P3O-5 High Frequency and High Temperature SAW Devices Based on Nanocrystalline Diamond Film on Langasite Substrate

The new layered structure diamond/langasite is studied for high temperature, high frequency and relatively high electromechanical coupling (K
²
) SAW devices. Theoretical study was performed in order to calculate the evolution, as a function of diamond film thickness, of phase velocity and K
²
of the Rayleigh mode and its higher modes. The calculation results show that phase velocities up to 12000 m/s and up to 9500 m/s are obtained for the mode 2 and mode 1, respectively, while the mode 0 exhibits high K
²
values up to 2.9 %. In order to realize the considered layered structure, first experiments were performed to investigate the nanocrystalline diamond (NCD) films deposition on the non conventional substrate, Langasite. Characterization results show that NCD films with good properties (low grain size, low surface roughness, satisfactory purity) can be grown on Langasite. However, in the considered conditions only an optimal NCD thickness estimated around 1 mum permits to avoid film delamination and cracking when returning to room temperature