# Coupled D33 Mode-Based High Performing Bio-Inspired Piezoelectric MEMS Directional Microphone
## 4 Results and Discussion
The higher electrical sensitivity results in higher acoustic sensitivity, as the acoustic sensitivity is a linear product of the mechanical and electrical sensitivities. Owing to this fact, two unit cells of regular and proposed D33 modes were simulated by reflecting the piezoelectric circuit model, as presented in Equation (1). With satisfactory simulation results, the unit cells of the regular and proposed D33 modes were simulated by varying the parameters of the IDT structure. The parameters with the highest induced voltage were used in a bio-inspired piezoelectric MEMS directional microphone for the experimental validations. Before fabricating the device, a comprehensive simulation was adopted to understand the effectiveness of the proposed D33 mode. The designed device with N-unit cells of the proposed D33 mode was simulated with the electrical circuit physics of COMSOL. Due to the use of N-unit cells of the proposed D33 mode, the whole diaphragm of the developed directional microphone was covered by them; as a result, the proposed D33 mode resulted in higher electrical sensitivity. The improved electrical sensitivity enhanced the acoustic sensitivity of the developed device. Figure 9: Experimental results at 1 kHz frequency: (**a**) acoustic sensitivity of the microphone when varying the acoustic pressure from 0.02 Pa to 3.56 Pa at 1 kHz frequency, and (**b**) directivity measurements at 1 kHz frequency and 1 Pa of sound pressure. In the experimental validations, the measured sensitivity was found to be in compliance with the simulated sensitivity. Taking this one step-further, the backside of the developed directional microphone was open; as a result, the device's directionality allowed it to be described with the ideal pressure-gradient sensor [13]. Owing to this fact, the developed device showed fully compliant directionality by varying the incidence angle of the incoming sound, in contrast to the ideal pressure-gradient sensor. Moreover, due to the lack of fabrication facilities, the authors had to fabricate the device using multi-user MEMS processes (MUMPs) and validate the uniqueness of the proposed D33 mode using their recommended piezoelectric material (i.e., AlN). To understand how the proposed D33 mode performed with other piezoelectric materials, such as PZT-5H and zinc oxide (ZnO), we simulated the unit cells of regular and proposed D33 modes, as shown in Figure 2a,b, with varying ITD widths from 5 \(\upmu\)m to 60 \(\upmu\)m and spacings between IDTs from 30 \(\upmu\)m to 90 \(\upmu\)m. In these simulations, at first, the AlN was replaced with PZT-5H and then with ZnO. The simulated results are shown in Figure 10a,b, respectively, for the variation of the width of IDTs and the spacings between IDTs. In both results, the unit cells of the proposed D33 mode showed higher performance than that achieved by the unit cells of the regular D33 mode. The PZT-5H based simulations showed worse performance than ZnO, which was due to the ferroelectric properties of PZT as discussed in Section 1. However, the ZnO did not show ferroelectric properties at room temperature. In conclusion, the acoustic functionalities demonstrated in this paper showed substantial improvements compared to the literature regarding the rigid diaphragm and D33 mode-based piezoelectric microphones. The improved characteristics of the developed bio-inspired piezoelectric MEMS directional microphone can be useful for applications n virtual reality for positioning and readjusting cameras. However, to do that, the present study should be extended by using advanced experimental measurements, such as frequency response, noise analysis and the realization of sound source localization, which is proposed as the future work.