Detection of particle traps by spatially resolved optical emission spectroscopy over grooved electrodes in radio frequency discharges

Abstract

Isophotal maps of spatially resolved optical emission signal from neutral, excited argon are used to detect regions of enhanced electron induced excitation over topographically contoured, rf-coupled electrodes in argon discharges. By aligning a lengthwise groove in one such electrode with the optical axis, it is possible to monitor the plasma homogeneity inside, along and above the groove. The use of a grooved electrode, previously shown to "trap" particles, is also shown to produce enhanced excitation in localized, well defined regions, depending on the discharge pressure and sheath thickness. At low and intermediate pressures (<100 mTorr) a single "bright" spot is noted above the center of the groove. Higher pressure operation causes two bright spots to form, symmetrically placed, close to the groove sidewalls. Laser light scattering is used to simultaneously detect the coordinates of suspended particles during the discharge. A correlation is noted between these bright spots and the location of trapped particles. These results suggest that trap formation is related to changes in the local ionization rate which alter the local potential profile.