Piezoelectric AlN films, 1.36.2 ?m thick, have been grown on bare and metallized Al2O3(0001) substrates by reactive radio-frequency-sputtering technique at 180 °C. The films were uniform, stress-free, highly c-axis oriented normal to the surface, and extremely adhesive to the substrates. Surface acoustic wave (SAW) delay lines, showing harmonic modes with operating frequencies up to about 2.44 GHz, were obtained just using conventional optical lithography at 7.5 ?m linewidth resolution. Four interdigital transducer (IDT)/counter electrode configurations were obtained locating the IDTs either on the AlN free surface or at the Al2O3/AlN interface, with and without an Al thin metal film opposite the IDTs. The temperature induced shift of the fundamental and harmonic operating frequencies of the four configurations was measured at different temperatures in the range from -25 to 70 °C. The first order temperature coefficient of delay (TCD) of the four structures was experimentally evaluated for different film thickness values and for SAWs propagating along and normal the Al2O3 a-axis. Eight AlN thicknesses, i.e., the temperature-compensated points (TCPs), were experimentally estimated at which the TCD is equal to 0 ppm/°C. These TCPs were found to be in good agreement with those theoretically evaluated. The SAW propagation along the four coupling structures was investigated in terms of phase and group velocity, electromechanical coupling coefficient, electrical potential, and IDT capacitance and radiation resistance for different film thickness values and SAW propagation directions. The numerical simulation of the mechanical and electrical behaviors of the coupling structures showed how the electroacoustic transduction efficiency, the IDT directivity, and bandwidth can benefit from having different electrical boundary conditions. The obtained results confirm the AlN feasibility to the implementation of SAW devices for application to gigahertz range, good electromechanical coupling, thermally stable electroacoustic devices.
Analysis of the acoustoelectric behavior of microwave frequency, temperature-compensated AlN-based multilayer coupling configurations
Caliendo C
2008
Abstract
Piezoelectric AlN films, 1.36.2 ?m thick, have been grown on bare and metallized Al2O3(0001) substrates by reactive radio-frequency-sputtering technique at 180 °C. The films were uniform, stress-free, highly c-axis oriented normal to the surface, and extremely adhesive to the substrates. Surface acoustic wave (SAW) delay lines, showing harmonic modes with operating frequencies up to about 2.44 GHz, were obtained just using conventional optical lithography at 7.5 ?m linewidth resolution. Four interdigital transducer (IDT)/counter electrode configurations were obtained locating the IDTs either on the AlN free surface or at the Al2O3/AlN interface, with and without an Al thin metal film opposite the IDTs. The temperature induced shift of the fundamental and harmonic operating frequencies of the four configurations was measured at different temperatures in the range from -25 to 70 °C. The first order temperature coefficient of delay (TCD) of the four structures was experimentally evaluated for different film thickness values and for SAWs propagating along and normal the Al2O3 a-axis. Eight AlN thicknesses, i.e., the temperature-compensated points (TCPs), were experimentally estimated at which the TCD is equal to 0 ppm/°C. These TCPs were found to be in good agreement with those theoretically evaluated. The SAW propagation along the four coupling structures was investigated in terms of phase and group velocity, electromechanical coupling coefficient, electrical potential, and IDT capacitance and radiation resistance for different film thickness values and SAW propagation directions. The numerical simulation of the mechanical and electrical behaviors of the coupling structures showed how the electroacoustic transduction efficiency, the IDT directivity, and bandwidth can benefit from having different electrical boundary conditions. The obtained results confirm the AlN feasibility to the implementation of SAW devices for application to gigahertz range, good electromechanical coupling, thermally stable electroacoustic devices.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
