HIGH-PRESSURE SCANNING ELECTRON-MICROSCOPY OF INSULATING MATERIALS - A NEW APPROACH

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.