| As an essential component of modern communication systems, microwave filters have been widely used in mobile communications, radar, satellite positioning and navigation, electronic countermeasures, wireless remote sensing, etc. With the electromagnetic environment becoming increasingly complex and the increasingly crowded channels, limited spectrum resources are unable to meet the growing demand. More attention was focused on tunable filter, which is based on BST ferroelectric thin films, due to the advantages of fast tuning speed, small size, compatibility with microelectronic technology and adaption of multi-channel. Therefore, this thesis focuses on the design and manufacture of a microstrip tunable band-pass filter based on ferroelectric thin film. The main contents and conclusions are as follows:1. Prepared BST ferroelectric thin films on a single-crystal sapphire substrate by using RF magnetron sputtering. The annealing process was improved because there was difference between thermal expansion coefficients of the substrates and that of the film, which was to avoid the micro-cracks in the surface of the film. The capacitance versus voltage(C-V) curve of BST ferroelectric thin films was tested by using a metal- insulator-metal(MIM) capacitor structure. The results showed that BST ferroelectric thin films had a dielectric constant of 185.5, and a dielectric loss of less than 0.02. Moreover, the tunability of the films’ dielectric constant was 45%.2. According to the design experience and methods of the microwave device described by the gro up members before, parameters for the design of filter were found. A five-order comb- line band-pass filter was designed. By using planar circuit simulation software ADS and three-dimensional electromagnetic simulation software HFSS, it was found that the center frequency of the device was 900 MHz, 3dB bandwidth was about 10%, in-band insertion loss was less than 2dB(without considering the films’ dielectric loss), out-band rejection was less than-50 dB, voltage standing wave ratio(VSWR) which was near the center frequency was less than 1.25.3. Detailed preparation process of microstrip tunable band-pass filter based on ferroelectric thin film was studied. When patterned the BST thin film dielectric layer and the upper electrode, simulation results showed how the lithography process influenced the device’s properties. Meanwhile, simulations on device performance for changes of the errors were investigated when thickening the upper electrode by electroplating process. The tunable bandpass filter was successfully fabricated on a sapphire substrate. After packaged, it was tested. Comparing the tested results with simulated results, the center frequency of the device shifted to a higher frequence of 1.15 GHz, and insertion loss was less than-13 dB, which was somewhat larger than the design. However, other properties were in agreement with the design requirements. When the filter was measured under 10 V and 20 V DC bias, compared to 0 V DC bias, the center frequency of the device are shifted 70 MHz and 180 MHz for ward to higher frequence direction, respectively. The total frequence tuning rate was 14.6%.4. The structure of the device was optimized to promote its performance, and electromagnetic field distribution on surface of the device was described. Only changing portion of the tuning capacitor of the original structure, it virtually eliminated the effects of dielectric loss of BST ferroelectric thin films to insertion loss of the device. While investigating the dynamic electromagnetic field distribution on surface of the device, it would be easy to explain how this could be. Finally, how to improve device performance by promoting the device fabrication processes was investigated. |
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