Detection by Induction in the Environmental SEM

Abstract

The advent of environmental SEM (ESEM) has revealed the true mechanism of detection, when the current mode is used. The ESEM allows the introduction of gas in the specimen chamber with a pressure at least sufficient to maintain specimens in their wet state. The gas is also used to suppress charge accumulation on insulating specimens and, furthermore, it can be used as a detection and amplification medium. The resolving power of the instrument is not impaired by the presence of gas, whereas new contrast mechanisms are now possible. The foundations of this technology have been outlined in an extended survey previously.It has been shown that the products of interaction between various signals and gas can be used for imaging. Ionization, in particular, was first used for imaging by Danilatos. A comprehensive theoretical survey of the gaseous detector device can be found elsewhere. Here, the principle of pulse induction by moving charge carriers between electrodes is highlighted. Any charge moving between two electrodes induces a charge of opposite sign on each electrode with a concomitant current flow in the external circuit. The current flow stops when the charge carriers terminate their motion. Therefore, pulses are generated by each electron in the beam and in the signals emerging from the beam-specimen interaction. The sum of all pulses corresponding to a given pixel element results in the total signal pulse carrying information from the specimen. This way of looking at signal formation would be superfluous, if one could always arrive at the same result by simply counting the number of charge carriers that arrive at a particular electrode. However, the creation of ions in the gas of an ESEM and the possibility of imaging non-conducting surfaces has compelled us to resort to first principles in order to properly understand, explain and evaluate new phenomena. First, the ions are more than 1000 times slower than electrons, and they may limit the frequency response of the system; the simple counting of electrons that arrive at the anode, while the corresponding ions have not yet reached the cathode, provides an incorrect signal intensity. Second, microscopists would not use the “specimen absorbed current mode” with insulating specimens in vacuum SEM, whereas a new situation exists in ESEM.