The first electron microscope prototype was developed in 1933 by the German engineers Ernst Ruska and Max Knoll. It was based on the concepts and findings of French physicist Louis de Broglie. Even though it was primitive and not fit for practical use, the device was still capable of magnifying objects by four hundred times. Reinhold Rudenberg had patented the electron microscope in 1931. Consequently M. von Ardenne at 1938 created a scanning transmission electron microscope (STEM) by adding scan coils to a transmission electron microscope. The first STEM micrograph was of a ZnO crystal imaged at an operating voltage of 23 kV at a magnification of 8000 times, and a spatial resolution between 50 and 100 nm. The micrograph contained 400 x 400 scan lines and took 20 min to record, since the film was automatically scanned in synch with the beam. The device had two electrostatic lenses, with the scan coils positioned between them. The device also had a viewing CRT. Nevertheless, the first practical electron microscope was built at the University of Toronto in 1938, by Eli Franklin Burton and students Cecil Hall, James Hillier and Albert Prebus. Zworykin et al at 1942 having employed in the RCA Laboratories in the United States illustrated the first SEM, which was used to test the surface of a solid specimen. The electron optics of the device consisted of three electrostatic lenses with scan coils positioned between the second and third lenses. The electron gun was placed at the bottom so the specimen chamber was at a comfortable height for the operator. This was a usual way in the early days. It did suffer from the slight problem though, that the specimen might fall over the column. A resolution of approximately 50 nm was accomplished with this first SEM. By means of comparison with the fast developing TEM, this figure was considered unexciting and further development lapsed. In the late 1940s C. W. Oatley, then a lecturer in the Engineering Department of Cambridge University, England, became interested in conducting research in the field of electron optics and decided to reinvestigate the SEM as a complement to the work being done on the TEM by V. E. Cosslett, also in Cambridge at the Physics Department. One of Oatley’s students, Ken Sander, began work on a column for a transmission electron microscope using electrostatic lenses however Ken took ill after about one year and had to leave for a time. Dennis McMullan then took up this work in 1948, and he and Oatley built their first SEM. On 1952 this instrument had achieved a resolution of 50 nm, however by far the important thing about this SEM is that it produced the first micrographs viewing the striking three-dimensional imaging character of the contemporary SEM. Ken Smith who started at 1952 and took over SEM1, made a number of developments to the electron optical system, and enhanced the efficiency of secondary electron collection followed Dennis McMullan. He illustrated for the first time that an established image could be produced using the true low-energy component of the total secondary emission.

Wells who started at 1953 and built a second SEM, also incorporating electrostatic lenses; unlike SEM1, nonetheless, this instrument had the gun at the bottom of the column–a design considered better for experimental work. Wells pioneered the use of the scintillator backscattered (BSE) detector as an alternative to the secondary electron multiplier used in SEM1 and applied SEM to many new types of specimen including a large-scale study of fibers. He was also the first to use stereographic pairs to produce SEM micrographs with quantifiable depth information at 1960. The next important step was taken by Everhart who started at 1955 and improved the secondary electron (SE) detector by using a scintillator to convert electrons to photons, which were then transmitted by a light pipe directly interfaced to the photo multiplier tube. Thornley followed up this idea at 1957 and their pioneering work resulted in the publication of a much-quoted paper: “Wide-band detector for micro-microampere low-energy electron currents” (Everhart and Thornley (1960)). Substitute of the electron multiplier with the new scintillator with photo multiplier combination amplified the amount of signal collected and produced an enhancement in signal-to-noise ratio. Consequently, weak contrast mechanisms such as voltage contrast, which was discovered by Oatley and Everhart at 1957, could be better examined. The term “voltage contrast” developed from the fact that as the voltage utilized to a specimen was altered, the image contrast changed. Image analysis was also improved in this early period of research when both Everhart and Wells made the first quantitative studies of the effects of beam penetration on image creation in the SEM. Peter Spreadbury started at 1956, built a simple SEM utilizing a CRT as a display unit. Gary Stewart who started at 1958 and fitted an ion gun to the SEM specimen chamber to allow ion bombardment of the specimen opened up new fields of application. After his research, Gary went on to the Cambridge Instrument Company to pioneer the production of the Stereoscan. Alec Broers who started at 1961 and improved the ion beam optics of the instrument and added a magnetic objective lens to improve resolution later extended the ion beam work. He used this system to conduct some of the earliest experiments in electron beam micro fabrication. Another breakthrough was achieved by Haroon Ahmed who started on 1959 and modified the SEM built by Wells, which was SEM2, to enable the examination of thermionic emitters at temperatures exceeding 1000K. Fabian Pease who started at 1960 built the first SEM to achieve a resolution of 10 nm. This was an all-magnetic lens SEM, the fifth to be constructed. Although modern electron microscopes can magnify objects up to two million times, they are still based upon Ruska’s prototype and his correlation between wavelength and resolution. The electron microscope is an integral part of many laboratories. Researchers use it to examine biological materials such as microorganisms and cells, a variety of large molecules, medical biopsy samples, metals and crystalline structures, and the characteristics of various surfaces.

There are two main types of electron microscope, the transmission electron microscope (TEM) and the scanning electron microscope (SEM). The TEM consists of a number of lenses, an electron source, and a system that projects an image onto a fluorescent screen or photographic plate. The lenses must be electric or magnetic lenses since ordinary glass lenses for light microscopes will not focus a beam of electrons. The electron source is a filament of tungsten that releases electrons when heated. A high voltage electric field approximately 50,000 to 100,000 volts employed between a pair of metal plates accelerates the electrons, and electric or magnetic lenses condense the electrons into a narrow beam. The electron beam then passes through the specimen and individual electrons are scattered in various directions depending upon the density of the material they encounter. Denser material scatters the electrons more so that fewer of them reach the device used to detect the electrons. Once the electrons pass through the specimen, the objective lens focuses them using magnetic fields. Specimens must be sliced extremely thin for use in a TEM because only electrons that pass through the specimen are recorded. Modern TEMs are capable of magnifications of between 1,000X to about 1,000,000X. The SEM works by scanning a tightly focused electron beam over a sample. Electrons in the beam scatter off of the sample and onto a cathode ray tube, or screen. Each point on the sample corresponds to a pixel or picture element on the screen. The more electrons that hit a particular element of the screen, the brighter the pixel appears. As the electron beam scans over the entire sample, a complete image of the sample is displayed on the monitor. SEMs have a range of magnification of between 20X to 200,000X. The SEM is particularly useful since it can produce detailed three-dimensional images of the surface of objects, while a TEM can only produce two-dimensional images. Samples scanned by an SEM do not need to be thinly sliced, as do TEM specimens, but they must be dehydrated to prevent electrons from being scattered by water molecules in the sample. The vessels that house electron microscopes must be evacuated to very high vacuum to prevent the scattering of the electrons off of air molecules. The invention of the electron microscope was made possible by a number of theoretical and experimental advances in physics and engineering. The main concept on which the electron microscope is founded—that electrons have a wavelike nature—was hypothesized by French physicist Prince Louis Victor de Broglie in 1923. In 1927, de Broglie’s hypothesis was experimentally verified by American physicists Clinton J. Davisson and Lester H. Germer, and independently by English physicist George Paget Thomson. In 1932 German engineers Max Knoll and Ernst Ruska built the first transmission electron microscope. In 1938 Ruska and German engineer Bodo von Borries built the first model of the commercial TEM for the Siemens-Halske Company in Berlin, Germany. The English engineer Sir Charles Oatley invented the SEM in its present form in 1952. Electron microscopes have proven powerful research tools for investigating the basic structure of matter, specifically in the fields of biology and solid-state science. They have helped to reveal the surface structures of various materials and confirmed the shapes and behaviors of bacteria as well as animal and human cells and cell components. They are important in research examining the effects of various manipulations or treatments of these various types of subject matter. Scientists and publishers often add color to the highly detailed electron micrographs to increase interest, to help distinguish portions of the image, and to highlight important areas. Electron microscopes have given scientific and lay media remarkable pictures, such as the “faces” of insects, the shapes of microscopic organisms, and the surface structure of molecules of new, high-tech alloys and other substances. They are also becoming important to regular clinical pathology at medical centers.

The SEM operates by scanning a tightly focused electron beam over a sample. Electrons in the beam scatter off of the sample and onto a cathode ray tube, or screen. Every point on the sample corresponds to a pixel or picture element on the screen. The more electrons that hit a particular element of the screen, the brighter the pixel appears. As the electron beam examines over the entire sample, a complete image of the sample is displayed on the monitor. SEMs have a range of magnification between 20X to 200,000X. Normally, the TEM resolution is approximately an order of magnitude better than the SEM resolution, nevertheless, since the SEM image relies on surface processes rather than transmission it is able to image bulk samples and has a much greater depth of view, and so can produce images that are a good representation of the 3D structure of the sample. SEM reveals new levels of detail and complexity in the amazing world of microorganisms and miniature structures. The scanning electron microscope, which is designed for precisely analyzing the surfaces of solid objects, makes use of a beam of focused electrons utilizing approximately 5–25 kilovolts energy, as an electron probe that is scanned in a regular manner over the specimen. The electron source and electromagnetic lenses that produce and focus the beam are comparable to those described for the transmission. Scanning electron microscope can amplify objects 100,000 times and is used to detect the minute gunpowder particles present on the hand of a person who has recently fired a gun. These particles can also be chemically analyzed to identify their origin from a particular type of bullet. Also, X-rays released from the electron beam/sample interaction can be detected by an Energy Dispersive Spectroscope (DES), which allows samples to be analyzed for their elemental composition. In the forensic field, SEM/DES is used to detect and analyze gunshot residue and other trace materials, such as paint, glass and light filaments. An electron microscope requires a good vacuum for the production and proliferation of the electron beam, which in the past meant that the specimen under inspection had to be placed also in vacuum. Nonetheless, it is now possible to view specimens inside gaseous surroundings so that wet samples in a water vapor environment can also be examined. The conductive coating of specimens according to traditional practice is no more essential because the gaseous layer around the specimen becomes ionized and suppresses charge buildup. The gas itself can also be used as detection medium giving rise to novel detection and imaging techniques. These innovations have resulted in the scanning electron microscope. The entire work has been based on the improvement and use of a prototype SEM, which has generated results of high quality. In many respects, these results still remain unique and unrivaled to date. The SEM is rapidly gaining approval by the scientific, technical and industrial community as evidenced from the large number of publications arising from its use. The many attempts and history that have led to the present development of SEM have been reviewed and surveyed elsewhere. These works include all main modes of imaging, electron dynamics and gas dynamics. The optimum design and integration of detectors with electron optics and differential pumping comprises the basic philosophy of SEM. Vital to these is also the establishment of new detection methods such as the gaseous detection device (GDD). With this, both the secondary and the backscattered electron signal can be detected, in a variety of ways. The progress of a proper theoretical and practical background has also constituted the basis for much of the progress in the past, present and future.

Other proof present at a crime scene may include skin, hair, blood, or semen. A breakthrough in technology permits scientists to study the deoxyribonucleic acid (DNA), or genetic material, of these substances to ascertain whether they belong to the victim or to a suspected assailant. As a result of a high-technology method known as the polymerase chain reaction (PCR), a laboratory can quickly clone, or multiply, the DNA from a tiny sample of any of these substances. This procedure generates enough DNA to compare with a sample of DNA taken from a suspected criminal. The use of DNA for such identification purposes is commonly referred to as DNA fingerprinting. With the use of the scanning electron microscope these things can readily be determined.

Human bite marks can also serve as circumstantial evidence. Such bites may be found upon the body of a homicide victim or within pieces of food or other objects, such as chewing gum, found at the crime scene. A forensic scientist can fill the impressions caused by these bites with liquid plastic. Upon hardening, the cast formed is an extremely accurate replica of the assailant’s teeth, which can be compared with a cast made from the teeth of the suspect.

District attorneys routinely call upon medical examiners to give expert testimony in a trial, especially concerning the findings of an autopsy and the results of laboratory tests. Expert testimony is that given by a specialist who has been recognized by the court as having expert knowledge about evidence in the case. Such testimony is governed by different rules than the testimony of ordinary witnesses in a trial. Ordinary testimony is restricted to statements concerning what the witness actually saw or heard. An ordinary witness is prohibited from stating opinions about the case and from quoting statements made by other people. In contrast, an expert witness is allowed to express an opinion about the validity of the evidence in a case and may quote the statements of other experts in support of an opinion.

In order to fast track the investigation and other tedious, long-straining methods of resolving crimes and obscure incidents as well as verification of DNA of persons, bite marks, and fingerprints among others, the scanning electron microscopes are being utilized. This instrument has been such a great helped for the investigators and other forensic experts.

In topography, the surface skin of an object and its texture. The detectable features are restricted to a few manometers. In morphology, the shape, size and arrangement of the particles making up the object that are lying on the surface of the sample or have been exposed by grinding or chemical etching, these detectable features are limited to a few manometers. For its composition, the elements and compounds, and their relative ratios in areas–1 micrometer in diameter can be determined. For its crystallographic information, the arrangement of atoms in the specimen and their degree of order, only useful on single-crystal particles >20 micrometers can also be known.

The following are the detailed explanations of how a regular SEM functions: 1. The electron gun being the source at the top produces a stream of monochromatic electrons. 2. The first condenser lens normally controlled by the coarse probe current knob condensed the stream. This lens is used to both form the beam and restrict the amount of current in the beam. It works in conjunction with the condenser aperture to eliminate the high-angle electrons from the beam. 3. The beam is then constricted by the condenser aperture, which eliminates some high-angle electrons. 4. The second condenser lens forms the electrons into a thin, tight, coherent beam and is usually controlled by the fine probe current knob. 5. A user selectable objective aperture further eliminates high-angle electrons from the beam. 6. A set of coils then scan the beam in a grid fashion, dwelling on points for a period of time determined by the scan speed ranging in microsecond. 7. The final lens, known as the Objective, focuses the scanning beam onto the part of the specimen desired. 8. When the beam strikes the sample and settles for a few microseconds, interactions occur inside the sample and are detected with various instruments. 9. Before the beam moves to its next dwell point these instruments count the number of interactions and display a pixel on a CRT whose intensity is determined by this number. The more reactions the more vivid the pixel. 10. This process is repeated until the grid scan is completed and then repeated, the entire pattern can be scanned 30 times per second.

If the scanning electron microscope has advantages it also has disadvantages. Some of these are pseudo-colored SEM image of the feeding basket, real electron microscope images carries grey-scale information. The first degree filter setae carry in v-form two rows of second degree setae, pointing towards the inside of the feeding basket. The purple ball is one micrometer in diameter. In order to display the total area of this fascinating composition one would have to overlay more than 7000 times this image.

Scanning electron microscopes are costly to purchase and maintain. They are dynamic rather than static in their operation requiring particularly stable high-voltage supplies, very stable currents to each electromagnetic coil or lens, incessantly driven ultra-high-vacuum systems, and a cooling water supply circulation through the lenses and pumps. Since they are extremely sensitive to vibration and external magnetic fields, microscopes aimed at achieving high resolutions must be housed in buildings or even underground with special services.

The samples have to be viewed in vacuum, as the molecules that compose air might scatter the electrons. Recent advancements have permitted hydrated samples to be imaged using an environmental scanning electron microscope. Scanning electron microscopes usually picture conductive or semi-conductive materials at their best. Non-conductive materials can be imaged by an environmental scanning electron microscope. A usual preparation method is to coat the sample with a several-nanometer layer of conductive material, such as gold, from a sputtering machine. Nevertheless, this process has the potential to interrupt delicate samples. The samples have to be prepared in several ways to give proper detail, which may result in artifacts purely as the result of treatment. The problem of distinguishing artifacts from material arises, especially in biological samples. Scientists maintain that the results from various preparation methods have been compared, and as there is no reason that they should all produced similar artifacts, it is therefore logical to believe that electron microscopy features correlate with living cells.

An electron gun sends out a beam of high-energy electrons after the air is thrust out of the column. This beam passes through in a descending manner by a series of magnetic lenses planned to direct the electrons to a very fine spot. Near the bottom, a set of scanning coils moves the focused beam back and forth across the specimen, row by row. As the electron beam hits each spot on the sample, secondary electrons are bumped free from its surface. A detector tallies these electrons and transmits the signals to an amplifier. The conclusive picture is developed from the number of electrons emanated from each spot on the sample. In a standard SEM, electrons are thermionically given off from a tungsten or lanthanum hexaboride cathode and are hastened in the direction of an anode. On the other hand, electrons can be given off via field emission. Tungsten is used because it has the highest melting point and lowest vapor pressure of all metals, thus permitting it to be heated for electron emission. The beam passes through pairs of scanning coils in the objective lens that ward off the beam horizontally and vertically hence it scans in a raster way over a rectangular area of the sample surface. The electrons lose energy by frequent dispersion and assimilation inside an interaction volume of the specimen as soon as the primary electron beam interacts with the sample. The size of the interaction volume depends on the atomic number of the specimen, the specimen’s density, and the beam accelerating voltage. The energy exchanges among the electron beam, the sample results in the emission of electrons, and electromagnetic radiation that can be detected to create an image.

The secondary electrons originate within a few nanometers from the surface because of their low energy. A scintillator-photomultiplier tool, detects these electrons and the resultant signal is rendered into a two-dimensional intensity distribution that can be viewed and saved as a digital image. This procedure relies on a raster-scanned primary beam. The clarity of the signal depends on the number of secondary electrons reaching the detector. If the beam goes into the sample perpendicular to the surface, then the activated region is unvarying about the axis of the beam and a certain number of electrons “escape” from within the sample. As the angle of incidence rises, the escape distance of one side of the beam reduces, and additional secondary electrons will be emitted. Thus steep surfaces and edges tend to be brighter than flat surfaces that result in images with a well-defined, three-dimensional appearance.

Backscattered electrons consist of high-energy electrons originating in the electron beam that are echoed or backscattered out of the specimen interaction volume. Backscattered electrons may be used to identify contrast among areas with different chemical compositions, especially when the average atomic number of the various regions is different, since the brightness of the BSE image tends to increase with the atomic number. Backscattered electrons can also be used to form an electron backscatter diffraction image that can be used to find out the crystallographic composition of the specimen.

There are smaller number backscattered electrons released from a sample than secondary electrons. The number of backscattered electrons exiting the sample surface upward may be significantly lesser than those that go after courses toward the sides. Furthermore, in contrast with the case of secondary electrons, the collection efficiency of backscattered electrons cannot be considerably enhanced by a positive bias usual on Everhart-Thornley detectors. This detector positioned on one side of the sample has low collection efficiency for backscattered electrons suitable to small acceptance angles. The nature of the SEM’s probe, energetic electrons, generates it distinctively suited to examining the optical and electronic properties of semiconductor materials. The high-energy electrons from the SEM beam will infuse charge carriers into the semiconductor. As a result, beam electrons lose energy by promoting electrons from the valence band into the conduction band, leaving behind holes.

Cathodoluminescence, the emission of light when atoms excited by high-energy electrons go back to their ground state, is analogous to UV-induced fluorescence, and a number of materials such as zinc sulphide and some fluorescent dyes, exhibit both phenomena. Cathodoluminescence is frequently experienced in daily life as the light emission from the inner surface of the cathode ray tube in television sets and computer CRT monitors. In the SEM, CL detectors either collect all light emitted by the specimen, or can analyze the wavelengths emitted by the specimen and display a spectrum or an image of the cathodoluminescence in real color.

The spatial resolution of the SEM depends on the size of the electron spot that depends on the magnetic electron-optical system, which produces the scanning beam. The resolution is also restricted by the size of the interaction volume, or the degree to which the material interrelates with the electron beam. The spot size and the interaction volume are both very large in contrast to the distances between atoms, so the resolution of the SEM is not high enough to image down to the atomic scale. The SEM has advantages including the capability to image a comparatively large area of the specimen, the ability to image bulk materials and the diversity of analytical modes accessible for measuring the composition and nature of the specimen.

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 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, 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 experimental finding that the electron probe diameter was increased inside a much wider spread of electrons surrounding, like a skirt, the broadened and weakened imaging probe as the gas pressure increased. This seemed similar to the well known “top-bottom effect” or beam spread through the thin specimen sections used in transmission electron microscopy. Thus, it seemed that the gas seriously compromised both contrast and resolution. 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. These isolated attempts clearly did not produce sufficient or satisfactory results to impress the microscopy users or the manufacturers at large.

Undertaking an investigation and implementing proper differential pumping techniques together with re-designing and re-configuring the BSE detector later re-appraised the condition. 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) that allows detection of the secondary electrons (SE) and (b) the determination that the electron probe diameter remained constant with pressure increase.

It was found that the gas acted not only as a conditioning medium or 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. A simple biased electrode detected these signals 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.

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 or contamination deposition, while such effect or contamination was prevented by heating the platinum wire and by a special detection configuration. This finding was further supported by a complete investigation using an analytical formulation and numerical computation of the scattering and distribution of electrons inside the gas. 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. The fundaments for a recognized new instrument were thus 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 site, or in vacuum.

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 was proposed by another worker as an alternative technique for imaging also wet conducting specimens in an moist environment. However, it was also incorrectly perceived that the gaseous ionization was obscuring and obstructing the image contrast observed. It was only the “Theory of GDD in an ESEM” based on correct physical principles, or displacement or induction current in conductors by all moving charges in the gas that restored electron microscopy to its scientific grounds.

After a few years of 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 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 world-wide with the addition of new manufacturers, at a fast rate.

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 and expectation. 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. The same principles in another form or another major innovation is to supply the environmental gas in the chamber by what has been termed “reverse flow pressure limiting aperture (RF-PLA)”, 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 radio-frequency gaseous detection device (RF-GDD) 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 of ESEM that must be addressed by the manufacturer and other workers, if it is to create a Universal SEM (USEM), which works seamlessly between vacuum and any gaseous pressure without compromise.

The foregoing is evolution of key technical issues of electron microscope 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 re-emerge in the most unexpected ways and forms.