Graded Sc-doped AlN structures for residual stress suppression and bandwidth improvement in FBAR filters

We report a combined theoretical and experimental study on thin-film bulk acoustic resonators (FBARs) using graded scandium-doped aluminum nitride (AlScN) as the piezoelectric layer. Two doping profiles, linearly graded and central-maximum, were investigated for their impact on electromechanical coupling (\(\:{\text{k}}_{t}^{2}\)), bandwidth, and quality factor (Q). Using the transfer matrix method, we show that linear doping enhances \(\:{\text{k}}_{t}^{2}\) and bandwidth, while central-maximum doping yields a higher Q. X-ray diffraction (XRD) analysis reveals that residual stress induced by Sc doping affects frequency response and causes device cracking. Graded doping structures effectively mitigate stress gradients, improving bandwidth uniformity and production yield. This work provides a viable pathway for engineering high-performance FBAR filters in broadband RF applications.