LASER-INDUCED FLUORESCENCE MEASUREMENT OF THE DYNAMICS OF A PULSED PLANAR SHEATH

Using laser-induced fluorescence (LIF) the ion density near the edge o f an expanding plasma sheath has been measured. These measurements uti lized a transition of N2+ [the P12 component of the X2SIGMA(g)+(nu = 0 ) double-line arrow pointing right B2SIGMA(u)+(upsilon = 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 ex panding sheath was produced by applying a -5 kV pulse to a polished pl anar electrode in the plasma source ion implantation device [J. R. Con rad et al., J. Vac. Sci. Technol. A 8, 3146 (1990)]. The laser beam wa s aligned normal to the surface and was reflected off the center of th e electrode. The LIF diagnostic used here is nonperturbing whereas pre vious 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 comparis on between experimental measurements and theoretical predictions. It h as been found that the sheath edge moves approximately 16 times faster than the ion-acoustic velocity during the early par-t of the pulse, t < 1 mus, and then slows to approximately the ion-acoustic velocity af ter 6 mus. In addition to the LIF measurements, a biased probe was use d far from the cathode to determine the sheath edge location. Good agr eement 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 mo del show moderate agreement with the data, substantial discrepancies a re observed. These discrepancies are attributed to a number of physica l phenomena that are not included in the present model.