  Properties of the solar radio spectrum, as well as the temporal profiles of
flare emission, indicate the thermal nature of the sub-terahertz (sub-THz)
component observed as the growth of radio emission in the frequency range of
100-1000 GHz. The sub-THz flare onset can be ahead of the impulsive phase for
several minutes. However, the origin of the pre-impulsive and impulsive sub-THz
emission remains unclear. The present work is devoted to a detailed analysis of
the M4.0 X-class solar flare observed on March 28, 2022 with the Bauman Moscow
State Technical University Radio Telescope RT-7.5 at 93 GHz. We supply these
data with multiwavelength solar observations in the X-ray (GOES, GBM/Fermi),
extreme ultraviolet (AIA/SDO), and microwave ranges. The differential emission
measure (DEM) responsible for EUV emission is determined by solving the inverse
problem based on the AIA/SDO data. Using the DEM and assuming a thermal
free-free emission mechanism in pre-impulsive and impulsive phases, we
calculated the millimeter emission flux of coronal plasma of the flare source,
which turned out to be much smaller than the observed values. We concluded that
electrons accelerated in the corona and heat fluxes from the coronal loop top
cannot be responsible for heating the sub-THz emission source located in the
transition region and upper chromosphere. A possible origin of chromospheric
heating in the pre-impulsive phase of the solar flare is discussed.
