Research Of BST Thin Film Varactor At High Frequencies Characterization

With the rapid growth of communication systems including satellite, LTE, opticalnetwork, it is desirable to design better microwave circuit in various frequency bands.As the important part in microwave components, tunable thin film becomes more andmore important. Knowledge of material properties is important for making highperformance devices. Due to frequency dependence of material properties, lowfrequency measurement results cannot represent the true material properties at thehigher frequency ranges. Moreover, parasitic due to the conductors and peripheralcircuit become unintelligible at microwave frequencies. Therefore, it is necessary tomodel the device under the test precisely and de-embed the parasitic completely in orderto extract accurately the material properties.In this thesis, the high property varactors were manufactured with bariumstrontium titanate (BaxSr1-xTiO3) thin film. Several conventional measurementtechniques are reviewed. Geometry dependent properties in BST varactors werediscussed. The major conclusions are as follow:(1) We have produced high quality BST varactors through RF magnetronsputtering, electron beam evaporation, micro-fabricated techniques.(2) Especially, test methods of different structure of parallel plate capacitor wereexpounded: concentric circles measurement technique, one port measurement techniqueusing GSG probes, two port measurement technique using GSG probes. Each of thesetechniques was compared advantages and disadvantages.(3) Capacitor density and quality factor variations with top electrode area in thinfilm BST parallel plate capacitors were discussed. Quality factor increases with theincrease of electrode thickness at high frequencies. The results show that both of thecapacitor density and the quality factor are geometry dependence. At low frequencies,capacitor density decreases with the increase of the top electrode area and quality factorincreases with the increase of the top electrode area. At high frequencies, capacitordensity and quality factor decrease with the increase of the top electrode area. For the same area with different perimeter, capacitor density increases with the increase of theperimeter, quality factor is independent.