Fabrication of high frequency SAW resonators using AlN/Diamond/Si technology G.F. Iriarte (1,*), J.G. Rodríguez (1), R. Ro (2), R. Lee (2), O. A. Williams (3), D. Araujo (4), M.P. Villar (4), F. Calle(1) (1) ISOM-ETSIT, Polytechnic University of Madrid, Spain; (2) I-Shou University, Taiwan; (3) Fraunhofer IAF, Germany, and Cardiff University, UK; (4) University of Cadiz, Spain (*)E-mail: gonzalo.fuentes@upm.es 1. Background, Motivation and Objective High-frequency surface acoustic wave (SAW) devices are needed more and more as the demand of large volume data transmission is required. In order to increase the frequency we can either decrease the wavelength, which depends on the size of the interdigital transducers (IDTs), or increase the phase velocity, which depends on the material. The first option entails the introduction of e-beam lithography on the manufacturing process. The alternative choice requires a careful selection of a piezoelectric layer and a substrate with a high phase velocity. The AlN/diamond bi-layer system is a perfect combination for high-frequency SAW devices. Diamond is an ideal candidate due to its sound velocity, around 11.000 m/s. Furthermore, diamond shows several advantages for MEMS (micro-electro-mechanical systems) on Si in terms of Young modulus (1300 GPa for diamond and below 200 GPa for Si) or friction coefficient [1] Recent progresses in the growth of diamond by CVD (chemical vapor deposition) allowed to control the incorporation of impurities and dopants [2] and the microstructure (nanocrystalline diamond, NCD and ultra-nano-crystalline diamond UNCD [3]), both affecting the acoustic behavior of the material. This permits the development of devices oriented to biological [4], aggressive [5] or high radiative [6] environments. Thus, the AlN/diamond bilayer structure together with e-beam lithography processing is a perfect combination to increase the operating frequency of surface acoustic wave (SAW) devices for cell phones, satellite services and wireless systems. 2. Statement of Contribution/Methods AlN thin films have been deposited at room temperature using a home-built reactive sputtering system. For process details please refer to [7]. The films were synthesized on top of polished micro and nanocrystalline diamond substrates in order to process high frequency devices. Nanocrystalline diamond (NCD) films are grown by chemical vapor deposition on Si substrates. Nevertheless, the principal disadvantage these NCD films is their high rough surfaces, that degrades the piezoelectric film quality and hence the SAW response. In this work we present different approaches to achieve high frequency SAW devices on NCD/Si substrates, namely: smoothing the AlN surface by different methods or removing the Si substrate to use the back side diamond surface. We also propose herewith a method to wet-etch a small area of the Si substrate and thereafter deposit AlN on the backside of the free-standing diamond. In order to get well oriented AlN, the diamond substrates have to be planarized as the high surface roughness worsens the quality of the sputtered piezoelectric film. Both chemical-mechanical polishing and SF 6 -O 2 reactive ion etching have been used to smooth the NCD. The AlN film has also been grown on the nucleation face of the diamond film once the Si substrate has been etched out (diamond 573 978-1-4577-1252-4/11/$26.00 ©2011 IEEE 2011 IEEE International Ultrasonics Symposium Proceedings 10.1109/ULTSYM.2011.0138