Laser-induced fluorescence measurement of the dynamics of a pulsed planar sheath

Abstract

Using laser-induced fluorescence (LIF) the ion density near the edge of an expanding plasma sheath has been measured. These measurements utilized a transition of N+2 [the P12 component of the X 2Σ+g(ν=0)→B 2Σ+u(ν=0) band] in a N2 plasma. The strength of the laser-induced fluorescence was used as a measure of the temporally and spatially varying ion density. The expanding sheath was produced by applying a −5 kV pulse to a polished planar electrode in the plasma source ion implantation device [J. R. Conrad et al., J. Vac. Sci. Technol. A 8, 3146 (1990)]. The laser beam was aligned normal to the surface and was reflected off the center of the electrode. The LIF diagnostic used here is nonperturbing whereas previous researchers have used Langmuir probes, which perturb the plasma, to make their measurements. As such, the data reported here represent a benchmark measurement of pulsed sheaths and allow a better comparison between experimental measurements and theoretical predictions. It has been found that the sheath edge moves approximately 16 times faster than the ion-acoustic velocity during the early part of the pulse, t<1 μs, and then slows to approximately the ion-acoustic velocity after 6 μs. In addition to the LIF measurements, a biased probe was used far from the cathode to determine the sheath edge location. Good agreement is found when the LIF and probe data are compared. The LIF data also are compared to the predictions of a simulation that is based on a time-varying two-fluid model of the sheath [G. A. Emmert and M. A. Henry, J. Appl. Phys. 71, 113 (1992)]. While the predictions of the model show moderate agreement with the data, substantial discrepancies are observed. These discrepancies are attributed to a number of physical phenomena that are not included in the present model.