WO2015141767A1

WO2015141767A1 - Concentration meter and method for measuring concentration

Info

Publication number: WO2015141767A1
Application number: PCT/JP2015/058205
Authority: WO
Inventors: 豊文梅川
Original assignee: 豊文梅川
Priority date: 2014-03-19
Filing date: 2015-03-19
Publication date: 2015-09-24
Also published as: JP6395089B2; JPWO2015141767A1

Abstract

In the present invention, the effect of a target substance present in the atmosphere is excluded when measuring the concentration of the target substance in a sample gas by means of a TDLAS method. The present invention presupposes a device (method) for measuring the concentration of a subject substance in a gas on the basis of a TDLAS method. The sample gas fills a measurement cell at a sufficiently lower pressure than atmospheric pressure (for example, 1/10). By means of a laser light-emitting element (variable-wavelength laser diode), laser light of a wavelength in a predetermined band around the absorption wavelength of the subject substance contained in the sample gas is caused to enter the measurement cell, and the laser light that has passed through the measurement cell is received by a light-receiving element. The second derivative equivalent waveform of the received signal waveform is obtained, only the absorption signal corresponding to the laser light that has passed through the measurement cell is extracted therefrom, and on the basis of the extracted signal, the concentration of the subject substance in the sample gas is calculated by means of a computation means.

Description

Densitometer and concentration measuring method

The present invention relates to a densitometer, and more particularly to a densitometer using the TDLAS method (modulation spectroscopy).

The higher the purity of various gases such as ammonia and hydrogen chloride used in semiconductor manufacturing, the less damage to the workpiece. However, the present situation is that impurities such as moisture are mixed in the manufacturing process even if it is slight.

As described above, the moisture concentration in the gas may be required to be as small as possible depending on the application, but in order to improve this point, it is necessary to first measure the moisture concentration of the target gas.

Also, low concentration moisture management is important in various furnaces (sintering furnace, nitriding furnace, non-oxidation superheated furnace, ammonia furnace).

There is a mirror-cooled dew point meter as a device for measuring the moisture concentration in gas. That is, as shown in FIG. 7, when the mirror surface is cooled while flowing the measurement gas G over the mirror surface 200, condensation occurs on the mirror surface when the temperature of the mirror surface and the dew point of the wet gas become equal. This state is detected by comparing the intensity received by the light receiving element 202 by reflecting the light from the light source 201 to the mirror surface and the intensity received directly by the light receiving element 203 without solving the mirror surface. Therefore, the amount of saturated water vapor at the temperature indicated by the thermometer 204 when the dew condensation occurs is the moisture concentration contained in the gas.

In the above mirror-cooled dew point meter, when the humidity is low, the amount of condensation decreases, so it takes time to reach equilibrium and the responsiveness deteriorates. Therefore, measurement with a moisture concentration of 40 ppm (dew point temperature −50 ° C.) or less is not always easy and cannot be applied when high accuracy measurement is required.

In addition, in atmosphere gases other than nitrogen gas, for example, gas whose dew point changes due to interaction with water such as hydrogen chloride, the influence of the interaction must be quantitatively understood in advance in order to correct the measured dew point. There is. Furthermore, there is a limit to the types of gas that can be measured with a mirror-cooled dew point meter because, in principle, measurement cannot be performed when the boiling point / sublimation point of the target substance is lower than the boiling point / sublimation point of the atmospheric gas.

There is a TDLAS (Tunable Diode Laser Absorption Spectroscopy) method as a method for measuring the gas concentration. A measurement cell filled with atmospheric gas (sample gas) containing the target substance is irradiated with a laser around the absorption wavelength of the target substance from the wavelength tunable laser diode, and the concentration of the target substance is obtained from the magnitude of the waveform of the absorption signal It is about to try.

The TDLAS method is characterized by being able to measure ppm-level gas and moisture in high sensitivity and in real time (responsiveness is a few seconds) and is not affected by other gas components or coexisting substances.

In this method, since the laser light emitting element and the light receiving element are arranged outside the measurement cell (usually in the atmosphere), the target substance is not contained in the atmosphere or the target substance concentration in the measurement cell is This method is based on the premise that the concentration of target substances in the atmosphere is negligibly small.

Here, the moisture concentration at an atmospheric temperature of 25 ° C. and a relative humidity of 60% is 20000 ppm. On the other hand, when measuring 10 ppm (dew point temperature –60 ° C) of water in a 200 mm long measurement cell, the concentration ratio is 1/2000. The equivalent is that the optical path length of the atmosphere is 0.1 mm. Therefore, in order to suppress the influence of the moisture concentration in the atmosphere to a value that can be ignored, it is necessary to shorten the optical path length in the atmosphere to about 10 ⁻⁴ to 10 ⁻³ mm.

However, the part where the optical path is exposed to the atmosphere includes the light emitting element and collimating lens, the collimating lens and the incident window of the measuring cell, and the gap between the emitting window and the light receiving element of the measuring cell. It is impossible to do.

The present invention has been proposed in view of the above-described conventional problems, and a concentration meter and concentration measurement that can accurately measure the concentration of a target substance even when the measurement cell is arranged in the air in the TDLAS method. It is intended to provide a method.

In order to achieve the above object, the present application adopts the following means. First, the present invention presupposes an apparatus (method) for measuring the concentration of a target substance in a sample gas based on the TDLAS method.

The measurement cell is filled with sample gas at a pressure sufficiently lower than atmospheric pressure (for example, 1/10).

The laser light of a predetermined band around the absorption wavelength of the target substance contained in the sample gas is incident on the measurement cell from the laser light emitting element (wavelength variable laser diode). The laser beam that has passed through the measurement cell is received by a light receiving element.

The signal waveform obtained from this light receiving element is input to the lock-in amplifier to obtain a waveform corresponding to the second derivative. The signal thus obtained is a combination of a gentle absorption signal in a wide wavelength band that has passed through the atmosphere and a sharp absorption signal in a narrow wavelength band that has passed through the measurement cell, and the two signals have different waveforms. . Therefore, only the absorption signal corresponding to the laser light transmitted through the measurement cell is extracted, and the concentration of the target substance in the sample gas is calculated by the calculation means based on the extracted signal.

Even if the measurement cell is placed in the atmosphere by the above method (apparatus), the concentration of the target substance contained in the sample gas in the measurement cell should be taken into account in the atmosphere. It becomes possible to measure without. In addition, it is possible to measure the concentration with high accuracy and in a short time regardless of the type of the atmospheric gas.

The figure explaining the TDLAS method. The figure which shows the waveform of the output signal of each part shown in FIG. The block diagram of the densitometer of this invention. The figure which shows the waveform of the output signal of a light receiving element. Reference signal waveform diagram. The figure explaining a fitting process. The figure which shows a dew point meter.

<TDLAS method>
FIG. 1 is a conceptual diagram showing the principle of concentration measurement by the TDLAS method.

From one end of the measurement cell 100 of a predetermined length filled with the sample gas containing the target substance, a triangular wave (or sawtooth wave) of about 10 Hz having a specific current bias as shown in FIG. Laser light from a light emitting element (wavelength variable laser diode) 10 driven by a drive signal on which a sine wave of about 10 kHz is superimposed is incident. Accordingly, this laser beam has a wavelength shown in FIG. 1C in which the wavelength λ changes in accordance with the magnitude of the triangular wave (the wavelength becomes shorter as the current increases), and the wavelength λ changes in accordance with the sine wave. Become. The incident laser light is absorbed in the vicinity of a specific wavelength corresponding to the target substance in the measurement cell 100 as shown in the upper part of FIG. The concentration of the target substance is measured from the magnitude of the signal waveform corresponding to the second derivative of the absorption signal waveform.

<Principle of the present invention>
The magnitude of the absorption signal mainly depends on the optical path length during measurement, the pressure of the measurement gas (sample gas), and the concentration of the target substance. The half width of the absorption signal is proportional to the pressure of the measurement gas. The lower the pressure, the narrower the absorption wavelength band and the sharper the waveform of the absorption signal. The half-value width means the width between the point where the increase rate of the absorption signal is the largest and the point where the decrease rate is the largest. When the second derivative of the absorption signal is taken, it appears as the width between the valley peaks.

Thus, since the wavelength band of the absorption signal is proportional to the pressure of the measurement gas, the half width of the waveform of the absorption signal obtained in an environment of 1/10 of the atmospheric pressure is as shown in FIG. 2 (Aa). The half-value width of the absorption signal waveform obtained from the atmospheric pressure (FIG. 2 (Ab)) is 1/10. Incidentally, the half width of the waveform of the absorption signal near 1392.53 nm due to moisture is 0.02 nm at atmospheric pressure and 0.002 nm at 1/10 of atmospheric pressure.

When the pressure in the measurement cell 100 is 1/10 of the atmospheric pressure and the environment in which the measurement cell 100 is placed is atmospheric pressure, the optical path extends over both the atmosphere and the measurement cell as described above. The signal shown in (Ab) overlaps with the signal shown in FIG. 2 (Aa). When this signal waveform is individually differentiated to the second order, it becomes FIG. 2 (Ba) and (Bb). Here, the half-value width appears as the width of the base end (valley peak) of the rise, and this width Corresponds to the pressure of the measurement gas, and the height (size) corresponds to the concentration of the target substance.

As will be described later, in the present invention, a signal waveform shown in FIG. Here, by extracting only the waveform of the absorption signal in the measurement cell 100 from the signal waveform shown in FIG. 2 (Ca), as shown in FIG. 2 (Cb), the size of the extracted waveform ( The concentration of the target substance contained in the sample gas is obtained from the center height between the valley peaks.

<Device>
FIG. 3 is a block diagram showing an outline of the present invention. In the following examples, the target substance is moisture.

The measurement gas 100 having a predetermined length is filled with a sample gas at a pressure sufficiently lower than the atmospheric pressure, for example, 1/10. In the above description, the sample gas filled in the measurement cell 100 is not stationary, but is always kept at the above-described pressure and in a predetermined amount (300 to 1000 ml / min).

The output from the sweep signal generation circuit 11 that outputs the scanning triangular wave (or sawtooth wave) and the sine wave modulation signal output from the modulation signal generation circuit 12 are superimposed by the LD driver 13, and the result is shown in FIG. A drive signal as shown is formed.

This drive signal is input to a laser light emitting element (wavelength tunable laser diode) 10 and a laser beam close to the absorption wavelength of the target substance (here, moisture) is emitted. This light is incident on the measurement cell 100 via the collimating lens 15.

Therefore, the laser light-emitting element 10 driven by the output of the LD driver 13 has a wavelength shortened as shown in FIG. 1C according to the triangular wave current shown in FIG. Correspondingly, laser light whose wavelength changes is output.

When moisture is a measurement target, the absorption wavelength of the laser beam has a peak at 1392.53 nm. Therefore, the waveform of the absorption signal in the measurement cell 100 is a waveform having a peak at 1392.53 nm as shown in FIG.

The above means absorption of laser light in the measurement cell 100, but before reaching the measurement cell 100, between the light emitting element 10 and the collimator lens 15, or between the collimator lens 15 and the measurement cell 100. There is a slight gap (about several millimeters as a whole) between the entrance window and between the exit window of the measuring cell 100 and the light receiving element 20, and the pressure is atmospheric pressure.

As described above, the absorption intensity of the laser light is proportional to the moisture concentration and the optical path length, and is also related to the pressure and temperature (here, the light emitting element 10 to the light receiving element 20 are placed in a constant temperature bath, and the inside of the measurement cell 100 And the atmospheric temperature are the same and can be regarded as constant, and the pressure in the measurement cell 100 is kept constant). Further, the half width of the wavelength band of the absorption signal is proportional to the pressure of the measurement atmosphere as described above.

Therefore, the waveform of the absorption signal corresponding to the moisture concentration in the atmosphere portion appears in a wide range of the wavelength band of the laser beam as shown in FIG. 2A, and the waveform of the absorption signal due to the moisture concentration inside the measurement cell 100 is obtained. Appears in a narrow band as shown in FIG.

The lock-in amplifier 22 obtains a signal having a waveform corresponding to the second derivative of the signal waveform from the light receiving element 20. By the second derivative, a waveform having a wide wavelength band as shown in FIG. 2 (Ca) and a sharp waveform having a narrow band are combined, and the overlapped signal is input to the calculation means 23.

FIG. 4B shows the output of the lock-in amplifier 22 in the apparatus of FIG. 3 when the optical path length in the atmosphere is 1 mm, the length of the measurement cell 100 is 300 mm, 1/10 atm, and the moisture concentration is 20 ppm. Yes, equivalent to FIG. 2 (Ca). On the other hand, FIG. 4 (a) shows the output of the lock-in amplifier 22 due to 10000 ppm of water in the atmosphere with an optical path length of 1 mm, which is equivalent to FIG. 2 (Bb).

In the calculation means 23, a portion corresponding to the absorption signal in the measurement cell 100 is extracted from the signal obtained from the lock-in amplifier 22 as described above, and the moisture concentration is determined.

First, the reference pattern Ra of the secondary differential waveform of the absorption signal obtained in the measurement cell 100 shown in FIG. 5A and the reference pattern Rb of the secondary differential waveform of the absorption signal in the atmosphere shown in FIG. It is stored in the storage means of the calculation means 23.

Here, since the half-value width of the secondary differential waveform corresponding to the absorption signal is proportional to the pressure, if the pressure in the measurement cell 100 is fixed, the valley of the reference pattern Ra on the measurement cell side will be described. The trough width (half-value width) is a value corresponding to the pressure in the measurement cell 100 that has been fixed. Therefore, in the fitting process described later, the height may be adjusted according to the moisture concentration.

On the other hand, the bandwidth of the reference pattern corresponding to the atmosphere is almost constant around 1 atm, and the height corresponds to the humidity at that time. Therefore, the reference pattern Rb is adjusted to a standard humidity at 1 atm, for example, a moisture amount of 60%.

The two reference patterns are fitted to the waveform of the output signal from the lock-in amplifier 22. In the fitting process, the reference pattern Rb corresponding to the atmosphere is first finely adjusted in the wavelength direction and height (atmosphere humidity) direction with respect to the output signal waveform from the lock-in amplifier 22 (FIG. 6A). (Thick broken line in FIG. 6B). Next, the apex Q ₀ of the absorption waveform of the measurement cell 100 and the apex P ₀ of the reference waveform Ra of the measurement cell 100 are matched, and the height (size) direction of the reference waveform Ra is adjusted to adjust both end points P of the reference waveform. _10, P ₂₀ is to overlap the reference waveform Rb of the air, the fitting is completed (thin broken line in FIG. 6).

When the fitting is completed in this way, the reference signal of the low-pressure portion after completion is extracted, and the position corresponding to the intermediate position between valley peaks (1/2 of P ₁ -P ₂ ) and the peak peak P ₀ are extracted. Find the height h. From this height h, the calculation means 22 calculates the water concentration in the measurement cell 100. As a specific method for calculating the moisture concentration from the height h, a theoretical formula can be used, but the relationship between the height (size) h at a predetermined pressure and the moisture concentration is stored in a table. It can respond.

In the above description, only one type is stored in the storage means as the reference pattern Ra corresponding to the measurement cell 100. However, since the absorption signal waveform differs somewhat depending on the concentration, the low concentration region, the medium concentration region, and the high concentration It is also possible to store about three types of reference patterns of areas.

The signal shown in FIG. 6B is equivalent to the waveform schematically shown in FIG. Further, the signal shown in FIG. 6C is equivalent to the waveform schematically shown in FIG. Further, the reference pattern Rb corresponding to the atmosphere is also stored in the calculation means, but only the reference waveform Ra corresponding to the measurement cell 100 may be stored. In this case, the reference waveform Ra is directly fitted to the composite waveform shown by the solid line in FIG. 6C, but this method may be used if a slight error enlargement is allowed.

As described above, when the moisture concentration is calculated, a value can be obtained with high accuracy even if the moisture concentration is 40 ppm or less.

In the above description, moisture is the target substance, but the present invention is not limited to this.

As described above, according to the present invention, the concentration of the target substance (for example, moisture) in the atmospheric gas (sample gas) introduced into the measurement cell can be accurately determined in a short time even in the situation where the target substance exists in the atmosphere. Can be measured. In addition, the concentration can be measured with high accuracy and in a short time regardless of the type of atmospheric gas, and the industrial applicability is extremely high.

DESCRIPTION OF SYMBOLS 10 Laser light emitting element 11 Sweep signal generation circuit 12 Modulation signal generation circuit 13 LD driver 15 Collimating lens 20 Light receiving element 22 Lock-in amplifier 23 Calculation means 100 Measurement cell Ra Reference pattern Rb Reference pattern

Claims

Based on the TDLAS method, in a device that measures the concentration of the target substance in the sample gas,
A measurement cell of a predetermined length filled with a sample gas at a pressure sufficiently lower than atmospheric pressure;
A laser emitting element that emits laser light having a wavelength in a predetermined band around the absorption wavelength of the target substance contained in the sample gas with respect to the measurement cell;
A light receiving element that receives laser light incident from the laser light emitting element through a sample gas filled in the measurement cell;
A lock-in amplifier that receives a signal from the light receiving element and outputs a waveform obtained by second-order differentiation of the waveform of the input signal;
A densitometer comprising: an arithmetic unit that extracts an absorption waveform of a target substance in a sample gas from a waveform of a signal output from the lock-in amplifier, and calculates a concentration from the magnitude.
The calculation means stores the output of the lock-in amplifier when the target substance has a predetermined concentration at the pressure of the measurement cell as a first reference waveform, and the first reference waveform is stored in the lock-in amplifier. The concentration meter according to claim 1, wherein the absorption waveform of the target substance in the sample gas is extracted by fitting to a portion corresponding to the absorption waveform of the output measurement cell.
The calculation means stores an output of the lock-in amplifier when the target substance has a predetermined concentration under atmospheric pressure as a second reference waveform, and outputs the second reference waveform as an output of the lock-in amplifier. Of the target substance in the sample gas by fitting the first reference waveform to the portion corresponding to the absorption waveform of the measurement cell of the output of the lock-in amplifier. The densitometer according to claim 1, wherein an absorption waveform is extracted.
Based on the TDLAS method, in the method of measuring the concentration of the target substance in the sample gas,
Filling the sample gas with a pressure sufficiently lower than atmospheric pressure in the measurement cell of a predetermined length; and
Entering a laser beam having a wavelength in a predetermined band around the absorption wavelength of the target substance contained in the sample gas from the laser light emitting element to the measurement cell;
Receiving a laser beam incident from the laser light emitting element through a sample gas filled in the measurement cell by a light receiving element;
A step of outputting a second-order differentiated waveform of the signal from the light receiving element by a lock-in amplifier;
A concentration measuring method comprising: a step of extracting an absorption waveform of a target substance in a sample gas from a waveform of a signal output from the lock-in amplifier, and calculating a concentration based on the waveform of the signal output from the lock-in amplifier. .
The calculation means stores the output of the lock-in amplifier when the target substance has a predetermined concentration at the pressure of the measurement cell as a first reference waveform, and the first reference waveform is stored in the lock-in amplifier. The concentration measurement method according to claim 4, wherein the absorption waveform of the target substance in the sample gas is extracted by fitting a portion corresponding to the absorption waveform of the output measurement cell.
The calculation means stores an output of the lock-in amplifier when the target substance has a predetermined concentration under atmospheric pressure as a second reference waveform, and outputs the second reference waveform as an output of the lock-in amplifier. Of the target substance in the sample gas by fitting the first reference waveform to the portion corresponding to the absorption waveform of the measurement cell of the output of the lock-in amplifier. The concentration measuring method according to claim 4, wherein an absorption waveform is extracted.