EEDF and plasma parameters in a spherical ICP

The electron energy distribution function (EEDF) was measured in the center of inductively coupled plasma (ICP) contained in a spherical Pyrex vessel. The ICP was energized with a screened equatorial induction coil symmetrically driven at 3 MHz. The EEDF and its integrals, the effective electron temperature T/sub e/ and the plasma density n, were inferred from probe measurements carried out over a wide range of total rf power P=60-500 W delivered through a matching-balancing transformer to the induction coil, at argon gas pressure p between 0.3 and 30 mTorr. The measurements showed that at relatively high argon pressure (10 mTorr and higher), the EEDF processes a two-temperature structure with depletion of its high energy part starting at an electron energy /spl epsiv//sub 1/ close to the excitation energy /spl epsiv/*. At lower pressure (p<10 mTorr), /spl epsiv//sub 1/>/spl epsiv/*, and in the collisionless regime (p/spl les/3 mTorr and lower, where the ICP is in the domain of strong anomalous skin effect) /spl epsiv//sub 1/ is close to eV/sub /spl omega//. Such depletion at /spl epsiv/>/spl epsiv//sub 1/ is typical for gas discharge in the near-collisionless regime when electron and ion escape to the wall is the main energy loss process. However, in spherically bounded plasma, the EEDF depletion in the plasma center at /spl epsiv/>eV/sub /spl omega// appeared to be much steeper since all electrons coming to the plasma center from the wall were normally reflected there. Another peculiarity revealed from EEDF at low argon pressure was a strong increase in the electron temperature with rf power. That is quite opposite to typical low-pressure das discharge plasma. This increase in Te was accompanied by a change in the discharge color and is similar (has the same blue color and the same nature) to the blue mode of helicon plasma in argon gas. In both cases, the change in the discharge color is caused by a significant rise in electron temperature due to gas heating and rarefication at large rf power density. Reduction in argon density requires a higher electron temperature to satisfy the ionization balance in an ICP. A significant rise of electron temperature with rf power occurs only at very low gas pressure when electron temperature is strongly depends on the product pR.