# Coupled D33 Mode-Based High Performing Bio-Inspired Piezoelectric MEMS Directional Microphone
## 1 Introduction  
Virtual reality (VR) offers a simulated, realistic environment similar to the real world [1]. The multi-projected environment (MPE) is one of the kinds of VR used to visualize simulated scenes or motions of the real world [2]. In MPE and some other VR applications such as realistic teleimmersion, there is always a need to rotate and project cameras for the realistic simulation of a specific scene [3]. Recent studies [3, 4, 5] have suggested the use of stereo and channel configurations of MEMS omnidirectional microphones for thisparticular kind of VR. However, these configurations render the directionality of the incoming sound, and those wavelengths match the inter-distance between the incorporated omnidirectional microphones, which is a fundamental drawback [6]. The MEMS directional microphone inspired by the ears of the fly _Ormia archaea_ offers improved functionalities in all aspects such as directionality and noise [7,8]. The understanding of this fly's hearing mechanism is that two eardrums, known as tympana, are mechanically coupled from the middle [7]. As a result, the ear closest to the sound source senses the incoming sound first and shows a phase difference with the other ear [7,8]. This mechanism does not depend on any particular wavelength of the incoming sound, because this fly localizes the incoming sound using the minute inter-aural time difference (ITD) and inter-aural intensity difference (IID) between the ears [7,8]. Inspired by this fly's hearing mechanism, bio-inspired MEMS directional microphones have been reported on regularly with multiple modifications. Beyond this, sensing modification has drawn significant attention regarding the improvement of the microphone's functionalities at a reduced size. Of the reported sensings, which include optical [9, 10, 11, 12], capacitive [13] and piezoelectric sensing [14, 15, 16], piezoelectric sensing with optimally selected materials and transducer modes has shown good performance, such as a high signal-to-noise ratio (SNR) made of low noise and moderate sensitivity. As previously mentioned, the acoustic sensitivity is the multiplied outcome of the mechanical sensitivity and electrical sensitivity [14]. The mechanical sensitivity forms the ratio of the mechanical deflection to the applied mechanical stimuli [14,17,18]. As the device dimensions are limited in MEMS technology, the enhancement of the acoustic sensitivity does not allow the microphone's dimensions to be increased. As a result, the improvement of the acoustic sensitivity has been regularly reported on using various piezoelectric sensing techniques, such as lead zirconate titanate (PZT) with D31 mode/top-to-bottom electrodes [14], aluminum nitride (AIN) with D31 mode [16] and AlN with D33 mode/inter-digitated electrode (IDTs) [15]. Among them, the thickness of the piezoelectric film limits the performance of the D31 mode regarding its vertical polarization [19, 20, 21]. In contrast, the thickness of the piezoelectric film does not have an impact on the D33 mode; rather, the spacing between the IDTs plays a role in this mode regarding its in-plane polarization [19, 20, 21]. Even though the D33 mode is better in terms of the enhancement of sensing signals, special attention needs to be paid to the selection of the IDT's specifications [22]. For instance, a piezoelectric material which has ferroelectric properties needs poling after deposition for dipole alignment [19,22]. During polarization, charges become trapped in the piezoelectric material near the IDT. Those charges form an opposite internal bias to the aligned dipoles at a higher IDT spacing, which causes signal degradation [22]. Besides piezoelectric materials, the current understanding of the D33 mode implies that it includes two main electrodes (MEs), and in-between them, IDTs are patterned with a user-defined spacing [15]. Moreover, the regular D33 mode is limited to a single span [15,17,19,21,22,23].