Scanning electron microscopy -Images, applications and developments

The principles and equipment of scanning electron microscopy are described in [P 865] .

This second article describes image formation, contrast sources, recent instrument developments and various applications.

As the main source of contrast results from the large variation in the intensity of secondary electron emission as a function of the angle of incidence of the primary beam, the current secondary electron image visualizes the micro-relief of the sample. With an excellent separating power, often less than 5 nm, and a great depth of field, it can be used to observe the topography of many types of surfaces in materials engineering (fractures, deposits, corroded surfaces, samples with microstructures revealed by appropriate preparation, etc.), in electronic microcomponent engineering and in biology. The Everhard-Thornley detector, or secondary electron detector, is used here.

The images acquired by the scanning electron microscope, in digital form, lend themselves very easily to image processing and analysis.

Numerous additional observations, based on other significant contrasts, can be made on certain types of samples, with less separating power:

• imaging of chemical, crystalline and magnetic contrasts on quasi-planar samples of a wide range of solid materials, using a backscattered electron (BSE) detector;

• imaging of the extreme surface of samples, using detectors placed in the microscope column (In-Lens);

• potential contrast and induced current imaging for semiconductors and microcircuits;

• local elemental microanalysis, by X-ray spectrometry (EDS), or by locating elemental traces using cathodoluminescence;

• imaging of crystal orientation, texture or grain boundaries, using a camera sensitive to electron backscatter diffraction (EBSD).

In recent years, new generations of instruments have joined the ranks of conventional microscopes:

• or by enabling samples to be observed in a low partial vacuum (controlled-pressure microscopes and environmental chamber microscopes), which has extended the possibilities of observation to non-conductive materials, "soft" matter, hydrated samples, living micro-organisms, etc. ;

• or by using a complementary...