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ENVIRONMENTAL SCANNING ELECTRON MICROSCOPE (ESEM)

ESEM Short History and its Future

PUBLICATIONS ON ESEM

PATENTS ON ESEM

VISCOELASTICITY (DYNAMIC MECHANICAL PROPERTIES OF KERATIN FIBRES)

PUBLICATIONS ON VISCOELASTICITY OF KERATIN

SHORT BIOGRAPHY

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ENVIRONMENTAL SCANNING ELECTRON MICROSCOPE (ESEM) (51)

 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 evolved 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. 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.

Among the countless new applications and possibilities 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.

To allow a brief exposition of the history below not every instance is provided with references and the reader should resort to the literature provided herewith: The many attempts and history that have led to the present development of ESEM have been reviewed and surveyed elsewhere ( 9, 28, 36, 38, 40, 45) and a bibliography is presented by ( 49).

The idea of shooting high-energy electron beams in the atmosphere seems to have started with Nikola Tesla in the early twentieth century, and this was later used in experimental electron beam welders, pressure gauges and probably weapons tests. Manfred von Ardenne (1940), founder of the scanning electron microscope (SEM), imaged specimens in a gaseous environment by passing a high-energy electron beam through an electron "transparent" (i.e. at high energy) film. The alternative separation of the high vacuum electron gun chamber from a gaseous specimen chamber via open diaphragms (pressure limiting apertures - PLA) was used by various laboratories employing high voltage transmission electron microscopes in UK, USSR, USA and France. These instruments are not well suited for imaging the surface of bulky specimens. One attempt to image a wet specimen with a SEM was by injecting a very localized vapor jet to maintain a gaseous gradient just above the specimen (Lane) while the vapor was fast pumped away permitting the use of a conventional secondary electron detector before breakdown occurred by the high bias of the detector; this was clearly an unstable system with limited use. Another attempt used a wide angle backscattered electron (BSE) detector with a single PLA (Robinson), but again images had to be hurriedly obtained before the electron gun was flooded with water vapor and instrument operation disrupted. Most disappointing was an erroneous experimental finding that the electron probe diameter was increased inside a much wider spread of electrons surrounding, like a skirt, the alleged broadened and weakened imaging probe as the gas pressure increased (Moncrieff et al). This seemed similar to the well known "top-bottom effect" (i.e. beam spread) through the thin specimen sections used in transmission electron microscopy. Thus, it seemed that both contrast and resolution were seriously compromised by the gas. This compromise was thought to be further aggravated by the very use of BSE, the resolution of which was generally associated with a large signal volume in the specimen; specimens with low atomic number, such as biological specimens, were thought to produce only poor resolution in this mode of detection. All those prior isolated attempts clearly did not produce sufficient or satisfactory results to impress the microscope users or the manufacturers.

The situation was later reappraised by undertaking an investigation and implementing proper differential pumping techniques together with re-designing and re-configuring the BSE detector (Danilatos et al). It was that time when the term environmental scanning electron microscope (ESEM) was first introduced.  The image contrast was significantly improved with stable instrument operation at high pressures. In fact, the pressure could be increased to ambient levels giving rise to an Atmospheric Scanning Electron Microscope (ASEM). However, the ASEM was far ahead of its time and work concentrated mostly at the lower range of pressures preferred for an environmental scanning electron microscope (ESEM). Despite the obtained optimum design and versatility of BSE detectors and routine instrument operation, the ESEM was still not ready to be accepted by the microscopy community as the question of resolution kept resurfacing. This critical barrier was crossed by two concurrent developments: (a) The invention of the gaseous detection device (GDD) (Danilatos 1983) that allows detection of the secondary electrons (SE) in a gaseous environment and (b) the determination that the electron probe diameter remained constant with pressure increase (as opposed to the Moncrieff et al finding).

It was found that the gas acted not only as a conditioning medium (e.g. to maintain the wet state of an object) but it also acted simultaneously both as an electrical charge conductor and a detector. All ionizing signals including the secondary electrons could be found and detected inside the gas itself. These signals were detected by a simple biased electrode with a variable voltage inside the variable pressure gaseous environment . It was then a matter of separating the BSE from the SE, which was shortly afterwards achieved by thorough experimental and theoretical investigations. Furthermore, the gas as a detection medium was found to have far reaching implications because not only the electrons and ions but also the photons generated in the gas could be used for imaging. The interplay of a host of physical parameters such as the nature and pressure of gas, geometry and configuration of electrodes, applied electrode bias, type of signal-gas interaction and hardware for signal-product detection opened a novel vista of possibilities and investigations. The potential for novel contrasts and uses was indeed immense, as subsequent work has shown.  A comprehensive theory of the Gaseous Detection Device in the ESEM provides an indispensable reference.

With regard to resolution, a specially designed experimental device allowed the observation of the electron probe profile by scanning the beam across the edge of a heated platinum wire. This clearly showed that whilst the beam current decreased with gas pressure, the diameter remained constant, contrary to the prior understanding. The beam broadening reported by previous workers was probably due to an edge effect and/or contamination deposition, whilst such effect/contamination was prevented by heating the platinum wire and by a special detection configuration. This finding was further supported by a thorough investigation using an analytical formulation and numerical computation of the scattering and distribution of electrons inside the gas (Danilatos 1988). Therefore, it was conclusively found that the diameter of the imaging probe remained constant in the oligo-scattering regime of the pressure-distance range used in an ESEM. Thus, the fundaments for a recognized new instrument were created. The ESEM became a fully fledged instrument for the examination of any specimen, wet or dry, insulating or conducting, untreated or treated biological or inorganic specimens close to the natural state and in situ, or in vacuum (ElectroScan 1988). The Foundations of the ESEM constitutes a prime reference for this technology.

The Gaseous Detection Device has further shed light on the physical processes behind image formation and contrast, in particular with regard to the so called "specimen absorbed current". The latter mode of detection was possible only with conducting specimens in vacuum SEM and this had been proposed by another worker as an alternative technique for imaging also wet conducting specimens in an moist environment earlier on (Shah and Beckett 1979). However, it was also incorrectly perceived that the gaseous ionization was obscuring and obstructing the image contrast that was observed. Thus, it was only the "Theory of GDD in an ESEM" ( 36), based on correct physical principles (e.g. displacement or induction current in conductors by all moving charges in the gas) that restored electron microscopy to its scientific grounds.

These works not yet grasped or known by the established electron microscope manufacturers at the time, it further required the formation of a small venture capital company, namely, ElectroScan Corporation in USA, to devote its efforts in the production of the first commercial ESEM. After a few years of company research and development, the first instruments appeared on the market and ESEM started to be applied beyond the confines of the prototype ESEM in Sydney. A few more years were needed to achieve the "critical mass" of users that made the new technique acceptable and broke the barrier of skepticism or disbelief. When a few years later a traditional electron microscope manufacturer (Philips/FEI) took over the operation to further commercialize the instrument, ESEM assumed its rightful place among the electron microscopes in the world. Since then, the most diverse applications have appeared in the literature, which, in turn, gave a new impetus to ESEM that is now spreading worldwide ( UK, UK, UK,  GE, GE,  US, US, US, US, US, US, US, US,   CH, TR, SA).  This has been improved with the addition of new manufacturers ( ZEISS). Other manufacturers have, in the meantime, followed with systems called "WET SEM", "LV-SEM", "Natural SEM", "ECO-SEM", "VP-SEM", etc., all being equivalent, namely, all having operated at around 100 Pa pressure or less with the use of some BSE detector, clearly a pre-ESEM technology, probably due to patent restrictions and other manufacturing limitations.  Nevertheless, all commercial versions of an ESEM have presented from the outset certain limitations not found on the prototype ESEM.  These limitations are still waiting to be rectified.

The first commercial ESEM did not mark the end of research and development of the instrument. Studies of the dynamics of gas flow in the ESEM by the use of the Direct Simulation Monte Carlo computational techniques, borrowed and adapted from the latest aerospace science, have produced some remarkable results unexpected by, or even running against, common intuition. This work has resulted in overcoming certain limitations that have accompanied the ESEM: The pair of PLAs employed in the instrument creates a tunnel vision effect that limits the field of view at low magnifications, which is a clear disadvantage over conventional SEM. Hence, in practice, it has been common sense to use larger final PLAs with larger pumping capacity with high beam kV to slightly improve, but not overcome, the limited field of view in the commercial ESEM. However, this tendency has prevented the use of very low accelerating voltage beam desirable in many applications with significant gas pressure, in clear conflict of interests. Fortunately, it has now been demonstrated that it is possible to restore the wide field of view of a SEM in an ESEM with a much smaller final PLA and smaller pumps if a system of PLAs, probe forming aperture, scanning coils and objective lens are properly designed and integrated. In one form, this constitutes one immediate task for the manufacturer ( new patent). The same principles in another form (i.e. another major innovation) is to supply the environmental gas in the chamber by what has been termed "reverse flow pressure limiting aperture (RF-PLA)" ( 53 , new patent), whereby the gas supplied literally pumps itself in the opposite direction through the PLA, via a supersonic annular jet with core pumping action at the PLA. This will provide some distinct advantages when it will be incorporated in the commercial ESEM. A third line of innovation is the proposed radiofrequency gaseous detection device (RF-GDD) ( 52, or new patent) whereby the d-c bias is replaced by a high frequency a-c voltage on the detector electrode. A further improvement regards the detection of x-ray signals where the spatial resolution remains seriously compromised on account of the electron skirt surrounding the electron probe. Several suggestions to overcome this problem have been made in the interim time but this problem requires the wider participation of several research scientists in the face of the more complex instrumentation involved for this mode of detection; good progress has already been reported in that area too. These are the most immediate needs/tasks of ESEM that must be addressed by the manufacturer and other workers, if it is to create a Universal SEM (USEM), which  would work seamlessly between vacuum and any gaseous pressure without compromise.

This breakthrough of ESEM, now enjoyed by thousands of microscopists around the world, did not come about easily. What is now common practice and makes common sense was not obvious at first. The introduction of gas in an electron microscope seemed an anathema by the generally established ideas that a "clean" high vacuum was imperative to maintain  both the electron probe and the specimen undisturbed. It was generally perceived that a gaseous environment would deteriorate the resolution by an assumed broadening of the electron beam spot and would exclude the secondary electron detection mode of imaging. Indeed, many workers had introduced gas in their electron microscopes but none of these works was sufficient to allow the production of a broadly accepted instrument.

In summary, it was the developments in the 1980s that allowed the manufacture of a commercial  ESEM .  These works encompass all main modes of imaging, electron dynamics and gas dynamics. The optimum design and integration of detectors with electron optics and differential pumping constitutes the basic philosophy of ESEM. Central to these is also the introduction of new detection methods such as the  gaseous detection device (GDD)  (36). With this, both the secondary and the backscattered electron signal can be detected, in a variety of ways. The advancement of a proper theoretical and practical background has also constituted the basis  (28) for much of the progress in the past, present and future. A detailed discussion of these developments can be found in the works listed below. Abstracts for the same works can be found in the  Index of ESEM terms.

The foregoing is a historical outline of key technical issues that had to be overcome in the past via a tortuous path of development. It has taken more than half a century for von Ardenne’s "universal-elektronenmikroskop", first used in the study of metal oxides, to reemerge in the most unexpected ways and forms. The road map ahead is now clear.