The use of hollow cathodes in deposition processes: A critical review

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The first report of a discharge in a hollow cathode was by F. Paschen in 1916. That study showed that such a system was capable of producing a high electron flux and relatively low ion and neutral temperatures. About 40years later, the work of Lidsky and others showed that hollow cathode arc discharges were one of the best plasma sources available at that time. The term “hollow cathode discharges” has commonly been used in reference to almost any discharge in a cathode with a cavity-like geometry, such that the plasma was enclosed or partially bound by the electrode walls that were at the cathode potential. Just as the magnetic field trapping of the electrons in a magnetron cathode results in an increase in the plasma density, in the hollow cathode, the reduced electron loss due to the geometry of the cathode also results in a higher plasma density. At least 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 (we will call this a discharge in a hollow cathode or D-HC). Even this type of plasma has a higher density than a normal planar parallel-plate or magnetron system because the hollow geometry strongly reduces the loss of electrons. Using an adequate combination of gas pressure and applied power with a given hollow cathode diameter, or separation of the cathode surface, the negative glow of the plasma can expand to occupy the majority of the interior volume of the cathode. Under this condition the plasma current can, for the same voltage, be 100 to 1000 times the value of the “simple” D-HC discharge, and the plasma density is correspondingly larger (we call this a hollow cathode discharge or HCD). If the cathode is not cooled, the discharge can transform into a dispersed arc as the electrode temperature increases and thermal-field electron emission becomes an important additional source of electrons (we will call this a hollow cathode arc or HCA). The accepted explanation for the HCD phenomenon involves the existence of high-energy “pendulum” electrons, which are reflected from the sheaths on either side of the cathode; the long trajectory of this electron is understood to produce a large number of secondary electrons, with this resulting in the high plasma density and plasma current. We describe the structure of a parallel-plate discharge, particularly the gas phase and cathode surface excitation and ionization collision processes. Using this description, we discuss some of the problems associated with the conventional hollow cathode model and we propose a new explanation that has important implications for the physics and applications of hollow cathodes.In the last section of this review, we describe how hollow cathodes have and can be used to deposit thin films and nanostructured coatings. We provide an extensive and approximately chronological listing of how hollow cathodes have been successfully used to deposit materials, mainly by sputtering and plasma enhanced chemical vapour deposition based techniques.