新型铁电薄膜可调微波器件的设计、制备与应用研究

Tunable microwave devices (such as phase shifter, resonator, filter, etc.) provide added functionality, smaller form factor, lower cost, and lighter weight, and are in great demand for future communications and radar applications. Ferroelectric thin films (such as BaxSr1?xTiO3, BST), have attracted much attention due to their simple processes, low power consumption, high power handling, small size, and fast tuning for microwave applications. Nowadays, there are an extensive number of reports on the integration of ferroelectric thin films for microwave applications. However, there are still many challenges to be solved. For example, the major disadvantage of using ferroelectric thin films for tunable microwave devices is their relatively high dielectric loss tangent which leads to microwave dissipation. The electrical means of improving the total quality factor (Q factor) of tunable microwave devices have not been implemented. Varactors are crucial components in RF and microwave devices. We propose a new structure of ferroelectric thin film hybrid varactor, which integrates the features of both structures of coplanar and parallel plate varactors and hopefully maintain the merit of their component structures. The first part of the plan is to investigate in detail on how the presence of the high resistivity DC bias electrodes affects the tunability and quality factor. Secondly, the Q factor and tunability of the hybrid varactor are expected to be improved by design optimization of the varactor structure or alternative materials (such as SrRuO3, LSMO) for low conductive electrode. Hopefully it might prove its potential of commercialization afterwards. The material physics of ferroelectric thin film will also be studied; of particular interest will be the mechanism of dielectric permeability and to find ways to decrease the dielectric loss in microwave frequency. We will also continue our state-of-art research in integrating our proposed the hybrid varactor into phase shifter devices, device physics on materials properties and electromagnetic theory to develop high performance prototype microwave devices. The proposed project has great potential for ferroelectric based microwave device applications.

近年来，可调微波器件（如移相器、共振器、滤波器等）在先进雷达和移动通讯系统有着巨大的应用需求。铁电材料在可调微波器件应用上吸引了广泛的关注。虽然目前已经有大量基于铁电薄膜集成微波应用的报道，但是实现铁电薄膜在可调微波器件上的商业应用，还有许多挑战需要克服。本项目中，我们提出了一种混合型变容二极管（hybrid varactor）结构，它集合了传统的平行板和共面型变容二极管结构的优点。我们将采用封闭形式的场分析对混合变容二极管结构进行电磁理论分析；利用射频模拟软件（HFSS）设计和优化变容二极管的结构；利用BaxSr1-xTiO3 铁电薄膜及合适的高电阻电极层材料（如SrRuO3, LSMO等）制作铁电混合型变容二极管；设计、制备基于混合型变容二极管的可调微波电路，如移相器、滤波器等。本项目旨在发展微波器件的微型化、高可调性、宽频带和低成本等，实现铁电薄膜材料在可调微波器件方面的商业应用。

钛酸锶钡（BST）材料被普遍认为是最有前途的铁电微波可调器件材料。采用BST薄膜的移相器以其成本低廉、响应速度快、频带宽、体积小、重量轻、控制简单等诸多优点，势必对高频有源相控阵雷达的研究有重大推动作用。我们提出了一种混合型变容二极管结构，该结构中高电阻氧化物层作为直流偏压的底电极。利用电磁场的分析方法，导出了高电阻薄膜在微波TEM波中的扰动。在导电率和薄膜厚度足够低的条件下，导电薄膜对于微波场可以看作是“透明”的。利用Ansoft公司的高特性全波电磁场模拟软件（HFSS），对混合型变容二极管进行结构设计和仿真。以高电阻的ZnO超薄层作为直流偏压的底电极，在单晶LaAlO3基片上制备了混合型BST铁电薄膜变容二极管，并进行了微波性能的测试。实验测试结果与理论分析相一致，混合型变容二极管相比传统的平行板型或共面型变容二极管，集合了后两种结构的优点，既能在低偏压下获得高的可调性，也具有较低的电容。这些特性使得混合变容二极管更适合于移相器或其它微波传输器件中的集成应用。我们研究了复合多层铁电薄膜的微波介电性能，利用BFO材料的多铁性，制备的BST/BFO/BST多层结构的微波介电常数具有电场和磁场的双可调性，此结构在可调微波器件上具有潜在应用。我们对MgO掺杂Ba0.25Sr0.75TiO3薄膜的介电常数和介电损耗因子的影响做了比较系统的研究。研究表明，MgO掺杂的BST薄膜的介电损耗要低于纯的BST薄膜，并且在掺杂浓度为5 mol% 时，获得最佳的实验结果。在介电常数为4.7，厚度为1.66 mm的PCB板上，制备了基于变容二极管载入的传输线超材料微波移相器，电路中采用GaAs变容二极管作为可调部分。从0到6 V直流偏压，在7.2 GHz时移相器提供了0-200o连续相移，对应的品质因数为F= ~57o/dB。该移相器结构显示了在移相键控方面具有潜在应用。