Second-harmonic currents in rf-biased, inductively coupled discharges

Capacitively-coupled plasmas generate strong current or voltage signals at harmonics of their driving frequencies. Inductively coupled plasma (icp) systems generally do not, unless they are equipped with capacitively-coupled rf bias, which generates strong signals at harmonics of its driving frequency. Recently, however, at an asymmetric, rf-biased electrode, a current component was detected at the second harmonic of the inductive source frequency, not the rf-bias frequency. The origin of this current is here investigated (in argon discharges at 1.3 Pa) by comparison with measurements made at a symmetric electrode and predictions made by two numerical models. The first simulates the sheath at the rf-biased electrode; the second models the plasma. Because capacitive coupling from the inductive source was minimized by a Faraday shield, the nonlinearity of the sheath contributes negligible second-harmonic current. Modulation of the photon flux in the plasma, however, produces a second-harmonic current photoemitted from the rf-biased electrode. The external circuitry and nonlinear inductive coupling produce a second-harmonic sheath voltage, which in turn generates second-harmonic current both directly and through a transit-time effect. The second model simulates how electrons emitted from the electrode-and then reflected at the quartz dielectric window of the inductive source-are deflected by the electric and magnetic fields in the plasma. It also gives predictions for the transit-time effect. Magnetic deflections and the transit-time effect usually dominate the electric deflection. Together these three mechanisms produce a second-harmonic current that has a Fourier amplitude approximately half the current that is elastically reflected at the icp window. These results suggest it may be possible to use the second-harmonic current to determine the elastic reflection coefficient at the window.