Low-Frequency Piezoelectric Accelerometer Array for Fully Implantable Cochlear Implants ^†

Abstract

We demonstrate a low-volume, stress-free, piezoelectric micro-electromechanical system (MEMS) cantilever array for fully implantable hearing aids. The 12-element spiral-matrix is sensitive to the lower part of audible frequency range (300–700 Hz) through the proper resonant frequency of the individual spirals tuned by dimensions of the cantilevers. The obtained high Q-factors (117–254) provide high frequency selectivity. The generated open circuit voltage signals could be sufficient for the direct analog conversion of the signals for cochlear multielectrode implants. By comparing different geometries we have also demonstrated that the initial stress, which is derived from silicon-dioxide (SiO₂) and aluminum-nitride (AlN) layers, could be drastically reduced simply by the spiral geometry. The results of vibration measurements have shown a good agreement with the calculated resonant frequencies.

1. Introduction

A fully implantable cochlear implant could offer several advantages for patients suffering severe hearing loss [1]. However, the miniaturization, complexity as well as self-powering, signal processing and cochlear multielectrode system are required from such a fully implantable device [2]. Correspondingly, these requirements are to be fulfilled in order to eliminate the use of external unit, i.e. microphone, sound processor, transmitter and battery, which are used in conventional semi-implantable hearing aids. In this work, instead of the direct detection of the sound (microphone), we demonstrate an accelerometer array, which could be mounted on one of the vibration transmitting bones, like anvil (incus), in the middle ear.

2. Materials and Methods

The demonstrated test device (Figure 1) was fabricated by standard bulk micromachining techniques [3]. Archimedean spiral geometry ensures the reduced device footprint formed by deep-reactive ion etching (DRIE). The uniformity of cantilever thickness is controlled by Si-on-Insulator (SOI) wafer. Aluminum nitride (AlN), as a biocompatible piezoelectric material, was deposited by reactive radio frequency (RF) sputtering from aluminum target. The accomplished sensor is mounted on a printed circuit board (PCB) and then placed on a purpose designed 3D printed chip holder. The acceleration feedback signal is ensured by a miniaturized calibrated accelerometer.

In the presented results (

Table 1. Calculated and measured resonant frequencies, peak output voltages for each piezoelectric cantilever.

Chanel No.	1	2	3	4	5	6	7	8	9	10	11	12
Resonant frequency exp. (Hz)	281	45	365	374	389	408	417	425	497	526	663	672
Resonant frequency calc. (Hz)	283	337	366	348	377	379	424	473	471	543	679	687
Peak voltage (mV)	6.89	8.80	9.60	9.53	5.71	4.47	2.64	5.02	3.40	3.05	4.40	3.14

) sinusoidal excitation was applied at a feedback controlled acceleration of 1 g.

In addition, we have designed a test structure with different number of deposited layers for comparison by a 3D optical surface profiler and shown that the initial stress can be minimized by using spiral-shaped cantilever.

3. Results and Conclusions

In conclusion, the manufactured 2 mm × 2 mm cantilevers can satisfy the size constraint for middle ear implants and can provide a new generation of implantable hearing aids. Moreover, a multi-contact, Fermat-spiral-shaped cantilever array are under construction which may open the way to collect more than one ambient frequency with a single cantilever (Figure 2).