High‐pressure scanning electron microscopy of insulating materials: A new approach

Abstract

SUMMARY A new SEM technique for imaging uncoated non‐conducting specimens at high beam voltages is described which employs a high‐pressure environment and an electric field to achieve charge neutralization. During imaging, the specimen surface is kept at a stable low voltage, near earth potential, by directing a flow of positive gas ions at the specimen surface under the action of an electric bias field at a pressure of about 200 Pa. In this way charge neutrality is continuously maintained to obtain micrographs free of charging artefacts. Images are formed by specimen current detection containing both secondary electron and backscattered electron signal information. Micrographs of geological, ceramic, and semiconductor materials obtained with this method are presented. The technique is also useful for the SEM examination of histological sections of biological specimens without any further preparation. A simple theory for the charge neutralization process is described. It is based on the interaction of the primary and emissive signal components with the surrounding gas medium and the resulting neutralizing currents. Further micrographs are presented to illustrate the pressure dependence of the charge neutralization process in two glass specimens which show clearly identifiable charging artefacts in conventional microscopy.

Charge Contrast Imaging of Exceptionally‐preserved Fossils

“…The production of SE in the VP‐SEM is complex and significantly different from high vacuum mode (the presence of the gaseous ions is especially important). Studies by Danilatos (1991), Farley and Shah (1991) and Thiel et al. (1997) have detailed the physics of electron‐gas scattering in the VP‐SEM; for information on issues related to secondary electron emission from dielectrics consult Thiel and Toth (2005) and papers therein.…”

Transient analysis of gaseous electron‐ion recombination in the environmental scanning electron microscope

“…The PIs generated in the cascade by the SEs, PEs and BSEs drift towards the sample, under the influence of E , neutralizing any excess charge build‐up on the surface of insulating materials (Moncreiff et al ., 1978; Danilatos, 1988). Neutralization of excess charge by the ions allows imaging and microanalysis of wet or hydrated (Meredith & Donald, 1996; Robinson, 1975b, 1978), biological (Shah & Beckett, 1979; Schnarr & Füting, 1997) and insulating (Toth et al ., 2002c, 2003) specimens without the need for conductive coatings (Robinson, 1975a; Moncreiff et al ., 1978; Danilatos, 1988; Farley & Shah, 1991). PIs impacting the sample surface and stage can generate further SEs that initialize electron cascades (von Engel, 1965; Nasser, 1971).…”

“…The amplifiers also contain finite input capacitances, C 1 (GSED electronics) and C 2 (ISC electronics), that act in parallel to the resistances described above. The coaxial cables used to transmit the induced signals will also have finite resistances, R 4 and capacitances C 4 (Raether, 1964; Danilatos, 1990b; Farley & Shah, 1991). The GSED preamplifier also contains two low‐pass filters which extract high frequency shot and thermal noise from signals (Philips Electron Optics, XL30 ESEM®, Eindhoven).…”

Quality Improvement of Environmental Secondary Electron Detector Signal Using Helium Gas in Variable Pressure Scanning Electron Microscopy

“…environmental secondary electron detector (ESED), have a fairly long history of study (Danilatos, 1990;Farley and Shah, 1991;Fletcher et al ., 1997Fletcher et al ., , 1999Mohan et al ., 1998). Recently, major SEM manufacturers have started producing them according to the requirements of the market.…”