Influence of an addition of H2 in a N2 CCP discharge
Résumé
The present work involves the study of Capacitively Coupled Plasmas (CCPs) produced at 13.56 MHz radio-frequency (RF) and 0.2 to 1.2 mbar pressures. The main characteristics of the CCP source, which is described with more detail in reference [1], are the following: (i) the discharge is confined laterally by a grounded cylindrical metallic grid; (ii) the gas is continuously injected into the chamber through the driven electrode, ensuring a uniform gas flow; and (iii) gases are pumped by a rotary-vane vacuum pump. In a parent abstract [2] we study CCPs produced in pure N2. Here we analyse the effect (upon both the electrical discharge and the plasma) of adding a small amount of H2 (up to 5%) to the RF nitrogen plasma. The measured discharge parameters are: (i) the applied voltage Vrf (which is kept constant as we vary the pressure and the concentration of H2); (ii) the self-bias potential Vdc of the polarized electrode; and (iii) the absorbed RF power. We observe that, for the same Vrf value, the effective RF absorbed power depends on both the pressure and the gas mixture composition. The plasma is studied by Optical Emission Spectroscopy (OES) from UV to near Infra-Red. The bands of the nitrogen second positive system (SPS), the nitrogen ionic first negative system (FNS) and the hydrogen line Hβ (486.1 nm) are recorded as a function of the H2 concentration, for different pressures. A small amount of argon is added to the gas mixture to act as an actinometer. For OES measurements, the 811.5nm Ar line is chosen because it doesn't overlap with the nitrogen first positive system and because its excitation energy (13.07 eV) is close to the 12.74 eV of Hβ. For a given value of Vrf, our measurements show variations of the argon line intensity for different initial hydrogen concentrations. This behaviour reveals that the addition of H2 into N2 may affect the plasma chemistry. We will present the changes, for different plasma conditions, of the SPS bands of N2 and the FNS bands of N2+, the latter allowing to monitor the influence of H2 on the plasma ionisation. The variation of the H2 dissociation degree, for various work conditions, will also be presented.