A distributed carbon cathodic vacuum arc

Abstract

We report here on a new form of steady state vacuum arc whereby an anomalous emission is observed from a thermally insulated carbon cathode. The arc is distributed over a diffuse area instead of being confined to cathodic spots. It occurs when the power per unit area is sufficient for the active region cathode surface to reach a critical temperature. At ignition the discharge evolves mainly as microscopic spots over the surface of the initially cold cathode. The rise in the cathode temperature, required for the carbon plasma around the cathode surface to be sufficient for the formation of a distributed arc, is caused by the reverse ion bombardment and Joule heating of the thermally insulated cathode. Note-worthy features of such an arc are its significantly lower current density, marginally higher arc voltage than that of the cathodic spot arc and reduced amplitude of voltage oscillations. The macroparticle content of its associated plasma is also lower than in the case of the cathodic spot arc. In order to achieve an understanding of the emission mechanism of such an arc, the emanating plasma stream is characterized by its electron density, temperature and the energy distribution of the ions. The spatial temperature profile of the cathode surface is also monitored during deposition. Since stable arcs of the above-mentioned type have also been observed on other materials such as undoped Si, the prospect of using such arcs as a source of ions for deposition of thin films is encouranging.