Operating Procedures: Optimizing Imaging
5-15
C O N F I D E N T I A L
– limited rights
User Manual
Revision A
Feb 2018
Accelerating Voltage and Beam Currents
The choice of accelerating voltages – in the UI named High Voltage (HV) – and
beam currents for the active display is possible via the toolbar dropdown list
boxes. The electron beam current and HV are independent, so any change of
voltage will not influence the current.
Changing High Voltage
Besides a normal list box’s behavior an intermediate value can be
entered into the toolbar editable HV text box or set by the
Column
module /
High Voltage
adjuster on the Beam control page. This
provides a calculated range of beam current / spot values. Default
values in the list box are set in the
Preferences
dialog /
Presets
section.
Electron Beam Current / Spot size
This is one of the basic operating parameters. Functionally, it represents the electron beam diameter (usually
presented as the
Spot
size, expressed as a relative number from 1 to 20), the final lens aperture diameter, and its
opening angle as set by the condenser. It is considered to be close to ideal when a spot’s edges just touch its
neighbouring ones. If the spot is too large, overlaps occur and the image appears out of focus. If it is too small,
electronic noise appears in the image.
A suitable beam current value for a particular magnification can be determined easily when good focus and
astigmatism correction are achieved.
Changing Electron Beam Current
Either electron
Beam Current
or
Spot Size
can be shown within the User Interface. This can be set
in the
Preferences
dialog /
General
section or by right-clicking on the toolbar text box and
selecting the desired context menu proposal.
An intermediate value can be set by the
Column
module /
Beam current
/
Spot size
adjuster on the
Beam Control page. Default values in the list box are set in the
Preferences
dialog /
Presets
section.
Table 5-2:
Spot sizes and recommendation of their use
SEM column Use case
Spot size
Best use
Standard / OptiPlan / Immersion
1 – 4
Very low currents, charging and sensitive samples
Immersion
3 – 7
Ultra high resolution
Standard / OptiPlan
4 – 6
High resolution
Standard / OptiPlan / Immersion
6 – 12
Standard imaging
Standard / OptiPlan / Immersion
12
and more
High current imaging, X-ray analysis with SDD detectors
Standard / OptiPlan
10 – 17
High current imaging, X-ray analysis, EBSD detector,
cathodo-luminiscence
Standard / OptiPlan
18 – 20
Very high currents, fast analysis