Cryogenic developments and signal amplification in environmental scanning electron microscopy

• Main Author: Fletcher, A. L.
• Published: University of Cambridge 1998
• Subjects: 530.0724
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599080

This thesis describes the development of a cryogenic imaging system for an environmental scanning electron microscope (ESEM). The ESEM is an important new development in electron microscopy since it enables specimens to be viewed in a small pressure of gas - generally this gas is water vapour, although an alternative must be used for cryogenic applications. The presence of the gas also contributes to the imaging mechanism, a process whereby the signal electrons are amplified by the gaseous molecules prior to detection. The purpose of the cryogenic system was to image the complicated, four phase microstructure of ice cream. Although viewed routinely by conventional electron microscopy techniques, the harsh temperature and pressure regimes involved (around -120°C at 10<SUP>.6</SUP> torr) increase the likelihood of introducing artefacts. Therefore, a methodology was developed for imaging ice cream with ESEM in a small pressure of an alternative imaging gas at a much warmer temperature of -80°C. In order to stabilise the ice phase in the samples at a higher temperatures, a system was designed for mixing gases, so that a small amount of water vapour could be mixed into the imaging gas. This system lifted the temperature restrictions of ice cream imaging so it can, in principle, now be imaged at its storage temperature of -20°C. In the search for alternative imaging gases to water vapour, questions were raised about the fundamental way in which the signal electrons interact with the gas. In order to understand the electron amplification properties of the different gases, a Faraday cage was designed and the electron amplification was investigated. We suggest that the ratio of the peak amplification to the plateau amplification gives a semi-quantitative method of determining the imaging quality of the gas. Furthermore, by isolating experimentally the effects of different components of the signal, it was found that the low energy secondary electrons dominate the signal at low pressure, whereas the effect of backscattered electrons becomes more important as the pressure is raised. In addition, the performance of two ESEM detector designs were compared. The new gaseous secondary electron detector (GSED), which was designed to reduce the contribution of some sources of signal, was found to achieve its aim, but some of its overall contrast was sacrificed when compared to the original environmental secondary detector (ESD).