X-ray or gamma ray systems or devices – Specific application – Diffraction – reflection – or scattering analysis
Reexamination Certificate
1999-11-23
2002-03-19
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Diffraction, reflection, or scattering analysis
C378S070000, C378S084000
Reexamination Certificate
active
06359964
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an X-ray analysis apparatus which includes:
a sample location for receiving a sample to be examined,
an X-ray source for irradiating the sample by means of X-rays,
a detector for detecting X-rays generated in the sample,
an influencing device which is constructed as a single mechanical unit, serves to influence the X-rays, and is arranged in a beam path between the X-ray source and the detector,
said device including a monochromatizing element and an X-ray mirror with a second-order reflecting surface, at least one of said two X-ray optical elements being situated in said beam path,
a frame for positioning at least the influencing device in the beam path between the X-ray source and the detector.
DESCRIPTION OF PRIOR ART
An influencing device for influencing the X-rays in an apparatus of this kind is known from “Proceedings of the Fifth European Powder Diffraction Conference”, May 25-28, 1997, Materials Science Forum, Vols. 278-281(1998), pp. 227-235, entitled “X-ray Optics for Materials Research”.
The cited article, notably the
FIGS. 2 and 3
and the accompanying description, discloses an influencing device which is to be arranged in an X-ray beam originating from a line-shaped X-ray source, such as the focal line of an X-ray tube for diffraction purposes. The influence exerted on the X-ray beam by the known device consists in the parallelization and monochromatization of the beam. To this end, the known device includes a graded multilayer X-ray mirror of parabolic shape which is arranged in the beam path of the X-rays and is succeeded by a germanium monochromator crystal having two reflecting surfaces. Both X-ray optical elements (the mirror and the monochromator) are accommodated in a single housing, so that the known influencing device is constructed as a single mechanical unit.
For X-ray analysis of materials to be examined it is sometimes desirable to irradiate the sample to be examined by means of an X-ray beam with very little divergence. This situation occurs, for example for the analysis of thin layers, for measurement of reflectivity, and for powder diffraction. Analysis of thin layers is a technique frequently used for the examination of materials for integrated electronic circuits. In such case it aims to determine the layer thickness as well as the mass density of the layer by means of a single measurement; furthermore, often it is also desirable to determine crystal defects and the content of a given chemical phase in the relevant layer. During all such measurements the X-ray beam is made to strike the (flat) sample surface at a suitably defined small angle. During measurement of reflection from a thin layer, the X-ray beam is also made to strike the (flat) sample surface at a suitably defined small angle; this small angle of incidence is varied so as to control the penetration depth of the X-rays in the layer and hence render the analysis of the various quantities in the layer dependent on the depth in the layer. Making an X-ray beam strike at such a small angle in a suitably defined manner, requires a high degree of parallelism of the incident X-ray beam. In the case of powder diffraction it may occur that the sample has a rough or a slightly curved surface; irradiation by means of a parallel beam then ensures that the measurements are not dependent on said surface condition. Moreover, in the case of irradiation by means of a parallel beam the measurements are not sensitive to displacements of the sample in the beam.
The above measurements require a divergence which is less than from 0.03° to 0.07°, depending on the application. According to a known method of realizing an extremely parallel X-ray beam, the X-ray beam emanating from a narrow X-ray focus (for example, a line focus having a width of 40 m) is made to be incident on a narrow gap (for example, having a width of 40 &mgr;m) which extends parallel to the line focus. When the distance between the line focus and the gap amounts to, for example 100 mm, a divergence of the X-ray beam of the order of magnitude of 0.025° is thus achieved. Such a small divergence, however, is achieved by intercepting the major part of the X-rays, so at the expense of the radiation intensity, so that measurements take much more time or a much poorer signal-to-noise ratio has to be accepted.
When use is made of an X-ray mirror, the X-ray beam originating from the narrow line focus can be converted into a practically parallel beam, so that the loss of intensity is substantially smaller. This is because in that case all radiation incident on the X-ray mirror from the line focus contributes to the intensity in the outgoing beam. This can be illustrated on the basis of the following numerical example: it is assumed that the line focus emits X-rays in a plane perpendicular to the line focus at an angle of 180° (line focus on a flat anode).
Comparing the combination formed by the line focus and the gap with the combination consisting of the line focus and the X-ray mirror, a further calculation reveals that the first mentioned combination utilizes a fraction of 1.3×10
−4
of the emitted radiation with a divergence of 0.023°. For the latter combination it is assumed that the distance between the mirror and the line focus is 100 mm, so that the line focus is seen at an angle of 0.025° from the mirror, also being the divergence of the X-ray beam reflected by the mirror. In this situation it is readily possible to realize a configuration in which the mirror is seen at an angle of 1° from the line focus. It appears that in this situation a fraction of {fraction (1°/180°)}=5.5×10
−3
of the emitted radiation is used with a divergence of 0.025°. The yield of the mirror is, therefore, approximately 20 times higher, taking into account a reflectivity of 50% of the X-ray mirror.
In X-ray analysis it is sometimes also desirable to irradiate the sample to be examined by means of an X-ray beam which has a very small divergence and has also been monochromatized, meaning that only one of the two lines of an X-ray doublet is used (for example, the spectral lines K
1
and K
2
of a copper anode), so that the other line must be removed from the beam spectrum. This situation occurs, for example in the case of high-resolution measurements during X-ray diffraction, such as measurements on perfect monocrystals (for example, pure silicon as used in the semiconductor industry) or measurements on nearly perfect structures, such as thin films in the semiconductor industry, or measurements on multilayer structures for X-ray reflection.
The desired monochromatization is realized in a known manner by reflection of the X-ray beam by a crystal monochromator. Such a monochromator can pass radiation only with a divergence of about 3×10
−3
°. The gain obtained by using a mirror therefore can be estimated as follows. Using an X-ray mirror a fraction of 1° of the radiation emitted in an angle of 180° can be collected by the mirror, which fraction is indicated as the useful yield of the line focus. A first situation consisting of the combination of the line focus and a crystal monochromator can be compared with a second situation consisting of the combination of the line focus and the mirror followed by the crystal monochromator. In the first situation 0.3% (0.003° of 1°) of the useful yield of the line focus is used. In the second situation the whole useful yield is collected by the mirror. Because an X-ray has a reflection efficiency of about 50% and because of imperfections in the reflecting surface of the mirror, eventually 35% of the radiation collected by the mirror is reflected, having a divergence of 0.025°. Only a part of that reflected fraction having a divergence of 0.003° (so about 12%) is accepted by the monochromator so that eventually about 4.2% (35%×12%) of the useful yield of the line focus will leave the monochromator. By this numerical example it is shown that the yield of the combination mirror-monochromator is about 14 times the yield of monochromator
Hodben Pamela
U.S. Philips Corporation
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