Thin Solid Films 441 (2003) 32–37 0040-6090/03/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090(03)00911-8 Structural, morphological and acoustic properties of AlN thick films sputtered on Si(001) and Si(111) substrates at low temperature C. Caliendo *, P. Imperatori , E. Cianci a, b c Istituto di Acustica, CNR, Via del Fosso del Cavaliere 100, Rome 00133, Italy a Istituto di Struttura della Materia, CNR, Via Salaria km. 29,500, Monterotondo, Rome 00016, Italy b Istituto di Fotonica e Nanotecnologie, CNR, Via Cineto Romano 42, Rome 00156, Italy c Received 15 November 2002; received in revised form 12 May 2003; accepted 10 June 2003 Abstract Polycrystalline AlN thick films were deposited on Si(001) and Si(111) substrates by reactive radio frequency sputtering technique at low temperature. The structure and the morphology of the films were investigated by X-ray diffraction, scanning electron microscopy and atomic force microscopy techniques. These measurements showed that the AlN films were highly c-axis oriented, with low surface roughness. The surface acoustic wave (SAW) properties of the films were investigated: a mean value of 3.8=10 CyN was estimated for the piezoelectric strain constant d ; the phase velocities of SAWs propagating in y12 33 polycrystalline AlNy(001)w110xSi and AlNy(111)w1–10xSi structures, for different film thicknesses, were calculated and found to be in good agreement with the theoretical velocities evaluated for SAWs propagating in single crystal AlNySi structures.  2003 Elsevier B.V. All rights reserved. Keywords: Aluminum nitride; Sputtering; X-ray diffraction; Piezoelectric effects 1. Introduction Aluminum nitride (AlN) exhibits many useful prop- erties such as high electrical resistivity (approx. 10 V 14 cm), wide energy gap (6.2 eV), high acoustic velocity (approx. 5500 mys and 11354 mys for transversal and longitudinal bulk waves, respectively, and 5607 mys for surface waves), high piezoelectricity, high thermal con- ductivity (2.85 W cm K at room temperature), y1 y1 chemical stability, transparency in the visual and infrared regions, low thermal expansion coefficient (4.2=10 y6 K and 5.3=10 K for the direction along and y1 y6 y1 perpendicular to the c-axis, respectively) w1–8x. Among all its properties, the high acoustic velocity makes AlN attractive for potential applications in high-frequency surface and bulk acoustic wave (SAW and BAW) devices to be used in mobile phones, cordless head- phones, alarm and security systems, military equipment, sensors, etc. The increase of the operation frequencies *Corresponding author. Tel.: q39-0649934044; fax: q39- 062066061. E-mail addresses: caliendo@idac.rm.cnr.it (C. Caliendo), imperat@mlib.cnr.it (P. Imperatori). of SAW devices can be achieved using high-resolution lithography techniques andyor high acoustic wave veloc- ity materials. The first solution is highly demanding in terms of fabrication costs and precision; the second one is easier if high-quality fast materials can be grown by standard thin film deposition techniques w9x. In this view, the high acoustic velocity and the high quality of piezoelectric AlN films, sputtered at low temperature, allow the realization of high-frequency acoustic devices onto non-piezoelectric substrates, such as silicon. More- over, the opposite temperature coefficient of delay of Si and AlN allows the realization of zero-temperature- coefficient acoustic devices to be used as sensors and actuators, for which low thermal drift, high sensitivity and high signal-to-noise ratio are demanded w10,11x. In our previous works w12–14x we already studied the growth of AlN films on different substrates using the radio frequency (RF) reactive sputtering technique, focussing on their structural and acoustic properties. In this article we present our latest results on AlN films deposited at low temperature on Si(001) and Si(111) substrates, after the optimisation of the growth process. The crystalline structure and the surface morphology of