Diamond underlayer microstructure effect on the orientation of AlN piezoelectric layers for high frequency SAW resonators by TEM

Abstract

Due to its mechanical properties, diamond is very attractive as an active material for the fabrication of SAW resonators for high frequency applications. In this work, the synthesis of piezoelectric AlN films by reactive sputtering at room temperature has been optimized on thick diamond layers grown on Si and alumina substrates in order to process high frequency devices. The effect of diamond underlayer microstructure is evaluated by TEM. Two sets of samples are studied, AlN/NCD/Alumina and AlN/MCD/Si. The orientation of the AlN grains is shown to improve with the film thickness and the diamond grain size. For NCD underlayer, the AlN deposited on top is more oriented. Moreover, above 1 μm from the AlN/diamond interface, a high degree of the c-axis orientation (perpendicular to the AlN/diamond interface) is demonstrated even though two different grain lattice orientations are shown to coexist: one with the (21̄1̄0) planes remaining parallel to the TEM-preparation lamella and the other with (011̄0) planes. The AlN/diamond interface is smooth down to the nm-scale.