Process Optimization and Characterization of Dielectric, Piezoelectric, and Ferroelectric Properties of 30% ScAlN Thin Films.

Abstract

Scandium-doped aluminum nitride (ScAlN) is a promising replacement for aluminum nitride in MEMS vibration and acoustic sensors due to its higher piezoelectric coefficients, and for RF MEMS due to its enhanced piezoelectric response and ferroelectric switching capability. However, poor process conditions often lead to degraded film performance. In this work, we optimized the growth conditions of ScAlN thin films deposited by reactive pulsed-DC magnetron sputtering system by studying the impact of N₂ flow rate, target-substrate distance, substrate temperature, and substrate bias on film stress, crystallinity, and surface morphology. Based on stress measurements, XRD rocking curves along the c-axis (002), and roughness with abnormally oriented grain (AOG) formation probability extracted from AFM and SEM images, an optimized deposition recipe was developed that balances stress, crystallinity, and AOG density. With this optimized recipe, samples were fabricated for dielectric, ferroelectric, and piezoelectric coefficient ( $d_{33,f}$ and $d_{31,f}$ ) measurements. To verify scalability, $d_{33,f},{\epsilon }_r$ , and $\text{tan}\left(\delta \right)$ were measured on 100, 150, and 200 mm substrates. Dual beam laser interferometry results showed $d_{33,f}$ values of $18\text{pm}/\text{V},{\epsilon }_r$ of 18, and lowest $\text{tan}\left(\delta \right)$ of 0.4%. Cantilever-based $d_{31,f}$ measurements yielded a value of -6.22 pC/N. The optimized ScAlN films also exhibited remanent polarization, $P_r=130\mu \text{C}/{\text{cm}}^2$ and coercive field, $E_c=3.5\text{MV}/\text{cm}$ . These results are expected to formulate a basis for future thin film development and applications in MEMS resonators and inertial sensors.