| dc.description.abstract | Ferroelectric varactor has the advantages of high capacitance density, high tunability, low operating voltage, and low complexity in fabrication. In this research, we have designed two different types of capacitor structures for parallel-plate capacitors based on barium-strontium-titanate (BST) thin films, developed the fabrication flows, and characterized the performance of the ferroelectric varactors.
The first capacitor structure has its width and length defined by the Pt bottom electrode and Pt/Au top electrode, respectively, with BST thin film as the insulating layer stacking in between the top and bottom electrodes. Fabrication complexity of this structure is low as only three masks are required. We have fabricated ferroelectric parallel-plate capacitors based on this capacitor structure in several fabrication runs. In this thesis, we describe the measurement results of the ferroelectric varactors produced in four of these runs. Measurement results show that, 2:1 tunability can be reached under a bias voltage of 6–7 V for the ferroelectric varactors based on the first capacitor structure. The breakdown voltage of the varactors is about 7 V. The maximum capacitance density is approximately 20 fF/µm2. Under 0-V bias, the quality factor at 2.4 GHz is about 15.
Though the former capacitor structure has the advantage of simple fabrication flow, the breakdown voltage of the ferroelectric varactor based on it is low, which limits the capacitance tuning range. Low breakdown voltage is usually due to the step coverage of thin films is poor. To solve this problem, the second type of capacitor structure is designed. In the first type of capacitor structure, it is the BST thin film that presents at the step where bottom and top electrodes overlap. In the second type of capacitor structure, the BST thin-film at the step is replaced by silicon-nitride thin film, which possesses higher breakdown field and better step coverage capability. Five masks are required to fabricate ferroelectric capacitors based on this capacitor structure. Measurement result shows that the breakdown voltage is improved from the original 7 V to 14 V. The tunability reaches 2:1 and 3:1 at bias voltage of 5 V and 12 V, respectively. The maximum capacitance density is around 19 fF/µm2. Under 0-V bias, the quality factor at 1.8 GHz is greater than 20.
We have successfully developed the fabrication flows for two types of capacitor structures for ferroelectric varactors and compared the measurement results. The first capacitor structure favors simple fabrication process whereas the second possesses high breakdown voltage. Through the advance of the fabrication process, the breakdown voltage is increased by a factor of two, which greatly enhances the practicability of the ferroelectric varactors | en_US |