Paper TF-WeE3
The Hollow Cathode Discharges; How They Have Been used to Produce Thin Films and the Novel Toroidal Planar Hollow Cathode System

Wednesday, December 10, 2014, 6:20 pm, Room Makai

In 1916 F. Paschen first report the hollow cathode discharge he demonstrated that the system was capable of producing a high electron flux with relatively low ion and neutral temperatures. Approximately 40 years later Lidsky showed that hollow cathode arc discharges were one of the best plasmas sources available at that time. The term "hollow cathode discharges" has been used in reference to almost any discharge in a cathode with a cavity-like geometry, such that the plasma was enclosed by the walls which are at the cathode potential. Just as trapping of electrons in a magnetron cathode by the magnetic field results in an increase in the plasma density, in the hollow cathode the geometry of the cathode also produces a high plasma density. In general, three types of discharge can be established in a hollow cathode; at low power and / or at relatively low gas pressures the plasma is a “conventional” discharge characterized by low currents and medium to high voltages (a Discharge in a Hollow Cathode or D-HC). However, even this simple plasma has a higher density than a normal planar parallel electrode system because the hollow geometry reduces the loss of electrons. If the combination of gas pressure, applied power and hollow cathode diameter is correct, the negative glow of the plasma almost completely occupies the interior volume of the cathode. Under this condition the plasma current can, for the same voltage, be 100 to 1000 times the values for the “simple” D-HC discharge and the plasma density is very large (this is the Hollow Cathode Discharge or HCD). If the temperature of cathode can increase so that Thermal-Field electron emission becomes an important additional source of electrons the discharge can change into a dispersed arc (this is the Hollow Cathode Arc or HCA). The accepted explanation for the HCD phenomenon involves the existence of high energy “pendulum” electrons reflected from sheath to sheath on either side of the inside of the cathode; the long trajectory of these electron is thought to produce an increased number of secondary electrons, which produces the high plasma density and plasma current. We will discuss some of the problems associated with the well-accepted model and we will propose a new explanation which has some important implications.

Finally, we will describe how hollow cathodes can be used to deposit thin films and nanostructured coatings, including the use of our novel toroidal planar hollow cathode to produce bismuth thin films, nanoparticles and bismuth/a-C:H nanocomposites.