Hermetically-Encapsulated Unidirectional Accelerometer Contact Microphone for Wearable Applications

We report on a novel wafer-level-packaged accelerometer contact microphone to pick up acoustic vibrations orthogonal to its contact surface without responding to airborne acoustic emissions. The design uses a high-bandwidth low-noise unidirectional capacitive accelerometer with a deformable frame suspending a proof-mass, designed to have larger displacement at the edge to allow for the implementation of shock stops. The sense electrodes use a nano-gap to provide high electrical sensitivity with high stiffness design and nano-gap damping electrodes enable near-critical damping in a low vacuum encapsulation to achieve a high bandwidth above 14 kHz. We demonstrate that the miniature accelerometer contact microphone placed on the throat can record human speech and heart sounds with high fidelity while rejecting surrounding noises.