Scanning Electron Microscopy - Coursework Example

? | Type here Select and compare a SE and BSE image taken from the same region of a sample. 1a. Highlight differences between the two images, referring to specific features in the images. The above micrographs represent a Secondary electron image (SE) and a Backscatter electron image (BSE) of the same region of an asbestos sample. The SE image appears more three dimensional than the BSE image. The cluster of fibers at the center in the SE image appears more rounded, while the one in the BSE appears flat. The contours and topography of the SE image are clearly visible, while the BSE image looks flatter and plane. There is a greater contrast in the BSE image than the SE one. The fibers in the BSE image are clearer, but those in the SE image look clumped. The SE image provides a better view of the finer details of the specimen, unlike the BSE image. There is a greater edge highlight in the SE image. 2b. Using the properties or characteristics of the SE and BSE, explain the observed differences. Clearly state the contrast mechanisms in each image. Secondary Electron images (SE) Vs. Backscattered Electron images (BSE) Secondary electron images are formed from the low energy electrons that are formed near the surface of the sample (Johnson). The brightness is affected by the surface topology of the specimen. For backscattered electron images, higher energy electrons formed deeper in the material are used to form the image. The result of these images is less contrast due to surface topology and more contrast due to different chemical composition (Johnson). This explains the 3D nature of the SE image in comparison to the flat BSE image, and the higher contrast of the BSE image in comparison to the SE image. Secondary electrons have lower energy compared to backscattered electrons, and so, they interact with the outer regions of the specimen by inelastic collisions. Therefore, only the surface topology of the specimen is clearly defined. This is the reason why the fibers in the SE looked clumped. The contrast in the BSE image is because of the production of backscatter electrons produced due to collisions of high energy electrons of the specimen. Parts of the specimen with higher atomic number cause higher backscatter than the lighter atomic number elements, resulting in a greater contrast, enabling a better study of the chemical composition of the specimen. The greater edge highlight in the SE image is because raised surfaces yield more secondary electrons. Images of a tilted TEM grid are provided showing a large difference in depth of field (file names DOF 1, 2, 3). 3 Calculate the depth of field from the images provided. Explain how you arrived at your answer. Compare SEM figures with the depth of field that would be available from an optical microscope for the same magnification. Large depth of field is one of the most important characteristics of SEM. The sharpness of the images recorded at low magnifications depends more on depth of field available than on small beam size (Lyman 1990). We know that depth of field, Where, d = minimum resolution of SEM W = Working distance D = aperture size Accordingly, the depth of field from the given images is computed as follows: Taking the following assumptions, d = minimum resolution of SEM= 3.5 nm = 3.5 ?10-9 m W = Working distance = as given in image in mm ?10-3 m D = aperture size= 200?m = 200?10-6 m Depth of field for first image with WD=13.0 mm= 13.0?10-3 m = 0.455?10-6 m = 4.55?10-7 m Depth of field for second image with WD=14.3 mm= 14.3?10-3 m =0.5005?10-6 m = 5?10-7 m Depth of field for second image with WD=44.3 mm= 44.3?10-3 m =1.55?10-6 m Comparison of SEM figures with the depth of field that would be available from an optical microscope for the same magnification The depth of field of SEM can be as great as 300 times that of the optical microscope. At low magnifications, below 300 to 400X, the image formed by the SEM is inferior to that of the optical microscope (Abbaschian et al 2008). At the same magnification, the depth of field that would be available from an optical microscope for the same magnification would be: For the first image, DOF=4.55?10-7 m / 300 = 0.015?10-9 m = 1.5?10-11 m (approx.) For the second image, DOF=5 ?10-7 m / 300 = 0.016?10-9 m = 1.6?10-11 m (approx.) For the first image, DOF=1.55?10-6 m / 300 = 0.0516?10-8 m = 5.16?10-10 m (approx.) Images obtained from a sample at 2, 15 and 30 kV are provided (file names Circuit 1, 2, 3). 4a. Describe differences in the images and explain these differences based on interaction volume changes with kV. The contribution from both SE and BSE must be taken into account. Image obtained at 2 kV This image is extremely light visibly reduced contrast. The fine circular granules and the horizontal dark lines are almost invisible. There is little distinction between the sample and the substrate. The image has very low resolution. This indicates that there are extremely less or no secondary electrons at all because of the low kV, i.e. 2kV. Lack of secondary electrons makes it difficult to view the surface topology and the contours of the specimen. The interaction volume is fairly low. Image obtained at 15kV Compared to the image obtained at a lower accelerating potential of 2 kV, the second image obtained at 15 kV exhibits better resolution and topographical details. The substrate and the specimen can be easily contrasted and identified. The circular granules and the horizontal dark lines can be easily identified. Finer details of the specimen can be easily studied. The edges are easily highlighted and the shape can be studied. This indicates sufficient emission of secondary electrons because of a higher accelerating voltage (15 kV in this case) of the beam. The interaction volume is higher. Image obtained at 30kV The image obtained at 30 kV has the highest contrast. There is lesser edge highlighting than the image obtained t 15 kV. The contours and topography of the specimen including the circular granules are less visible. This is probably because of increase in backscatter of electrons and re-absorption of secondary electrons by the sample itself. The beam has a higher acceleration voltage and penetrates deeper into the specimen, making the image appear more contrasted yet with lesser detail. 4b. Estimate the depth of penetration of 2, 15 and 30 kV electrons for the sample used. To estimate the depth of penetration of 2, 15 and 30 kV, a copper sample was used and a Monte Carlo simulation was performed using Casino v2.42. The intent of this software is to assist scanning electron microscope users in interpretation of imaging and microanalysis and also with more advanced procedures including electron-beam lithography (Drouin et al 2007). Depth of penetration at 2kV Using the simulation, the average depth of penetration was computed as 25.5nm. Depth of penetration at 15kV Using the simulation, the average depth of penetration was computed as 550 nm. Depth of penetration at 30kV Using the simulation, the average depth of penetration was computed as 1811.1nm. Compare the high resolution images ‘Gold on carbon.jpg’ (FE-SEM image) and ‘Gold on carbon2.jpg’ (tungsten filament SEM image). 5a List all the instrument parameters used to capture the tungsten filament SEM image and explain why the selected instrument conditions were used. To capture the tungsten filament SEM image, a tungsten filament was used as the electron source. The electrons in the SEM beam of incident electrons are produced by a field emission cathode (FE) or by a thermal emission source like a heated tungsten filament. The tungsten filament is used for the thermionic emission of electrons because of the fact that it has the lowest vapour pressure and the highest melting point compared to all other metals. This makes it possible to heat it long enough to release electrons. Electric current is used to heat up the filament. A typical tungsten filament electron gun for scanning electron microscopy has a diameter of about 100 micrometers. In operation, the filament will be heated by passing electrical currents through it. When the heat generated is enough to overcome the work function of tungsten, the electrons can escape from the material. Optimum filament temperature for the thermionic emission of electrons is around 2700 degrees Kelvin. The accelerating voltage is generally between -500 Volts and -50,000 Volts DC, is applied to the Wehnelt cylinder. Resistive self-biasing is usually used where an adjustable bias resistance connects the filament to the accelerating voltage. The biasing brings the filament slightly more positive than the Wehnelt. The anode is connected to electrical ground. Without the Wehnelt and anode, electrons emitted from the filament would tend to stay in the area of the filament. This forms a 'space charge' or a cloud of electrons whose mutual repulsion resists any further emission from the filament. The anode, being at ground potential, is more positive than the filament and attracts the electrons away from the filament - providing the primary acceleration for the electron beam. But the current flow that would result would be very low and dependent on the accelerating voltage. By adding the grid, or Wehnelt, we have a way of controlling the space charge of the filament, shaping the beam and increasing the beam current. The equipotential lines between the various parts of an electron gun, form a contour map of the intensity, or flux, of the electrical field. The gradient, or direction, of the electrical field will always be perpendicular to these lines and is the direction of the most rapid change of the potential. Electrons leaving the filament will be accelerated along the gradient towards the most positive area, the anode. This beam of electrons will be focused by the shape of the field gradient to a cross-over just before the anode, forming the first optical image of the source and ensuring that a larger percentage of the electrons will pass through the aperture of the anode (Sampson 1996). (Image courtesy: Sampson 1996) 5b Explain why different conditions were used for the FE-SEM image and comment on the benefits of field emission sources. Note: An inlens detector is a secondary electron detector located inside the pole piece. FE-SEM requires high vacuum conditions because the emission of electrons depends on the work function of the metal used. This can be affected by adsorbed gases, making it necessary to use high vacuum, about 10-9 Torr. The vacuum further enables the electron movement through the column, preventing electron scattering and discharge. In case of the field emission source used in FE SEM, the filament is not heated. Instead, the emission of electrons is caused placing the filament in an electrical potential gradient. The FES also uses a wire of tungsten, but it is into a sharp point. It is important to have a minute tip radius, about 100 nm, so that the electrical field produced can be concentrated to a very high range, so that the electrons can be emitted when the work function of tungsten is lowered. Benefits of field emission sources As is evident from the images given, the FE SEM produces clearer and cleaner images. FE SEM images have lesser electrostatic distortions and the spatial resolution is less than 2nm, which is 3 to 6 times better than SEM (EAC). For automated operation over several hours, Field emission sources are suitable because of their stability and long filament lifetime (Francus 2004) The image produced is clearer, and of higher magnification with better contrast. Compare figures ‘Pollen 1, 2 & 3’. 6a. Explain the benefits of using a mixed signal image. Using a mixed signal image combines both the merits of SE and BSE imaging. As is evident from the given images, the SE image enables a better viewing of the topology and contours of the pollen, but compromises on finer details. There is edge highlighting in the image. The BSE image gives a better idea of the details and composition of the specimen, but the topology of the specimen is reduced. By using a mixed signal, the edge highlighting is fairly reduced, and both the topology and the finer details and composition are much clearer. 6b. Explain what image averaging and image integration is. How do these noise reduction methods improve a SEM image? Image average and image integration are noise reduction methods to improve the quality and clarity of a SEM image. These techniques use computational modelling and noise reduction software. Image averaging In this technique, the images themselves averaged to produce an average first image and an average second image that are then correlated (Kompenhans et al 2007). Few or several identically framed shots are taken consecutively and then averaged (Freeman 2008). Image integration It is the process of averaging a series of images over a period of time (Sprawls). By integrating a series of images, which have a random distribution of image noise with respect to time, the images can be improved and the noise can be fairly reduced. It increases the signal to noise ratio. These methods greatly reduce the image noise in SEM images and produce high definition images. SEM images are mostly distorted due to noise because of the secondary electron detector that is homogenously spread throughout the image. By averaging frames or by averaging the detector signal of each pixel, this noise can be reduced. Another source of noise in SEM imaging is gray level fluctuations that occur because of electrostatic charges, variation in the alignment of target, electron beam and secondary electron detector. Background clutter also contributes to noise in SEM images to a large extent (Pham et al 2007). All these can be nullified using image integration and image averaging. Use appropriate images from the portfolio to answer the following; 7a. Explain what sample charging is. Explain methods to reduce charging. Sample charging occurs when there is build up of charge because of excess electrons on the sample surface. Typically the majority of the electrons in the beam go into the sample and out through the electrical ground of the SEM. A small fraction of these electrons generate useful imaging electrons, e.g. secondary or backscattered electrons. This build-up of electrons or “charge” creates an electric field which deflects the electron beam in undesirable ways (Rice). When this charge is released randomly, the detector is excited in unpredictable ways. It is of the following types: General charging-entire image becomes bright as charge builds up over the entire scan area. Edge charging-edges become bright due to charge build-up. Area charging-some areas become bright or dark, changing continuously. Line by line charging-bright lines occur across the image. Residual charging-electrons from a previous scanning session are left over and add to the current scan. Methods to reduce charging: Sample charging can be reduced by balancing the incoming electron beam to the outgoing electrons. Sample charging can also be eliminated by the Focused Ion Beam (FIB) Technique. Other methods include accelerating the voltage and turning it down to decrease the sample potential, reducing the number of electrons hitting the sample by reducing the spot size, maintaining low vacuum so that gas in the chamber absorbs excess electrons, coating the sample with a conductive layer (Rice). The charging can also be reduced by increasing the scan rate. 7b. Explain methods/approaches that can be used to eliminate or reduce edge highlighting. Edge highlighting can be reduced by using an efficient backscattered electron detector. This detector appears to have approximately the same limit of spatial detect-ability as a secondary electron detector. It has the advantages over a secondary electron detector of a reduction of charging artifacts, improved flat surface contrast and reduced edge highlighting (Robinson 1974). Another approach towards reducing edge highlighting would be to use a high accelerating voltage electron beam. 7c. Explain why figure ‘Geopolymer 2’ exhibits less charging than ‘Geopolymer 1’ given the SEM operating conditions are the same. Note: figure ‘Geopolymer 1’ was collected with a slow scan rate and figure ‘Geopolymer 2’ was collected with a fast scan rate and a 120 frame average. Increasing the scan rate to very high levels diminishes the time the sample is exposed to the incoming beam. It also increases the signal to noise ratio. That is the reason for less charging in the second image than the first image. Though both the images were taken under similar SEM operating conditions, increasing the scan rate reduced the amount of time for exposure to the electron beam, thereby eliminating the build-up of charge on the specimen surface. Energy Dispersive Spectroscopy You will be provided with x-ray spectra collected from copper at 5, 10 and 20 keV. 1a. Measure the L/K ratio, resolution, zero point and gain from the copper spectra. Using WinPLOTR, the X-ray spectra are plotted as follows: The FWHM for Copper at the three different keV is found as follows: Calculation of the L/K ratio: Energy (keV) K energy (keV) K intensity (counts) K FWHM (keV) L energy (keV) L intensity (counts) L FWHM (keV) L/K Background (Counts) 5 0.934 1126 0.081 X X X X 28.12 10 0.932 2837 0.078 8.038 25.8 0.167 0.0091 45.84 20 0.929 6428 0.081 8.037 1947 0.154 0.3029 66.27 Calculation of resolution: Since the smallest value of FWHM is considered as the maximum resolution, the resolution for the Cu X ray spectra is as follows: Resolution for 5 keV: 0.081 Resolution for 10 keV: 0.078 Resolution for 20 keV: 0.081 Calculation of gain: The gain is calculated as the ratio of maximum signal intensity K to Background: Gain= Gain for 5 keV= =40.04 Gain for 10 keV= =61.88 Gain for 20 keV= =96.99 1b. Explain the variation in L/K ratio just measured. Indicate the relative importance for each reason provided. The increase in the L/K ratio is proportional to the increase in energy. Both the L and the K peaks depend considerably on voltage provided. At 5 keV, the L peak is absent and the K peak is fairly minute. This is because the voltage for this peak is lesser than the cut off voltage. At 10 keV, the L peak is still very minute and unnoticeable. This is because the voltage at 10 keV is very close to the cut off voltage, and so, a very small number of electrons are excited to the required level. The K peak is quite apparent. At 20 keV, the voltage is quite high, exciting a large number of electrons to the required level. Both the peaks, L and K, are highly pronounced. Because of the high excitation of electrons at high voltage, it is most appropriate to apply higher voltages for spectral analysis of elements. A BSE image from a sample with regions of different average atomic number will be provided. In addition, selected x-ray maps and spot mode spectra from the same area will be made available. 2 Discuss the relative merits of the BSE image, x-ray maps and spot mode spectra. The given micrographs show a mineral sample in various imaging modes. The image taken in BSE clearly shows the varying composition of the specimen, but the exact elemental composition of each element cannot be deciphered. The BSE method gives a rough idea of the presence of varying elemental composition in the specimen. This method can be employed as a primary detection method, as it is faster. Based on the atomic number, the areas are highlighted, the brightest being the area having highest atomic number. The X-ray map gives a better idea of the composition of the sample. The distribution of varying elements can be visually demonstrated. This method is slower than BSE, but enables qualitative analysis of the specimen. The spot mode spectra give the exact elemental composition of the specimen using X-ray spectra analysis. The regions of carbon, oxygen, silicon, chlorine, copper and silver are clearly demarcated in the given images. Using the spot mode, the particular area to be examined can be selected and analysed. Bibliography Abbaschian, R., Lara Abbaschian, Robert E. Reed-Hill. Physical Metallurgy Principles. India: Cengage Learning, 2008. Drouin, D., Alexandre Real Couture, Dany Joly, Xavier Tasteti, Vincent Aimezi, Raynald Gauvin. "CASINO V2.42—A Fast and Easy-to-use Modeling Tool for Scanning: Electron Microscopy and Microanalysis Users". Scanning 29 (2007): 92–101. Francus, Pierre. Image analysis, sediments and paleoenvironments. New York: Springer Science, 2004. Freeman, M. The Complete Guide to Night & Lowlight Digital Photography. New York: Sterling Publishing Company, Inc., 2008. Johnson, Katrina."Analysis of Screw Fracture Surface Using Scanning Electron Microscope (SEM) Techniques". 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