Measurement of individual ion density using floating harmonic method in Ar/He mixed inductively coupled plasma

Argon and helium ion density were obtained in Ar/He mixed gas discharge. Experiments were performed at pressures of 10, 25, 50 mTorr in planar type inductively coupled plasma. The electron temperature, electron density, and ion saturation current were measured using the floating harmonic method and electron energy distribution function to obtain the ion density. Experimental results show that the relative density of argon ion ([n_Ar+]/[n_Ar+]+[n_He+]) increases with increasing the flow rate portion of argon ([Ar]/[Ar]+[He]). The results were reasonable considering the low ionization energy of argon and the penning ionization effect by He^*. At 25 mTorr, when the flow rate portion of the argon is 90%, the relative ion density of the argon is maximized near the coil. On the other hand, when the flow rate portion of the helium is 90%, the relative ion density of the argon is maximized near the discharge center. The reason is that helium discharge has a relatively long energy relaxation length compared to argon, and as the portion of the helium increases, the electron relaxation length is extended. Also, because helium ions diffuse better than argon ions, the relative ion density of helium at the edge is high.