Lithium Niobate Microphone With High SNR Potential

Abstract

A bimorph lithium niobate (LiNbO3, LN) transducer has been proposed as a microphone. Surface electrodes sense the lateral in-plane electric field in thin LN films resulting from out-of-plane deformation due to acoustic pressure. The bimorph is implemented using LN films with opposing polarization, achieved with a wafer bonding approach. The article summarizes our work on an LN microphone using an LN diaphragm with $300~\mu $ m thickness and a quarter-inch diameter. Laser Doppler vibrometer (LDV) measurements are performed using piezoelectric excitation of the diaphragm to characterize mode shapes of the diaphragm. Pitch-catch acoustic measurements are performed in the air using tone burst waveforms. We characterized acoustic sensitivity and noise floor. Lumped element and finite element analysis (FEA) are used to predict the acoustical performance. This is the first work demonstrating LN material in microphone applications. We ultimately envision using a bimorph LN film with $2~\mu $ m thickness in total on the silicon substrate, where an approximately $1\times 1$ mm diaphragm is formed via a backside through wafer etch. A rigorous lumped element model is used to simulate the LN microelectromechanical systems (MEMS) microphone with a 73.58-dB signal-to-noise ratio (SNR). Improving MEMS microphone SNR beyond the current state-of-the-art is challenging. LN microphones may be a viable path.