. L. Paradis, “rf-sputtered aluminium nitride films on sapphire”, Applied Physics Letters, vol. 24, (4), pp. 155 – 156 (1974)

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[12] S. Yoshida, S. Misawa, Y. Fujii, H. Hayakawa, S. Gonda, and A. Itoh, “Reactive molecular beam of epitaxy of aluminium nitride”, Journal of Vacuum Science Technology, vol. 16, (4), pp. 4724 – 4728 (1995)

[4] J. A. Ruffner, P. G. Clem, B. A. Tuttle, D. Dimos and D. M. Gonzales, “Effect of substrate composite on the piezoelectric response of reactively sputtered AlN thin films”, Thin Solid Films, 354, pp. 256 – 261 (1993) [5] G. Carlotti, F. S. Hickernell, H. M. Liaw, L. Palmieri, G. Socino and E. Verona, “The elastic constants of sputtered AlN films”, IEEE Ultrasonic Symposium, pp. 353 – 356 (1995)

[13] R. Jenkins and R. L. Snyder, “Introduction to XRay Powder Diffractometry”, John Wiley & Sons. Inc. (1996) [14] F. S. Ohuchi and P. E. Russell, “AlN thin films with controlled crystallographic orientations and their microstructure”, Journal of Vacuum Science Technology A, vol. 5, (4), pp. 1630 - 1634 (1998)

[6] M. Penza, M. F. De Riccardis, L. Mirenghi, M. A. Tagliente and E. Verona, ”Low temperature growth of r.f. reactively planar magnetron-sputtered AlN films”, Thin Solid Films, 259, pp. 154 – 162 (1995)

[15] C. Morros, M.H.Corbett, G. Catalan, J.M. Gregg & R.M. Bowman, "Piezoresponse measurement and imaging of electromechanical PZT and PZN-BT thin films" Ferroelectric Thin Films IX, Materials Research Society Symposium Proceedings 665, CC11.8.1-CC11.8.6 (2001)

[7] I. Ivanov, L. Hultman, K. Jarrendahl, P. Martensson, J. E. Sundgren, B. Hjorvarsson and J. E. Greene, “Growth of epitaxial AlN(0001) on Si(111) by reactive magnetron sputter deposition”, Journals of Applied Physics,vol. 78, (9), pp. 5721 – 5726 (1995)

[16] M. A. Dubois, P. Muralt and L. Sagalowicz, “Aluminium nitride thin films for high frequency applications”, Ferroelectrics, vol. 224, pp. 243-250 (1999)

[8] F. J. Hickernell, R. Xian and F. S. Hickernell, “Statistical modelling for the optimal deposition of sputtered piezoelectric films”, IEEE Trans. Ultrason., Ferroelect., Freq. Contr., pp. 615 – 623 (1997) [9] M. Ishihara, K. Yamamoto, F. Kobai and Y. Koga, “Aluminium nitride thin films prepared by radical-assisted pulsed laser deposition”, Vacuum, 59, pp. 649 – 656 (2000) [10] R.D. Vispute, V. Talyansky, R.P. Sharma, S. Choopun, M. Downes, T. Venkatesan, Y.X. Li, L.G. Salamanca-Riba, A.A. Iliadis, K.A. Jones and J. McGarrity, “Advanced in pulsed laser deposition of nitrides and their integration of oxides”, Applied Surface Science, 127 – 129, pp 431 – 439 (1998)

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