Glow-discharge sheath electric fields: Negative-ion, power, and frequency effects.

Abstract

Using Stark-mixed laser-induced fluorescence, space-time--resolved maps of sheath electric fields in discharges through ${\mathrm{BCl}}_{3}$ and Ar are measured as a function of concentration, power density (0.14--0.41 W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$), and frequency (dc to 10 MHz). Sheath oscillations are observed throughout this frequency range and are discussed qualitatively in terms of characteristic times for ion and electron transport. Double layers are observed in low-frequency discharges containing more than 5% ${\mathrm{BCl}}_{3}$ and are attributed to a buildup in negative-ion concentration; it is argued that different ratios of negative-ion to electron density in the plasma and sheath are responsible for double-layer formation at the plasma-sheath boundary. These maps should prove useful as input for particle transport simulations in plasma-processing and light-source applications. They also provide stringent tests for self-consistent sheath theories.