In about the second half of 1985, a venture capital company, ElectroScan Corporation, was formed by a group of people north of Boston USA, in order to commercialize the idea of introducing gas in the specimen chamber of a scanning electron microscope (SEM). The first engineering attempt was based on advice and a patent by Alan Nelson (US4720633) , which however soon failed to produce any image. By November of 1985, the group sought the assistance by Danilatos in Sydney who already had produced a working prototype with numerous publications. Under Danilatos guidance, the first image in a gaseous environment was obtained at the ElectroScan premises within three days, i.e. on the 5 February 1986. A new commercial instrument , the environmental scanning electron microscope or ESEM was thus born, and within a few years of manufacturing developments it was to be fully marketed by 1988.
The first imaging was obtained with the use of correct differential pumping between two thin-wall apertures as opposed to the stacking of a series of apertures without pumping between them as per Nelson patent. The latter stack of apertures traps air over an extended transition region in which the primary electron beam is catastrophically scattered and lost. A series of apertures like that may have a better engineering separation of the vacuum region in the column from the pressurized specimen chamber than a single aperture of the same diameter. However, such a series of apertures constitutes the worst approach for a working ESEM at (a) the lowest beam kV, (b) minimum beam loss and (c) highest gas pressure.
Initial imaging at ElectroScan was based on the use of a scintillating backscattered electron (BSE) detector but Danilatos insisted also on the use and development of a new detection principle already pioneered and practiced on the prototype since 1983. The latter consisted of the use of the ionization of gas by various signalsas a detector for those signals. Since the secondary electron (SE) imaging constituted the basic mode of detection in most conventional SEMs, the company work concentrated on making the gaseous secondary electron detector the basis of the new instrument, for which a new patent (US4785182) application was lodged.
ElectroScan also designed and built their own home grown electron optics column that integrated differential pumping in the objective lens (US4823006). Unfortunately, whilst differential pumping was very good, the geometry of a flat bottom pole-piece placed severe limitations on the integration of detectors and specimen movement. In this respect, the commercial instrument was severely constrained from reproducing the results already on record by the Danilatos prototype. The optimal BSE detectors (17) already developed by Danilatos (see also design shown here ) could not be used on this commercial ESEM and thus this mode of detection was compromised. The first offered scintillating BSE detector, or the later use of third party BSE detectors of an earlier era have resulted in the commercial ESEM lagging seriously behind the Sydney prototype ESEM ever since. It has not been appreciated that both BSE and SE modes of detection are highly relevant in ESEM developments as opposed to the lopsided development of one only over the other.
Danilatos ceased to be an adviser to ElectroScan in 1993, whilst ElectroScan was bought by Philips shortly afterwards and by FEI later on. The introduction of the Philips conical objective lens was a step in the right direction with much potential, but none of the later developments were done under any instructions or guidance by Danilatos. The optimal BSE detectors were not incorporated as they could and should, whilst new issues emerged as described below.
In recent years Danilatos advised LEO (now Zeiss) on ways to enter the ESEM market with an ESEM of their own make capable of using the gaseous secondary electron mode in the extended pressure range, together with optimal BSE detectors. Success in this direction will be measured by the extent of such implementations.
Users of commercial ESEMs should be aware that any deviations of performance from the reported results by the Danilatos prototype ESEM are the responsibility of the corresponding manufacturer and that such deviations do not represent any inherent weakness of the principles involved. Through the entire research period, including the times when Danilatos received financial support from some companies, all Danilatos works and reports are based on pure science away from any bias arising from commercial interests or other considerations. These works have aimed to establish the theoretical and practical physical limits in the design and construction of an ESEM. Hence the manufacturers should use such works as a benchmark and the users should satisfy themselves that their instruments live up to their expectations.
The commercial availability of ESEM has given a tremendous impetus in many fields of applications and for the first time the international scientific community have enjoyed possibilities un-thought of previously with the vacuum SEM. However, when some commercial design aspect is not what it should be, it may not be easy to rectify immediately, as the production of a particular model results in the installation of a significant number of units in the market. The only or best reaction then would be for clients provide a feedback somehow to the manufacturer in the hope that the identified problem will be rectified with the next generation of microscopes. Some users have expressed certain concerns on instrument performance and Danilatos occasionally offered some assistance when the users sought it. Some key problems that need fixing are included herewith:
At the University of Technology (UTS) using a Philips XL30 ESEM, it was found that imaging became impractical or impossible as the pressure reached 1000 Pa and acceptable work was mostly performed with the aid of a cooling stage to lower the saturation water vapor pressure close to 600 Pa and/or with the use of the highest beam kV. Upon visual examination of the aperture bullet system, it was thought that the geometry was in serious deviation from the optimum design and a computational determination of the gas flow was undertaken. In Fig. 1 the pressure (or density) gradients are shown along the axis of the bullet system: A 0.5 mm bottom pressure limiting aperture (PLA1) is shown on the left side, with the vacuum of the column on the right side separated by the second (upper) PLA2. Significant gas layers develop throughout the bullet system between the two apertures, with the highest values of gas density inside and around the vicinity of the bottom PLA1 which is also shown at an enlarged view in Fig. 2.
Fig. 1 Gas density gradients inside the "bullet" of the UTS ESEM
Fig. 2 Gas density gradients (enlarged view) through the pressure limiting aperture (PLA) of the UTS (previous) "bullet". On the left side (in red) is the specimen chamber at 10 mbar pressure. The electron beam has to overcome the stagnating gas inside the PLA cylinder after already having undergone significant losses in the intermediate space between the two PLAs.
Fig. 3 Gas density gradients inside the QLD ESEM. There is less overall stagnating gas than the UTS bullet.
The same work was repeated on an early ElectroScan U3 ESEM model used by the University of Queensland (QLD) and the corresponding gas density in the bullet is shown in Fig. 3.
Last, the gas flow density was computed for a single thin aperture having maximum conductance downstream of the flow (free space). The density contours are shown in Fig. 4. The latter is taken from a rigorous investigation of the gas flow in the complete range of pressures up to atmospheric pressure for different gases including water vapor. In practice, this situation can be closely achieved with proper care of the aperture construction (e.g. conical shape) , mounting and configuration, together with various detectors .
An integration of the gas density along the axis between the apertures yields the total mass thickness which the electron beam has to overcome before it enters the specimen chamber (on the left of PLA1). The amount of total electron beam loss along the axis in this region has been computed for a series of conditions and the results have been presented in detail twice in (54) and (56) . These results are summarized in Fig. 5 showing the total electron beam loss for the three cases of instrument design mentioned above. It is immediately seen that the older commercial model performs better than the newer, but both perform worse than the physical limit obtained with a thin aperture, as used on the prototype ESEM.
Fig. 5 Comparative presentation of the electron beam losses for the three cases of PLAs presented in the previous figures
The results from the work on thin apertures have been used to establish the electron beam transmission (or loss) in the complete accelerating voltage range, gas pressure range and working distance range for different gases. Whilst already published works can lead to these results, much work still remains unpublished in a comprehensive form, which however can be available to interested parties, e.g. manufacturers [See latest paper or (57) ]. By such means we have established the physical limits which every ESEM should tend to approach, i.e. the closer to this limit the better. In fact, the Prototype ESEM has been shown to operate practically at this limit, which has resulted in the production of the best raw images emanating from the best physics and not from any image processing technique. One would then expect to have obtained even better results by modern computer image processing more than 20 years after the pioneering work took place. Unfortunately, this is not born out by the subsequent commercial instruments like the ones presented herewith [as it should be stressed again that the later commercial models perform worse than the earlier , and all of them worse than the original prototype]. Clearly, the thick wall PLA1 (as in Figs. 1, 2 and 3) has a similar effect as the series of apertures used in the Alan Nelson (US4720633) patent. It is not clear if this has been a deliberate engineering attempt to decrease the gas leak through the thick aperture (at the expense, however, of electron beam transmittance), or a persistent engineering oversight, or some other reason. In any case, it seems like another commercial behavior operating against proven science and practice, but with some very disappointing results for ESEM.
The consequence of using a cylindrical instead of a thin wall PLA1 has far more destructive effects in addition to the beam loss. The cylindrical geometry has a large inside surface which is prone to quick contamination. Any debris in contact with the surface gets irradiated and firmly stuck against the surface. At low pressure, or if the user wishes to revert to vacuum operation, any contamination inside the PLAs will create serious astigmatism on account of charging. The charging can be so great by the incident beam that even at increased pressure the gaseous ionization may not suffice to balance off the amount of charging and hence imaging becomes problematic and a general malady for the instrument over all. Furthermore, the position and shape of PLA2 together with the overall "bullet" (apertures-evacuation assembly) design are critical to prevent contamination of the upper column, which can drastically reduce the lifetime of the electron gun and affect normal/efficient electron probe formation. Users who find it difficult to focus and get clear images or experience short gun lifetime should become aware of the causes of their difficulties. Manufacturers should address these problems both on new instruments and on the ones already in place out on the market. Solutions do exist for those who wish to have them.
Another tested improvement (also solution to above problems) regards the " field of view" which has been compromised for a long time in ESEM including the early prototype: The pair of apertures used creates a "tunnel" vision at low magnifications, which inhibits, or makes difficult the observation of a large area before the user zooms-in over a small feature of interest. The practice of increasing the diameter of PLA1, as a means to increase the field of view, quickly reduces the working pressure range on account of increased gas flow and increased gas density gradients in the bullet. As a result, either very high beam accelerating voltage must be used at low pressure or imaging is impossible at elevated pressure. This problem has been overcome with a recent patent (US6809322) that allows the use of a much smaller (and thinner) PLA1 with simultaneous much greater field of view than hitherto used. This at the same time increases the useful pressure range and decreases the accelerating voltage needed for the beam. The practical consequences of such means are enormous: Better resolution on "soft" specimens with much less beam damage at higher magnifications/resolutions. Furthermore, smaller PLA (without compromising the wide field of view) means less probability to contaminate the upper column, better imaging, long electron gun life, etc...
ElectroScan also developed and sold two important accessories for the ESEM, namely, a hot stage with which specimens could be heated at more than 1000 degrees C, and a cooling stage using the Peltier principle. The hot stage took a big proportion of the R&D effort, presumably prompted by demand in that area of application. The cooling stage was mainly prompted by the difficulty experienced by the engineers to operate the ESEM at room temperature and high pressure. However, there is also a third ancillary device in great need, namely, a microinjector device that allows deposition of liquid droplets of the smallest possible size in a controlled stop/start manner. A commercial version of such a device became available by the use of a capillary needle connected to a syringe at ambient pressure outside the microscope. However, this approach invariably resulted in the flooding of the specimen stage with water, since the flow could not be stopped or controlled in any way: By pulling the syringe plug back, the back pressure at the plug remains at saturation vapor pressure at ambient temperature, whilst the pressure at the specimen was at saturation pressure at the cooling stage temperature which was always lower than at room temperature. If the pressure at the plug is greater than the pressure at the tip of capillary needle (near the specimen), it is impossible to stop the flow of water from the syringe to the specimen. Clearly, that approach was bound to fail, because the water could not be sucked back and could not be stopped or controlled. Because of this phenomenon, a solution to the problem was found and published by Danilatos and Brancik (27) a long time earlier: The syringe was connected to a pipe loop that opens back out to room (ambient) pressure. Water flowing in the pipe loop supplies on its way the back of the capillary needle, or water can be freely sucked back in the syringe emptying the back of the capillary needle. In this manner, water could be freely pushed back-and-forth between the syringe and the back of the capillary needle, hence a controlled start-stop situation could be easily achieved. Such a microinjector was successfully used for a long time and many hours of video recordings at true TV scanning rates were obtained during the years 1980-1985. Some still images from those recordings have been published . Also, some excerpt recordings were copied to VHS cassettes and retained by ElectroScan. They were shown at the Albuquerque EMSA Meeting in 1986, as they were also used to promote the ESEM at the outset of its commercial development. To date, no such video recordings at true TV scanning rates have ever been shown from any commercial ESEM .
The necessity to use a cooling device on a regular basis is disappointing, because it is very restrictive as to the ease and range of applications possible in the commercial ESEM. Having to control the temperature at a low point prior to observations and imaging in the ESEM is just another unnecessary complication, even detrimental to many applications. For example, biological studies of fresh and living specimens can yield problematic results due to the cooling device alone. The cooling device should only be used in extreme situations, whilst for routine operation the ESEM should be usable at room temperature without further ado.
As pointed out before, the use of third party scintillating BSE detectors is most unfortunate, since those have been superseded by specially designed and devoted ESEM detectors since 1979. Third party detectors, in general, are suitable for SEM but not for ESEM, because they cannot fulfill the stringent requirements of the latter. The manufacturer should have developed and equipped every commercial ESEM with indigenous (own or homegrown) design BSE detectors along with the SE gaseous detector. The BSE certainly should not be offered as an "option" of extraneous detectors of dubious performance for ESEM. This is an additional reason why the commercial ESEM has not duplicated, let alone surpassed, many of the results obtained by the first laboratory prototype ESEM. There has been a long and vigorous discussion on the contrast and resolution obtained by BSE and SE signals and related detectors throughout the 1970s and afterwards. The best conclusion is that these modes of detection complement rather compete against each other. Very high resolutions have been demonstrated also with BSE detectors. Certain misunderstandings have been exploited by manufacturing expediency in promoting only one mode, as it has happened with the commercial ESEM. Since patents were owned only on the gaseous SE detection and not on BSE mode, the manufacturer identified the SE detector with the ESEM technology in general, and a generation of users have fallen victim of this misunderstanding too. Good BSE detectors are, in effect, absent throughout the hitherto existence of commercial ESEM. A significant percentage of applications would have been far better implemented in the BSE mode, whilst the lone SE mode has led many users to erroneous conclusions about the true capability of ESEM. Actually, the contrast and resolution in ESEM is not limited by either of those modes of detection in most applications of untreated specimens, but the electron beam radiation effects often become the limiting factor.
The development of a "helix detector" [also mirrored here]] in a research institute is highly commendable. Actually, this and scores of other potential developments and improvements have been envisaged in the " Theory of the Gaseous Detection Device " or (36 ) and elsewhere. Many other research institutes should follow this example. Manufacturers should also take up specialized developments such as this but not before the commercial instruments have overcome their existing fundamental problems and limitations. The "helix" detector certainly does not solve any of the problems and limitations outlined herewith, and statements like "However, until the development of the Helix detector, ESEM could not be applied at the very highest SEM magnifications that are essential for nanotechnology" are mere misrepresentations of the basic ESEM capabilities in a proper design. A specialized detector development should not be given precedence over eliminating other existing basic flaws, the cart should not be placed before the horse and ESEM should not be downgraded to operating only at low vacuum. The manufacturers need better direction on the grounds of a comprehensive understanding of ESEM instead of pursuing haphazardly a few developments here and there. This brings us to face the cause of many problems closely: The self-portrayed "road mappers (on page 10)" of ESEM have not yet produced a machine to its full potential. More work is needed to extricate right from wrong.
The basic gaseous detection device followed by the many modifications as described in the Theory of the Gaseous Detection Device can serve the ESEM for many generations of commercial models to come, whilst we are not short of novel detection ideas if the manufacurers were willing to expand even more.
It has been shown that the resolving power of an ESEM is determined by the electron probe diameter in the same manner as with a SEM. Test specimens of gold particles on carbon can be equally resolved by both instruments . Manufacturers compete in the "battle to resolve the Angstrom" and the "nano-" has become the latest catchword to impress the world with various "nano-technologies". However, the physics of electron beam-specimen interactions has not changed an iota no matter how smart an instrument claims to be. The actual resolution may be spoiled in most applications by the very radiation effects that accompany the highest magnification available. For example, if we look at Figs. 100 and 101 of Foundations... the edge definition of an ordinary specimen at moderate magnifications appears blurred following irradiation. The unaware user would in vain try to sharpen the focus in this case, or may think the instrument is faulty. This and other types of irradiation effects have been reported not only with ESEM but also with SEM. However, ESEM is more demanding in this area because it claims the privilege to look at natural specimens. Apparently, blasting of a wetting specimen by the electron beam should not usually be the case of study (e.g. see "Video Tour EVOŽ Tour 4 of 5" on coffee grain, which is also directly mirrored here). When one often or easily sees "bubbling" on the image, one can suspect that either the instrument or the operator is not performing well (i.e. optimally). Specimens should be kept as unspoiled as possible. The only way to achieve this is by the use of the absolute minimum radiation dosage at the maximum magnification. Actually, a glance in various journals immediately reveals that the vast majority of applications have not been done or have not demanded the use of the ultimate resolving power of the instrument in use. Therefore, it is imperative that the "sensitivity" of microscopes rather is of prime consideration and should not be compromised or overlooked by the manufacturers in their planning to improve their instruments. An optimum beam transfer [see latest paper or (57 )] together with the supply of the most sensitive detectors requiring minimum electron beam intensity and accelerating voltage are the prime factors that have brought radiation effects under control at the minimum possible physical limit. This is an aspect of paramount importance in the practice of this technology, an aspect not to be overlooked at all times.
Another flaw that has plagued the commercial instruments is the problematic control of relative humidity, of wetting and drying a specimen in a controlled way. There are a number of publications from the use of commercial instruments that bear evidence of this limitation, of which every user must be aware. This has been caused by the lack of a specimen exchange chamber (airlock). The entire specimen chamber of commercial ESEM is evacuated each time a specimen is changed and it is practically impossible to remove the ambient gas without initially lowering the relative humidity well below the 100% level. Some users have devised a routine of cycling the pumping a number of times before they can reach a saturation (wet) state in the chamber, but this imposes severe limitations on many applications, especially in the examination of fresh and live biological samples where relative humidity matters. This is time consuming and detrimental both to the application and to the instrument in which huge pressure differentials on the PLA (and gas flow) increases the chances of contamination. The prototype ESEM has overcome these limitations right from the outset because the modified JEOL JSM-2 SEM is equipped with an airlock, which is appropriately connected to the pumping system of the ESEM (see OPERATION AND APPLICATIONS as well as other numerous publications by Danilatos). Specimens are inserted in the specimen chamber within a minute and never have to undergo any drying condition whatsoever. The airlock contains a quantity of water which allows lowering of the ambient pressure monotonically to the 100% (wet) condition, and then the specimen is inserted in the main chamber which is continuously maintained at 100% relative humidity without interruption with each specimen change. This is speedy and safe both for the specimen and the overall operation of the instrument, as is outlined in relevant publications ( 28 ). Incorporation of a water reservoir inside the commercial chamber would again limit the scope of applications and relative humidity control, hence the inclusion of an airlock properly integrated with the pumping manifold is the best solution to an existing market problem. Perhaps manufacturing expediency, once more, determined the elimination of a classical airlock from routine instruments as those instruments were addressing the needs of the bigger industrial market applications (presumably) than lesser market of biological applications. The validity of such an argument may be questioned, as it may also be questioned why an instrument has to have only one or the other specimen transfer mechanism and not both. The whole problem is reduced to a problem of engineering efficacy. It is finally reduced to a question of whether the engineering management is in tune with and qualified to address these problems.
It is disappointing that good workers, while they genuinely strive to understand the new physics of ESEM, accept their commercial instruments uncritically. Indeed, it is strange that some users, especially of latest commercial ESEM, seem oblivious to the severe limitations of their instruments and have actually become subservient to them (after all, they have to justify their budget expenditure). They wouldn't even think of modifying their instrument or risk losing their warranty (after all, their assumption is that they got the "latest" technology in the field!). Thus, a whole generation of well intentioned workers is trapped. This may explain why we often come across some pompous paper titles hinting at thorough and deep-going scientific work, whilst the authors cannot even untangle themselves from the artificial constraints of their machines. None of these works has improved the commercial ESEM one iota, even in full view of the latter being downgraded to low vacuum SEM. Pompous titles yes, but real progress none. It now requires some courageous individuals to get out of this trap, scientists should start speaking out, at least some of those still operating the older models that provide better outcomes. The rest of the truth must be sought elsewhere when we examine events behind the academic world .
Based on the evidence given above, one should appreciate why there are several commercial ESEMs around that represent a serious deviation from the goals and achievements originally obtained and set. With any description of "how ESEM works" [also mirrored here ], it must be made clear that the description applies to a particular commercial ESEM type and not to ESEM in general. The instructions may well be based on a retrogressive design: That is an example of the artificial limitations imposed on working pressure range, beam kV, field of view, image noise (hence dependence on image averaging), working distance, temperature (hence dependence on cooling), charging of large insulating specimens, etc. The ESEM, in general, is not like SEM with "two added degrees of difficulty" but a particular commercial brand may be so. It is ironic that the truth (fortunately) is exactly in the opposite direction: ESEM has created new degrees of freedom and ease of operation! A more general description of how ESEM works can be found here .
A proper ESEM is the one that allows the user to operate at room temperature, under fully wet conditions, at low kV (less than 5 kV), with sufficient space to allow most applications and ancillary devices (like a microinjector), and a large field of view. These, in turn, allow excellent contrast and resolution with minimum or no specimen damage, all of which, in turn, allow an unlimited number of applications. However, ill-conceived commercial instruments may only have an opposite effect on the market, which, in turn, is used as internal company argument against normal progress. The ESEM, in general, should not be identified with any single form of commercial make. The term ESEM was introduced prior and independently of the commercial developments and there should be a clear distinction of terminology regarding the technology per se and various commercial forms of ESEM.
It is very striking if one considers the achievements at the very outset or ( 1 ) of this work, let alone the breakthroughs that followed in the decade after that. When imaging could be obtained at pressures in excess of 50 mbar (5000 Pa) in the year 1978 with the simplest modifications to a 1968-make of an old SEM and with meagre resources and a shoestring budget, whilst present day "state of the art" machines are constrained to low vacuum and pressure with crude operational parameters, the scientific community becomes seriously concerned. This exposition clearly shows that beyond the general understanding of the foundations and principles involved, success also involves the art of adequate precision in the design and construction of an ESEM. This clearly indicates that the engineering crudeness factor of various design parameters on commercial instruments is at least one order of magnitude out of the scientifically prescribed one. Any scientist or user of this technology must wonder why this is so. Three broad causes can be proposed: (a) The commercial engineers have never experienced the levels of achievement by the ESEM Research Laboratory , (b) they may have not read or have never understood the published works , (c) there are other obscure reasons outside scientific logic.
Clearly, an ESEM optimally designed to operate at high pressure is automatically superior in performance at low pressure or low vacuum without further ado: An optimal ESEM design is good both for low and high pressure work. The internal argument used by manufacturing personnel that "there is not high demand for high pressure work" is very poor indeed: Firstly, it is absolutely false to claim that there is not high demand in the market for high pressure work. Secondly, even if the previous claim were to be correct in some way, you do not go about compromising and corrupting the engineering design because allegedly there is no high demand in the high pressure range. This is ludicrous and obscure thinking (yet it happens!). How can you expect a demand for high pressure work if the current offerings to an unaware market are ineffective? Furthermore, you simply cannot expect the market forces alone to ensure optimum operation of the ESEM product, since this is not an every day commodity like milk and bread, or cars. The market forces to operate for this commodity (ESEM) can have a time lag of 20-30 years and have no immediate practical consequence. The first commercial ESEM did not come about because customers were demanding it from the manufacturers. It was the initiative of a small group of people who promoted it in the first place and the market followed. After that, supply and demand go hand in hand but the key is to maintain a healthy supply. A good product will increase demand. Bad reputation will stifle the market. Hence the manufacturers are obliged to live up to the expectations of this newly created market, if progress is to made and if they are genuine in their commitment to serve science and technology. In parallel, scientists must press ahead with progress despite all odds.
Herewith, we have exposed several instances of artificial technological mishaps, unnecessary and avoidable throughout ESEM's commercial existence. We can also understand why an ESEM should not simply be identified with the commercial detector of "GSED" (a misnomer for the gaseous detection device) as is often inappropriately done. For what is the use of the latter detector in an machine that results in massive electron beam losses during transfer, or incurs frequent contamination and image deterioration, or sustains only short electron gun life, or desiccates and kills fresh specimens prior to achieving 100% relative humidity, or blasts delicate specimens under forced high accelerating voltage beam operation, or has to operate only with a cooling device mainly at freezing conditions, or provides poor BSE imaging, or is cumbersome to operate, or has long down times and requires frequent service requests? ESEM is the synergy of a host of technological musts. In short, we have demonstrated several clear cases for improvements waiting for implementation for a long time now. There is no technological justification of why these problems have appeared at all. The explanation must be sought in the management of human resources by the manufacturing world and in the tolerance or misguidance demonstrated by the academic world . The historical reasons for the delay in the implementation of what otherwise should have been plain and natural ESEM development during the last 20 years constitutes a topic to be dealt with separately, when we look behind the commercial world.
All obstacles above have a solution and so scientists and users of ESEM should not lose heart in their continued efforts to do the best out of this unique technology even in the present form available to them, whilst striving to steer the commercial ESEM in the right direction. Manufacturers must realize that there is a technological "vacuum" in the market which should be filled with a fully fledged ESEM in accordance with proven specifications (e.g. optimum design specifications ), which still remain beyond the reach of mainstream users. Bygone previous patent limitations, the key now is the "know-how" for ESEM excellence. It remains to be seen which manufacturer will be the first to fill this vacuum...
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