Parallel Loss Channels in Superconducting Epitaxial Aluminum Resonators

Superconducting epitaxial aluminum (epi-Al) on silicon and sapphire has demonstrated low-loss performance that is desirable for linear circuit elements in quantum computing.~ Most often, it is process artifacts that limit the performance of devices fabricated from epi-Al.~ Two common artifacts are photoresist residue that is impossible to observe with optical microscopy and line edge defects on the aluminum sidewalls.~ Superconducting quarter-wave resonators exhibit both saturable power dependence akin to conventional two-level-systems, and power independent loss that strongly impacts yield and is fabrication process dependent.~ Correlations between detailed electron microscopy, and resonator quality factor measurements with values above and below 10\textasciicircum 6 will be discussed.