Impact of Asymmetric Piezoelectricity of 100 nm Al0.7Sc0.3N Thin Film on Ferroelectrically Switchable Multi-Layer Bulk Acoustic Wave Resonators

In this paper, we investigate the impact of asymmetric piezoelectricity of the 100nm Al
0.7
Sc
0.3
N film on the mode-switchable film bulk acoustic resonator (FBAR), which composes of two stacked ferroelectrically switchable Al
0.7
Sc
0.3
N layers. A high overtone bulk acoustic wave mode test structure is designed and fabricated to extract the piezoelectric properties of the 100nm Al
0.7
Sc
0.3
N film at positive and negative polarization states. It is found that the film exhibits different electromechanical coupling constant $(K_{\text{t}}^2)$ of 11.8% and 7.9% at different polarities. The theoretical model built upon the extracted material parameters suggests that two resonant frequencies at around 10GHz and 18.5GHz can be obtained in a FBAR with stacked 100nm Al
0.7
Sc
0.3
N dual films in between 50nm Mo electrodes. The effective coupling coefficient ($K_{{\rm{eff}}}^2$) can be tuned between 19.7% and 12.7% at the 10GHz mode by aligning both Al0.7Sc0.3N films at positive or negative polarization, respectively. While switching to make the two films at opposite polarities will boost the frequency to be 18.5GHz with $K_{{\rm{eff}}}^2$ of 18%.