Getting the most out of a scanning electron microscope (SEM) requires optimisation of the operating conditions to suit both the type of data required and the sample being analysed. The general trends of how resolution, depth of focus or count rates will change as you change apertures or accelerating voltage are quite well understood, with plenty of resources and text books available to explain these concepts [ 1][ 2].
Characterising the performance of an SEM in a more specific way than what is provided in a specification sheet or in a text book has many benefits. These include enabling direct comparison of different SEM models and providing additional tools for troubleshooting instrument and environmental performance issues. In addition to these improved instrument management tools, characterisation has the capacity to improve research outputs by refining the instructions and guidance provided to users of a system.
Detailed is an approach to characterisation of the SEMs in the Monash Centre for Electron Microscopy, along with some general findings and examples of the research and training aids it has helped to develop.
Resolution was measured by fitting an Error function to the average of at least 200 edge profiles measured over at least eight directions. Here resolution is defined as the full-width-half-maximum of the Gaussian derivative of the fitted Error function.
As condenser lens strength is altered, significant increases to probe current can be achieved with only relatively minor sacrifices to resolution as shown in Figure 1, making sample navigation and microscope alignment much easier.
Fig. 1.
Resolution (blue) and probe current (red) of SEM micrographs of gold on carbon as function of spot size. For all conditions, accelerating voltage was 15 kV, working distance was 4.6 mm and a 30 µm aperture was used.
For analytical SEM using EDX, higher probe currents are required.