FEI Quanta FEG 250, Руководство пользователя

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System Options: Beam Deceleration
9-69
BEAM DECELERATION APPLICATIONS
• The BDM enables detection of the BSE when the electron energy
is under the detection limit of the detector.
• The BDM expands the electron energy range under the minimum
HV limit.
• The BDM improves the microscope resolution at low accelerating
voltages. A conventional microscope resolution is limited by a
chromatic aberration at low electron energies. The higher is the
Immersion Ratio, the smaller are the aberrations and a loss of
resolution at low electron energies is well compensated.
• The BDM enables to detect electrons heading nearly parallel to a
surface which accentuates a surface roughness.
Application Restrictions
• In the LoVac mode the chamber environment is rather electrically
conductive and the BDM is not available.
• The sample tilt causes an electrical field deformation, which adds
not correctable aberrations (a chromatic aberration and an image
distortion). An acceptable sample tilt is about a few degrees, for a
higher immersion ratios preferably less.
FIGURE 9-57 SIGNAL DISTORTION AND IMAGE ABERRATIONS
FOR TILTED AND ROUGH SAMPLE (TIN BALLS)
AT HIGH IMMERSION RATIO
BEAM DECELERATION MODULE
The Beam Deceleration module is accessible from the Beam
Deceleration Control page. It has following features:
• The On button switches the BDM on / off. This is available only in
the HiVac mode and with the Beam On. When switching the beam
off, the BDM switches off automatically.
• The Stage Bias (bias) reflects the negative voltage applied to a
stage. Minimal value is 50 V, maximal is 4 kV.
When the BDM is on, a hook mark is placed behind the HV. In this
case this value represents energy of electrons reaching a sample
surface (it is also stored within the TIF file header).
The imRatio = (HV + bias) / HV.