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Text of the page (random words):
out 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 sample preparation edit a spider sputter coated in gold having been prepared for viewing with an sem low voltage micrograph 300 v of distribution of adhesive droplets on a post it note no conductive coating was applied such a coating would alter this fragile specimen sem samples have to be small enough to fit on the sample stage secured with a conductive paint the relatively low vacuum 0 076 to 20 torr of variable pressure and environmental sem obviates the need for cleaning even those with water or oily films allowing observation in a natural state preparation is identical to the steps used in light microscopy polished samples sem techniques can be used to examine integrated circuit condition and attachment points foreshortening is achieved by tilting any specimen sputter coating is used to apply a thin coating of metal or metal alloy to nonconductive specimens 19 conductive materials in current use for specimen coating include gold gold palladium alloy platinum iridium tungsten chromium osmium 17 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 20 21 biological samples edit since the sem specimen chamber is under high vacuum a sem specimen must be completely dry or cryogenically cooled 22 hard dry materials such as wood bone feathers dried insects or shells including egg shells 23 can be examined with little further treatment but living cells and tissues and whole soft bodied organisms require chemical fixation to preserve and stabilize their structure fixation is usually performed by incubation in a solution of a buffered chemical fixative such as glutaraldehyde sometimes in combination with formaldehyde 22 24 25 and other fixatives 26 and optionally followed by postfixation with osmium tetroxide 22 the fixed tissue is then dehydrated because air drying causes collapse and shrinkage this is commonly achieved by replacement of water in the cells with organic solvents such as ethanol or acetone and replacement of these solvents in turn with a transitional fluid such as liquid carbon dioxide by critical point drying 27 the carbon dioxide is finally removed while in a supercritical state so that no gas liquid interface is present within the sample during drying 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 samples may be sectioned with a microtome if information about the organism s internal ultrastructure is to be exposed for imaging if the sem is equipped with a cold stage for cryo microscopy cryofixation may be used and low temperature scanning electron microscopy performed on the cryogenically fixed specimens 22 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 28 low temperature scanning electron microscopy lt sem is also applicable to the imaging of temperature sensitive materials such as ice 29 30 and fats 31 freeze fracturing freeze etch or freeze and break is a preparation method particularly useful for examining lipid membranes and their incorporated proteins in face on view the preparation method reveals the proteins embedded in the lipid bilayer materials edit according to goldstein et al there are many ways to section the samples including blade sawing wire sawing abrasive cutting fracturing best for more brittle materials shearing spark erosion and microtomy the surface to be prepared for microstructural analysis is polished using a graded sequence of abrasive materials ceramic and geological samples like metals may require etching to permit microstructural features to be imaged or analyzed in the sem however the electron beam path from the specimen surface to ground must remain unbroken to ensure the specimen does not act like an electron mirror when sufficient charge builds up equal to that of the incident beam which is referred to as charging according to goldstein et al the best and simplest way to overcome charging problems is to deposit a thin metal layer on the surface of the sample this is achieved via vacuum evaporation coating or sputter coating for elements with atomic numbers 8 through 20 only carbon aluminum and chromium are suitable coating materials 19 539 545 648 669 fib is analogous to sem but with the use of positively charged ions instead of electrons besides imaging fib can be used for precision cutting etching or depositing conductive materials on the sample surface 19 553 scanning process and image formation edit schematic of an sem the sem consists of an electron column with the electron gun electron lenses and vacuum pumps plus a control console with viewing screen and electron beam controls the electron gun generates and accelerates electrons between 0 1 to 30 k ev electron lenses focus the electron beam to less than 10 nm at the sample where it interacts with the specimen to a depth of about 1 micrometre generating signals used for point by point imaging deflection coils sweep the electron beam across the specimen in a raster scan fashion with the backscattered and secondary electrons collected by the electron detector forming an image that can be processed and displayed digitally sharpness and feature visibility are controlled by electron probe size electron probe current electron probe convergence angle and electron beam accelerating voltage according to goldstein et al most older sems use tungsten or lab 6 thermionic emitters but increasingly new microscopes are equipped with cold thermal or schottly field emission sources because these provide enhanced performance reliability and lifetime 19 21 60 mechanisms of emission of secondary electrons backscattered electrons and characteristic x rays from atoms of the sample low temperature sem magnification series for a snow crystal the crystals are captured stored and sputter coated with platinum at cryogenic temperatures for imaging magnification edit magnification in an sem can be controlled over a range of about 6 orders of magnitude from about 10 to 3 000 000 times 32 changing magnification is achieved by adjusting the length of the scan on the specimen with higher magnification requiring a smaller sample area or pixel 19 108 113 detection of secondary electrons edit 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 due to their low energy these electrons originate from within a few nanometers below the sample surface 18 the electrons are detected by an everhart thornley detector 33 according to goldstein et al the development of the e t detector provided the first efficient use of the rich secondary backscattered electron signal with a large solid angle of collection high amplifier gain low noise and robust low maintenance performance light is emitted when an energetic electron strikes scintillator material which is conducted via a light guide to a photomultiplier which provides high gain little noise degradation high bandwidth at a fast response rate 19 128 129 detection of backscattered electrons edit comparison of sem techniques top backscattered electron analysis composition bottom secondary electron analysis topography backscattered electrons bse consist of high energy electrons originating in the electron beam that are reflected or back scattered out of the specimen interaction volume by elastic scattering interactions with specimen atoms 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 18 according to goldstein et al when the e t detector is biased negatively only backscattered electrons are detected all secondary electrons are rejected including those that are emitted from the specimen in the direction of the e t detector within the line of sight solid angle for direct geometric collection dedicated backscattered electron detectors are designed to greatly increase the solid angle of collection these include the passive scintillator backscattered electron detectors the backscattered to secondary electron conversion detector and the solid state diode detector 19 129 133 backscattered electrons can also be used to form an electron backscatter diffraction ebsd image that can be used to determine the crystallographic structure of the specimen 19 10 beam injection analysis of semiconductors edit voltage contrast and charge collection sem microscopy are used to characterize the electrical performance of integrated circuits and semiconductor devices charge collection uses the currents or voltages induced in the specimen by the electron beam electron beam induced current ebic is used to study schottky barriers diffused and ion implanted p n junctions 19 546 cathodoluminescence edit color cathodoluminescence overlay on sem image of an ingan polycrystal the blue and green channels represent real colors the red channel corresponds to uv emission cathodoluminescence can be used to detect impurities in minerals or the presence of excess electron hole pairs in semiconductors 19 16 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 over the last decades cathodoluminescence was most commonly experienced 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 an emission spectrum or an image of the distribution of cathodoluminescence emitted by the specimen in real color x ray microanalysis edit 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 analysis of the x ray signals may be used to map the distribution and estimate the abundance of elements in the sample 19 297 353 complementary techniques edit many sem based research studies are supported by complementary nanoscale techniques such as atomic force microscopy afm and its electrical imaging modes these methods provide insights that go beyond surface morphology for example afm can probe the sample s surface topography at the nanometer scale using a sharp tip in contact or tapping mode conductive afm c afm enables mapping of local electrical conductivity useful in studying resistive switching materials and semiconductors kelvin probe force microscopy kpfm measures surface potential variations which is valuable for analyzing charge distributions in electronic or photovoltaic materials when used alongside sem these techniques offer a comprehensive understanding of both structural and functional properties of materials resolution of the sem edit a video illustrating a typical practical magnification range of a scanning electron microscope designed for biological specimens the video starts at 25 about 6 mm across the whole field of view and zooms in to 12000 about 12 μm across the whole field of view the spherical objects are glass beads with a diameter of 10 μm similar in diameter to a red blood cell a sem is not a camera and the detector is not continuously image forming like a ccd array or film unlike in an optical system the resolution is not limited by the diffraction limit fineness of lenses or mirrors or detector array resolution the focusing optics can be large and coarse and the se detector is fist sized and simply detects current instead the spatial resolution of the sem depends on the size of the electron spot which in turn depends on both the wavelength of the electrons and the electron optical system that produces the scanning beam the resolution is also limited by the size of the interaction volume the volume of specimen material that interacts with the electron beam 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 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 depending on the instrument the resolution can fall somewhere between less than 1 nm and 20 nm as of 2009 the world s highest resolution conventional 30 kv sem can reach a point resolution of 0 4 nm using a secondary electron detector 34 environmental sem edit main article environmental scanning electron microscope conventional sem requires samples to be imaged under vacuum because a gas atmosphere rapidly spreads and attenuates electron beams as a consequence samples that produce a significant amount of vapour e g wet biological samples or oil bearing rock must be either dried or cryogenically frozen processes involving phase transitions such as the drying of adhesives or melting of alloys liquid transport chemical reactions and solid air gas systems in general cannot be observed with conventional high vacuum sem in environmental sem esem the chamber is evacuated of air but water vapor is retained near its saturation pressure and the residual pressure remains relatively high this allows the analysis of samples containing water or other volatile substances with esem observations of living insects have been possible 35 the first commercial development of the esem in the late 1980s 36 37 allowed samples to be observed in low pressure gaseous environments e g 1 50 torr or 0 1 6 7 kpa and high relative humidity up to 100 this was made possible by the development of a secondary electron detector 38 39 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 the first commercial esems were produced by the electroscan corporation in usa in 1988 electroscan was taken over by philips who later sold their ...