Scanning electron microscopy -Images, applications and developments

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

This second article [P 866v2] 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 surface in materials engineering (fractures, deposits, corroded surfaces, samples of microstructures revealed by appropriate preparation...), electronic microcomponent engineering and biology.

Images acquired by scanning, 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 contrast, crystalline contrast, magnetic contrast on quasi-planar samples of many solid materials;

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

• local elemental microanalysis by X-ray spectrometry or elemental trace detection by cathodoluminescence.

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 observation possibilities to non-conductive materials, "soft" matter, living micro-organisms, etc. ;

• or by allowing a complementary ion beam to penetrate inside the sample (double-column scanning electron microscopy).