【作者】 杨卫明；

【导师】 于军；

【作者基本信息】 华中科技大学 ， 微电子学与固体电子学， 2008， 博士

【摘要】 相控阵天线用铁电薄膜移相器是一种基于铁电材料的新型移相器,具有扫描速度快、精度高、驱动功率小、波控系统简单、工作温度范围宽、多目标跟踪与处理、价格低廉等优点,是目前军事、航天等领域雷达系统的研究热点之一。本文研究了Ba_1-x-yPb_xSr_yTiO₃（BPST）铁电材料的制备、表征、电学特性以及基于BPST薄膜的电容器加载共面波导移相器的仿真设计等内容。在分析铁电移相器工作原理的基础上论证了铁电材料应用于移相器的可行性,设计出BST与PST的固溶体BPST作为移相器用可调谐介质,指出0.3≤y≤0.6的组分具有较大的移相器应用价值,为实验提供了可调谐介质材料的选择依据。采用固相反应法制备了BPST陶瓷材料,研究了其微结构与介电特性。发现随Pb浓度的提高,BPST陶瓷在室温下由立方相转变为四方相,晶格畸变程度逐渐加剧。介电特性分析发现Ba_0.5Sr_0.5TiO₃、Ba_0.4Pb_0.1Sr_0.5TiO₃、Ba_0.3Pb_0.1Sr_0.6TiO₃与Ba_0.2Pb_0.2Sr_0.6TiO₃适合于铁电移相器的应用,在10kHz测试频率下它们的室温介电常数分别为1235、1431、1140和1593;介电损耗分别为0.0029、0.0047、0.0041和0.0062。采用第一性原理计算了BPST的晶格结构,发现在Ba_0.7-xPb_xSr_0.3TiO₃晶格中,随Pb含量的提高,Ti离子距钙钛矿结构氧八面体中心的偏移量Δc越大;而在Ba_1-x-yPb_xSr_yTiO₃(_0.4≤y≤0.6)晶格中,当Δc=0时晶格能量最低。采用射频磁控溅射法制备了BPST薄膜,研究了薄膜的制备工艺、介电特性、漏电流特性与元素化学态。在所讨论的BPST薄膜中,发现Ba_0.2Pb_0.2Sr_0.6TiO₃薄膜最适合应用于铁电薄膜移相器。室温时,在1MHz测试频率下Ba_0.2Pb_0.2Sr_0.6TiO₃薄膜的介电常数为514、介电损耗为0.014、偏置电场为375kV/cm时调谐率为51%、FOM为36.43。薄膜漏电流特性分析发现,不含Pb薄膜的漏电流密度最小,含Pb薄膜的漏电流密度相对较大,并且Pb含量越高,漏电流密度越大。在375kV/cm电场作用下,Ba_0.2Pb_0.2Sr_0.6TiO₃薄膜的漏电流密度为6.7μA/cm²。采用XPS研究了薄膜的元素化学态,发现Ba_0.2Pb_0.2Sr_0.6TiO₃薄膜的表层中有少量的非钙钛矿结构的Ba与Ti;在薄膜内部,Ba、Sr、Ti等金属元素均为单一的钙钛矿结构结合状态;Pb元素在薄膜内部出现了少量单质的化学状态,这与薄膜内部缺氧严重有关。通过元素含量计算发现薄膜中Ba、Ti含量偏高、Sr含量偏低;薄膜内部Pb元素含量与化学式符合较好,而表层中Pb含量偏低;薄膜表层与内部的钙钛矿结构氧均偏低。采用ADS仿真得到优化的Ba_0.2Pb_0.2Sr_0.6TiO₃/MgO基电容器加载共面波导移相器的结构参数:叉指宽度s、叉指间隙g、叉指交叠长度l0、单侧叉指数目m分别为5μm、10μm、45μm、1或2。通过使用不同的薄膜调谐率T仿真了具有20个周期性重复的移相单元、信号线长度为1cm的电容器加载共面波导移相器在偏置电压调控下的性能,结果显示:移相器的最大S11为-17.4dB、最小S21为-0.75dB。当T由0变化到15%时,在22GHz～23GHz频率处可获得的最大移相量为330°,适合应用于相控阵天线。更多还原

【Abstract】 Ferroelectric films based shifter for phased array antenna using has attracted much more attention in recent years because it has the merits such as rapid scanning, high accuracy, low drive voltage, simple control system, multi-object tracking, and low cost, etc. In this dissertation, the design, fabricating and characterization of ferroelectric Ba_1-x-yPb_xSr_yTiO₃ （BPST） ceramics and thin films are discussed. Meanwhile, the structure and performance of the BPST thin film based coplanar waveguide （CPW） phase shifter with capacitance loaded are simulated by the Advanced Design System （ADS）.The principle of ferroelectric phase shifter is discussed in this thesis. The feasibility of ferroelectric materials application on phase shifter is proved. The BPST materials are designed for the phase shifter using, and the calculation result shows that suitable BPST components are those that the subscript y is in the range of 0.3 to 0.6.The BPST ceramics are fabricated by conventional solid state reaction method. The microstructure and dielectric properties are studied. With the Pb content increasing, the BPST lattice changes from cubic phase to tetragonal one at room temperature （RT）, and the lattice distortion becomes more severe. There are four BPST components, such as Ba_0.5Sr_0.5TiO₃, Ba_0.4Pb_0.1Sr_0.5TiO₃, Ba0.3Pb0.1Sr0.6TiO₃ and Ba_0.2Pb_0.2Sr_0.6TiO₃, which are suitable for phase shifter using in all the ceramics investigated. When measured at 10 kHz, the dielectric constants of above four components are 1235, 1431, 1140 and 1593, respectively. The loss tangents are 0.0029, 0.0047, 0.0041 and 0.0062, respectively. The microstructure is calculated based on the first principle. Results show that the the offset of Ti ion （denoted as ?c, which represents the displacement of Ti ion that offsets from the center of oxygen octahedron） increases with the Pb content increasing in the Ba0.7-xPbxSr0.3TiO₃ ceramics. In other BPST lattices, the total energy is lowest when ?c=0.The BPST thin films are fabricated by RF-magnetron sputtering. The sputtering techniques, dielectric performance, leakage current and elemental chemical states are investigated. When measured at 1 MHz, the RT dielectric constant and loss tangent of the Ba_0.2Pb_0.2Sr_0.6TiO₃ thin film are 514 and 0.014, respectively. The tunability is 51% under a bias electric field of 375kV/cm, and the FOM is 36.43. With the Pb content increasing, the leakage current density increases in the BPST thin films. Under an electric field of 375kV/cm, the leakage current density of Ba_0.2Pb_0.2Sr_0.6TiO₃ thin film is 6.7μA/cm2. The chemical bonding states of all elements that appears in the BPST thin films are investigated by XPS analysis. The results show that there are few non-perovskite bonded Ba and Ti on the surface of Ba_0.2Pb_0.2Sr_0.6TiO₃ thin film. The interior elements of Ba, Sr, and Ti are all bonded into perovskite structure, but there is few elemental lead inside of the thin film. The elements’content calculation shows that the Ba and Ti content is a little excess, while Sr content is a little deficiency both on the surface and inside of the thin films. The Pb content is consistent with the stoichiometric proportion inside of the thin films, but there is a little deficiency on the surface. The perovskite bonded oxygen content is a little deficiency both in the inside and on the surface of the BPST thin films.The structure and performance of the CPW phase shifter with capacitance loaded based on Ba_0.2Pb_0.2Sr_0.6TiO₃/MgO are simulated by the ADS. The optimal structure of phase shifter unit are as follows: the interdigitated capacitor’s （IDC） finger width （s）, gap between the fingers （g）, long of the finger （l0）, unilateral finger numbers （m） are 5μm, 10μm, 45μm, and （1or 2）, respectively. The performance simulation of the phase shifter under various bias electric fields is studied. Results show that the maximal S11 and minimum S21 value of the CPW phase shifter are -17.4dB and -0.75dB respectively. The phase shifter can obtain a 330o phaseshift by changing the tunability from 0 to 15% in the frequency range of 22 to 23GHz. It’s suitable for the phased array antenna using.更多还原