Development of piezoelectric thin film resonator and its impact on future wireless communication systems

The bulk acoustic wave filter composed of piezoelectric thin film resonators has many features superior to those of other small filters such as a surface acoustic wave (SAW) filter and a ceramic filter. As it has no fine structure in its electrode design, it has a high Q factor that leads to low-loss and sharp-cut off characteristics and a high power durability particularly in the high-frequency range. Furthermore, it has the potentiality of integrated devices on a Si substrate. In this paper, we review the recent developments of piezoelectric thin film resonator filters in the world, including our development for mobile communication applications. After describing the feature and history of the piezoelectric thin film resonator filters, our technologies are introduced in focusing on the resonator structures, the piezoelectric thin film and electrode film materials, the cavity structures, the filter structure and its design rules and characteristics, comparing with SAW filters. The competition and coexistence between the piezoelectric thin film resonator filters and the SAW filters are also described. In this paper, we describe the development of a piezoelectric thin film resonator from the standpoint of researchers who have a long experience of SAW filter development.