X-ray or gamma ray systems or devices – Specific application – Fluorescence
Reexamination Certificate
1999-09-24
2001-07-17
Dunn, Drew (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Fluorescence
C378S045000, C378S049000
Reexamination Certificate
active
06263042
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an apparatus for X-ray analysis, including a sample location for accommodating a sample to be analyzed, means for generating X-rays in the sample, a position-sensitive detector for detecting X-rays generated in the sample, a focusing X-ray mirror which is arranged in the beam path between the sample and the detector in order to focus X-rays emanating from the sample on the detector in the form of a line focus, the detector and the X-ray mirror being arranged relative to one another in such a manner that a position-sensitive direction of the detector extends parallel to the line focus.
An apparatus of this kind is known from the published international patent application No. WO 97/13142. The X-ray analysis apparatus disclosed therein includes a fluorescence unit with a focusing X-ray mirror and a position-sensitive detector (PSD) for detecting fluorescent radiation generated in a sample to be examined. The beam of fluorescent radiation emanating from the exposed area on the specimen diverges in two directions (in the plane of drawing as well as in the direction perpendicular thereto). This beam is captured by the focusing X-ray mirror whose surface is shaped as a cylinder at that area. By virtue of this shape it is achieved that the beam is focused to a given extent, so that a significant part of the beam lands on the PSD. The focusing X-ray mirror then serves as an analysis element, i.e. every position on the PSD corresponds to a given position on the mirror, which itself corresponds to a given wavelength of the fluorescent radiation.
The (known) operation of such an analysis element (usually a monocrystal or a multilayer mirror which is known per se) is based inter alia on the well-known Bragg relation 2d.sin
=n&lgr; (d=spacing of the reflecting planes of the crystal or the mirror,
=angle of incidence of the X-rays on the reflecting planes, n=the order of the reflection, and &lgr;=the wavelength of the X-rays). This relation demonstrates that only radiation which is incident on the analysis element at the correct angle
is reflected.
Because of the proportioning and the positioning of the X-ray mirror relative to the sample in the known apparatus, a relation exists between the reflected wavelength and the position on the PSD. The intensity of the incident radiation is determined in each element of the PSD; thus, reading out all elements of the array yields the intensity spectrum as a function of the wavelength of the fluorescent radiation.
In contemporary X-ray analysis there is a need for determination of the properties of thin layers on a sample, such as the layer thickness and the concentration of a given chemical element in a layer of an integrated electronic circuit (IC). To this end, according to a generally known method the sample is irradiated by means of exciting radiation so that fluorescent radiation is generated in the sample. Information concerning said layer properties can be acquired by measurement of the fluorescent radiation. However, using the conventional X-ray fluorescence (XRF) method, one measurement can yield only information concerning the product of the concentration and the layer thickness, but no information concerning each of these quantities individually. The latter information can be acquired by means of an existing detection method which is known as Grazing Exit XRF (GEXRF). In the case of GEXRF the sample (for example, an IC) has a smooth and plane surface. The intensity of the fluorescent radiation of one given, desired wavelength (notably a characteristic wavelength of a chemical element which is relevant to the analysis) is then measured as a function of the exit angle relative to the surface of the sample; in this respect only small exit angles (Grazing Exit) are of importance, i.e. angles of the order of magnitude of from 0° to approximately 7°. In order to measure the intensity variation as a function of the exit angle, it is necessary to scan the angular range by means of a slit which must be very narrow because of the very severe requirements imposed on the angular resolution in the case of GEXRF. Consequently, this narrow slit transmits only a low X-ray power, so that the duration of the measurement is comparatively long. This is a drawback, notably because such measurements will usually be performed for the manufacture of ICs where the throughput time in the (comparatively expensive) dust-free manufacturing rooms represents an important production factor.
Because of the tendency towards the study of samples with ever smaller details, as is the case for ICs, there is also a need for realizing a high positional resolution in GEXRF. This would be possible only by measuring exclusively the fluorescent radiation originating from the desired small region on the sample, which means that fluorescent radiation is generated only in said small region. However, this has the drawback that the overall power of the fluorescent radiation is also significantly reduced, so that these measurements would require even more time. The gravity of this problem will be illustrated on the basis of the following numerical example.
X-rays are generated in a conventional X-ray tube X-rays while assuming that the X-ray power P
x
delivered by the anode is proportional to the electric power P
e
taken up by the tube with a proportionality factor c; this can be written as P
x
=c.P
e
. The anode emits this X-ray power in a solid angle 2&pgr; (i.e. in a semi-sphere). If only a part in a solid angle &OHgr; thereof s used, the X-ray power obtained must be multiplied by &OHgr;/(2&pgr;) in order to obtain the useful X-ray power P
s
in the sample region of interest. This can be written in the form of a formule:
P
s
=
cP
e
Ω
2
π
(
1
)
Some possibilities for forming an exciting spot on the sample can now be compared; the proportionality factor c is then irrelevant, because its value is the same for all possibilities considered.
For a conventional fluorescent tube it holds that &OHgr;=1.6 staradian. This value has been determined on the basis of the fact that the X-ray beam formed by such a tube typically has half an angle of aperture &agr; amounting to approximately 41.5°; this value of &OHgr; follows directly therefrom according to the relation &OHgr;=2&agr;(1−cos&agr;). Using a typical value P
e
=4 kW, it follows therefrom that P
s
=10
3
c.
In order to realize the desired small exciting spot (for example, of the order of magnitude of 1 mm), the use might be considered of a microfocus tube which typically has a focal spot of approximately 50 &mgr;m on the anode and an electric power P
e
amounting to 40 W. In this tube the distance from the focal spot to the sample typically amounts to 22 mm, resulting in a solid angle value &OHgr; of 1.6×10
−3
staradian for a desired spot size of 1 mm. Using the values thus determined there is obtained p
s
=10
−2
c, being 10
5
times smaller than that for a conventional fluorescent tube. It will be evident from the foregoing numerical examples that this choice does not offer a solution to the described problem concerning the long measuring times.
SUMMARY OF THE INVENTION
The apparatus which is known from said international patent application does not offer a solution to these problems either, because this apparatus is arranged to execute a wavelength-dispersive measurement and not to execute an angular scan.
It is an object of the invention to provide an X-ray analysis apparatus which is suitable for picking up an intensity distribution of fluorescent radiation as a function of the exit angle in GEXRF conditions, without the measuring times becoming inadmissibly long.
To achieve this, the apparatus of the kind set forth is characterized in that the X-ray mirror is proportioned and arranged relative to the surface of the sample in such a manner that the mirror captures the X-rays which emanate from the sample surface in grazing exit conditions, and that the direction o
De Bokx Pieter K.
den Hartog Sander G.
Dunn Drew
U.S. Philips Corporation
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