基于ScAlN薄膜的超高频声表面波谐振器的构建与损耗机理研究

Scandium-doped aluminum nitride (ScAlN) thin film based layer structures offer high velocity and large electromechanical coupling coefficient simultaneously, and thus have great potential to operate in the ultra-high frequency band above 3GHz. Those excellent performances make it have a very bright prospect to meet the need of high frequency and wide-band for future mobile communication applications. However, the excellent SAW characteristics is only obtainable when ScAlN thin film combined with fast substrate such as diamond and 6H-SiC, and the quality factor Q of the device also urgent needs to be improved. Therefore, inspired by IHP-SAW technology, a new ScAlN thin film-based composite multilayer structure with high velocity and low velocity alternately attached to ScAlN is proposed. Firstly, theoretical analysis of the SAW characteristics and loss mechanism for the multiple SAW modes is performed on the proposed ScAlN-based layere structure, so that provides guidance for selection of SAW mode, optimizing the design of high and low impedance alternating layers, ScAlN thin film thickness and Sc concentration as well as transverse guided wave structure. By the implementation of this project, the ScAlN-based layer structure with excellent SAW performance is obtainable, and the loss mechanism of SAW propagating is revealed. Furthermore, a SAW resonator operating in ultra-high frequency range with large K2 , high Q and excellent TCF is designed, which lays a foundation for the research of high frequency and wide-band SAW filter for application to communication system.

掺钪氮化铝（ScAlN）薄膜层状结构SAW器件兼具较高的声速V和较大的机电耦合系数K2，具有能够工作于3GHz以上的超高频频段的巨大潜力，有望满足未来移动通信的高频、宽带化发展需求。然而，ScAlN薄膜仅在单晶金刚石、6H-SiC等高速基底上才具有优异的SAW特性，器件的品质因数Q也亟待提高。基于此，本项目拟研究一种基于ScAlN薄膜的新型多层复合SAW器件结构，该结构借鉴IHP-SAW技术，在ScAlN压电薄膜的底部交替沉积了高声速层和低声速层。通过理论分析ScAlN层状结构中SAW传播特性及损耗机理，以指导SAW模式选择、ScAlN薄膜的厚度和掺杂浓度、形成IHP-SAW的高低声速层以及横向导波结构的优化。通过本项目的实施，获得综合性能优异的SAW基片，揭示传播损耗的物理机制，并设计出能够工作在超高频频段的高K2、高Q值且TCF优良的SAW谐振器，为高频宽带SAW滤波器的发展奠定基础。

SAW/BAW微声学滤波器作为射频前端的核心元器件，全球年需求量约500亿只。随着5G通信时代的到来以及载波聚合(CA)和多输入多输出(MIMO)等技术方案的普及，对SAW/BAW滤波器的提出了更高的要求，亟待提高器件的工作频率和增大带宽。掺钪氮化铝（ScAlN）薄膜层状结构SAW器件兼具较高的声速V和较大的机电耦合系数K2，具有能够工作于3~4GHz以上的超高频频段的巨大潜力，有望满足未来移动通信的高频、宽带化发展需求。本项目研究了ScAlN薄膜新型多层复合结构中SAW 传播特性与损耗机理，首先通过优化ScAlN薄膜欧拉角，调控声场分布，使得SAW模式发生转换，优选SH型LSAW模式的K2高达12.8％，几乎是已报道结果的5倍。其次，通过研究材料参数与结构配置对器件性能影响的规律关系，揭示了传播损耗的物理机制，为指导SAW模式选择、优化设计高频、高K2、高Q值的SAW谐振器提供了理论指导。研究如何调控高声阻抗层AlN和低声阻抗层SiO2结构配置和材料参数形成IHP-SAW，实现Q值提高到1000以上。进一步结合 SAW和 BAW技术的共同优势，提出一种基于ScAlN薄膜异质层状结构BAW/SAW压电声波混合谐振器，优化设计获得更高的K2（~21%），并且工作频率高达4GHz以上的宽频范围的频段覆盖，谐振频率fr=4.83 GHz处Qr为3458，反谐振频率fa=5.32GHz处Qa为1348，频率稳定性较好（TCF约为-17.5ppm/oC）。相比于工艺复杂的体声波滤波器，该结构降低了器件的制作难度和生产成本，对于面向5G频段的高性能声学滤波器具有良好的应用前景。最后，基于优化设计的结构配置和材料参数，项目采用脉冲直流磁控溅射系统制备Sc掺杂20%的ScAlN薄膜层状结构压电基底，制备出工作频率高达3.15GHz的超高频SAW谐振器。通过本项目的实施，为形成高性能层状结构SAW器件设计自主知识产权以及开发满足未来移动通信的高频、宽带、低损耗SAW滤波器的奠定了坚实的基础。