Neutral radical species takes important parts in plasma processes such as etch, deposition, and etc. For example, the concentration of the radical species determines the rates of etch and deposition reactions. So, many researchers has studied the characteristics of the radical species in experimental and theoretical manners.
In this study, the characteristics of F radical species which was created in an electron cyclotron resonance(ECR) $SF_6$/Ar discharge was examined expermentally. Also, a brief modeling was performed for the prediction of the effects of discharge conditions on the radical density and its spatial distribution.
In theoretical study, we established a simple model which included a transport phenomena of the radical atoms. For a simplicity, any reaction through which a radical atom is created or disappeared was not considered. The equations of continuity and momentum for the radical atoms were solved numerically with proper boundary conditions. The two dimensional distributions of F radical atom density and fluid velocity were obtained at various $SF_6$ flow rate, pump speed and substrate radius.
In experimental study, we used an optical emission actinometry(OEA) method for the characterization of F radical atoms in the ECR $SF_6$/Ar discharge. OEA method has been used so far for the determination of a spatial profile of a radical species concentration as well as an average relative concentration. But, the method implies an assumption that a standard atom, so called actinometer, has a uniform spatial profile of concentration. If this assumption is not valid, the spatial profile of radical species obtained by the OEA method would be incorrect. So, a new method for the determination of the spatial profile of radical atom concentration is required.
We suggested a new method, so called plasma parameter normalization method (PPNM) for the purpose. In this method, relative concentration of a neutral atom species is given as the ratio of the emission intensity of the neutral species to the plasma parameter which is a mulitiplication of plasma density by excitation rate coefficient. If the spatial profiles of the emission intensity and the plasma parameter are measured, we can derive the spatial distribution of the neutral atom concentration.
In this study, we have determined the relative concentration of F radical atoms and its spatial profile by using the PPNM as well as the OEA method. The results obtained by using the two methods, PPNM and OEA, were compared with each other. The spatial profile of the actinometer Ar atoms was also derived by the PPNM. In the measurements of the F average relative concentration with $SF_6$ flow rate varied, the results from PPNM and OEA method showed good agreement except at low flow rate. The discrepancy between the two results appearing at $SF_6$ flow rate of 5 sccm may be due to a high energy electron effect. So, the PPNM was proven to be invalid for the average concentration measurement if the energy distribution of the electron deviates seriously from Maxwellian.
However, in the derivation of the spatial profile of the F relative concentration, erroneous results were appeared in OEA measurements. From the results of PPNM for the actinometer Ar atom, the invalid results from the OEA were proven to be due to a nonuniform distribution of the actinometer atom concentration. So, OEA method is improper for the determination of the spatial profile of a radical species concentration.
In the characteristics of the F radical species, theoretically predicted and experimentally confirmed was the fact that the spatial profile of the F radical concentration(SPRC) is dependent not only on the $SF_6$ flow rate but also the pump speed. As the $SF_6$ flow rate be caused by the increases and so does the pressure, the SPRC becomes nonuniform, which may decrease of the diffusion coefficient of the radical atoms due to the increase of the collisionality.
The spatial profile of the Ar atom concentration(SPAC) shows the uniformity change different from that for the F radical atoms with respect to $SF_6$ flow rate. The uniformity of the SPAC increases while that of the SPRC decreases in a certain range of $SF_6$ flow rate. We suggested a dissimilar change of the temperature of each species, F and Ar, as a reason for the difference in the uniformity change of the SPRC and the SPAC. A different kind of reactions in which a neutral species takes part, would make the temperature variation of a species different from the other.
The effect of the pump speed on the uniformity of the spatial density profile was predicted theoretically and an evidence of the effect was found experimentally. Although a clue for the effect of the pump speed was found, we need further consideration on the effect of the pump speed for the thorough understanding.
In conclusion, we suggest the PPNM as a diagnostic method for the determination of the spatial profile of neutral species including molecular species as well as atomic species. And also, we notify the effect of various reactions occuring in a discharge on the uniformity of the spatial distribution of a neutral species.
