Reconstruction of the time-averaged sheath potential profile in an argon radiofrequency plasma using the ion energy distribution

Abstract

Charge-exchange collisions and radio frequency (RF) excitation combine to give peaks in the ion energy distribution measured at the ground electrode of an argon plasma in a capacitive reactor. These peaks are used as a diagnostic to reconstruct the profile of the time-averaged potential in the sheath. Particle-in-cell !?!c) simu!aiions show that t h e method is accurate. The method is applied to investigate the sheath thickness as a function of excitation frequency at constant plasma power. The time-averaged potential is found to be parabolic in both exoerimental measurements and numerical simulations.

Sheath thickness in very-high-frequency plasma chemical vapor deposition of hydrogenated amorphous silicon

“…A fit to simultaneously find the constants a and b in t s ϰ f a p b yields as a best result: aϭ0.5 and bϭ0.85, with an error of about 10%. These cannot be compared to literature values [6][7][8][9][10][11][12] as no simultaneous fit is reported there. Figure 4 also includes data at constant pressure ͑0.4 mbar͒ as a function of frequency, taken from Ref.…”

“…Data on sheath thickness reported in literature are comparable. [6][7][8][9][10][11][12] It has to be noted that an apparent frequency dependence exists, albeit only slightly. A fit to simultaneously find the constants a and b in t s ϰ f a p b yields as a best result: aϭ0.5 and bϭ0.85, with an error of about 10%.…”

“…From the energy of the ions in these peaks the potential profile in the sheath can be reconstructed. 12 With the assumption of a constant time-and place-averaged charge density n i throughout the sheath, the energy E j of charge exchange peak number j is given by 18,24 …”

“…5 The data presented here suggest that there may exist a certain plasma parameter in which the effects of frequency and pressure variations are reflected. Various authors have reported on the dependence of the thickness of the sheath between the grounded electrode and the light-emitting plasma bulk (t s ) versus frequency f or pressure p. [6][7][8][9][10][11][12] It is known, that an increase in frequency reduces t s while a pressure decrease enlarges t s , according to t s ϰ f a , with Ϫ0.5ϽaϽϪ1.0, and t s ϰp b , with Ϫ0.3ϽbϽϪ0.55, respectively. Thus, the combination of a frequency increase and a pressure decrease can result in a constant t s .…”

“…Here we will use two recently developed methods [12][13][14][15][16][17][18] to determine the sheath thickness. The first method is based on the phenomenon that the deposition rate decreases when a gap exists between the glass substrate and the grounded a͒ Present address: Department of Solid State Physics III, Research Institute for Materials, University of Nijmegen, P. O.…”