Plasma molding over surface topography: Energy and angular distribution of ions extracted out of large holes

Abstract

Plasma molding over surface topography was investigated by measuring the energy and angular distribution of ions extracted from a hole in contact with a high density plasma. Holes with diameter larger than as well as smaller than the local sheath thickness were studied in argon or deuterium gas. When the hole diameter (10 μm) was much less than the sheath thickness, the plasma was not perturbed by the presence of the hole. The ion energy distribution (IED) had multiple peaks due to ions sampling the time-varying potential while crossing the sheath. The ion angular distribution (IAD) was Gaussian, peaking at zero angle with respect to the surface normal. These results agree with reported studies. At the other extreme, when the hole diameter (1270 μm) was larger than the sheath thickness, plasma “leaked” into the hole. The IED had a single peak since ions now experience an average sheath potential. The IAD was quite broad extending beyond 30° off normal. When the hole diameter (508 μm) was comparable to the sheath thickness, the shape of the IED and IAD was in-between the two extremes mentioned above. The IAD became more isotropic with increasing power, suggesting that the plasma leaked only partly through the hole (the plasma–sheath meniscus was located inside the hole). For all cases, increasing pressure resulted in lower ion energy in argon plasmas due to ion–neutral collisions. Increasing pressure had little effect on the ion energy for deuterium plasmas, for hole diameter less than 508 μm. This is due to the smaller ion–neutral collision cross section for deuterium.