Second harmonic currents in rf-biased, inductively coupled plasmas

Measurements at the rf-biased electrode of an inductively coupled plasma (icp) have detected a current at the second harmonic of the icp source frequency. The origin of this harmonic has been investigated by modeling and experiments in argon at rf bias voltages of 50 V to 1.4 kV. In this voltage range, nonlinear inductive coupling [1] and nonlinear sheath impedance contribute only small fractions of the second harmonic current, which is instead predominantly produced by two other phenomena. One is second harmonic modulation of the vacuum ultraviolet photon flux in the plasma, which in turn modulates the photocurrent emitted at the rf-biased electrode. Another is deflection of emitted and reflected electrons by the rf electric and magnetic fields induced in the plasma. Models and literature data explain the size and behavior of the observed second harmonic current. The harmonic measurements, when analyzed by the models, provide accurate values or useful estimates for conditions at the dielectric window beneath the icp source, including the electron elastic reflection coefficient, electron-induced emission yield, electric and magnetic fields, and sheath voltage.

[1] V. A. Godyak, R. B. Piejak, and B. M. Alexandrovich, Phys. Rev. Lett. 83, 1610 (1999).