鈦酸鍶鋇的介電可調性及其相關之介電系統

Abstract

在電子工業上，鐵電性材料鈦酸鍶鋇近年來已有相當廣泛的應用， 文獻上有對鈦酸鍶鋇電容之介電可調性提出數種應用。然而針對此性質之 相關研究並不多見。本論文之主要目的在在研究以鈦酸鍶鋇製作之數種電 容其介電性質及介電可調性。介電之材料採用一快速燒結方法製得。研究 中發現鍶含量的添加除了改變鈦酸鍶鋇系統之居里溫度外，其效應包括： 使此材料變得更耐火、增加成份的波動、進而影響到其介電可調性。本論 文中提出一個可調式電容的等效電路模型可解釋鈦酸鍶鋇電容其在直流電 場作用下介電損失的行為。根據此型的解釋，過大的漏電流導致介電損失 隨直流電場的增加而增加而影響到其應用。研究發現不同晶粒大小的鈦酸 鍶鋇造成其鐵電性－順電性相變及極化鬆弛時間分佈的擴散，並且發現一 個介電損失行為轉變的溫度，此溫度的變化可由其等效電路模型暸解。本 論文中另提出一個修正型的偏壓方程式來預測鈦酸鍶鋇電容在直流電場作 用下介電常數的變化。利用此方程式的分析結果顯示鐵電性－順電性相轉 變的擴散是造成鈦酸鍶鋇電容其介電可調性下降的主要原因。 本論文中所研究的鈦酸鍶鋇相關系統包括：鈦酸鍶鋇薄膜電容、複合物電 容及利用鈦酸鍶鋇製作的可調式移相器。利用射濺鍍系統在鍍有鈀金屬之 電極的矽晶圓上鍍覆鈦酸鍶鋇薄膜電容，研究顯示其在400kV/cm電場作用 下，介電調性可高達36﹪，並且隨頻率的增加而降低。利用鑄造的方法可 製作鈦酸鍶鋇－矽橡膠及石墨－矽橡膠之複合介電系統。鈦酸鍶鋇－矽橡 膠複合物電容之介電可調性與複合物中之電場分佈有相當大的關係，本研 究中推導了幾種理論模型的介電可調性，並因此而了解實際複合物的介電 可調性。經由石墨－矽橡膠之電阻率量測，找出此系統的濾通臨界值 約23.8 %使得此系統由絕緣體轉變為導體。其頻率響應在接近此臨界值時 ，系統由電容性轉變為電感性。經由微波反射的實驗，亦觀察到其類似於 磁性的電感反應。此外，本研究在電路板上利用鈦酸鍶鋇製作兩種可調式 移相器；一為單阜反射式，另一為雙阜穿透式。單阜移相器在偏壓200伏 特時，其相位角變化高達100度。而相同的偏壓下雙阜 移相器，其相位 角變化仍高達40度，並且其可作為一低損失(<-0.5dB)的帶通濾波器，由 於相位角與偏壓的線性關係使得由鈦酸鍶鋇製作的可調式移相器有良好的 控制性。 Ferroelectric barium strontium titanate (Ba1-xSrxTiO3, BST) has been applied in many electrical applications. Several special applications were issued for thedielectric tunability of BST. In literature, few studies for this properties were reported. Capacitors made of BST ceramics are used in this thesis to investigate the dielectric characteristics and dielectric tunability. The bulk samples doped with 1.0 mol% MgO and 0.05 mol% MnO2 were prepared with a non-isothermal sintering process. In addition to the shift of Curie temperature, the increase of Sr molar fraction makes the samples being more refractory, induces composition fluctuation, and influences the dielectric tunability of BST capacitor. An equivalent tunable capacitor circuit model is proposed to explain the dielectric loss behavior underDC biasing field. In this model, high leakage current indicates an increase of dielectric loss under DC biasing. Samples with various grain sizes reveal variousferroelectric to paraelectric transition broadenings and various distributions ofrelaxation times in BST dielectrics. A dielectric loss behavior transformation temperature is observed in BST systems. This phenomenon is explained with the proposed circuit model. A new modified bias equation(MBE) is proposed to predict the DC field dependence of dielectric properties. The results of analalyzed with this MBE equation indicate that the reduction of dielectric tunability is due to the transition broadening which is attributed to the material inhomogeneuity. The BST related dielectric systems had been studied in this thesis includeBST thin film capacitors, composite capacitors and high frequency tunable phase shifters incorporated with BST dielectrics. The BST thin film was deposited on a Pd electroded Si-wafer with an RF magnetron sputtering system. The dielectric tunability of BST thin film is 36 % under 400 kV/cm bias. The BST/Si-rubber and graphite/Si-rubber composites with various BST or graphite volume fractions were prepared with a casting method. The dielectric tunability of BST/ Si-rubber compositedepends strongly on the BST fraction and the connectivity of the BST phase. A percolation threshold of about 23.8% is observed for the graphite/Si-rubbersystem. There is a capacitive to inductive feature transition near the percolationthreshold for the AC response of this conductor- insulator composite system. It isobserved that the inductive feature yields a magnetic response at microwave region(2~18GHz). An one-port and a two-port electrically-tunable phase shifters which were incoporated with BST dielectrics were implemented on the microstrip circuit. The phase shift for the one-port shifter is 100 degree at 2 MHz and 200V bias voltage which is larger than that of two port shifter of about 40 degree. It is observed that the two-port phase shifter acts as a band pass filter(10 MHz ~ 100 MHz) with low loss(< -0.5dB). The phase shift increases with the applied DC voltage and decreases withthe increasing frequency.