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In general, magnetic hysteresis can affect
the stability of the magnetic fields. Therefore, hysteresis removal, also known as lens cleaning, should be addressed during optimization of microscope conditions.
In SEM systems, the function of DeGauss is used to reduce hysteresis of the magnetic field in the objective lens. Therefore, this function needs to be performed manually or automatically:
i) Before making the electron optical axis alignment.
ii) After changing focus widely. In this case, the accuracy of magnification or alignment of the electron optical axis may degrade due to hysteresis of the focusing magnetic field.
iii) After changing magnification greatly.
iv) After changing accelerating voltage.
v) After changing working distance.
vi) After changing operation mode.
In TEM systems, hysteresis factor affects the TEM operations under changed lens conditions once magnetized lenses retain a certain amount of residual magnetism. This means that the level of current used does not always specify lens strength. This effect can particularly affect:
i) The total magnification. That is, the total magnification redouts may be in error.
ii) The standard focus.
iii) The magnification calibration. On most microscopes, one gets the best calibration data by calibrating at the highest magnification, and then works down the range because of inherent hysteresis in the electromagnetic lenses.
iv) Electron beam position on the specimen. The instability of beam position can be caused by the magnetic hysteresis of condenser lenses. A method to overcome such effect from magnetic hysteresis is to increase the beam diameter beyond the edges of the detector area.
i) and iii) can be corrected by pressing the STANDARD FOCUS bottom a number of times during the operation. In this way, the applied current is essentially returned to zero a number of times. This reduces lenses to the standard level of magnetization.
Compensation of magnetic hysteresis in condenser lens systems can be performed by:
i) Increasing voltage of the condenser lenses to the maximum value a couple of times.
ii) Resetting the condenser lens.
iii) Activating the beam tilt wobbler.
iv) Lens relaxation by current oscillation.
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