  We present a comprehensive study of internal quality factors in
superconducting stub-geometry 3-dimensional cavities made of aluminum. We use
wet etching, annealing and electrochemichal polishing to improve the as
machined quality factor. We find that the dominant loss channel is split
between two-level system loss and an unknown source with 60:40 proportion. A
total of 17 cavities of different purity, resonance frequency and size were
studied. Our treatment results in reproducible cavities, with ten of them
showing internal quality factors above 80 million at a power corresponding to
an average of a single photon in the cavity. The best cavity has an internal
quality factor of 115 million at single photon level.
