Structural and electroacoustic studies of AlN thin films during low temperature radio frequency sputter deposition

<jats:p>AlN is a material used in a wide variety of applications such as electroacoustic devices, blue diodes, IR windows, thermal conductors, metal–insulator–semiconductor structures, integrated circuit packaging, etc. In this work thin piezoelectric AlN polycrystalline films have been grown on Si and SiO2 using rf magnetron sputter deposition in an Ar/N2 gas mixture. The structural properties of the films have been optimized by varying the deposition parameters, such as process pressure, gas mixture, substrate temperature, discharge power, etc. [K. Tominaga et al., Jpn. J. Appl. Phys., Part 1 35, 4972 (1996); H. Okana et al., ibid. 31, 3446 (1992); K. Kazuya, T. Hanabusa, and K. Tominaga, Thin Solid Films 281-282, 340 (1996)]. It was found that the best film texture was obtained for a particular set of parameters, namely process pressure of 4 mTorr, substrate temperature 350 °C, discharge power 350 W, and a gas mixture of 25% Ar and 75% N2. The films as examined by x-ray diffraction exhibited a columnar structure with a strong (001) texture, and a full width at half maximum (FWHM) rocking curve of 1.6°. Atomic force microscopy measurements indicated a surface roughness with a rms value of 8 Å. Classical nonapodized transversal surface acoustic wave filters operating at a frequency of 534 MHz were fabricated to characterize the electroacoustic properties of the films. The measurements indicated a coupling coefficient of 0.37% and a phase velocity of 4900 m/s. Further, thin epitaxial films were grown on (001)α-Al2O3 (sapphire) under the same deposition conditions except the substrate temperature. The films exhibited a (001)AlN//(001)α-Al2O3 plane orientation with a (002) rocking curve FWHM value of about 0.4°, showing a relatively good alignment of the c axis. The in-plane orientation was [110]AlN//[120]α-Al2O3 corresponding to a rotation of the AlN film of 30° with respect to the (001)α-Al2O3 surface. Cross-sectional transmission electron microscopy studies indicated a population of both thread and edge dislocations with decreasing concentrations with film thickness.</jats:p>