Paper PS1-ThA1
Spatial Density Distribution of Low-energy Electrons in a 2f-CCP by Laser Absorption and Optical Emission Spectroscopy

Thursday, October 23, 2008, 2:00 pm, Room 304

Optical emission spectroscopy (OES) is widely used as the tool of plasma diagnostics and plasma characteristics. The OES is restricted to the phenomena caused by higher energy electrons over the threshold of the electronic excitation of the target molecule mostly greater than ~10 eV, while the mean energy of electrons in a low-temperature radio frequency plasma is typically 3-5 eV in the bulk plasma, A simple in-situ method to determine spatiotemporally resolved transport of low energy electrons in a two-frequency capacitively coupled plasma (2f-CCP) is presented by using OES and laser absorption spectroscopy in pure Ar.¹ The method employs the long-lived metastable atom Ar(1s₅) and short-lived excited Ar(2p₉). Due to the large cross section of metastable atoms with electrons,² the net excitation rate of Ar(2p₉) obtained by OES is expressed as a function of the electron density at the peak energy ~3.3 eV and the metastable density. The spatiotemporal measurement of the electron density distribution with energy of ~3 eV is demonstrated in a typical condition in a 2f-CCP, driven at 100 MHz and biased at 500 kHz in pure Ar at 25, 50 and 100 mTorr. The density shows almost no dependence on time in the bulk plasma and has a sharp peak in the sheath in front of the bias electrode at higher pressure. The influence of photon reabsorption will be further discussed.

¹M. Ishimaru, T. Ohba, T. Ohmori, T. Yagisawa, T. Kitajima and T. Makabe, Diagnostics for low-energy electrons in a two-frequency capacitively coupled plasma in Ar , Appl. Phys. Lett. 92, 071501 (2008)
²A. A. Mityureva and V. V. Smirnov, Opt. Spectrosc. 97, 508 (2004).