Thu, 3 Nov One is the "physics" question what does theory say? Here the ultimate limit is diffraction.
Thu, 3 Nov One is the "physics" question what does theory say? Here the ultimate limit is diffraction. Regardless of the source type, a too-small aperture will cause a diffraction spreading of the beam.
The simplest way to understand this is as a manifestation of the Heisenberg uncertainty principle -- when the lateral location of the electron is too tightly constrained by the aperture, then the lateral uncertainty in the momentum increases and there is broadening.
So if one were operating a SEM in a difraction-limited mode, increasing the aperture size would be the most sensible way to increase the beam current. Usually the dominant limit to resolution is spherical aberration and that term varies as aperture size to the third power.
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The simple quadrature approximation shown as equation 2. There is an optimum aperture size for any given beam current and either a smaller or larger aperture than that optimum will increase the beam diameter.
Now for the practical. What actually happens when you turn a knob or change an aperture on a particular instrument depends on the details of how that instrument was designed and is being operated. Consequently, for modest changes in beam current, just turning the "Spot" control or whatever you call the control that regulates the condenser current is the simple practical expedient.
Now, back to the original question that was asked. That is a practical reality in many situations. But it is a mechanism that needs to be considered given the limited information provided. So far from being a "nuance," depending on the instrument being referenced, this may in fact be the most pertinent answer to the question posed.Product and Process Design Principles - Seider - Ebook download as PDF File .pdf), Text File .txt) or read book online.
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