Ultra-miniature monolithic FBAR filters for wireless applications

This paper presents ultra-miniature monolithic transmit (Tx) filters for 1900 MHz PCS using thin film bulk acoustic resonators (FBAR) technology. Conventional FBAR filters often include multiple tuning inductors, which have prevented the miniaturization and the monolithic integration of the filters. However, the developed filter has a novel circuit configuration using a single integrated inductor to minimize the size. The FBAR block consists of seven FBARs electrically coupled in a ladder topology. A meander type inductor is used for simplicity of fabrication processes and is optimized to increase the receive (Rx) band rejection performance. The filters are hermetically packaged at a wafer-level, which enables us to achieve a size of 1.0 mm × 1.0 mm × 0.7 mm. The packaged filters showed high performance with insertion loss less than 3.3 dB and Rx band rejection over 35 dB.