1. Introduction

In recent years, vibration energy harvesters have drawn more attention in the world. It can be used in many fields ranging from implanted devices and wearable electronic devices to mobile electronics and self-powered wireless network nodes^[1-3]. There are three ways to convert vibration energy into electrical energy, namely electromagnetic, piezoelectric and electrostatic^{[4, 5]}. Among these three harvesters, the electromagnetic harvester cannot produce a larger current but relative low voltage, so the design of management power circuit is more difficult. Because of the need for an external voltage source, the electrostatic harvester is impossible to be used in practical applications. Therefore most of the researchers have been working on the piezoelectric harvester, which has a high output voltage and a high efficiency of electromechanical energy conversion of piezoelectric.

As the key part of the piezoelectric vibration harvester, the piezoelectric materials mainly include lead zirconate titanate (PZT) piezoceramic, polyvinylidene fluoride (PVDF) polymer, polycrystalline zinc oxide (ZnO), aluminum nitride (AlN) and so on. AlN has proven to be the superior piezoelectric ceramic when it comes to energy harvester devices owing to its high energy density, moderate voltage levels, low dielectric constant and especially its compatibility with CMOS processes^{[6, 7]}; furthermore it can be deposited easily at low temperatures and has already been widely used in the MEMS, microelectronic and microsensor industries^[4].

A number of works have been devoted to fabricating the single beam harvester based on the AlN film^{[4-6, 8-15]}, but the harvester array based on the AlN film was rarely reported, the max output was about 1 $\mu$W. This paper presents the fabrication process and the measurement results of the harvester array based on the AlN film. The characterizations, such as load characteristics and frequency characteristics, have been done in the final part.

2. Energy harvester cantilever design and fabrication

Based on the piezoelectric effect, the vibration energy harvester array converts the environmental vibration energy in the surrounding to electric energy. The vibration energy harvester array consists of five cantilevers ended with one attached mass. The three advantages of the design are that the grooves between cantilevers allow the air to flow smoothly and reduce the air damping ratio, the cantilever arrays connected in series make the currents increase greatly, one attached mass should guarantee the consistency of the five cantilevers resonant frequency. Equation (1) in Ref. [16] shows that the air damping ratio decreases with decreasing the width of cantilever when keeping other parameters constant. The currents increasing can be explained by Kirchhoff's current law in Ref. [17]. The one attached mass ensures that the 5 cantilevers can vibrate in phase and the piezoelectric elements can be connected directly in parallel or series. The structure in this paper is different from the single cantilever structure reported in reference^{[4, 5, 10-13, 18, 19]}, but alike to the structure in Ref. [3]. The schematic structure is shown in Fig. 1. The fabricating process of the vibration piezoelectric energy harvester arrays is shown in Fig. 2.

$\begin{equation} \xi_{\rm sq}=\frac{\nu b^2}{2\rho_{\rm a}g_0^3h\omega}, \end{equation}$

(1)

where $\nu$, b, $\rho_{\rm a}$, g, h and $\omega$ are the air viscosity, the cantilever width, the air density, the distance of cantilever to the fixture, the thickness of the cantilever and the response frequency, respectively.

N-type 100 mm diameter, double-sided-polished, (100) CZ silicon (Si) of resistivity 2-4 $\Omega$$\cdot$cm silicon on insulator (SOI) wafers are prepared for fabricating vibration energy harvester arrays. The thickness of the device layer that is the top layer of the SOI wafer is about 50 $\pm$ 1 $\mu$m, and the bottom silicon layer of the SOI wafer is about 450 $\pm$ 2 $\mu$m. The thermal buried oxide is a thickness 1.0 $\pm$ 0.05 $\mu$m between the device layer and bottom silicon layer. After the wafers were cleaned in RCA1 and RCA2 solutions, a 300 nm thickness silicon oxygen layer as a dielectric layer was grown on both sides of the SOI wafers. Then patterned stacks of 25 nm AlN (seed layers), 200 nm Mo (bottom electrode) and 1.0 $\mu$m AlN (crystalline piezoelectric material) were deposited by the magnetron system. Following that, Al as the top electrode material was deposited and patterned and the deep reactive ions etch (DRIE) system was used to produce grooves. In the end, the SiO₂ layer between the SOI wafer was removed by the reactive ions etch (RIE) after the cantilever structure was released by DRIE. Finally, the wafer was diced and wire bond, then individual devices can be electrically tested. Figure 3 shows the picture of the fabricated vibration piezoelectric energy harvester arrays.

3. Results and discussion

The power out of piezoelectric vibration piezoelectric energy harvester arrays is mainly dependent on the properties of AlN and the structure of the harvester array. So the X-ray diffraction (XRD) is utilized to characterize the AlN film, as shown in Fig. 4. The X-ray rocking curve measurement indicates the film has the crystal orientation (002) and the intensity reaches 1.1 $\times$ 10$^{5}$ counts, the full width height maximum (FWHM) is 1.9$^\circ$. The FWHM of the diffraction peak in the XRD pattern refers to the general quality of the AlN film. The smaller the FWHM is, the higher the piezoelectric quality is.

The vibration piezoelectric energy harvester can be equivalent to the two-port network as shown in Fig. 5. While $V_{\rm s}$ is set as an exited source, $V_{\rm o}$ is the voltage value dissipated on the equivalent load, while $Z_{\rm s}$ and $Z_{\rm l}$ is equivalent to the impedance of the harvester array and the load respectively. The power out is formatted as follows:

$\begin{equation} P_0 =\frac{V_0^2}{Z_{\rm l}}, \quad {\rm that \, is}\; p_0 =V_{\rm s}^2 \frac{Z_{\rm l}}{\left( {Z_{\rm s} +Z_{\rm l}} \right)^2}. \end{equation}$

(2)

From Eq. (1), an conclusion can be drawn that the max power dissipated on the load can be attained when and only when $Z_{\rm s}$ and $Z_{\rm l}$ are equal.

The power out and open circuit voltage of the harvester arrays were detected when a series of different load impedances were applied, as shown in Fig. 6. The varying of the resonant frequency and the output voltage with the applied acceleration were obtained as shown in Fig. 7. The equivalent impedance of 80 k$\Omega$ and the maximum power output of 30.4 $\mu$W can be firstly obtained based on Fig. 6. As far as we know, the power output of the proposed device is really high compared to the harvester arrays based on AlN thin films. It can be seen in Fig. 7 that the resonant frequency varied little with acceleration increasing, but open voltage increased lineally with acceleration increasing. The bandwidth is a little narrow, so the future work will focus on the bandwidth broadening. The main reason for the power output in this paper, more than that of the harvester in Ref. [3] is that the value of 0.9g is applied in our experience, while the power output is proportional to $g^{2}$. The relations between power out and accelerations will be presented in other papers.

4. Conclusions

In this work the fabrication of piezoelectric vibration energy harvester arrays based on AlN thin film deposited by pulsed-DC magnetron sputtering is presented. The maximum power output is about 30.4 $\mu$W at an acceleration of 0.9g for the 80 k$\Omega$ resistance when the resonant frequency is 204 Hz, its power density reaches about 1.4 mW/cm³. The findings will be applied widely in wireless sensor networks (WSN) and other fields such as portable electronic products. In the next step we will further focus our attention on the improvements of AlN properties and increase the high efficiency of electromechanical energy conversion to increase the power density and decrease the resonance frequency.