Smart ultrasonic sensors systems: Investigations on aluminum nitride thin films for the excitation of high frequency ultrasound

Aluminum nitride is a promising material for the use as a piezoelectric sensor material for resonance frequencies higher than 50 MHz and contains the potential for high frequency phased array application in the future. This work represents the fundamental research on piezoelectric aluminum nitride films with a thickness of up to 10 µm based on a double ring magnetron sputtering process. The deposition process of the aluminum nitride thin film layers on silicon substrates was investigated and optimized regarding their piezoelectric behavior. Therefore a specific test setup and a measuring station were created to characterize the sensors. Large single element transducers were deposited on silicon substrates with aluminum electrodes, using different parameters for the magnetron sputter process, like pressure and bias voltage. Afterwards acoustical measurements were carried out in pulse echo mode up to 500 MHz and the piezoelectric charge constants (d33) were determined. As a result, two parameter sets were found for the sputtering process to obtain an excellent piezoelectric charge constant of about 7.2 pC/N maximum. Additionally the sputtering process with these parameters was used to deposit sensors on various substrate materials and with different electrode sizes.