Imaging with Backscattered Electrons

Backscattered Electron (BSE) imaging involves the detection of electrons originating from deeper within the sample. The BSE electrons are higher energy, since they result from elastic interactions between primary beam and sample which change the trajectory of the incident electrons such that they are scattered back from the sample surface. An SE detector is ineffective at measuring these electrons since its remote position from the sample results in very low signal. Instead a BSE detector will typically consist of p-n semiconductors arranged immediately below the SEM objective lens to maximise signal collection.

The scattering interactions which generate the BSE signal are much stronger in samples made of heavier elements. Consequently, a BSE image contains information about sample composition where heavier phases appear brighter. While simple operation of BSE detectors shows atomic number contrast if the sensors are arranged into two or more segments, the signals from each sensor can be combined to show topographical information.

Backscattered Electrons (red) are generated from below the sample surface and produce a greyscale image containing information about sample composition. 20 kV, 1 nA, 15 seconds to acquired.

Imaging with BEX

BEX combines BSE sensors with X-ray sensors which are placed in the same geometry as a standard BSE detector, immediately below the objective lens. The X-ray sensors, typically silicon drift detectors (SDDs), measure characteristic X-ray emissions generated as the sample is irradiated by the SEM electron beam in the same manner as an EDS detector. Software algorithms process the collected X-ray signal to automatically identify which elements are present. These elements are then assigned colours which are layered along with the signal from the BSE detector to provide a final image.

When compared to SE or BSE imaging a BEX imaging system provides more information about sample composition and elemental distribution in the same acquisition time, with the same operating conditions.

In contrast to an EDS detector, a BEX imaging system has sensors with an incredibly high solid angle. This ensures X-ray information can be collected at normal imaging speeds and with typical imaging beam currents (~1 nA). The position of the SDD, near the objective lens, also eliminates shadowing effects from sample topography and allows for sample investigation at a wider range of working distances.

BEX coloured electron image

Backscattered electron image

Backscattered Electrons (red in a.) and X-rays (blue in a.) are detected and combined to produce a BEX colour image showing sample composition b). 20 kV, 1 nA, 15 seconds to acquired.