Investigation Of High Voltage Resistance BST Thin Film Varactors

With the rapid development of communication technology, single-band communication system has been unable to meet the needs of modern communications. The miniaturization and integration of communication device claims that the communication system can scan the band independently, and transfer the data farthest. Therefore, there is an urgent need of tunable microwave devices that can work at different frequency to meet the needs of the communication system. A variety of new high-frequency, high-voltage electronic devices also appeared in radar systems and high-power communications jamming system, the prominent feature of these devices is the large operating voltage, sometimes with a larger operating current. Therefore, the electronic components used in large power circuit must have a high withstand voltage and high power carrying capacity. Based on the needs of wideband and high-power communications, research on high-voltage tunable microwave devices is particularly important.In this thesis, the effect to the leakage current of the preparation conditions of barium strontium titanate(BaxSr1-xTiO3,BST) thin film varactors was studied, high withstand voltage, high tunability BST thin film varactors were prepared, the high frequency performance test methods of BST thin film varactors were explored, and indentified the foundation of BST thin film varactors used in high-frequency power circuit. The main contents and conclusions are as follows.1. Mn2+ doped BST ceramic sputtering targets were prepared by solid-state reaction method. 2% molar ratio of C4H6MnO4 was added before sintering for Mn2+ doped. After Mn2+ acceptor doping, the concentration of free electrons can be greatly reduced, thereby reducing leakage current and dielectric loss of the BST thin film, the withstand voltage was improved.2. BST thin film varactors were prepared under different substrate temperature, sputtering pressure and oxygen-argon flow ratio by RF magnetron sputtering system and microfabrication techniques. The process parameters was optimized and high withstand voltage, high tunability BST thin film varactors were prepared. The tunability rate of BST thin film prepared under optimum conditions reached 53.7% when the electric field is 2000kV/cm, and has a good frequency stability, the leakage current density is not more than 1.0×10-4A/cm2 when the applied electric field is 3000kV/cm, the operating voltage was up to 140 V.3. The high frequency S-parameter of BST thin film varactor was tested through a single port and dual-port test methods by a vector network analyzer, and then the capacitance and Q-factor was extracts from the S-parameter. Because of the parasitic parameters, the capacitance value of the varactor increase with the increasing frequency, the Q-factor decreases with the increasing frequency, the Q-factor is about 2 at 4GHz, the Q-factor decreases sharply at high frequencies caused by the top and bottom electrodes of the varactor conduction losses.