Sensor Verification
Metrological verification of ion beam currents in vacuum processing relies on comparing instrument readouts with reference current standards. A Faraday cup calibration determines the accuracy of the collected electrical charge by measuring the current under controlled beam conditions. This ensures that the ion dose delivered during circuit board etching or doping is precise and repeatable.
Aperture Alignment
Geometric alignment ensures that the entire ion beam enters the collector without being clipped by the edges. During the Faraday cup calibration, the aperture of the sensor is positioned relative to the ion source to eliminate measurement errors from partial beam capture. This setup is verified using a secondary grid that monitors secondary electron emission.
If emission occurs, the calculated dose is incorrect.
Loss Prevention
Secondary electron retention is necessary to prevent artificial inflation or deflation of the measured current. The Faraday cup calibration uses suppressor rings held at a negative voltage to force ejected electrons back into the collector cup. Without this suppression, the current reading is too high, leading to under-exposure of the circuit board during processing.
Thus, the bias voltage is adjusted during calibration to find the stable plateau where the current measurement becomes independent of minor voltage fluctuations.
