Low temperature aluminum nitride deposition on aluminum by rf reactive sputtering

Abstract

Thin film bulk acoustic resonators formed by sputter deposition of piezoelectric films of aluminum nitride on silicon substrates offer an attractive approach to monolithic integration of high Q passive circuit elements with active devices. Such resonators could provide low cost, high performance filters for the steadily growing number of consumer wireless communication devices. Fabrication of these resonators requires a well-characterized low temperature deposition process for aluminum nitride. This material has been studied for use in sensors and actuators, as well as acoustic wave devices. Radio frequency reactive magnetron sputtering produces high quality films, and provides reasonable deposition rates for fabricating high frequency resonators. This work describes aluminum nitride films deposited by radio frequency reactive magnetron sputtering with power varied from 2 to 6 W/cm2, pressure from 0.5 to 1.5 Pa, and N2 flow from 50% to 100% of the total flow. Measured results include x-ray diffraction spectra and measurements of piezoelectric coefficients. X-ray diffraction measurements indicate highly (002) c-axis oriented films. Piezoelectric coefficients e31 were measured by depositing aluminum nitride capacitors on silicon beams and measuring charge generation as each beam was deflected a known distance. Film quality was found to depend strongly on deposition power, with a smaller dependence on pressure and gas mixture. Deposition at higher pressures is preferred for resonator fabrication due to reduction in stress when compared with low pressure deposition.