Smooth and oriented AlN thin films deposited by laser ablation and their application for SAW devices

Aluminum nitride films of up to 1.2 mu m thickness were deposited by excime r laser ablation onto c-, r-, m-, and a-cut sapphire substrates. The films were found to be of (00.1), (11.0), (10.0) and (00.1) orientation on c-, r- , m- and a-cut sapphire substrates, respectively. XRD pole figure measureme nts showed good in-plane alignment of all three sample sets. AFM measuremen ts reveal that all films were atomically smooth in the initial stages of gr owth. Furthermore, for AlN(11.0) on r-cut sapphire the film surface remaine d smooth, with an r.m.s. roughness of about 1 nm up to a film thickness of 1.2 mu m. AlN films with the polar c-axis parallel to the film surface are particularly favourable for SAW device applications, since the c-axis direc tion yields the strongest coupling coefficient. A smooth surface, as for Al N(11.0), is essential to prevent scattering of propagating surface acoustic waves. A SAW r.f. filter was fabricated with interdigital transducers of 5 .6 mu m wavelength on top of a 1.2 mu m AlN(11.0) film on r-cut sapphire. T he resonance frequency of 1.068 GHz corresponds well to theoretical expecta tions for the SAW velocity in this propagation geometry.