Combined assessment of piezoelectric AlN films using X-ray diffraction, infrared absorption and atomic force microscopy J. Olivares ⁎ , S. González-Castilla, M. Clement, A. Sanz-Hervás, L. Vergara, J. Sangrador, E. Iborra Grupo de Microsistemas y Materiales Electrónicos, ETSIT, Universidad Politécnica de Madrid, Ciudad Universitaria, 28040 Madrid, Spain Available online 17 January 2007 Abstract In this work we present the comparative study of sputtered AlN films by X-ray diffraction (XRD), infrared absorption in the reflectance mode (R-IR), atomic force microscopy (AFM) and the measurement of their piezoelectric response. The aim of this comparison is to achieve a good understanding of the information provided by each technique to relate it with the piezoelectric behaviour of the films. A large set of AlN films with different characteristics has been evaluated. We have observed that films with perfect c-axis orientation, as measured by XRD, exhibit an excellent piezoelectric response. The presence of traces of non-0002 reflections in the XRD patterns of some films, indicative of the existence of tilted grains, is related to a significant reduction in their piezoelectric response. In such cases, R-IR and AFM measurements are significantly more sensitive for the detection of tilted grains than conventional XRD measurements. © 2006 Elsevier B.V. All rights reserved. Keywords: Aluminium nitride; Microstructure; Phonons; Surface microscopy 1. Introduction Thanks to their high acoustic velocity [1] and high piezoelectric coefficients (d 31 ∼ 2.65 pm/V and d 33 ∼ 5.53 pm/ V) [2], films of polycrystalline c-axis-oriented AlN are being used as active layers in many piezoelectric devices, such as surface acoustic wave (SAW) [3] and bulk acoustic wave (BAW) [4,5] devices, and microelectromechanical systems (MEMS) [6,7]. Aluminium nitride thin films can be deposited at low temperatures on a large variety of substrates by reactive sputtering, which allows to grow highly oriented polycrystalline films with piezoelectric properties very similar to those of epitaxial films. For this reason, much work has been dedicated in the last decade to study the reactive sputtering of AlN using various sputtering methods such as DC, pulsed DC and RF in diode or magnetron configuration. Most of the piezoelectrically driven devices require AlN films containing a high volume of microcrystals oriented with the c-axis normal to the surface and with the same crystallographic polarity, as this ensures the highest piezoelectric response [8,9]. The crystalline properties of AlN films are very dependent on the deposition conditions and the nature of the substrate; therefore, it would be advisable to control these properties during production in order to guarantee the required perfect c- axis orientation that provides the highest piezo-response. In a previous work [10] we have shown that XRD patterns of apparently well c-axis-oriented AlN films provide signs (almost imperceptible non-0002 peaks) of the existence of critical defects, related to a significant degradation of the piezoelectric response. Further R-IR measurements [11] have clearly revealed the existence of a significant volume of misoriented grains in this kind of films. The existence of these grains might be associated with some kind of defects that induce the growth of grains with opposite piezoelectric polarization reducing the overall piezoelectric response. These defects in the piezoelectric orientation of grains cannot be observed by XRD or R-IR. In this work we have examined the surface of the AlN films using AFM in search of any morphological change that explains the results obtained by XRD and R-IR. We will show that the combination of these three techniques, along with the piezo- response measurements, allows to achieve a more comprehen- sive understanding of the information provided by each technique separately. Diamond & Related Materials 16 (2007) 1421 – 1424 www.elsevier.com/locate/diamond ⁎ Corresponding author. Tel.: +34 913367366x3319; fax: +34 913367216. E-mail address: olivares@etsit.upm.es (J. Olivares). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.11.065