  We report the observation of omega phase formation in Nb thin films deposited
by high-power impulse magnetron sputtering (HiPIMS) for superconducting qubits
using transmission electron microscopy (TEM). We hypothesize that this phase
transformation to the omega phase with hexagonal structure from bcc phase as
well as the formation of {111}<112> mechanical twins is induced by internal
stress in the Nb thin films. In terms of lateral dimensions, the size of the
omega phase of Nb range from 10 to 100 nm, which is comparable to the coherence
length of Nb (~40 nm). In terms of overall volume fraction, ~1 vol.% of the Nb
grains exhibit this omega phase. We also find that the omega phase in Nb is not
observed in large grain Nb samples, suggesting that the phase transition can be
suppressed through reducing the grain boundary density, which may serve as a
source of strain and dislocations in this system. The current finding may
indicate that the Nb thin film is prone to the omega phase transition due to
the internal stress in the Nb thin film. We conclude by discussing effects of
the omega phase on the superconducting properties of Nb thin films and
discussing pathways to mitigate their formation.
