  When exposed to high magnetic fields, certain materials manifest an exotic
superconducting (SC) phase that has attracted considerable attention. A
proposed explanation for the origin of the high-field SC phase is the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. This state is characterized by
inhomogeneous superconductivity, where the Cooper pairs have finite
center-of-mass momenta. Recently, the high-field SC phase was observed in FeSe,
and it was deemed to originate from the FFLO state. Here, we synthesize FeSe
single crystals with different levels of disorder. The level of disorder is
expressed by the ratio of the mean free path to the coherence length and ranges
between 35 and 1.2. The upper critical field \textit{B}$_{\rm{c}2}$ was
obtained by both resistivity and magnetic torque measurements over a wide range
of temperatures, which went as low as $\sim$0.5 K, and magnetic fields, which
went up to $\sim$38 T along the \textit{c} axis and in the \textit{ab} plane.
In the high-field region parallel to the \textit{ab} plane, an unusual SC phase
was confirmed in all the crystals, and the phase was found to be robust against
disorder. This result suggests that the high-field SC phase in FeSe is not a
conventional FFLO state.
