training for advanced research


Glossary terms about Electron beam

Electron beam
A stream of electrons (cathode rays).

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30 pages mention Electron beam

Acceleration voltage vs. specimen type
With a higher accelerating voltage the electron beam penetration is greater and the interaction volume is larger. 
Background information - What is scanning electron microscopy?
The SEM uses an electron beam instead of a torch, an electron detector instead of eyes, and a viewing screen and camera as memory. 
Concepts - introduction
Much logically flows from this, such as that a vacuum is needed to generate the electron beam, that electrons are used for imaging, and that we need to understand the interactions of electron beam with the sample in order to interpret our images. 
As the specimen surface is scanned by the incident electron beam, backscattered electrons (BSE) are generated, the yield of which is controlled by the topographical, physical and chemical characteristics of the sample. 
Electromagnetic lenses, apertures and beam size
The condenser lens converges the cone of the electron beam to a spot below it, before the cone flares out again and is converged back again by the objective lens and down onto the sample. 
Electron column
The electron column focuses and illuminates the specimen using the electron beam generated by the electron gun. 
Electron gun
Thermoelectrons produced by the filament are accelerated through the application of a voltage between the filament and the anode, thus creating an electron beam that streams down the microscope column. 
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). 
Electron-matter interactions
When the electron beam hits a sample it interacts with the atoms in that sample. 
Focused ion beam (FIB) technology
The advantage of dual beam machines is that they allow specimens to be imaged in detail using the electron beam, without damaging the surface of the specimen with the ion beam. 
Generating an image
So far only the secondary electrons produced by interaction of the primary electron beam with the sample have been discussed. 
High vacuum mode and pump system
A high vacuum minimises scattering of the electron beam before reaching the specimen. 
Today, many SEMs are commercially available and they all contain the same essential features: an electron optical column in which an electron beam is generated under high vacuum, and focussed to a tiny spot on the specimen surface. 
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 condenser lens controls the intensity of the electron beam reaching the specimen. 
Principles of SEM operation
A scanning electron microscope is a machine comprised of an electron generating component called the gun, a column through which the electron beam travels, a series of lenses to shape the electron beam, the sample chamber at the base, and a series of pumps to keep the system under vacuum. 
Recommended reading
H. electron beam analysis of materials (2nd ed. 
Risk assessment example: using a SEM
Notes: : Interactions between the electron beam and sample/stage generate ionizing radiation during normal operation. 
Role of sample height
There are three reasons why the value of the external Z control (mechanical control) and the WD provided on the image screen are different. The ‘on screen’ value of the WD is only an accurate measurement if the electron beam is focused accurately onto the specimen surface. 
Sample insertion and beam activation
While you are waiting for the machine to reach its correct vacuum you can select the electron beam voltage that you will need to use (but don’t turn it on yet). 
Sample preparation
Magnetic materials can be investigated, but care is needed so that the powder is not attracted to the  
Scan rate and signal to noise ratios
The image quality of the SEM is limited by the spot size and the ratio of the signal (S) produced by the electron beam to the noise (N) imparted by the electronics of the instrument in the displaying of this signal (S/N). 
Specimen chamber
The specimen chamber is maintained at high vacuum that minimises scattering of the electron beam before reaching the specimen. 
The electron gun
The electron gun is used to provide a large, stable current in a small electron beam
Troubleshooting: edge effect, charging, sample damage
Irradiating a specimen with an electron beam results in a loss of the beam energy to the sample in the form of heat. 
Variable Pressure or Low Vacuum scanning electron microscopy (LVSEM)
This charging artefact is the result of electrons from the electron beam building up in a nonconductive sample. 
What is an SEM (Parts of the machine)?
the computer that drives the microscope, with the additional bench controls ancillary equipment that, for example, analyses composition. 
What is astigmatism?
To allow accurate imaging, the electron beam (probe) should be circular in cross section when it reaches the specimen. 
What is resolution?
Resolution is dependent on the wavelength of the beam we use to see the material, and this explains why the electron beam, with its much shorter (smaller) wavelengths is able to provide better detail than the light microscope. 
What the SEM can't do
Na+) are volatile under the electron beam because the negative electron beam exerts a force on charged material.