Patent Number: 
Section: description

FIG. 1 schematically illustrates the essential components of a layer-thickness measuring device 11, in which case the illustration of an evaluation unit, a screen for visualizing a measurement object recorded by a video camera, and also an input keyboard and printer has been dispensed with. This layer-thickness measuring device 11 is used for example for measuring bonding pads, contacts which are provided in part with a selective coating, conductor tracks and functional coatings on small areas. A layer-thickness measuring device 11 with the apparatus 12 according to the invention is preferably used to determine or check layer thicknesses whose measurement area or the functional areas are smaller than 100 xcexcmxc3x97100 xcexcm, in particular smaller than 50 xcexcmxc3x9750 xcexcm. X-rays are generated in an X-ray tube 13 and are directed via an anode 14 to a measurement object 16. The X-rays excite a fluorescent radiation in a layer of the measurement object 16. The intensity of this fluorescent radiation depending on the energy (spectrum) is a function of the layer thickness. This or the parameter of the layer system is utilized by the system of the emitted radiation being registered with the aid of a detector 17. The apparatus 12 according to the invention is provided between the X-ray tube 13 and the measurement object 16, which apparatus, in accordance with the exemplary embodiment, comprises two mutually opposite reflecting areas 18. These reflecting areas 18 serve for focussing rays and forwarding rays, with the result that the X-rays pass to the measurement area of the measurement object 16. The reflecting areas 18 are preferably arranged directly relative to the anode 14 or to an exit flange 21 near the anode 14. Furthermore, a collimator 23 is provided at the lower end 22 of the reflecting areas 18 which are assigned to one another, as a result of which it is possible to image a measurement region 24 as shown in FIG. 3 on a measurement object. The collimator 23 is advantageously a slit collimator whose slit width is adjustable. The reflecting areas 18 are designed as elongate, rectangular areas, as can be gathered from FIG. 1 and FIG. 2. The length of the reflecting areas 18 is essentially determined by the construction and also by the degree of total reflection. X-rays which do not run parallel between an axis of the measurement region 24 and the anode 14 are deflected at least once by total reflection. The width of the reflecting areas 18 is at least one and a half times as large as the maximum functional area to be checked. It is advantageous to use silicon wafers for the reflecting areas 18. This cost-effective base material can be adapted in a simple manner to the corresponding size of the apparatus 12 according to the invention. Further semiconductor materials such as, for example, germanium, gallium arsenide or the like are also suitable for the reflecting areas 18. The reflecting areas 18, which are preferably produced from a silicon wafer, are advantageously applied to holding elements 26, 27 as shown in FIG. 3. These are advantageously bonded on in a strain-free manner, so that the planarity of the reflecting area 18 can be maintained. As an alternative, the reflecting areas 18 can also be fixed in a stress-free manner on the holding elements 26, 27 by means of clamping or the like. As shown in FIG. 3, an adjusting unit 28 engages on one of the two holding elements 27, by means of which adjusting unit a holding element 27 can be adjusted relative to the stationary element 26. The holding element 26 advantageously accommodates the reflecting area 18 parallel to the central axis 29 of the apparatus 12. The slit width can be adjusted by the adjusting unit 28. It likewise becomes possible to adjust the angularity of the holding element 27 relative to the element 26. As an alternative, it is likewise possible to provide a mirror-inverted arrangement. Likewise, provision may alternatively be made for an adjusting unit 28 to be provided on each of the holding elements 26, 27, as a result of which the holding elements 26, 27 can be arranged either parallel to one another and/or at an angle to one another, thereby forming a uniform or tapering slit towards the measurement object 16. The adjusting unit 28 is designed in such a way that slit widths in a range of from 10 to 100 xcexcm, for example, can optionally be adjusted. For this purpose, it is possible to provide precision-mechanical adjusting mechanisms, piezo-electric actuators, and also electrically, hydraulically, pneumatically operated actuating drives. At an end pointing towards the measurement object 16, a flattened portion 31 is provided on the holding element 26. This flattened portion makes it possible for there to be a sufficient aperture width 32 available for the emitted fluorescent radiation in order to detect the emitted fluorescent radiation. The reflecting area 18 may, for example, have a noble metal vapour-deposited on it. This makes it possible to increase the critical angle for total reflection, which is 1.5 mrad for silicon, to 4.5 mrad by means of a platinum coating. This in turn has an advantageous effect on the transmission of the X-rays. As an alternative, in the case where coated reflecting areas are used, it is conceivable that the base material may comprise a quartz surface or a plastics material which satisfies the requirement of planarity and has a coating. The coating may advantageously be provided at least at the input of the reflecting areas 18, so that the number of captured and reflected rays is as large as possible. The coating may be continued completely over the course along the reflecting areas 18, or else be provided only partly. Likewise, the coating or the material of the coating may also change depending on the applications. By way of example, by reducing the critical angle for total reflection, it is possible to reduce the divergence at the output of the reflecting areas 18, which makes it possible to obtain focussing of the radiation and, as a result, an intensity increase on the measurement region 24 of the measurement object 16. To that end, it is conceivable, for example, for a coating not to be provided in a region near the lower end 22 of the reflecting area 18 or for a coating that prevents total reflection to be provided, as a result of which the radiation emerging below the reflecting area 18 is focussed precisely to the size of the measurement region 24 of the measurement object 16. The irradiation of edge regions outside the measurement region 24 can thereby be reduced considerably. The invention""s configuration of the apparatus 12 enables the measurement region to be adjusted depending on the measurement task. The collimator 23 can likewise be adapted to this measurement region, so that the focussing of the radiation enables an intensity increase on a predetermined measurement region. As an alternative, it may be provided that the reflecting areas 18 are designed to be at least slightly concave. Likewise, the concave design may taper towards the lower end 22, yielding a kind of meslithone-shaped configuration of the reflecting areas 18. In this case, however, account should be taken of the dimensions, which can also lie in the micrometer range. The aperture width of the reflecting areas 18 at the input of the apparatus 12 essentially corresponds to the outlet opening for the X-rays emitted via the anode. Likewise, it is also possible to provide a slightly larger or smaller aperture width relative to the diameter of the primary spot of the X-rays. Furthermore, the apparatus 12 may also have openings and receptacles which serve for arranging an optical system in order to visualize the measurement object 16 using a video camera. In accordance with the exemplary embodiment, the apparatus 12 is provided by two reflecting areas 18 which are arranged relative to one another and are arranged parallel or at an acute angle relative to one another. It may also be provided that, instead of these two reflecting areas 18, three or more reflecting areas are arranged in a suitable manner relative to one another in order to enable the transmission of X-rays to the measurement region 24 of a measurement object 16, so that an intensity increase is made possible by the focussing of the X-rays. However, in contrast to what is known from the prior art, it is not necessary to use a closed, tubular arrangement in order to focus the X-rays to the measurement region by total reflection. Further geometrical configurations of the reflecting areas 18 which enable the total reflection of the X-rays are likewise conceivable.