X-ray or gamma ray systems or devices – Specific application – Fluorescence
Reexamination Certificate
2001-03-19
2002-04-09
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Fluorescence
C378S044000, C378S206000, C378S150000
Reexamination Certificate
active
06370221
ABSTRACT:
The invention relates to a method of setting a position of an object of measurement in layer thickness measurement by X-ray fluorescence according to the precharacterizing clause of claim
1
.
Setting the correct position of the object of measurement with respect to the primary X-radiation and with respect to the detector is crucial for the correctness of the measurement when measuring thin layers or multiple layers. For such layer analysis, an X-ray fluorescence radiation of individual elements of a specimen is detected and converted into layer thickness(es) and composition(s). Apparatuses which have an X-ray tube in a housing which is substantially opaque to X-rays are used, with emergence of an X-ray beam being provided through an opening. The extent of the X-radiation is restricted to a specific surface area of the specimen by a collimator. In this surface area, an object of measurement is positioned at a defined distance from the collimator on a table which is movable with respect to the measuring head, comprises an X-ray tube, collimator and the other components required here. The measuring head has, furthermore, a proportional counting tube or a detector, which serves for recording the fluorescence radiation of the irradiated area of the surface.
The distance between the collimator and the surface of the object of measurement has to be set to a specific distance or exact measurement, in order that the fluorescence radiation can be recorded with sufficient intensity.
DE 40 03 757 discloses an adjustment in which the collimator itself is used directly as part of the adjustment. In this case, it is provided that the tip of the collimator is moved against the specimen, with the collimator yielding correspondingly on account of a resilient suspension. Subsequently, further relative movement between the specimen and apparatus is stopped and the device draws the collimator away from the specimen again. The distance between the collimator and the specimen can be set by the amount of spring deflection of the collimator. This apparatus has the disadvantage that damage to the surface can occur. In addition, there are inaccuracies in the setting of the distance on account of production inaccuracies and the paths to be moved along, with a cumulative effect of the errors occurring.
A brochure from the company Veeco Instruments Inc., 1997 edition, likewise discloses an apparatus for measuring thin layers by X-ray fluorescence analysis. In this case it is provided that a beam of an optical recording device is projected into the beam of the X-radiation, in order to be able to view the object of measurement. In the case of this method, a laser beam is used for setting the critical distance for the reproducibility of the measurements. This laser beam falls obliquely on the surface of the object of measurement. During upward and downward movement of the object of measurement, the point of impingement of the laser beam shifts for example from right to left on the surface. Cross wires are superimposed in the recording device and are adjusted to the X-ray beam. As soon as the laser of the laser beam projected onto the surface of the object of measurement coincides with the cross wires, the exact working distance is set. This upward and downward movement of thd object of measurement in relation to the measuring head can be performed manually by an operator, with considerable deviations in the said setting being obtained in these measurement results.
Furthermore, this brochure discloses an automatic laser focusing which is intended to increase the reproducibility of the exact setting. This automatic laser focusing method of setting the measuring distance with respect to a surface of an object of measurement has the disadvantage that the surface impinged is only indistinctly visible in the case of highly reflective surfaces, which leads to an inaccurate height setting. The finite size and unsharpness of the specimen surface impinged by the laser leads to setting errors. Furthermore, an additional laser and corresponding shielding are required.
It is also disadvantageous in the case of both methods mentioned that tilting of the specimen surface cannot be recorded.
The invention is therefore based on the object of providing a method of setting a point of impingement of an X-radiation on an object of measurement defined by a distance of a collimator from the surface of the object of measurement which, on its own, makes an exact setting of this distance possible.
This object is achieved according to the invention by the features of claim
1
. The steps provided according to the invention for carrying out the method allow an automatic setting of the surface of an object of measurement at a defined distance from the collimator to take place, with a high degree of accuracy of reproduction being obtained for the position of the surface of the object of measurement with respect to the collimator. In addition, additional sources of error can be eliminated by recording the changes in brightness of the image points of an image, as is the case for example with laser focusing with regard to the point of impingement. Furthermore, the accuracy of reproduction in comparison with manual focusing can also be significantly improved. By evaluating the changes in brightness of the image points during the changing of the distance between the surface of the object of measurement and the collimator, automatic setting can take place without additional apparatus. For this purpose, the electronic recording device, which has a beam projected into the beam of the X-radiation, is used in order that the exact setting of the distance between the collimator and the object of measurement is carried out. By ascertaining the maximum difference in brightness of the image points of the images recorded, a fixed, defined distance of the surface of the object of measurement from the collimator can be set. The beam of the electronic recording device is advantageously adjusted in such a way that the focal point lies in a measuring plane which is at the exact distance from the collimator. When a maximum difference in brightness is ascertained, it can be ensured that a sharp image is recorded by the recording device, and that, as a result, the defined distance has been set. The advantageous assignment of the change in brightness of the image points in a measuring plane to a Z coordinate makes it possible that, after ascertaining a maximum difference in brightness of the image points of an image while moving over the path, an exact setting of the distance can be carried out by positioning the surface of the object of measurement and the collimator with respect to each other.
The object of the invention is similarly achieved by an alternative method according to the features of claim
2
. The recording of the differences in brightness of the image points of at least one measuring plane and the ascertainment of the maximum take place in analogy with the method according to claim
1
. As a difference from the latter, an assignment of the image in a measuring plane to a Z coordinate is not envisaged. The maximum difference in brightness of the image points of an image is advantageously ascertained and the distance between the surface and the collimator is changed once more, a change in direction being envisaged here. During the changing of this distance, the difference in brightness of the image points of an image in respective measuring planes in turn approaches the maximum. As soon as a comparison establishes that the current maximum coincides with the maximum ascertained when the distance was changed the first time, the changing of the distance is interrupted, whereby a focusing of the image and consequently a defined distance of a collimator from the surface of the object of measurement is set.
According to an advantageous refinement of the invention, it is provided that individual measurements for ascertaining changes in brightness of the image points of an image are carried out during a changing of the distance between the collimator and
Kaiser Karl-Heinz
Rössiger Volker
Helmut Fischer GmbH Institut fur Elektronik und Messtechnik
Kim Robert H.
Yun Jurie
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