Growth of piezoelectric aluminium nitride for layered SAW devices

Abstract

A piezoelectric film combined with a high velocity substrate such as diamond seems very promising for SAW devices operating at high frequencies. In this work, we investigate the optimisation of growth parameters to produce AlN films with required properties for SAW devices: high resistivity, low roughness and good piezoelectricity coupling. AlN films are deposited by reactive DC magnetron sputtering on [100] silicon substrates under various deposition conditions including N/sub 2/ concentration in Ar-N/sub 2/ gas mixture, sputtering pressure (3/spl times/10/sup -3/ to 9/spl times/10/sup -3/ mbar), DC power (100-400 W) and substrate temperature (100 to 600/spl deg/C). The growth duration was modulated to obtain a constant film thickness (2 /spl mu/m) to permit better comparison. X-ray diffraction shows that the AlN films deposited in the range of 60-80% N/sub 2/, 400/spl deg/C, and 6/spl times/10/sup -3/ mbar, exhibit columnar structure textured in [002] orientation corresponding to wurtzite structure with the c-axis oriented perpendicular to the surface. AlN films elaborated in optimum conditions exhibit low surface roughness ( 10/sup 14/ /spl Omega/.cm). The stoichiometric composition determined by energy dispersive X-ray spectroscopy (EDXS) reveals a weak presence of oxygen in the Al/sub 1/N/sub 1/ films. The best compromise is obtained for the sample grown with 75% N/sub 2/. A SAW filter is formed by development of IDT of 32 /spl mu/m wavelength on AlN/Si and AlN/sapphire structures. For AlN film grown with 75% N/sub 2/, frequency responses measured by network analyser show a central frequency of 156.4 MHz and 176 MHz corresponding to phase velocities of 5004.8 and 5632 m/s respectively for silicon and sapphire substrates. Also, we have formed a layered structure AlN/diamond SAW device which does not exhibit an operating frequency due to high surface roughness of the diamond film.