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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Environmental scanning electron microscope | 5/7 | https://en.wikipedia.org/wiki/Environmental_scanning_electron_microscope | reference | science, encyclopedia | 2026-05-05T10:04:32.049506+00:00 | kb-cron |
=== Specimen charging === The electron beam impinging on insulating specimens accumulates negative charge, which creates an electrical potential tending to deflect the electron beam from the scanned point in conventional SEM. This appears as charging artifacts on the image, which are eliminated in the SEM by depositing a conductive layer on the specimen surface prior to examination. Instead of this coating, the gas in the ESEM being electrically conductive prevents negative charge accumulation. The good conductivity of the gas is due to the ionization it undergoes by the incident electron beam and the ionizing SE and BSE signals. This principle constitutes yet another deviation from conventional vacuum electron microscopy. Charge contrast imaging is a scanning electron microscope imaging mode which can produce images of otherwise invisible microstructures in insulating materials and in fossils. The technique has been used to detect changes in minerals which reflect compositional differences. Since charge is suppressed in ESEM at sufficient gas pressure, it is possible to regulate the amount of charging by varying the pressure between the SEM vacuum and some low pressure while observing the charging contrast variation effects. See examples with a series of images, also four images of the same field, and with color superposition.
=== Contrast and resolution === As a consequence of the way ESEM works, the resolution is preserved relative to the SEM. That is because the resolving power of the instrument is determined by the electron beam diameter which is unaffected by the gas over the useful travel distance before it is completely lost. This has been demonstrated on the commercial ESEMs that provide the finest beam spots by imaging test specimens, i.e. customarily gold particles on a carbon substrate, in both vacuum and gas. However, the contrast decreases accordingly as the electron probe loses current with travel distance and increase of pressure. The loss of current intensity, if necessary, can be compensated by increasing the incident beam current which is accompanied by an increased spot size. Therefore, the practical resolution depends on the original specimen contrast of a given feature, on the design of the instrument that should provide minimal beam and signal losses and on the operator selecting the correct parameters for each application. The aspects of contrast and resolution have been conclusively determined in the referenced work on the foundations of ESEM and other related works. Further, in relation to this, we have to consider the radiation effects on the specimen.
=== Specimen transfer === The majority of available instruments vent their specimen chamber to the ambient pressure (100 kPa) with every specimen transfer. A large volume of gas has to be pumped out and replaced with the gas of interest, usually water vapor supplied from a water reservoir connected to the chamber via some pressure regulating (e.g. needle) valve. In many applications this presents no problem, but with those ones requiring uninterrupted 100% relative humidity, it has been found that the removal of ambient gas is accompanied by lowering the relative humidity below the 100% level during specimen transfer. This clearly defeats the very purpose of ESEM for this class of applications. However, such a problem does not arise with the original prototype ESEM using an intermediate specimen transfer chamber, so that the main chamber is always maintained at 100% relative humidity without interruption during a study. The specimen transfer chamber (tr-ch) shown in the diagram of ESEM gas pressure stages contains a small water reservoir so that the initial ambient air can be quickly pumped out and practically instantaneously replaced with water vapor without going through a limited conductance tube and valve. The main specimen chamber can be maintained at 100% relative humidity, if the only leak of vapor is through the small PLA1, but not during violent pumping with every specimen change. Once the wet specimen is in equilibrium with 100% relative humidity in the transfer chamber, within seconds, a gate valve opens and the specimen is transferred in the main specimen chamber maintained at the same pressure. An alternative approach involving controlled pumping of the main chamber may not solve the problem entirely either because the 100% relative humidity cannot be approached monotonically without any drying, or the process is very slow; inclusion of a water reservoir inside the main chamber means that one cannot lower the relative humidity until after all of the water is pumped out (i.e. a defective control of the relative humidity). See "Operation" on page 238 of the ESEM Foundations.
== Radiation effects == During the interaction of an electron beam with a specimen, changes to the specimen at varying degrees are almost inevitable. These changes, or radiation effects, may or may not become visible both in SEM and ESEM. However, such effects are particularly important in the ESEM claiming the ability to view specimens in their natural state. Elimination of the vacuum is a major success towards this aim, so that any detrimental effects from the electron beam itself require special attention. The best way around this problem is to reduce these effects to an absolute minimum with an optimum ESEM design. Beyond this, the user should be aware of their possible existence during the evaluation of results. Usually, these effects appear on the images in various forms due to different electron beam-specimen interactions and processes. The introduction of gas in an electron microscope is tantamount to a new dimension. Thus, interactions between electron beam and gas together with interactions of gas (and its byproducts) with specimen usher a new area of research with as yet unknown consequences. Some of these may at first appear disadvantageous but later overcome, others may yield unexpected results. The liquid phase in the specimen with mobile radicals may yield a host of phenomena like specimen damage but also enabling the direct imaging of molecular structures.