Low Motional Impedance Bulk Acoustic Resonators Based on Metamaterials

X. Rottenberg, R. Jansen, V. Rochus, B. Figeys, B. Nauwelaers, H.A.C. Tilmans
IMEC, BE

Keywords: MEMS, metamaterials, bulk acoustic resonator

Summary:

This paper presents the implementation of propagating acoustic metamaterials in electrostatically transduced Bulk Acoustic Resonators (BARs) to tune by design their characteristic properties. Novel in this paper is our focus on lowering the observed motional impedance (Rm). This parameter is indeed important to take into account in the design of low phase noise oscillators based on these BARs. We report on the modelling, simulation and experimental verification of the implementation of periodic square patterns of square holes, i.e., vacuum/air inclusions, in order to decrease Rm for the first width extensional resonance mode of electrostatically transduced bar-type BARs. The Rm is proportional to the characteristic acoustic impedance of the resonator material, i.e., √(Eeffeff), where Eeff is the effective Young’s modulus and eff the effective mass density of the resonator material. The larger the proportion of hole area in the equivalent metamaterial is, the lower eff and Eeff are, and the lower Rm is. In the theoretical limit of holes filling the whole area, Eeff, eff and Rm approach zero.