RF filter design using LTCC and thin film BAW technology

SAW technology is commonly used to provide RF front-end selectivity in many mobile phones and radios. Their small size, high rejection and low insertion loss gives this technology a significant advantage over competing approaches. However, although photolithographic advances continue to be made which have allowed SAW devices to operate at frequencies beyond several gigahertz, manufacturing progress has been slow. Also SAW RF power handling decreases with increasing frequency. In contrast thin film bulk acoustic wave (BAW) technology may be quite robust at frequencies up to and beyond 10 GHz. Thin film BAW filters can handle very reasonable power levels without the need for exotic metalization schemes. Thin film BAW resonators have higher Q and lower temperature coefficients than SAW devices using lithium tantalate. In this paper we describe the development of an RF filter based on thin film BAW resonators. Aluminum nitride, stacked reflector type BAW resonators were used, fabricated on a sapphire substrate and using pure aluminum metalization. The effective performance of the BAW resonators was enhanced with integrated inductors and capacitors implemented within a low temperature, co-fired ceramic (LTCC) module. The LTCC module also conveniently functions as a hermetic housing for the BAW die through the use of flip-chip die attach. As a vehicle for this demonstration we selected the CDMA transmitter inter-stage filter application. The main RF requirements are to provide a 60 MHz pass band response at 1880 MHz and to provide 32 dB rejection beginning 20 MHz above the pass band. The complete filter form factor is 10 mm×6 mm ×2 mm and its frequency response achieves the required 3.5 dB maximum insertion loss in the Tx band over the temperature range of -40C to +85C. Although this application does not require the filter to handle "high" input power, we evaluated the lifetime potential for thin film BAW technology using this device by applying 1-3 watts of RF power at the maximum energy dissipation frequency near the upper pass band edge