SEM Quick Start
Revision No.2
May 2014
Page 6 of 16
The pressure of the sample chamber can be viewed under the
Vacuum
tab on the right sidebar. Pressure units can be cycled through to
the user’s preference by left-clicking the vacuum pressure status boxes.
The microscope requires a vacuum below
0.009 Pa
before extra high
tension (EHT) can be enabled, which is required to produce an image.
Although the software allows enabling EHT at this pressure, better images
are usually produced at lower pressures (~ 10
-4
Pa).
1.2.3
Accelerating Voltage
The voltage used to accelerate electrons toward the sample has an
important impact on the produced image. Before turning on EHT, the
operator is advised to ensure that the accelerating voltage is set to a
reasonably-low value (~
2.0 kV
, ± 1.0 kV). The accelerating voltage can
be changed most easily by double-clicking the
EHT
textbox at the bottom
of the main camera window. When EHT is off, this textbox will show
EHT = 0.00 kV
. After checking that both the vacuum chamber pressure
and accelerating voltages are at reasonable values, EHT can safely be
enabled by clicking the
EHT
status box at the bottom-right of the
SmartSEM
window. If successfully enabled, the status box will change
accordingly. Note that it is common practice to change EHT values in
small increments (no more than ±
5 kV
) in hopes of prolonging the
lifetime of the filament responsible for emitting electrons.
As a general rule, lower EHT values gives greater information
about the surface of a sample with cost to resolution. Conversely, higher
accelerating voltages yield higher resolutions while obfuscating details
about the surface. It is also important to use lower voltages (in the absence
of variable-pressure modes) on samples that conduct electrons poorly to
decrease the effects of charging.
It is important to be aware of the effects accelerating voltage has
on the final image. The beginner is encouraged to experiment with
different EHT values while attempting to produce an image (Section 1.3).
The more experienced user will also find this practice to be beneficial
when working with new sample types. Once familiar with the basic
process of producing an image (focusing, stigmating, etc.), practice saving
images (Section 1.3.9) of the same scene at different accelerating voltage
