FEI Scios 2, User'S Operation Manual

Page: 207 / 224

System Options: Beam Deceleration
C O N F I D E N T I A L – FEI Limited Rights Data 7-29
Beam Deceleration
The Beam Deceleration (BD) mode is based on a negative voltage
(bias) applied to a stage (i.e. a sample). The electrical field
between the sample and the nearest surface over (a column
bottom or a detector) is formed, acting as the additional
electrostatic lens. Its power is described by the Immersion Ratio
(imRatio) parameter.
Detection Principles
The Beam Deceleration influences both primary and detected
electrons.
As the sample is at the negative potential with relation to the
ground and detectors, the initial SE and BSE energy (when leaving
the surface) is accelerated by the Stage Bias before the detection.
The higher the Immersion Ratio, the lower is the difference
between SE and BSE energies when detected.
Signal electrons are accelerated upwards and deflected towards the column axis.

The SE’s have a low initial speed and they are usually absorbed into the detector central hole, continue through and
can be detected by in-column detectors (T2 and T3). Equally like the BSE heading upright.

Conversely the BSE heading nearly parallel to a surface (which normally cannot be detected) are driven to a
detector.
By changing the Stage Bias an output angle of electrons leaving a surface can be changed.
FIGURE 7-10
Typical trajectories of secondary (red) and back scattered (green) electrons
Detectors most convenient for the Beam Deceleration are BSE ones, placed closely under or directly inside the
column. Their efficiency depends on their active area – the smaller is the inner diameter of the active area, the
better. The standard ETD can also be used, but its efficiency is low.