  Dielectric losses are one of the key factors limiting the coherence of
superconducting qubits. The impact of materials and fabrication steps on
dielectric losses can be evaluated using coplanar waveguide (CPW) microwave
resonators. Here, we report on superconducting CPW microwave resonators with
internal quality factors systematically exceeding 5x106 at high powers and
2x106 (with the best value of 4.4x106) at low power. Such performance is
demonstrated for 100-nm-thick aluminum resonators with 7-10.5 um center trace
on high-resistivity silicon substrates commonly used in quantum Josephson
junction circuits. We investigate internal quality factors of the resonators
with both dry and wet aluminum etching, as well as deep and isotropic reactive
ion etching of silicon substrate. Josephson junction compatible CPW resonators
fabrication process with both airbridges and silicon substrate etching is
proposed. Finally, we demonstrate the effect of airbridges positions and extra
process steps on the overall dielectric losses. The best quality fa ctors are
obtained for the wet etched aluminum resonators and isotropically removed
substrate with the proposed ultrasonic metal edge microcutting.
