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

AIN 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 AIN polycrystalline films have been grown on Si and SiO2 using rf magnetron sputter deposition in an Ar/N-2 gas mixture. The structural properties of the film 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 degreesC, discharge power 350 W, and a gas mixture of 25% Ar and 75% N-2. The films as examined by x-ray diffraction exhibited a columnar structure with a strong (001) texture, and a fall width at half maximum (FWHM) rocking curve of 1.6 degrees. Atomic force microscopy measurements indicated a surface roughness with a rms value of 8 Angstrom. 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)alpha -Al2O3 (sapphire) under the same deposition conditions except the substrate temperature. The films exhibited a (001)AlN//(001)alpha -Al2O3 plane orientation with a (002) rocking curve FWHM value of about 0.4 degrees, showing a relatively good alignment of the c axis. The in-plane orientation was [110]AlN//[120]alpha -Al2O3 corresponding to a rotation of the AIN film of 30 degrees with respect to the (001)alpha -Al2O3 surface. Cross-sectional transmission electron microscopy studies indicated a population of both thread and edge dislocations with decreasing concentrations with film thickness. (C) 2001 American Vacuum Society.