FUNDAMENTAL-STUDIES OF MIXED-GAS INDUCTIVELY-COUPLED PLASMAS

The effects of adding foreign gases to the central-gas flow or the int ermediate-gas flow of an argon inductively coupled plasma are presente d. In particular, the influence of up to 16.7% added helium, nitrogen or hydrogen on radially-resolved electron number density, electron tem perature, gas-kinetic temperature and calcium ion emission profiles is examined. It is shown that these gases affect not only the fundamenta l parameters and bulk properties of the plasma, but also how energy is coupled and transported through the discharge and how that energy int eracts with the sample. For example, added helium causes an increase i n the gas-kinetic temperature, most likely due to the higher thermal c onductivity of helium compared to argon but, in general, does not appe ar to affect significantly either the electron temperature or electron concentration. The shift in the calcium ion emission profile towards lower regions in the discharge with added helium may be attributable t o higher droplet desolvation and particle vaporization rates. In contr ast, the addition of nitrogen or hydrogen to an Inductively Coupled Ar gon Plasma (Ar ICP) results in dramatic changes in all three fundament al plasma parameters: electron number density, electron temperature, a nd gas-kinetic temperature. The net effect of these molecular gases (N -2 or H-2) on calcium ion emission and on the fundamental plasma param eters is shown to be dependent on the amount of gas added to the plasm a and whether the gas is introduced as part of the central- or interme diate-gas flow. In general, nitrogen added to the central-gas Bow caus es a significant reduction in the number of electrons throughout most of the discharge (over an order of magnitude in certain regions), main ly in the central and upper zones of the ICP. A drop of 3000-5000 K in the central channel electron temperature and a smaller drop in the ga s-kinetic temperature are also observed when N-2 is added to the centr al-gas flow. In contrast, the introduction of nitrogen in the intermed iate flow causes about a 1 x 10(15) electrons cm(-3) increase in the e lectron concentration in the low, toroidal regions of the plasma and a n increase in the gas-kinetic temperature of around 1000 K throughout most of the discharge. As seen with the addition of nitrogen to the ce ntral-gas flow, the electron temperature is found to increase in the t oroidal zones of the plasma when N-2 is added to the intermediate flow . These combined effects cause a 20-fold depression in the calcium ion emission intensity only a 1.7-fold depression when N-2 is added to th e central- or intermediate-gas flows, respectively. On the other hand, hydrogen causes a depression in the electron concentration in the upp er areas of the plasma when this gas is added to the central flow but increases the number of electrons in the same region when added to the intermediate flow. Hydrogen also causes a dramatic effect on the elec tron and gas-kinetic temperatures, significantly increasing both of th ese parameters throughout the discharge. An increase in the calcium io n emission intensity, accompanied by a downward shift, elongation and broadening of the calcium ion emission profile is also observed with H -2 addition.