Localization of Dielectric Failures in Aluminium Nitride MEMS Actuators Using Lock-in Thermography

Aluminium Nitride (AlN) as a piezoelectric material has certain advantages compared to PZT, like linearity, inherent polarity and the absence of lead as a potentially hazardous substance. Its weaker piezoelectric coefficient can partially be compensated by a higher driving voltage, as AlN usually has a high dielectric strength. Yet, while developing our quasistatic MEMS micro mirrors based on AlN, we frequently encountered dielectric breakdowns between 30V and 80 V, while expecting reliable operation up to 150 V. By means of lock-in thermography, we were able to localize hot spots at the border of the top electrode that would lead to breakdown events. A cumulation of such hot spots was examined by focused ion beam (FIB) slicing and we discovered AlN voids close to the border. An inspection of the Pt bottom electrode by SEM and dark field microscopy showed no significant defect density in this process stage. Our explanation for the failure is therefore a diffusion of ions into the AlN layer that would be mobilized by the electric field across the piezo layer and lead to heating and dielectric breakdown. To conclude the study, we plan further analysis by TEM and EDX.