X-ray or gamma ray systems or devices – Specific application – Telescope or microscope
Patent
1998-07-13
2000-12-26
Church, Craig E.
X-ray or gamma ray systems or devices
Specific application
Telescope or microscope
378145, G21K 700
Patent
active
061671126
DESCRIPTION:
BRIEF SUMMARY
Th e invention relates to an X-ray microscope with zone plates for a condenser-monochromator and for a microscope objective.
In X-ray microscopy, substantial progress has been made over recent years in the wavelength region of approximately 0.2-5 nm. X-ray microscopes have been developed which are being operated on brilliant X-ray sources. Electron storage rings emit strongly focused X-ray radiation. Also included in the development are compact X-ray sources which are intended for the use of X-ray microscopes in the laboratory. Such X-ray sources can consist of hot microplasmas (typical diameter of the radiating region: 10-50 .mu.m) which are generated with the aid of pulsed laser beams. They radiate their X-ray light in all spatial directions.
At present, only microscope zone plates are used as highly resolving objectives in X-ray microscopes. Microscope zone plates are rotational symmetrical circular transmission gratings with grating constants which decrease outward, and typically have diameters of up to 0. 1 mm and a few hundred zones. The numerical aperture of a zone plate is determined very generally by the diffraction angle at which the outer, and thus finest zones diffract vertically incident X-ray beams. The achievable spatial resolution of a zone plate is determined by its numerical aperture. Over recent years, it has been possible for the numerical aperture of the X-ray objectives used to be substantially increased, with the result that their resolution has improved. This trend to higher resolution will continue.
It is known from the theory of microscopy that the numerical aperture of the illuminating condenser of a transmitted-light microscope should always be approximately matched to the numerical aperture of the microscope objective, in order also to obtain an incoherent object illumination from incoherently radiating light sources, and thus to obtain a virtually linear relationship between object intensity and image intensity. If the aperture of the condenser, by contrast, is less than that of the microscope objective, a partially coherent image is present, and the linear transformation between object intensity and image intensity is lost for the important high spatial frequencies, which determine the resolution of the microscope.
A condenser of high light-gathering power must be used for it to be possible to use the X-ray sources in a simple and matched [sic] way for bright-field microscopy, phase contrast microscopy and, in particular, dark-field microscopy. Normally, use is also made as condensers of diffracting optical systems, for example zone plates, since these may be used to render the X-ray radiation monochromatic at the same time. Such zone plates are to have a diffraction efficiency that is as high as possible, in order to focus as much of the captured radiation as possible onto the object.
Such "condenser zone plates" are normally used at the first diffraction order, at which all condenser zone plates implemented to date have their highest diffraction efficiency. It is difficult in this case to achieve the previously required matching of the numerical aperture of the condenser zone plates to that of the microscope zone plate (X-ray objective). In order to realize the matching, the condenser zone plate must have the same fine zones on the outside as does the microscope zone plate itself. The microscope zone plates built with the highest light-gathering power meanwhile have zone widths of only 19 nm (corresponding to a 38 nm period of the zone structures). Zone plates with such fine zone structures can so far be produced only using methods of electron beam lithography, in which the zones are produced successively. Holographic methods, which produce the pattern of a zone plate in one step in a "parallel" fashion and thus in a short time are ruled out, since a suitably shortwave UV holography does not exist. Consequently, it would also be possible to produce condenser zone plates with matched numerical apertures only using methods of electron beam lithography, and this must be descri
Bastian Nieman
Church Craig E.
Schneider Gerd
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