In situ quantitative field emission imaging using a low-cost CMOS imaging sensor

Abstract

Spatially resolved field emission measurements represent an important factor in further development of existing field emitter concepts. In this work, we present a novel approach that allows quantitative analysis of individual emission spots from integral current-voltage measurements using a low-cost and commercially available CMOS camera. By combining different exposure times to extrapolate oversaturated and underexposed pixels, a near congruence of integral current and image brightness is shown. The extrapolation also allows parallel investigation of all individual tips participating in the total current with currents ranging from a few nanoampere to one microampere per tip. The sensitivity, which is determined by the integral brightness-to-current ratio, remains unchanged within the measurement accuracy even after ten full measurement cycles. Using a point detection algorithm, the proportional current load of each individual tip of the field emitter array is analyzed and compared at different times during the initial measurement cycle. Together with the extracted I–V curves of single emission spots from the integral measurement, the results indicate the effect of premature burnout of particularly sharp tips during conditioning of the emitter.

Quantitative Field Emission Imaging for Studying the Doping-Dependent Emission Behavior of Silicon Field Emitter Arrays

“…The potential of the camera is set to ground. The FEA is pulled to a negative potential by a regulation circuit [36,39] The measurements are performed in a vacuum setup with a controlle pressure of 10 −5 mbar. This relatively high pressure for field emission measur chosen to investigate the emission behavior of the FEAs under real application [37].…”

“…The potential of the camera is set to ground. The FEA is negative potential by a regulation circuit [36,39] which allows curren measurements, but limits the extraction voltage to a maximum potential of −1…”

“…During post-processing, the images are fed through a spot detection algorithm [36] that also analyzes the gray values of the spots generated by the electron emission for saturated pixels. In the case of pixel saturation, the entire spot is replaced by extrapolating the data captured at a lower exposure time.…”

“…This is done by multiplying the integral gray value of each spot by the quotient of integral current and integral brightness over the entire image. This gives a distribution of the individual tip emission currents over the entire array for each data point [36,40].…”

“…To overcome these drawbacks, we recently reported on a low-cost and easy-to-use method for uniformity measurements of FEAs based solely on the commercially available Raspberry Pi HQ camera [36]. It was shown that the use of multiple shutter speeds can extend the dynamic range of the CMOS sensor to allow spatially resolved mapping of emission currents ranging over many orders of magnitude, without sacrificing near realtime and simultaneous observation of the entire array during the integral measurement.…”