Towards Unparallelled CMOS-compatible Air-coupled pMUT Performance with 30% Sc-doped AlN through an Analysis of Residual Stress Effects

Aluminum Nitride (AlN) offers a CMOS-compatible, stable, and lead-free solution for piezoelectric micromachined ultrasonic transducers (pMUTs), if not for its limited e31,f piezoelectric coefficient. Even though increasing scandium (Sc) doping content in ScAlN is known to enhance the electromechanical coupling factor $\left({{{\text{K}}_{\text{t}}}^2}\right)$ and overall acoustic performance, the outcome is highly dependent on the stress of the ScAlN film especially for air-coupled pMUTs. This study aims to compare pMUT performance (in terms of $\left({{{\text{K}}_{\text{t}}}^2}\right)$) due to increasing Sc content from 20% to 30% in relation to stress and considering its effects on frequency and static deformation of the membrane. Results show that 30% Sc devices achieved an average $ {{\text{K}}_{\text{t}}}^2 > 6\% $ at -50 MPa, on par with PZT-based pMUTs. Compared to 20% Sc, the 30% Sc-doped pMUTs demonstrated a 50% increase in transmit pressure sensitivity and overall 6 dB increase in two-way sensitivity.