JP2003222591A - Gas measurement device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small and inexpensive ammonia gas measurement device having high measurement accuracy. <P>SOLUTION: This gas measurement device is provided with: a first laser beam source for outputting a wavelength of λ1; a second laser beam source for outputting a wavelength of λ2; a nonlinear crystal for generating and outputting a wavelength of λ3 absorptive for at least a measurement object gas by inputting the two wavelengths; a filter for passing the wavelength of λ3 out of light outputted from the nonlinear crystal; a gas cell with the measured gas for inputting the laser beam passing the filter introduced; and a light receiving element for receiving the light passing the gas cell. The concentration of the measured gas is measured with the pressure in the gas cell lowered to 0.3 atm. or below. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a concentration measuring device for ammonia gas, which is an important index in semiconductor processes and the like.

[0002]

2. Description of the Related Art The presence of ammonia gas influences the quality of semiconductor processes, especially in the exposure process. The concentration of ammonia gas that affects quality is 1 to 0.1 ppb
It is an order. Therefore, the ammonia gas measuring device is required to have high sensitivity and high accuracy. FIG. 8 is a schematic configuration diagram showing an example of a conventionally used ammonia gas measuring device.

In the figure, reference numeral 21 denotes a diffusion scrubber, which has a pure water circulating device 22 for circulating pure water which is an ammonia absorbing liquid. Although not shown in the figure, this diffusion scrubber is made of porous Teflon (registered trademark) in a glass tube.
It has a double-tube structure in which a tube is inserted, the absorption liquid of the pure water circulation device 22 circulates on the outside, and the atmospheric gas collected in a clean room or the like flows on the inside. Two
Reference numeral 3 is a gas collection pump for sucking the atmospheric gas.

The absorbing liquid which has absorbed the atmospheric gas is sent to an automatic switching valve 27 arranged on the analysis means 24 side via a switching valve 25 and a sample pump 26. The automatic switching valve 27 concentrates the absorption liquid in the concentration column 28 and sends it to the separation column 30 side together with the eluent from the eluent pump 29. The detector 31 analyzes the gas component contained in the atmospheric gas for each component separated by the separation column 30.

In addition to the above-mentioned analysis device, a semiconductor infrared analyzer shown in FIG. 9 and a microphone infrared analyzer shown in FIG. 10 are known as means for measuring ammonia gas and the like.

[0006]

By the way, in the measuring device shown in FIG. 8, there is a problem that it takes time for concentration and separation, the device becomes large in size, and maintenance is very difficult and very expensive. In the conventional device shown in (1), when measuring the concentration of a trace amount of a specific gas in the gas to be measured, it is often difficult to measure the concentration of the target gas due to the influence of other gases, and especially when the water content is wide. Adversely affect the area.

The present invention has been made to solve the above problems, and an object of the present invention is to realize a gas measuring apparatus which enables highly sensitive analytical measurement and is downsized.

[0008]

In order to solve such a problem, the present invention provides a first laser light source which outputs a wavelength of λ1 and a second laser which outputs a wavelength of λ2. A light source, a nonlinear crystal that inputs these two wavelengths and generates and outputs at least an absorption wavelength λ3 for the gas to be measured, a filter that passes the wavelength λ3 of the light output from this nonlinear crystal, and this filter A gas measuring device having a gas cell into which a gas to be measured for inputting laser light that has passed through is introduced, and a light receiving element for receiving light transmitted through this gas cell, wherein the pressure in the gas cell is 0.3 atm or less. It is characterized in that the measured gas concentration is measured in a state of being lowered to.

According to a second aspect, in the gas measuring apparatus according to the first aspect, either or both of the λ1 and λ2 are swept to set the wavelength of λ3 to 2.2115 to 2.2.
It is characterized by sweeping in a range including 14 μm.

According to a third aspect of the present invention, in the gas measuring apparatus according to the first or second aspect, the wavelength of λ3 is FM-modulated near the absorption peak value of the output signal from the light receiving element,
It is characterized in that the 2f signal is used as an output signal.

According to a fourth aspect of the present invention, in the gas measuring device according to any of the first to third aspects, a half mirror is provided in front of the gas cell to split the light having a wavelength of λ3, and the split light is incident. A reference gas cell is provided, a measurement target gas having a known concentration or a gas containing a measurement target gas having a known concentration is introduced with the internal pressure of the reference gas cell being 0.3 kg / cm ² , and the wavelength of the measurement laser (λ3) is calibrated. It is characterized by doing so.

According to a fifth aspect of the present invention, in the gas measuring device according to any of the first to fourth aspects, when introducing the gas to be measured into the gas cell, a nozzle is provided at the gas inlet and the gas is introduced from the tip of the nozzle. Characterized by introducing while blowing out.

According to a sixth aspect of the present invention, in the gas measuring apparatus according to any of the first to fifth aspects, the measuring laser (λ3) is reflected to the light receiving element side after being reflected an odd number of times in the gas cell. Characterize.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION First, referring to FIG.
The relationship between the wavelength and the amount of absorption in various gases in the range of 5 μm will be described. Such a relationship is generally known. Here, targeting gas in the atmosphere, water vapor (humidity): 10%, CO ₂ ; 330 ppm, N ₂ O; 0.3 ppm,
CH ₄ ; containing 1.6 ppm, was indicated as ammonia gas being present at a concentration of 1 ppb. In the figure, the horizontal axis represents wavelength and the vertical axis represents absorption amount (no unit). The portion surrounded by the alternate long and short dash line shows the distribution range of each gas.

According to the figure, H ₂ O has absorption over the entire range, and CO ₂ has N of about ₂ to 2.15 μm and 2.5 μm.
₂ O is _2, 2.1 to 2.185, 2.25 to 2.32,
In the vicinity of 2.45 to 2.453 μm, CH ₄ is 2.14 to
In the vicinity of 2.5 μm, NH ₃ contains 2 to 2.09 and 2.125.
It can be seen that there is absorption at around 2.48 μm.

FIG. 7 shows the wavelength range shown in FIG. 6 narrowed down to a specific range. Here, the wavelength range on the horizontal axis is 2.2.
It is set to 08 to 2.218 μm. As can be seen from the figure, absorption in CO ₂ , N ₂ O, and CH ₄ is not seen in this wavelength range, but only in H ₂ O and NH ₃ .

Further, the wavelength range 2.2 shown by the arrow A is 2.2.
115 to 2.2124 μm and wavelength range indicated by arrow B
Focusing on 2.213 to 2.21385 μm, this range
Then H _TwoThere is no influence of absorption by O, and the wavelength range
H in the range of 2.215 to 2.214 μm_TwoO's
Ammonia is not affected in this wavelength range because it has almost no effect.
Can be regarded as being absorbed only by
It

By the way, at present, one laser light source that can output the above wavelength range is not commercially available. FIG. 1 is a configuration diagram of a main part of an ammonia gas measuring apparatus that outputs lasers in the above wavelength range using two laser light sources. Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a configuration diagram showing an example of an embodiment of the present invention. In the figure, 1
Is a first laser light source emitting a wavelength λ1 and 2 is a second laser light source emitting a wavelength λ2. The laser light from these light sources enters the non-linear crystal 3, and is emitted together with λ3 generated by the non-linear crystal 3 based on the wavelengths λ1 and λ2.

Numeral 4 is a filter which makes the light emitted from the nonlinear crystal 3 incident, and mainly transmits light having a wavelength of λ3. The light of wavelength λ3 that has passed through the filter 4 passes through the gas cell 5 and illuminates the light receiving element 6. For example, air collected from a room in which an exposure apparatus for a semiconductor process is installed is gastightly introduced into the gas cell into the gas cell through a nozzle 50, and the introduced gas is sucked by a vacuum pump 51 and discharged to the outside.

Although not shown in the figure, a pressure gauge for monitoring the internal pressure is attached to the gas cell, and the output of the vacuum pump is adjusted so that the pressure in the gas cell becomes 0.3 atm or less. (The reason why the pressure in the gas cell is 0.3 atm or less will be described later).

The wavelength λ3 generated by the nonlinear crystal 3 has a relationship of 1 / λ3 = (1 / λ1)-(1 / λ2), where (λ1 <λ2). The output light of the nonlinear crystal 3 has three wavelengths λ
Since the light of 1, λ2 and λ3 are mixed, the passage of λ1 and λ2 is blocked by the filter. Then, by sweeping either or both of λ1 and λ2, the wavelength of λ3 can be swept within the range of the absorption wavelength of ammonia described above.

When λ3 is calculated with λ1; 852 nm, λ2; 1386 nm, λ3 = 2211 nm, and either or both of λ1 and λ2 are swept based on the wavelength in the vicinity to obtain a range of 2.215 to 2.214 μm. To sweep.

The general theory of gas absorption spectroscopy is represented by the following equation. Gas absorption cross-section a cm ² Concentration per unit volume N cells / cm ^{3 If the} cell length is L cm, gas absorption is expressed by the following equation (1) according to the Lambert-Beer equation. Note that a is a value defined as an absorption area of one molecule for a specific wavelength.

Here, a is the absorption coefficient (absorption cross section) at each wavelength of each molecule, and is a function of the wavelength λ determined by various gases existing in the gas and the vibration mode of each gas. a _1m is n for a certain gas (for example, a _1m is oxygen gas, a _2m is nitrogen gas, a _3m is carbon dioxide gas ...)
Shows the absorption function for each vibration mode.

[0025] think about the shape of the individual in _{a m.} It a _m is a finite release time from the excited state, or resulting from by broadening due to pressure of the atoms in the gas.
However, in reality, the influence of the gas pressure is dominant. Broadening due to pressure is roughly classified into two types. The first is caused by collisions between atoms and molecules and has a Lorentzian distribution. When there are many kinds of gas, the Lorentz width (α _L ) is calculated from the partial pressure by the following equation (2).

Where α _{A and} α _S are line widths of individual spectra (widths 3 dB below the top of a certain spectrum), T and T ₀ are gas temperature and standard temperature (296 k) P ₀ is
1 atm (1.013 × 10 ⁵ pa) P ′ is the partial pressure of gas, and P is the total pressure. The increase in line width other than due to collision is caused by Doppler shift. The line width at that time is expressed by the following equation (3).

Here, k _B is the Voltsma constant C is the speed of light. Considering the absorption at the i-th line based on the above-mentioned preamble, it is expressed as the following equation (4). Here, S _i is a waveform function indicating a change in line waveform due to temperature, and K _i is a waveform function indicating a change in line due to other parameters. From this, the absorption coefficient at point j for all absorptions is expressed by the following equation (5).

A) Specific example of calculation By calculating the above-mentioned formula (4), the line shape can be calculated depending on the type of gas at any pressure and temperature. However, since this is an analytically complicated calculation, NH here is used.
_{3 In} order to calculate the absorption line at 1963.028 nm efficiently, the calculation is performed under the following conditions as an example. NH ₃ 1ppb Components other than NH ₃ H ₂ O 1% (relative temperature 20%) CO ₂ 100ppm N ₂ O 1ppm concentration cell length 100m

A-1 Calculation result Absorption amount (2) Signal detection method Considering the worst point of the signal obtained when the signal is detected by sweeping the wavelength by Δλ in the vicinity of the NH ₃ absorption point, the pressure dependence of the disturbance slope is considered G _H2O = -0.016P G _CO2 = 5.4 x 10 ^-3 P ² G _NO2 = 0

Ammonia G _NH3 = 8 × 10 ^-4 (1-P) + 1.5 × 10 ^-4 Disturbance signal Sd ω Ammonia signal Maximum point of 2ω S / N ratio Numerator = 0 = 0 Smaller is better. The lower the pressure, the better the S / N. However, in reality, since the frequency band is different between ω and 2ω, the S / N value can be expected to be 20 times or more, so P = 0.1 and S / N = 10.
As a result, it is possible to measure up to 0.1 ppb. However, S / N here is a value only for the detector part.

FIG. 2 (a, b, c) shows the composition conditions of a certain gas at a temperature of 300 K (Kelvin) as H ₂ O 1 × 10.
_{^{-2 (10000ppm), CO 2 1}} × 10 -4 (10
0 ppm), CH ₄ 1 × 10 ⁻⁶ (1 ppm), N ₂ O
1 × 10 ⁻⁶ (1 ppm), NH ₃ 1 × 10 ⁻⁶ (1 pp
As m), the calculation result of the relationship between the wavelength (nm) of each gas and the absorption (no unit) at each pressure (in the cell) is shown by using the above-mentioned calculation formula.

According to FIG. 2 (a), the spectral absorption of H ₂ O alone was observed at 1 atm, and 0.1% shown in FIG. 2 (b) was observed.
Absorption of all gases is recognized at atmospheric pressure, and spectral absorption of all gases is more clearly recognized at 0.01 atmosphere shown in FIG. 2 (c). The NH ₃ spectrum is shown in Fig. 2.
Although it is clearly observed at 0.1 atm of (b), this tendency becomes remarkable from about 0.3 atm. Based on this result, the pressure in the cell is set to 0.3 or less in this invention.

In FIG. 3, the wavelength of λ3 is FM-modulated by at least one of the light sources 1 and 2 in the vicinity of the absorption peak value of the output signal from the light receiving element 6, and the peak value is accurately measured by the waveform diagram which outputs the 2f signal. be able to.

In FIG. 4, the light incident on the gas cell 5 is reflected by the inner wall of the gas cell and is reciprocated while shifting the traveling direction.
Finally, the light reflected by an odd number of times (three times in the figure) is received by the light receiving element 6.
The light is received at. With such a configuration, the sensitivity can be improved and the influence of the Doppler shift of the gas flowing in the cell can be removed. Further, according to such a configuration, by using the semiconductor laser as the light source, the size is reduced as compared with the conventional example,
Cost reduction is possible.

FIG. 5 shows the filter 4 in the apparatus shown in FIG.
The light of λ3 emitted from is split by the half mirror 7, and the split light is made incident on the reference gas cell 5a. In this case, the influence of the fluctuation of the light source can be removed by introducing the gas to be measured into the reference cell and comparing it with the signal detected by the gas cell 5. If the same gas as the gas to be measured (here, ammonia gas) is introduced into this reference gas cell, wavelength calibration and sensitivity calibration can be performed simultaneously. In addition, the concentration of known ammonia gas was introduced while the internal gas pressure of the reference gas cell was 0.3 atm or less,
The absorption wavelength can be accurately calibrated by narrowing the width of the absorption spectrum.

The above description of the present invention has been presented only with reference to particular preferred embodiments for purposes of illustration and illustration. Accordingly, the invention is subject to many modifications without departing from its essence,
It will be apparent to those skilled in the art that variations can be made. In the embodiment, the gas to be measured is ammonia, but other gas satisfying the same condition may be used. The scope of the present invention, which is defined by the description in the section of the claims, includes modifications within the scope,
Modifications shall be included.

[0037]

As described above, according to the first aspect of the present invention.
According to the first laser light source that outputs a wavelength of λ1,
A second laser light source that outputs two wavelengths, a non-linear crystal that inputs these two wavelengths and generates and outputs an absorption wavelength λ3 for at least the gas to be measured, and a light output from the non-linear crystal that has the λ3 A gas measuring device comprising a filter that passes a wavelength, a gas cell into which a gas to be measured that inputs laser light that has passed through this filter is introduced, and a light receiving element that receives light that has passed through this gas cell, wherein the gas cell Since the measured gas concentration is measured while the internal pressure is reduced to 0.3 atm or less, highly accurate measurement, downsizing, and cost reduction can be achieved.

According to the second aspect, either or both of λ1 and λ2 are swept to set the wavelength of λ3 to 2.2115.
Since the sweep is performed in the range including .about.2.214 .mu.m, high S / N measurement can be realized.

According to the third aspect, since the wavelength of λ3 is FM-modulated in the vicinity of the absorption peak value of the output signal from the light receiving element and the 2f signal is used as the output signal, highly sensitive measurement is performed. be able to.

According to claim 4, a half mirror is provided in front of the gas cell to split the light of wavelength λ3, and
A reference gas cell for injecting divided light is provided, and a measurement target gas having a known concentration or a gas containing a measurement target gas having a known concentration is introduced with the internal pressure of the reference gas cell being 0.3 kg / cm ² or less, and a measurement laser ( Since the wavelength of λ3) is calibrated, the wavelength calibration and the sensitivity calibration can be performed accurately at the same time.

According to the fifth aspect, when the gas to be measured is introduced into the gas cell, a nozzle is provided at the gas introduction port and the gas is introduced from the tip of the nozzle while blowing out the gas.
The gas is cooled and accurate measurement can be performed.

According to the sixth aspect, the measuring laser (λ3) is reflected to the light receiving element side after being reflected an odd number of times in the gas cell, so that the influence of the Doppler shift caused by the gas flow can be eliminated. Good S / N can be measured.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an example of an embodiment of a gas measuring device of the present invention.

FIG. 2 shows the calculation results of the relationship between the pressure in the cell and the wavelength of various gases and the absorptivity under a certain gas composition condition at a temperature of 300 K (kelvin).

FIG. 3 is a waveform diagram for FM-modulating a wavelength and outputting a 2f signal.

FIG. 4 is a configuration diagram showing another embodiment of the present invention.

FIG. 5 is a configuration diagram showing another embodiment of the present invention.

FIG. 6 is a diagram showing absorption characteristics with respect to wavelengths of various gases.

FIG. 7 is a partially enlarged view of FIG.

FIG. 8 is a configuration diagram showing a conventional example.

FIG. 9 is a configuration diagram showing another conventional example.

FIG. 10 is a configuration diagram showing another conventional example.

[Explanation of symbols]

1 light source 1 2 light source 2 3 Non-linear crystal 4 filters 5 gas cells 5a Reference gas cell 6 Light receiving element 7 Half mirror

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2G020 AA03 BA02 BA12 CB23 CB42                       CB55 CC32 CC48 CD04 CD22                       CD39                 2G057 AA01 AB02 AB06 AC03 BA05                       BB01 BB10 BC05 DA03 DC07                       GA01 JA20                 2G059 AA01 BB01 CC20 DD12 EE01                       EE12 FF08 GG01 GG09 HH01                       HH08 JJ01 JJ02 JJ13 JJ22                       JJ24 KK01 LL03 MM05 MM14

Claims (6)

[Claims] 1. A first laser light source for outputting a wavelength of λ1,
A second laser light source that outputs a wavelength of λ2, a nonlinear crystal that inputs these two wavelengths and generates and outputs an absorption wavelength λ3 for at least the gas to be measured, and a light output from this nonlinear crystal A gas measuring device comprising a filter that passes a wavelength, a gas cell into which a gas to be measured that inputs laser light that has passed through this filter is introduced, and a light receiving element that receives light that has passed through this gas cell, wherein the gas cell A gas measuring device, characterized in that the concentration of a gas to be measured is measured while the internal pressure is reduced to 0.3 atm or less. 2. Either or both of λ1 and λ2 are swept to set the wavelength of λ3 to 2.2115 to 2.214 μ.
2. The gas measuring device according to claim 1, wherein sweeping is performed in a range including m. 3. The gas measuring device according to claim 1, wherein the wavelength of λ3 is FM-modulated in the vicinity of the absorption peak value of the output signal from the light receiving element, and the 2f signal is used as the output signal. 4. A half mirror is provided in front of the gas cell to split light having a wavelength of λ3, and a reference gas cell for making the split light incident is provided, and the internal pressure of the reference gas cell is set to 0.3 kg / cm ² or less. The gas measurement according to claim 1 or 2, wherein a measurement target gas having a known concentration or a gas containing a measurement target gas having a known concentration is introduced to calibrate the wavelength of the measurement laser (λ3). apparatus. 5. When introducing the gas to be measured into the gas cell, a nozzle is provided at the gas introduction port, and the gas is introduced while being blown out from the tip of the nozzle.
The gas measuring device according to any one of 1 to 4. 6. The gas measuring apparatus according to claim 1, wherein the measuring laser (λ3) is reflected to the light receiving element side after being reflected an odd number of times in the gas cell.