The SAW characteristics of sputtered aluminum nitride on silicon

Epitaxial aluminum nitride (AlN) films grown by chemical vapor deposition (CVD) on sapphire have been used to fabricate high frequency surface acoustic wave (SAW) resonators and filters with near theoretical performance capabilities. The required substrate temperature of above 1000°C is high and incompatible with a typical semiconductor IC processing. An alternative method for the preparation of AlN films at a lower temperature is by reactive sputtering, which can produce highly-oriented fine grain polycrystalline films. Such AlN films are expected to give lower coupling factors and higher propagation losses which could limit their feasibility for SAW applications. We have carried out experiments to evaluate the effect of the material parameters of sputtered AlN on SAW characteristics. AlN films were reactively sputtered on (100) silicon covered with a thin CVD dielectric layer. Arrays of SAW transducer electrodes were patterned on the AlN surface for evaluation of SAW conversion loss and propagation characteristics over the range from 50 MHz to 1.5 GHz. A key material parameter affecting the SAW characteristics was found to be the degree of preferred orientation of the AlN grains as measured by X-ray diffraction from the (002) planes. There was a strong correlation between the diffraction intensity and the oxygen content in the films. The better the oriented the AlN grains, the stronger the SAW response, the higher the SAW phase velocity, and the lower the propagation losses over the entire range of the measured film-thickness to wavelength ratios. Above 500 MHz the propagation losses of the well oriented films followed a frequency squared dependence only slightly higher than the reported values for the best epitaxial films. The coupling factors deduced from the transducer characteristics approach the theoretical values reported for epitaxial AlN films