Low-power inductive RF plasma sources for technological applications

Abstract

The objective of the present paper is to develop an analytic theory of cylindrical low-power RF plasma sources operating at an industrial frequency (f=13.56 MHz, ω=8.52×107 s−1). Inductive surface exciters of electromagnetic fields (exciting antennas) are considered that are positioned either at the side surface of the cylinder or at one of its end surfaces. In the latter case, the plasma flows out of the source through the opposite end surface of the cylinder. A study is made of elongated systems in which the length L of the cylinder exceeds its diameter 2R and of planar disk-shaped systems with L<2R. Simple analytic expressions are derived for electromagnetic fields excited by the antenna in the source plasma. The equivalent plasma resistance and the equivalent RF power deposited in the plasma are calculated for systems with prescribed parameters, i.e., in a non-self-consistent model. Up to now, such sources have been investigated mainly through the numerical solution of the complicated general electrodynamic equations. In the Introduction, the problem is formulated in general terms and the geometry of the sources, as well as the characteristic parameters of the source plasma, is discussed. In Section 2, plasma sources operating without an external magnetic field are investigated. In Section 3, helicon plasma sources in a sufficiently strong external magnetic field are considered. Analytic predictions are compared with the results from solving the problem numerically without using the helicon approximation. Section 4 gives a brief discussion of an electron cyclotron resonance-based RF plasma source. In the Conclusion, the main results of the paper are summarized and the technological efficiency of the sources under consideration is estimated at a qualitative level.