Growth of piezoelectric aluminium nitride for layered saw devices

A piezoelectric film combined with high velocity substrate as diamond, seems very promising for Surface Acoustic Wave (SAW) devices operating at high frequencies. In this work, we investigate on the optimisation of growth parameters to perform AIN films with required properties for SAW devices: high resistivity, low roughness and good piezoelectricity coupling. AIN films are deposited by reactive DC magnetron sputtering on (100) silicon substrate in various deposition conditions including N-2 concentration in Ar-N-2 gaz mixture, sputtering pressure (from 3.10(-3) to 9.10(-3) mbar), DC power (from 100 to 400W) and substrate temperature (from 100 to 600degreesC. The duration of the growth was modulated to obtain a constant film thickness (2mum) to permit a better comparison. X ray diffraction shows that the AIN films deposited in the range of 60-80% N-2,400degreesC, and 6.10' (3) mbar, exhibit columnar structure textured in (002) orientation corresponding to wurtzite structure with the c-axis oriented perpendicular to the surface. AIN films elaborated in optimum conditions exhibit low surface roughness (<5mn) and high electrical resistivity (>10(14) Omega.cm). The stoechiometric composition determined by Energy Dispersive X-ray Spectroscopy (EDXS) reveals a weak presence of oxygen in the Al1N1 films. The best compromise is obtained for the sample elaborated with 75% N-2 The SAW filter is formed by development of IDT of 32 pm wavelength on the AIN/Si and AIN/Sapphire structures. For AIN film grown with 75 %N-2, the frequency responses measured by network analyser show a central frequency of 156,4 MHz and 176 MHz corresponding to a phase velocity of 5004,8 and 5632 m/s respectively for silicon and sapphire substrates. Also, we have performed a layered structure AIN/diamond SAW device which have not exhibit a operating frequency due to high surface roughness of diamond film.