The ESEM

Location:

RoselandsNew South WalesAustralia

Revenue:

$2.4 Million

Employees:

12
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Description:

The Environmental Scanning Electron Microscope (ESEM) is one of the major innovations and a fundamental advance in the field of electron microscopy. ESEM allows the examination of practically any specimen under any gaseous conditions, unlike conventional SEM, which operates in vacuum. An electron microscope requires a good vacuum for the generation and propagation of the electron beam, which in the past meant that the specimen under examination had to be placed also in vacuum. That condition limits the SEM, since either the specimens have to be modified and compromised by various treatments or the microscope's operational parameters must be constrained. The ESEM allows the examination of any specimen, wet or dry, insulating or conducting in situ and close to its natural state, while the environmental gas medium produces completely novel possibilities of operation and imaging. The implications of this technique go beyond the realms of microscopy, as the principle has applications to modern nanotechnologies and microengineering. Focused electron and ion beams in a controlled gaseous environment have much wider applications while capable of imaging. Physics, chemistry, biology, materials science and industrial technology can greatly benefit after the invention of ESEM. It is no exaggeration that ESEM represents, after many decades of conventional electron microscopy, the next biggest revolution in microscopy following the advent of SEM itself. The viewing of wet samples has become possible because water can be maintained in the liquid phase as long as the surrounding vapor is saturated. The saturation water vapor pressure varies with the specimen temperature from 609 Pa at the freezing point to around 2000 Pa at room temperature, a pressure range easily obtainable in a properly designed ESEM. The conductive coating of specimens to suppress charge accumulation according to conventional practice is no more necessary because the gaseous layer around the specimen becomes ionized and suppresses charge accumulation. Furthermore the gas itself can also be used as detection medium ( 11, 12 , 36 ) giving rise to novel detection and imaging techniques . These developments have been based on work carried out on a prototype ESEM which has yielded results of high quality. In many respects, these results still remain unique and unsurpassed to date having set the benchmarks of ESEM performance. These innovations have resulted in a new instrument that is also now commercially available and is undergoing continual improvements. Users should compare performance and results obtained by available commercial ESEMs with the early achievements by the the prototype ESEM. The ESEM is based on a number of changes to the instrument of conventional SEM, namely, the introduction of differential pumping and new detection systems. A minimum of a pair of apertures with pumping between them is placed at the end of the objective lens, thus separating the electron optics column from the specimen chamber. Gas introduced around the specimen first flows through the bottom aperture and most of it is removed with a pump before only a small fraction of gas escapes in the column. The latter can easily be handled by the vacuum system of the SEM. Depending on the type of electron gun used in the generation of the electron beam, additional differential pumping stages may be introduced. Initially, the tungsten type of electron gun operates only with one pair of pressure limiting apertures plus the usual pumping at the column. With a LaB6 electron gun, one additional aperture is required to increase the vacuum by one order of magnitude. With a field emission electron gun, one additional pumping stage is required, but such graduated pumping stages are already typical in SEM technology. The extra pressure limiting apertures in the column can be integrated with the ones acting as probe forming or spray apertures along the column simply taking care that no fugitive gas goes around the apertures. Recent studies on the gas flows have resulted in the possibility to optimize to a minimum the pumping requirements in order to allow durable and uninhibited operation of the ESEM with any type of electron gun, exactly as in conventional SEM. The geometry and total configuration of pressure limiting apertures are critical in the design of a commercial ESEM, if the latter is to perform optimally. The detection systems used in the ESEM have involved optimum design backscattered electron detectors that allow best signal-to-noise ratio (best signal output) even at the highest pressure used. Further, a novel secondary electron detection system has been developed based on the ionization of the environmental gas. The secondary electron produced by the beam-specimen interaction are accelerated in the gas by a suitable bias electrode. These energetic electrons collide with the gas molecules and release new electrons from the gas, a process that is repeated many times giving rise to an avalanche amplification of the secondary electrons. This induces an electron current in the associated electrode followed by additional amplification by the electronics of the microscope and the formation of a scanned image in the usual manner of a SEM. Furthermore, the collisions of the electrons in the gas also produce excitation and photon emission in an avalanche form, which is used for detection by light detectors to produce an amplified signal for the formation of images in the usual way of a SEM. Because the backscattered electrons also produce ionization and excitation in their collisions with the environmental gas, suitable means of electrode configurations have been devised to separate the different types of signal. Additionally, cathodoluminescence mode of detection has been demonstrated in the use and operation of ESEM. The total configuration of such detection systems is critical in the design of a commercial ESEM, if the latter is to perform optimally. Among other fundamental works (e.g. 28 and 36), detailed studies of charge distribution ( 38) and critical issues ( 45 ) must be thoroughly understood if further progress is to be achieved. The integration of proper differential pumping, pressure limiting apertures and detection systems results in freeing enough space below the final aperture for the placement and manipulation of specimens. In an optimum design, the specimen-aperture distance allowable should be maximum whilst the incident electron beam undergoes collisions and suffers losses. The incident beam eventually becomes lost beyond a certain travel distance in the gas and no imaging is possible. The condition whereby the beam retains an imaging capability has been termed the " oligo-scattering regime" ( 28 ). Thus, it is important to understand these initial processes before the probe strikes the specimen: Electrons are removed exponentially with the distance and the gas pressure from the beam spot. These scattered electrons are distributed over a wide area being orders of magnitude greater than the beam spot, hence their effect is the creation of uniform noise which is separated out from the signal generated by the minute beam spot. The resolving power of the instrument is generally said to be same as the probe diameter, and hence ESEM has the same resolving power as a SEM, all depending on the initial size of the spot in vacuum. The removal of electrons from the useful beam ultimately decreases the resolution of a specimen feature with low contrast. The latter can be compensated for by an increase in the beam current which is usually accompanied with an increased beam spot. However, it should be recognized that this amount of deterioration is of the order of beam spot variation, which is generally small and with no significant practical effect in most applications apart from some specialized applications which strive to extract the last Angstrom of resolution. Very high resolutions of uncoated polymers have been achieved with an early commercial model of ESEM. The SEM as well as the ESEM deal with the surface examination of bulk specimens. In many applications, specimens characterized by low contrast features are those with low atomic number such as organic materials. On such materials, resolutions of the order of the beam spot are difficult to achieve on account of radiation effects ( 18) becoming very pronounced as we increase the magnification. As the beam energy is dissipated in a smaller viewing area on the sample, specimen damage becomes a limiting factor well before we reach the specified resolving power of a given machine. Typically, the manufacturer backs the resolution of an instrument with micrographs of gold-on-carbon particles with least particle spacing separation. This is done also by the the use of maximum (or design optimum) electron beam keV accelerating voltage typically of 20 or 30 keV. It has been shown that such specifications can be the same both in SEM and ESEM. However, reproduction of such conditions on bulk organic specimens with low contrast are unlikely to be reached before the specimen is destroyed by radiation. Therefore, it is imperative to achieve the best possible contrast on low atomic number specimens in the manner it was done with the original ESEM Research Laboratory prototype. This understanding becomes all the more important in the selection of a commercial ESEM which should be designed to operate at the physical limits prescribing the best of opposing parameters: i.e. the maximum useful specimen distance, the maximum gas pressure with the minimum keV and spot size allowable by the chosen instrument. A concise description of how the ESEM works is presented in wikipedia , and video description in German here. Among the countless new applications and possibilities (see e.g. high quality imaging of living specimens , pollen , amphiphilic particles , swelling of the hydrogel ) there is also one novel use of the ESEM that has probably escaped the attention of users: The gas flow can be visualized and imaged as, for example, is shown by imaging the gaseous jet itself flowing through an aperture. Gas dynamics can be visualized and studied by this unique method. Despite the fact that the commercial instruments are yet to implement the best of designs and potential of the technology , the ESEM has gained acceptance by the scientific, technical and industrial community as evidenced from the large number of publications ( 49) arising from its use (already listed back in 1993, with unaccounted large numbers of works since then). Many other workers have continued on the path of development and understanding of this new technology. Among those, a recent investigation by Scott Morgan has set an exemplary standard in this regard. An understanding of the special historical background of this technology may help accelerate future developments.

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