WHAT IS THE S.E.M.?
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The Scanning Electron Microscope (SEM) is a microscope that uses electrons rather than light to form an image. There are many advantages to using the SEM instead of a light microscope.
The SEM has a large depth of field, which allows a large amount of the sample to be in focus at one time. The SEM also produces images of high resolution, which means that closely spaced features can be examined at a high magnification. Preparation of the samples is relatively easy since most SEMs ony require the sample to be conductive. The combination of higher magnification, larger depth of focus, greater resolution, and ease of sample observation makes the SEM one of the most heavily used instruments in research areas today.
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Forces in a cylindrical Magnetic Lens
Beam's Path through the Column
A beam of electrons is generated in the electron gun, located at the top of the column, which is pictured to the left. This beam is attracted through the anode, condensed by a condenser lens, and focused as a very fine point on the sample by the objective lens. The scan coils are energized (by varying the voltage produced by the scan generator) and create a magnetic field which deflects the beam back and forth in a controlled pattern. The varying voltage is also applied to the coils around the neck of the Cathode-ray tube (CRT) which produces a pattern of light deflected back and forth on the surface of the CRT. The pattern of deflection of the electron beam is the same as the pattern of deflection of the spot of light on the CRT.
The ehe electron beam hits the sample, producing secondary electrons from the sample. These electrons are collected by a secondary detector or a backscatter detector, converted to a voltage, and amplified. The amplified voltage is applied to the grid of the CRT and causes the intensity of the spot of light to change. The image consists of thousands of spots of varying intensity on the face of a CRT that correspond to the topography ohy of the sample.
SEM Ray Diagrams
These schematics show the ray traces for two probe-forming lens focusing conditions: small working distance (left) and large working distance (right). Both conditions have the same condenser lens strength and aperture size. However, as the sample is moved further from the lens, the following occurs:
- the working distance S is increased
- the demagnification decreases
- the spot size increases
- the divergence angle alpha is decreased
The decrease in demagnification is obtained when the lens current is decreased, which in turn increases the focal length f of the lens. The resolution of the specimen is decreased with an increased working distance, because the spot size is increased. Conversely, the depth of field is increased with an increased working distance, because the divergence angle is smaller.
While all these signals are present in the SEM, not all of them are detected and used for information. The signals most commonly used are the Secondary Electrons, the Backscattered Electrons and X-rays.
Using a Vacuum
When a SEM is used, the column must always be at a vacuum. There are many reasons for this. If the sample is in a gas filled environment, an electron beam cannot be generated or maintained because of a high instability in the beam. Gases could react with the electron source, causing it to burn out, or cause electrons in the beam to ionize, which produces random discharges and leads to instability in the beam. The transmission of the beam through the electron optic column would also be hindered by the presence of other molecules. Those other molecules, which could come from the sample or the microscope itself, could form compounds and condense on the sample. This would lower the contrast and obscure detail in the image.
A vacuum environment is also necessary in part of the sample preparation. One such example is the sputter coater. If the chamber isn't at vacuum before the sample is coated, gas molecules would get in the way of the argon and gold. This could lead to uneven coating, or no coating at all.
Sample Chamber
The photo on the left shows the sample chamber located at the base of the column. The second photo shows the lens and detectors located inside the sample chamber.
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Objective Lens
The lens that focuses the beam of electrons towards the sample is in the center of the picture. The parts off to the right of the sample are different detectors. One is for detecting the secondary electrons and the other is for detecting the backscattered electrons. The operator has the power to choose and switch detectors for use on each sample. Using the secondary electron detector produces a clear and focused topographical image of the sample. The backscatter electron detector produces an image that is useful when determining the make-up of the sample. Each element in the sample appears as a different shade, from almost white to black.
Stage
| A prepared sample is mounted on a specimen stub and placed on the stage. |
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Specimen Preparation
There are two basic types of SEM's. The regular SEM, which we have in the Iowa State Materials Science Department, requires a conductive sample. An environmental SEM can be used to examine a non-conductive sample without coating it with a conductive material. Three requirements for preparing samples for a regular SEM such as in the Iowa State Materials Science Department are:
1) Remove all water, solvents, or other materials that could vaporize while in the vacuum.
2) Firmly mount all the samples.
3) Non-metallic samples, such as bugs, plants, fingernails, and ceramics, should be coated so they are electrically conductive. Metal What is an Electron Microscope?
| The development of the Scanning Electron Microscope in the early 1950's brought with it new areas of study in the medical and physical sciences because it allowed examination of a great variety of specimens. As in any microscope the main objective is for magnification and focus for clarity. An optical microscope uses lenses to bend the light waves and the lenses are adjusted for focus. In the SEM, electromagnets are used to bend an electron beam which is used to produce the image on a screen. By using electromagnets an observer can have more control in how much magnification he/she obtains. The electron beam also provides greater clarity in the image produced. |
The first modern scanning electron microscope,
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