Sponsored Article: What is BEX?
Imaging Technologies in SEM
Imaging with Secondary Electrons
Historically, the most common modality is Secondary Electron (SE) imaging, which involves the detection of low energy electrons (~50 eV) that are ejected from the sample due to inelastic interactions between the primary beam and sample surface.
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. 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. Simple operation of BSE detectors shows atomic number contrast if the sensors are arranged into two or more segments, and combining the signals from each sensor can yield topographical information.
Imaging with BEX
BEX combines BSE sensors with X-ray sensors which are sited immediately beneath the objective lens, in the same geometry as a standard BSE detector. The X-ray sensors, typically silicon drift detectors (SDDs), measure characteristic X-ray emissions generated 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 and under 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.
Want to learn more?
The world’s first BEX imaging detector, Unity, is now available from Oxford Instruments and is being demonstrated on stand 211 throughout mmc.