PRIMARY CONSIDERATIONS FOR IMAGE-ENHANCEMENT IN HIGH-PRESSURE SCANNING ELECTRON-MICROSCOPY .2. IMAGE-CONTRAST

In the preceding paper the effect of the primary beam scattering on contrast in high‐pressure SEM techniques such as moist‐environment ambient‐temperature SEM and environmental SEM was examined. Here, the effect of the scattering of the emissive components on the image formation and contrast in high‐pressure SEM was considered and evaluated experimentally. A specimen current amplifier was used to detect the absorbed current signal together with the ionic signal components generated by the emissive and primary electrons. It was shown that space charge accumulations of ionic carriers can build up above the specimen when imaging in the presence of a gas and that this can act to suppress the secondary electron emission. The ionic carrier concentration is also discussed. Full image contrast can be restored by the application of an electric bias field above the specimen surface. Since ionic charge carriers contribute to the signal current, new image‐contrast mechanisms were considered and a theoretical model for contrast generation is therefore proposed. The predictions of this model were tested experimentally for a specimen substrate of uniform emissivity under varying electric bias field strengths. It was also found that under sufficiently high electric fields low‐energy charge carriers gain enough energy to ionize the host gas molecules and therefore multiply the signal current. Micrographs are presented to illustrate these image formation processes as a function of electrode bias. The new detection system successfully extends the signal information of high‐pressure SEM and a range of applications is discussed to exploit these possibilities. 1990 Blackwell Science Ltd