Experimental diagnosis of electron density and temperature in capacitively coupled argon plasmas: Triple-frequency discharges and two-dimensional spatial distributions

An approach combining optical emission spectroscopy with a collisional radiative model (OES-CRM) has been used to diagnose electron density and electron temperature in low pressure capacitively coupled argon plasmas. The electron density and electron temperature obtained by the OES-CRM show reasonable agreement with the results measured by a Langmuir probe. The method was first applied to diagnosis in triple-frequency (2, 13.56, and 27.12 MHz) plasmas. Compared with dual-frequency (2 and 27.12 MHz) discharges, it was found that in triple-frequency discharges, the high frequency source controls electron density more independently and with less influence on electron temperature as the intermediate frequency power increases. Next, the method was extended to a two-dimensional diagnosis based on the use of a charge coupled device camera and optical bandpass interference filters. The results showed that the axial and radial distributions of the electron density are more uniform at a lower radio frequency (RF) power. The axial uniformity of the electron density is better at a lower discharge frequency while the radial profiles of the electron temperature are flatter at a higher frequency. In all the cases, the electron temperature is highly uniform within the bulk plasma. Moreover, a mode transition from an α to a γ mode is observed at 13.56 MHz with the increasing RF power, and this is accompanied by a significant enhancement in electron density and a sharp reduction in electron temperature.