Time Dependent Study of the Positive ion Current in the Environmental Scanning Electron Microscope (ESEM)

Abstract

<jats:p>The Environmental Scanning Electron Microscope (ESEM) is capable of image generation in a gaseous environment at sample chamber pressures of up to 20 torr. in an ESEM, low energy secondary electrons emitted from a sample surface, by virtue of the primary electron beam, are accelerated towards the positively biased metallic ring (typically +30 to +550V) Gaseous Secondary Electron Detector (GSED). As these electrons accelerate towards the ring they undergo ionizing collisions with gas molecules producing positive ions and additional electrons known as environmental secondary electrons. The environmental electrons further ionize the gas on their way to the ring producing a cascade amplification of the original signal. The amplified signal induced in the ring is used to form an image. The electric field generated between the GSED ring and the grounded stage causes the positive ions produced in the cascade to drift towards the sample, effectively neutralizing negative charge build up on the surface of a non-conducting sample.</jats:p>

Quantification of electron-ion recombination in an electron-beam-irradiated gas capacitor

“…We note that while an abrupt change in the magnitude of a microscope operating parameter (e.g. I PE , V e and P ) initiates a change in I ion , the time it takes the latter to reach a steady state can be in excess of a few seconds [22,35]. The magnitude of this response time depends on parameters such as sample capacitance, sample-electrode geometry, gas type, I PE , V e and P .…”

“…We note that the present model of self-damping (including equation ( 7)) is merely a description of the steady state that the gas cascade converges to under a given set of microscope operating conditions. The model is not concerned with the kinetics of the convergence process, governed by ion mobilities [22,35] (the time dependence of self-damping is discussed briefly in the appendix), nor is it concerned with changes in δ or cascade amplification efficiency caused by sub-surface charge trapped in dielectric specimens [14]. Grounded conductors were used in the present experiments in order to characterize self-damping behaviour in the absence of charging artefacts.…”