X-ray or gamma ray systems or devices – Specific application – Fluorescence
Reexamination Certificate
1999-07-13
2001-01-09
Church, Craig E. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Fluorescence
C378S050000
Reexamination Certificate
active
06173037
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of examining a sample by means of X-ray fluorescent analysis, which sample comprises a substrate which is provided with a thin layer and contains a first chemical element, said thin layer containing the same chemical element and a second chemical element which does not form part of the substrate, which method includes the following steps: irradiation of the sample by means of primary X-rays, measurement of the intensity of comparatively hard X-ray fluorescent radiation and of comparatively soft X-ray fluorescent radiation, both types of radiation being generated in the sample in response to the irradiation by means of the primary X-rays. The invention also relates to an apparatus for carrying out said method.
DESCRIPTION OF PRIOR ART
A method of this kind is known from Japanese patent No. 2706601. The cited Japanese patent describes an X-ray fluorescent analysis method for determining the thickness of a thin layer on a substrate and for determining the concentration of a chemical element present in said thin layer. To this end, the sample is irradiated by means of X-rays (primary radiation) emanating from an X-ray source. The sample is formed by a substrate which consists of a first chemical element, in this case being silicon, and on which there is provided a thin layer which consists of the first chemical element (i.re. silicon) and a second chemical element, in this case being tungsten. In the sample the primary radiation generates X-ray fluorescent radiation which contains characteristic radiation of the chemical elements present in the specimen, so silicon and tungsten. This characteristic radiation comprises comparatively hard X-rays (for example having a wavelength of the order of magnitude of from 0.02 to 1 nm) as well as comparatively soft X-rays (for example, having a wavelength of the order of magnitude of from 2 to 15 nm).
According to the known method the apparatus response is determined by means of known reference samples. Using the apparatus response, the layer thickness and the concentration of one of the chemical elements in the layer, for example tungsten, is determined on the basis of the measured intensities of the comparatively hard as well as the comparatively soft fluorescent radiation. Such determination is performed by means of a calculation method which is known per se for the theoretical calculation of the intensity of X-ray fluorescent radiation; this calculation method is also known as the “fundamental parameter method”.
(This theoretically calculated intensity of X-ray fluorescent radiation is the intensity which occurs immediately after the radiation emanates from the sample; therefore, the effect of the entire measuring channel from the sample to the detector has not been taken into account therein.) According to the cited Japanese patent the combination of said two types of X-ray fluorescent radiation (i.e. the comparatively hard and the comparatively soft fluorescent radiation) is formed by one of the combinations of X-ray lines K with M, K with N and L with N of tungsten, or one of the combinations of X-ray lines K with M, K with N and L with N of silicon.
Even though the known method is capable of determining the thickness of the thin layer as well as the concentration of one of the chemical elements present therein, it is not capable of providing other information concerning these determinations. It is notably not possible to obtain an idea of the accuracy, and hence of the reliability, of the determinations, or to obtain an impression of a concentration gradient, if any, of one of the chemical elements present in the thin layer.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method of the kind set forth which also enables further information to be obtained in respect of the determination executed, for example the degree of accuracy of determination of the layer thickness and/or the concentration in the thin layer or a concentration gradient present in said layer.
To this end, the method according to the invention is characterized in that it also includes the following steps: measurement of the intensity of X-ray fluorescent radiation whose wavelength deviates from said comparatively hard and comparatively soft X-rays, generated in the sample in response to the irradiation by means of the primary X-rays, determination of the thickness of the thin layer and/or the concentration of the first or the second chemical element on the basis of the measured intensities of said three types of X-ray fluorescent radiation, determination of the reliability and/or the accuracy of the determination of at least one of said two quantities (thickness and concentration) on the basis of the measured intensities of said three wavelengths of the X-ray fluorescent radiation.
The invention is based on the recognition of the fact that further information as regards the determinations and/or the sample can be obtained by measurement of the intensity of at least one wavelength other than said two wavelengths. The intensity of said third wavelength can be simply derived when the equipment for measuring said two wavelengths has already been provided. Merely the calculation of the desired quantities will then require some adaptations in order to derive the additional information; this will be explained on the basis of the embodiment according to the invention.
In a further version of the method according to the invention, the wavelength of said X-ray fluorescent radiation whose wavelength deviates from said comparatively hard and comparatively soft X-rays lies between the wavelengths of said comparatively hard and comparatively soft X-rays. Consequently, in many cases it is possible to utilize the same equipment of the X-ray spectrometer, notably the analysis crystals and the X-ray detector, as used for the measurement of the intensity of the comparatively hard and the comparatively soft X-rays. Furthermore, it may be assumed that the comparatively hard X-rays contain information mainly concerning the layer thickness and that the comparatively soft X-rays contain information mainly concerning the concentration. If the third wavelength were chosen so as to have the same order of magnitude as said comparatively hard or comparatively soft X-rays, only little additional information would be obtained. As a result of said choice, therefore, it is achieved that as much additional information as possible concerning the thin layer is obtained.
The invention will be described in detail hereinafter with reference to the Figures. Therein:
REFERENCES:
patent: 2706601 (1998-01-01), None
Church Craig E.
Piotrowski Tony E.
U.S. Philips Corporation
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