Amplification and noise in high‐pressure scanning electron microscopy

Abstract

SUMMARY High‐pressure scanning electron microscopy (HPSEM) is a promising new family of techniques. The present knowledge of these techniques is reviewed and a new set of criteria developed for optimizing signal detection in HPSEM with a view to preserving specimen integrity. For this purpose, amplification of contrast signals generated in HPSEM was examined by computing the effect of ionization over a range of pressures and biasing fields, routinely used for this technique. The influence of secondary ionization due to ion impact was included in the calculations. To check the calculated results, the experiments were performed in the HPSEM apparatus in a nitrogen atmosphere. A divergence between the experimental values and the calculated values was found. This was removed by taking into account the effect of recombination of charge carriers. Inductive currents generated in the HPSEM environment are transient and do not affect the conclusions of this study. The gas pressure, biasing fields and beam current for preserving specimen integrity and obtaining good micrographs were selected from the data. Experimental measurements of noise are reported, and criteria for optimizing the signal‐to‐noise ratio for performing HPSEM are discussed. The application of these criteria has shown that specimen current detection can be more advantageous than other methods and configurations of detection and was capable of obtaining high/medium‐resolution micrographs.

Electron‐gas interactions in the environmental scanning electron microscopes gaseous detector

“…Working within such a model it is possible to calculate the total number of SEs generated (and also the number of positive ions generated in the cascade), and hence the amplification of the system. Similar approaches have been taken by several other workers, namely, Moncrieff et al (1978), Durkin and Shah (1991), and also Danilatos (1990). Full details of the derivation will not be given in this paper, and for more information the interested reader is directed to the above authors, or indeed the more extensive texts of von Engel (1955) or Meeks and Craggs (Dutton 1978).…”

An improved model for gaseous amplification in the environmental SEM

“…More detailed discussions of this formulation can be found in the references by Moncrieff (Moncrieff et al, 1978), Farley (Farley & Shah, 1990a,b) and Durkin (Durkin & Shah, 1993). Their early work on 'high-pressure' scanning electron microscopy involved experimental conditions under which the TGC model is more applicable.…”

“…By this process, all lowenergy electron signals are amplified according to the reaction ðgÞ þ e ¹ → ðgÞ þ þ 2e ¹ where the liberated electron generally has only a few electronvolts of kinetic energy. Several authors (Moncrieff et al, 1978;Danilatos, 1990;Farley & Shah, 1990a,b;Durkin & Shah, 1993) have advanced a model for electron-specimen-gas interactions that is based on the physics of the Townsend gas capacitor (TGC) (von Engel, 1965). These treatments represent an idealized experimental configuration and take into account the contributions of low-energy electrons originating from the sample as well as those created in the gas itself.…”

A critical overview of ESEM applications in the biological field

“…Water vapor is the most common gas used in ESEM both for its amplifying efficiency and useful thermodynamic properties. Many authors provided a detailed description of the cascade effect (Danilatos, 1988(Danilatos, , 1990Durkin and Shah, 1993) and its relations with image intensity and resolution Stokes et al, 2000).…”