Relative calibration of a retarding field energy analyzer sensor array

Arrays of Retarding Field Energy Analyzer (RFEA) sensors distributed over a planar electrode surface are frequently used to measure ion distribution functions spatially resolved across a substrate surface. Typically, no sensor-to-sensor calibration is performed. To obtain realistic information on the spatial distribution of the ion flux, we developed a procedure to calibrate the ion flux measured by each sensor relative to a global reference sensor. To achieve this, we systematically changed the positions of all sensors of such an array to determine sensor specific calibration factors. The effects of the neutral gas pressure, RF power, gas mixture and time on these calibration factors are investigated in a large area low pressure CCP. We find that such a relative calibration is essential for obtaining accurate ion flux profiles, i.e. without such a calibration unrealistic results are measured. The calibration factors are almost independent of external control parameters, but change over time, i.e. such a calibration must be repeated periodically.