  We present microwave-frequency NbTiN resonators on silicon, systematically
achieving internal quality factors above 1 M in the quantum regime. We use two
techniques to reduce losses associated with two-level systems: an additional
substrate surface treatment prior to NbTiN deposition to optimize the
metal-substrate interface, and deep reactive-ion etching of the substrate to
displace the substrate-vacuum interfaces away from high electric fields. The
temperature and power dependence of resonator behavior indicate that two-level
systems still contribute significantly to energy dissipation, suggesting that
more interface optimization could further improve performance.
