In general, the sputtered area diameter increases with
increasing gap distance as depicted by the relationship between
sputtered area diameter and Gd in Fig. 4(a). The shape of gas flow
(Fig. 1(d)) could be used for predicting the size of sputtered area.
When the gap distance is increased, the dense gas profile reached at
the irradiated area before being evacuated and bounced at the
target is expanded. This is the reason why the sputtered area
diameter increased when the gap distance is increased.