Metallization design investigations for graphene as a virtually massless electrode material for 2.1 GHz solidly mounted (BAW-SMR) resonators

The performance characteristic of Bulk Acoustic Wave (BAW) resonators is strongly determined by their mechanical and electrical losses. Extrinsic losses are mainly induced by viscous and ohmic losses of the electrode. The ideal electrode material is a highly conductive and light material, which at first glance might comprise a conflict of objectives. However, the solution lies in graphene-a promising candidate as an alternative electrode material due to its outstanding electrical and mechanical properties and its virtually massless character. In this publication we show that graphene is highly suitable as an electrode material for BAW resonators and, moreover, is able to reduce mechanical losses to a minimum, resulting in a strong enhancement of the quality factor at the parallel resonance frequency. With a variation of the metallization design, which is needed for the graphene contacting, we come to the conclusion that the actual resonating area can be directly adjusted by a metal bar structure. At this juncture, the metal/graphene area ratio reveals an impact on both the viscous losses and the additional resonating modes. These modes can be successfully suppressed with a thin metal bar structure with a bar width of smaller than 6 mm, resulting in improved admittance behavior, more than a triplication of the quality factor at parallel resonance and an increase in the effective coupling coefficient. These findings highlight the advantages of graphene and other 2D conductive materials for alternative electrodes in electroacoustic resonators for radio frequency (RF) applications.