The hollow cathode. A high-performance tool for plasma-activated deposition

The hollow cathode will be presented as a plasma source for reactive evaporation processes. The hollow cathode generates an arc discharge plasma. This contains a high portion of directed electrons with an enhanced mean energy, the so called low-voltage electron beam (LVEB). The mean energy of the LVEB, in the range of 11 eV, results in a very effective ionization of the gas and vapor particles. Consequently, very high plasma densities can be achieved, which corresponds to high particle densities in high-rate deposition processes. Furthermore, a high self bias potential of about 16 V is obtained on insulating substrates. For the coating of heat-sensitive substrates with high deposition rates a process with a low ratio between thermal load and deposition rate is necessary. The heat flux on plastic substrates has been measured at the reactive Al evaporation process. The overall thermal load of about 4 W/cm2 related to a deposition rate of 100 nm/s is low in comparison to other processes. This makes the hollow cathode a favorable tool for the plasma-activated high-rate deposition. The oxide layers deposited by this process show dense and glassy structures even at comparatively low condensation temperatures. This is caused by the high ion current densities in the order of 30 mA/cm2. The low ion energy determined by the self bias potential results in relatively low compressive stress below 100 MPa. The low thermal load and the moderate intrinsic stress of the layers makes the hollow cathode plasma-activated deposition (HAD) process the method of choice for the deposition of oxides as abrasion resistant layers on plastic films and sheets