Nanocrystalline diamond films for surface acoustic wave devices

Surface acoustic wave (SAW) devices are critical components in wireless communication systems. In this paper the microstructural characteristics of NCD films deposited in Ar/H2/CH4 microwave discharges and the potentialities of AlN/NCD/Si layered structures for high frequency SAW device achievement are investigated by both experimental and theoretical approaches. A model describing the SAW propagation in such a structure is developed and used in order to determine the acoustic phase velocity (V) and the electromechanical coupling coefficient (K2). A comparison between the phase velocity estimated from the high frequency characterisation of SAW devices and numerical calculations demonstrates the relevance of the model and shows that acoustic velocity up to 9500 m/s may be reached. According to theoretical results, the best combination of V and K2 is obtained for the IDT/AlN/NCD/Si configuration, khAlN = 3 and khNCD ≥ 4. The paper points out that nanocrystalline diamond is a promising material for high frequency SAW devices, since high-velocity smooth films with the required thickness may be deposited in a few hours and can be used as-grown.