A method of forming a semiconductor device to have a gap between wirings formed
on a substrate, which gap is filled with a gas having a thermal conductivity equal
to or higher than three times that of air at zero degrees Celsius. In the method,
the following steps are performed: (A) forming a wiring and a filling layer filled
between wirings, on a substrate; (B) forming a gas permeable film on the wiring
and the filling layer; (C) removing the filling layer through the gas permeable
film so as to form a gap between the wirings; (D) filling a gas having a thermal
conductivity equal to or higher than three times that of air at 0.degree. C. through
the gas permeable film into the gap; and (E) forming a gas impermeable film on
the gas permeable film.
