压电效应能够实现机械能与电能之间的转换，以锆钛酸铅为代表的压电材料广泛应用在各类电子元器件中。但铅元素危害极大，压电材料的无铅化对绿色和可持续发展具有重要意义，其中铌酸盐体系居里温度高、压电系数大，研究最广泛。随着MEMS (Micro-Electro-Mechanical System)技术和大规模集成技术的发展，对电子器件的微型化需求促进了对薄膜的研究。本文以(K,Na)NbO3 (KNN)和NaNbO3 (NN)为例，对铌酸盐薄膜的化学溶液法（溶胶-凝胶法）制备工艺进行了优化，并通过元素掺杂提升电性能，在此基础上制备了基于(K,Na)NbO3薄膜的压电MEMS器件，获得了较高的机电响应。另外，研究了衬底的力学和电学边界条件对NaNbO3外延薄膜结构、铁电性和压电性的影响，并解析了其结构-性能之间的关系。首先对KNN薄膜的溶胶配制工艺进行了优化，制备了均匀致密的薄膜。通过Mn掺杂进一步改善薄膜质量和电性能，系统研究了KNN基薄膜晶体结构、微观形貌、畴结构以及电性能与Mn掺杂量的关系，并对性能提升的机制进行了分析。进一步地，在直径2英寸的SOI晶圆上沉积了高质量的KNN基大尺寸薄膜，其具有（100）择优取向，室温下多相共存，表现出优异的电性能。在此基础上采用传统的MEMS工艺首次制备了基于无铅铁电薄膜的微型压电超声换能器（pMUTs）。对器件的振动特性进行了评估，其主共振频率与器件面积呈反比，发射灵敏度在半径800 μm的pMUTs中最高，达到1250 nm V-1，优于目前报导的大部分PZT/AlN基的pMUTs，在距离探测方面具有潜在应用。对于NN薄膜，通过调控薄膜中的应变能和静电能，首次在NN薄膜中观察到反铁电相特征的双电滞回线。NN薄膜中可逆的反铁电-铁电相变一方面来源于晶格失配应变引入的P4bm反铁电相，另一方面，导电性较弱的Nb:SrTiO3衬底不能提供足够的电荷屏蔽退极化场，铁电相不稳定，与反铁电相之间形成竞争关系。最后，通过调控NN薄膜中衬底诱导的晶格失配应变，实现了薄膜压电性的提升。当NN薄膜面内应力由拉应力转变为压应力时，极化方向趋向于向面外方向偏转，在NN薄膜中产生了非180?畴和非本征贡献的压电响应，增大d33,eff*。面内压应变最大的NN薄膜d33,eff*达到702 pm V-1，远高于当前商用PZT薄膜的报导。且薄膜具有良好的电绝缘性，在实际应用中被击穿的风险较低。
The piezoelectric effect enables the conversion of mechanical energy into electrical energy and vice versa. Lead zirconate titanate is a commonly used piezoelectric material in electronic components. However, the use of lead is highly hazardous, which necessitates the study of lead-free piezoelectric materials for sustainable and eco-friendly development. Among various lead-free piezoelectric systems, niobate-based materials are most extensively researched as they possess high Curie temperature and large piezoelectric coefficient. With the development of MEMS (Micro-Electro-Mechanical System) and large-scale integration technology, the demand for miniaturization of electronic devices promotes the research on thin films. This study focuses on optimizing the chemical solution deposition process of niobate-based films, such as (K,Na)NbO3 (KNN) and NaNbO3 (NN), and enhancing the electrical properties of the films through Mn-doping. In addition, piezoelectric MEMS devices based on (K,Na)NbO3 films were fabricated, exhibiting high electromechanical response. This study also investigated the impact of mechanical and electrical boundary conditions, induced by substrates, on the structure, ferroelectric, and piezoelectric properties of NaNbO3 epitaxial films. Furthermore, the relationship between the structure and properties of the films was also explored. Initially, the optimization of the preparation process for the solution of (K,Na)NbO3 was carried out, leading to the successful deposition of homogeneous and compact KNN films. The electrical properties and quality of the films were further enhanced through Mn doping. Through a systematic analysis, the impact of Mn doping on the crystal structure, domain configuration, morphologies, and electrical properties of KNN-based films was thoroughly investigated. Additionally, the mechanisms behind the improvement in electrical properties were also scrutinized.In addition, high-quality KNN-based films were deposited onto 2-inch diameter silicon-on-insulator (SOI) wafers, exhibiting a preferred orientation of (100), coexistence of multiple phases at room temperature, and favorable electrical properties. Utilizing this achievement, piezoelectric micromachined ultrasonic transducers (pMUTs) based on lead-free ferroelectric films were produced for the first time through traditional MEMS processing. The fundamental resonance frequency of the device was found to be inversely proportional to its area, while the transmitting sensitivity peaked at 1250 nm V-1 for pMUTs with the radius of 800 μm, surpassing most of the PZT/AlN-based pMUTs that have been previously reported. The pMUTs based on KNN films in this study hold significant potential for use in applications such as range-finding devices.Besides, the double hysteresis loop, which is the characteristic of antiferroelectrics, was observed for the first time in NaNbO3 films by modulating the strain energy and electrostatic energy in the film. On one hand, the reversible antiferroelectric-ferroelectric phase transition in NaNbO3 films comes from the P4bm antiferroelectric phase introduced by the lattice mismatch. On the other hand, the Nb:SrTiO3 substrate with weak conductivity cannot provide enough charge shielding depolarization field, and the ferroelectric phase is unstable. As a consequence, there is a competitive relationship between the antiferroelectric phase and ferroelectric phase. Finally, the modulation of piezoelectricity of NN films was realized by introducing lattice mismatch strain into NN films through substrates. When the in-plane stress in NN films changes from tensile stress to compressive stress, the polarization direction tends to be turned to the out-of-plane direction, generating non-180° domain and extrinsically contributed piezoelectric response in the NN films. As a result, d33,eff* of the film was increased. The d33,eff* of NN film with the largest in-plane compressive strain reaches a value of 702 pm V-1, which is significantly higher than that reported for commercial PZT films. Furthermore, the NN film also demonstrates good electrical insulation and a low risk of breakdown, making it a promising material for practical applications.
