High Quality Co-Sputtering AlScN Thin Films for Piezoelectric Lamb-Wave Resonators

• Published in: Journal of microelectromechanical systems Vol. 31; no. 3; pp. 328 - 337
• Main Authors: Shao, Shuai, Luo, Zhifang, Lu, Yuan, Mazzalai, Andrea, Tosi, Carlo, Wu, Tao
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.06.2022; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: AlScN; Aluminum nitride; co-sputter; Coupling coefficients; Crystal structure; Crystallinity; Deposition; Doping; Etching; Ferroelectricity; ICP etch; III-V semiconductor materials; Lamb waves; Lamb-wave resonator; Performance evaluation; Piezoelectricity; Resonant frequencies; Resonators; Scandium; Sputtering; Stress; stress control; Thin films; Transducers
• ISSN: 1057-7157; 1941-0158
• DOI: 10.1109/JMEMS.2022.3161055

Doped AlN thin films, especially high Sc-ratio AlScN film, have been reported to significantly improve the piezoelectric properties and draw attention for high performance resonators, transducers and integrated ferroelectric applications. However, many demonstrated devices were limited by poor film stress control, abnormal oriented grains and lack of a good etching profile. Compared to costly single alloy target, co-sputtered AlScN films can benefit customized doping concentrations and provide a unique solution for high Sc-ratio AlScN film quality and device studies. In this work, the optimized co-sputtering deposition and ICP etching processes of 500 nm AlScN thin film were developed and released AlScN Lamb-wave resonators were demonstrated. The influence of stress control by changing N 2 process gas on the crystalline orientation, abnormal orientation grains, film roughness and piezoelectric property of AlScN thin films were discussed in detail. The AlScN film with a high Sc content requires a lower deposition pressure to obtain good crystalline quality. Al 0.85 Sc 0.15 N thin films with FWHM of 1.75°, an average stress of −14.5 MPa and a stress range of 156 MPa were obtained. 130 nm/min etching rate and 77° sidewall profile were achieved by optimized ICP etching of Al 0.78 Sc 0.22 N film. Lamb-wave resonators were fabricated based on both Al 0.78 Sc 0.22 N and Al 0.85 Sc 0.15 N thin films, achieving a quality factor of over 1000 at resonant frequency of approximately 300 MHz. The electromechanical coupling coefficients were improved by 152% and 80% compared to pure AlN devices.[2021-0210]