5.6 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Environmental scanning electron microscope | 2/7 | https://en.wikipedia.org/wiki/Environmental_scanning_electron_microscope | reference | science, encyclopedia | 2026-05-05T10:04:32.049506+00:00 | kb-cron |
The specimen chamber sustaining the high-pressure gaseous environment is separated from the high vacuum of the electron optics column with at least two small orifices customarily referred to as pressure-limiting apertures (PLA). The gas leaking through the first aperture (PLA1) is quickly removed from the system with a pump that maintains a much lower pressure in the downstream region (i.e. immediately above the aperture). This is called differential pumping. Some gas escapes further from the low pressure region (stage 1) through a second pressure limiting aperture (PLA2) into the vacuum region of the column above, which constitutes a second stage differential pumping (stage 2). A schematic diagram shows the basic ESEM gas pressure stages including the specimen chamber, intermediate cavity and upper electron optics column. The corresponding pressures achieved are p0>>p1>>p2, which is a sufficient condition for a microscope employing a tungsten type of electron gun. Additional pumping stages may be added to achieve an even higher vacuum as required for a LaB6 and field emission type electron guns. The design and shape of a pressure limiting aperture are critical in obtaining the sharpest possible pressure gradient (transition) through it. This is achieved with an orifice made on a thin plate and tapered in the downstream direction as shown in the accompanying isodensity contours of a gas flowing through the PLA1. This was done with a computer simulation of the gas molecule collisions and movement through space in real time. We can immediately see in the figure of the isodensity contours of gas through aperture that the gas density decreases by about two orders of magnitude over the length of a few aperture radii. This is a quantitatively vivid demonstration of a first principle that enables the separation of the high-pressure specimen chamber from the low pressure and vacuum regions above. By such means, the gas flow fields have been studied in a variety of instrument situations, in which subsequently the electron beam transfer has been quantified.
=== Electron beam transfer ===
By the use of differential pumping, an electron beam is generated and propagated freely in the vacuum of the upper column, from the electron gun down to PLA2, from which point onwards the electron beam gradually loses electrons due to electron scattering by gas molecules. Initially, the amount of electron scattering is negligible inside the intermediate cavity, but as the beam encounters an increasingly denser gas jet formed by the PLA1, the losses become significant. After the beam enters the specimen chamber, the electron losses increase exponentially at a rate depending on the prevailing pressure, the nature of gas and the acceleration voltage of the beam. The fraction of beam transmitted along the PLA1 axis can be seen by a set of characteristic curves for a given product p0D, where D is the aperture diameter. Eventually, the electron beam becomes totally scattered and lost, but before this happens, a useful amount of electrons is retained in the original focused spot over a finite distance, which can still be used for imaging. This is possible because the removed electrons are scattered and distributed over a broad area like a skirt (electron skirt) surrounding the focused spot. Because the electron skirt width is orders of magnitude greater than the spot width, with orders of magnitude less current density, the skirt contributes only background (signal) noise without partaking in the contrast generated by the central spot. The particular conditions of pressure, distance and beam voltage over which the electron beam remains useful for imaging purposes has been termed oligo-scattering regime in distinction from single-, plural- and multiple-scattering regimes used in prior literature. For a given beam accelerating voltage and gas, the distance L from PLA1, over which useful imaging is possible, is inversely proportional to the chamber pressure p0. As a rule of thumb, for a 5 kV beam in air, it is required that the product p0L = 1 Pa·m or less. By this second principle of electron beam transfer, the design and operation of an ESEM is centered on refining and miniaturizing all the devices controlling the specimen movement and manipulation, and signal detection. The problem then reduces to achieving sufficient engineering precision for the instrument to operate close to its physical limit, corresponding to optimum performance and range of capabilities. A figure of merit has been introduced to account for any deviation by a given machine from the optimum performance capability.
=== Signal detection ===
The electron beam impinges on the specimen and penetrates to a certain depth depending on the accelerating voltage and the specimen nature. From the ensuing interaction, signals are generated in the same way as in an SEM. Thus, we get secondary and backscattered electrons, X-rays and cathodoluminescence (light). All of these signals are detected also in the ESEM but with certain differences in the detector design and principles used.
==== Secondary electrons ==== The conventional secondary electron detector of SEM (Everhart–Thornley detector) cannot be used in the presence of gas because of an electrical discharge (arcing) caused by the kilovolt bias associated with this detector. In lieu of this, the environmental gas itself has been used as a detector for imaging in this mode:
===== Gaseous detection device =====