P0-15 Temperature Compensated Bulk Acoustic Wave Resonator and its Predictive 1D Acoustic Tool for RF Filtering

The design of bulk acoustic wave "BAW" resonators should take into account the stringent requirements of thermal effects. In this paper, we propose a thermal model, allowing the reduction of the Temperature Coefficient of Frequency "TCF" with a slightly modified process while retaining a good coupling and quality factor. For a significant reduction of the TCF, a SiO₂ layer is added above the upper electrode. A TCF of +1.5 ppm/degC was obtained at 2.08 GHz. The present accuracy of the ID model is approximately 1 ppm/degC. Most material parameters were extracted by comparison between the modeling and the measurement of BAW resonators with different stacks. To improve the dispersion of the TCF, the uniformity of each layer is introduced into the model. Firstly, the reduction of the top SiO₂ Bragg layer thickness dispersion from 2 % to 0.2 % is achieved, using an ion etching. The compensation of BAW resonators in our process has been improving for PCS diversity standard. This model was used to design a Personal Communications Service "PCS" RX diversity filter (1.93-1.99 GHz). The compensated loaded and unloaded resonators were used in the filtering function and present TCFs between 0.8 and -7 ppm/degC with a coupling factor of approximately 5.2 %.