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Glossary terms about Focus

Focus
In scanning electron microscopy: The process of bringing the electron beam to its smallest diameter cross section at the sample surface. The production of a sharp, well defined image, rather than a fuzzy, blurry image.

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18 pages mention Focus

Applications and practical uses - what the SEM can do
using a focussed ion beam). 
Background information - What is scanning electron microscopy?
By using a focussed beam of electrons, the SEM reveals levels of detail and complexity inaccessible by light microscopy. 
Beam Adjustment
focus the image. 
Electromagnetic lenses, apertures and beam size
A series of electromagnetic lenses and apertures are used to reduce the diameter of the source of electrons and to place a small, focused beam of electrons (or spot) onto the specimen. 
Electron column
The electron column focuses and illuminates the specimen using the electron beam generated by the electron gun. 
Electron-beam lithography (E-beam lithography or EBL)
Upon irradiation of focused electron beam, electron-sensitive resists undergo chain-scission or crosslinking, resulting in solubility switch of materials during the subsequent development process (remove/retain exposed material in development depending on the tone of the resist). 
Focused ion beam (FIB) technology
The beam is focussed to an extremely fine probe size (<10 nm) onto the surface of a specimen. 
History
In 1923, Busch demonstrated that a beam of electrons could be focused by magnetic or electric fields. 
Layout
The SEM uses a beam of high energy electrons generated by an electron gun, processed by magnetic lenses, focused at the specimen surface and systematically scanned (rastered) across the surface of a specimen. 
Magnetic lens system
The objective lens brings the electron beam into focus (de-magnifies) on the specimen. 
Operation unit
The operation keyboard controls critical operations such as focussing in high magnification and astigmatism correction. 
Resolution
the image must be properly focused and the image must have adequate resolution. 
Role of sample height
However, the true working distance (WD) is measured electronically as the point of focus on the sample surface to the SEM column above. 
Sample preparation
Magnetic materials can be investigated, but care is needed so that the powder is not attracted to the  
SEM challenge
The specimen is mounted flat and in focus
Spot size
These three parameters interact and need to be considered carefully to achieve the best image because they also affect other parameters such as field of focus and strength of the electron signal. 
Types of SEM
focused ion beam (FIB)  
What is astigmatism?
The image in the centre shows a correctly focused image that has also been corrected for astigmatism.