X-ray or gamma ray systems or devices – Specific application – Fluorescence
Reexamination Certificate
2002-02-20
2003-06-10
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Fluorescence
C378S045000
Reexamination Certificate
active
06577704
ABSTRACT:
The invention relates to an X-ray fluorescence analysis device and to a method for carrying out such analyses and the use of this device. In particular, the device is appropriate to the trace element analysis, and comprises possible application fields, particularly both in the environmental analysis and thin film technology (semiconductor technology).
The highest detection sensitivity with the X-ray fluorescence analysis one achieves in that X-radiation having an angle of incidence at which total reflection occurs is directed upon a sample on a sample carrier wherein the angle of incidence has to be lower than the critical angle &thgr;
c
of the total reflection which is caused by the matter properties of the sample carrier material. The critical angle &thgr;
c
results from the refractive index (n=1−&dgr;+i&bgr;; 1−&dgr;→real part [dispersion], i&bgr;→imaginary part [absorption] of the sample carrier material with &thgr;
c
={square root over (2&dgr;)} (&thgr;
c
[rad] as an radian measure). The greater is the photon energy of the used radiation the lower is the critical angle &thgr;
c
, and being approximately 0.1° with e.g. Mo K type irradiation for a quartz carrier. However, the lower is the angle of incidence &thgr;
c
the greater is the projection of the primary beam section upon the sample carrier. Therefore, the sensitivity of detection (related to the number of atoms to be detected per surface) of the total reflection X-ray fluorescence analysis (TXRF) is essentially limited by the obtained photon flux density of the exciting primary radiation per surface unit.
Increasing the photon flux density of the X-radiation by means of focussing polychromatic or monochromatic radiation is possible but does not merely have any practical significance due to the small angles &thgr;
c
.
With the total reflection the radiation portion refracted within the sample carrier is degraded toward the surface wave exponentially fading into the depth, and the penetrating depth (several nanometers only, int. al. <10 nm) of the X-radiation into the sample is controlled in a dispersive manner. Typically, more than 95% of the incident radiation will be reflected.
By the limitation of the sample carrier volume excited with the sample to a few nanometers a very high signal-to-background ratio is to be written down. The portions of the background generated in this area in the vicinity of the surface are reaching the detector in an unattenuated form, however, and will be consequently measured therewith.
A measuring arrangement based on this findings is described in EP 0 456 897 A1. There, an X-ray source is used wherein the X-radiation thereof is directed upon a sample carrier by means of at least one reflecting unit under the conditions of total reflection. The fluorescence radiation thus excited is measured by a detector, and the sample is respectively analysed. A second detector senses the X-radiation reflected on the sample carrier, and the control of the critical angle of the total reflection is allowed to be readjusted by means of an angle resolved type measurement with respective handling the reflector unit and the sample carrier.
The respective sample is immediately placed on the sample carrier. A periodically repeating sequence of one or more layers made of materials having a different refractive index is formed on the reflector unit. On that occasion, layers are mentioned wherein having such a pair of layers, at least one layer is relatively large-atomic. Thus, so called multi-layer mirrors are used to diffract the X-radiation on the reflector unit. However, the fluorescence excitation exclusively occurs under the conditions of the total reflection on the sample carrier in acceptance of the mentioned and well known drawbacks.
Therefore, it is an object of the invention to increase the detection sensitivity of most different samples.
This object preferably is achieved by the characterizing features of the present invention. Advantageous embodiments and further developments of the solution will be apparent from the description of the invention provided herein.
With the device according to the invention the radiation from an X-ray source for the fluorescence excitation of a sample which can be solid or fluidic is directed upon a multi-layer system serving as a sample carrier. The multi-layer system comprises a number of at least two layers which are made of materials having a different x-ray optical refractive index, respectively. The period thickness d of one period meets the BRAGG relationship in consideration of the incident angle &thgr;
in
of radiation with &lgr; upon the surface of the multi-layer system. The period thickness d and the sequence of individual layers is allowed to be constant for the respective adjacent periods in the multi-layer system wherein in this case it is allowed to be said of periodically built-up multi-layer systems.
In addition, there is also the way to modify the structure of the multi-layer system by changing one or several individual layer thicknesses such that in contrast to the periodical structure a higher reflectivity and/or a lower angular acceptance is achieved with the predetermined &thgr;
m
and wavelength &lgr; of the incident X-radiation photons. In this case it is being said of a periodically constructed multi-layer systems.
With the use of one or several reflector unit(s) it has to be taken account of the period thickness d
j
of each surface element A
j
of the multi-layer system on a reflector unit which has to be selected such that the X-radiation photons reflected from this multi-layer system and directed upon the multi-layer system arranged on the sample carrier meet the BRAGG relationship in the sample location. On that occasion, influencing the angle of incidence of the radiation can take place by moving the sample carrier and/or with at least one reflector unit which is arranged in the optical path between the X-ray source and the surface of the multi-layer system.
The radiation is allowed to be directed upon the used multi-layer system on the sample carrier in consideration of the BRAGG relationship
m*&lgr;=
2
d
eff
*sin &thgr;
m
wherein
m is a whole number 1, 2, 3 . . .
d
eff
=
d
(
1
-
δ
_
sin
2
θ
m
)
wherein
d is a period thickness of the multi-layer system, and
δ
_
=
1
d
∑
i
=
1
m
d
i
δ
i
is a mean value weighed according to the thickness of the real parts of the refractive index of all layers of one period, with an angle of incidence being substantially greater than it is the case with the total reflection. On that occasion, the photon flux density/surface element of the exciting X-radiation can be greater by the factor sin &thgr;
m
/sin &thgr;
c
as it is the case with the total reflection as the reflected radiation portion is in the same procentual range as with the total reflection. Herein, &thgr;
m
is the BRAGG angle of the m
th
maximum which is greater than the critical angle &thgr; of total reflection, and which is substantially determined by the period thickness d of the layer pairs of the multi-layer system, and which is not determined by the &dgr;-values of the layers.
The highest reflectivities of multi-layer systems can be found within the first order (m=1).
Depending on the amount of reflectivity a continuous wave field is forming on the surface of the multi-layer system arranged on the sample carrier which can be employed for the fluorescence excitation of the sample. With an appropriate choice of the period number, period thickness and layer materials in the multi-layer system reflectivities can be achieved which almost are corresponding to those with total reflection(R>90%).
The mulitlayer system is allowed to be formed on a substrate, e.g., with well known methods of thin-film technology (plasma enhanced chemical vapour deposition PLD, chemical vapour deposition technique CVD, sputtering or other). The individual layers and the substrate should be composed
Dunn Drew A.
Fraunhofer-Gesellschaft zur Forderung der ange-wandten Forschung
Kiknadze Irakli
Leydig , Voit & Mayer, Ltd.
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