Highly miniaturized RF bandpass filter based on thin-film bulk acoustic-wave resonator for 5-GHz-band application

Highly miniaturized RF bandpass filter using a thin-film bulk acoustic wave resonator (TFBAR) is designed and fabricated for 5-GHz-band application. The topology of the fabricated filter is based on a ladder-type configuration that has a common trimming inductor connected concurrently with two shunt TFBARs and the trimming inductor is directly connected to the ground. The role of unit the TFBAR's physical characteristics such as the size and thickness of the TFBAR determine the performance of the TFBAR filter, including the effect of the electrical impedance-matching characteristics of the TFBAR filter. The shape of the fabricated TFBARs are tetragons with unparallel sides and the sizes of resonator are smaller than 70/spl times/70 /spl mu/m/sup 2/ to ensure the ranges of impedance-matched filter performance. The insertion loss and bandwidth of fabricated TFBAR filters are less than 2.8 dB and 160 MHz at 3 dB. The out-of-band rejection is over 30 dB. The actual size of filter is smaller than 700/spl times/600 /spl mu/m/sup 2/, including signal and ground pad sizes.