Investigation Of Low Loss BZN/BST Thin Films And Varactor Technologies

Microwave tunable devices can improve the system performances and reduce thenumber of components by controlling the frequency, phase and amplitude of microwavesignal, thus becoming the development tendency of modern communication systems.Among the electric tuning technologies, dielectric thin film varactors have severalproperties that make them attractive for microwave tunable applicaions including fastresponse speed, high power handling capability, relatively low loss, and low cost.Therefore, it is very important to investigate the thin film varactor technologies. Most ofthe literature on microwave applications of tunable dielectrics has focused on theferroelectric BaxSr1-xTiO3(BST) thin films. BST thin films indicate large dielectrictunability whereas the loss of these films is relatively high, e.g. tan δ>0.01forBa0.5Sr0.5TiO3thin films at room temperature. Recently, cubic pyrochloreBi1.5Zn1.0Nb1.5O7(BZN) thin films have gained a great deal of attention because of theirvery low dielectric loss and a certain dielectric tunability. However, the tunability ofthese films is relatively low and it requires very high electric fields to achieve a hightunability. In this dissertation, BZN/BST bilayered thin films were investigated to utilizethe optimum combination of the high tunability of BST and the low loss of BZN, andthe low loss BZN/BST thin film varactor technologies were developed.The BZN/BST composite thin films were prepared on Pt/Ti-coated sapphiresubstrates by radio frequency magnetron sputtering. Based on the dielectric connectiontheory, a model for the relationship between dielectric properties and constructionparameters of bilayered thin films was established. Through optimum control thethickness ratio of bilayered thin films, BZN/BST composite films with optimumcombination of low loss and high tunability were obtained. The dielectric loss of thesefilms was~0.78%, and the relative tunability was~46%at an electric filed of1MV/cm.The interfacial effects of BZN/BST bilayered thin films were investigated. TheFermi level of BZN films was deeper in the band gap than that of BST films, and thus apotential barrier was established at the BZN-BST interface. The electrons ejected from trapcenters in BST need higher energy to overcome the potential barrier for transporting into conduction band of BZN. This resulted in an increase of the threshold electric field of PFemission for BZN/BST at positive biases, and accordingly the currents in BZN/BSTbilayered films were decreased. In addition the barrier height of top contact was increased bythe insertion of a BZN layer between BST and top Pt-electrodes, resulting in a current reductionof BZN/BST composite thin films.It was proposed a capacitor construction of several capacitors in parallel toimprove the device Q-factor. Capacitors with new construction showed higher Q-factorthan that of conventional parallel capacitors, and it was also helpful to improve thereliability of the devices, particular in the large capacitance situation.The electrodes were patterned by a lift-off process. The mixed liquor compositedof HF as etching liquor, NH4F as complex catalyst, HNO3as cosolvent, and dilutiondeionized water perfectly etched the dielectric films.The dielectric properties of BZN/BST thin film varactors were investigated. Thevaractors indicated a high Q-factor of~150at frequency of100MHz andfrequency-independent capacitance and tunability. The Q-factor of varactors wasrestricted by the conductive loss of electrodes at high frequencies, and thus decreased asfrequency increased. Incorporate electrodes with higher conductivity and largerthickness would decrease the conductive loss, and hence improve the high Q-factor ofthe BZN/BST varactors. The phase shifter employing BZN/BST varactors exhibitedlower insertion loss than that of employing semiconductor diodes at designed frequencyof445MHz, demonstrating the potential of BZN/BST varactors for RF applications.At last, the demonstration of an X-band distributed phase shifter implemented withBZN/BST thin films was presented. In the form of two identical varactors loading atboth sides of the unit line exhibited better impedance matching and low insertion loss ofthe phase shifter of a transmission line periodically loaded with varactors. The returnloss of the phase shifter was better than-13dB at12GHz, the maximum insertion losswas-4dB and the differential phase was continuously variable from0to65°.