Signal Detection in CamScan Low Vacuum Microscopes
The conventional Thornley-Everhart Secondary Electron detector, which relies on a high voltage supply for its operation, cannot function in a low vacuum.
An alternative detector has to be used and CamScan provide two types of electron detector for operating in these conditions.
- Specially designed LEASE - Low Energy Amplified Secondary Detector.
- Conventional Back Scattered Electron Detector.
LEASE Detector
The Lease detector obtains images from those electrons that
have interacted with the gaseous environment around the sample.
During Lease operation the sample and its immediate surroundings
are held at a potential that enhances the collection of these
low energy electrons which carry information about the sample
surface and topography.
The detector is not critically dependent on working distance
and can image over a wide range of beam voltages from <
5 to >30 Kv. The column and detectors do not limit the
field of view of the microscope and very low magnification
images (<10x) can be obtained. There is no problem in operating
at the standard analytical position for EDX and WDX indeed
working distances greater than 50mm can be used.
Chamber pressures up 300 Pascals can be used but for most
applications excellent results will be achieve with a setting
of less than 50 Pascals and a resolution <5nm can be achieved
at 30 kV and 8mm WD from an appropriate sample.
The microscope resolution will depend on the normal factors
of kV, WD, probe current and signal/noise
Back Scattered Electron Detector
A conventional back scattered electron detectors of either
the Semiconductor or Scintillator design can be used as complementary
detectors.
It has been common practice for many years to use these detectors
for low vacuum SEM imaging They provide the additional benefit
of both compositional (Z) and topographical contrast information
in both High and Low Vacuum modes.
