MPACVD diamond films for surface acoustic wave filters

Diamond, due to its high surface acoustic wave (SAW) velocity, offers much attraction for realisation of high frequency SAW filters. To launch elastic waves in solid, the interdigital transducers (IDTs) and a thin layer of suitable piezoelectric material such as ZnO are deposited on diamond film. The most significant requirements of diamond films for SAW filters concern a high resistivity, a high purity, a surface roughness of approximately 10 nm and a large thickness to obtain good characteristics of frequency response (insertion loss, rejection,...). In this work, diamond films deposited on silicon by microwave plasma assisted chemical vapour deposition (MPACVD) in CH4-H-2 gas mixture, are characterised as a function of time deposition and N-2 adjunction in order to determine their capacity to be used as substrates for SAW layered structure filters. The purity, morphology and roughness of the films are characterised by Raman spectroscopy, scanning electron microscopy (SEM), and profilometry. The electrical properties are determined with the four points probe. It is first shown that the films resistance and roughness increase with thickness when the diamond purity, in regard to Raman spectroscopy, remains constant. Secondly, we point out that nitrogen addition in CH4-H-2 gas mixture improves the resistance and the thickness of diamond films while the roughness drastically decreases. These results have a crucial importance for the realisation of good functioning SAW filters. (C) 2001 Elsevier Science B.V. All rights reserved.