Electrical and spectroscopic characterizations of a low pressure argon discharge created by a broad-band helical coupling device

This paper presents the spatial characterization of Ar plasma induced by a broad-band helical coupling device at low pressure (15 Pa). The electric field analysis outside the helix but within the coupling device is carried out by electrostatic probe for several excitation frequencies ranging from 13.56 to 433 MHz. The results prove that this helical cavity behaves as a slow-wave plasma source. At 13.56 MHz, the spatial investigation of electric field with and without plasma led to show a flat profile along the helix structure. At this frequency, the plasma is studied by spatially resolved emission spectroscopy and by means of a double Langmuir probe in order to evaluate the densities of some characteristic species (electron and Ar emitter) and the electron and gas temperatures. These temperatures are invariants along the discharge with average values about 400 +/- 50 K and 2.3 +/- 0.6 eV for gas and electron, respectively. Consequently, the electron energy distribution function and argon ground state density are also constant along the discharge. No variation of the argon metastable density, estimated by means of a simplified kinetic model, could be shown and an average value equal to 9 x 10(17) m(-3) is obtained. In contrast, the density profiles of electron and Ar emitter show a minimum situated in the middle of the helical structure and two maxima located at the positions where the RF power is transmitted to the gas and at the end of the helix. The electron density values vary from 3 x 10(16) to 10(17) m(-3). According to the low pressure conditions and the flat evolution of the electric field, these hollow shaped density profiles can be explained as a consequence of a non-linear phenomenon.