Thermal Characterization of Ferroelectric Aluminum Scandium Nitride Acoustic Resonators

This work presents the high-temperature characterization of ferroelectric wurtzite Aluminum Scandium Nitride films and acoustic devices. Compared to the room temperature measurements, we observe up to 3 times reduction in the measured coercive field of the sputtered Mo (100nm)/Al0.7Sc0.3N (900nm)/Mo (100nm) capacitors at elevated temperatures up to 600K. Such coercive field reduction translates into significantly lower polarization switching voltages required for a given ferroelectric film thickness. Here, we study the dependency of the coercive field on the device temperature and propose to use temperature control to fine-tune the coercive field and the associated polarization switching voltage. Furthermore, two types of acoustic devices are cofabricated on the same piezo-stack on silicon-on-insulator (SOI) platform: (i) Al
_0.7
Sc
_0.3
N thin film bulk acoustic resonator (FBAR) released from the front side by etching the Si device layer, with an overall resonant stack thickness of 1.1 μ m. (ii) Al
_0.7
Sc
_0.3
N-on-Si composite FBAR (CFBAR), released from the backside, with the resonant stack including a passive 3.5 μm-thick Si device layer. The hightemperature frequency responses of the two acoustic devices are studied showing an excellent temperature coefficient of frequency (TCF) match between COMSOL finite element analysis (FEA) and measured results. Finally, the frequency tunability of the ferroelectric FBAR is tested at 600K with the DC bias ranging from 0 to -100V, showing 3 times higher frequency tuning than at room temperature under the same DC bias conditions. This proves that a larger tuning range and polarization switching can be achieved with lower DC voltages at higher temperatures.