Measurement of effective kt2, q, Rp, Rs vs. Temperature for Mo/AlN FBAR resonators

FBAR resonators are used to build microwave filters. For optimization, k_t² and the motional parameters of the resonators often need to be different than those of the classic piezoelectric resonator with "thin" electrodes. A solution to obtaining parameter adjustment is to provide electrodes of non-negligible acoustic thickness. In this experiment, membrane FBAR resonators are fabricated as a sandwich structure, by RF sputtering. A bottom Molybdenum (Mo) electrode layer 0.02 um to 0.6 um thick is reactively sputtered onto the wafer. An Aluminum Nitride (AlN) piezoelectric layer, 3 microns to 0.5 micron thick, is sputtered on the Mo. A top Mo layer, of the same thickness as the bottom Mo layer, completes the stack. The proportion of Mo/AIN is chosen to hold the parallel resonance frequency constant at 1840 MHz. As the Mo thickness increases, k_t decreases, Q increases, the parallel resistance R_p decreases, and the series resistance R_s increases, all relative to those of the "thin" electrode resonator. To determine the temperature coefficients, FBARS with various Mo/AIN proportions were fabricated. The resulting FBAR die were probed at wafer level with a Cascade 10000 Probe Station and an Agilent 8722C Network Analyzer, controlled by an HPVEE^R program to automatically acquire the s - parameter data vs. frequency. The temperature was varied from ∼20°C to ∼120°C by a heating stage on the Probe Station. To get an accurate temperature at the die site, minimizing the thermal loading of the die by the probes is important. Over this temperature range, the s-parameter data were acquired to determine the temperature coefficients. The temperature coefficient of frequency for the composite resonator lay in the -20 to -40 ppm/°C range. Temperature coefficients of frequency -26 ppm/°C for AlN, and -50 ppm/°C for Mo were inferred.