11E-2 Review and Comparison of Bulk Acoustic Wave FBAR, SMR Technology

Abstract

Since FBAR (free-standing bulk acoustic resonator) and solidly mounted resonator SMR-BAW devices came onto the scene as serious competition to the entrenched SAW technology (for cell phone applications), there has been much speculation and discussion as to which of these two "disruptive" technologies is the superior one. The one fundamental difference between SMR-BAWs and FBARs, is the means by which the acoustic energy is trapped. For FBAR the air/crystal interface on both faces of the resonator ensures that the main mode of interest (the thickness extensional mode, TE1) is appropriately trapped In the SMR-BAW, Bragg reflectors underneath the resonator, effectively trap this mode. This addition of a Bragg reflector will degrade the effective coupling coefficient as well as creating additional Q loss mechanisms. The science of improving Q must focus on the edges of the resonator for FBAR, while for SMR devices, both the edges and the vertical design of the Bragg reflector must be optimized. That said, compared to conventional SAW technology, both BAW and FBAR provide temperature stability, power handling capability, good ESD (electro-static discharge), and -most important-high Q's. One of the key drivers for the success (in terms of high Q and coupling coefficients) of both BAW and FBAR was the exploitation of AlN as the piezoelectric material and high acoustic impedance, electrode materials such as molybdenum, tungsten or ruthenium. These are the technology differentiators that create the inherent superiority of BAW/FBAR over both the conventional SAW technology and the newer and enhanced flavors of SAW. The need for large coupling coefficients and high Q's becomes an important value proposition as one considers the proliferation of new bands and services.