3.2.2.3. Secondary Electron Imaging
A scintillator-photomultiplier detector or "Everhart- Thornley" detector is used to detect the secondary electrons emitted from the specimen. The low energy secondary electrons are emitted from the sample in all directions and possess relatively low energies. They are initially gathered by a charged collector grid (or cage), which can be biased from -50 to +300 V (Figure 3.2.2.3). This draws the secondary electrons towards the scintillator, which is a thin plastic disk coated with a short-persistence phosphor that is highly efficient at converting the energy contained in the electrons into ultraviolet light photons (4000 Å). The response time of the phosphor is fast and permits high resolution scanning. The outer layer of the scintillator is coated with a thin layer [10-50 nm] of aluminum, positively biased at approximately 10 KeV, which accelerates the electrons to the scintillator surface. The charged collector grid, in addition to collecting secondary electrons from the sample, helps to alleviate some of the negative effects of the scintillator aluminum layer bias which can actually distort the incident beam.
The aluminum layer also acts as a mirror to reflect the photons produced in the phosphor layer down the light pipe, which consists of a Plexiglas or polished quartz pipe, and out through the specimen chamber wall, where they are amplified into an electronic signal by way of a photocathode and photomultiplier tube (PMT). The photocathode converts the UV photons back into electrons, which travel down the PMT towards an anode striking the walls of the tube as they go. The PMT is coated with material (usually an oxide) that has a very low work function and thus electron striking the walls generate more freed electrons producing "gain." The result is a cascade of electrons that eventually strike the anode. The number of cascade electrons produced by the PMT depends on the voltage applied across the cathode and anode of the PMT. One can increase the gain by increasing the voltage to the PMT (this is essentially what is accomplished when adjusting the contrast.) The amplified electrical signal is sent to further electrical amplifiers, which increase the electrical signal from the PMT by a constant amount thus increasing or brightness.
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Figure 3.2.2.3. A secondary electron scintillator-photomultiplier detector following the design of Everhart and Thornley. Low-energy secondary electrons (trajectories shown by dashed lines) are collected by applying a suitable bias to the Faraday grid. These electrons are further accelerated in order to give them sufficient energy to scintillate the phosphor (after Potts 1987). |
The topographical aspects of a secondary electron image depend on how many of electrons actually reach the detector. When the incident electron beam intersects the edges of topographically high portions of a sample at lower angles, it puts more energy into the volume of secondary electron production. Thus, high points produce more secondary electrons, generating a larger signal. Faces oriented towards the detector also generate more secondary electrons. Secondary electrons that are prevented from reaching the detector do not contribute to the final image and these areas will appear as shadows or darker in contrast than those regions that have a clear electron path to the detector. It should be noted that those backscattered electrons directed at the scintillator will also contribute to the signal that reaches the scintillator and form part of the the secondary electron image.
The MBX SEM system can produce magnification from 100x to 80,000x, although magnifications above about 4000x are rarely employed. Originally the SE image produced by the MBX microprobe was displayed on a 105 x 105 mm viewing screen with 800 line definition. This system has been replaced by computer acquisition using the MAC G-3 monitor for display.
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Copyright 1997-2003, James H. Wittke
Last update: 01/18/2006 01:47 PM.