Spiral-Shaped Piezoelectric MEMS Cantilever Array for Fully Implantable Hearing Systems

Abstract

Fully implantable, self-powered hearing aids with no external unit could significantly increase the life quality of patients suffering severe hearing loss. This highly demanding concept, however, requires a strongly miniaturized device which is fully implantable in the middle/inner ear and includes the following components: frequency selective microphone or accelerometer, energy harvesting device, speech processor, and cochlear multielectrode. Here we demonstrate a low volume, piezoelectric micro-electromechanical system (MEMS) cantilever array which is sensitive, even in the lower part of the voice frequency range (300⁻700 Hz). The test array consisting of 16 cantilevers has been fabricated by standard bulk micromachining using a Si-on-Insulator (SOI) wafer and aluminum nitride (AlN) as a complementary metal-oxide-semiconductor (CMOS) and biocompatible piezoelectric material. The low frequency and low device footprint are ensured by Archimedean spiral geometry and Si seismic mass. Experimentally detected resonance frequencies were validated by an analytical model. The generated open circuit voltage (3⁻10 mV) is sufficient for the direct analog conversion of the signals for cochlear multielectrode implants.

Keywords: Archimedean spiral; aluminum nitride (AlN); artificial basilar membrane; cochlear implant; energy harvesting; finite element analysis; frequency selectivity; micro-electromechanical system (MEMS); piezoelectric cantilever.

Figure 1

(a) Tensile stress distribution along the beam calculated by finite element analysis to exclude “fragile” geometries upon selection; (b) Layout of the selected 4 × 4 spirals. Each of the spirals fit into a square area of 2 mm × 2 mm. Five geometrical parameters of the Archimedean spirals were varied to tune the natural frequency: number of the half turns, width (W), starting radius at the clamping point (R₀), radius of the proof mass cylinder (r₀), and c parameter, describing the rate of the radius change from the edge towards the center.

Figure 2

Schematics of the fabrication procedure: (A) 4” Si-on-Insulator (SOI) wafer; (B) thermal oxidation (300 nm); (C) deposition and lift-off of the bottom Ti/Au contact and pads; (D) radio frequency (RF) sputter deposition and patterned etching of aluminum nitride (AlN) (830 nm); (E) deposition and lift-off of top Ti/Au contact; (F) deep reactive ion etching (DRIE) of the spiral beam from the front side; (G) first DRIE Bosch process from the back-side and the strip of the photoresist mask; (H) second DRIE Bosch process from the back-side through the Al hard mask with a slightly modified pattern to obtain perforated Si frame; (I) etching of the Al masking layer; (J) hydrofluoric acid (HF) etching of the buried oxide and removal of the top protective photoresist layer.

Figure 3

Experimental setup for the characterization of the piezoelectric cantilever arrays: (a) Wire bonded cantilever array on the printed circuit board (PCB); (b) 3D printed sample holder with a calibrated accelerometer mounted on a shaker; (c) the shaker was controlled by a function generator through a power amplifier. The output voltage signals of the cantilevers and the calibrated accelerometer were collected by a programmed data acquisition card (DAQ). The signals were in situ visualized by an oscilloscope during the automatized frequency sweeps. Current source was used to feed the accelerometer.

Figure 3

Experimental setup for the characterization of the piezoelectric cantilever arrays: (a) Wire bonded cantilever array on the printed circuit board (PCB); (b) 3D printed sample holder with a calibrated accelerometer mounted on a shaker; (c) the shaker was controlled by a function generator through a power amplifier. The output voltage signals of the cantilevers and the calibrated accelerometer were collected by a programmed data acquisition card (DAQ). The signals were in situ visualized by an oscilloscope during the automatized frequency sweeps. Current source was used to feed the accelerometer.

Figure 4

Tilt-view scanning electron microscope (SEM) images of two typical suspended spiral-shaped cantilevers (1,3) (a) and (3,4) (b) with contacted AlN layer (darker region) on their top surfaces, and a wafer thick 3D-micromachined Si seismic mass beneath. The darker circle in the center corresponds to a Ti/Au disc applied for laser reflection tests. Truncated shape of the tip mass is due to the increasing underetching ratio during the DRIE.

Figure 5

3D optical surface profiler images taken on a portion of a suspended spiral with (a) and without (b) seismic mass at its end. Characteristic height profiles along the dashed lines for cantilever with (c) and without seismic mass (d).

Figure 6

Piezoelectric output voltage during continuous sinusoidal excitation at a feedback controlled acceleration of 1 g. Depending on the geometry, the base frequency of the cantilevers falls in the range of 281–673 Hz. Inset shows the sinusoidal output waveform of channel 1 at resonance.

Figure 7

(a) Deflection distribution along the spiral cantilever at a driving acceleration of 1 g calculated by finite element analysis; (b) stroboscopic snapshot taken from the backside during shaking of the microspiral under optical microscope. The live slow-motion video clearly showed the off-axis wobbling movement of the seismic mass.

Figure 8

Experimental resonance frequencies for each cantilever as a function of the calculated first bending mode frequency using a simplified rectangular beam model.