Comparison of argon electron-cyclotron-resonance plasmas in three magnetic field configurations. II. Energy distribution of argon ions

Abstract

A gridded energy analyzer (GEA) has been implemented on a 2.45 GHz magnetic-mirror electron-cyclotron-resonance plasma tool to characterize the energy distribution function of Ar ions downstream from the cyclotron-resonance region. The microwave power is applied through an alumina window located near one mirror peak, and the GEA is located 14 cm beyond the second mirror peak at the substrate location. The mirror peak-to-peak separation is 50 cm. The dependence of the ion distribution function and current density upon pressure, microwave power, and magnetic field configuration were determined. Three magnetic field configurations were studied: symmetric mirror with a mirror ratio of 2; asymmetric mirror (i.e., divergent field), and minimum-B mirror. The operating pressure was found to have a small effect on the average ion energy until the pressure dropped below 0.05 mTorr. For pressures from 0.05 to 1.0 mTorr the energy distribution function usually exhibited a large low-energy peak with a high-energy tail or peak. The upper edge of this high-energy ion group corresponded to the midplane plasma potential measured by a Langmuir probe and ranged from 15 to 25 eV. As pressure increased, the position of the low-energy peak dropped from 10 to 5 eV. At the GEA position downstream from the mirror midplane, the maximum current density was found to be 4.5 mA/cm2 for a symmetric mirror magnetic field profile. For an asymmetric mirror configuration the current density was found to be only 1 mA/cm2, increasing to 3.5 mA/cm2 as the downstream mirror coil current was increased. The minimum-B field gave a current density similar to that of the asymmetric case.