5.5 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Environmental scanning electron microscope | 4/7 | https://en.wikipedia.org/wiki/Environmental_scanning_electron_microscope | reference | science, encyclopedia | 2026-05-05T10:04:32.049506+00:00 | kb-cron |
===== Adapted detectors ===== Despite the above developments, devoted BSE detectors in the ESEM have played an important role, since the BSE remain a useful detection mode yielding information not possible to obtain with SE. The conventional BSE detection means have been adapted to operate in the gaseous conditions of the ESEM. The BSE having a high energy are self-propelled to the corresponding detector without significant obstruction by the gas molecules. Annular or quadrant solid-state detectors have been employed for this purpose but their geometry is not easily adaptable to the requirements of ESEM for optimum operation. As a result, not much use has been reported of these detectors on genuine ESEM instruments at high pressure. The "Robinson" BSE detector is tuned for operation up to around 100 Pa at the usual working distance of conventional SEM for the suppression of specimen charging, whilst electron collection at the short working distance and high pressure conditions make it inadequate for the ESEM. However, plastic scintillating materials being easily adaptable have been used for BSE and made to measure according to the strictest requirements of the system. Such work culminated in the use of a pair of wedge-shaped detectors saddling a conical PLA1 and abutting to its rim, so that the dead detection space is reduced to a minimum, as shown in the accompanying figure of optimum BSE detectors. The photon conduction is also optimized by the geometry of the light pipes, whilst the pair of symmetrical detectors allow the separation of topography (signal subtraction) and atomic number contrast (signal addition) of the specimen surface to be displayed with the best ever signal-to-noise-ratio. This scheme has further allowed the use of color by superimposing various signals in a meaningful way. These simple but special detectors became possible in the conditions of ESEM, since bare plastic does not charge by the BSE. A very fine wire mesh with appropriate spacing has been proposed as a GDD when gas is present and to conduct negative charge away from the plastic detectors when the gas is pumped out, towards a universal ESEM. Since the associated electronics involve a photomultiplier with a wide frequency response, true TV scanning rates are available. This is an attribute to maintain with an ESEM that enables the examination of processes in situ in real time. In comparison, no such imaging has been \ reported with the electron avalanche mode of the GDD yet (as of March, 2025). The use of scintillating BSE detectors in ESEM is compatible with the GDD for simultaneous SE detection, in one way by replacing the top plane electrode with a fine tip needle electrode (detector), which can be easily accommodated with these scintillating BSE detectors. The needle detector and cylindrical geometry (wire) have also been surveyed.
==== Cathodoluminescence ====
Cathodoluminescence is another mode of detection involving the photons generated by the beam-specimen interaction. This mode has been demonstrated to operate also in ESEM by the use of the light pipes after they were cleared of the scintillating coating previously used for BSE detection. However, not much is known on its use outside the experimental prototype originally tested. Clearly, ESEM has advantages in this detection mode compared to SEM, since the natural surface of any specimen can be examined in the imaging process. Cathodoluminescence is a materials property, but with various specimen treatments required and other limitations in SEM \ this mode of detection has not been popular in the past. However, this is now available for the examination of untreated specimen surfaces.
==== X-rays ====
The characteristic elemental X-rays produced also in the ESEM can be detected by the same detectors used in the SEM. However, there is an additional complexity arising from the X-rays produced from the electron skirt. These X-rays come from a larger area than in SEM and the spatial resolution is significantly reduced, since the “background” X-ray signals cannot be simply “suppressed” out of the probe interaction volume. However, various schemes have been proposed to solve this problem. These methods involve spot masking, or the extrapolation technique by varying the pressure and calibrating out the effects of skirt, whereby improvement has been achieved.
==== Specimen current ==== In vacuum SEM, the specimen absorbed current mode is used as an alternative mode for imaging of conductive specimens. Specimen current results from the difference of electron beam current minus the sum of SE and BSE current. However, in the presence of gas and the ensuing ionization, it would be problematic to separate this mode of detection out of the generally operating gaseous detection device. Hence this mode, by its definition, may be considered as unsustainable in the ESEM. Shah and Becket assumed the operation of the specimen absorbed current mode if the conductivity of their specimen was assured during the examination of wet botanical samples; in fact, Shah by 1987 still considered the ionisation products in gas by SE and BSE as a formidable obstacle, since he believed that the ionisation did not carry any information about the specimen. However, he later embraced to correct role of gaseous ionisation during image formation.