Retarding field energy analyser ion current calibration and transmission

Accurate measurement of ion current density and ion energy distributions (IEDs) is often critical for plasma processes in both industrial and research settings. Retarding field energy analysers (RFEAs) have been used to measure IEDs because they are considered accurate, relatively simple and cost effective. However, their usage for critical measurement of ion current density is less common due to difficulties in estimating the proportion of incident ion current reaching the current collector through the RFEA retarding grids. In this paper an RFEA has been calibrated to measure ion current density from an ion beam at pressures ranging from 0.5 to 50.0 mTorr. A unique method is presented where the currents generated at each of the retarding grids and the RFEA upper face are measured separately, allowing the reduction in ion current to be monitored and accounted for at each stage of ion transit to the collector. From these I-V measurements a physical model is described. Subsequently, a mathematical description is extracted which includes parameters to account for grid transmissions, upper face secondary electron emission and collisionality. Pressure-dependent calibration factors can be calculated from least mean square best fits of the collector current to the model allowing quantitative measurement of ion current density.