  We study the backreaction of quantum fields induced through the vacuum
polarization and the conformal anomaly on the collapse of a thin shell of dust.
It is shown that the final fate of the collapse process depends on the physical
properties of the shell, including its rest and gravitational masses.
Investigating the conditions for the formation of black holes, we notice that
quantum effects modify the geometry and structure of Schwarzschild space-time
in such a way that black holes have two horizons, an inner and an outer
horizon. If the gravitational mass of the shell is about that of an ordinary
star, then in most cases, the semi-classical collapse will terminate in a
singularity, and in general, quantum fluctuations are not strong enough to
prevent the creation of the singularity. Although under certain conditions, it
is possible to form a non-singular black hole, i.e., a regular black hole. In
this way, the collapse stops at a radius much larger than the Planck length
below the inner horizon, and the shell bounces and starts an expansion.
