Power handling and temperature coefficient studies in FBAR duplexers for the 1900 MHz PCS band

Abstract

Duplexers for 1900 MHz PCS handsets based on FBARS have been realized by micro-machined thin film AlN devices. A major advantage of the FBAR duplexer is a 10-fold reduction in physical volume compared to that of dielectric types. However, since the RF input power to the transmit (Tx) filter remains at levels up to +29 dBm input, the Poynting power density is /spl sim/1 kWatts/cm/sup 2/ with concomitantly large RF strain RF levels. The in-band insertion loss for the entire multi-element Tx ladder filter is/spl sim/3 dB (343 mW dissipated maximum), which results in a volume power dissipation per FBAR/spl sim/1 MWatts/cm/sup 3/. Power densities of this order can lead to 1) frequency shifts due to heating, 2) long term degradation, 3) strain levels approaching the fracture limit of the thin films comprising the FBAR, and 4) thermal destruction of the Tx filter FBARS. We discuss two methods to measure the temperature coefficient of frequency: 1) probing individual FBAR resonators on a hot chuck, or 2) heating packaged duplexers in an oven. The measured resonator frequency temperature coefficient is/spl sim/27 ppm//spl deg/C, while the duplexer Tx response shows a somewhat lower value. Self-heating temperatures can be estimated from this and the observed frequency shift, and were also measured by infrared microscopy. Next, we present preliminary results on the duplexer power handling capabilities, based on a small sample of parts. The Tx devices will withstand up to +36 dBm (4 Watts) input power without destruction. Above this, catastrophic failures can be observed. A scanning electron micrograph example illustrating a catastrophic failure in the duplexer Tx filter will be presented. Finally, we will discuss the effects of high power (+30 to +36 dBm) on duplexer performance. At present we do NOT observe any long term, cumulative effects which could lead to catastrophic failures. Our observations support a model in which device characteristics shift slightly with time.