Superconducting RF Resonators for Electron Spin Resonance

This research project focuses on the development and implementation of superconductive resonators fabricated from Yttrium Barium Copper Oxide (YBa2Cu3O7, abbreviated as YBCO). The motivation for employing YBCO lies in its superior properties compared to other superconducting materials; notably, it has a relatively high critical temperature and is operational within a wider magnetic field range, making it an ideal candidate for use in Radio Frequency (RF) systems, including Electron Spin Resonance (ESR) applications. The primary objective of this project is to engineer a planar superconductive resonator exhibiting a high-quality factor (Q-factor) that can operate effectively at liquid nitrogen temperature (approximately 77 K). The experimental work for the project is staged meticulously, with the initial focus on the creation of a detailed microfabrication process flow. This stage takes place in the state-of-the-art cleanroom facilities at EPFL, and particular attention is given to process optimization strategies that preserve the integrity and superconducting properties of the YBCO material. With an understanding that environmental factors can adversely affect the superconducting properties, the project also explores encapsulation techniques, aiming to protect the material from environmental degradation. Throughout each stage of the fabrication process, advanced metrology techniques—including but not limited to scanning electron microscopy (SEM) and atomic force microscopy (AFM)—are employed. The data gathered serve not only as a quality control measure but also facilitate improvements in the fabrication process. After the successful microfabrication, the resonators undergo a series of characterizations in both Direct Current (DC) and Alternating Current (AC) modes. In these evaluations, the Q-factor and resonant frequency of the devices are extracted under varying experimental conditions. Parameters such as temperature, input power, coupling distance between the resonator and the external circuit, and design features like dimensions and layout are systematically varied to study their impact on performance. In the final stage of the project, the YBCO-based resonator is incorporated into an ESR system for a series of groundbreaking experiments focused on a single crystal BDPA. These experiments are designed to explore the utility and enhanced sensitivity of the superconducting resonator in ESR measurements. Special attention is paid to how variables like temperature, input power, and modulation intensity affect the ESR signal, with the results thoroughly documented and analyzed.