Structural characterization of highly textured AlN thin films grown on titanium
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lsed direct current (dc) reactive ion beam sputtering system has been used to synthesize highly c-axis oriented aluminum nitride (AlN) thin films on (0002)-oriented 200-nm thin titanium layers deposited on a Si-(111) substrate. After a systematic study of the processing variables, high-quality polycrystalline films with preferred c-axis orientation have been grown successfully on the Ti (0002) layer using an Al target under a N2/(N2 + Ar) ratio of 70%, a 2 mTorr processing pressure, and keeping the temperature of the substrate holder at ambient temperature (no substrate heating). The crystalline quality of the AlN and the underlaying Ti thin films was characterized by high-resolution x-ray diffraction. Best o- full width at half maximum values of the (0002) reflection for 1-mm thin AlN layers are 0.56 . Hence, the AlN layers show a high degree of orientation in the (0002) direction, which directly translates into a high Q value piezoelectric response. Atomic force microscopy measurements were used to study the surface morphology of the Ti layer in an attempt to understand its impact on the quality of the AlN films deposited on top of them. Transmission electron microscopy cross-section analysis has been carried out to investigate the AlN/Ti interface. Our observations reveal the presence of crack-free layers with a smooth surface and extremely low defect density. Even local epitaxy phenomena have been identified at the AlN/Ti interface. The processing conditions used to synthesize AlN layers on Ti at room temperature are efficient in reducing the dislocation density and in-plane residual strain. Such AlN/Ti bilayers can be applied to manufacture novel electroacoustic device structures (such as bulk acoustic wave filters) on silicon substrates in further investigations. I. INTRODUCTION
The demand for cheap jet high performance components operating in the microwave region has increased substantially in recent years, particularly for mobile and wideband communication systems. As a consequence, the thin film electroacoustic technology has attracted substantial interest. Traditionally, electroacoustic devices such as resonators, filters, and delay lines have been fabricated on single crystalline piezoelectric substrates such as quartz, LiNbO3, LiTaO3, etc. For surface acoustic wave (SAW) devices, thin piezoelectric films allow the fabrication of electroacoustic devices on nonpiezoelectric substrates. It has been demonstrated that combining thin piezoelectric films with high acoustic quality substrates, i.e., ZnO/sapphire,1–3 ZnO/diamond,4–10 and AlN/diamond,8,11,12 enables the fabrication of microwave SAW devices using optical lithography. Moreover, it opens up the possibility to intea)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0068
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http://journals.cambridge.org
J. Mater. Res., Vol. 25, No. 3, Mar 2010 Downloaded: 04 Apr 2015
grate these traditionally off-chip devices with standard Si technology. For bulk acoustic wave (BAW) devices, the direct relationship
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