Mechanical Vibration/Thermal Effects on EMs
- Practical Electron Microscopy and Database -
- An Online Book -

http://www.globalsino.com/EM/  



 

This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

 

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The resolution of electron microscopes (EMs), especially for TEM, is partially limited by:
        i) The electrical stability of the EMs systems, e.g. the stabilities of the high voltage and the lens currents;
        ii) External disturbances e.g. mechanical vibration, contamination, charging, fluctuation of stray magnetic fields, and the nonuniform magnetic properties of the pole-piece material used.

For EM installation, we need to carefully check the vibration condition of the site. For TEM, we need to have the best anti-vibration mount where the microscope is placed. The residual vibration on the anti-vibration mount with amplitude of one micron can be delivered to the specimen stage with a relative displacement of 0.1 nm. Therefore, 0.1 nm resolution will be the best for the microscope assuming the other parts do not limit the microscope further.

Shock absorbers and a shield box for the microscope high-voltage circuit can be installed to avoid degrading high-voltage stability through mechanical vibration or stray electromagnetic fields.

In most cases in TEM experiments, specimen drift originates from thermal difference between the stage and specimen holder itself. This type of drift is mostly linear and thus can be modeled and corrected after data acquisitions. It will disappear when the specimen-stage assembly reaches thermal stability. For instance, for TEMs with side-entry specimen stages, they are susceptible to vibrations and thermal drift (The rod serves as a transmitter of vibration and heat), which are considerably higher than that for top-entry specimen stages.

Comparing with side-entry specimen stages, previously top-entry specimen stages were much less susceptible to vibrations and thermal drift, and thus enabled higher specimen stability (especially for HRTEM) since they were not connected directly to the outside. Therefore, it was theoretically and empirically suggested that resolution with top-entry holders was slightly higher than that with side-entry stage, for instance, 0.24 nm for the top entry stage versus and 0.27 nm for side entry stage at an accelerating voltage of 200 kV before.

We could also consider mechanical and electrical instabilities as a kind of incoherent aberration.

 

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