Sound source localization by Ormia ochracea inspired low–noise piezoelectric MEMS directional microphone

The single-tone sound source localization (SSL) by majority of fly Ormia ochracea’s ears–inspired directional microphones leaves a limited choice when an application like hearing aid (HA) demands broadband SSL. Here, a piezoelectric MEMS directional microphone using a modified mechanical model of fly’s ear has been presented with primary focus to achieve SSL in most sensitive audio bands to mitigate the constraints of traditional SSL works. In the modified model, two optimized rectangular diaphragms have been pivoted by four optimized torsional beams; while the backside of the whole structure has been etched. As a result, the SSL relative to angular rotation of the incoming sound depicts the cosine dependency as an ideal pressure–gradient sensor. At the same time, the mechanical coupling leads the magnitude difference between two diaphragms which has been accounted as SSL in frequency domain. The idea behind this work has been analytical simulated first, and with the convincing mechanical results, the designed bio–inspired directional microphone (BDM) has been fabricated using commercially available MEMSCAP based on PiezoMUMPS processes. In an anechoic chamber, the fabricated device has been excited in free-field sound, and the SSL at 1 kHz frequency, rocking frequency, bending frequency, and in-between rocking and bending frequencies has been found in full compliance with the given angle of incidence of sound. With the measured inter-aural sensitivity difference (mISD) and directionality, the developed BDM has been demonstrated as a practical SSL device, and the results have been found in a perfect match with the given angle of incidence of sound. Furthermore, to facilitate the SSL in noisy environment, the noise has been optimized in all scopes, like the geometry of the diaphragm, supportive torsional beam, and sensing. As a result, the A-weighted noise of this work has been found less than 23 dBA across the audio bands, and the equivalent-input noise (EIN) has been found to be 25.52 dB SPL at 1 kHz frequency which are the lowest ever reported by a similar device. With the developed SSL in broadband–in addition to the lowest noise–the developed device can be extended in some audio applications like an HA device.


Supplementary
. Designed  Higher sensor noise due to high dielectric loss tangent as shown in Figure 5 (a) in the main article.
Lower sensor noise due to low dielectric loss tangent as shown in Figure 5 (a) in the main article. SSL Outfit to localize 90° incidence of sound.

Fabrication and device parameters:
The designed bio-inspired piezoelectric MEMS directional microphone was fabricated using a commercially available Multi-users MEMS processes (MUMPs) through PiezoMUMPs-an extension of the MEMSCAP Inc [SR_5]. Fig. S1(a) shows the cross-sectional view (not to scale); where AA´ is the cross-section line shown in Fig. S1(a) and Fig. S1(b).
The fabrication starts with an n-type double side polished Silicon-On-Insulator (SOI) wafer which has 150 mm diameter with (100) lattice orientation [SR_5]. The wafer is composed of a 400 μm-thick handling substrate, a 1 μmthick Oxide, and a 10 μm-thick Silicon. Then, the top surface of the Silicon layer was doped using phosphosilicate glass (PSG) layer and annealed at 1050 °C for 1 hour in Argon. Next, the PSG layer was washed out using chemical wet processing. After that, a 0.2 μm thermal oxide was grown on top of the Silicon layer which defined as the Pad Oxide in Fig. S1(a). Then, a 0.5 μm-thick aluminum nitride (AlN) was patterned, wet etched followed by a solvent resist strip. Next, on top of the AlN layer, a 0.02 μm-thick chrome and a 1 μm-thick aluminum were patterned using the liftoff process [SR_5]. Then, the combination of chrome and aluminum was patterned as D33 electrodes which is defined as electrode which shown in Fig. S1 Fig. S1(b). Fig. S1(c) shows the zoomed view of the supportive torsional beam's structure which was used to anchored the diaphragms; where, lt, and wt are the length of each torsional beam, and width of each torsional beam, respectively. Note that, the incorporated torsional beams are identical, thus, only one pair of torsional beams was used to describe it. Fig. S1(d) shows the D33 electrode structure; where el, ew, and es are the length of the main electrodes, width of the main electrodes, and gap of the main electrodes, respectively. In-between the main electrode's gap, the interdigitated electrodes were patterned which is indicated using box in Fig. S1(d). Fig. S1(e) shows the zoomed view of the interdigitated electrodes of the fabricated device; where il, is, and iw are presenting the length, spacing, and width of the interdigitated electrodes, respectively. It is noted that the value of all parameters which are discussed above are listed in Table S1.

Experimental setup:
The extended view of experimental setups which were shown in Figure 4(b) and Figure 6 in the "main article" is shown in Fig. S2. In Fig. S2(a) Further, the signal from each diaphragm was connected to a charge amplifier (SR570, Stanford Research Systems), and then each sensing signal was recorded using a lock-in amplifier (SR830, Stanford Research Systems). Whereas, the sound was generated using a function generator (DS345, Stanford Research Systems). The schematic view of until this point is shown in Figure 6 in the "main article". Further, the experimental setup is shown in Fig. S2(c) was modified to demonstrate the SSL both in frequency domain and azimuth angle. To do that, the authors had to place to the rotational stage on a table in order to apply the sound source in different angles as shown in Fig. S2(c). Moreover, before placing the device, the table was calibrated to control the reflections as much as possible. Then, the sensing signal was directly connected to a lock-in amplifier in order to check the credibility of the developed device without having the amplification. After that, the signal was connected to a data acquisition device (DAQ-6009, National Instruments) to interface with the LabVIEW 2015 version software. In the LabVIEW software, two logic were developed to demonstrate the SSL in frequency domain and azimuth angle simultaneously. Using Fig. S2(c), the SSL in frequency domain and azimuth angle were perfectly achieved which are shown in "SV_2.mp4", and "SV_3.mp4", respectively. where, Kb, T, tanδ, and ω are the Boltzmann constant, room temperature, dielectric loss tangent, and frequency in radian/s, respectively.

A-weighted noise:
The A-weighted broadband noise can be defined as [SR_6],