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In this work, the effect of a diamond nucleation process on freestanding aluminium nitride (AlN)/diamond surface acoustic wave (SAW) device performances was studied. Before diamond deposition, silicon (Si) substrates have been mechanically nucleated, using an ultrasonic vibration table with submicron diamond slurry, and bias-enhanced nucleated (BEN). Freestanding diamond layers obtained on mechanically scratched Si substrates exhibit a surface roughness of R/sub MS/=13 nm, whereas very low surface roughness (as low as R/sub MS//spl les/1 nm) can be achieved on a freestanding BEN diamond layer. Propagation losses have been measured as a function of the operating frequency for the two nucleation techniques. Dispersion curves of phase velocities and electromechanical coupling coefficient (K/sup 2/) were determined experimentally and by calculation as a function of normalized thickness AlN film (kh/sub AlN/=2/spl pi/h/sub AlN///spl lambda/). Experimental results show that the propagation losses strongly depend on the nucleation technique, and that these losses are weakly increased with frequency when the BEN technique is used.