Dual Band MEMS Directional Acoustic Sensor for Near Resonance Operation

Abstract

In this paper, we report on the design and characterization of a microelectromechanical systems (MEMS) directional sensor inspired by the tympana configuration of the parasitic fly Ormia ochracea. The sensor is meant to be operated at resonance and act as a natural filter for the undesirable frequency bands. By means of breaking the symmetry of a pair of coupled bridged membranes, two independent bending vibrational modes can be excited. The electronic output, obtained by the transduction of the vibration to differential capacitance and then voltage through charge amplifiers, can be manipulated to tailor the frequency response of the sensor. Four different frequency characteristics were demonstrated. The sensor exhibits, at resonance, mechanical sensitivity around 6 μm/Pa and electrical sensitivity around 13 V/Pa. The noise was thoroughly characterized, and it was found that the sensor die, rather than the fundamental vibration, induces the predominant part of the noise. The computed average signal-to-noise (SNR) ratio in the pass band is about 91 dB. This result, in combination with an accurate dipole-like directional response, indicates that this type of directional sensor can be designed to exhibit high SNR and selectable frequency responses demanded by different applications.

Keywords: MEMS sensor; acoustic sensor; bio-inspired; resonant sensors.

Figure 1

Schematic diagrams of two-wing MEMS sensors showing simulated frequency response in three different configurations: (a) symmetric bridge; (b) different wing sizes; (c) asymmetric leg pivot points (offset from the center). The vibration amplitudes are normalized for clarity.

Figure 2

MEMS sensor design: (a) Schematic diagram of the design sensor with major dimensions (all dimensions are in micrometers). (b) Simulated mechanical sensitivity of the tip of the wings for the symmetric bridge and three different offsets. The peaks at higher frequencies are from the left wing (shorter bridge) and the peaks at lower frequencies are from the right wing (longer bridge).

Figure 3

Fabricated MEMS sensor: (a) Optical microscopy image of the Gen 4-2 sensor. Metallized pads and tracks appear brighter than the substrate gray. The numbers on the image represent: ① Wing; ② Bridge; ③ Pivot point; ④ Substrate; ⑤ Capacitive comb fingers. (b) Scanning electron microscope image of the comb finger capacitors.

Figure 4

Sensitivity measurements: (a) Mechanical sensitivity measured using laser vibrometry at each wing tip separately. (b) Electric sensitivity of both wings measured using lock-in amplifiers (blue and red lines). Addition and subtraction of the wings output signals are also shown (black and green line, respectively).

Figure 5

Phase and directionality characterization: (a) Measured phase frequency response for both wings separately (left side scale) and phase difference (right side scale). (b) Directional response measured at the peaks and valley between peaks of the subtraction of the two wing output signals, showing in Figure 4b, solid black line.

Figure 6

Mechanical and electrical noise: (a) Measured intrinsic bending vibration of the sensor in absence of any sound stimulus. (b) Measured electrical noise spectral density of the instrument, readout electronics with a 22 pF fixed capacitor connected to the input of the charge amplifier (readout only) and readout electronics with the MEMS sensor connected. The inset shows a zoom in of the area highlighted by the square, where the vertical scale is linear for better view of the effect.

Figure 7

Schematic diagram of the experimental setups used to perform measurements of mechanical sensitivity, electric sensitivity, and noise. Details in the text.