Patent Number: 055090435
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to an X-ray analysis apparatus, comprising an X-ray source, a wavelength-dispersive system of crystals, an object carrier, and an X-ray detection system. The invention also relates to a crystal monochromator and to a crystal analyzer for such an apparatus. 2. Description of the Related Art An X-ray analysis apparatus of this kind is known from U.S. Pat. No. 4,567,605. So as to achieve notably a high resolution, the apparatus described therein comprises a dispersive element in the form of a 4-crystal monochromator. For specific applications, for example examination of thin layers, be it imperfect as well as epitaxial layers and the like, the comparatively low radiation intensity of the known 4-crystal monochromators may become objectionable. Increasing the radiation intensity by using a high-intensity radiation source makes the apparatus expensive and substantially limits the service life of the radiation source. SUMMARY OF THE INVENTION It is an object of the invention to provide an X-ray analysis apparatus enabling operation with a comparatively high radiation intensity. To achieve this, the X-ray analysis apparatus of the kind set forth in accordance with the invention is characterized in that reflective crystal end faces of a dispersive crystal do not extend parallel to diffractive crystal lattice planes in the crystals. Because the crystal end faces in the monochromator in accordance with the invention do not extend parallel to the crystal lattice planes in the crystals, a larger acceptance angle is realized for an X-ray beam to be monochromatized. (The phenomenon that the crystal end faces used do not extend parallel to the crystal lattice planes is referred to as asymmetry in the context of the present invention). As a result, for analysis in an X-ray diffractometer an effective X-ray beam with a substantially higher radiation intensity can be generated and a higher detection efficiency can be realized in the X-ray spectrometer. Such asymmetry results in a resolution which is less high, but that is not objectionable for different examinations. For many types of examination the high resolution of the known 4-crystal monochromator can be sacrificed for a high intensity then required. The use of the monochromator in accordance with the invention enables faster analysis with a better signal-to-noise milo. In a preferred embodiment reflecting crystal end faces form pan of a 4-crystal monochromator. In the case of an adapted angle between the crystal end faces and the crystal lattice planes, such a monochromator undergoes hardly any or no exterior geometrical modifications relative to the known monochromator, so that it can be included in an X-ray analysis apparatus without requiting complex adaptations. The four crystal end faces preferably enclose the same angle with respect to the relevant crystal lattice planes, but for specific applications deviations therefrom are feasible. The crystals consist of, for example monocrystalline germanium, the diffractive crystal lattice planes being formed by (220) or (440) lattice planes. Because the (220) lattice planes already produce a higher intensity, it is advantageous to use an asymmetrical monochromator in accordance with the invention in the (220) position. In a further preferred embodiment, the angle between the crystal end faces and the crystal lattice planes amounts to, for example from approximately 15.degree. to 23.degree. for the (220) position. Such a monochromator produces an effective X-ray beam having an intensity which is approximately x times higher than that of the known symmetrical monochromator. Calculations and measurements have demonstrated that x=4 for 15.degree. For such an asymmetry angle the (440) crystal plane mode still acts as the high resolution mode. Calculations have also demonstrated that x=15 for 20.6.degree. . In order to realize a monochromator which can be fully exchanged, the angle is chosen so that the crystal end faces, measured in the diffraction direction, are large enough to accept the entire incident beam. On the other hand, the value of the angle can also adapted to a desired effective beam intensity for specific examinations. The monochromator carrier may be constructed so that different measurement modes can be selected by rotation of the crystal pairs, for example an asymmetrical (220) position for high intensity and a (440) position for high resolution. However, upon changing over from one measurement mode to the other in this manner it may occur that no detection of a reflection can be observed. This is because a range of zero intensity is traversed during rotation of the crystal pairs. In the case of a small alignment error (i.e. the angles between the X-my beam and the crystal end faces deviate slightly from the prescribed value), no reflection will occur any more for any angular rotation. Alignment of the experimental arrangement then becomes very difficult. Therefore, in a preferred embodiment the monochromator holder is constructed as a changer system whereby several monochromators can be alternately positioned in the beam path. Because rotation of the crystal pairs is thus avoided, the alignment problem no longer occurs. A monochromator carrier in the form of a changer may also comprise asymmetrical crystals as well as symmetrical crystals with a (220) position as well as a (440) position for the crystals, so that crystal rotation is no longer necessary. Even though the present description often refers to a monochromator for the sake of clarity, the use of the invention is by no means restricted to what is customarily referred to as a monochromator in an X-ray analysis apparatus. An asymmetrically ground crystal system can also be used as an analyzer in an apparatus of this kind. This is because incoming radiation, now already diffracted from a specimen to be examined, is also discriminated therein in respect of wavelength and/or direction. It may again be advantageous to sacrifice a pan of the resolution for a gain in radiation intensity. An X-ray monochromator suitable for an X-ray analysis apparatus in accordance with the invention is provided with crystals whose crystal end faces do not extend parallel to diffractive crystal lattice planes. Different crystal lattice planes can be chosen for this purpose; however, crystal lattice planes which already produce a comparatively high effective beam in a symmetrically ground crystal (i.e. a crystal in which the crystal end face extends parallel to the relevant crystal lattice planes), are most suitable for this purpose.