Radiant energy – Electron energy analysis
Reexamination Certificate
1999-06-02
2002-05-21
Anderson, Bruce (Department: 2881)
Radiant energy
Electron energy analysis
C250S311000, C250S3960ML
Reexamination Certificate
active
06392228
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a National Stage Application of International Application No. PCT/FR97/01555, filed Sep. 3, 1997. Further, the present application claims priority under 35 U.S.C. § 119 of French Patent Application No. 96/11146 filed on Sep. 12, 1996.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an energy filter, also called velocity fitter, a transmission electron microscope and associated method for filtering energy
2. Description of Background and Relevant Information
The invention is especially applicable to TEM (Transmission Electron Microscope) or to combined TEM-STEM (Scanning Transmission Electron Microscope) as well as to electron sources. It could be used for specific STEM microscopes.
A notable shortcoming of transmission electron microscopes, during the formation of images or of diffraction diagrams lies in the presence of chromatic aberrations. The latter, essentially due to faulty adjustable electromagnetic lenses of the microscope, are detrimental to contrast and to resolution. Chromatic aberrations can be reduced to a certain extent by applying an electron acceleration voltage that is both high and stable, and by observing very narrow samples.
However, a manner, particularly efficient and accurate, to improve the picture consists in eliminating a portion of the dispersed electrons in a non-elastic way using an energy filter.
The electrons having undergone a given loss of energy may also be employed to form the picture. By selecting a characteristic loss of a type of interaction or of a chemical element, we can obtain a filtered picture providing the mapping of this type of interaction or of this element.
Energy filtering also enables to form the picture of samples that would be too thick to be observed with a conventional transmission electron microscope.
An energy filter usually comprises spatial dispersion means for the electrons of the beam transmitted by the sample in relation to their energy, as well as a filtering slot enabling selection of an energy window. Besides filtering pictures or diffraction diagrams, energy filters are also employed for spectral analysis of energy losses. Energy filters can be installed in an electron microscope either inside the column of the microscope as an integral part of the instrument, or as an accessory below the visualisation screen. We shall find recent reports on several types of filters known in the articles by Bernard Jouffrey: <<Energy loss spectroscopy for transmission electron microscopy>> in Electron Microscopy in Materials Science, World Scientific, 1991, pp. 363-368 and by Harald Rose and Dieter Krahl: <<Electron optics of imaging energy filters>>, in Energy Filtering Transmission Electron Microscopy, Springer, 1995, pp. 43-55.
For example, the article in the magazine Optik, vol. 96, no4, pp. 163-178 by Uhlemann and Rose, describes a mandolin-type magnetic energy filter.
A parameter determining energy filters is energy dispersion D, expressed in &mgr;m/eV: the greater this parameter, the greater the selective power of the filter. In order to increase this dispersion D, various energy filters have been suggested, which cause the electrons of the beam to follow sufficiently long an optical path. Indeed, the dispersion D increases in particular with the length of the distance covered. Thus, in so-called &OHgr; systems, while remaining in a fixed vertical plane, the beam propagating along the optical axis of the system is first deviated laterally, runs then along an optical path substantially parallel to the optical axis in the propagation direction, then is deviated towards the optical axis of the microscope before it is brought back in alignment with its initial direction.
The problem of the filters employed usually lies in their space requirements. Good dispersion D of the filter is indeed obtained by causing the electrons to run a distance over sufficient height. Vertical space requirements of the current filters range generally between 25 and 50 cm, for a dispersion D not exceeding 6 &mgr;m/eV.
The European patent application EP-40.538.938 suggested an electron beam instrument provided with an energy selective device. The latter causes the electrons to follow a path in a dispersion plane not containing the optical axis of the instrument. The vertical space requirements of the energy selective device are then reduced considerably for a given path length. In the specific embodiment disclosed in said document (FIG.
3
), the energy filter comprises four beam deviating elements located in the dispersion plane, in the respective corners of a substantially rectangular figure which accepts two orthogonal planes of symmetry. The filter also comprises a first deflecting element deviating the beam of the optical axis of the microscope towards one of the deviating elements in the dispersion plane, and a second deflecting element deviating the beam coming from another deviating element in alignment with the optical axis.
SUMMARY OF THE INVENTION
The present invention relates to an energy filter capable of producing a wide dispersion D while exhibiting small vertical space requirements, notably increasing the dispersion properties of the filter disclosed in the document EP-0.538.938.
Another object of the invention is such a filter that can be used in conventional electron microscopy at high as well as at low voltage, stigmatic in the first order and affected by small aberrations only.
An additional object of the invention is an energy filter enabling high acceleration voltages.
The invention also relates to a transmission electron microscope provided with an energy filter generating wide dispersion D while exhibiting reasonable vertical space requirements, whereas this microscope can be notably of TEM or TEM-STEM type.
The invention also relates to an energy filtering method for an electron beam propagating along an optical axis, generating wide dispersion over small height along the optical axis, whereas this method can be applied to imaging, diffraction or spectrometry.
To this end, the invention relates to an energy filter receiving during operation an electron beam oriented along an optical axis in a propagation direction. The energy filter comprises:
a deflecting system that deviates in a dispersion plane not containing the optical axis, the beam received along the optical axis, and
a dispersing system that guides the beam sent from the deflecting system on an optical path inscribed in the dispersion plane and returning to the deflecting system, and which generates a spatial dispersion of the electrons of the beam in relation to their energy,
whereby the deflecting system brings back in alignment with the optical axis in the propagation direction the beam coming from the dispersing system.
According to the invention, the deflecting system comprises a single deflecting element ensuring inverse deviations of the beam, whether outgoing or incoming.
The energy filter according to the invention is different with respect to the existing systems in that it comprises a single deflecting element which, both, deviates the beam in a dispersion plane not including the optical axis and provides inverse deviations of the beam, whether outgoing or incoming. The vertical space requirements of this energy filter are therefore reduced considerably, while the latter remains particularly efficient, thus ensuring wide dispersion, small aberrations and other suitable optical properties.
The expressions <<deflecting system>> and <<dispersing system>> are generic expressions referring to the main technical effect of each of both systems. It is however extremely difficult to prevent the deflecting system from also causing dispersion, even if the latter is rather reduced. Similarly, the dispersing system generates deflections of the beam that follow energy dispersion.
The outgoing and incoming paths between the deflecting and dispersing systems are generally collinear, although slight discrepancies may appear i
Jouffrey Bernard
Souche Cécile
Anderson Bruce
Centre National de la Recherche Scientifique
Greenblum & Bernstein P.L.C.
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