Planar tunable HTS microwave filter with patterned ferroelectric thin film

A planar tunable HTS microwave filter with patterned ferroelectric thin film is presented in this paper. The advantages of using patterned ferroelectric thin film in comparison with ferroelectric thin film deposited on the entire substrate for fabricating a planar tunable microwave device are the reduction of the device’s insertion loss and unintentional tuning. Patterning of the ferroelectric thin film using conventional methods such as wet etching or ion beam milling can damage the substrate surface or the HTS thin film and adversely affects the quality of the HTS thin film, resulting in the fabricated device having poor performance. The tunable filter presented here was fabricated using HTS YBa 2 Cu 3 O 7−δ and patterned ferroelectric Ba 0.1 Sr 0.9 TiO 3 thin films. The patterned ferroelectric thin film of the tunable filter was fabricated using a lift-off method with YBa 2 Cu 3 O 7−δ film as a refractory masking layer. This fabrication method has the advantages of not damaging the substrate surface or HTS thin film, and allows the ferroelectric thin film to be processed at the high temperature required for optimum properties. The fabricated tunable filter had a central frequency of 11.74 GHz and insertion loss of 1.6 dB when it was in the electrically unbiased state. With the applied voltage at 200 V, the insertion loss decreased to 0.35 dB and the filter was able to achieve a frequency tunability of 1.6% as the central frequency was tuned to 11.93 GHz.