  Irradiation of cuprate high-$T_c$ superconductors with light ions of moderate
energy creates point defects that lead to a reduction or full suppression of
the critical temperature. By shaping the ion flux with a stencil mask,
nanostructures for emerging superconducting electronics can be fabricated. The
3-dimensional shape of such defect landscapes is examined, based on
calculations of full collision cascades and atom displacements. A relation
between the calculated defect density and experimental values of the critical
temperature $T_c$ in thin YBa$_2$Cu$_3$O$_{7-\delta}$ films is etablished that
allows to determine the distribution of local $T_c$'s and its 3-dimensional
visualization. The results confirm that, using 75 keV He$^+$ ion irradiation
and a stencil mask, well-defined patterns of non-superconducting material in
the superconducting matrix can be produced with low blurring.
