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

Abstract

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 ε 33 S , 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 ε 33 S 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.

Sputtered thin film piezoelectric aluminum nitride as a functional MEMS material

“…Figure 10a shows that the effective transverse coefficient has no dependency of the rf bias power. This is in good agreement with earlier published results from (Karakaya et al 2008). The mean transverse coefficients are between -1.0 and -1.2 C/m 2 .…”

“…For the transverse piezoelectric coefficient the results are ambiguous. Ababneh et al (2010) reported an increase in the coefficient for thicker films, whereas Karakaya et al (2008) stated a decrease. Our investigations showed that the e 31,f coefficient has the tendency to increase for thicker films.…”

“…The use of such layers can affect the physical properties like thermal conduction or stress and strain and influences the device function itself. In the case of AlN it was shown that the intrinsic layer stress can be adjusted by varying deposition parameters like pressure, gas mixture and bias voltage and still possesses good piezoelectric properties (Dubois and Muralt 2001;Karakaya et al 2008;Lanz et al 2006;Loebl et al 2003;Ababneh et al 2010). However, data about the characterization of the longitudinal and transversal piezoelectric coefficient at the same time is missing.…”

The influence of sputter deposition parameters on piezoelectric and mechanical properties of AlN thin films

“…Most groups discussed compressive stress in AlN thin films. This can mainly be attributed to the incorporation of Ar in the film induced by a bias voltage V s , according to the explanations given in the literature [25,26]. This influence can be excluded in the present investigation due to the absence of any bias voltage, thus rendering a tensile stress of the film.…”

Fabrication and Characterization of d33 Mode (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) Energy Harvester

“…(10,11) Different piezoelectric materials have been developed in MEMS energy harvesters, such as PZT, (12) AlN, (13) ZnO, (14) PVDF, (15) and PMN-PT (16,17) single-crystal materials. Compared with other materials, the PMN-PT material exhibits outstanding piezoelectric properties that considerably surpass those of PZT ceramics by a factor of 4 to 5.…”