Characterization of Interfaces for Quantum Computing Devices with Electron Energy Loss Spectroscopy

Amorphous non-superconducting layers in superconducting qubits are known to host two-level system (TLS) losses, which can limit qubit performance. For instance, niobium (Nb), a widely used superconductor in qubits, develops an amorphous oxide layer upon exposure to oxygen and forms an amorphous interlayer when deposited on a silicon substrate. In this study, we use Scanning Transmission Electron Microscopy (STEM) and Electron Energy Loss Spectroscopy (EELS) to investigate two critical interfaces in superconducting qubits: Nb/Si and Nb/capping metal layers. For the Nb/Si interface, we will demonstrate the existence of an amorphous layer, characterize its composition, and assess the effects of various surface treatments on its properties. Additionally, we will show that this interlayer can trap contaminants depending on the fabrication process. At the Nb/capping metal interface, we will investigate the formation of Nb oxide and evaluate how different metals and alloys function as oxygen diffusion barriers. This work aims to provide insights into optimizing qubit fabrication to reduce TLS losses.