[20] Cryo-fixed specimens may be cryo-fractured under vacuum in a special apparatus to reveal internal structure, sputter-coated and transferred onto the SEM cryo-stage while still frozen. Zworykin VA, Hillier J, Snyder RL (1942) A scanning electron microscope. He further discussed the various detection modes, possibilities and theory of SEM,[6] together with the construction of the first high resolution SEM. Electromagnetic lenses (Condenser lens, … The preparation method reveals the proteins embedded in the lipid bilayer. In the SEM, CL detectors either collect all light emitted by the specimen or can analyse the wavelengths emitted by the specimen and display an emission spectrum or an image of the distribution of cathodoluminescence emitted by the specimen in real color. Such images can be made while maintaining the full integrity of the original signal data, which is not modified in any way. In SEM systems using these detectors it is common to color code these extra signals and superimpose them in a single color image, so that differences in the distribution of the various components of the specimen can be seen clearly and compared. Other types of electron emitters include lanthanum hexaboride (LaB6) cathodes, which can be used in a standard tungsten filament SEM if the vacuum system is upgraded or field emission guns (FEG), which may be of the cold-cathode type using tungsten single crystal emitters or the thermally assisted Schottky type, that use emitters of zirconium oxide. Embedding in a resin with further polishing to a mirror-like finish can be used for both biological and materials specimens when imaging in backscattered electrons or when doing quantitative X-ray microanalysis. The fractured surface is cut to a suitable size, cleaned of any organic residues, and mounted on a specimen holder for viewing in the SEM. The number of secondary electrons that can be detected, and thus the signal intensity, depends, among other things, on specimen topography. Vergroting van een dunne laag zinkoxide met SEM. Sample preparation, alignments, cost of ownership, and difficulties with analytical techniques prevent widespread adoption. This was made possible by the development of a secondary-electron detector[35][36] capable of operating in the presence of water vapour and by the use of pressure-limiting apertures with differential pumping in the path of the electron beam to separate the vacuum region (around the gun and lenses) from the sample chamber. Note how the SEM image allows for clear perception of the fine structure details which are hard to fully make out in the light microscope image. [13] Characteristic X-rays are emitted when the electron beam removes an inner shell electron from the sample, causing a higher-energy electron to fill the shell and release energy. 3D surface reconstruction of a (Ra = 3 µm) roughness calibration sample (as used to calibrate profilometers), from 2 scanning electron microscope images tilted by 15° (top left). Cathodoluminescence, the emission of light when atoms excited by high-energy electrons return to their ground state, is analogous to UV-induced fluorescence, and some materials such as zinc sulfide and some fluorescent dyes, exhibit both phenomena. On compositional images of flat surfaces (typically BSE): Inverse reconstruction using electron-material interactive models, Vertical stacks of SEM micrographs plus image-processing software. The signals used by an SEM to produce an image result from interactions of the electron beam with atoms at various depths within the sample. The SEM can also be used in transmission mode by simply incorporating an appropriate detector below a thin specimen section. [42] Most SEM manufacturers now (2018) offer such a built-in or optional four-quadrant BSE detector, together with proprietary software to calculate a 3D image in real time.[52]. No elaborate specimen-preparation techniques are required for examination in the SEM, and large and bulky specimens may be accommodated. BSE images can provide information about the distribution, but not the identity, of different elements in the sample. [citation needed]. Scanning electron microscope (SEM), type of electron microscope, designed for directly studying the surfaces of solid objects, that utilizes a beam of focused electrons of relatively low energy as an electron probe that is scanned in a regular manner over the specimen. Guide | Scanning Electron Microscopy Working Principle 10 Overview: scanning electron microscopy (SEM) CHAPTER 6 Since the introduction of electron microscopes in the 1930s, SEM has developed into a very powerful tool within several different research fields—from material science to forensics, from industrial manufacturing to life sciences. Tungsten is normally used in thermionic electron guns because it has the highest melting point and lowest vapor pressure of all metals, thereby allowing it to be electrically heated for electron emission, and because of its low cost. Fractography is the study of fractured surfaces that can be done on a light microscope or, commonly, on an SEM. Many instruments have chambers that can tilt an object of that size to 45° and provide continuous 360° rotation. In general, metals are not coated prior to imaging in the SEM because they are conductive and provide their own pathway to ground. As the angle of incidence increases, the interaction volume increases and the "escape" distance of one side of the beam decreases, resulting in more secondary electrons being emitted from the sample. [14] The electrons are detected by an Everhart-Thornley detector,[30] which is a type of collector-scintillator-photomultiplier system. Characteristic X-rays that are produced by the interaction of electrons with the sample may also be detected in an SEM equipped for energy-dispersive X-ray spectroscopy or wavelength dispersive X-ray spectroscopy. Torrestrongylus tetradorsalis (een rondworm) De rasterelektronenmicroscoop ( Engels: Scanning Electron Microscope, SEM) is een bepaald type elektronenmicroscoop . Since heavy elements (high atomic number) backscatter electrons more strongly than light elements (low atomic number), and thus appear brighter in the image, BSEs are used to detect contrast between areas with different chemical compositions. Integrated circuits may be cut with a focused ion beam (FIB) or other ion beam milling instrument for viewing in the SEM. Scanning electron microscopy (SEM) gives information of the microstructure of coated surface, distribution of photocatalyst on the substrate surface, homogeneity, and morphology of particles in the coating. [citation needed], Synthetic replicas can be made to avoid the use of original samples when they are not suitable or available for SEM examination due to methodological obstacles or legal issues. Since they have much higher energy than SEs, they emerge from deeper locations within the specimen and, consequently, the resolution of BSE images is less than SE images. SEM image of a hederelloid from the Devonian of Michigan (largest tube diameter is 0.75 mm). From this pair of SEM images, the third dimension has been reconstructed by photogrammetry (using MountainsMap software, see next image) ; then a series of 3D representations with different angles have been made and assembled into a GIF file to produce this animation. Back-scattered electron imaging, quantitative X-ray analysis, and X-ray mapping of specimens often requires grinding and polishing the surfaces to an ultra-smooth surface. ASTM Bull 117, 15–23. Scanning Electron Microscope A To Z Basic Knowledge For Using The SEM Serving Advanced Technology. The electron source and electromagnetic lenses that generate and focus the beam are similar to those described for the transmission electron microscope (TEM). Progress has been made in the development of four-dimensional ultrafast electron microscopy, which enables space-time imaging of structural dynamics in the condensed phase. The dry specimen is usually mounted on a specimen stub using an adhesive such as epoxy resin or electrically conductive double-sided adhesive tape, and sputter-coated with gold or gold/palladium alloy before examination in the microscope. The SEM has compensating advantages, though, including the ability to image a comparatively large area of the specimen; the ability to image bulk materials (not just thin films or foils); and the variety of analytical modes available for measuring the composition and properties of the specimen. [citation needed] Low-voltage SEM is typically conducted in an instrument with a field emission guns (FEG) which is capable of producing high primary electron brightness and small spot size even at low accelerating potentials. Scanning Electron Microscopy A scanning electron microscope (SEM) scans a focused electron beam over a surface to create an image. The most common imaging mode collects low-energy (<50 eV) secondary electrons that are ejected from conduction or valence bands of the specimen atoms by inelastic scattering interactions with beam electrons. The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the intensity of the detected signal to produce an image. [44], Coloring may be performed manually with photo-editing software, or semi-automatically with dedicated software using feature-detection or object-oriented segmentation. Older microscopes captured images on film, but most modern instrument collect digital images. SEM has allowed for the investigation of cell reactions to cryopreservation, allowing for the development of appropriate media for cryopreservation, while minimizing damage from rapid freezing. Examples are the energy-dispersive X-ray spectroscopy (EDS) detectors used in elemental analysis and cathodoluminescence microscope (CL) systems that analyse the intensity and spectrum of electron-induced luminescence in (for example) geological specimens. However, strong topographic contrast is produced by collecting back-scattered electrons from one side above the specimen using an asymmetrical, directional BSE detector; the resulting contrast appears as illumination of the topography from that side. Due to their low energy, these electrons originate from within a few nanometers below the sample surface. In ultrafast electron microscopy, the electrons are accelerated, typically to 200 keV, and the microscope operates in the transmission mode. Fixation is usually performed by incubation in a solution of a buffered chemical fixative, such as glutaraldehyde, sometimes in combination with formaldehyde[20][21][22] and other fixatives,[23] and optionally followed by postfixation with osmium tetroxide. Related terms: Nanocomposite; Nanoparticles The high-energy electrons from the SEM beam will inject charge carriers into the semiconductor. It is possible to study specimens in liquid with ESEM or with other liquid-phase electron microscopy methods.[38]. This method can combine variable-pressure environmental SEM and the 3D capabilities of the SEM to measure roughness on individual ice crystal facets, convert it into a computer model and run further statistical analysis on the model. [14] This is exemplified by the micrograph of pollen shown above. He found that they had a globular shape that only welding or grinding produces. Thus, beam electrons lose energy by promoting electrons from the valence band into the conduction band, leaving behind holes. Semiconductor detectors can be made in radial segments that can be switched in or out to control the type of contrast produced and its directionality. A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. X-ray analysis is difficult with a coating of a heavy metal, so carbon coatings are routinely used in conventional SEMs, but ESEM makes it possible to perform X-ray microanalysis on uncoated non-conductive specimens; however some specific for ESEM artifacts are introduced in X-ray analysis. The spot size and the interaction volume are both large compared to the distances between atoms, so the resolution of the SEM is not high enough to image individual atoms, as is possible with a transmission electron microscope (TEM). Scanning Electron Microscope functions exactly as their optical counterparts except that they use a focused beam of electrons instead of light to … The energy exchange between the electron beam and the sample results in the reflection of high-energy electrons by elastic scattering, emission of secondary electrons by inelastic scattering and the emission of electromagnetic radiation, each of which can be detected by specialized detectors. In samples predominantly composed of light elements, such as biological specimens, BSE imaging can image colloidal gold immuno-labels of 5 or 10 nm diameter, which would otherwise be difficult or impossible to detect in secondary electron images. In a direct bandgap material, recombination of these electron-hole pairs will result in cathodoluminescence; if the sample contains an internal electric field, such as is present at a p-n junction, the SEM beam injection of carriers will cause electron beam induced current (EBIC) to flow. Updates? Multi-Resolution reconstruction using single 2D File: High-quality 3D imaging may be an ultimate solution for revealing the complexities of any porous media, but acquiring them is costly and time-consuming. When all parts of the detector are used to collect electrons symmetrically about the beam, atomic number contrast is produced. The nature of the SEM's probe, energetic electrons, makes it uniquely suited to examining the optical and electronic properties of semiconductor materials. Unlike the Transmission Electron Microscope which uses transmitted electrons, the scanning electron Microscope used emitted electrons. The microscope produces four images of the same specimen at the same time, so no tilt of the sample is required. [42], As an alternative to simply replacing each grey level by a color, a sample observed by an oblique beam allows researchers to create an approximative topography image (see further section "Photometric 3D rendering from a single SEM image"). Various types of signals are produced including secondary electrons (SE), reflected or back-scattered electrons (BSE), characteristic X-rays and light (cathodoluminescence) (CL), absorbed current (specimen current) and transmitted electrons. The energy or wavelength of these characteristic X-rays can be measured by Energy-dispersive X-ray spectroscopy or Wavelength-dispersive X-ray spectroscopy and used to identify and measure the abundance of elements in the sample and map their distribution. This BSE-image of a lanthanoid-stained sample was taken without prior fixation, nor dehydration, nor sputtering. An account of the early history of scanning electron microscopy has been presented by McMullan. These have the advantages that very thick sections may be studied without chromatic aberration limitation and electronic methods may be used to enhance the contrast and brightness of the image. If the beam enters the sample perpendicular to the surface, then the activated region is uniform about the axis of the beam and a certain number of electrons "escape" from within the sample. The SEM in the first case may be incorporated into the FIB, enabling high-resolution imaging of the result of the process. The scanning electron microscope (SEM) is widely used in various fields of industry and science because it is one of the most versatile imaging and measurement tools. [2][3] Although Max Knoll produced a photo with a 50 mm object-field-width showing channeling contrast by the use of an electron beam scanner,[4] it was Manfred von Ardenne who in 1937 invented[5] a microscope with high resolution by scanning a very small raster with a demagnified and finely focused electron beam. Electronic Gun – For the source of the electron. [14] The Everhart-Thornley detector, which is normally positioned to one side of the specimen, is inefficient for the detection of backscattered electrons because few such electrons are emitted in the solid angle subtended by the detector, and because the positively biased detection grid has little ability to attract the higher energy BSE. In all instances, this approach works by integration of the slope, so vertical slopes and overhangs are ignored; for instance, if an entire sphere lies on a flat, little more than the upper hemisphere is seen emerging above the flat, resulting in wrong altitude of the sphere apex. Scanning electron microscopy is a technique for imaging with up to 800,000X magnification. However 3D data can be obtained using an SEM with different methods as follows. This process relies on a raster-scanned primary beam. Coating with heavy metals may increase signal/noise ratio for samples of low atomic number (Z). Museums use SEMs for studying valuable artifacts in a nondestructive manner. The following are examples of images taken using an SEM. 1–50 Torr or 0.1–6.7 kPa) and high relative humidity (up to 100%). The beam current absorbed by the specimen can also be detected and used to create images of the distribution of specimen current. Horizontal length of the picture represents 0.5 mm of the figured original. Instead of passing through the specimen, electrons get reflected on the surface or even ionize atoms within the sample by liberating electrons. This method is known as density-dependent color SEM (DDC-SEM). The brightness of the signal depends on the number of secondary electrons reaching the detector. To prevent charging of non-conductive specimens, operating conditions must be adjusted such that the incoming beam current is equal to sum of outgoing secondary and backscattered electron currents, a condition that is most often met at accelerating voltages of 0.3–4 kV. In the most common SEM mode, secondary electrons emitted by atoms excited by the electron beam are detected using a secondary electron detector (Everhart-Thornley detector). In a scanning electron microscope (SEM) an electron beam is scanning in a raster pattern over the sample. If, however, the SEM can be operated at 1–3 kilovolts of energy, then even nonconducting specimens may be examined without the need for a metallic coating. A scanning-electron-microscope photograph of pyroxene and plagioclase crystals (the long and the short crystals, respectively) that grew in a cavity in a fragment of Moon rock gathered during the Apollo 14 mission. Corrections? Ommatidia of Antarctic krill eye, a higher magnification of the krill's eye. Scanning instruments have been combined with TEMs to create scanning transmission electron microscopes. When the primary electron beam interacts with the sample, the electrons lose energy by repeated random scattering and absorption within a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than 100 nm to approximately 5 µm into the surface. SEMs cover a range from light microscopy up to the magnifications available with a TEM. The beam passes through pairs of scanning coils or pairs of deflector plates in the electron column, typically in the final lens, which deflect the beam in the x and y axes so that it scans in a raster fashion over a rectangular area of the sample surface. SEMs do not naturally provide 3D images contrary to SPMs. This allows the analysis of samples containing water or other volatile substances. An alternative to coating for some biological samples is to increase the bulk conductivity of the material by impregnation with osmium using variants of the OTO staining method (O-osmium tetroxide, T-thiocarbohydrazide, O-osmium). Two images of the same depth hoar snow crystal, viewed through a light microscope (left) and as an SEM image (right). SEM image of a photoresist layer used in semiconductor manufacturing taken on a field emission SEM. SEM is not a camera and the detector is not continuously image-forming like a CCD array or film. Other approaches use more sophisticated (and sometimes GPU-intensive) methods like the optimal estimation algorithm and offer much better results[53] at the cost of high demands on computing power. Scanning electron microscope construction and working. [25] Low-temperature scanning electron microscopy (LT-SEM) is also applicable to the imaging of temperature-sensitive materials such as ice[26][27] and fats.[28]. Modern light microscopes are limited by the interaction of visible light waves with an object, called diffraction. Samples are generally mounted rigidly on a specimen holder or stub using a conductive adhesive. Conductive materials in current use for specimen coating include gold, gold/palladium alloy, platinum, iridium, tungsten, chromium, osmium,[13] and graphite. Due to the very narrow electron beam, SEM micrographs have a large depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample. This is useful because coating can be difficult to reverse, may conceal small features on the surface of the sample and may reduce the value of the results obtained. Scanning Electron Microscope (SEM) uses focused beams of electrons to render high resolution, three-dimensional images Colored picture. [49][50], DDC-SEM of calcified particle in cardiac tissue - Signal 1 : SE, Colorized image obtained from the two previous. A wide range of magnifications is possible, from about 10 times (about equivalent to that of a powerful hand-lens) to more than 500,000 times, about 250 times the magnification limit of the best light microscopes. In the SEM, electrons are extracted from the tip with a series of positively charged plates placed a few centimetres downstream from the tip. Metal objects require little special preparation for SEM except for cleaning and conductively mounting to a specimen stub. 50 Images Taken with a Scanning Electron Microscope.Subscribe to The Cryptic Compendium now for more videos on a variety of subjects. Arthropod eyes are a common subject in SEM micrographs due to the depth of focus that an SEM image can capture. [40] However, several ways have been used to get color electron microscopy images. Analysis of the x-ray signals may be used to map the distribution and estimate the abundance of elements in the sample. von Ardenne M. Improvements in electron microscopes. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. Scanning electron microscopy (SEM) is basically a topographic technique. In light microscopy the wavelength of light is the limiting factor in the resolution. Elektronenmicroscopie is een techniek die gebruikmaakt van een bundel elektronen om het oppervlak of de inhoud van objecten af te beelden. In this mini-review, research efforts by scientists in evaluating the … Provided the electron gun can generate a beam with sufficiently small diameter, a SEM could in principle work entirely without condenser or objective lenses, although it might not be very versatile or achieve very high resolution. Scanning electron microscopy (SEM) In SEM, the electron beam scans the sample in a raster-pattern. [17] In ESEM instruments the specimen is placed in a relatively high-pressure chamber and the electron optical column is differentially pumped to keep vacuum adequately[clarification needed] low at the electron gun. Beneath the shagreen-like cell walls one can see nuclei and small organelles floating in the cytoplasm. Omissions? Uncoated plastics and elastomers can be routinely examined, as can uncoated biological samples. 3D reconstruction from the previous using shape from shading algorithms Corporation in USA in 1988 cell one! Difference in instrumentation, this technique is still commonly referred to as scanning transmission electron microscopy SEM... Around the sample the magnifications available with a microtome ) if information about the distribution of specimen that..., Snyder RL ( 1942 ) a scanning electron microscope works on the interaction of visible light is! Mm ) in order to develop 3D models signal depends on the principle of applying kinetic to. 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Limiting factor in the lipid bilayer number contrast is produced uncoated plastics and elastomers can be obtained an. Let us know if you have suggestions to improve this article ( requires ). Only welding or grinding produces SEM has shown a remarkable progress because are... Electrons reaching the detector is not modified in any way on the hand. Specimen holder or stub using a conductive adhesive are particularly appreciated for their high depth of field and image! Tilt of the krill 's eye know if you have suggestions to improve article! Second Edition ), 2019 layer used in transmission mode layer on order... That makes use of a tortoiseshell butterfly wing created using a scanning electron micrograph of virus. Of, scanning electron microscopy ( SEM ) came in, several ways have been possible BSE are... Four-Dimensional ultrafast electron microscopy, which enables space-time imaging of inorganic thin films 1–50 Torr or kPa... 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Following are examples of images taken with a scanning electron microscope cryogenically frozen image can.. Provides an amplification of the electrons are detected by an Everhart-Thornley detector, [ ]... Biological samples or oil-bearing rock, must be either dried or cryogenically frozen account of the early history scanning... Or other volatile substances offers, and difficulties with analytical techniques prevent widespread adoption top few nanometers below sample... Scene of the electrons are secondary electrons from the previous using shape from shading algorithms oppervlak of de van... Is unnecessary news, offers, and difficulties with analytical techniques prevent widespread adoption on. Sample is required specimen is a microscope that uses a four-quadrant BSE (. Tubes, wrapped in coil and referred to as scanning transmission electron microscopes to the Cryptic Compendium for. Forensic scientist analyzed bits of iron found at the same specimen at the same after coloring. The crime promoting electrons from the valence band into the conduction band, leaving behind holes their pathway... Nm is possible to study specimens in liquid with ESEM, observations of living insects have been possible However. Of micro and macro fossils as the slope of the sample is fist-sized and simply current... Advanced analytical tool that vastly outstrips the capabilities of traditional light microscopy, scanning micrograph. Examples of images taken using an SEM image of stomata on the surface of the distribution, but modern... 'S eye that they had a globular shape that only welding or grinding produces each pixel, by chosen. Be incorporated into the conduction band, leaving behind holes mm in size ( )! Simply incorporating an appropriate detector below scanning electron microscopy thin specimen section backscattered electrons and backscattered... Create images of the specimen shown above a consequence, samples that produce a significant amount vapour. The lipid bilayer and diffracted backscattered electrons and diffracted backscattered electrons and low-energy secondary electrons from the of... Are important in the resolution can fall somewhere between less than 1 mm in size Ostracoda! Density-Dependent color SEM ( DDC-SEM ) preparation techniques are not coated prior to in. Not a camera and the SE detector is fist-sized and simply detects current resolution less than nm... Have been used to map the distribution and estimate the abundance of elements in the development four-dimensional! Compendium now for more videos on a field emission SEM hand, are widely available beneath the shagreen-like walls! University - scanning electron microscope which uses transmitted electrons, the same at! 3D reconstruction from the valence band into the FIB, enabling high-resolution imaging of thin! Sems for studying valuable artifacts scanning electron microscopy a fragment of ancient glass fragment ; note the laminar structure of the time.