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Niobium is commonly used for superconducting quantum systems as readoutresonators, capacitors, and interconnects. The coherence time of thesuperconducting qubits is mainly limited by microwave dissipation attributed totwo-level system defects at interfaces, such as the Nb/Si and Nb/air interface.One way to improve the Nb/air interface quality is by thermal annealing, asshown by extensive studies in 3D superconducting radio frequency (SRF)cavities. However, it is unclear how the microstructure and chemistry of theinterface structures change during heat treatment. To address this knowledgegap, we comprehensively characterized Nb films deposited on Si wafers byphysical vapor deposition, including (1) an Nb film from a transmon and (2) anNb film without any patterning step, using an aberration-corrected transmissionelectron microscope. Both Nb films exhibit columnar growth with strong [110]textures. There is a double layer between the Nb film and Si substrate, whichare amorphous niobium silicides with different Nb and Si concentrations. Afterin-situ heating of the heterostructure at 360{\deg}C inside the microscope, thecomposition of the double layers at the Nb-Si interface remains almost the samedespite different thickness changes. The initial amorphous niobium oxide layeron Nb surface decomposes into face-centered cubic Nb nanograins in theamorphous Nb-O matrix upon heating.

Multi-modal electron microscopy study on decoherence sources and their stability in Nb based superconducting qubit