Ultrananocrystalline Diamond as Material for Surface Acoustic Wave Devices

Abstract

Diamond is one of the most promising materials for SAW applications due to the highest sound velocity and thermal conductivity. Unfortunately single crystals and epitaxial CVDfilms are expensive and beyond that conventional CVD grown microcrystalline diamond films feature large facet structures with high roughness inapplicable for this application. Ultra-Nanocrystalline diamond (UNCD) films grown in a microwave plasma enhanced chemical vapor deposition (MPECVD) system on Si substrates possess a smooth surface making it an ideal material for SAW applications. Moreover, due to its nanocrystalline structure, the film properties of the UNCD material can be tailored in a wide range to adjust them to the specific needs of a SAW filter. For this task a profound understanding of the growth process of UNCD and the dependency of the film performance from the film properties is needed. In addition, a simple and quick method to characterize the properties of the UNCD films is necessary. Laser-induced SAW pulse method, which is fast and accurate, is demonstrated to measure the mechanical and structural properties of the UNCD films. AFM measurements were done to correlate the SAW pulse method results with the surface roughness of the deposited films. Another advantage of the UNCD films is, that highly C-axis textured aluminum nitride (AlN) films can be grown directly on UNCD films by DC-sputtering. Using this technique a feasibility study for SAW devices has been successfully performed.