S.A. Wight and C.J. Zeissler
Objective: To directly measure electrons scattered out of the primary electron beam as a function of vacuum and instrument conditions in the environmental scanning electron microscope using phosphor imaging plate technology.
Problem: The analytical resolution of the environmental scanning electron microscope (ESEM)) is limited by the scattering of primary electrons out of the electron probe. Interactions of the primary beam electrons with the gas molecules in the chamber scatter a fraction of the beam electrons into a broad low intensity skirt around the focused probe. Electrons in the skirt contribute unwanted signals that originate relatively far away from the intended quantitative analysis area. It is important to understand the size and magnitude of the electron skirt to test simulations and generate correction schemes.
Approach: Phosphor imaging plate technology is adapted for use as a position sensitive electron detector. The phosphor grains are sensitive to single electron stimulation. The plates are capable of storing several orders of magnitude differences in intensity information. Grains can be as small as 25 micrometers, which is more than sufficient resolution for measuring electron distributions that are reported to be hundreds to thousands of micrometers in diameter. A plate optimized for tritium exposure was chosen for its thin phosphor layer and lack of a protective coating. The thin phosphor layer is more sensitive to electrons and a coating would inhibit the relatively low energy electrons of interest in the ESEM.
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Figure 1: Graphical representation
of the primary and scattered electrons at short and long beam-gas path-length
and high and low pressures of water vapor. |
The plate is physically placed in the ESEM chamber and pumped to a specimen chamber pressure of 266 Pa (2 Torr) or 1330 Pa (10 Torr) of water vapor. A Uniblitz vacuum compatible shutter is adapted into the electron beam path to limit the dose of electrons to the phosphor imaging plate. The 20 kV electron beam is allowed to strike the plate for a short time under scattering conditions. The beam-gas path-length is either short (2 mm) or long (10mm). The plate is retrieved from the ESEM chamber and scanned by the imaging system to produce an intensity distribution of the probe and the scattered electrons simultaneously. Results and Future Plans:
The addition of the shutter to control the dose of electrons to the plate
is successful and allows data to be collected at different instrument
conditions. Four images from the phosphor imaging plate system are presented
in Figure 1 for a two millisecond exposure at both high (1330 Pa) and
low (266 Pa) pressures of water vapor in the specimen chamber for both
long (10 mm) and short (2 mm) beam-gas path-lengths. There is a shadow
in the short images that is not in the long images from x-rays generated
in the column that may have implications for quantitation. |
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Last Updated
March 5, 2002
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