DETECTION OF PARTICLE TRAPS BY SPATIALLY-RESOLVED OPTICAL-EMISSION SPECTROSCOPY OVER GROOVED ELECTRODES IN RADIO-FREQUENCY DISCHARGES

Isophotal maps of spatially resolved optical emission signal from neut ral, excited argon are used to detect regions of enhanced electron ind uced excitation over topographically contoured, rf-coupled electrodes in argon discharges. By aligning a lengthwise groove in one such elect rode with the optical axis, it is possible to monitor the plasma homog eneity inside, along and above the groove. The use of a grooved electr ode, previously shown to ''trap'' particles, is also shown to produce enhanced excitation in localized, well defined regions, depending on t he discharge pressure and sheath thickness. At low and intermediate pr essures ( < 100 mTorr) a single ''bright'' spot is noted above the cen ter of the groove. Higher pressure operation causes two bright spots t o form, symmetrically placed, close to the groove sidewalls. Laser lig ht scattering is used to simultaneously detect the coordinates of susp ended particles during the discharge. A correlation is noted between t hese bright spots and the location of trapped particles. These results suggest that trap formation is related to changes in the local ioniza tion rate which alter the local potential profile.