Radiant energy – Electron energy analysis
Patent
1987-01-29
1989-03-07
Anderson, Bruce C.
Radiant energy
Electron energy analysis
250281, 250282, H01J 3705, H01J 4946
Patent
active
048108839
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a device for providing an energy filtered charged particle image particularly to such a device which is able to act as an energy band-pass filter whilst maintaining the spatial integrity of the image passing through the filter.
BACKGROUND ART
Charged particle emission such as electron emission from solid surfaces may be used to advantage in an imaging mode. In particular if an energy filtered image is produced the chemical morphology of a surface or the information about the excitation processes can be determined.
An engery filtered image is constructed from electrons with energies centered about E.sub.o and a band-pass of +.delta.E. Techniques for producing energy filtered image fall into the following three catagories.
(1) Those in which the excited area on the object is much smaller than the collection area of the analyser aperture (the microprobe method). Examples of this group are Auger electron spectroscopy and backscattering in the scanning electron microscope.
(2) Those in which the area of the analyser aperture is much smaller than the excited area on the object (the selected area method). Examples are field-emission spectroscopy and the scanning photoelectron spectroscopies.
(3) Those in which the area of excitation (or its image) is much the same as the area of the analyser aperture (the whole-image method), for example the energy-selecting electron microscope and the magnetically-collimated emission spectromicroscope.
This disclosure is concerned with an imaging band-pass analyser for whole-image energy-filtering. Whole image techniques provide rapid signal collection over the total image area and thus can be particularly time efficient.
One problem in designing fixed band-pass energy analyser which operate in a magnetic field, is that there are no elementary transmission low-pass energy filters which will preserve an image. Consequently a retarding-field electron mirror is used which can in principle maintain the image geometry. To produce a usable filter however, the reflected filtered electron stream must be separated from the incoming stream. This can be achieved by using crossed electrostatic and magnetic fields in the region preceding the filter. In a cross-field region, electrons acquire a drift velocity, perpendicular to both field directions, irrespective of the initial direction of motion of the electrons. Consequently, electrons can be drifted into the electron mirror and out again along different paths.
Transmission high-pass energy filters based on electrostatic saddle fields are well known and capable of very high energy discrimination when applied to narrow beams. For uniform retardation across a wide stream, in the absence of a magnetic field, planar grids are needed with consequent obstruction of the image and sensitivity to the spatially varying micropotentials. When a magnetic field is coaxial with the electrostatic saddle field the effects of off-axis transverse electrostatic field components are largely suppressed, as will be discussed later. Thus it becomes possible, in principle, to dispense with grids.
A band-pass energy filter can therefore be constructed be directing the image electrons into an electron mirror then, using crossed field deflection, to a high-pass filter. Using a single electron mirror however results in a reversal of the beam direction in the analyser. The use of a second electron mirror, either before of after the high-pass filter, restores the direction of the initial beam. An advantage of using such a folded electron beam path is that the length of the analyser is reduced and it requires a smaller region of uniform magnetic field in which to operate.
As mentioned the low-pass electron mirror necessitates the use of crossed electrostatic and magnetic fields which serve to transport the image electrons from one stage of the analyser to the next. Unfortunately, in uniform crossed fields, this arrangement suffers from a severe defect. The drift imposed by the crossed fields is a function of the
REFERENCES:
patent: 3437805 (1969-04-01), Brown
Tam et al., "Magnetically Collimated Electron Impact Spectrometer", Rev. Sci. Instr. 50(3), Mar. 1979.
Microscopie Electronique, 1970, Septieme Congris International De Microscopie Electronique, Grenoble (1970), pp. 179-180.
Grope et al., "Design of an ExB Filter with Improved Velocity Separating Capability", J. of Phys. E, Sci. Instrum., vol. 10, No. 5, May 1977.
Eyerton et al., "Modification of a Transmission Electron Microscope to Give Energy Filtered Images and Diffraction Patterns, and Electron Energy Loss Spectra", J. Phy. E., Sci. Instrum., vol. 8, 1975.
Lee, "A Nondispersive Electron Energy Analyzer for ESCA", Rev. Sci. Instrum., vol. 44, No. 7, Jul. 1973.
Anderson Bruce C.
Guss Paul A.
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