Ion dynamics of rf plasmas and plasma sheaths: A time-resolved spectroscopic study

Abstract

A new plasma diagnostic technique, time-resolved laser-induced fluorescence (TRLIF) spectroscopy, has been developed to monitor ion dynamics in rf glow discharges. Time-dependent ion concentrations are measured by pulsing a dye laser synchronously with the applied rf potential and then detecting laser-induced fluorescence. In this initial work, Cl2/N2 plasmas have been investigated. In the electrode sheaths, the ion response to the applied potential was measured directly. This response governs ion transport to microelectronic device surfaces in plasma etching and deposition. At frequencies below the ion plasma frequency (∼1 MHz), we found that the electrode sheaths expand and contract with period f−1. The sheath front travels at a speed which depends linearly upon both frequency and sheath thickness, but the sheath expansion begins only after a critical voltage is applied. Characterization of the sheath in terms of its thickness, expansion speed, and critical expansion voltage promises to be a useful means by which ion transport can be modeled. In the discharge center, TRLIF was used to measure ground-state ionic lifetimes, which are essential input for kinetic models of plasma reactors. In pure N2 discharges the N+2 loss rate increases linearly with pressure but the loss mechanism is not clear. In N2/Cl2 plasma mixtures, N+2 rapidly charge exchanges with Cl2 (k≥6×106 sec−1 Torr−1) to form Cl+2 .