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Direct Measurement of Electron Beam Scattering in the Environmental Scanning Electron Microscope Using Phosphor Imaging Plate Technology

S.A. Wight and C.J. Zeissler

Objective: To measure electron scattering in the environmental scanning electron microscope (ESEM) specimen chamber using phosphor imaging plate technology.

Problem: The scattering of electrons from the primary electron beam, under relatively high-pressure conditions (266 Pa) in the ESEM sample chamber, degrades the analytical accuracy of elemental analysis in the ESEM. The degree of this degradation is poorly known. To date, attempts to experimentally measure the spatial distribution of the scattered electrons have been limited to observing secondary effects such as the intensity of x-rays produced from copper targets positioned at various distances from the primary-electron-beam interaction point. A more accurate distribution of the scattered electron intensity can be obtained from a direct measurement of both the scattered and unscattered electrons over a large area with single electron sensitivity.

Approach: Advances in phosphor imaging plate technology can be exploited for this application. Phosphor imaging plates are sensitive to single electron stimulation, are capable of storing intensity information over several orders of magnitude, and can measure large areas several centimeters in size. The plates have a minimum digitization dimension of 25 micrometers, which is sufficient for measuring electron scattering in the ESEM chamber which covers areas hundreds to thousands of micrometers in diameter. The challenge is to merge phosphor imaging plate and ESEM measurements to accurately determine the electron scattering.

A plate configuration optimized for tritium mapping is ideally suited for mapping of the relatively low energy electrons, 1 to 20 keV, of interest in the ESEM. The plate is placed in the ESEM chamber that is pumped to a pressure of 266 Pa (2 Torr) of water vapor. The phosphor plate is exposed to the 20 keV electron beam for a short time, < 1s, under scattering conditions. After exposure, the plate is retrieved from the ESEM chamber and the intensity distribution of the electron beam and the scattered electrons is recorded.

Figure 1. Phosphor Plate Image of Primary

Figure 1. Phosphor Plate Image of Primary
Electron Beam and Scattered Electrons with
Linescan Overlay (25 cm wide)

 Results and Future Plans: A phosphor plate image of the primary beam and the scattered electron distribution is shown in Figure 1 with a linescan through the center overlaid at the bottom of the image. The horizontal line on the right hand side of the image is an artifact of the measurement system. The image shows that the scattered electron intensity is significant and extends out approximately 300 mm from the primary beam. Future plans include an electron beam shutter adaptation to reduce the saturation of the primary beam and provide better control of electron dose, and characterization of the phosphor signal as a function of electron energy in the energy range of interest. These measurements will be used to improve theoretical scattering models for the electrons in the ESEM chamber and hence the accuracy of elemental analysis in the ESEM.


Last Updated March 5, 2002

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