Suppression of spurious lateral modes in thickness-excited FBAR resonators

Thin film bulk acoustic wave resonators (FBAR) utilize thickness-excited modes in which the resonant frequency is determined by the thickness of the structure and the wave velocity of the mode used. Unfortunately, other resonant modes also may be excited in the device. Some of these correspond to low-frequency, laterally-excited modes and, although a relatively small amount of the total energy is absorbed by these modes, their harmonics may produce an undesirable response around the fundamental resonance frequency of the desired thickness mode. This work explores various ways of suppressing the spurious effects caused by lateral-excited modes by studying their dependence of the electrode geometry. The origin of the lateral-excited modes is discussed in detail, and the results from a number of different electrode geometries are compared. A new elliptical electrode shape for suppression of spurious modes is developed and demonstrated.