  Many superconducting qubits are highly sensitive to dielectric loss, making
the fabrication of coherent quantum circuits challenging. To elucidate this
issue, we characterize the interfaces and surfaces of superconducting coplanar
waveguide resonators and study the associated microwave loss. We show that
contamination induced by traditional qubit lift-off processing is particularly
detrimental to quality factors without proper substrate cleaning, while
roughness plays at most a small role. Aggressive surface treatment is shown to
damage the crystalline substrate and degrade resonator quality. We also
introduce methods to characterize and remove ultra-thin resist residue,
providing a way to quantify and minimize remnant sources of loss on device
surfaces.
