Development of High-Quality-Factor Ferroelectric Thin-Film Varactors with Thick Electrodes

As the semiconductor technology progresses, the performances of high-frequency (including RF, microwave, and millimeter-wave) active components have been greatly enhanced over time. However, the advances in high-frequency passive components have lagged far behind. As a result, the mediocre performance of high-frequency passives is now becoming a major bottleneck in circuit design. Ferroic materials, possessing special electromagnetic properties, can be used for implementing multi-functional high-frequency passives, which could make a fundamental impact on the research and development of integrated passive components. Among the ferroic materials, ferroelectrics are those possessing a tunable permittivity that can be controlled by applied electric fields and therefore can be used to fabricate variable capacitors (varactors). Compared to the existing varactor technologies, such as semiconductor-based varactor diode and MEMS varactor, ferroelectric-based varactor is more suitable for RF circuit application, therefore worthy of further research and development. The development of ferroelectric-based varactor would facilitate the realization of tunable RF circuits, making the design of wireless front-end more flexible.Either from academic point of view or industry perspective, there is still plenty of work to be done on research and development of ferroelectric-based varactors. For parallel-plate ferroelectric varactors, the bottom electrode is one of the major sources of high-frequency losses. Thin bottom electrode with poor conductivity is a major inhibitor that makes it difficult for the RF losses to go down, as a result blocking ferroelectric varactors from being applied in the electronic circuits at commercial RF frequency bands (below 10 GHz). Aiming to solve this problem, we propose this project - Development of High-Quality-Factor Ferroelectric Thin-Film Varactors with Thick Electrodes. The strategy we propose in this project is to first use thin bottom electrodes for the growth of ferroelectric thin film, making sure we get a good oxide-metal interface and therefore better film quality and lower dielectric losses, and then, with other fabrication processes, to expose the bottom electrodes, after which the electrodes can be made thick by electroplating so as to reduce the loss caused by the metal electrodes. Here we propose two separate fabrication processes for exposing the bottom electrodes: (a) etching of the substrate below the bottom electrodes (i.e. making via-holes) and (b) performing laser lift-off on the parallel-plate capacitor structure and transferring it to another substrate. We will use these two techniques for fabricating high-quality-factor ferroelectric varactors. We expect to get a capacitor tuning ratio greater than 50% with a bias voltage of 5 V or less and a quality factor of 100 or above at 2.4 GHz. With specified performance, the ferroelectric varactors will be able to replace the commercially available GaAs-based varactor diodes. The specified performance is also superior to those of the semiconductor-based varactors in current integrated circuit technologies offered by the foundries. Finally, we will apply the developed ferroelectric varactors in RF circuits such as phase shifters and tunable antennas to demonstrate their practicality.