  The Spectrometer/Telescope for Imaging X-rays (STIX) is the HXR instrument
onboard Solar Orbiter designed to observe solar flares over a broad range of
flare sizes, between 4-150 keV. We report the first STIX observations of
microflares recorded during the instrument commissioning phase in order to
investigate the STIX performance at its detection limit. This first result
paper focuses on the temporal and spectral evolution of STIX microflares
occuring in the AR12765 in June 2020, and compares the STIX measurements with
GOES/XRS, SDO/AIA, and Hinode/XRT. For the observed microflares of the GOES A
and B class, the STIX peak time at lowest energies is located in the impulsive
phase of the flares, well before the GOES peak time. Such a behavior can either
be explained by the higher sensitivity of STIX to higher temperatures compared
to GOES, or due to the existence of a nonthermal component reaching down to low
energies. The interpretation is inconclusive due to limited counting statistics
for all but the largest flare in our sample. For this largest flare, the
low-energy peak time is clearly due to thermal emission, and the nonthermal
component seen at higher energies occurs even earlier. This suggests that the
classic thermal explanation might also be favored for the majority of the
smaller flares. In combination with EUV and SXR observations, STIX corroborates
earlier findings that an isothermal assumption is of limited validity. Future
diagnostic efforts should focus on multi-wavelength studies to derive
differential emission measure distributions over a wide range of temperatures
to accurately describe the energetics of solar flares. Commissioning
observations confirm that STIX is working as designed. As a rule of thumb, STIX
detects flares as small as the GOES A class. For flares above the GOES B class,
detailed spectral and imaging analyses can be performed.
