X-ray or gamma ray systems or devices – Specific application – Fluorescence
Reexamination Certificate
2001-12-07
2003-09-30
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Fluorescence
C378S084000, C252S506000, C501S152000
Reexamination Certificate
active
06628748
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a device and method for analyzing atomic and/or molecular elements by means of wavelength dispersive x-ray spectrometric devices, comprising at least a mirror or focussing device with multi-layer structures, particularly a device wherein fluorescent radiation induced by incident primary x-rays or electron beams from a sample to be analyzed is directed No a mirror or focussing device before the radiation reaches a measuring or analysis detector.
Such apparatus and methods are used in scientific analyses, but also in the industry for detecting atomic and/or molecular elements in various applications for example for detecting or analyzing very small amounts of impurities or disturbances in a sample. A particularly important area of application in the industrial field is, for example, the examination of semiconductor wafers (silicon wafers, germanium wafers), which form the basis for the manufacture of highly integrated semiconductor circuits.
In this process, x-ray or electron beams of any type are directed onto a sample whereby, as part of the radiation reflected from the sample fluorescent light is emitted. The fluorescent light is generated by the incident x-rays by known physical processes. Before the fluorescent light beam reaches a measuring or analysis arrangement for example in the form of a fluorescence radiation selective detector, it is directed onto a suitable crystal from which it is reflected onto the measurement and analysis detector. The crystals are effective as analyzers. The crystals can be manufactured artifically and may consist of thin alternating layers of two or more materials with different x-ray optical properties. The incident fluorescent light radiation is reflected by these crystals but only that part of the radiation for which the Bragg equation
n&lgr;=
2
d
sin &THgr;
is fulfilled. Herein is
λ
(
nm
)
=
1.24
E
(
keV
)
wherein n=a natural number (n=1,2,3,4 . . . ); &lgr; is the wavelength of the x-radiation; d is the periodicity (lattice parameter) of the analysis crystal; 2&THgr; is the refraction angle and E is the energy of the x-radiation. If the effects of the refraction are taken into consideration, which effects are very small for x-radiation, a modified equation is obtained from which the wavelength of the reflected x-radiation can be determined with the giver, angles &THgr; and the lattice parameter d of the analyzer based on the first equation or, respectively, the modification thereof. With a variation of the angle, the wavelength of the reflected rays can be selected in a controlled manner.
The advantage of the artificial crystals which, consisting of many uniformly changing layers—so-called multi-layer structures, is that the materials of the multi-layer can be selected so as to optimize the operation. This is an important advantage of the artificially manufactured multi-layer structures as compared to material crystals.
The intensity of the reflected light depends to a large degree on the material used for the multi-layer structures. It is also possible to vary the lattice parameters within wider limits as it is possible with natural crystals.
It is therefore a particular advantage of the multi-layer structure acting as an analyzing device that the analysis of light elements is facilitated with a uniform intensity and without unhealthy side effects. This is an additional advantage when compared with natural crystals, if natural crystals can be used at all for the analysis of light elements.
So far the multi-layer structure or, respectively, the individual layers of the multi-layer structure has been adjusted to the atomic or molecular element that was expected from the sample being examined. Of high importance in the semiconductor industry is, for example, the determination of the boron content in oxygen-containing materials such as boron phosphorus silicate since this material is generally used during the manufacture of microelectronic components.
So far, a multi-layer structure of molybdenum boron carbide layers has been used for the detection of boron. Such a layer is for example described in U.S. Pat. No. 4,785,470.
Such a molybdenum boron carbide multi-layer and the tungsten carbon multi-layers, which have been used for that purpose, have in the energy range of 183 V only a reflectivity of about 35.4% or, respectively, 10% at an optimal angle of &THgr;=26.5° (with a tungsten carbon multi-layer structure). Furthermore, the use of tungsten carbon multi-layer structures for the detection of boron in samples, which also contain oxygen, has been found problematic. This is essentially because the emission line of oxygen with a value of E=525 eV has essentially three times the energy of the emission line of boron with E=83 eV. Accordingly, the multi-layer structure reflects in accordance with the equation given earlier, the oxygen line in the third Bragg order (n=3) at about the same angle as the boron line in the first Bragg order (n=1). Since the earlier referred to tungsten-carbon multi-layer has for E=525 eV at &THgr;=26.7° in the third order still a reflectivity of 0.24%, a wavelength dispersive separation of the boron and oxygen lines and, consequently, a clear detection of the two elements is insufficient with this multi-layer if at all possible.
The result is improved if molybdenum-boron carbide multi-layers (Mo—B
4
C) are used as they are for an optimum detection of boron in commercial x-ray fluorescence spectrometers. In comparison with a W—C multi-layer a clearly increased reflectivity of 35.4% in the first Bragg order is achieved. At the same time, the reflectivity of such a Mo—B
4
C multi-layer for 525 eV in the third Bragg order is reduced to 0.16% so that the oxygen line is somewhat suppressed.
It is however a disadvantage that W—C— as well as Mo—B
4
C multi-layers have also a significant reflectivity for E=90 eV. This is also very important for the semiconductor industry since the silicon-L-emission lines are about at 90 eV. Computations reveal that a Mo—B
4
C multi-layer with d=8 nm at an angle of &THgr;=25.9° have, in addition to the desirable high reflectivity at E=183 eV for the optimal detection of boron, also an undesirable reflectivity of about 3.2° at E=90 eV. This results with boron-containing samples such as boron phosphor silicate (BPSG) disposed on silicon wafers in an increased background signal which is disadvantageous for the x-ray spectrometric detection limit of boron.
It is the object of the present invention to provide a device and method for an improved x-ray analysis for the detection of boron wherein the device and the method can utilize known means and procedures so that available analysis equipment can essentially be continued to be used and the equipment can be easily and inexpensively installed and operated in research laboratories and industrial plants.
SUMMARY OF THE INVENTION
In a device and a method for the analysis of atomic and molecular elements by way of wavelength dispersive x-ray spectrometric structures including at least one mirror or focussing device having a multi-layer structure onto which fluorescent radiation generated by primary x-ray or electrons beams from a sample to be examined is directed and the reflected fluorescence radiation is supplied to a measuring device for determining the nature of impurities contained in the sample, the multi-layer structure consists of at least a lanthanum layer and a boron carbide layer.
With the device according to the invention, the detection of boron is greatly facilitated particularly in the energy range of 180 eV. The particularly favorable x-ray optical properties of the materials forming the layer pairs such as lanthanum and boron carbide provide, in comparison with the earlier mentioned known analyses, for an increased reflectivity for the boron line as well as a substantially improved suppression of the oxygen-K— as well as the silicon-L-li
Bormann Rüdiger
Michaelsen Carsten
Bach Klaus J.
Dunn Drew A.
GKSS-Forschungszentrum Geesthacht GmbH
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