The authors 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 bias conditions, either via electron-beam-induced generation of an unstable hot spot or via the beam-induced growth of a stable hot spot. 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 hot spot regime confirms the basic features of common hot spot models.

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