Relationship between center-peaked plasma density profiles and harmonic electromagnetic waves in very high frequency capacitively coupled plasma reactors

An understanding of the factors that control radial plasma uniformity in very high frequency (VHF) capacitively coupled plasma (CCP) sources is important for many plasma processes in semiconductor device manufacturing. Here, we report experimental measurements and high-resolution self-consistent numerical simulations that illustrate the plasma density profile and the higher harmonic wave content in two types of VHF-CCP test-bench reactors. A distinct sharp center peak superimposed on a broad center peak in argon plasma was observed for driving frequencies of 60 and 106 MHz. Experimental measurements and numerical simulations of the electric field power spectrum reveal the presence of UHF waves when the electron density is over 5 × 1016 (#/m3). The presence of UHF waves closely correlates with the occurrence of a distinct and sharp-center-peaked electron density. The numerical simulations show that specific frequency bands in the UHF spectrum are amplified in the plasma and lead to the evolution of the sharp-center-peaked electron density.