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United States Patent |
5,109,396
|
Ohsugi
,   et al.
|
April 28, 1992
|
Total reflection X-ray fluorescence apparatus
Abstract
A total reflection X-ray fluorescence apparatus comprises a base material
having an optically flat surface for totally reflecting X-rays radiated at
a small glancing angle, a first detector such as an SSD for detecting
fluorescent X-rays emerging from a specimen located near the optically
flat surface of the base material and a second detector such as a
scintillation counter for detecting an intensity of an X-rays coming from
the base material.
Inventors:
|
Ohsugi; Tetsuya (Yokohama, JP);
Kyoto; Michihisa (Yokohama, JP);
Nishihagi; Kazuo (Neyagawa, JP)
|
Assignee:
|
Sumitomo Electric Industries, Ltd. (Osaka, JP)
|
Appl. No.:
|
597027 |
Filed:
|
October 15, 1990 |
Foreign Application Priority Data
| Oct 19, 1989[JP] | 1-272123 |
| Oct 19, 1989[JP] | 1-272124 |
Intern'l Class: |
G01N 023/203 |
Field of Search: |
378/44-58,82-86,90
|
References Cited
U.S. Patent Documents
4349738 | Sep., 1982 | Baecklund | 378/49.
|
4649557 | Mar., 1987 | Hornstra et al. | 378/84.
|
4916720 | Apr., 1990 | Yamamoto | 378/46.
|
9642811 | Feb., 1987 | Georgopoulos | 378/84.
|
Primary Examiner: Howell; Janice A.
Assistant Examiner: Porta; David P.
Attorney, Agent or Firm: Stevens, Davis, Miller & Mosher
Claims
We claim:
1. A total reflection X-ray fluorescence apparatus comprising:
a base material having an optically flat surface for totally reflecting
X-rays radiated thereonto at a given glancing angle;
monochromator means for monochromatizing X-rays radiated from an X-ray
source and radiating monochromatized X-rays onto said optically flat
surface at said given glancing angle;
first detection means for detecting fluorescent X-rays emerging from a
specimen located near the optically flat surface of said base material;
and
second detection means for detecting an intensity of X-rays reflected from
said base material.
2. An apparatus according to claim 1, wherein said first detection means
comprise a semiconductor X-ray detector.
3. An apparatus according to claim 1, wherein said second detection means
comprise a scintillation counter.
4. An apparatus according to claim 1, wherein said monochromator means use
a diamond type crystal body, and the X-rays radiated from said X-ray
source are reflected by a crystal surface thereof.
5. An apparatus according to claim 1, wherein said X-ray source comprises a
rotating anode type X-ray source.
6. An apparatus according to claim 1, wherein said monochromator means use
a crystal of lithium fluoride (LiF), and the X-rays radiated from said
X-ray source are reflected by the crystal surface thereof.
7. An apparatus according to claim 1, wherein said monochromator means use
a crystal of ethylenediamine ditartarate (EDDT), and the X-rays radiated
from said X-ray source are reflected by the crystal surface thereof.
8. An apparatus according to claim 1, wherein said monochromator means use
a crystal of pentaerylthritol (PET), and the X-rays radiated from said
X-ray source are reflected by the crystal surface thereof.
9. An apparatus according to claim 1, further comprising a support means
for supporting said monochromator means and for changing a fixing angle of
said monochromator means so as to radiate said X-rays onto said base
material in any wavelength range after monochromatization thereof.
10. An apparatus according to claim 1, wherein said monochromator means
radiate monochrome X-rays having a higher energy than that of an X-ray
absorption spectrum of an element to be detected from said specimen onto
the optically flat surface.
11. An apparatus according to claim 1, wherein said X-ray source uses
tungsten, and said monochromator means use a crystal of lithium fluoride
(LiF).
12. An apparatus according to claim 1, further comprising positioning means
for setting coordinates on the optically flat surface and positioning said
monochromator means.
13. A total reflection X-ray fluorescence apparatus, comprising
a base material having an optically flat surface for totally reflecting
X-rays radiated thereonto and a given glancing angle;
detection means for detecting fluorescent X-rays emerging from a specimen
located near the optically flat surface of said base material;
monochromator means for monochromatizing X-rays radiated from an X-ray
source and radiating monochromatized X-rays onto said optically flat
surface at said given glancing angle; and
wherein said monochromator means radiate monochrome X-rays onto the
optically flat surface, said monochrome X-rays having a higher energy than
that of an X-ray absorption spectrum of an element to be detected from
said specimen.
14. An apparatus according to claim 13, wherein said X-ray source uses
tungsten, and said monochromator means use a crystal of lithium
fluoride(LiF).
15. An apparatus according to claim 7, wherein said monochromator means use
a diamond type crystal body, and the X-rays radiated from said X-ray
source are reflected by a crystal surface thereof.
16. An apparatus according to claim 13, wherein said monochromator means
use a crystal of lithium fluoride(LiF), and the X-rays radiated from said
X-ray source are reflected by the crystal surface thereof.
17. An apparatus according to claim 13, wherein said monochromator means
use a crystal of ethylenediamine ditartarate(EDDT), and the X-rays
radiated from said X-ray source are reflected by the crystal surface
thereof.
18. An apparatus according to claim 13, wherein said monochromator means
use a crystal of pentaerythritol(PET), and the X-rays radiated from said
X-ray source are reflected by the crystal surface thereof.
19. An apparatus according to claim 13, further comprising a support means
for supporting said monochromator means and for changing a fixing angle of
said monochromator means so as to radiate said X-rays onto said base
material in any wavelength range after monochromatization thereof.
20. An apparatus according to claim 13, wherein said first detecting means
comprise a semiconductor X-ray detector.
21. An apparatus according to claim 13, wherein said second detection means
comprise a scintillation counter.
22. An apparatus according to claim 13, wherein said X-ray source comprises
a rotating anode type X-ray source.
23. A total reflection X-ray fluorescence apparatus comprising:
a base material having an optically flat surface for totally reflecting
X-rays radiated thereonto at a given glancing angle;
detection means for detecting fluorescent X-rays emerging from a specimen
located near the optically flat surface of said base material;
monochromator means for monochromatizing X-rays radiated from and X-ray
source and radiating monochromatized X-rays onto said optically flat
surface at said given glancing angle; and
positioning means for setting coordinates on the optically flat surface and
positioning said monochromator means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a total reflection X-ray fluorescence
apparatus used in a trace analysis of particles located near a surface
such as surface contaminations by total reflection X-ray fluorescence.
2. Related Background Art
Total reflection X-ray fluorescence will be briefly described below.
When X-rays are radiated on an optically flat surface at a small glancing
angle, the X-rays are reflected at the same angle as the glancing angle
without being absorbed by a radiated material. In other words, the X-rays
are totally reflected. In this case, if a specimen is placed on the
surface by which the X-rays are totally reflected, since X-rays other than
those radiated on the specimen are totally reflected, fluorescent X-rays
emerging from the specimen can be detected in a state wherein scattered
X-rays can be apparently ignored. Therefore, a spectral measurement with a
high S/N ratio can be attained (Nippon Kinzoku Gakkai Kaiho, vol. 24, No.
11 (1985) pp. 956-961). Such an analysis method is called the total
reflection X-ray fluorescence.
Qualitative/quantitative analysis of the specimen is performed based on the
result of the spectral measurement. As analysis examples, for
qualitative/quantitative analysis of a specimen placed on a wafer surface,
"Progress in X-ray Analysis 19" (Agune Technical Center) pp. 217-226,
Technical Reports of University of Electro-Communications, Osaka, "Natural
Science Edition" 22 (1986), from p. 87and the like are known, and for
qualitative/quantitative analysis of a solution dripped on a wafer
surface, "Progress in X-ray Analysis 19" (Agune Technical Center) pp.
237-249, and the like are known.
According to the prior art technique, it was difficult to make X-rays
radiate at a small glancing angle to the optically flat surface on which a
specimen is placed so as to meet conditions for total reflection of
X-rays.
Also, if X-ray total reflection conditions are satisfied, X-rays enter to a
depth of about 100 .ANG. of a surface portion of an optically flat
surface. For this reason, X-rays detected by the above-mentioned total
reflection X-ray fluorescence include fluorescent X-rays radiated from a
base material having the optically flat surface, and X-rays unique to an
X-ray source target (Mo, W, or the like) and continuous X-rays (white
X-rays). Since those X-rays are detected as a background level of the
analysis result, they induce an increase in detection lower-limit density
of a small amount of an element placed as a specimen on the optically flat
surface of the base material, and also induce a decrease in quantitative
precision of the element.
Further, although the conventional total reflection X-ray fluorescence
apparatus can designate an analysis position on a surface for totally
reflecting X-rays, it cannot be discriminated by only analysis of the
designated position whether the designated region can represent an
attaching state of a particle on the entire surface for totally reflecting
X-rays or corresponds to a region where a large amount of particles is
locally present. For this reason, the surface for totally reflecting the
X-rays is divided into a plurality of regions. The amount of particles
cannot be determined unless analysis is independently performed for all
the divided regions to confirm an attaching state of a particle, resulting
in much time and poor work efficiency.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to eliminate the above
drawbacks.
In order to achieve the above object, an apparatus according to a first
aspect of invention comprises a base material having an optically flat
surface for totally reflecting X-rays radiated at a small glancing angle,
first detection means for detecting fluorescent X-rays emerging from a
specimen located near the optically flat surface of the base material and
second detection means for detecting on intensity of an X-rays coming from
the base material.
Also a method according to a second aspect of the invention comprises the
step of approaching at least one of said optically flat surface and said
X-ray beam so that said X-ray beam can coincide with said optically flat
surface, holding the positions of said optically flat surface and said
X-ray beam just before the intensity of said X-ray beam changes due to the
approaching between said optically flat surface and said X-ray beam,
inclining said optically flat surface in predetermined angle on an axis
intersecting said X-ray beam on said optically flat surface.
Also an apparatus according to a third aspect of the invention comprises
monochromator means for monochromatizing the X-rays radiated from X-ray
source and radiating the monochromatized X-rays onto the optically flat
surface at a small glancing angle instead of the second detection means of
the apparatus according to the 1st invention.
Further, an apparatus according to a fourth aspect of the invention
comprises positioning means for setting coordinates on the optically flat
surface and positioning the base material and a surface inspection system
for performing state inspection of the optically flat surface and
obtaining inspection data of each point on the optically flat surface in
correspondence with the coordinates instead of the second detection means
of the apparatus according to the 1st invention.
According to the total reflection X-ray fluorescence apparatus of the first
aspect of the invention, since intensity of X-rays can be detected by
second detection means, a precise positioning of a base material to X-rays
to be radiated thereon is obtained.
According to the method of the second aspect of the invention, the
positioning of the base material with a predetermined angle can be made
easily.
According to the total reflection X-ray fluorescence apparatus of the third
aspect of the invention, since only X-rays in a specific wavelength
monochromatized by the monochromator means are radiated on the optically
flat surface of the base material on which a specimen is placed,
unnecessary continous X-rays are removed from X-rays to be radiated from
the base material, and background components in a detected spectrum can be
eliminated. Therefore, a small amount of an element can be easily
identified, and its quantitative precision can be improved. Further, it is
possible to identify a particle attached on the surface, a particle buried
in the neighborhood of the surface, and an element of linear material
formed perpendicularly to the surface according to the apparatus.
According to the total reflection X-ray fluorescence apparatus of the
fourth aspect of the invention, supplemental information necessary for
determining an amount of particle on the basis of an analysis result
obtained by the total reflection X-ray fluorescence and supplemental
information necessary for efficiently performing the total reflection
X-ray fluorescence can be obtained by the surface inspection system within
a short period of time. Therefore, analysis with high precision can be
performed with high work efficiency on the basis of this information.
The present invention will become more fully understood from the detailed
description given hereinbelow and the accompanying drawings which are
given by way of illustration only, and thus are not to be considered as
limiting the present invention.
Further scope of applicability of the present invention will become
apparent from the detailed description given hereinafter. However, it
should be understood that the detailed description and specific examples,
while indicating preferred embodiments of the invention, are given by way
of illustration only, since various changes and modifications within the
spirit and scope of the invention will become apparent to those skilled in
the art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing a total reflection X-ray fluorescence
apparatus according to the present invention,
FIGS. 2A to 2C are flow charts showing a method for adjusting glancing
angle of X-rays which can be used for the present invention,
FIG. 3 is a graph showing an analysis result obtained by using the total
reflection X-ray fluorescence apparatus according to the present
invention,
FIG. 4 is a graph showing an analysis result obtained by using a
conventional total reflection X-ray fluorescence apparatus,
FIG. 5 is a schematic diagram of a total reflection X-ray fluorescence
apparatus according to the present invention,
FIGS. 6A and 6B are views showing analysis result by the total reflection
X-ray fluorescence apparatus according to the present invention,
FIGS. 7A, 7B and FIG. 8 are views showing inspection results by a surface
inspection system according to the present invention, and
FIG. 9 is a view showing an inspection result by the surface inspection
system according to the present invention, which result is different from
that shown in FIG. 8.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The first embodiment of the present invention will now be described with
reference to FIGS. 1 to 4.
X-rays radiated from an X-ray source such as an X-ray emission tube are
converted to a collimated X-ray beam by a slit 2, and the X-ray beam is
monochromatized by an monochromator means 3. For X-ray source, a sealed
X-ray tube having Mo as its target and an X-ray source having a W rotating
anode target (see FIG. 2) can be used as an exciting source. The latter
X-ray source can generate higher power of X-rays than the former X-ray
tube since the latter X-ray source uses a rotating target. As the
monochromator means 3, a crystal of lithium fluoride (LiF) can be used,
and X-rays radiated from the X-ray source 1 are reflected (diffracted) by
its crystal plane of (200), thus monochromatizing X-rays. For the
monochromator means 3, a crystal of topaz, Si, NaCl, calcite CaCO3, Ge,
.alpha.-quartz, graphite, InSb and pentaerythritol, etc. can be used. The
X-rays monochromatized are radiated on an optically flat surface of a base
material 6 at a small glancing angle. Note that the monochromator means 3
can change its fixing angle to the glancing angle of X-rays, so that X-ray
monochromatized in any wavelength range after the monochromatization can
be radiated on the base material 6. Therefore, only X-rays unique to a
target used as the X-ray source 1 can be radiated on the base material 6.
A specimen 7 is placed on the surface of the base material 6, which
surface is irradiated with the X-rays. As the specimen 7 is fixed by an
chuck utilizing electrostatic force provided with the base material 6, the
warpage of the base material 6 having a specimen 7 thereon is straightened
to allow the surface of the speciment 7 to be flat. The base material 6 is
placed on a stage 8a of the positioning table 8, and the positioning table
8 is subjected to positioning control by a controller 10.
In this case, the positioning for the specimen 7 may be performed by
positioning the monochromator means 3 controlled by controller 10 instead
of positioning the positioning table 8.
X-rays reflected by the base material 6 are incident on a scintillation
counter 11 via a slit 5. The scintillation counter 11 measures an X-ray
intensity. A prior art type of scintillation counter having a phototube
can be used for the scintillation counter 11. The measured X-ray intensity
is input to a central processing unit 12 comprising, e.g., a CPU. On the
basis of this intensity, a table position control instruction is output
from the central processing unit 12 to the controller 10 to perform
positioning control of the base material 6, thus satisfying X-ray total
reflection conditions.
Next, a method for adjusting a glancing angle of X-ray to a base material
which can be applied for the present invention is explained with reference
with FIGS. 2A to 2C.
The optimized angle of X-rays is about 0.06 degree where the base material
6 having an optically flat surface is a Si wafer for example. The specimen
7 is attached on the optically flat surface of the base material 6. The
positioning of the base material 6 is performed as follows.
First X-rays radiated from an X-ray source are caused to be directly
incident into a scintillation counter 11 and an intensity of the X-rays is
measured by the scintillation counter 11 (FIG. 2A). After that, the
optically flat surface of the base material 6 is coincided with the X-rays
so that the X-rays may contact wholly with the optically flat surface
(FIG. 2B). If the coincidence between them could not be perfectly made,
the intensity of X-rays should be changed due to scattering or absorption
of the X-rays. Therefore, it is possible to make a precise coincidence by
monitoring the intensity changes. After achieving coincidence, the stage
8a of positioning table 8 is precisely inclined by 0.06 degree on an axis
perpendicular to the direction of X-rays radiation on the optically flat
surface so as to obtain the glancing angle of 0.06 degree which meets a
condition of total reflection for X-rays (FIG. 2C). The critical angle
(maximum angle) of .theta.c which meets the condition of total reflection
can be calculated by the following equation;
.theta.c={(5.4.times.10.sup.10)Z.rho./A}.sup.1/2 .lambda.
where Z is an atomic number of constitutional element, A is the atomic
weight and .lambda. is the wavelength of the glancing beam. Generally, an
optimized angle which varies for different materials and at which a
maximum intensity of fluorescent X-rays can be obtained is approximately
one third of the critical angle. It is experimentally confirmed by the
present inventors that the optimized angle for GaAs is 0.09 degree and
that for InP is 0.07 degree for example (Peter Wobrauschek and Hannes
Alglnger, Analytical Chemistry, Vol. 44B, No. 5 (1989) 483).
A semiconductor X-ray detector 13 such as a SSD (Solid State Detector) is
arranged above the base material 6 to oppose the optically flat surface of
the base material 6. The semiconductor X-ray detector 13 utilizes a
phenomenon in which X-rays radiated onto a Si diode applied with an
inverse voltage results in current flow to generate pulses. Concretely, Si
material is doped by Li to form a P-i-n type diode and the P-i-n type
diode is applied with an inverse voltage for use. The Si(Li) itself is
sealed in a vacuum tube and X-rays are incident thereon through a window
made of beryllium foil several microns thick. Since the diode has Li doped
therein, a certain number of electron-hole pairs corresponding to the
energy of X-rays incident on the i-layer of the diode is generated to
output pulses. The energy of the X-rays can be detected by measuring the
wave height (voltage) and the intensity of the X-rays can be detected by
counting the number of pulses.
The SSD published by K. Nishihagi et al in the Extended Abstracts of
Electro Chemical Society of Vol. 89-2 and the Si(Li) type SSD made of Link
Co. Ltd. (detection area: 80 mm.sup.2, diameter: 10 mm) may be used for
the semiconductor X-ray detector 13. The semiconductor X-ray detector 13
detects fluorescent X-rays radiated from the specimen 7 placed on the base
material 6. This detection output is amplified by a preamplifier 15 and a
linear amplifier 16, and is extracted as a pulse output having a peak
value proportional to the magnitude of a fluorescent X-ray energy. The
preamplifier 15 may be directly connected to the semiconductor X-ray
detector 13 to prevent the S/N ratio from lowering. Further, the
semiconductor X-ray detector 13 and the preamplifier 15 may be cooled by
liquid nitrogen in order to reduce thermal noise as much as possible. This
pulse output is converted into a digital output by an A/D converter 17,
and the digital output is accumulated according to energy by a
multichannel analyzer. The accumulated output is then subjected to data
processing in the central processing unit 12.
Further, if air exists on the path of fluorescent X-rays between the
specimen 7 and the semiconductor X-ray detector 13, the detection
intensity is lowered due to scattering and absorption by particulate
matter in the air. In this case, as fluorescent X-rays of Ar existed in
the air is excited by X-rays, it becomes difficult to detect fluorescent
X-rays of Cl and K since their energy values are very close to that of Ar.
Accordingly, it is preferable that the chamber in which the fluorescent
X-rays are measured a vacuum atmosphere. In this case, the degree of
vacuum is preferably below 0.1 Torr. For example, a peak energy intensity
for Si is 0.1 cps where the degree of vacuum is 760 Torr, but the peak
energy intensity increases to 2.2 cps where the degree of vacuum is 0.01
Torr. Since the vacuum atmosphere prevents the energy intensity from
lowering, an efficiency for detecting light elements is improved.
FIG. 3 shows an analysis result of the above-mentioned total reflection
X-ray fluorescent apparatus. This analysis was performed using tungsten
(W) for the X-ray source, and a spectral crystal of lithium fluoride (LiF)
as the monochromator means, and X-rays were spectrally diffracted by a
(200) plane of the spectral crystal. Contamination particles as a specimen
became attached onto an optically flat finished surface of a silicon
wafer, and were analyzed. Note that this analysis was performed by an
energy dispersion type detection method. In FIG. 3, energy values of
detected fluorescent X-rays are plotted along the abscissa, and X-ray
counts (detection frequency) of the respective energy values are plotted
along the ordinate, thus representing an analysis result. Upon evaluation
of this analysis result, peaks of the count numbers appear at energy
values unique to Si, K, Ca, Cr, Fe, Ni, and Zn, respectively, and almost
no continuous X-rays (white X-rays) are detected. Therefore, peaks of
these elements can be prevented from being concealed behind a background
level caused upon detection of continuous X-rays, and these elements can
be easily identified. Since peak areas and corresponding element densities
have predetermined correlations, amounts of elements can be determined by
performing calibration of a specimen whose density is known.
FIG. 4 shows an analysis result by a conventional total reflection X-ray
fluorescent apparatus. In this analysis, molybdenum (Mo) was used for an
X-ray source, and X-rays radiated from the X-ray source were radiated on
an optically flat surface of a silicon wafer without being monochromatized
into monochromatized X-rays. Note that contamination particles attached as
a specimen onto the silicon wafer are not the same as those attached in
FIG. 3. Upon evaluation of this analysis result, peaks of counts appear at
energy values unique to Si, Ar, Cr, Fe, W, and Zn, respectively, and these
elements can be identified. However, in this analysis result, since
detected X-rays include continuous X-rays, these continuous X-rays are
present as a background level of the peak values. For this reason, peaks
other than those of Ar and Fe are not easily discriminated from the
background level, and peaks may be erroneously judged. When a small amount
of element is to be detected, since its peak is concealed behind the
background level, a detection lower-limit density of the small amount of
element is increased, and the corresponding element cannot often be
identified. When a detected element is to be quantitatively measured, a
value from which the background level is subtracted must be used as a peak
area. However, an error occurs depending on the way of setting the
background level, resulting in a decrease in quantitative precision.
In contrast, when the total reflection X-ray fluorescence apparatus
according to the present invention is used, only monochrome X-ray
components in a specific wavelength range monochromatized by the
monochromator means are radiated on the optically flat surface of the base
material on which the specimen is placed. Therefore, continuous X-rays are
removed from the X-rays radiated from the base material, and can be
prevented from being detected by the semiconductor X-ray detector.
Therefore, in the obtained analysis result, since the background level
caused upon detection of continuous X-rays can be greatly reduced, as
shown in FIG. 3, a small amount of an element can be easily identified,
and peak areas can be precisely obtained, thus improving quantitative
precision.
In the above embodiment, the spectral crystal of lithium fluoride (LiF) is
used for the monochromator means. In place of this crystal, a spectral
crystal of, e.g., ethylenediamine ditartarate (EDDT), pentaerythritol
(PET), or the like, or a diamond type crystal body may be used. In
particular, when the diamond type crystal body is used and X-rays are
monochromatized by its (111) plane, X-rays in a wavelength range to be
extracted can be efficiently monochromatized, and line-spectral monochrome
X-ray components with a uniform wavelength can be radiated on the
optically flat surface of the base material. Therefore, the
above-mentioned identification and quantitative precision can be further
improved.
X-rays monochromatized by the monochromator means and radiated on the base
material may be any unique X-rays radiated from the X-ray source as long
as they have a higher energy than that of an X-ray absorption spectrum of
an element to be detected form a specimen. For example, when K.alpha. rays
as unique X-rays of tungsten (W) are monochromatized by the monochromator
means and are radiated on the base material, elements corresponding to
atomic numbers below lanthanum (La) are detected since they have a high
energy value. However, when L.beta.1 rays are monochromatized and radiated
on the base material, only elements having atomic numbers equal to or
smaller than that of zinc (Zn) are detected. Therefore, elements
constituting the base material can be appropriately selected according to
X-rays to be radiated so as not to detect a corresponding element. Thus, a
background level can be further reduced, and analysis precision can be
improved. In particular, the present invention is effective for detection
of a transition-metal element on a GaAs or InP wafer.
The value of the X-ray fluorescence is unique for each element and an
energy gap between elements is small where the elements have light
weights. For example, in case of detecting Na and Mg, the energy value of
K.alpha. rays for Na is 1.0410 keV and that of K.alpha. rays for Mg is
1.2536 keV. Therefore, the energy gap between them is 212.6 eV. In this
case, it is impossible to identify peaks if the semiconductor X-ray
detector 13 does not have a resolution power to be able to identify the
energy gap below 212.6 eV. Accordingly, it is preferable that a
semiconductor X-ray detector 13 with resolution power below 200 eV is used
for identifying light elements. It is generally known that the energy
resolution power of the semiconductor X-ray detector 13 is inversely
proportional to the size of the detecting portion of the semiconductor
X-ray detector 13. For example, a semiconductor X-ray detector 13 with a
size of 150 mm.sup.2 .phi. has a resolution power of 250 eV and a
semiconductor X-ray detector 13 with a size of 80 mm.sup.2 .phi. has a
resolution power of 150 to 180 eV. Accordingly, it is enough to identify
light elements if the size of the detecting portion of the semiconductor
X-ray detector 13 is 80 mm.sup.2 .phi..
The second embodiment of the present invention will be described below with
reference to FIGS. 5 to 9.
The apparatus illustrated in FIG. 5 can be mainly constituted by an X-ray
analysis system A for performing a total reflection X-ray fluorescence,
and a surface inspection system B for inspecting a state of a surface
where the total reflection X-ray fluorescence is performed. In the X-ray
analysis system A, X-rays are radiated from an X-ray source 1 such as an
X-ray emission tube, and are collimated to a fine X-ray beam by a slit 2
as described before. The X-ray beam is radiated on an optically flat
surface of a base material 6 at a small glancing angle after
monochromatization by the monochromater means 3 fixed with a member
inclinable to the X-rays. The base material 6 is, for example, a silicon
wafer, and a specimen 7 is attached to a surface of the base material 6
where X-rays are radiated. The base material 6 is placed on a stage 8a of
a positioning table 8, and is positioned by the positioning table 8. The
positioning table 8 has a predetermined coordinate system (x,y,z), and is
subjected to positioning control by a position controller 10 according to
the coordinate system. Therefore, when the base material 6 is placed on
the stage 8a, a coordinate system is set on the optically flat surface of
the base material 6 for totally reflecting X-rays.
X-rays radiated on the surface for totally reflecting X-rays (total
reflection surface) of the base material 6 are reflected by the surface,
and are incident on a scintillation counter 11 via a slit 5. The
scintillation counter 11 measures an X-ray intensity. The scattered X-ray
intensity is input to a first central processing unit 12 comprising a CPU,
a ROM, a RAM, and the like, and a stage position control instruction is
output based on the input intensity. The base material 6 is positioned on
the basis of the control instruction to satisfy X-ray total reflection
conditions. On the total reflection surface of the base material 6,
elementary analysis of a particle by the total reflection X-ray
fluorescence is performed for a position (region) according to the control
instruction. Information associated with a coordinate position
corresponding to the control instruction at that time is stored in the RAM
of the first central processing unit 12.
A semiconductor X-ray detector 13 such as a SSD (Solid State Detector) is
arranged above the base material 6 to oppose the optically flat surface
(total reflection surface) of the base material 6. The semiconductor X-ray
detector 13 detects fluorscent X-rays radiated from a specimen 7 attached
onto the base material 6. The detection output is amplified by a
preamplifier 15 and a linear amplifier 16, and is extracted as a pulse
output having a peak value proportional to a magnitude of the fluorescent
X-ray energy. The pulse output is converted into a digital output by an
A/D converter 17. The digital output is accumulated by a multichannel
analyzer, and is then subjected to data processing in the first central
processing unit 12.
The total reflection X-ray fluorescence apparatus according to the present
invention has the surface inspection system B for performing surface
inspection of the total reflection surface such as an attaching condition
of the specimen 7 to the total reflection surface of the base material 6,
a distribution and particle size of attached specimen particles, scratches
or defects on the total reflection surface, and the like. The surface
inspection system B obtains inspection information of regarding each point
on the total reflection surface in correspondence with coordinates set on
the surface of the base material 6, and can store the information. The
surface inspection system B comprises a stage 14 on which the base
material 6 is placed, a laser 18 for radiating a laser beam to be radiated
on the base material 6 placed on the stage, a reflection mirror 19 for
reflecting the laser beam to radiate it on the base material 6, a
photomultiplier 20 for detecting a reflection light intensity of the laser
beam reflected by the base material 6, and a second central processing
unit 21 for receiving an output signal digitized by A/D converter 17 from
the photomultiplier 20. The reflection mirror 19 changes its position to
be able to scan the laser beam on the entire total reflection surface of
the base material 6. The laser beam radiated onto the base material 6 is
reflected by the base material 6. If foreign matter is present on the base
material 6 or in the neighborhood of the surface, the laser beam is
scattered by the foreign matter, and an intensity of reflected light is
changed. Therefore, information such as a distribution condition of the
specimen 7 attached to the total reflection surface can be obtained based
on the change in reflected light intensity. The reflected light is focused
by an elliptic reflection mirror 22, and is incident on the
photomultiplier 20. The light incident on the photomultiplier 20 is
converted into an electrical signal according to its intensity, and the
electrical signal is amplified and output. The output signal from the
photomultiplier 20 is converted into digital data by an A/D converter 23,
and is input to the second central processing unit 21. The signal input to
the second central processing unit 21 is processed in correspondence with
the coordinates set on the total reflection surface, and the processed
signal is stored as inspection information such as a distribution
condition, particle size, and the number of particles of the specimen 7
attached to the total reflection surface, the number of particles of the
specimen present in a region subjected to the total reflection X-ray
fluorescence, a ratio of the number of particles to a total number of
particles of the specimen attached to the entire total reflection surface,
scratches or defects on the total reflection surface, and the like. The
surface inspection system B is connected to the X-ray analysis system A
via the position controller 10, as shown in FIG. 5. Note that as the
surface inspection system B, a surfscan 4500 available from TENCOR
INSTRUMENTS can be used.
As described above, in the total reflection X-ray fluorescence apparatus of
the present invention with the surface inspection system B, the base
material 6 is placed on the stage 14 to perform surface inspection of the
total reflection surface of the base material 6 before or after X-ray
analysis is performed for the base material 6 placed on the stage 8a.
Therefore, in order to discriminate whether a region on the base material
6 where the total reflection X-ray fluorescence is performed represents an
attaching state of a particle on the entire total reflection surface or
corresponds to a region where a large amount of particle is locally
present, no time-consuming operation for dividing the total reflection
surface into a plurality of regions and independently performing total
reflection X-ray fluorescence on all the divided regions to confirm an
attaching state and the like of a specimen is required unlike in the
conventional apparatus. A coordinate region most suitable for performing
the X-ray fluorescence can be searched from the inspection result by the
surface inspection system B, and efficient X-ray analysis can be
performed. For example, when the total reflection X-ray fluorescence is to
be performed for a portion having a highest distribution density of the
specimen 7, a region having a highest distribution density is searched
from the inspection result of the surface inspection system B, and the
X-ray analysis can be performed for only this region. After the total
reflection X-ray fluorescence is performed, whether the region on the base
material 6 subjected to the fluorescence represents an attaching state of
a particle on the entire total reflection surface or corresponds to a
region where a large amount of particle is locally present can be easily
discriminated. Furthermore, since a ratio of a specimen attached to a
portion subjected to the analysis to the entire specimen can be obtained
based on the inspection result of the surface inspection system B, the
amount of the specimen 7 can be easily determined with high precision
using this ratio.
In the total reflection X-ray fluorescence apparatus according to the
present invention with the surface inspection system B, as described
above, since the X-ray analysis and the surface inspection can be
performed as a series of operations, opportunity for foreign matter to
become additionally attached to the base material can be reduced, and
analysis with higher precision can be performed.
FIGS. 6A and 6B show analysis results of the X-ray analysis system A
described above, and FIGS. 7A and 7B show inspection results of the
surface inspection system B. FIG. 6A shows an analysis result of the total
reflection X-ray fluorescence as follows. That is, N.sub.2 gas was sampled
from a pipe for introducing N.sub.2 gas to, e.g., a semiconductor
manufacturing device. The sampled N.sub.2 gas was blown on a silicon wafer
used as the base material for two hours to specimen contamination
particles included in the N.sub.2 gas as a specimen, and the total
reflection X-ray fluorescence was performed for the contamination
particles. FIG. 6B shows an analysis result for the silicon wafer surface
before the N.sub.2 gas is blown to attach the contamination particles. In
these figures, energy values of fluorescent X-rays are plotted along the
abscissa, and X-ray counts (detection frequency) of the respective energy
values are plotted along the ordinate. Note that this analysis was
performed by an energy dispersion type detection method.
Upon examination of this analysis result, peaks appear at energy values
unique to Cr, Fe, Cu, and Zn, and corresponding metal elements are
identified by them. Upon comparison with the analysis result shown in FIG.
6B, it can be determined that these metal elements are contained in the
N.sub.2 gas. Since these elements constitute elements of stainless steel
and brass, it can be estimated that contamination sources of the N.sub.2
gas are pipes and their joints. Since the axis of ordinate represents the
counts, a peak area for each element reflects a corresponding element
density, and the amount of the element can be determined by performing
calibration using a specimen whose density is known.
FIG. 7A shows an inspection result of a distribution condition of
contamination particles attached to the surface of the above-mentioned
silicon wafer after the N.sub.2 gas is blown, using the surface inspection
system A, and FIG. 7B shows an inspection result of the silicon wafer
surface before the N.sub.2 gas was blown. As can be seen from these
inspection results, a very small amount of contamination particles are
attached to the silicon wafer surface before the N.sub.2 is gas blown, but
a large amount of contamination particles are uniformly distributed and
attached to the surface of the silicon wafer after the N.sub.2 gas is
blown. These results can demonstrate that these contamination elements are
particles containing the metal elements identified in FIGS. 6A and 6B.
The analysis position in FIG. 6A corresponds to an area having a diameter
of 6 mm at the central portion of the wafer, and it can be determined
based on the inspection result of the surface inspection system B that the
number of contamination particles attached to that portion is about 10% of
the total number of contamination particles. Therefore, if attaching
efficiency of the contamination particles can be detected, a density of
each element contained in the N.sub.2 gas can be calculated based on a
peak area of the corresponding element and the total amount of blown
N.sub.2 gas.
FIG. 8 shows another inspection result of the surface inspection system B,
which is different from that in FIGS. 7A and 7B. This inspection result
was obtained as follows. That is, a magnet was attached to the central
portion of the lower surface of the silicon wafer, and the N.sub.2 gas was
similarly blown on the upper surface. As can be seen from FIG. 8, since
the magnet is attached, contamination particles are concentrated around
the magnet. Contamination particles about 20% of the overall contamination
particles are attached to an area having a diameter of 6 mm at the central
portion of the wafer. Therefore, coordinates of the area where the
contamination particles are concentrated and attached are read out, and
the positioning table 8 is controlled by the position controller 10 so
that the total reflection X-ray fluorescence is performed for this area.
When the total reflection X-ray fluorescence is then performed, an
analysis result having larger peak values than those in the analysis
result shown in FIG. 6A can be obtained, and contamination elements can be
detected with high sensitivity. Also precision of quantitative analysis
can also be improved.
Note that the surface inspection system B can detect the total number of
contamination particles attached to the wafer or the number or particles
in units of particle sizes. FIG. 9 shows these detection results. The
particle sizes of contamination particles can be an important clue to
clear up the cause of contamination.
In the above embodiment, the stage 8a used for the total reflection X-ray
fluorescence and the stage 14 used for surface inspection of the total
reflection surface are independently arranged, and the base material 6 is
transferred between these two stages. Therefore, when the base material 6
is transferred, the base material 6 is preferably transferred in parallel
without changing its orientation so that the coordinates set on the total
reflection surface during the total reflection. X-ray fluorescence can be
directly used in surface inspection of the total reflection surface. That
is, the relative positional relationship between the X-Ray analysis system
A and the base material 6 is the same as the positional relationship
between the surface inspection system B and the base material 6. Even when
different coordinates are set for the X-ray analysis and surface
inspection, a coordinate setting reference need only be provided to the
base material 6, so that the different coordinates can be converted based
on the reference as if common coordinates were set. As the reference, when
a semiconductor wafer is used as the base material 6, a flat orientation
can be used.
Alternatively, the surface inspection system B may be added to the X-ray
analysis system A to have one common stage, so that coordinates common to
the X-ray analysis and surface inspection can be set. In this case, a
transfer process of the base material 6 can be omitted, and additional
attachment of a contamination material on the base material 6 during
transfer can be avoided.
Further, although the surface inspection system B uses a laser 18 and a
photomultiplier 20 to make an inspection by scanning a laser beam from
laser 18 two-dimensionally, a CCD instead of the photomultiplier 20 may be
used to make an inspection by linear scanning with laser beams enlarged in
certain direction and radiated onto the CCD.
Additionally a lamp instead of the laser 18 and a CRT instead of the
phohomultiplier 20 may be used to make an inspection based on image data
obtained by CRT.
From the invention thus described, it will be obvious that the invention
may be varied in many ways. Such variations are not to be regarded as a
departure from the spirit and scope of the invention, and all such
modifications as would be obvious to one skilled in the art are intended
to be included within the scope of the following claims.
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