The present invention relates to an X-ray beam device for X-ray analytical applications such as X-ray Diffractometry, High Resolution X-Ray Diffractometry, X-ray Reflectometry comprising an X-ray Source, typically a point focus X-ray source, and an x-ray focusing optical assembly for focusing monochromatic x-rays on a sample.
The invention will be more particularly adapted for High Resolution X-ray Diffraction applications. High Resolution X-ray Diffraction is a well established technique for carrying out the analysis of high quality thin layer of near perfect single crystals materials.
High resolution diffraction analysis requires to illuminate the sample with an X-ray beam having a very small divergence and a high energetic resolution. This is necessary to determine the parameters such as lattice spacing with a sufficient precision. Typically for High Resolution X-ray Diffraction applications in the case where Copper radiation is used, the energy resolution required is such that the incident x-ray beam needs to be filtered so that only Kα1 line is transmitted to the sample. The Kα2 line from the Copper Kα doublet has consequently to be filtered.
The X-ray Beam Device according to the invention typically comprises a high brilliance X-ray source which can be either a sealed tube X-ray source or a rotating anode X-ray source with a point focus geometry. Laboratory X-ray sources have isotropic radiations and the characteristic X-ray spectrum emitted by an X-ray source with a Copper target is composed of several parasitic X-ray peaks and bremsstrahlung x-rays. Moreover the Kα1 and Kα2 lines are merged in the Kα doublet.
X-ray multilayer optical elements composed of a Bragg reflective structure can be used to collect the beam emitted by an x-ray source in order to monochromatize and adapt the x-ray beam in real space and in angular space toward a sample. However the use of a multilayer optical element to filter the X-ray beam emitted by a source will be unsufficient for high resolution diffraction applications as for Copper X-ray beam radiation the Kα1 and Kα2 lines are merged after the reflection on the multilayer optical element.
Natural crystals (e.g. Si, Ge, LiF) can be used as X-ray optical elements in order to monochromatize an x-ray beam to the level of energetic resolution required for High Resolution X-ray Diffraction. However the efficiency of such natural crystals is limited when curved crystal elements necessary for focusing the X-ray beam emitted from the source are used (curvature deteriorates the very small lattice spacing structure). Such natural crystals typically have a corresponding angular acceptance of 10 to 30 Arcseconds. FIG. 1 is a schematic graph illustrating an example of angular acceptance ΔθM of a crystal monochromator in terms of reflectivity.
One optical system arrangement typically used for high resolution x-ray diffraction applications is composed of an x-ray multilayer conditioning optic that is used to convert the divergent X-ray beam emitted by the X-ray source into a parallel beam before entering a crystal monochromator arranged downstream the multilayer optic. Such multilayer conditioning optic will be referred to a collimating optic in the description of the invention. The collimating effect of a multilayer conditioning optic is referring to the optical effect where a divergent beam collected by the optic is converted in a substantially parallel beam having a divergence lower than 1 milliradians typically.
FIG. 2 is illustrating a type of multilayer conditioning optic 20 producing a collimating effect wherein the optical element comprises a reflective surface being shaped according to two curvatures Cx and Cy corresponding to two different directions which are the sagittal direction (direction Y in FIG. 2) and the meridional direction (direction X in FIG. 2). These directions that will be referred to in the description of the invention can be defined with respect to the general direction of propagation of the X-ray beam:—the meridional direction being the mean direction of propagation of the x-ray beam (and more precisely the mean direction between the mean direction of propagation of the beam before and after its reflection on the optical assemblies concerned), and the sagittal direction being the horizontal transverse direction of this meridional direction (the vertical being defined by the mean normal to the part of the reflective surface of the optical assemblies which will be described and used for reflecting the incident x-ray beam). The incoming x-ray beam is after a single reflection on the optic 20 collimated in two-dimensions which are the sagittal plane (a sagittal plane being the plane defined by the sagittal direction and by the mean direction of propagation of the x-ray beam at the exit or at the entry of the concerned optical element) and in the meridional plane (the meridional plane being the plane defined by the meridional direction and the mean normal to the part of the reflective surface of the optical assemblies which will be described and used for reflecting the incident x-ray beam).
Such optical system arrangement with a collimating multilayer optic 20 arranged upstream a crystal monochromator is efficient for applications requiring a few hundred microns or a millimiter sized x-ray beam at the sample position. The X-ray spot size at the sample position as it is defined in the invention is given by the dimensions Ws and WT of the beam in the two directions perpendicular to the general direction of propagation of the beam. As an indication, Ws and WT dimensions are illustrated in FIG. 2 for the x-ray beam at the exit of the two-dimensional collimating mirror 20.
However such collimating optical systems are limited for applications requiring a small spot dimension at the sample position (in the order of few hundred microns or less than 100 microns). Indeed the flux will be reduced proportionally to the surface of the spot.
Another known optical system arrangement is to use a two-dimensional focusing optic with a multilayer coating adapted upstream the crystal monochromator in order to collect the divergent x-ray beam emitted from the source and to focus such beam on a small spot at the sample. However, some intensity is lost due to the divergence of the X-ray beam incoming the monochromator in the scattering plane of the monochromator (the scattering plane of the monochromator is the plane including the incident beam and the diffracted beam) resulting from the focusing effect of the upstream multilayer optic.
FIG. 3 illustrates an optical arrangement known from state of the art where an optical element 21 with a multilayer coating collects an incident X-ray beam X1 emitted by a point source 10 and conditions such beam towards a crystal monochromator M by collimating the beam X2 in one dimension and focusing the beam X2 in the other dimension. As illustrated in FIG. 3, the x-ray beam X2 outcoming from the optical element 21 is collimated in the meridional plane of the optical element 21 corresponding to the scattering plane of the monochromator, and is focused in the sagittal plane of the optical element 21 corresponding to the sagittal plane of the Monochromator. The focusing in the sagittal plane of the Monochromator M enables to concentrate a higher intensity beam on a reduced spot size at the sample 40. This focusing effect, leading to a slightly divergent beam in the sagittal plane is possible due to a larger tolerance of the crystal monochromator in this plane (compared to the tolerance on beam divergence in the scattering plane which is the angular acceptance as shown in FIG. 1). This difference of tolerances on incident beam divergence for a crystal monochromator is referenced for example in the Document entitled “Parallel Beam Coupling into channel-cut monochromators using curved graded multilayers” from M. Schuster and H. Gobel published in J. Phys. D: Applied Physics 28 (1995) A270-275 (a maximum divergence value of 1.95° in the sagittal plane is given as an example for a Germanium(022) crystal monochromator for the Copper Kα1 line).
However, the overall usefull flux of such optical arrangement that is illustrated in FIG. 3 is limited due to a reduced divergence of the x-ray beam diffracted by the crystal in the scattering plane of the crystal monochromator. Indeed the divergence of the X-ray beam in this scattering plane of the monochromator will be limited to the reflection width of the crystal ΔθM which is typically between 10 and 30 Arcseconds. As it will be illustrated later in the description of the invention, even for High Resolution X-ray Diffraction applications it can be accepted to have a higher divergence of the incidence X-ray beam that is illuminating the sample compared to the angular acceptance ΔθM of a traditional crystal monochromator. However it is not possible to increase the angular acceptance of a traditional crystal monochromator while keeping a high energetic resolution.
An object of the present invention is to increase the usefull flux compared to traditional x-ray beam device for High Resolution X-ray Diffraction Applications in particular for applications requiring a small spot (in the order of few hundred microns or less than 100 microns).
A further object of the present invention is to achieve very small spot dimensions on the sample, in particular spot dimensions smaller than 50 microns for X-ray analytical applications requiring such high spatial resolution as in semiconductor metrology applications.