Two-dimensional CT images of two-frequency capacitively coupled plasma

Abstract

Two-dimensional images of two-frequency capacitively coupled plasma (2f-CCP) in Ar and Ar/CF4(5%) in an axisymmetric parallel plate reactor are investigated by using 2D-t optical emission spectroscopy. Spatially averaged electron density is obtained by microwave interferometry. Results are presented in the form of 2D profiles of the net excitation rate of Ar(3p5)(εex=14.5 eV) and Ar+(4p4D7/2)(εex=35.0 eV) used as a probe. Large area uniformity of plasma production driven at very high frequency (VHF) (100 MHz) and that driven at high frequency (HF) (13.56 MHz) at low pressure (∼25 mTorr) are compared and discussed under a low frequency (LF) (700 kHz) bias voltage on the wafer. The time modulation of the net excitation rate and the electron density indicate that the LF bias is considerably influential in the production of the plasma and in the confinement of high energy electrons at HF. Functional separation between plasma production in a gas phase and ion acceleration to the wafer is achieved in 2f-CCP excited at VHF (100 MHz). The addition of a small amount of CF4 to the Ar plasma improves the uniformity of the radial profile of the excitation at HF (13.56 MHz).

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