The effect of the built-in stress level of AlN layers on the properties of piezoelectric vibration energy harvesters

In this paper we investigated the effects of built-in stress on the dielectric and piezoelectric properties of sputtered AlN layers, meant to be implemented in micromachined piezoelectric vibration energy harvesters. Test structures including cantilevers, 4-point bending beams and metal-insulator-metal capacitors were manufactured with reactive sputtered AlN layers in a thickness range of 400-1200 nm. Various bias conditions during the deposition process allowed controlling the built-in stress level in the layers, from tensile to compressive. The clamped dielectric permittivity epsilon(S)(33), the voltage response and the piezoelectric coefficient e(31) of the deposited AlN layers were measured by performing capacitance, voltage-deflection and 4-point bending measurements, respectively. In addition, we obtained from electrical impedance analyses the generalized electromechanical coupling (GEMC) and the quality factors of the fabricated test cantilevers, which are the critical parameters directly connected to the performance of the device in terms of energy harvesting. It is found that the permittivity epsilon(S)(33) and the piezoelectric constant e(31) were not significantly affected by the different stress levels for a given layer thickness. However, the GEMC and the quality factor were found to be decreasing for structures that have a larger residual stress. We concluded that large residual stress has to be avoided in order to optimize the output power of AlN-based vibration harvesters.