Recent progress in aluminum nitride for piezoelectric MEMS mirror applications: enhancements with scandium doping

Piezoelectric microelectromechanical systems (MEMS) mirrors enable precise and rapid beam steering with low power consumption, making them essential components in light detection and ranging (LiDAR) and advanced optical imaging systems. Lead zirconate titanate (PZT) offers a high piezoelectric coefficient suitable for such applications. However, its elevated processing temperatures (typically 500 °C-700 °C), lead content that raises contamination concerns during complementary metal-oxide-semiconductor (CMOS) integration, and hysteresis-induced nonlinearity limit its broader integration into MEMS mirrors. In contrast, aluminum nitride (AlN), with low deposition temperatures (below 400 °C) and contamination-free composition, offers CMOS compatibility, environmental stability, and low hysteresis, making it a promising lead-free alternative. However, its intrinsically low piezoelectric coefficient limits actuation efficiency for large scan angles. To overcome this limitation, scandium (Sc) doping has emerged as an effective strategy to enhance the piezoelectric response of AlN. Sc-doped AlN (AlScN) enables relatively large scan angles in MEMS mirror applications due to its significantly enhanced piezoelectric coefficients and reduced mechanical stiffness, while retaining essential advantages, such as CMOS compatibility and environmental robustness. This review comprehensively examines the recent progress in AlN and AlScN for MEMS mirror applications. We focus on its impact on piezoelectric properties, fabrication techniques, and mirror performance. Furthermore, we provide a comparative assessment of AlN- and AlScN-based MEMS mirrors, highlighting their respective advantages, limitations, and application potentials. Finally, this review summarizes recent developments and research trends, providing insights into their performance benefits and directions for future research.