FEI Quanta FEG 250, Руководство пользователя

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System Options: Beam Deceleration
9-67
Beam Deceleration
The Beam Deceleration mode (BDM) method is based on a negative
voltage
Stage Bias ( bias ) applied to a stage (i.e. a sample). The
electrical field between the sample and the nearest surface above (a
column bottom or a detector) is formed, acting as the additional
electrostatic lens. Its power is described by the Immersion Ratio
(imRatio) parameter (see below).
DETECTION PRINCIPLES
The Beam Deceleration influences both primary and signal electrons.
As the sample is at the negative potential according 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 is the
Immersion Ration, 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 have a low initial speed and they are usually
absorbed into the detector central hole, 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 distribution of electrons
leaving a surface could be obtained.
FIGURE 9-56 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 the active area inner
diameter the better. The standard ETD could also be used, but its
efficiency is low.