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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a stress evaluation apparatus for
evaluating the physical property of a substance such as semiconductor and
more particularly, it relates to a stress evaluation apparatus for
evaluating internal stress existing in the substance.
2. Description of the Prior Art
In a conventional method of evaluating stress of a semiconductor member or
the like which is manufactured through a process involving thermal
expansion and internally provided with thermal stress, employed is a
stress evaluation apparatus such as that disclosed in "Applied Physics",
The Japan Society of Applied Physics, Vol. 50, No. 1, 1981;
"Characterization of Semiconductors by Laser-Raman Spectroscopy" p. 69.
FIG. 1 is a schematic block diagram showing a conventional stress
evaluation apparatus for measuring the Raman spectrum of scattered light.
Referring to FIG. 1, a light source 1 supplys excitation light such as
Ar.sup.+ laser beam or He-Ne laser beam. Excitation light 9 supplied from
the light source 1 is reflected by a mirror 10, and converged by a lens
11a on a measuring point of a substance 3 to be evaluated. Scattered
light 12 from the measuring point of the substance 3 to be evaluated is
converged by an another lens 11b, and spectro-analyzed by a spectroscope
13 such as a double monochromator. Spectro-analyzed light is detected by a
detector 14, inputted in a microcomputer 15, and transmitted to a recorder
16.
In the conventional stress evaluation apparatus of the above structure, the
Raman spectrum is measured in the following manner: The excitation light 9
supplied from the light source 1 is changed in direction by the mirror 10
and focused by the lens 11a, to be converged on/applied to the measuring
point of the evaluated substance 3. The scattered light 12 from the
measuring point of the evaluated substance 3 passes through the lens 11b,
to be converged on an entrance slit of the spectroscope 13. The light is
spectro-analyzed by the spectroscope 13 so that the Raman spectrum thereof
is detected by the detector 14, to be inputted in the microcomputer 15 as
an electric signal and stored in the same. Then the electric signal is
subjected to data processing by the microcomputer 15 and transmitted to
the recorder 16, which in turn records the Raman spectrum as a waveform or
a peak wave number thereof.
Measurement of the Raman spectrum through the aforementioned apparatus is
now described with reference to a flow chart. FIG. 2 is a flow chart
showing conventional Raman spectrum measurement processing.
A substance to be evaluated is set at a step 601 and an optical system
including lenses, mirror etc. is adjusted at a step 602, while conditions
for Raman spectrum measurement are set at a step 603. Then, the Raman
spectrum of light scattered from the evaluated substance is measured at a
step 604. Description is now made on the routine of the Raman spectrum
measurement processing with reference to another flow chart.
FIG. 3A is a flow chart showing a spectrum measurement routine in case of
employing a photomultiplier as the detector 14.
First, the wave number of a spectroscope is set at a measurement start wave
number .omega..sub.1 at a step 701. Referring to a step 702, Raman
scattering intensity corresponding to the wave number is measured. In this
case, intensity of Raman scattered light is converted into a voltage
signal by the photomultiplier, to be measured. At a step 703, data on the
Raman scattering intensity thus obtained and the set wave number of the
spectroscope are transferred to a microcomputer. Referring to a step 704,
these data are A-D converted in the microcomputer to be stored in a memory
as digital signals. At a step 705, the wave number of the spectroscope is
compared with a measurement end wave number .omega..sub.2, so that the
wave number of the spectroscope is increased by .DELTA..omega. at a step
706 if the same is smaller than the measurement end wave number
.omega..sub.2. Measurement of Raman scattering intensity corresponding to
each wave number is repeated as shown in FIG. 3B until the wave number of
the spectroscope exceeds the measurement end wave number .omega..sub.2, to
be stored in the memory of the microcomputer as a digital signal. When the
wave number of the spectroscope exceeds the measurement end wave number
.omega..sub.2, the process is advanced to processing as shown in FIG. 2.
At a step 605, a spectral waveform is outputted to a recorder on the basis
of the data stored in the memory. Thereafter a peak wave number is read
from the outputted spectral waveform at a step 606. Although the peak wave
number is read from the recorded spectral waveform by an operator, the
peak wave number value may be calculated by the microcomputer to be
outputted to the recorder.
Description is now made on a method of evaluating internal stress existing
in a substance from the Raman spectrum measured in the aforementioned
manner. Raman scattered light results from excitation light striking the
evaluated substance and partially losing its energy as vibration energy
for component atoms and molecules etc. of the substance, to be different
in wavelength from the original excitation light. The energy variation
corresponds to the energy of lattice vibration and molecule vibration of
the evaluated substance, and depends on stress existing therein. This
variation corresponds to change in wave number in a peak of the measured
Raman spectrum. FIG. 4 shows such a phenomenon with respect to silicon,
for example. Referring to FIG. 4, a one-dot chain line 80 denotes the peak
wave number of the Raman spectrum of single crystal silicon having no
stress, which peak wave number is 520.5 cm.sup.-1. However, in case of
silicon internally having stress such as SOI (silicon on insulator:
polysilicon deposited on silicon oxide) structure recrystallized by
irradiation of laser beam, the peak number of its Raman spectrum as
measured is shifted to a lower wave number side as shown by a solid line
81. This is because tensile stress exists in the SOI structure. Further,
compressive stress exists in SOS (silicon on sapphire), which is
polysilicon deposited on a sapphire substrate, and hence the peak wave
number thereof is shifted to a higher wave number side as shown by a
dotted line 82.
Thus, stress existing in a substance is evaluated through the fact that the
stress corresponds to variation in peak wave number of the Raman spectrum.
As hereinabove described, stress existing in a substance is generally
evaluated through difference between peak numbers of Raman spectra of a
substance having no stress and the same substance internally having
stress. However, such a value is influenced not only by the value of the
stress but also by temperature difference in the substance. The results of
measurement of relation between the peak wave numbers of Raman bands of
silicon samples and sample temperatures are described in Physical Review
B, Vol. 1, No. 2, pp. 638-642 (1970): "Temperature Dependence of Raman
Scattering in Silicon" and Applied Physics Letters, Vol. 41(11), pp.
1016-1018 (1982): "Temperature Dependence of Silicon Raman Lines". For
example, when the output power of excitation light applied to the same
measuring point of an evaluated substance is changed, the peak wave number
of the measured Raman spectrum is varied as shown in FIG. 5. This is
because the temperature of the evaluated substance is varied with the
output power of the excitation light. Such a phenomenon may occur in case
of evaluating a substance having sectional structure as shown in FIG. 6,
even if excitation light of the same output power is employed. When a
silicon thin film 101 is irradiated with excitation light, temperature
rise by the irradiation is increased since a silicon oxide film 102 has
low thermal conductivity. When a silicon substrate 103 is irradiated with
the excitation light of the same output power, heat by the irradiation is
diffused in the interior of the silicon substrate 103 and hence
temperature rise thereof is small as compared with that of the silicon
thin film 101. Therefore, even if the silicon thin film 101 internally has
stress of the same level as the silicon substrate 103, difference is
caused in peak wave numbers of the Raman spectra employed for stress
evaluation since the two members are different in temperature rise by
irradiation from each other.
SUMMARY OF THE INVENTION
The present invention has been proposed to overcome the aforementioned
disadvantage, and an object thereof is to provide a stress evaluation
apparatus which can correct variation of peak wave numbers of Raman
spectra caused by temperature difference in measuring points of a
substance to be evaluated, in order to evaluate stress in high accuracy.
The stress evaluation apparatus according to the present invention is
adapted to evaluate stress existing in a substance by difference between
peak wave numbers in Raman spectra of scattered light. The inventive
stress evaluation apparatus comprises:
(a) a light source for emitting excitation light;
(b) an entrance optical system for guiding the excitation light to
measuring points of a substance to be evaluated;
(c) a scatter optical system for focusing scattered light from the
measuring points;
(d) scattered light measuring means for measuring peak wave numbers in
Raman spectra of the scattered light;
(e) temperature change means for changing temperatures of the measuring
points;
(f) statistic processing means for statistically processing a plurality of
measured values of the peak wave number varied with temperature change of
each measuring point; and
(g) arithmetic means for deciding relation between the temperature change
of the measuring point and the variation in peak wave number on the basis
of statistic values calculated by the statistic processing means to obtain
a peak wave number at a prescribed reference value.
The temperature change means in the present invention is adapted to change
the temperatures of the measuring points of the evaluated substance. Since
a plurality of peak wave numbers are measured as those varied with
temperature change of each measuring point, the measured values are
statistically processed per measuring point. Relation between temperature
change of the measuring point and the variation in peak wave number is
decided by the values statistically processed per measuring point. Thus,
the peak wave number at the prescribed reference value can be obtained by
the said relation. Consequently, the obtained peak wave number can be
employed for stress evaluation as data released from influence by
temperature change of the measuring point.
According to the present invention, a plurality of Raman spectra of the
substance to be evaluated are measured at different temperatures per
measuring point to perform statistic/arithmetic processing for eliminating
influence exerted by variation in peak wave number caused by temperature
change, thereby to obtain data through which stress can be evaluated in
high accuracy.
These and other objects, features, aspects and advantages of the present
invention will become more apparent from the following detailed
description of the present invention when taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram showing a conventional stress
evaluation apparatus;
FIG. 2 is a flow chart showing Raman spectrum measurement processing
through the conventional stress evaluation apparatus;
FIG. 3A is a flow chart showing a Raman spectrum measurement routine;
FIG. 3B is a diagram for illustrating FIG. 3A;
FIG. 4 illustrates variation in peak wave number of Raman spectra caused by
stress existing in substance;
FIG. 5 illustrates dependency of peak wave number of Raman spectrum on
output power of excitation light;
FIG. 6 is a sectional view showing a substance being in structure varied in
temperature change with irradiation of excitation light;
FIG. 7 is a general block diagram functionally showing a stress evaluation
apparatus according to an embodiment of the present invention;
FIG. 8 is a schematic block diagram showing exemplary structure of the
stress evaluation apparatus according to the present invention;
FIGS. 9 and 10A are flow charts showing Raman spectrum measurement
processing through the inventive apparatus; and
FIGS. 10B and 10C are diagrams for illustrating FIG. 10A.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 7 is a general block diagram functionally showing an embodiment of a
stress evaluation apparatus according to the present invention. As obvious
from FIG. 7, excitation light supplied from a light source 1 passes
through an entrance optical system 2 to be applied on a substance 3 to be
evaluated. The light scattered by the evaluated substance 3 is converged
by a scatter optical system 4, to be measured by scattered light measuring
means 5. The scattered light measuring means 5 is formed by
spectro-analyzing means 5a, detecting means 5b and analyzing means 5c. The
scattered light is spectro-analyzed by the spectro-analyzing means 5a to
be detected as a Raman spectrum by the detecting means 5b, and the
analyzing means 5c analyzes the same to obtain the peak wave number of the
Raman spectrum. On the other hand, temperature change means 8 is adapted
to change the temperature of a measuring point of the evaluated substance
3. Relation between the temperature change and variation of the peak wave
number obtained by the analyzing means 5c is statistically processed by
statistic processing means 6, so that the relation is decided and the peak
wave number at a prescribed reference value is obtained by arithmetic
means 7.
FIG. 8 is a schematic block diagram showing exemplary structure of a stress
evaluation apparatus based on the general block diagram as shown in FIG.
7.
Referring to FIG. 8, excitation light 9 supplied from a light source 1 is
reflected by a mirror 10, and converged by a lens 11a on a measuring point
of a substance 3 to be evaluated. Scattered light 12 from the measuring
point is converged by an another lens 11b, and spectro-analyzed by a
spectroscope 13. Spectro-analyzed light is detected by a detector 14,
inputted in a microcomputer 15 and transmitted to a recorder 16.
In the aforementioned stress evaluation apparatus, the output power of the
excitation light 9 supplied from the light source 1 is controlled by an
output controller 17. The excitation light 18 thus controlled in output
power is changed in direction by the mirror 10 and focused by the lens
11a, to be converged on/applied to the measuring point of the evaluated
substance 3. The scattered light 12 from the evaluated substance 3 is
focused by the lens 11b, to be incident on a slit of the spectroscope 13
and spectro-analyzed. The Raman spectrum of the spectro-analyzed light is
detected by the detector 14 and converted into an electric signal, to be
stored in the microcomputer 15. Thus, a Raman spectrum is measured with
respect to a measuring point. The output power of the excitation light 9
is changed by the output controller 17, in order to measure a plurality of
Raman spectra corresponding to different output power of the excitation
light per measuring point. In this embodiment, the microcomputer 15
supplies indication for output change of the excitation light 9 to the
output controller 17. The plurality of Raman spectra thus measured are
stored as electric signals in the microcomputer 15. The electric signals
are analyzed in order to obtain peak wave numbers of the respective Raman
spectra. Thereafter the respective peak wave numbers corresponding to the
output power of the excitation light are statistically processed as shown
in FIG. 5. Relation between the output change of the excitation light and
the variation in peak wave number is decided through a regression line or
spline function as shown in FIG. 5 per measuring point of the evaluated
substance. A peak wave number at a prescribed reference value is obtained
per measuring point through the decided relation, as data for stress
evaluation/comparison between measuring points. In this embodiment, a peak
wave number at an excitation light output power of 0 mW is obtained
through extrapolation, as shown in FIG. 5. The aforementioned statistic
processing and arithmetic are performed in the microcomputer 15, so that
the results thereof are transferred to the recorder 16 to be recorded
therein.
Description is now made on Raman spectrum measurement through the
aforementioned inventive stress evaluation apparatus, with reference to
flow charts. FIGS. 9 and 10A are flow charts showing Raman spectrum
measurement processing in the apparatus according to the present
invention.
First, a substance to be evaluated is set with reference to a step 301 and
an optical system including lenses, mirror etc. is adjusted at a step 302
while conditions for Raman spectrum measurement are set at a step 303.
Then, excitation light is set at a measurement start output power V.sub.0
at a step 304. Then the Raman spectrum is measured at a step 305 along the
aforementioned spectrum measurement routine as shown in FIG. 3A.
Completion of this processing means that the Raman spectrum corresponding
to an output power of the excitation light is measured in a given
measuring point. With reference to a step 306, the output power of the
excitation light is compared with a measurement end output power V.sub.f,
so that the output power of the excitation light is increased by .DELTA.V
at a step 307 if the same is smaller than the measurement end output power
V.sub.f. Measurement of Raman spectra corresponding to respective output
power is repeated until the output power of the excitation light exceeds
the measurement end output power V.sub.f, so that the Raman spectra are
stored in a memory of a microcomputer as digital signals. When the output
power of the excitation light exceeds the measurement end output power
V.sub.f, the process is advanced to processing as shown in FIG. 10A. At a
step 401, the Raman spectrum data at respective output power of the
excitation light, being stored in the memory, are analyzed to obtain a
peak wave number .omega..sub.P as shown in FIG. 10B. In this embodiment,
Lorentz fitting is performed on respective Raman intensity data shown by
symbol x in FIG. 10B, in order to obtain the peak wave number
.omega..sub.P from the parameter of an optimum Lorentzian line. At a step
402, the peak wave number .omega..sub.P thus obtained is statistically
processed in correspondence to each output power of the excitation light,
to decide relation between the same. For example, an optimum regression
line is obtained by a method of least squares as shown in FIG. 10C.
Referring to a step 403, the regression line as shown in FIG. 10C is
extrapolated thereby to obtain a peak wave number .omega..sub.P0 at an
output power 0 mW of the excitation light.
Thus, the peak wave number of a measuring point is obtained to serve as
data to be compared. All of the above processing can be realized by the
microcomputer.
Although the output controller 17 is employed upon emission of the
excitation light in order to control the output power of the excitation
light in the aforementioned embodiment, the output power of the light
source 1 may be controlled in place. Further, although the temperature at
the measuring point of the evaluated substance 3 is changed by controlling
the output power of the excitation light through the output controller 17,
light other than the excitation light, such as heating light 19, may be
externally applied to the evaluated substance 3 in order to change the
temperature of the measuring point. In addition, the light source is not
restricted to a laser beam source, but may be formed by any other light
source so far as the same is suitable for Raman spectrum observation.
Needless to say, the inventive apparatus is not restricted to evaluation
of semiconductor, but may be applied to evaluation of various other
substances. Further, the arithmetic may be performed through a method
other than the extrapolation as shown in the embodiment.
Although the present invention has been described and illustrated in
detail, it is clearly understood that the same is by way of illustration
and example only and is not to be taken by way of limitation, the spirit
and scope of the present invention being limited only by the terms of the
appended claims.
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