Enhanced hollow cathode plasma source for assisted low pressure electron beam deposition processes

•An ion source with optimum operating pressure is demonstrated.•Ion energy distribution is shown to vary with restrictor plate geometry.•Ion energy distribution is controlled using cathode voltage.•Thin film material properties are improved at room temperature with minimal plasma induced substrate heating and defect incorporation.

A self-sustaining hollow cathode plasma source has been demonstrated to operate at an order of magnitude lower deposition pressure than previously reported, enabling plasma ion-assisted electron beam deposition at pressures of ≈ 2.0E- 4 mbar.This method uses a restrictor plate to create a pressure differential in gas flow between a hollow cathode region and the main deposition area. The cathode operates in higher density plasma, enhancing the hollow cathode effect and enabling self-sustaining plasma formation at lower gas pressures in the deposition region. It is also demonstrated that ion energy distribution and current density at the substrate plane can be varied by changing the orifice geometry and/ or gas flow.It has been shown that by varying the restrictor orifice the proportion of thermalised ions to translational ions can be reduced, thereby reducing both substrate heating by typically 60% and defect incorporation into the deposited film. This is significant when coating low temperature materials such as plastics. Moreover the variation in ion current density over the calotte area can also be controlled by varying the geometry of the restrictor plate.The hollow cathode design described in this work utilises both the interior and exterior cathode surfaces, with the additional electrons generated removing the need for a separate neutralising source as verified by Langmuir probe measurement of electron and ion densities at the substrate plane.The effects on plasma assisted electron beam deposited TiO2 film optical and mechanical properties have been assessed and correlated with plasma source characteristics.In contrast to other hollow cathode plasma source configurations, this system has a converging magnetic field allowing electron concentration within the orifice.