Experimental and simulation studies of capacitively coupled structured electrode plasmas at intermediate pressures

In this research we demonstrate the effect of structural cavities in tailoring the radial ionization nonuniformities observed in large area capacitively coupled plasma (CCP) reactors. We carried out experiments and simulations with a CCP, equipped with a structured electrode, in the intermediate pressure regime of 1-2.5 Torr, operating in Ar/O₂ (ratio 99:1%). Experiments were carried out in an Oxford Instruments PlasmaPro-100 plasma enhanced chemical vapor deposition (PECVD) reactor. The structured electrode CCP was characterized at 150W, 13.56 MHz for a range of operating pressures. The optical measurements were carried out by capturing plasma images through a 750 nm (FWHM 1 nm) band-pass filter to detect optical emission from the Ar (2p1-1s2 transition), which is primarily produced through direct-impact excitation of ground-state argon atoms by electrons with energy of at least 15.4 eV. This energy corresponds to the high energy secondary electrons population, which primarily drives the intense ionization near the electrode surface. Therefore, the measured optical signals qualitatively indicate the ionization effects caused by these energetic electrons within and surrounding the cavity region. The fluid-kinetic simulations were performed using Quantemol-virtual tool built upon the hybrid plasma equipment model. Metastables play an important role in maintaining the discharge equilibrium at intermediate pressures. Therefore, these effects were considered in the simulations by incorporating the metastables in the reaction set and the corresponding plasma chemistry was accessed using Quantemol Database. The motivation of this work was to understand how to improve the deposition uniformity across the large diameter substrate in a PECVD tool, by locally controlling the ionization rates in a CCP plasma with the help of structural cavities embedded in the powered electrode.