JPH05164613A - Fourier-transformation infrared spectroscopic measuring method - Google Patents

Abstract

PURPOSE:To make it possible to measure sensitivity highly reliably by using a highly sensitive photoconductive semiconductor detector at low temperature, and arranging a bandpass filter, which transmits the light in an infrared absorbing band noted in an incident light path. CONSTITUTION:The dispersed light from a continuous infrared light source 1 is made to pass through an aperture 2, made to be the parallel light with a collimator mirror 3, cast into a Michelson interferometer 4 and split into the reflected light and the transmitted light with a beam splitter 5. The lights are reflected from a fixed mirror 6 and a movable mirror 7, respectively, and the waves are combined and interfered with the beam splitter 5. The liquid are condensed at a concave mirror 8, transmitted through a band pass filter 9, transmitted through a sample 10 and detected with a MCT detector 12, which is cooled with liquid nitrogen by way of an aperture 11. Carbon gas and steam in a sample chamber are purged with nitrogen gas, and infrared absorption is eliminated. As the filter 9, the filter, whose transmitted wave-number region agrees with the wave-number region of the infrared absorbing band noted in the sample 10, is used. Thus, the light of only the wave-number region of the noted spectrum is transmitted at a high transmittance. The light of the unnecessary wave-number region is cut, and the high sensitivity property of the detector 12 can be utilized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for performing Fourier transform infrared spectroscopic measurement on a sample having an infrared absorption band.

[0002]

2. Description of the Related Art In an FT-IR (Fourier transform infrared spectroscopy) apparatus, a TGS (triglycine sulfate) detector is generally used as an infrared light detector. This is because the TGS detector can be used at room temperature, has a wide measurement wavelength range, and has a linear output signal with respect to the received light intensity of the detector.

On the other hand, an FT equipped with an infrared microscope in the optical path
-In the IR device, weak infrared light from a minute portion of the sample is used, and since the optical path is long and is reflected by many mirrors arranged in the optical path, the detected light intensity is further increased by the loss of light. It becomes weak. In such a case, since a good low noise spectrum cannot be obtained with the sensitivity of the TGS detector, a highly sensitive MCT (mercury cadmite) detector used at a liquid nitrogen temperature of 77 K is used (for example, a special Kaihei 2-102424). This MCT detector is a kind of photoconductive semiconductor detector, and as other highly sensitive photoconductive semiconductor detectors used at liquid nitrogen temperature, InAs, InSb, PbS,
There are detectors such as PbSe, and the same applies to such detectors in the following description.

As shown in FIG. 6, when the received light intensity (detected light intensity) of the detector is small, the MCT detector has a linear relationship between the detected light intensity and the detector output signal intensity. At slightly larger values, this relationship becomes non-linear. When Fourier spectroscopy is performed using this non-linear portion, a background spectrum as shown in FIG. 7A is obtained. This spectrum was obtained in a state where the sample chamber was purged with nitrogen gas without mounting the sample, and FT-I
The wavelength dependence of the apparatus sensitivity in R measurement is shown. The MCT detector, although the detection sensitivity with wavenumber 450 cm ^-1 or less of the light is zero, apparently, not also become the responses 0 in wavenumber range 450cm ^-1 ~0cm ^-1 It can be seen that a ghost signal (folded spectrum) appears. This ghost signal appears as a result of using the non-linear portion in FIG. 6, and conversely, the degree of utilization of the non-linear portion can be examined by examining the degree of the ghost signal. Even if the FT-IR measurement is performed under the condition that such a ghost signal appears, a reliable measurement result cannot be obtained.

In order to prevent the ghost signal from appearing by using the MCT detector, it is necessary to reduce the detected light intensity. For example, when light is reduced by a mesh filter having a transmittance of about 6% as shown in FIG.
The background spectrum as shown in (1) is obtained, and the measurement in which the ghost signal does not appear can be performed.

However, if the light is dimmed in this way, the sensitivity is uniformly reduced in the entire wavelength range, and the high sensitivity of the MCT detector cannot be utilized at all.

On the other hand, as shown in FIG. 8, a silicon single crystal having a thickness of 2 mm has a transmittance of about 50%, and relatively well transmits light. Using this silicon single crystal as a sample, MCT
When FT-IR measurement is carried out by a usual method using a detector,
The integrated value of the received light intensity of the MCT detector at all wavelengths becomes too large, and the detector signal includes a strong nonlinear portion.

[0008] That is, when an attempt is made to perform high-sensitivity measurement using an MCT detector, the linearity of the signal is lost and quantitative analysis becomes difficult. It becomes impossible to take advantage of the high sensitivity of the detector.

For this reason, a normal F is applied to a sample such as a semiconductor crystal that has a high average transmittance of infrared light.
A highly sensitive MCT detector cannot be used in the T-IR measurement, and the MCT detector is used only when the sensitivity is improved by ignoring the quantitative property of the spectrum.

For example, substitutional carbon C in silicon single crystal
Localized vibration absorption of s is wavenumber 605cm ^-1Found in the vicinity
However, the absorption of this substitutional carbon Cs is strong as shown by a in FIG.
It overlaps with the nonon absorption band. This wave number part
Is enlarged and shown in FIG. As mentioned above, silicon single crystal
Transmits infrared light relatively well, but has strong phonon absorption
Wave number range 630-600 cm^-1In the transmitted light
The strength is extremely weak. Therefore, wave number 605 cm^-1To
High-sensitivity detector for detection of substitutional carbon Cs having a peak
Is desired, but the integrated intensity at all wavelengths of transmitted light is considerably large.
Therefore, the above-mentioned problem of non-linearity of the detection sensitivity arises.
Therefore, a highly sensitive MCT detector can be simply used.
Absent. Therefore, the measurement is performed using a normal TGS detector.
Therefore, the lower limit of detection of the concentration of substitutional carbon Cs is AST
M design: F123-81 standard
According to the above, it remained at about 0.05 ppma.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to provide a Fourier transform infrared spectroscopic measurement method capable of highly reliable and highly sensitive measurement in view of such problems.

[0012]

Means for Solving the Problems and Its Actions The output signal of the MCT detector has linearity with respect to the detected light intensity,
It is in the case where the detected light intensity is relatively weak as shown in FIG. 6 that high quantitativeness is maintained in the IR measurement, and the FT-IR device using the infrared microscope, which has a weak detected light intensity,
Highly sensitive MCT detectors are commonly used. In the case of a sample having a relatively good infrared light transmissivity, such as a silicon single crystal having a transmittance spectrum as shown in FIG. 8, it may be necessary to move from near point a to near point b in FIG. It is necessary to reduce the detected light intensity by the method of.

However, if this reduction is performed by a dimmer such as a mesh filter, the sensitivity is uniformly lowered in the entire wavelength range, and the merit of using the MCT detector is completely lost.

On the other hand, when the background spectrum of FIG. 2A is compared with the background spectrum of FIG. 2C, it is noted that the ghost signal with a wave number of 450 cm ⁻¹ or less is reduced. This ghost signal attenuation means that the linearity of the detector signal strength is ensured.

In the present invention, in a Fourier transform infrared spectroscopic measurement method for performing Fourier transform infrared spectroscopic measurement on a sample having an infrared absorption band, a high-sensitivity photoconductive semiconductor detector at low temperature is used and attention is paid to it. A bandpass filter that transmits light in the infrared absorption band is arranged in the optical path that is incident on the photoconductive semiconductor detector.

This sample is, for example, a trace amount of substitutional carbon C.
It is a silicon single crystal containing s. The photoconductive semiconductor detector is, for example, an MCT detector cooled with liquid nitrogen.

According to the present invention, the light is not uniformly extinguished over the entire wavelength range as shown in FIG. 7B, but a bandpass filter as shown in FIG. By transmitting only the light with a high transmittance and cutting the light in other unnecessary wave number ranges, the integrated intensity in the entire wavelength range of the detection light is reduced and the linearity of the detector sensitivity is secured, and , The detection light intensity in the measurement region of interest is made sufficiently strong, and the high sensitivity of the photoconductive semiconductor detector is fully utilized.

When the spectrum is reduced and Fourier spectroscopy is performed in this way, the background spectrum showing the wavelength dependence of the device sensitivity of the FT-IR is as shown in FIG. 2 (c). Comparing the background spectrum of FIG. 7 (c) obtained by dimming with a mesh filter and the background spectrum of FIG. 2 (c) obtained by dimming with a bandpass filter, the focused wave number range 700- Since it has high sensitivity only at 300 cm ⁻¹ and its sensitivity characteristic does not have nonlinearity, no ghost signal below 450 cm ⁻¹ appears.

2 (a), 2 (c) and 7 (c) 3
For the background spectrum of the species, 700-30
The characteristics are summarized in the wave number region of 0 cm ⁻¹ , as shown in Fig. 2 (a).
In Fig. 7 (c), the sensitivity is low, but the sensitivity is low. In Fig. 2 (c), the sensitivity is high and the linearity of the sensitivity is ensured. There is.

The localized vibrational absorption of the substitutional carbon Cs in the silicon single crystal appears at 605 cm ^-1 , and overlaps with the strong absorption of phonons as shown in FIG. Therefore, a very small amount of substitutional carbon C contained in the silicon single crystal.
When s is to be detected by FT-IR measurement, the transmitted light intensity becomes weak in the wave number region 630 to 600 cm ^{−1 where} strong phonon absorption occurs. Therefore, it is desirable to use a photoconductive semiconductor detector, for example, a high-sensitivity detector such as an MCT detector.

The localized vibration absorption spectrum of substitutional carbon Cs in a silicon single crystal is measured in the wave number range of 700 to 550 cm.
^Since only ^-1 is required, as shown in FIG. 2 (c), it is preferable to configure the device so as to have high sensitivity in the wave number range of 700 to 550 cm ^-1 , and to secure linearity of sensitivity. When the FT-IR measurement is performed using the bandpass filter having the characteristics shown in FIG. 2B in the arrangement shown in FIG.
It is possible to obtain the absorption spectrum of the sample under the sensitivity of the device as shown in.

[0022]

【Example】

 (1) Optical System of FT-IR Device FIG. 1 is a principle diagram of an optical system of the FT-IR device.

The divergent light emitted from the infrared continuous light source 1 is
It is collimated by the collimator mirror 3 through the aperture 2 and enters the Michelson interferometer 4. Michelson interferometer 4
Includes a beam splitter 5, a fixed mirror 6, and a movable mirror 7. The movable mirror 7 is linearly driven periodically by a voice coil or the like in the direction of the arrow in the figure. The infrared light incident on the beam splitter 5 is split into reflected light and transmitted light, the reflected light is reflected by the fixed mirror 6 and returns to the beam splitter 5, and the transmitted light is reflected by the movable mirror 7 to be a beam. Splitter 5
To the beam splitter 5, the beams are mixed and interfered by the beam splitter 5, condensed by the concave mirror 8, transmitted through the bandpass filter 9, transmitted through the sample 10 arranged in the sample chamber, passed through the aperture 11, and passed through the aperture 11. Detected in.

As the bandpass filter 9, a filter in which the transmitted wave number range matches the wave number range of the infrared absorption band of the sample 10 of interest is used. The sample chamber is purged with nitrogen gas in order to eliminate infrared absorption due to carbon dioxide or water vapor. Further, the MCT detector 12 is cooled with liquid nitrogen.

A known laser interference optical system for detecting the moving distance of the movable mirror 7 is not shown in FIG.

The interferogram detected by the MCT detector 12 is digitally converted and then Fourier-transformed to obtain a spectrum.

An experimental example using the above FT-IR apparatus will be described below. This experimental example shows the effect of the present invention in comparison with the conventional method.

(2) Experimental Example 1 FIG. 3 shows sample 1 in the wave number range of 700 to 540 cm ⁻¹ .
The transmittance | permeability spectrum at the time of carrying out FT-IR measurement without mounting 0 is shown. Ideally, the transmittance is 100%, but in reality, it deviates from the 100% line due to the problem of temporal stability of device performance and noise. This noise is due to device performance. The smaller the noise width, the more advantageous for high sensitivity measurement.

FIG. 3 (b) is a transmittance spectrum obtained by integrating signals for 10 minutes with a measurement resolution of 2 cm ⁻¹ by a normal FT-IR apparatus using a TGS detector, and the noise width is 0. It is about 02%. On the other hand, FIG.
An FT-I in which a T detector 12 is used and a bandpass filter having the characteristic shown in FIG. 2B is arranged in front of this T detector.
It is a transmittance spectrum obtained by integrating signals with a measurement resolution of 2 cm ⁻¹ for 10 minutes using an R device.

The noise width in FIG. 3 (a) is about 0.005%, which is about 1/4 of that in FIG. 3 (b).
Clearly, the advantage of higher sensitivity can be seen. The linearity of the sensitivity can be confirmed by the fact that no ghost signal is seen in FIG.

(3) Experimental Example 2 FIG. 4 shows the localized vibration absorption spectrum of the substitutional carbon Cs in a FZ method silicon single crystal having a thickness of 2 mm at room temperature, with the vertical axis being the absorbance and the difference spectrum from the reference crystal. It is shown in. The measurement resolution is 2 cm ^-1 .

FIG. 4 (b) is an absorption spectrum obtained by integrating signals for 10 minutes by a normal FT-IR apparatus using a TGS detector, and the noise width is about 0.02%. On the other hand, in FIG. 4A, the MCT detector 12 is used, and the signal is integrated for 10 minutes by an FT-IR apparatus in which a bandpass filter having the characteristic shown in FIG. 2B is arranged in front of this detector. It is the absorbance spectrum obtained by.

The substitutional carbon Cs concentration in the silicon single crystal used in this experiment was 0.04 ppma according to the concentration calculation method of ASTM: F123-81, which was the lower limit of concentration detection to 0.05 ppma (ASTM: F123-8).
It is almost the same concentration as 1). In FIG. 4B, the spectral shape is destroyed by noise, whereas in FIG.
In (a), it can be seen that there is less noise and the spectrum shape is good.

In the case of FIG. 4 (a), the absorption peak height can be recognized up to about 1/3 of that in the case of FIG. 4 (b) (0.04 ppma). Therefore, according to the present invention, the lower limit of detection of carbon Cs impurities in a silicon single crystal is about 0.01 pp.
It has been improved to ma.

(4) Experimental Example 3 FIG. 5 shows the result of performing the same measurement as in Experimental Example 2 on a CZ method silicon single crystal having a substitutional carbon concentration of about 0.04 ppma and a thickness of about 2 mm.

In FIG. 5 (b), it can be seen that the spectrum shape is destroyed by noise, whereas in FIG. 5 (a) there is less noise and the spectrum shape is good.

In the case of FIG. 5A, the height of the absorption peak can be recognized up to about 1/3 of that in the case of FIG. 5B (0.04 ppma). Therefore, according to the present invention, the lower limit of detection of carbon impurities in a silicon single crystal is about 0.01 ppma.
It has been improved to.

[0038]

According to the Fourier transform infrared spectroscopy measuring method of the present invention, the sensitivity and the SN ratio of the spectrum can be improved without lowering the quantitative analysis property of the obtained spectrum, and the SN ratio is TGS. It has an excellent effect that it is improved about 4 to 5 times as much as when a detector is used, and largely contributes to improvement of microanalysis accuracy.

Further, in the FT-IR measurement for a silicon single crystal containing a trace amount of substitutional carbon, the present invention was applied while the lower limit of detection when the TGS detector was used was 0.05 ppma at room temperature measurement. In this case, there is an excellent effect that the lower limit of detection can be set to about 0.01 ppma at room temperature measurement.

[Brief description of drawings]

FIG. 1 is a principle diagram of an optical system of an FT-IR apparatus.

FIG. 2 is a diagram showing the wave number dependence of FT-IR device sensitivity.

FIG. 3 is a transmittance spectrum diagram without a sample attached.

FIG. 4 is an infrared absorption spectrum diagram of a trace amount of carbon in a 2 mm-thick silicon single crystal grown by the FZ method.

FIG. 5 is an infrared absorption spectrum diagram of a trace amount of carbon in a silicon single crystal grown by the CZ method and having a thickness of 2 mm and a carbon concentration of 0.04 ppma.

FIG. 6 is a sensitivity characteristic diagram of a TGS detector and an MCT detector.

FIG. 7 is a diagram showing the wave number dependence of sensitivity of a conventional FT-IR apparatus.

FIG. 8 is a transmittance spectrum diagram of a 2 mm-thick silicon single crystal grown by the FZ method.

9 is an enlarged view of the wave number axis in the vicinity of point a in FIG.

[Explanation of symbols]

 1 Infrared continuous light source 4 Michelson interferometer 9 Bandpass filter 12 MCT detector

Claims (3)

[Claims] 1. In a Fourier transform infrared spectroscopic measurement method for performing Fourier transform infrared spectroscopic measurement on a sample having an infrared absorption band, a high-sensitivity photoconductive semiconductor detector at low temperature is used and the red A Fourier transform infrared spectroscopic measurement method, characterized in that a bandpass filter for transmitting light in the outer absorption band is arranged in an optical path incident on the photoconductive semiconductor detector. 2. The Fourier transform infrared spectroscopic measurement method according to claim 1, wherein the sample is a silicon single crystal containing a trace amount of substitutional carbon. 3. The Fourier transform infrared spectroscopic measurement method according to claim 1, wherein the photoconductive semiconductor detector is an MCT detector cooled with liquid nitrogen.