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Scanning Electron Microscope (SEM)

The scanning electron microscope (SEM) is a type of electron microscope capable of producing high

resolution images of a sample surface. Due to the manner in which the image is created, SEM images havea characteristic 3-dimensional quality and are useful for judging the surface structure of the sample

Scanning process

In a typical SEM configuration, electrons are thermionically emitted from a tungsten

or lanthanum hexaboride LaB6 cathode filament towards an anode; alternativelyelectrons can be emitted via field emission (FE). The electron beam, which typically

has an energy ranging from a few keV to 50 keV, is focused by two successivecondenser lenses into a beam with a very fine spot size (~ 5nm). The beam then

passes through the objective lens, where pairs of scanning coils deflect the beam

either linearly or in a raster fashion over a rectangular area of the sample surface. Asthe primary electrons strike the surface they are inelastically scattered by atoms in

the sample. Through these scattering events, the primary beam effectively spreads

and fills a teardrop-shaped volume, known as the interaction volume, extending

about 1m to 5m into the surface. Interactions in this region lead to thesubsequent emission of electrons which are then detected to produce an image. X-rays, which are also produced by the interaction of electrons with the sample, may

also be detected in an SEM equipped for Energy dispersive X-ray spectroscopy.

Detection of secondary electrons

The most common imaging mode monitors low energy (

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detector design that is only sensitive to the high-energy backscattered electrons.There are usually 2-10 times more backscattered electrons emmitted from a sample

than there are secondary electrons. The Everhart-Thornley detector has lowgeometric efficiency since it is located on one side of the sample and is highly

directional in its collection. Placement of a backscatter detector above the sample ina "doughnut" type arrangement, with the electron beam passing through the hole of

the doughnut, greatly increases the solid angle of collection and reduces thetrajectory effects associated with backscatter electrons.

Resolution of the SEM

The spatial resolution of the SEM depends on the size of the electron spot which in

turn depends on the magnetic electron-optical system which produces the scanningbeam. The resolution is also limited by the size of the interaction volume, or the

extent of material which interacts with the electron beam. The spot size and theinteraction volume are both very large compared to the distances between atoms, so

the resolution of the SEM is not high enough to image down to the atomic scale, as ispossible in the transmission electron microscope. The SEM has compensating

advantages, though, including the ability to image a comparatively large area of thespecimen; the ability to image bulk materials (not just thin films or foils); and the

variety of analytical modes available for measuring the composition and nature ofthe specimen. In general, SEM images are much more easy to interpret than TEM

images, and many SEM images are actually beautiful, quite apart from their scientificvalue.

Scanning Tunneling Microscope (STM)

A scanning tunneling microscope (STM) is used to obtain atomic-scale images of metal surfaces.

The STM is a high-resolution non-optical microscope which employs principles ofquantum mechanics. A fine probe is moved over the surface of the material under

study, and a voltage is applied between probe and the surface. Depending on thevoltage and its characteristics electrons will "tunnel" (this is a quantum-mechanical

effect) from the probe to the surface (or vice-versa depending on the polarity)resulting in a weak electric current. The size of this current is highly dependent on

the distance between probe and the surface. By scanning the probe over the surfaceand measuring the current, one can thus reconstruct the surface structure of the

material under study. Adjustments of the distance between probe and surface are

done using a servo loop and converse piezoelectricity. It is even possible to moveand position individual atoms, which makes the scanning tunneling microscope an

important tool in nanotechnology. The scanning tunneling microscope was developedat IBM Zrich in 1981 by Gerd Binnig and Heinrich Rohrer who shared half of the

Nobel Prize in physics in 1986 for their achievement. The other half went to Ernst

Ruska for for his fundamental work in electron optics, and for the design of the firstelectron microscope.

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