Infrared spectrum measuring device and concentration measuring device

Abstract

To provide an infrared spectrum measurement device capable of stably measuring an infrared spectrum of a prescribed measurement wave number region for a long time, and a concentration measurement device with the same.SOLUTION: An infrared spectrum measurement device includes a Fourier transform infrared spectrometer 10, a sample part 20, a refrigerator 22 for a sample, a cold filter 80, a photodetector 30, a helium refrigerator 40 for executing a refrigeration cycle by a helium refrigerant to cool an MCT detection element 32 of the photodetector 30 and the cold filter 80, a control part 50 for controlling the helium refrigerator 40 so as to make the MCT detection element 32 to be a prescribed setting temperature, and a signal processing part 60 for acquiring spectrum information based on a detection signal from the photodetector 30. The control part 50 has a setting change part 54 capable of changing the setting temperature to a plurality of temperatures within the range of 4 to 150K.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an infrared spectrometer capable of stably measuring infrared absorption spectra and the like for a long period of time, and relates to an infrared spectrometer suitable for measuring low concentration carbon, which is an impurity in silicon crystals, for example. [Background technology]

[0002] The Japan Electronics and Information Technology Industries Association (JEITA) standard EM-3503 states that the concentration of substitutional carbon atoms in silicon crystals, which are semiconductor materials, is measured using an infrared spectrophotometer to measure their specific infrared absorption peak (wave number 605 cm -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). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] JEITA Standard EM-3503 "Standard method for measuring the concentration of substitutional carbon atoms in silicon crystals by infrared absorption" [Non-Patent Document 2] SEMI Standard MF1391-1107 "Test method for substitutional carbon content in silicon by infrared absorption" Summary of the Invention [Problem to be solved by the invention]

[0004] In the field of semiconductor materials, there is a need to measure impurities in silicon crystals at lower concentrations. The inventors considered that in order to achieve this, firstly, it is important to make the wavenumber range of the absorption peak to be measured occur in a wavenumber range that shows good sensitivity characteristics of the analytical device, and secondly, it is important to be able to perform the required number of spectrum measurements under the same conditions as much as possible (identity of measurements). In other words, it is important to be able to stably measure the spectrum of a sample in the desired measurement wavenumber range for a long period of time.

[0005] An object of the present invention is to provide an infrared spectrometer 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 spectrometer to measure extremely low concentration substances in a sample. [Means for solving the problem]

[0006] The infrared spectrum measuring apparatus according to the present invention is an apparatus for measuring an infrared spectrum, comprising: a sample unit; an infrared spectrophotometer; a photodetector having a semiconductor structure; a refrigerator for cooling a semiconductor detection element of the photodetector by executing a refrigeration cycle using a predetermined refrigerant; a control unit for controlling the refrigerator so that the semiconductor detection element is at a predetermined set temperature; and a signal processing unit for acquiring spectral 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 be able to change the set temperature to a number of temperatures within a range of 4 to 150K.

[0007] Here, the type of refrigerant used in the refrigerator is not limited. More preferably, the setting change unit changes the set temperature within a range of 4 to 77K.

[0008] According to the 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 a plurality of temperatures in the range of 4 to 150 K, so that it is possible to appropriately set the temperature condition of the semiconductor detection element such that the sensitivity characteristics of this infrared spectrum measuring device become good (for example, the characteristic shows resistance to temperature fluctuation) in the wave number range of the absorption peak to be measured. Then, the temperature of the semiconductor detection element is maintained at the set temperature during the spectrum measurement of the sample by the configuration of the refrigerator and the control section. As a result, it is possible to repeatedly and stably measure the spectrum in the specified measurement wavelength range for a long period of time.

[0009] The sensitivity characteristics of an infrared spectrum measuring device may fluctuate even under temperature control by a refrigerator and a control unit. The inventors have taken advantage of the fact that changing the cooling temperature of a semiconductor detection element changes the value of the band gap energy, shifting the detection band of the semiconductor detection element in the wavenumber direction and also changing the detection sensitivity of the semiconductor detection element, and have decided to determine in advance the temperature conditions of the semiconductor detection element such that even if fluctuations in the sensitivity characteristics of the infrared spectrum measuring device occur, the effect on the spectrum measurement in the measured wavenumber range is small. Specifically, based on the detection signal of the photodetector with nothing installed in the sample section, sensitivity characteristic data of the spectrum measuring device for each cooling temperature is obtained, and these sensitivity characteristics Based on the data, determine the optimal cooling temperature set point.

[0010] That is, in the infrared spectrum measuring device, 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 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.

[0011] 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.

[0012] In 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.

[0013] Here, the "temperature conditions under which the fluctuation of the sensitivity characteristic of the infrared spectrum measuring device becomes small in a predetermined measurement wavenumber range" may be temperature conditions under which the increase or decrease in sensitivity in the wavenumber direction in the predetermined measurement wavenumber range becomes small (in other words, temperature conditions under which the wavenumber dependency of the sensitivity characteristic in the predetermined measurement wavenumber range becomes small, or temperature conditions under which the sensitivity characteristic in the wavenumber direction in the predetermined measurement wavenumber range is flat). If the temperature control of the semiconductor detection element is performed under such temperature conditions, even if the sensitivity characteristic of the infrared spectrum measuring device changes due to temperature fluctuations or the like, the sensitivity characteristic in the predetermined measurement wavenumber range will hardly change.

[0014] Alternatively, the "temperature conditions under which the variation in the sensitivity characteristics of the infrared spectrum measuring device is small in a predetermined measurement wavenumber range" may be temperature conditions under which a predetermined temperature range is determined for each temperature condition under which the sensitivity characteristics are obtained, and the variation in the multiple sensitivity characteristics obtained under temperature conditions within the temperature range is small in the predetermined 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 predetermined measurement wavenumber range will hardly change.

[0015] 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 the former criterion (the increase or decrease in sensitivity to wavenumber changes in a predetermined measurement wavenumber range is small) and the latter criterion (the variation in sensitivity characteristics in a predetermined measurement wavenumber range is small for all of the multiple sensitivity characteristics acquired under temperature conditions within a predetermined temperature range).

[0016] 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 easy to maintain the sensitivity characteristics of the measurement device in a good state in a predetermined measurement wavenumber range.

[0017] The infrared spectrum measuring device further includes an optical filter disposed between the sample portion and the semiconductor detection element, The optical filter comprises: 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 the higher wavenumber side than the predetermined measurement wavenumber range and all or a part of light on the 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.

[0018] 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 detection element. 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.

[0019] The infrared spectrum measuring device further 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 a filter changer that changes the optical filter to another optical filter.

[0020] 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 measurement under wavenumber range restriction (e.g., narrow band) by the optical filter and spectrum measurement in a wide band without such wavenumber range restriction. Also, by switching the optical filter to another optical filter, spectrum measurement in a different wavenumber range can be performed.

[0021] The infrared spectrum measuring device is characterized in that it further comprises a sample refrigerator for cooling the sample in the sample section, in addition to a refrigerator for cooling the semiconductor detection element.

[0022] With this configuration, the sample is cooled by the sample refrigerator, which makes it possible to suppress phenomena that could interfere with the measurement of the spectral peak to be measured (for example, large broadband absorption peaks associated with lattice vibrations when the sample is a crystalline material) and infrared radiation from the sample itself.

[0023] The concentration measuring device of 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 above-mentioned infrared spectrum measuring device, and 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 the concentration value of the substance based on the spectral peak.

[0024] In particular, the concentration measuring device is 605 cm -1 It is preferable to measure the carbon atom concentration in the silicon crystal by setting the set temperature of the semiconductor detection element to a temperature of 4 K or more and less than 77 K, using a wave number range including and around the wave number range as the predetermined measurement wave number range.

[0025] The concentration measuring device having the above configuration includes a refrigerator and a control unit (including a setting change unit for the set temperature) in the same manner as the infrared spectrum measuring device, so that the temperature condition of the semiconductor detection element can be appropriately set so that the sensitivity characteristics of this concentration measuring device become good (for example, the characteristic shows resistance 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 predetermined 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. [Brief description of the drawings]

[0026] [Figure 1] 1 is a diagram showing an overall configuration of a spectrum measuring device according to an embodiment; [Diagram 2] 2 is a diagram showing a detailed configuration of a control unit and a signal processing unit of the measuring device. FIG. [Diagram 3]4 is a flow diagram for determining the cooling temperature set point using the measurement device. [Figure 4] FIG. 4 is a diagram showing a schematic diagram of sensitivity characteristics of a spectrum measuring device for each cooling temperature. [Diagram 5] FIG. 4 is a diagram for explaining an example of a criterion for determining a set value of the cooling temperature. [Figure 6] 4 is a flow diagram for calculating the concentration value of a specific substance in a sample using the measurement device. [Figure 7] 5A and 5B are diagrams for explaining processing of a detection signal when calculating a concentration value of the specific substance. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The spectrum measuring device according to this embodiment is a device that can measure various spectra caused by substances in a sample with high sensitivity in a predetermined 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 concentration 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 above-mentioned JEITA standard (EM-3503) is about 0.04 ppma to 7 ppma, whereas the spectrum measuring device according to this embodiment can measure up to 0.004 ppma, which is 1 / 10 of the lower measurement limit of the JEITA standard, and can maintain such measurement for a long time.

[0028] 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.

[0029] 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 so that beam splitter 12 combines two light beams with different optical path lengths from fixed mirror 13 and movable mirror 14 to generate an infrared interference wave.

[0030] 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 a semiconductor material sample is detachably 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.

[0031] The helium refrigerator 22 for the sample cools the semiconductor material of the sample portion 20, and has the effect of suppressing the generation of large broadband absorption peaks associated with lattice vibrations of the silicon crystals, as well as the generation of infrared radiation from the semiconductor material itself.

[0032] The cold filter 80 is disposed on the optical path immediately before the photodetector. The cold filter 80 is disposed in a measurement wavenumber range (for example, 630 to 580 cm) when measuring the absorption peak of the carbon atom. -1 ) and is designed not to transmit light with lower wave numbers (longer wavelengths). The cutoff is 580 cm -1 The cold filter 80 may be a filter having a cutoff at a wavenumber lower than this, or a filter having a cutoff at a wavenumber lower than this. Infrared radiation (thermal radiation light, etc.) from the optical elements on the light source side, including the infrared light source 11, and the semiconductor material (sample) is room temperature radiation, but its influence is large in spectrum measurement at extremely low temperatures. Since it can cause temperature fluctuations in the MCT detection element 32 and the generation of thermal noise, the influence is suppressed by cutting off all or part of this light with an optical filter, and the accuracy of the spectrum measurement is improved. Note that, for example, in the measurement wavenumber range (630 to 580 cm -1) can be substituted with a bandpass filter that transmits only a specific narrow band of light that matches the wavelength of the light emitted by the laser. It is also possible to combine two filters, a cold filter (short pass filter) and a long pass filter. 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 can be placed in the room temperature area.

[0033] The photodetector 30 receives the infrared interference wave transmitted through the sample and the cold filter 80, and outputs a detection signal according to the light intensity. In this embodiment, the photodetector 30 is, for example, a mid-band MCT detector.

[0034] The MCT detection element 32 of a semiconductor structure constituting the mid-band MCT is preferably attached to the cooling section of the helium refrigerator 40 and 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 temperature measured at 77K is actually too cold, and the original appropriate cooling temperature is 80K or 90K. When measuring the content of low-concentration carbon atoms in silicon crystals as in this embodiment, the cooling temperature range of the helium refrigerator 40 is preferably set to a temperature range of 4K or more and less than 77K.

[0035] 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.

[0036] 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, it is difficult to freely control the cooling temperature because the evaporation temperature of the liquid nitrogen or liquid helium becomes the cooling temperature of the photodetector 30. In this embodiment, a helium refrigerator 40 is used to cool the photodetector 30, so that the set value of the cooling temperature can be arbitrarily determined from a wide temperature range above the evaporation temperature (for example, from about 4 K to about 150 K), making it possible to control the temperature of the MCT detection element.

[0037] It should be noted that even if a refrigerator using a refrigerant other than helium (a liquefied gas such as nitrogen) is used, it is possible to similarly control the temperature within a cooling temperature range appropriate for that refrigerant.

[0038] Additionally, the cold filter 80, together with the MCT detection element, is similarly cooled and temperature controlled by the helium refrigerator 40. By cooling the cold filter 80, infrared radiation from the filter itself, which is a cause of noise signals, is suppressed.

[0039] 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.

[0040] Although not shown, the spectrum measuring device of this embodiment may be configured to automatically and continuously measure multiple samples by 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 required for the temperature to reach the desired temperature again becomes longer than usual. If a mechanism such as an autosampler is used, such time loss can be eliminated, and long-term and multiple sample measurements can be performed more stably without being affected by disturbances caused by opening and closing the sample chamber, etc.

[0041] 2, a configuration for determining a set temperature for controlling the temperature of the photodetector 30 will be described. The control unit 50 includes a temperature control unit 52 that controls the temperature of the MCT detection element 32 via the helium refrigerator 40 to a set temperature, and a setting change unit 54 that can change the set temperature.

[0042] The signal processing unit 60 in FIG. 2 is configured to include 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 .

[0043] The Fourier transform unit 62 performs a Fourier transform on the interferogram data formed from the detection signal from the photodetector 30. The spectrum information acquisition unit 64 processes the data after the Fourier transform to calculate desired spectrum data. The concentration information acquisition unit 66 acquires the concentration value of a specific substance (such as carbon atom) in the sample based on the calculated spectrum data.

[0044] 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. In addition, the temperature condition selection unit 70 determines whether the fluctuation in the sensitivity characteristic of the spectrum measurement device is within a predetermined measurement wavenumber range (for example, 630 to 580 cm) based on the sensitivity characteristic for each set temperature. -1 The temperature condition selected by the temperature condition selection unit 70 is sent to the control unit 50 and becomes the set temperature for controlling the temperature of the MCT detection element 32 when acquiring spectrum information of the sample.

[0045] In the helium refrigerator 40 in FIG. 2, a cold filter 80 is disposed together with the photodetector 30. The cold filter 80 is provided so as to be swingable 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 not subject to such wavenumber range restriction. Note that a filter switcher configured to be able to appropriately switch between a plurality of optical filters including the cold filter 80 may be used to perform spectrum measurements in different wavenumber ranges.

[0046] 2, the control unit 50 may be replaced by a setting change unit having only the function of setting a preselected temperature condition as the set temperature for temperature control, in which 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 measurement device in the measurement wavenumber range, and is set by measuring the sensitivity characteristic of the spectrum measurement device using the same infrared spectrophotometer 10 and photodetector 30 configuration.

[0047] When the cooling temperature of the photodetector 30 changes, the sensitivity characteristics (detection sensitivity and detection band) of the MCT detection element 32 change. By utilizing this, it is possible to optimize the sensitivity characteristics by adjusting the cooling temperature. In this embodiment, the optimal set temperature in a predetermined measurement wavenumber range is determined by the following procedure.

[0048] Pre-measurement procedure The procedure for the preliminary measurement is shown in Figure 3. pieces Considering that there are differences in the individual measurement devices, it is preferable to perform preliminary measurement (resetting of temperature conditions) for each measurement device periodically, taking into consideration that the optical characteristics specific to each measurement device change over time.

[0049] 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 on 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).

[0050] 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 manner, all the sensitivity characteristics under temperature control at the predetermined set temperatures (for example, T1 to T5) are acquired.

[0051] In this preliminary measurement, the setting change unit 54 may change the set temperature, for example, in the range of 4K or more and 150K or less. The range is preferably 4K or more and less than 77K. The pitch for changing the set temperature may be, for example, 0.1K to 10K. By making the pitch finer around a temperature condition in which the change in the sensitivity characteristic of the spectrum measuring device relative to temperature change is large, the change in the sensitivity characteristic can be obtained accurately. Note that when the accuracy of temperature control of the photodetector 30 is low, there is no point in making the pitch too fine, so it is advisable to set the pitch according to the accuracy of the temperature control.

[0052] 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 measurement wavenumber range (for example, 630 to 580 cm) shown in FIG. -1 ), and select a temperature condition in which the wavenumber dependency of the sensitivity characteristic in this wavenumber range is small (criterion 1). In other words, select a 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). In the example of Figure 4, the wavenumber dependency of the sensitivity characteristic at set temperatures T3 and T4 is small, so the temperature conditions are narrowed down to T3 and T4.

[0053] In this embodiment, a temperature condition in which the change (variation) in the sensitivity characteristic with respect to temperature change is small is further selected (criterion 2). Explaining this with reference to FIG. 5, first, a predetermined temperature range (T28 to T32) including the set temperature T3 and a predetermined temperature range (T38 to T42) including the set temperature T4 are set. The difference between the maximum and minimum temperatures in each temperature range is made the same. Then, a target wave number according to the target substance (wave number 605 cm for carbon atom) is set. -1 5, the set temperature T3 has smaller variation in sensitivity for each temperature, and therefore the set temperature T for temperature control is determined to be T3.

[0054] In addition, because a change in the sensitivity of the photodetector 30 also affects the S / N ratio, a high S / N ratio may be added to the selection criteria (criterion 3). For example, the optimum temperature conditions may be determined by comprehensively applying all of criteria 1 to 3.

[0055] As a modification of the above-mentioned criterion 2, the average value of the sensitivity in the measurement wavenumber range may be calculated for each temperature, and the sensitivity with the smallest change (variation) in the average value within the set temperature range may be selected.

[0056] Even under temperature conditions where the sensitivity of the photodetector 30 is somewhat lower, the photodetector 30 can be measured in a predetermined measurement wavenumber range (for example, the target wavenumber of 605 cm -1 The flatter the sensitivity characteristic in the measurement wavenumber range (band around the measurement wavenumber range), the easier it is to obtain accurate measurement values ​​even in the presence of temperature fluctuations in the photodetector 30. In other words, the smaller the gradient of the sensitivity characteristic in the measurement wavenumber range, the more resistant it is to temperature fluctuations.

[0057] In addition, in the peak detection in this measurement, the vertical axis value of the absorption spectrum at the peak position is not simply read, but a baseline is drawn for the peak spectrum, and the difference from the baseline to the peak top is read. -1) is flat in order to improve the accuracy of the measured values. 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 the range so as to include at least the entire peak.

[0058] Note that 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 expanded 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 expanded detection band on the low wavenumber side as described above can be cut in advance by the cold filter 80. Therefore, even if the detection band is expanded unnecessarily to the low wavenumber side by changing the set temperature, the effect of this on the accuracy of the spectrum measurement can be eliminated.

[0059] Measurement procedure 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 executes temperature control 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.

[0060] Next, a reference sample is placed in the sample section 20 (step S24), and a spectrum is similarly acquired (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 acquired (steps S22 to S23).

[0061] Here, the Fourier transform infrared spectrophotometer 10 is used to measure the wavelength of about 700 to 500 cm-1 The transmission spectrum is measured in the wave number range of 1000 to 10000. The signal (SB signal) obtained by single beam measurement using the Fourier transform infrared spectrophotometer 10 is shown in FIG. 7A as a background transmission spectrum I0, a reference sample transmission spectrum I1, and a reference sample transmission spectrum I2. R The transmission spectrum of the measured sample is shown as I1.

[0062] Next, the spectrum information acquisition unit 64 acquires the transmission spectra I R , I1 are divided by the background spectrum I0 to obtain the transmittance spectrum (I R / I 0、 I1 / I0) is calculated (step S25). R / I 0、 I1 / I0) are shown respectively. They are often expressed as "%T". Next, the transmittance spectrum (I R / I 0、 The absorption spectrum of the reference sample and the measurement sample is shown in FIG. 7(C). The vertical axis represents absorbance (Abs).

[0063] The reason why it is difficult to measure the carbon concentration in silicon crystals is that the absorption peak at 605 cm -1The detection light in the wavenumber range around the measurement point becomes dark, which is not appropriate. Also, there was no photodetector with an appropriate detection band. Moreover, since the absorption peak due to the carbon concentration overlaps with a large absorption peak in a wide band due to silicon lattice vibration, as shown in the absorption spectrum of the measurement sample in FIG. 7(C), the carbon peak height is usually calculated by subtracting the absorption spectrum of a reference sample (reference wafer) whose carbon concentration can be assumed to be zero from the absorption spectrum of the measurement sample. Here, the identity of both measurements is a condition for obtaining an appropriate difference spectrum, and is the most important factor that determines 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. In addition, since it is necessary to extend the measurement time (for example, by increasing the number of integration times of the Fourier transform infrared spectrophotometer 10) to obtain a predetermined signal-to-noise ratio, it is also important to be able to stably perform long-term spectrum measurement.

[0064] In this embodiment, in the preliminary measurement, temperature conditions are selected that reduce the fluctuation in the sensitivity characteristics of the spectrometer in the measurement wavenumber range, and the selected temperature conditions are used as the set value of the cooling temperature in the main measurement. As a result, it becomes possible to perform stable spectrum measurement for a long time in the main measurement, and it becomes possible to obtain the identity between the spectrum measurement of the reference sample and the spectrum measurement of the measurement sample in the measurement wavenumber range, and it becomes possible to calculate an accurate absorption peak by taking the difference between the two spectra.

[0065] That is, in the procedure of Fig. 6, the absorption spectrum of only the specific substance (carbon atom) is calculated by subtracting the absorption spectrum of the reference sample from the absorption spectrum of the measurement sample (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 multiplied by "measurement sample thickness / reference sample thickness" can be subtracted from the absorption spectrum of the measurement sample.

[0066] Finally, the concentration information acquisition unit 66 acquires the measurement wavenumber range (for example, 630 to 580 cm) of the absorption peak of the specific substance in FIG. -1 ), a baseline is drawn at the peak wavenumber, and the absorbance Apeak at the peak top and the absorbance Abase of the baseline at the peak wavenumber are read, and the "peak height" is calculated as the difference between the two (Apeak-Abase). Then, an 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). The full half width of the absorption peak is measured, for example, at 6 cm -1 If it is greater than , it is inappropriate, so the measurement conditions may be reviewed.

[0067] Regarding the sensitivity characteristics of mid-band MCT, it is more sensitive than broadband MCT, so if possible, it is desirable to use mid-band MCT as a photodetector. However, at the cooling temperature of liquid nitrogen (77K), the cutoff of mid-band MCT is 650-600 cm -1 Located at 605cm -1 However, in the case of a mid-band MCT cooled by a helium refrigerator, the cutoff can be reduced to 500 cm. -1 It can be extended to less than

[0068] When the temperature of the MCT fluctuates, the cutoff wavenumber position shifts slightly, so peak detection in the wavenumber range near the cutoff is difficult. However, in wavenumber ranges away from the cutoff, the change in sensitivity characteristics becomes more gradual, so it is very effective to detect peaks in such wavenumber ranges.

[0069] 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-range MCT by the helium refrigerator was adjusted to set the cutoff of the mid-range MCT to 500 cm -1 Expand it to less than. In addition, the detection band of the carbon atom peak (605 cm -1The cooling temperature of the mid-band MCT is actively fine-tuned so that the temperature range (around 1000 s) is exactly the range where the change in the wavenumber direction of the sensitivity characteristic of the entire measurement device including the mid-band MCT is gradual (flat). By using a mid-band MCT with its temperature adjusted in this way, data on the carbon concentration in silicon crystals can be obtained repeatedly and stably over long periods of time.

[0070] Since the peak to be detected occurs in a specific narrow wavenumber band, the measurement sensitivity can be improved by limiting the wavenumber band of the light received by the mid-band MCT to that narrow wavenumber band. For example, the cutoff characteristics of the temperature-controlled mid-band MCT can be used to prevent unnecessary light on the low wavenumber side (longer wavelength side) from being detected. In addition, 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 the mid-band MCT in combination with a band-pass filter.

[0071] In addition, the long-pass filter and band-pass filter installed in front of the photodetector may be cooled and temperature-regulated by a helium refrigerator in the same way as the cold filter, so as 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.

[0072] In addition, by applying the present invention to the case of using a wideband MCT that can detect a wavenumber range lower than that of the midband MCT, there is an advantage that, for example, a peak in a measurement wavenumber range in a low wavenumber range can be measured with good sensitivity. Furthermore, the present invention is not limited to an MCT detector, and can also be applied to other semiconductor detectors such as various photodiodes, and infrared spectrophotometers equipped with semiconductor detectors whose sensitivity characteristics change when the cooling temperature is changed within the range of 4K to 150K. [Explanation of symbols]

[0073] 10 Fourier transform infrared spectrophotometer (infrared spectrophotometer) 20 Sample Section 22 Helium refrigerator for samples (refrigerator for samples) 30 Photodetector 32 MCT detector (semiconductor detector) 40 Helium refrigerator (refrigerator) 50 Control section 54 Setting change section 60 Signal Processing Section 68 Sensitivity characteristic acquisition section 70 Temperature condition selection section 80 Cold Filter (Optical Filter) 82 Filter changer

Claims

1. 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 150K.

2. 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 measuring device becomes small in a predetermined measurement wavenumber range; 4. The infrared spectrum measuring device according to claim 1, wherein the temperature condition is determined based on sensitivity characteristics of the infrared spectrum measuring device obtained under a plurality of temperature conditions.

3. 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. 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. 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. 6. The infrared spectrometer according to claim 1, an optical filter disposed between the sample portion and the semiconductor detection element; The optical filter comprises: 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. 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.

8. 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. 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;

10. 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.

11. The concentration measuring device according to claim 10, 605cm －１ The wave number range including and before and after the predetermined measurement wave number range is set as the predetermined measurement wave number 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.

Images