  We present low-temperature scanning electron microscopy (LTSEM)
investigations of superconducting microbridges made from ultrathin NbN films as
used for hot electron bolometers. LTSEM probes the thermal structure within the
microbridges under various dc current bias conditions, either via
electron-beam-induced generation of an unstable hotspot, or via the
beam-induced growth of a stable hotspot. Such measurements reveal
inhomogeneities on a micron scale, which may be due to spatial variations in
the NbN film or film-interface properties. Comparison with model calculations
for the stable hotspot regime confirm the basic features of common hot spot
models.
