scanning electron microscope full report
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15-02-2011, 11:35 AM
The optical microscope was invented in the early 17th century. Although this discovery that revolutionized science, these were the first microscopes little more than one type magnifying glasses. The centuries many scientists then looked for new versions of the microscope with an improved resolution. In the year 1900, the maximum resolution achievable with a light microscope reached: 2 micrometers.
This maximum resolution we can do as follows:
The wavelength is important for the resolution on ee optical system, especially when there is an optical imaging system. In the latter case, the resolution is always of the same magnitude as the wavelength of the radiation used. This can be demonstrated eg by using the Abbe equation and the Rayleigh condition.
A point light source at infinity by a thin lens opening angle focused into an image with a typical intensity gradient. This image has a first minimum diameter
δ = 0.61 λ / (n sinα)
Two point light sources at infinity by a thin lens as separate images when seen at a distance d from each other so that the image maximum of one point corresponds to the first image of the other minimum point.
By limitations on the values of λ, α and n is the resolution of the optical microscope is limited. For the best microscopes α is about 70 °. The shortest wavelength of visible light is blue (λ = 450nm) and the typical high-resolution lenses are submerged in oil (n = 1.56). These values give us a complete resolution to 202nm as already mentioned.
The next big step in microscopy was the use of an electron beam instead of light to obtain magnified images. The electrons were found to follow a curved path to form magnifying "lenses". The technology was further developed and improved and in 1935 the first SEM was a fact.
SEM is the abbreviation for Scanning Electron Microscopy. Electron microscopy is a direct technique that can be used to the size and shape of particles. Also, information about the composition and internal structure can be provided. The resolution of an SEM is 3 to 10nm. This resolution is much better than that of an ordinary optical light microscope is higher magnifications can be achieved thereby. Details on the atomic scale can be visualized.
SEM provides a lot of advantages over most microscope techniques, including resolution. Resolution is defined as the shortest distance between two points that still separate the two issues can be observed. Rayleigh empirically posited that the greatest resolution of two interference patterns occur when the maximum intensity of one pattern coincides with the first minimum intensity of the second pattern. The resolution of two spatially distributed sources with an angular separation of
a = a_0 = λ / D
with D the diameter of the opening of the diffraction pattern and a0 the "resolving power" or the angular resolution. a0 indicates the minimum distance between two point sources.
Optical microscopes use light with a wavelength of 400-700 nm. The wavelength of an electron is determined by the formula of De Broglie
h = mv
The microscope is a potential difference applied to the electron acceleration. If the accelerating voltage is high then the electrons reach a velocity substantially equal to the speed of light. In that case, relativistiche effects are not neglected. Relativistic mass change provides an energy that can be used to the wavelength of the electron accelerator to calculate:
λ = √ (1.5 / (V + 〖10〗 ^ (-6) V ^ 2))
Using this formula we find for the wavelength of an electron a range of 0:0009 to 0:0086 nm nm depending on the value of accelerating voltage. This shorter wavelength gives the SEM a better angular resolution can be achieved with relatively longer wavelengths of visible light.
The large depth of focus of an SEM in comparison with the depth of focus of light-optical systems, is one of the features that explain the popularity of SEM. Depth of focus is the vertical distance that the image on the video screen in focus. This depth of focus T is a function of the angle of the primary beam. We can calculate T as follows:
The CRT video screen is approximately 10cm x 10cm.
The CRT display has 1000 horizontal scan lines.
The resolution is therefore 0.1 mm CRTscherm
Preparation at this level corresponds in turn with a resolution of
δ = 0.1 mm / M
We calculate the depth of field T as follows:
T = δ / (tan (α_p)) ⋍ δ / (α) = 0.1 mm / (M. Α_p)
This means that all preparation detail a distance T away from the point where the beam has a minimum diameter, sharp.
One of the most common method for generating an electron beam is Lanthaanhexaboride heating a filament. LaB6 is a single crystal. The electrons flow from the filament. The life span is nine months. A better option is a tungsten filament. The life span of this filament is about 80h. The probe of the hairpin tungsten filament is smaller allowing a better resolution is achievable.
For the electron beam to create a potential difference is applied tssen the filament, which acts as a cathode and an anode. Wehnelt a disc is present in the microscope and serves as the anode. Furthermore, this disc as a function of the electron beam to diminish, as the hole of the anode.
Tungsten is a metal with a work function φ and a Fermi level Ef. When the metal is heated, the atoms begin to vibrate in the lattice and give their thermal energy to the electrons in the top of the conduction. The overall thermal enrgie is kT with k the Boltzmann constant and T is the temperature in Kelvin. When the thermal energy transferred is greater than φ, electrons are emitted from the metal surface. The majority of the electron emission occurs at the tip of the filament because the temperature is the highest. A current is sent through the filament, which heats the filament to a temperature of 2800K. The power difference, about 500 to 3000 V between anode and cathode electrode is large enough to allow neurons could be accelerated to the anode. This creates an electron beam. Be generated once the bundle is going to be a Wehnelt disc used for the emissive control. The disc is a conductor that surrounds the filament. He has a small hole just below the tip of the filament. Only through this hole, the electrons reach the anode. Emissieconrole This can happen because his potential more negative than the filament itself. The control is very important in a good beeldkwaileit of the SEM.Moreover, changes in the emission flow. Because of the potential of varying Wehnelt also varies the area where electrons are produced at the filament, and thus the strength of the electron current is adjusted. This variation is possible by using a bias resistor between the filament and the power generator. Once the beam is generated, it will pass through the lens system.
The first lenses in the SEM are electrostatic lenses and they are created by the elektronengun. Generally, an electrostatic lens formed by a conductor with a circular opening at a constant potential to keep. This opening is centered along the optical axis of the microscope. When the potential is negative, the electrons are withdrawn by the same potential at the surface and those that do not go back along the optical axis are detected when passing through the opening.
The effect of the cathode and Wehnelt disc together is a curved electric field whose field lines to create a cross-over forms. This crossover creates an effect similar to that of a convex lens. The anode, in turn, bends the electrons in the electron equivalent of a concave lens. The divergence of the electron beam caused by the concave lens simulates an electron source for the Wehnelt flat disc with a divergence angle α.
The remaining lenses in the SEM are magnetic. Magnetic lenses use an electric current through a bundle of twisted copper wire in order to get an inhomogeneous magnetic field B to create axial symmetry. Reulterende the force that accelerates electrons is given by the Lorentz force equation
F = q (v x w)
The direction of the force on the electrons is constantly changing by the inhomogeneity of the magnetic field and the deflection of the electrons.
When the electrons travel through the magnetic field, they rotate around the optical axis.Eenamaal the electrons through the center of the windings are gone, their spiaarlbeweging ensure that the Lorentz force them to withdraw the optical axis, so the electrons are focused on a different crossover point. The magnetic field can be controlled by copper d almost completely covered with a ferromagnetic material like iron. The iron acts as a shield, thereby preventing the magnetic field does extend beyond the lagnetische lateriaal. The strength of the magnetic field can be changed by the power to adjust. Variation in strength, also provides a variation in the convergence of the electron beam.
Electron interactions with the material
Once the electrons in the optical column have come to interact with the electrons and the sample going to be a generated image. In order to obtain this image, the SEM based on two types of electron scattering, and inelastic east symbolic.
Elastic scattering of primary electrons is called backscattering. Backscattered electrons (BSE) are electrons reacted with the surface of the specimen and scattered by elastic collisions in an angle θ> 90 °. BSE's have a final kinetic energy is practically equal to their initial kinetic energy. By the proportional relationship between the cross-section for BSE and Z2, with Z the atomic number, BSE's can be used to show contrast due to atomic or molecular surface variations: different elements in different gray shades.Heavier elements appear as a light color and light elements appear darker because heavy elements lichtverstrooires more efficient and therefore appear brighter.
Above applies to BSE that the depth from which electrons are BSE, decreases with increasing Z. Furthermore, the surface area to which the BSE electrons are much larger than the diameter of the primary beam.
BSE contrast is thus mainly characterized by:
The fact that BSE electrons come from an area larger than the diameter of the primary electron beam. This has an impact on the resolution, which will be lower than SE images with electrons.
The strong direction dependence: higher angles provide a stronger BSE signal.
Composition Contrast sensitivity provides high-Z Z-contrast.
In a light-optical equivalent though we were the preparation from the direction of the detector "highlights".
These electrons provide us information about sample composition and topography on the other hand. Backscattered electrons have the advantage of being less sensitive because of their higher energy to recharge and pollution. BSE images study in slow scan must always happen. We can distinguish between components and topographic images. The differences are highlighted in the figure below:
In a topographical image, the black and white disappear. When a component image appear brighter the heavier elements. This can be checked by EDX analysis.
Again we give some images:
Although the black-white contrast would have gone, we still see a cross. One possible explanation is that the escape of electrons is much smaller gaps. Another possible cause is the C-pollution accumulated there.
Inelastic scattering creates so-called secondary electrons. When the primary electrons from an electron beam interacting EIA the surface of the specimen, some electrons are absorbed by the atoms. This atomic excitation can result in secondary electrons, emitted when the atom de-exciteerd. Because these secondary electrons cover a larger energy range than the primary electrons, it is difficult to focus SE.
Scanning electron micrsocopie especially done by a narrow electron beam across the surface to send. This beam scans the entire surface and detect the production of secondary or backscattered electrons as a function of the position of the primary beam.Contrast is due to the orientation: parts of the surface which are entitled to the detector, appear as bright areas. As the detector is turned away, appear darker. Secondary electrons usually have low energies (5-50 eV) and radiate from the surface region of the sample.
The number of secondary electrons emitted from a sample surface is highly dependent on the angle of the electron beam relative to the surface. An electron beam with a larger angle affects the preparation, causing an intense SE signal. Since the energy of these electrons is very low, only electrons emitted from a thin layer. Therefore, these electrons are highly suitable for studying the topography. We take into account the fist governed contrast <brightness and show some images of an indent to clarify the operating principles:
The location of the detector can be deduced from the image. As mentioned earlier parts of the surface appear to be right on the detector, and clear away some of the detector is turned in, appear darker. The detector is located at top left. This contamination is clearly indent. The indent of a purer sample is given in:
The topography is clearly visible, as previously mentioned.
Further characteristics for SE-contrast are:
A limited-depth information on metals
The SE signal comprises:
An intense signal directly below the primary beam: SE images have higher resolution than BSE images
A less intense signal due to BSE electrons: this effect creates a loss of resolution
The effect of gecomineerd using SE and BSE's is an image of the specimen with both contrast the present topography and by variations in the chemical composition of the sample
One of the byproducts of electron microscopy to X-rays. This forms the basis for determining the composition of submicronschaal and Energy dispersive X-ray
Analysis (EDX). The interaction of an electron in an atom produces two types of X-rays: characteristic emission lines and Brems radiation. The forward characteristic atomic emission lines when the incident electron, a bound electron from one orbital to loosen it.The atom is unstable and has two possibilities to decay: X-ray fluorescence and Auger decay. The first possibility is the basis for the EDX analysis.
Characteristic X-rays are emitted from a surface when an electron beam incident on it.By collecting and analyzing the emitted X-rays, we can identify the elements in the specimen (qualitative analysis). We can also weight the various elements determined (quantitative analysis) are very finely focused electron beam. Therefore, one can easily analyze a very small part of the surface using the 'spot mode'. Furthermore, you can scan through a certain part also obtain an average elemental analysis.
We set the spot size to 6. This is unusual, but appropriate for this material.
The number of counts per second 'or CPS, we can adapt. Too little light will get a clear picture, with neither too much light (cf. the human eye). We perform several measurements on a steel inluitsel to clarify the operating principle. We measure again at the center of the inclusion, then the edge and once in the matrix.
The spectrum includes:
1) characteristic peaks of the elements
2) (white) background radiation or radiation Brehm
3) sompieken, which are simultaneously incident X-rays (same) element
4) escape peaks of characteristic X-rays excite Si
Following charts and decisions are obtained:
The matrix is a matrix of aluminum.
Inclusions of copper.
In the skin we find both elements of the matrix (Al) as elements of inclusions (Cu) back.
We begin with the preparation of the sample. The first step is to clean. Then we generate the spample electrically conductive by Au layer. The deposition of the Au layer can be done by evaporation or by sticking a film. The prepared sample we put into the microscope, where we provide electrical contact with the internship. We optimize the height of the sample in the microscope with the help ofthe so-called pinguïnetje. This is a plate with a protuberance that besides the sample sites. The sample should fit just below the nipple. The WD is 10mm. This working distance (WD) is an important parameter. The smaller the aperture of the objective lens and the larger the WD, the better depth perception. Next we connect the microscope and let this vacuum. If the vacuum is reached, we set the power and we saturate the filament. (We use a voltage of 25.0 keV and a tungsten filament.). Everything is now in readiness for the sample to study. For a good overview, we can play with different parameters. The first adjustment is the brightness and contrast. We use governed following:
SE: Contrast <Brightness
BSE: Contrast> Brightness
By contrast higher, we see more structure. The regulation of these two parameters can also be done automatically. These changes are purely electronic processing of the image and have nothing to do watch the amount of electrons.
Then we focus. It is useful to a selected area to work because the scanning is slow. The whole image scanning takes too long time in which you are not properly see what you're doing. In order to obtain a good focus, do your best to focus on increasing 4Mal as large as the increase which you want to focus. Finally, we apply to have the focus and astigmatism. Also this adjustment is best done at a sufficiently high magnification.Astigmatism is a major problem when using lenses. Astigmatism is an optical defect (aberration). The word comes from the Greek "a-" (not) and "stigma" (point or dot).Astigmatism is an optical system has different focal lengths for rays that move in mutually perpendicular planes. In the image of such a cross (+) than the horizontal line focus on a different distance than the vertical.
If all these steps were performed correctly, there would be a clear picture on the monitor to appear.
We conclude that the SEM is a very powerful tool. The magnifications and resolution that it can offer us achieve amazing images. Many of our signals because it can be studied and the signals each contain different information, the extensive capabilities of the SEM.Information about both the composition and the topography can be obtained.
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