A beam booster is an integral part of the GEMINI® electron optical column. The beam
booster always maintains a high beam energy through-out the entire column, regardless of the electron beam energy selected by the operator. Only after passing through the scanning system the electron beam is decelerated to its selected landing energy. The electron beam path has been designed to eliminate cross-over of beam electrons between source and specimen. Furthermore, the high beam energy throughout the column ensures that the GEMINI® column is extremely well protected against magnetic stray fields, even when operated at very low voltages. The tolerable magnetic stray field limit is therefore independent of the selected voltage. An electromagnetic, multi-hole aperture changer has been incorporated close to the electron source. In combination with a magnetic field lens to select the optimum beam aperture angle and to tune the probe current.
The combination of the high beam energy and cross-over free electron beam path also minimise the statistical Coulomb interactions between beam electrons, which tend to reduce the brightness and hence the resolution limit of the microscope. As a result, the SUPRA™ GEMINI® column always provides optimum beam brightness, especially when needed at the lowest electron probe energies.
In the GEMINI® column the electron beam rays only intersect once: at the focussing point on the specimen surface. Therefore, the broadening of beam energy spread, a problem with all electron optic configurations featuring one or more intermediate cross-over - irrespective of the electron source - has been completely eliminated. This is especially significant when ultra high resolution is needed at low voltage applications.
The innovative GEMINI® column concept provides superior beam brightness with ultra high resolution over the complete voltage range together with high probe currents for analytical applications.
Operating principle of the GEMINI® field emission column
U1 - extractor voltage at first anode
U2 - accelerator voltage at second anode
U3 - booster voltage | 
Probe current in nA related to landed beam energy |
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