Piezoelectric Multi-Channel Bilayer Transducer for Sensing and Filtering Ossicular Vibration

Abstract

This paper presents an acoustic transducer for fully implantable cochlear implants (FICIs), which can be implanted on the hearing chain to detect and filter the ambient sound in eight frequency bands between 250 and 6000 Hz. The transducer dimensions are conventional surgery compatible. The structure is formed with 3 × 3 × 0.36 mm active space for each layer and 5.2 mg total active mass excluding packaging. Characterization of the transducer is carried on an artificial membrane whose vibration characteristic is similar to the umbo vibration. On the artificial membrane, piezoelectric transducer generates up to 320.3 mV_pp under 100 dB sound pressure level (SPL) excitation and covers the audible acoustic frequency. The measured signal-to-noise-ratio (SNR) of the channels is up to 84.2 dB. Sound quality of the transducer for fully implantable cochlear implant application is graded with an objective qualification method (PESQ) for the first time in the literature to the best of the knowledge, and scored 3.42/4.5.

Keywords: MEMS; fully implantable cochlear implant; piezoelectric transducer.

Figure 1

Fully implantable cochlear implant concept illustration. a) Illustration of the system on ear anatomy. b) Bilayer transducer and interface electronics. c) Ambient sound. d) Fabricated transducer. e Filtered signal. f) Interface electronics and subblocks. g) Generated stimulation pulses.

Figure 2

Fabrication process flow of the bilayer transducer. a) SOI wafer, b) Thermal oxidation and bottom electrode deposition, c) PLD‐PZT deposition and patterning, d) Bottom electrode patterning, e) Parylene‐C deposition and patterning, f) Top electrode deposition and patterning, g) Thermal oxide patterning by RIE, h) Beam formation by DRIE, i) Backside thermal oxide patterning by RIE, j) Backside structure formation by DRIE, k) Buried oxide removal with RIE, and l) fabricated beam structure.

Figure 3

Acoustic characterization of transducer. a Schematic of the test setup. b Output waveforms of 8‐channel system under 50 to 100 dB SPL excitation in the hearing band. c 8‐channel output characteristic of the transducer under 70 dB SPL excitation. d Vibration characteristic of the artificial membrane and its comparison with the measured vibration amplitudes of the umbo under 80 dB excitation in the literature.

Figure 4

Speech test of the transducer. a Schematic of the test setup. b Time domain input waveform. c Spectrogram of the input signal. d Time domain output waveforms of each channel. e Spectrogram of the regenerated output signal.