JP7611563B2 - Infrared spectrum measuring device and concentration measuring device - Google Patents

Description

The present invention relates to an infrared spectrum measuring device that can stably measure infrared absorption spectra, etc. for long periods of time, and is suitable for measuring low-concentration carbon, an impurity in silicon crystals, for example.

Japan Electronics and Information Technology Industries Association (JEITA) standard EM-3503 describes a method for determining the concentration of substitutional carbon atoms in silicon crystal, a semiconductor material, from its specific infrared absorption peak (near a wave number of 605 cm ^-1 ) using an infrared spectrophotometer (see Non-Patent Document 1). In addition, SEMI standard MF1391-1107 describes a method for measuring the carbon content of silicon by infrared absorption while cooling the sample to 80K or less (see Non-Patent Document 2).

JEITA Standard EM-3503 "Standard method for measuring the concentration of substitutional carbon atoms in silicon crystals by infrared absorption" SEMI Standard MF1391-1107 "Test method for substitutional carbon content in silicon by infrared absorption"

In the field of semiconductor materials technology, there is a need to measure impurities in silicon crystals at even lower concentrations. The inventors believed that in order to achieve this, it is important, first, to ensure that the wavenumber range of the absorption peak or other components to be measured occurs in a wavenumber range that exhibits good sensitivity characteristics of the analytical device, and second, to be able to perform the required number of spectrum measurements under the same conditions as much as possible (measurement uniformity). In other words, it is important to be able to stably measure the spectrum of a sample for a long period of time in the desired measurement wavenumber range.

The object of the present invention is to provide an infrared spectrum measuring device capable of stably measuring infrared spectra in a desired measurement wavenumber range for a long period of time, and a concentration measuring device that uses such an infrared spectrum measuring device to measure extremely low concentration substances in a sample.

The infrared spectrum measuring device according to the present invention is a device for measuring infrared spectra, comprising: a sample unit, an infrared spectrophotometer, a semiconductor photodetector, a refrigerator that cools a semiconductor detection element of the photodetector by executing a refrigeration cycle using a predetermined refrigerant, a control unit that controls the refrigerator so that the semiconductor detection element is at a predetermined set temperature, and a signal processing unit that acquires spectral information based on a detection signal from the photodetector, wherein the infrared spectrum measuring device is configured to determine, for the set temperature of the semiconductor detection element, temperature conditions such that fluctuations in the sensitivity characteristics of the infrared spectrum measuring device become small in a predetermined measurement wavenumber range, and the control unit has a setting change unit configured to be able to change the set temperature to a plurality of temperatures in a range of 4 to 150 K.

The type of refrigerant used in the refrigerator is not limited. More preferably, the setting change unit changes the set temperature in a range of 4 to 77 K.

According to an infrared spectrum measuring device having this configuration, the setting change section of the control section can change the set temperature of the semiconductor detection element to multiple temperatures in the range of 4 to 150 K, so that it is possible to appropriately set the temperature conditions of the semiconductor detection element such that the sensitivity characteristics of this infrared spectrum measuring device are good (for example, exhibiting characteristics resistant to temperature fluctuations) in the wavenumber range of the absorption peak or the like to be measured. Then, the configuration of the refrigerator and control section maintains the temperature of the semiconductor detection element at the set temperature during the spectrum measurement of the sample. As a result, it is possible to repeatedly and stably measure the spectrum of a specified measurement wavelength range over an extended period of time.

The sensitivity characteristics of the infrared spectrum measuring device may vary even under temperature control by the refrigerator and the control unit. By utilizing the fact that changing the cooling temperature of the semiconductor detection element changes the value of the band gap energy, shifts the detection band of the semiconductor detection element in the wavenumber direction, and also changes the detection sensitivity of the semiconductor detection element, the inventors have decided to determine in advance the temperature conditions of the semiconductor detection element such that even if the sensitivity characteristics of the infrared spectrum measuring device fluctuate, the effect on the spectrum measurement in the measurement wavenumber range is small. Specifically, based on the detection signal of the photodetector in a state where nothing is installed in the sample section, sensitivity characteristic data of the spectrum measuring device for each cooling temperature is obtained, and based on this sensitivity characteristic data, an optimal setting value of the cooling temperature is determined.

That is, in the infrared spectrum measuring device, the control unit is configured to determine, as the set temperature of the semiconductor detection element, a temperature condition in which a fluctuation in sensitivity characteristic of the infrared spectrum measuring device becomes small in a predetermined measurement wave number range;
The temperature condition is determined based on sensitivity characteristics of the infrared spectrum measuring device obtained under a plurality of temperature conditions.

In addition, in the infrared spectrum measuring device,
the signal processing unit has a sensitivity characteristic acquisition unit that acquires sensitivity characteristics of the infrared spectrum measuring device under a plurality of temperature conditions, and a temperature condition selection unit that selects a temperature condition under which a variation in the sensitivity characteristics of the infrared spectrum measuring device becomes small in a predetermined measurement wavenumber range, based on the sensitivity characteristics for each of the temperature conditions;
The control unit is configured to determine the temperature condition selected by the temperature condition selection unit as the set temperature of the semiconductor detection element.

With an infrared spectrum measuring device having this configuration, the configuration of the sensitivity characteristic acquisition unit and the temperature condition selection unit makes it easy to set or reset the temperature conditions of the semiconductor detection element, for example, when starting a new measurement. Even if the wavenumber range of the absorption peak to be measured is changed, the optimal temperature conditions for the new measurement wavenumber range can be easily determined.

Here, "temperature conditions under which the fluctuation in the sensitivity characteristics of the infrared spectrum measuring device is small in a specified measurement wavenumber range" refers to temperature conditions under which the increase or decrease in sensitivity in the wavenumber direction in the specified measurement wavenumber range is small (in other words, temperature conditions under which the wavenumber dependency of the sensitivity characteristics in the specified measurement wavenumber range is small, or temperature conditions under which the sensitivity characteristics in the wavenumber direction in the specified measurement wavenumber range are flat). If the temperature control of the semiconductor detection element is performed under such temperature conditions, even if the sensitivity characteristics of the infrared spectrum measuring device change due to temperature fluctuations, etc., the sensitivity characteristics in the specified measurement wavenumber range will hardly change at all.

Alternatively, "temperature conditions under which the variation in the sensitivity characteristics of the infrared spectrum measuring device is small in a specified measurement wavenumber range" may refer to temperature conditions under which a specified temperature range is defined for each temperature condition under which the sensitivity characteristics are acquired, and the variation in the multiple sensitivity characteristics acquired under temperature conditions within that range is small in the specified measurement wavenumber range. If the temperature control of the semiconductor detection element is performed under such temperature conditions, even if the sensitivity characteristics of the infrared spectrum measuring device change due to temperature fluctuations or the like, the sensitivity characteristics in the specified measurement wavenumber range will hardly change.

Alternatively, the temperature conditions of the semiconductor detection element may be set by combining the above two temperature condition criteria. For example, the temperature conditions may be set to satisfy both criteria by applying both the former criterion (the increase or decrease in sensitivity with respect to wavenumber changes in a specified measurement wavenumber range is small) and the latter criterion (the variation in sensitivity characteristics in a specified measurement wavenumber range is small for all multiple sensitivity characteristics acquired under temperature conditions within a specified temperature range).

The above are merely examples of standards for determining the temperature conditions of a semiconductor detection element, but by using temperature conditions based on these standards, it is easier to maintain the sensitivity characteristics of the measurement device in a good condition in a specified measurement wavenumber range.

The infrared spectrum measuring device further includes an optical filter disposed between the sample portion and the semiconductor detection element,
The optical filter is
An optical filter that does not transmit all or a part of light on the lower wavenumber side than the predetermined measurement wavenumber range, or
an optical filter that does not transmit all or a part of light on a higher wavenumber side than the predetermined measurement wavenumber range and all or a part of light on a lower wavenumber side than the predetermined measurement wavenumber range;
The optical filter is configured to be cooled by the refrigerator together with the semiconductor detection element.

According to this configuration, the optical filter prevents all or part of the light with lower wave numbers than the predetermined measurement wave number range from proceeding to the semiconductor detector. This light may include, for example, infrared radiation (thermal radiation light, etc.) from the optical element on the light source side or the sample, which may cause temperature fluctuations and thermal noise in the semiconductor detector. Therefore, the optical filter suppresses the effects of this light, improving the accuracy of the spectrum measurement.
Furthermore, if an optical filter is used that does not transmit not only light on the low wavenumber side but also light on the high wavenumber side above the measurement wavenumber range, the measurement sensitivity can be further improved.
Furthermore, with this configuration, the optical filter is cooled and temperature-controlled by the refrigerator together with the semiconductor detection element, so that infrared radiation from the optical filter itself is prevented from entering the semiconductor detection element.

Furthermore, the infrared spectrum measuring device is characterized in that it includes a filter changer that offsets the position of the optical filter from the optical path between the sample portion and the semiconductor detection element. Or, the infrared spectrum measuring device is characterized in that it includes a filter changer that switches the optical filter to another optical filter.

According to this configuration, the filter changer appropriately offsets the optical filter between the sample section and the semiconductor detection element, so that a single spectrum measurement device can perform both spectrum measurements under wavenumber range restrictions (e.g., narrow band) imposed by the optical filter and spectrum measurements over a wide band that is not subject to such wavenumber range restrictions. In addition, by switching the optical filter to a different optical filter, spectrum measurements can be performed in different wavenumber ranges.

The infrared spectrum measuring device is also characterized in that it is equipped with a sample refrigerator for cooling the sample in the sample section, separate from the refrigerator for cooling the semiconductor detection element.

With this configuration, the sample is cooled by the sample refrigerator, which can suppress phenomena that can interfere with the measurement of the spectral peak to be measured (for example, large broadband absorption peaks caused by lattice vibrations when the sample is a crystalline material) and infrared radiation from the sample itself.

The concentration measuring device according to the present invention is a concentration measuring device that measures the concentration value of a substance in a sample, and is characterized in that it has the configuration of the infrared spectrum measuring device described above, and the signal processing unit is configured to read the spectral peak in the specified measurement wavenumber range from the acquired spectral information of the sample, and calculate the concentration value of the substance based on the spectral peak.

In particular, it is preferable that the concentration measuring device measures the carbon atom concentration in the silicon crystal by setting the wavenumber range around 605 cm ^-1 , including the wavenumber range, as the predetermined measurement wavenumber range, and by setting the set temperature of the semiconductor detection element to a temperature of 4 K or higher and lower than 77 K.

The concentration measuring device having the above configuration is equipped with a refrigerator and a control unit (including a setting change unit for the set temperature) like the infrared spectrum measuring device, so that the temperature conditions of the semiconductor detection element can be appropriately set so that the sensitivity characteristics of this concentration measuring device are good (for example, the characteristics are resistant to temperature fluctuations) in the wave number range of the absorption peak specific to the target substance. Here, good sensitivity characteristics do not simply mean a state of high sensitivity. For example, even if the sensitivity itself is moderate, it may be better if the sensitivity change in the wave number direction in a specified measurement wave number range is small and flat. Then, since the temperature of the semiconductor detection element is maintained at the set temperature during the spectrum measurement of the sample, the spectral peak of the substance in the sample can be repeatedly and stably measured for a long period of time. As a result, the concentration information of the substance based on the measured spectral peak can be calculated with high accuracy, and the measurement limit on the low concentration side of the substance can be improved compared to the conventional method.

1 is a diagram showing an overall configuration of a spectrum measuring device according to an embodiment; 2 is a diagram showing a detailed configuration of a control unit and a signal processing unit of the measuring device. FIG. 4 is a flow diagram for determining the cooling temperature set point using the measurement device. FIG. 4 is a diagram showing a schematic diagram of sensitivity characteristics of a spectrum measuring device for each cooling temperature. FIG. 4 is a diagram for explaining an example of a criterion for determining a set value of the cooling temperature. 4 is a flow diagram for calculating the concentration value of a specific substance in a sample using the measurement device. 5A and 5B are diagrams for explaining processing of a detection signal when calculating a concentration value of the specific substance.

The spectrum measuring device according to this embodiment is a device that can measure various spectra resulting from substances in a sample with high sensitivity in a specified measurement wavelength range using a dispersive or Fourier transform infrared spectrophotometer (FTIR). Here, as an example, a case will be described in which the concentration of substitutional carbon atoms contained at low concentrations in silicon crystal, which is a semiconductor material, is calculated based on the peak of the absorption spectrum measured by a Fourier transform infrared spectrophotometer. The impurity concentration range assumed in the aforementioned JEITA standard (EM-3503) is approximately 0.04 ppma to 7 ppma, whereas the spectrum measuring device of this embodiment makes it possible to measure down to 0.004 ppma, which is 1/10 of the lower measurement limit of the JEITA standard, and can maintain such measurements for a long time.

The spectrum measuring device in FIG. 1 includes a Fourier transform infrared spectrophotometer 10, a sample section 20, a helium refrigerator 22 for the sample, a cold filter 80, a photodetector 30 having an MCT detection element 32, a helium refrigerator 40 for the cold filter 80 and the photodetector 30, a control section 50, and a signal processing section 60.

The Fourier transform infrared spectrophotometer 10 has an infrared light source 11, a beam splitter 12 that splits the infrared light, a fixed mirror 13 that reflects one of the split lights, and a movable mirror 14 that reflects the other split light, and is configured such that the beam splitter 12 combines two light beams with different optical path lengths from the fixed mirror 13 and the movable mirror 14 to generate an infrared interference wave.

The sample section 20 is composed of a sample holder installed on the optical path of the infrared interference wave from the beam splitter 12, and the semiconductor material sample is removably held or placed on this sample holder. Note that the measurement target of the spectrum measuring device of the present invention is not limited to solid samples, and may be a liquid sample contained in a sample cell or a gas sample contained in a gas cell.

The helium refrigerator 22 for the sample cools the semiconductor material of the sample section 20, and has the effect of suppressing the generation of large broadband absorption peaks associated with lattice vibrations of the silicon crystal, as well as the generation of infrared radiation from the semiconductor material itself.

The cold filter 80 is disposed on the optical path immediately before the photodetector. The cold filter 80 is formed so as not to transmit light on the lower wavenumber side (longer wavelength side) than the measurement wavenumber range (for example, 630 to 580 cm ⁻¹ ) when measuring the absorption peak of the carbon atom. The cold filter 80 may have a cutoff of 580 cm ⁻¹ , or a filter having a cutoff of a lower wavenumber than this. Infrared radiation (thermal radiation light, etc.) from the optical elements and semiconductor material (sample) on the light source side, including the infrared light source 11, is normal temperature radiation, but its influence is large in spectrum measurement at extremely low temperatures. Since it can cause temperature fluctuations and thermal noise in the MCT detection element 32, the optical filter cuts all or part of these lights, thereby suppressing their influence and improving the accuracy of the spectrum measurement. In addition, for example, a band-pass filter that transmits only light in a specific narrow band that matches the measurement wavenumber range (630 to 580 cm ⁻¹ ) can be used instead. In addition, two filters, a cold filter (short pass filter) and a long pass filter, may be combined. In this case, the heat radiation from the long pass filter is cut by the cold filter in the latter stage, so the long pass filter may be installed in the room temperature section.

The photodetector 30 receives the infrared interference wave that has passed through the sample and the cold filter 80, and outputs a detection signal according to the light intensity. In this embodiment, for example, a mid-band MCT detector is used as the photodetector 30.

The MCT detection element 32 of the semiconductor structure constituting the mid-band MCT is attached to the cooling section of the helium refrigerator 40 and is cooled to a temperature of 4K or more and 150K or less. For example, in a cooling method using a Dewar vessel filled with liquid nitrogen, the cooling temperature cannot be intentionally changed because the cooling method directly uses the heat of evaporation of liquid nitrogen (77K), but the cooling temperature can be changed in the cooling method using the helium refrigerator 40 of this embodiment. Therefore, it may be found that the measurement at a temperature of 77K was actually too cold, and that the proper cooling temperature is a temperature of 80K or 90K. When measuring the content of low-concentration carbon atoms in silicon crystals as in this embodiment, it is particularly preferable to set the cooling temperature range of the helium refrigerator 40 to a temperature range of 4K or more and less than 77K.

The helium refrigerator 40 is configured to execute a predetermined refrigeration cycle using liquid helium as a refrigerant, and the control unit 50 is configured to control the helium refrigerator 40 so that the MCT detection element reaches a predetermined set temperature. As the helium refrigerator 40, for example, a GM refrigerator manufactured by Sumitomo Heavy Industries, Ltd. can be used.

As described above, in the conventional configuration in which liquid nitrogen or liquid helium is poured into a Dewar vessel to cool the photodetector 30, the evaporation temperature of the liquid nitrogen or liquid helium becomes the cooling temperature of the photodetector 30, making it difficult to freely control the cooling temperature. In this embodiment, a helium refrigerator 40 is used to cool the photodetector 30, so the cooling temperature can be set arbitrarily from a wide range of temperatures above the evaporation temperature (e.g., from about 4 K to about 150 K), making it possible to control the temperature of the MCT detection element.

In addition, even if a refrigerator using a refrigerant other than helium (a liquefied gas such as nitrogen) is used, it is possible to control the temperature within the cooling temperature range appropriate for that refrigerant.

The cold filter 80, together with the MCT detection element, is also cooled and temperature controlled by the helium refrigerator 40. By cooling the cold filter 80, infrared radiation from the filter itself, which causes noise signals, is suppressed.

The signal processing unit 60 processes the detection signal from the photodetector 30 to obtain interferogram data, and performs processing such as Fourier transform to calculate spectral information such as the absorption spectrum. The signal processing unit 60 is composed of, for example, a microcomputer built into the spectrum measuring device, or a personal computer separate from the device.

Although not shown, the spectrum measuring device of this embodiment may be configured to automatically measure multiple samples continuously using a mechanism such as an autosampler in the sample section 20. In this embodiment, cryogenic cooling is performed using a helium refrigerator, so if there is an influence of disturbance or temperature rise caused by opening the sample chamber, etc., the time it takes to reach the temperature again will be longer than usual. By using a mechanism such as an autosampler, it is possible to eliminate such time loss and perform long-term and multi-analyte measurements more stably without being affected by disturbances such as opening and closing the sample chamber, etc.

The configuration for determining the set temperature for controlling the temperature of the photodetector 30 will be described with reference to FIG. 2. The control unit 50 is configured to include a temperature control unit 52 that performs temperature control via the helium refrigerator 40 so that the MCT detection element 32 reaches the set temperature, and a setting change unit 54 that can change the set temperature.

The signal processing unit 60 in FIG. 2 is composed of a Fourier transform unit 62, a spectrum information acquisition unit 64, a concentration information acquisition unit 66, a sensitivity characteristic acquisition unit 68, and a temperature condition selection unit 70.

The Fourier transform unit 62 performs a Fourier transform on the interferogram data consisting of the detection signal from the photodetector 30. The spectrum information acquisition unit 64 processes the data after the Fourier transform to calculate the desired spectrum data. The concentration information acquisition unit 66 acquires the concentration value of a specific substance (such as carbon atoms) in the sample based on the calculated spectrum data.

The sensitivity characteristic acquisition unit 68, which is characteristic of this embodiment, acquires the sensitivity characteristic of the entire spectrum measurement device including the MCT detection element 32 for each set temperature changed by the setting change unit 54. The temperature condition selection unit 70 is configured to select temperature conditions based on the sensitivity characteristic for each set temperature such that the fluctuation of the sensitivity characteristic of the spectrum measurement device is small in a predetermined measurement wavenumber range (e.g., 630 to 580 cm ^-1 ). The temperature conditions selected by the temperature condition selection unit 70 are passed to the control unit 50, and become the set temperatures for controlling the temperature of the MCT detection element 32 when acquiring spectrum information of a sample.

In the helium refrigerator 40 in FIG. 2, a cold filter 80 is placed together with the photodetector 30. The cold filter 80 is provided so that it can be swung by a filter switcher 82, and the position of the filter is selected to be on the optical path or at an offset position depending on the swing position. By appropriately offsetting the cold filter 80 using the filter switcher 82, it is possible to smoothly switch from a measurement under the wavenumber range restriction by the filter to a measurement without such wavenumber range restriction. Note that a filter switcher configured to be able to appropriately switch between multiple optical filters including the cold filter 80 may be used to perform spectral measurements in different wavenumber ranges.

The control unit 50 is not limited to the configuration shown in FIG. 2, and may be a setting change unit that only has the function of setting a preselected temperature condition as the set temperature for temperature control, instead of the setting change unit 54. In this case, the signal processing unit 60 does not need to be provided with the sensitivity characteristic acquisition unit 68 and the temperature condition selection unit 70. The set value is based on temperature conditions that reduce the fluctuation of the sensitivity characteristic of the spectrum measuring device in the measurement wavenumber range, and is set by measuring the sensitivity characteristic of the spectrum measuring device using the same infrared spectrophotometer 10 and photodetector 30 configuration.

When the cooling temperature of the photodetector 30 changes, the sensitivity characteristics (detection sensitivity and detection band) of the MCT detection element 32 change. This can be used to optimize the sensitivity characteristics by adjusting the cooling temperature. In this embodiment, the optimal set temperature for a specified measurement wavenumber range is determined by the following procedure.

The procedure for pre-measurement is shown in Figure 3. Considering that there are individual differences in the measurement devices, the pre-measurement is performed for each measurement device. Also, considering that the optical characteristics specific to the measurement device change over time, it is preferable to perform the pre-measurement (resetting of temperature conditions) of the measurement device periodically.

First, the setting change unit 54 of the control unit 50 sets the set temperature T for controlling the temperature of the photodetector 30 to T1 (step S11). Next, the temperature control unit 52 controls the temperature of the photodetector 30 via the helium refrigerator 40 (step S12). Then, without placing a sample in the sample unit, the spectrum is measured using the Fourier transform infrared spectrophotometer 10, and this is treated as the sensitivity characteristic of the spectrum measurement device (step S13).

The setting change unit 54 changes to the next set temperature T2 (step S14) and similarly acquires the sensitivity characteristics (steps S11 to S13). In this way, all sensitivity characteristics under temperature control at the specified set temperatures (e.g., T1 to T5) are acquired.

In this preliminary measurement, the setting change unit 54 may change the set temperature, for example, in the range of 4 K or more and 150 K or less. Preferably, it is in the range of 4 K or more and less than 77 K. The pitch at which the set temperature is changed may be, for example, 0.1 K to 10 K. By making the pitch finer around temperature conditions where the sensitivity characteristics of the spectrum measuring device change significantly in response to temperature changes, the change in sensitivity characteristics can be obtained accurately. Note that if the accuracy of temperature control of the photodetector 30 is low, making the pitch too fine is meaningless, so it is advisable to set the pitch according to the accuracy of the temperature control.

FIG. 4 shows a schematic diagram of the sensitivity characteristics of the spectrum measuring device for each set temperature T1 to T5. The temperature condition selection unit 70 determines the set temperature for temperature control of the photodetector 30 based on the sensitivity characteristics (step S15). In this procedure, the sensitivity characteristics in the measurement wavenumber range (for example, 630 to 580 cm ⁻¹ ) shown in FIG. 4 are focused on, and a temperature condition in which the wavenumber dependency of the sensitivity characteristics in this wavenumber range is small is selected (criterion 1). In other words, a temperature condition in which the increase or decrease in sensitivity (vertical axis) in the wavenumber (horizontal axis) direction in the measurement wavenumber range is small (flat) is selected. In the example of FIG. 4, the wavenumber dependency of the sensitivity characteristics at the set temperatures T3 and T4 is small, and the temperature conditions are narrowed down to T3 and T4.

In this embodiment, a temperature condition with a small change (variation) in sensitivity characteristics with respect to temperature change is further selected (criterion 2). Explaining with reference to FIG. 5, first, a predetermined temperature range (T28-T32) including the set temperature T3 and a predetermined temperature range (T38-T42) including the set temperature T4 are set. The difference between the maximum temperature and the minimum temperature in each temperature range is set to be the same. Then, a temperature with a small variation in sensitivity for each temperature at a target wave number according to the target substance (wave number 605 cm ^-1 in the case of carbon atom) is selected. In the example of FIG. 5, the set temperature T3 has a smaller variation in sensitivity, and the set temperature T for temperature control is determined to be T3.

In addition, because changes in the sensitivity of the photodetector 30 also affect the S/N ratio, increasing the S/N ratio may be added to the selection criteria (criterion 3). For example, the optimal temperature conditions may be determined by comprehensively determining the criteria 1 to 3.

As a variation of the above-mentioned criterion 2, the average sensitivity of the measurement wavenumber range may be calculated for each temperature, and the sensitivity with the smallest change (variation) in the average sensitivity within the set temperature range may be selected.

Even under temperature conditions where the sensitivity of the photodetector 30 is somewhat low, a flat sensitivity characteristic in a predetermined measurement wavenumber range (for example, a band near the target wavenumber of 605 cm ⁻¹ ) makes it easier to obtain accurate measurement values even with temperature fluctuations in the photodetector 30. In other words, it can be said that a smaller gradient of the sensitivity characteristic in the measurement wavenumber range is more resistant to temperature fluctuations.

Furthermore, in peak detection in this measurement, instead of simply reading the vertical axis value of the absorption spectrum at the peak position, a baseline is drawn for the peak spectrum and the difference from the baseline to the peak top is read, so that it is important to improve the accuracy of the measurement value that the sensitivity characteristics in the vicinity of the target wavenumber (e.g., 605 cm ^-1 ) are flat. For this reason, the measurement wavenumber range in this embodiment should be appropriately determined according to the band in which the spectrum peak of the measurement target occurs, and it is preferable to determine it so as to include at least the entire peak.

When the set temperature of the temperature control of the photodetector 30 is changed, the detection band of the MCT detection element 32 may be extended unnecessarily to the low wavenumber side, as in the example of FIG. 4. In this embodiment, the cold filter 80 in an extremely low temperature state is also used, so that the light detected in the extended low wavenumber detection band as described above can be cut in advance by the cold filter 80. Therefore, even if the detection band is extended unnecessarily to the low wavenumber side by changing the set temperature, the effect on the accuracy of the spectrum measurement due to this can be eliminated.

Procedure for Main Measurement The procedure for the main measurement using the temperature condition (T=T3) selected in the preliminary measurement is shown in Fig. 6. First, the temperature control unit 52 controls the temperature of the photodetector 30 at the set temperature T3 (step S21). Next, with no sample placed on the sample unit 20 (step S22), the spectrum is measured using the Fourier transform infrared spectrophotometer 10, and this is treated as background data (step S23). Note that the sensitivity characteristic data for the set temperature T3 obtained in the preliminary measurement can also be used as background data.

Next, a reference sample is placed in the sample section 20 (step S24), and a spectrum is similarly obtained (steps S22 to S23). Here, for example, a low carbon content (almost no carbon) wafer is used as the reference sample. Other samples that serve as standards may also be used as the reference sample. A measurement sample wafer is then placed in the sample section 20 (step S24), and a spectrum is similarly obtained (steps S22 to S23).

Here, the transmission spectrum in the wave number range of about 700 to 500 cm ⁻¹ is measured by the Fourier transform infrared spectrophotometer 10. In Fig. 7A, the signals (SB signals) obtained by single beam measurement by the Fourier transform infrared spectrophotometer 10 are shown as the background transmission spectrum I ₀ , the reference sample transmission spectrum I _R , and the measurement sample transmission spectrum I ₁ , respectively.

Next, the spectrum information acquisition unit 64 divides the transmission spectra I _R and I ₁ of the reference sample and the measurement sample by the background spectrum I ₀ to calculate the transmittance spectra (I _R /I _0, I ₁ /I ₀ ) (step S25). The calculated transmittance spectra (I _R /I _0, I ₁ /I ₀ ) are shown in FIG. 7(B). They are often displayed as "% T". Next, the transmittance spectra (I _R /I _0, I ₁ /I ₀ ) are converted to absorption spectra (step S26). The absorption spectra of the reference sample and the measurement sample are shown in FIG. 7(C). The vertical axis represents absorbance (Abs).

The reason why it is difficult to measure the carbon concentration in silicon crystal is that the light source of a general FTIR is not suitable because the detection light in the wave number region around 605 cm ⁻¹ , which is the absorption peak, becomes dark. In addition, there was no photodetector with an appropriate detection band. Moreover, since the absorption peak derived from the carbon concentration overlaps with the large absorption peak in a wide band derived from silicon lattice vibration, as shown in the absorption spectrum of the measurement sample in FIG. 7(C), the absorption spectrum of the reference sample (reference wafer) whose carbon concentration can be assumed to be zero is usually subtracted from the absorption spectrum of the measurement sample to calculate the carbon peak height. Here, the identity of both measurements is a condition for obtaining an appropriate difference spectrum, and is the most important factor in determining the quantitative accuracy of the carbon concentration. In other words, it is important to be able to stably perform two measurements of the measurement sample and the reference sample. In addition, in the spectrum measurement of multiple samples, the identity of each measurement is also important. Furthermore, in order to obtain a predetermined signal-to-noise ratio, it is necessary to extend the measurement time (for example, by increasing the number of integration times of the Fourier transform infrared spectrophotometer 10), so it is also important that spectral measurements can be performed stably for a long period of time.

In this embodiment, in the preliminary measurement, temperature conditions are selected that reduce the fluctuation in the sensitivity characteristics of the spectrum measurement device in the measurement wavenumber range, and these temperature conditions are used as the set value of the cooling temperature in the main measurement. As a result, it is possible to perform stable spectrum measurement for a long period of time in the main measurement, and it is possible to obtain identity between the spectrum measurement of the reference sample and the spectrum measurement of the measurement sample in the measurement wavenumber range, and it is now possible to calculate an accurate absorption peak by taking the difference between the two spectra.

In other words, in the procedure of FIG. 6, the absorption spectrum of the reference sample is subtracted from the absorption spectrum of the measurement sample to calculate the absorption spectrum of only the specific substance (carbon atom) (step S27). FIG. 7(D) shows an enlarged view of the absorption peak (residual peak) occurring in the measurement wavenumber range. Note that if the thicknesses of the reference sample and the measurement sample are different, the absorption spectrum of the reference sample can be multiplied by "measurement sample thickness/reference sample thickness" and then subtracted from the absorption spectrum of the measurement sample.

Finally, the concentration information acquisition unit 66 draws a baseline in the measurement wavenumber range (e.g., 630-580 cm ^-1 ) of the absorption peak of the specific substance in Fig. 7 (D), reads the absorbance Apeak of the peak top and the absorbance Abase of the baseline at the peak wavenumber, and calculates the "peak height" which is the difference between the two (Apeak-Abase). Then, the absorption coefficient is calculated from the peak height and the thickness of the sample, and the concentration value of the specific substance is calculated (step S28). Note that the full half width of the absorption peak is measured, and if it is greater than, for example, 6 cm ^-1 , it is inappropriate, so the measurement conditions may be reviewed.

Regarding the sensitivity characteristics of mid-band MCT, it is more sensitive than wideband MCT, so it is desirable to use mid-band MCT as a photodetector if possible. However, at the cooling temperature of liquid nitrogen (77K), the cutoff of mid-band MCT is at 650-600 cm ^-1 , and the sensitivity rises sharply in the wavenumber range around 605 cm ^-1 , so it cannot be used for measurement. However, if the mid-band MCT is cooled by a helium refrigerator, the cutoff can be extended to less than 500 cm ^-1 .

When the temperature of the MCT fluctuates, the cutoff wavenumber position shifts slightly, making it difficult to detect peaks in the wavenumber range near the cutoff. However, in wavenumber ranges away from the cutoff, the change in sensitivity characteristics becomes more gradual, so it is very effective to be able to detect peaks in such wavenumber ranges.

In this embodiment, when measuring an absorption peak of a carbon atom occurring near the cutoff of the photodetector, the following procedure can be carried out.
First, the cooling temperature of the mid-band MCT by the helium refrigerator is adjusted to extend the cutoff of the mid-band MCT to less than 500 cm ⁻¹ .
In addition, the cooling temperature of the mid-band MCT is actively fine-tuned so that the detection band of the carbon atom peak (around 605 cm ^-1 ) is exactly in a band where the change in the wavenumber direction of the sensitivity characteristics of the entire measurement device including the mid-band MCT is gradual (flat).
By using a mid-band MCT whose temperature has been adjusted in this manner, data on the carbon concentration in the silicon crystal can be repeatedly and stably obtained over a long period of time.

In addition, since the peak to be detected occurs in a specific narrow wavenumber band, limiting the wavenumber band of the light received by the mid-band MCT to that narrow wavenumber band can improve measurement sensitivity. For example, the cutoff characteristics of a temperature-controlled mid-band MCT can be used to prevent unnecessary light on the low wavenumber side (longer wavelength side) from being detected. Also, unnecessary light on the high wavenumber side (shorter wavelength side) can be removed before the mid-band MCT by an appropriate long-pass or band-pass filter. For example, the cutoff characteristics of the mid-band MCT can be made more pronounced by using a mid-band MCT in combination with a band-pass filter.

The long-pass filter and band-pass filter placed in front of the photodetector may be cooled and temperature-regulated by a helium refrigerator in the same way as the cold filter, to suppress thermal radiation from the filter itself. The sample is also cooled to suppress thermal radiation from the sample as much as possible. Therefore, the photodetector is hardly affected by harmful thermal radiation from the sample and the filter in front of the photodetector.

The present invention can also be applied to cases where a wideband MCT capable of detecting wavenumber ranges lower than those of a midband MCT is used, offering the advantage of being able to measure peaks in the measurement wavenumber range, for example, in the low wavenumber range, with good sensitivity. The present invention is not limited to MCT detectors, and can also be applied to infrared spectrophotometers equipped with other semiconductor detectors, such as various photodiodes, whose sensitivity characteristics change when the cooling temperature is changed within the range of 4K to 150K.

10. Fourier transform infrared spectrophotometer (infrared spectrophotometer)
20 Sample section 22 Helium refrigerator for sample (refrigerator for sample)
30 Photodetector 32 MCT detector (semiconductor detector)
40 Helium refrigerator (refrigerator)
50 Control unit 54 Setting change unit 60 Signal processing unit 68 Sensitivity characteristic acquisition unit 70 Temperature condition selection unit 80 Cold filter (optical filter)
82 Filter changer

Claims (11)

A sample portion;
An infrared spectrophotometer;
a photodetector having a semiconductor structure;
a refrigerator that executes a refrigeration cycle using a predetermined refrigerant to cool a semiconductor detection element of the photodetector;
a control unit that controls the refrigerator so that the semiconductor detection element is at a predetermined set temperature;
and a signal processing unit that acquires spectrum information based on a detection signal from the photodetector,
The set temperature of the semiconductor detection element is determined to be a temperature condition such that the fluctuation of the sensitivity characteristic of the infrared spectrum measuring device is small in a predetermined measurement wave number range,
The control unit has a setting change unit configured to change the set temperature to a plurality of temperatures in a range of 4 to 150 K. 2. The infrared spectrometer according to claim 1,
the control unit is configured to determine a temperature condition for the set temperature of the semiconductor detection element such that a fluctuation in sensitivity characteristic of the infrared spectrum measurement device is small in a predetermined measurement wavenumber range;
4. An infrared spectrum measuring apparatus according to claim 1, wherein the temperature condition is determined based on sensitivity characteristics of the infrared spectrum measuring apparatus obtained under a plurality of temperature conditions. 2. The infrared spectrometer according to claim 1,
The signal processing unit includes:
a sensitivity characteristic acquisition unit that acquires sensitivity characteristics of the infrared spectrum measurement device under a plurality of temperature conditions;
a temperature condition selection unit that selects a temperature condition based on the sensitivity characteristic for each temperature condition such that a variation in the sensitivity characteristic of the spectrum measurement device is small in a predetermined measurement wavenumber range,
The control unit is configured to determine the temperature condition selected by the temperature condition selection unit as the set temperature of the semiconductor detection element.
An infrared spectrum measuring device comprising: 4. The infrared spectrometer according to claim 2,
The temperature condition under which the fluctuation in the sensitivity characteristic of the infrared spectrum measuring device becomes small in a predetermined measurement wave number range is as follows:
An infrared spectrum measuring apparatus characterized in that the temperature conditions are such that the increase or decrease in sensitivity in the wavenumber direction in the specified measurement wavenumber range is small. 5. The infrared spectrometer according to claim 2,
The temperature condition under which the fluctuation in the sensitivity characteristic of the infrared spectrum measuring device becomes small in a predetermined measurement wave number range is as follows:
an infrared spectrum measuring device, characterized in that a predetermined temperature range is determined for each temperature condition when the sensitivity characteristics are acquired, and the temperature conditions are such that a variation in the plurality of sensitivity characteristics acquired under temperature conditions within the temperature range is small in the predetermined measurement wavenumber range. 6. The infrared spectrometer according to claim 1,
an optical filter disposed between the sample portion and the semiconductor detection element;
The optical filter is
An optical filter that does not transmit all or a part of light on the lower wavenumber side than the predetermined measurement wavenumber range, or
an optical filter that does not transmit all or a part of light on a higher wavenumber side than the predetermined measurement wavenumber range and all or a part of light on a lower wavenumber side than the predetermined measurement wavenumber range;
The infrared spectrum measuring device is characterized in that the optical filter is configured to be cooled by the refrigerator together with the semiconductor detection element. 7. The infrared spectrometer according to claim 6,
13. An infrared spectrum measuring device comprising: a filter changer for offsetting the position of the optical filter from the optical path between the sample portion and the semiconductor detection element. 7. The infrared spectrometer according to claim 6,
An infrared spectrum measuring device comprising a filter changer for changing the optical filter into another optical filter. 9. The infrared spectrometer according to claim 1,
an infrared spectrum measuring device comprising a refrigerator for cooling a sample in the sample portion, in addition to a refrigerator for cooling the semiconductor detection element; A concentration measurement device for measuring a concentration value of a substance in a sample, comprising the infrared spectrum measurement device according to any one of claims 1 to 9,
The signal processing unit is configured to read a spectral peak in the specified measurement wavenumber range from the acquired spectral information of the sample, and calculate a concentration value of the substance based on the spectral peak. The concentration measuring device according to claim 10,
The wavenumber range including 605 cm ⁻¹ and its vicinity is set as the predetermined measurement wavenumber range,
The set temperature of the semiconductor detection element is set to a temperature of 4 K or more and less than 77 K,
A concentration measuring device for measuring the concentration of carbon atoms in a silicon crystal.

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