  We study the effect of random dilution of attraction centers on electron
pairing in a generalization of the negative $U$ Hubbard model where the
interaction $U_i$, defined on the two dimensional lattice, can be $-|U|$ and 0
with probability $c$ and $1-c$ depending on the lattice site $i$. Using the
determinant quantum Monte Carlo approach, we find the critical concentration
$c_0$ of negative centers which leads to superconductivity, and show how the
evolution of the local density on the two types of sites lends insight into the
formation of global phase coherence.
