In some cases, damage caused by sputtering was observed. The
dome-like structure at the edge of the sputtered area was
confirmed to be hard by touching its surface with the needle. The
dome-like structure was broken but there was no scale bouncing off
to other areas. Therefore, it was named a “bubble” due to its shape
and soft surface. Fig. 3(c) shows the SEM image for Si bubbles and
blisters at the edge of the sputtered area using the 2.5 sccm GFR
condition after a Td of 10 s. The bubble diameter size ranges from
0.05 to 5 mm in diameter. From the literature review, the bubble is
formed due to knock-in of Ar gas molecules during bombardment
[9]. If the bubble has a large of internal pressure, the bubble will
deform and rupture [10]. However, in our system, total ion energy is
lower than 1 keV which is too low to cause ion implantation