Investigation of Influencing Factors on the Measurement Signal of a CMOS Image Sensor for Measuring Field Emission Currents.

We use optical CMOS image sensors for spatially and time-resolved measurement of the emission currents of field emission cathodes. The measured signal depends, on the one hand, on the emission current that flows from the cathode surface through the vacuum to the sensor surface. On the other hand, it is influenced by other variables, such as the extraction voltage, which accelerates the electrons towards the sensor surface, and the exposure time set on the sensor. In this article, these influencing factors on the measured pixel signals of a CMOS image sensor are examined in detail. In the first step, an equation is formulated that describes the signal measured by the sensor as a function of the emission current from a field emission tip, with the acceleration voltage and the exposure time as parameters. In the next step, we explain how the sensor signal is determined from the captured images. We then conduct experiments with a segmented field emission array consisting of 2 × 2 individually addressable emitters, where the voltage and currents for each emitter are known. The sensor signals are then measured for various voltages and currents and compared with the theoretical predictions. Thus, we demonstrate that, for a known voltage, the sensor signals obtained from the images can be corrected using the theoretical correlation, allowing the sensor signal to be used to measure the emitter current. This method can also be applied to investigate field emission arrays with many tips, provided that the emission spots on the CMOS sensor images can be clearly distinguished.