Theoretical and experimental investigation of Lamb waves characteristics in AlN/TiN and AlN/TiN/NCD composite membranes

In this study, we present a theoretical and experimental investigations of the zero order quasi-symmetric (S0) Lamb waves mode propagating in AlN/TiN and AlN/TiN/NCD composite membranes. Theoretical analysis of S0 mode characteristics shows that The AlN/TiN membrane enables to achieve smooth dispersion curves, high velocity up to 10000m/s and electromechanical coupling coefficient K² up to 3.5%. Thanks to the excellent mechanical properties of the Nano-Crystalline Diamond (NCD), the AlN/TiN/NCD membrane exhibits excellent acoustic wave properties: acoustic wave velocity more than 12000m/s for diamond film thickness less than 1μm. An intrinsic K² coefficient up to 5% can be reached for in the zone of maximum phase velocity dispersion. These characteristic can be of great interest for sensors applications. From the experimental point of view, a highly oriented c-axis Aluminum Nitride thin films were successfully grown on metallic TiN buffer layer by low temperature sputtering deposition. One important factor in controlling the quality of AlN was the growth of cubic TiN with (111) as preferential crystal orientation. Different acoustic lamb waves devices were fabricated and characterized confirming the very good piezoelectric activity of the AlN. For the case of AlN/TiN/NCD, the addition of the NCD thin films enhances drastically the mechanical toughness of the structure compared to AlN/TiN membrane.