Performance of polygonal-shaped TFBARs and on-wafer tuning inductors

In this paper, two different shapes of thin film bulk acoustic resonators (TFBARs) are characterized with respect to electrode thickness and overall area, and gold plated on-wafer inductors are employed to tune the TFBAR filter performance. Air-gap type TFBARs are fabricated with aluminum nitride (AlN) as a piezoelectric material and platinum as top and bottom electrodes. An equivalent inductor model is employed for the tuning of fabricated TFBAR bandpass filters, designed based on the modified Butterworth-Van Dyke (MBVD) equivalent circuit. Fabricated inductors revealed inductance of 3 nH and Q factor of about 8 at 2 GHz. It is clearly shown that tuning inductors can enlarge the bandwidth of the TFBAR ladder filters by about 10 MHz and suppress the out-of-band rejection around 10 dB further.