Microfabrication of double proof-mass SOI-based matryoshka-like structures for 3-axis MEMS accelerometers

This work presents a micromachining process that allows the creation of hierarchical, matryoshka-like MEMS structures that can be used for multi-axis sensing. This novel vibration multi-axis MEMS sensor based on the capacitive open-loop operation can be widely deployed in the structural monitoring systems due to its simple fabrication and operating principle. The device is composed by a double proof-mass hierarchical design with separate sets of electrodes for in-plane differential measurements. The operation principle of this multi-axis device relies on the fact that accelerations in the zz direction will induce a change in the overlapping area of the xx and yy sensing electrodes, extracted from the single-ended capacitance measurement, while xx and yy accelerations will yield a differential capacitance change. To sense the direction of zz accelerations (capacitance decrease independently of the direction), out-of-plane parallel-plates were added to the device using suspended metallic membranes. The devices were fabricated through an in-house process using a seven-mask dicing-free MEMS process on a 10 μm-thick SOI wafer. The proposed devices were successfully validated using a two-degrees of freedom (DoF) setup that induces external accelerations in the three-orthogonal axes and reads the resulting output voltage of the device. It then possible to conclude that using the proposed fabrication process, it is possible to successfully produce functional multi-structure SOI-based devices that integrate suspended metallic membranes.