New capacitive micro-acoustic resonators machined in single-crystal silicon stacked structures

We present a new type of acoustic resonator technology aimed to undoing the technological locks encountered during the realization of capacitive silicon MEMS resonators exploiting true Bulk Acoustic Wave resonances instead of structural ones. The single-crystal silicon resonators are driven through a combination of a static bias and dynamic voltage applied across a 700 nm-thick electrostatic gap parallel to the surface of the resonator substrate. The electrostatic gap cavity is realized by a local over-thickening of the gold layers used in the gold-gold bonding process of a resonant plate from resistive silicon with an external electrode-support structure, also made from resistive silicon. Thus, the electrostatic actuation is actually applied through existing interconnections between the existing conductive layers. This technology aims to the realization of very thin gaps on surfaces in the range of a few square millimeters while avoiding the shallow machining in the silicon that would require an additional technological step. We detail this technology through the process steps and designs implemented during the so-called ORSEPEE R&T project from CNES. Finally, the first characterization results are provided.