  Lithium niobate is a promising material for developing quantum acoustic
technologies due to its strong piezoelectric effect and availability in the
form of crystalline thin films of high quality. However, at radio frequencies
and cryogenic temperatures, these resonators are limited by the presence of
decoherence and dephasing due to two-level systems. To mitigate these losses
and increase device performance, a more detailed picture of the microscopic
nature of these loss channels is needed. In this study, we fabricate several
lithium niobate acoustic wave resonators and apply different processing steps
that modify their surfaces. These treatments include argon ion sputtering,
annealing, and acid cleans. We characterize the effects of these treatments
using three surface-sensitive measurements: cryogenic microwave spectroscopy
measuring density and coupling of TLS to mechanics, x-ray photoelectron
spectroscopy and atomic force microscopy. We learn from these studies that,
surprisingly, increases of TLS density may accompany apparent improvements in
the surface quality as probed by the latter two approaches. Our work outlines
the importance that surfaces and fabrication techniques play in altering
acoustic resonator coherence, and suggests gaps in our understanding as well as
approaches to address them.
