JP2013050403A - Gas analyzer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas analyzer by a TDLAS system capable of widening a measurable range of concentration measurement of a target gas.SOLUTION: In a gas analyzer according to the present invention, in a case where an absorption line currently used is determined not to be suitable for concentration measurement of a target gas by an absorption line determining section 82, one piece of information of another absorption line is selected from a plurality of absorption lines stored in an information storage section 81. In the information storage section 81, setting values of a driving current and a control temperature of an LD1 corresponding to each of the plurality of absorption lines are stored. A controller 2 controls an LD driving section 3 and an LD temperature controlling section 5 on the basis of the setting values and starts next measurement. On other hand, in a case where the absorption line currently used is determined to be suitable for the concentration measurement, a concentration calculating section 83 calculates concentration on the basis of individual measuring values of transmission light intensity, a gas temperature and pressure corresponding to the absorption line while referring to information of absorption line strength and an absorption characteristics function corresponding to the absorption line stored in the information storage section 81.

Description

  The present invention relates to a gas analyzer that measures the concentration of a specific component in a gas to be measured using absorption of laser light.

  One method for measuring the concentration of a target gas in a gas (a gas to be measured) is absorption spectroscopy that utilizes the fact that gas molecules absorb only light of a specific wavelength. One of the features of absorption spectroscopy is that the concentration of the target component gas (target gas) can be measured without contacting the gas to be measured.

  In recent years, among the absorption spectroscopy methods, the TDLAS (Tunable Diode Laser Absorption Spectroscopy) method has been established, which uses a semiconductor laser diode as a light source and scans the wavelength of the laser beam for a predetermined absorption line (absorption spectrum) of the target gas. ing. In the TDLAS method, since the monochromaticity of the light source is high, it is possible to selectively receive light absorption of only the target gas. As a result, it is hardly affected by other gas components. In addition, since the laser diode can easily perform high-speed lighting and high-speed modulation driving, it has an advantage of excellent responsiveness.

Hereinafter, the theory of the TDLAS method will be briefly described. The laser light frequency is ν, the incident light intensity I ₀ (ν) of the laser light to the measurement gas at the frequency ν, the transmitted light intensity of the laser light at the frequency ν is I (ν), and the molecular number density of the target gas is c Lambert Bale, where L is the optical path length through the gas to be measured, S is the line intensity (absorption line intensity) of the absorption line of the target gas, and K (ν) is the profile function (absorption characteristic function) of the absorption line From the (Lambert-Beer) law, the following equation (1) holds.

The TDLAS method includes a method of measuring and integrating the light intensity over the frequency ν, and a method of measuring the light intensity at the center frequency ν ₀ of the absorption line. Only the latter method will be described here. At the center frequency ν ₀ , the expression (1) is expressed by the following expression (2).

The absorption line intensity S is generally approximated by multiplying the absorption line intensity S _ref in the standard state by a correction term related to the temperature T. The value of the absorption line strength S _ref can be easily known from HITRAN provided as a database of absorption lines. Further, the optical path length L is known. Accordingly, if even the absorption characteristic function K (ν ₀ ) can be accurately determined, the concentration of the target gas can be calculated from the equation (2).
It is known that the absorption characteristic function K (ν) is one of three types of functions depending on the pressure of the gas to be measured.

ここで、γ_Lは大気圧における吸収スペクトルの半値半幅であり、被測定ガスの種類、温度、及び圧力に依存する。
特に中心周波数ν₀においては次の(4)式となる。

[1] When the gas to be measured is atmospheric pressure In this case, the absorption characteristic function K (ν) is represented by a Lorentz function as shown in the following equation (3).

Here, γ _L is the half-width of the absorption spectrum at atmospheric pressure, and depends on the type of gas to be measured, temperature, and pressure.
Especially at the center frequency ν ₀ , the following equation (4) is obtained.

上式のγ_EDはドップラ幅と呼ばれ、次の(6)式で表される。

ここで、ｋ_Bはボルツマン定数、Tはガス温度、Mは目的ガスの分子量である。(6)式より、ドップラ幅γ_EDは、吸収線の中心周波数、分子量、及び温度に依存しており、1Torrよりも高真空領域では、吸収特性関数K(ν)は被測定ガスの圧力による影響を受けないことが分かる。
(5)式の吸収特性関数K(ν)は特に中心周波数ν₀においては次の(7)式となる。

[2] When the total pressure of the gas to be measured is in a vacuum region higher than 1 Torr In this case, the absorption characteristic function K (ν) is a Gaussian function as shown in the following equation (5).

Γ _{ED in the} above equation is called the Doppler width and is expressed by the following equation (6).

Here, k _B is the Boltzmann constant, T is the gas temperature, and M is the molecular weight of the target gas. From equation (6), the Doppler width γ _ED depends on the center frequency, molecular weight, and temperature of the absorption line, and in the vacuum region higher than 1 Torr, the absorption characteristic function K (ν) depends on the pressure of the gas to be measured. It turns out that it is not influenced.
The absorption characteristic function K (ν) in the equation (5) becomes the following equation (7) particularly at the center frequency ν ₀ .

[3] In the case of an intermediate pressure between atmospheric pressure and 1 Torr In this case, the absorption characteristic function K (ν) is called a Vogt function, and is expressed by a convolution function of the Lorentz function and the Gauss function. It becomes.

ここで、tは時間、aは周波数変調のための正弦波信号の変調振幅、ωは周波数である。 The above is a method for calculating the concentration by directly measuring the incident light intensity I ₀ (ν) and the transmitted light intensity I (ν) (hereinafter referred to as “direct absorption spectroscopy”). As a method effective when the frequency is low, spectroscopy (hereinafter referred to as “wavelength modulation spectroscopy”) that detects at a frequency that is an integral multiple of the laser modulation wave is known (see, for example, Non-Patent Document 1). When performing wavelength modulation spectroscopy, the frequency of light irradiated to the gas to be measured is modulated as shown in the following equation (8).

Here, t is time, a is the modulation amplitude of a sine wave signal for frequency modulation, and ω is frequency.

ここで、constは比例定数であり、検出器及び同期検出回路の感度によって変化する。比例定数constは、上記の直接吸収分光法など別の手段で予め既知となった濃度のガスを測定することにより決定される。 In the second harmonic synchronization detection, a signal component corresponding to a frequency 2ω that is twice the frequency of the laser light frequency-modulated by the equation (8) is extracted. The second harmonic detection signal signal (ν) at the frequency ν is defined by the following equation (9).

Here, const is a proportionality constant and changes depending on the sensitivity of the detector and the synchronization detection circuit. The proportionality constant const is determined by measuring a gas having a known concentration in advance by another means such as the direct absorption spectroscopy described above.

Japanese Patent Laid-Open No. 5-99845

Webster, “Infrared Laser Absorption: Theory and Applications in Laser Remote Chemical Analysis”, “Infrared Laser Absorption: Theory and Applications in Laser Remote Chemical Analysis” Wiley, New York, 1988

  The gas concentration measurement by the TDLAS method is used, for example, for monitoring water that is harmful in the semiconductor processing gas, monitoring carbon monoxide, nitrogen dioxide, etc. in the flue. The TDLAS method can calculate the concentration of the target gas without contacting the gas to be measured, so in-line measurement is possible with minimal changes to the exhaust lines and flues of existing semiconductor manufacturing equipment. It becomes. However, on the other hand, the optical path length may not be set freely. In such a case, for example, if the optical path length is too long, the laser beam is excessively absorbed by the target component gas and attenuated significantly, resulting in a decrease in measurement accuracy and the measurement itself being impossible. May be frustrated.

  Further, in a semiconductor manufacturing apparatus or the like, the concentration of the target gas may be measured under a situation where the pressure of the gas to be measured is suddenly and greatly reduced from near atmospheric pressure. In such a case, the concentration of the target gas may change greatly with a sudden change in pressure of the gas to be measured, and the transmitted light intensity may change greatly, thereby exceeding the detectable range of the laser light receiving unit.

  The problem to be solved by the present invention is to provide a gas analyzer using the TDLAS method, which can widen the measurable range of concentration measurement of the target gas.

The present invention made to solve the above problems
A laser irradiation unit that irradiates the gas to be measured with a laser beam; and a light receiving unit that receives the laser beam that has passed through the gas to be measured. From the intensity of the laser beam received by the light receiving unit, In the gas analyzer that calculates the concentration of the target gas contained in
Information storage means for storing information relating to a plurality of absorption lines of the target gas, and information for calculating the concentration of the target gas from the received light intensity in each of the plurality of absorption lines;
An oscillation wavelength adjusting means for setting an oscillation wavelength of the laser irradiation unit based on information on each absorption line;
Concentration calculating means for calculating the concentration of the target gas based on the intensity of the laser beam received by the light receiving unit and each concentration calculation information;
It is characterized by having.

  In the conventional gas analyzer using the TDLAS method, when measuring the concentration of a predetermined target gas, only one absorption line of the target gas is targeted. On the other hand, the gas analyzer according to the present invention is characterized in that a plurality of absorption lines are used. This produces the following effects.

  Even for the same target gas, the absorption coefficient changes if the absorption line is different. That is, the absorption amount (attenuation amount) of the laser beam by the target gas differs for each absorption line. Therefore, for example, when the transmittance of light received by the light receiving unit is extremely small (that is, the light absorption rate is extremely large), the laser oscillation wavelength is switched to an absorption line having a small absorption coefficient, and the transmittance is conspicuous. If it is large (that is, the light absorptivity is remarkably small), the concentration measurement can be performed with an appropriate transmittance by measuring the laser oscillation wavelength with an absorption line having a large absorption coefficient. Similarly, if the optical path length must be increased depending on where the gas analyzer is installed, switch to an absorption line with a small absorption coefficient, and if the optical path length must be reduced, an absorption line with a large absorption coefficient. By switching to, it becomes possible to cope with various use conditions.

In order to calculate the concentration, the absorption line intensity and the absorption characteristic function are required. Since these differ for each absorption line, they are stored in the information storage means corresponding to each absorption line and used when calculating the concentration. The concentration calculation information may be parameters (for example, γ _L , γ _ED , S _ref, etc.) for calculating these, instead of the absorption line intensity and the absorption characteristic function itself.

  The oscillation wavelength adjusting means can adjust the oscillation wavelength of the laser irradiation unit by controlling either one or both of a laser driving current flowing through the laser irradiation unit and a temperature of the laser irradiation unit.

  In the gas analyzer according to the present invention, when measuring the concentration of the target gas, it is possible to perform measurement by selecting an absorption line suitable for measurement from a plurality of absorption lines. Thereby, it is possible to cope with various use conditions with one gas analyzer. Further, it is possible to widen the measurable concentration range as compared with the conventional method and measure with high accuracy.

1 is a schematic block diagram of a moisture measuring apparatus that is an embodiment of a gas analyzer according to the present invention. The figure which shows an example of the signal waveform obtained by a direct absorption spectroscopy. The graph which shows the relationship between the wavelength of a laser beam and a water vapor transmission rate in a 1.3 micrometer band. The graph which shows the relationship between a water concentration and the transmittance | permeability when the wavelength of a laser beam is 1.3686 micrometers or 1.3692 micrometers. The graph which shows the relationship between a water concentration and the transmittance | permeability when the wavelength of a laser beam is 1.3686 micrometers or 1.3692 micrometers.

  A moisture measuring apparatus which is an embodiment of a gas analyzer according to the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic block diagram of a moisture measuring apparatus according to this embodiment. The apparatus of this embodiment measures the concentration of water (water vapor) in the gas to be measured by direct absorption spectroscopy. However, other gas concentrations can be used as long as the gas absorbs laser light at a specific wavelength. It is also possible to measure.

  In the moisture measuring apparatus of the present embodiment, the semiconductor laser diode (LD) 1 is scanned in wavelength within a predetermined wavelength range according to the drive current supplied from the LD drive unit 3 under the control of the control unit 2. The measurement cell 4 is irradiated with laser light. For example, a DFB (Distributed Feedback) type laser with a wavelength of 1.3 μm in which water molecules have absorption lines can be used as LD1, but other than this, oscillation occurs at a wavelength at which absorption lines of water molecules exist. Any tunable laser can be used. Of course, when measuring the concentration of another target gas instead of water, a wavelength tunable laser that oscillates at a wavelength where the absorption spectrum of the target gas exists may be used. The LD 1 is also temperature-controlled by the LD temperature adjusting unit 5 under the control of the control unit 2.

  The measurement gas is continuously introduced into the measurement cell 4, and the laser light emitted from the LD 1 is absorbed by the components contained in the measurement gas while passing through the measurement cell 4. Thus, the laser beam after receiving the absorption reaches the photodiode (PD) 6, and the PD 6 outputs a measurement signal corresponding to the received light intensity. The measurement signal output from the PD 6 is converted into a digital value by the first A / D converter 7 and input to the data processing unit 8 as transmitted light intensity I (ν).

When calculating the amount of water by the equation (2), in addition to the transmitted light intensity I (ν ₀ ) at the center frequency ν ₀ of the target absorption line, the incident light intensity I ₀ (ν ₀ ) at the center frequency ν ₀ is also calculated. Necessary. The incident light intensity I ₀ (ν ₀ ) can be obtained by the following method.

FIG. 2 shows an example of a signal waveform of transmitted light intensity I (ν), where the horizontal axis represents frequency deviation ν−ν ₀ and the vertical axis represents signal intensity. As shown in this figure, an absorption spectrum is observed at a frequency deviation of zero, that is, near the center frequency ν ₀ . Since the incident light intensity I ₀ (ν) is the signal intensity when there is no absorption by the target gas, an approximate curve is drawn from the transmitted light intensity I (ν) measured in the non-absorption region, and the approximate curve at the center frequency ν ₀ . Is obtained, the incident light intensity I ₀ (ν ₀ ) can be obtained.

As the incident light intensity I ₀ (ν ₀ ), the transmitted light intensity measured in advance in the absence of the target gas (moisture in this embodiment) can also be used.

A temperature sensor 9 and a pressure sensor 10 are disposed in the measurement cell 4, and the gas temperature T and pressure (total pressure) P of the gas to be measured are measured. The measurement signals output from the temperature sensor 9 and the pressure sensor 10 are converted into digital values by the second A / D converter 11 and the third A / D converter 12, respectively, and then input to the data processing unit 8. The
When the gas temperature of the gas to be measured is known, the temperature sensor 9 and the second A / D converter 11 can be omitted from the configuration of FIG. Similarly, when the pressure of the gas to be measured is known, the pressure sensor 10 and the third A / D converter 12 can be omitted from the configuration of FIG.

  The data processing unit 8 includes an information storage unit 81, an absorption line determination unit 82, and a concentration calculation unit 83. As described above, the present invention is characterized in that a plurality of absorption lines are used for measuring the concentration of the target gas. In the information storage unit 81, various information used for measuring the concentration of water molecules is stored corresponding to each absorption line. Here, the information used for the concentration measurement is, for example, a set value for the control unit 2 to control the LD driving unit 3 and the LD temperature adjusting unit 5, a determination condition in the absorption line determination unit 82, and a concentration in the concentration calculation unit 83. Absorption line intensity and absorption characteristic function used for calculation.

The absorption line determination unit 82 is for determining whether or not the absorption lines of water molecules targeted in the current measurement are suitable for the measurement. If the absorption line determination unit 82 determines that the absorption line is not suitable, the absorption line different from the absorption line targeted this time is selected from the information of the plurality of absorption lines held in the information storage unit 81. Select one piece of information. The information storage unit 81 holds setting values of the drive current and temperature control temperature of the LD 1 corresponding to each absorption line. This set value is sent to the control unit 2, and the control unit 2 controls the LD driving unit 3 and the LD temperature adjusting unit 5 based on the set value, and starts the next measurement.
When the absorption line determination unit 82 determines that the target absorption line is suitable for measurement, the concentration calculation unit 83 calculates the water concentration. The concentration calculation unit 83 refers to the measured values of transmitted light intensity, gas temperature, and pressure while referring to the information of the absorption line intensity and the absorption characteristic function corresponding to the absorption line held in the information storage unit 81. To calculate the concentration. The result is sent to the output unit 13, and the result is output to a monitor or the like.

  The measurement operation of the moisture measuring device of the present embodiment will be described with a specific example.

  FIG. 3 shows an absorption band near 1.3686 μm, which has a large absorption coefficient in the 1.3 μm band. In general DFB-type laser characteristics, when the semiconductor element is changed by 10 ° C., the oscillation wavelength changes by about 1 nm. Therefore, when an absorption line having a center wavelength of 1.3686 μm is used for concentration measurement, a practical candidate as another absorption line is an absorption line having a relatively small absorption coefficient within ± 5 nm, Absorption lines having center wavelengths of 1.3682 μm, 1.3689 μm, and 1.3692 μm shown in FIG. 2 can be preferably used. Of course, other absorption lines may be used.

In the following specific examples, the absorption lines used for concentration measurement are those having a center wavelength of λ ₁ = 1.3686 μm and λ ₂ = 1.3692 μm.

  4 and 5 show the water concentration and transmittance (transmitted light intensity / incident light intensity) when the optical path length L is 1000 cm, the gas temperature T is 300 K, and the wavelength of the laser light is 1.3686 μm and 1.3692 μm. It shows the relationship. FIG. 4 shows a change in ppm order at a relatively low concentration, and FIG. 5 shows a change in water vapor level naturally present in the atmosphere at around 10,000 ppm (1%). From these figures, at a wavelength of 1.3686 μm where the absorption coefficient is large, L = 1000 cm, it is suitable for measuring trace moisture below the ppm order, but the laser light is absorbed too much at the% order, and almost up to PD6. I understand that it does not reach. On the other hand, at a wavelength of 1.3692 μm, although the change in transmittance is small with respect to a change in moisture content on the order of ppm, the transmittance continues to change up to the% order. In such an environment, it is possible to perform measurement over a wide range and with high accuracy by using an absorption line of this wavelength.

を吸収線判定部８２の判定条件とし、この判定条件が満たされていれば、中心波長λ₁=1.3686μmの吸収線の選択が適切であるとして、次の式から水分濃度（水分子数密度）cを算出する。

上式のS₁及びK₁はν₁を中心周波数とする吸収線に対応する吸収線強度と吸収特性関数であり、情報記憶部８１に格納された情報と、ガス温度の測定値Tと、圧力の測定値Pとに基づいて算出される。 Next, selection of an absorption line used for measurement and calculation of concentration for each absorption line will be described. For example, assume that concentration measurement is initially performed for an absorption line having a center wavelength λ ₁ = 1.3686 μm (center frequency ν ₁ ) in a low-concentration moisture state. here,

Is determined as the determination condition of the absorption line determination unit 82. If this determination condition is satisfied, it is determined that the absorption line having the center wavelength λ ₁ = 1.3686 μm is appropriate, and the moisture concentration (water molecule number density) is ) Calculate c.

S ₁ and K _{1 in the} above equation are the absorption line intensity and the absorption characteristic function corresponding to the absorption line having ν ₁ as the center frequency, the information stored in the information storage unit 81, the measured value T of the gas temperature, Calculated based on the measured pressure value P.

On the other hand, if the moisture concentration rises and the condition of equation (10) is not satisfied, it is judged that the moisture concentration is too high for this absorption line, and the absorption line of λ ₂ = 1.3692 μm (center frequency ν ₂ ) is targeted. Switch to the measurement. At this time, if the setting of the drive current is not changed, and the temperature adjustment temperature setting of the LD temperature adjustment unit 5 is increased by about 6 ° C., the oscillation wavelength of the LD 1 changes as described above, and the absorption line of λ ₂ = 1.3692 μm. Can be measured.
Note that the absorption line may be switched not by setting the temperature control temperature of the LD temperature control unit 5 but by setting the drive current. Further, the absorption line may be switched by changing both settings.

を判定条件として用いる。この判定条件が満たされていれば、中心波長λ₂=1.3692μmの吸収線の選択が適切であるとして、次の式から水分濃度（水分子数密度）cを算出する。

ここで、S₂及びK₂はν₂を中心周波数とする吸収線に対応する吸収線強度と吸収特性関数であり、情報記憶部８１に格納された情報と、ガス温度の測定値Tと、圧力の測定値Pとに基づいて算出される。 On the other hand, when performing the measurement for the absorption line of λ ₂ = 1.3692 μm, the absorption line determination unit 82

Is used as a determination condition. If this determination condition is satisfied, the moisture concentration (water molecule number density) c is calculated from the following equation, assuming that the selection of the absorption line with the center wavelength λ ₂ = 1.3692 μm is appropriate.

Here, S ₂ and K ₂ are the absorption line intensity and the absorption characteristic function corresponding to the absorption line having ν ₂ as the center frequency, the information stored in the information storage unit 81, the measured value T of the gas temperature, Calculated based on the measured pressure value P.

If the moisture concentration decreases and the condition of equation (12) is not satisfied, it is determined that the moisture concentration is too low for this absorption line, and the temperature adjustment temperature setting of the LD temperature adjustment unit 5 is reduced by about 6 ° C. , Switch to measurement for the absorption line of λ ₁ = 1.3686 μm.

  Although one embodiment of the gas analyzer according to the present invention has been described above, it is obvious that modifications, corrections, additions, etc. as appropriate within the scope of the present invention are included in the scope of the claims of the present application. It is.

  For example, in the above embodiment, an absorption line suitable for measurement is automatically selected by the determination in the absorption line determination unit 82, but the apparatus shows an absorption line that can be selected for the output unit, and the analyst himself selects one of them. It can also be set as the structure which selects.

1 ... Semiconductor laser diode (LD)
2 ... Control unit 3 ... LD drive unit 4 ... Measurement cell 5 ... LD temperature control unit 6 ... Photodiode (PD)
7 ... 1st A / D converter 8 ... Data processing part 81 ... Information storage part 82 ... Absorption line determination part 83 ... Concentration calculation part 9 ... Temperature sensor 10 ... Pressure sensor 11 ... Second A / D converter 12 ... 1st Three A / D converters 13 ... output section

Claims (6)

A laser irradiation unit that irradiates the gas to be measured with a laser beam; and a light receiving unit that receives the laser beam that has passed through the gas to be measured. From the intensity of the laser beam received by the light receiving unit, In the gas analyzer that calculates the concentration of the target gas contained in
Information storage means for storing information relating to a plurality of absorption lines of the target gas, and information for calculating the concentration of the target gas from the received light intensity in each of the plurality of absorption lines;
An oscillation wavelength adjusting means for setting an oscillation wavelength of the laser irradiation unit based on information on each absorption line;
Concentration calculating means for calculating the concentration of the target gas based on the intensity of the laser beam received by the light receiving unit and each concentration calculation information;
A gas analyzer characterized by comprising:   The oscillation wavelength adjusting unit adjusts an oscillation wavelength of the laser irradiation unit by controlling either one or both of a laser driving current passed through the laser irradiation unit and a temperature of the laser irradiation unit. The gas analyzer according to 1.   The absorption line suitable for measuring the target gas is automatically selected from the plurality of absorption lines based on the transmittance or absorption rate of light received by the light receiving unit. The gas analyzer described in 1.   The gas analyzer according to claim 1, wherein the target gas is moisture.   5. The absorption line having a center wavelength of 1.3686 [mu] m among the plurality of absorption lines used for measuring the moisture concentration, and the other absorption line is between 1.3636 [mu] m and 1.3736 [mu] m. The gas analyzer described in 1.   Among the plurality of absorption lines used for measuring the moisture concentration, one is an absorption line having a center wavelength of 1.3686 μm, and the other is any one or more of absorption lines having center wavelengths of 1.3636 μm, 1.3689 μm, and 1.3692 μm The gas analyzer according to claim 4, wherein

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