High-frequency surface acoustic wave devices based on AlN/diamond layered structure realized using e-beam lithography

We report in this paper on the study and the realization of surface acoustic wave devices based on an AlN/diamond layered structure intended for the X band (8 GHz). Both x-ray diffraction and transmission electronic microscopy, used for characterization of the structural properties of the AlN/diamond structure, have shown (002) highly oriented sputtered AlN films on free-standing chemical vapor deposition diamond films. Surface roughness of the AlN/diamond structure was measured by atomic force microscopy and showed a very low surface roughness, less than 1 nm. Low surface roughness is very important to reduce the acoustic propagation losses. SAW devices operating in the range of 8 GHz were realized by the combination of the high velocity of the AlN/diamond layered structure and the high lateral resolution obtained using e-beam lithography (EBL). Due to high electrical resistivity of the AlN film, interdigital transducers with submicronic resolution were patterned by an adapted technological EBL process. The analysis of device performances in terms of electromechanical coupling coefficient and temperature stability was carried out and discussed. The dispersion of both parameters as a function of wavelength was experimentally determined, and showed the obtention of an electromechanical coupling coefficient up to 1.4% and a temperature coefficient of frequency varying between 9 and 20 ppm/°C. The dispersion curves of phase velocity were also analyzed and experimental results show a good agreement with theoretical calculations.