JPH07122229A - Mass spectrometer - Google Patents

Abstract

(57) [Summary] [Object] To provide a mass spectrometer capable of accurately measuring a specific component in a plurality of components as an ion generation source detected by mass spectrometry. [Structure] A gap 38 through which a part of thermoelectrons passes is formed by two electron beam shield plates 34 and 36, and the gap is freely controlled. Further, the variable power sources 45a and 45b control the potential difference between the filament 32 and the electron beam shield plates 34 and 36. As a result, the energy of the electron beam that ionizes the sample gas and the energy distribution width are changed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mass spectrometer for detecting the components of a sample gas by analyzing the mass of atoms or molecules of the sample gas.

[0002]

2. Description of the Related Art Generally, a mass spectrometer is an ion source section for ionizing atoms and molecules of a sample gas to accelerate them in a certain direction, a separation section for separating generated ions according to mass (mass separation), and a mass. The detector is configured to detect the separated ions. In this mass spectrometer,
Although various types of ion sources (ionizing means) for ionizing atoms and molecules have been developed, so-called electron impact type ion sources are usually most often used. In the electron impact ion source, the thermoelectrons emitted from the filament for thermionic emission are accelerated, and the thermoelectrons collide with the sample gas introduced into the ionization chamber to ionize the atoms and molecules of the sample gas. Has been done.

[0003]

Here, considering the case where the thermoelectrons emitted from the filament ionize the atomic radicals represented by X and the molecules or molecular radicals represented by XY, respectively. Also produces an ion of X ⁺ . Usually, there is a difference of several eV between the X and XY ionization energies. However, the electron beam emitted from the filament has an energy distribution due to the voltage drop of the filament itself, and the width of this energy distribution is generally larger than the difference in ionization energy.

Therefore, when the sample gas is analyzed using the above-described mass spectrometer having the electron impact ion source to detect the ion X ⁺ , the ion is derived from the ionization of the atomic radical represented by X. It is difficult to distinguish between the one represented by XY and the one caused by dissociative ionization of the molecule or molecular radical represented by XY. Therefore, for example, there is a problem that the atomic radical X (specific component) cannot be accurately detected.

Further, it is known that the temperature in the vicinity of the thermionic emission filament used as the electron beam source of the electron impact ion source becomes extremely high. Due to this high temperature, a gas, such as Cl ₂ gas, which requires a small amount of energy for dissociation, is easily thermally dissociated in the vicinity of the filament to generate a Cl radical. Therefore, there is a problem that it is difficult to separate and detect the radicals generated by thermal dissociation in the vicinity of the filament and the radicals to be detected existing in the sample gas generated by dissociation by discharge or the like.

In order to solve these problems, Japanese Patent Publication No. 62-57069 discloses a mass spectrometer which adopts a method of ionizing using light having a constant energy, that is, a constant wavelength, instead of accelerating electrons. Is disclosed. However, this device has a problem in that molecules or radicals that can be ionized are limited because the wavelength that can be used as a light source is limited.

In view of the above circumstances, it is an object of the present invention to provide a mass spectrometer capable of detecting various components of a sample gas more accurately than before.

[0008]

A first mass spectrometer according to the present invention for achieving the above object has an electron beam source for emitting thermoelectrons and a thermoelectron emitted from the electron beam source. And an ionization chamber into which a sample gas to be analyzed is introduced, (1) The distance between the electron beam source and the ionization chamber, which is provided between the electron beam source and the ionization chamber, is freely controlled. Two electron beam shield plates that form a gap through which thermoelectrons pass (2) A potential difference control unit that controls the potential difference between the electron beam source and the electron beam shield plate is provided.

Here, it is preferable that an energy measuring means for measuring the energy of the electron beam passing through the gap is provided in front of the electron beam shield plate in the thermionic emission direction. Also,
A second mass spectrometer according to the present invention for achieving the above object, comprises: an electron beam source for emitting thermoelectrons; and a sample gas to be analyzed, into which thermoelectrons emitted from the electron beam source are introduced. A mass spectrometer equipped with an ionization chamber to be introduced, characterized in that it comprises a container in which the electron beam source is hermetically housed, having an opening through which thermoelectrons emitted from the electron beam source pass. It is a thing.

Here, it is preferable to provide gas supply means for supplying gas to the container or exhaust means for exhausting the inside of the container.

[0011]

The principle of analyzing a sample gas using the mass spectrometer of the present invention will be described with reference to FIG. FIG. 1 is a graph showing the relationship between electron beam energy and electron beam flux and electron impact ionization cross section, where the horizontal axis represents electron beam energy and the vertical axis represents electron impact ionization cross section and electron beam flux. .

Atom radicals X
And molecules or molecular radicals (hereinafter referred to as molecules) XY will be examined. The energy threshold of the ionization reaction for ionizing the atomic radical X shown in the reaction formula (1) to generate the ion X ⁺ is set to El, and the molecule XY shown in the reaction formula (2) is ionized to generate the ion X ^+. Let E2 be the ionization reaction energy threshold value for generating
The relationship is l <E2. Curve (I) and curve (I
I) shows this relationship.

X + e ⁻ → X ⁺ + 2e ⁻ (1) XY + e ⁻ → X ⁺ + Y + 2e ⁻ (2) The energy of the electron beam used for ionization is E
and the energy distribution of this electron beam is represented by the curve (II
The distribution state is conceptually shown as I). As shown in FIG. 1, if the relationship of El <Ee <E2 can be precisely set, only the reaction of the formula (1) can be caused to generate the ion X ^+, and thus only the target atomic radical X can be generated. It can detect and measure selectively.

The present invention has been made in consideration of the above principle. In the first mass spectrometer of the present invention, a gap through which thermoelectrons pass is formed by two electron beam shield plates. The distance of this gap can be freely controlled. For this reason,
The flux of the generated electron beam is controlled, and thus the energy distribution width of the electron beam can be precisely controlled. Furthermore, since the potential difference between the electron beam source and the electron beam shield plate can be controlled by the potential difference control means, the energy of the electron beam (the value corresponding to the peak of the curve (III) in FIG. 1) is controlled based on this potential difference. Can be set.

Therefore, the energy and the energy distribution width of the electron beam that ionizes the sample gas can be changed.
By controlling the electron beam energy so as to have the distribution of the curve (III) in FIG. 1, ionization can be caused only by the reaction of the equation (1). As a result, even if the common generation sources X and XY of the ion X ⁺ are present, only the atomic radical X, which is one of the generation sources, can be detected.

When the energy measuring means is provided, the energy of the electron beam passing through the gap can be measured,
It is possible to control the gap interval and the potential difference between the electron beam source and the electron beam shield plate according to the measurement result. Therefore, the electron beam having a desired energy can be introduced into the ionization chamber by controlling the gap distance and the potential difference between the electron beam source and the electron beam shield plate.

Further, since the electron beam source of the second mass spectrometer of the present invention is hermetically housed in the container, it is possible to substantially prevent the sample gas from entering the vicinity of the electron beam source. As a result, even if the sample gas contains a component that easily causes thermal dissociation, this component hardly penetrates into the vicinity of the electron beam source, so that thermal dissociation of this component is prevented and the component of the sample gas Can be detected more accurately than before.

Here, when the gas supply means is connected to the container, the gas can be supplied from the gas supply means to the container so as to make a pressure difference between the ionization chamber and the electron beam source. This can more reliably prevent the sample gas from entering the vicinity of the electron beam source. As a result, even if the sample gas contains a component that easily causes thermal dissociation, this component cannot enter the vicinity of the electron beam source, so that thermal dissociation of this component is prevented and the component of the sample gas is It can be detected more accurately than before.

When an exhaust means is connected to the container,
Even if the sample gas enters the vicinity of the electron beam source, the sample gas is exhausted and does not return to the ionization chamber again, so that the components of the sample gas can be detected more accurately than before.

[0020]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a schematic configuration diagram showing a mass spectrometer and a plasma chamber, FIG. 3 is an explanatory diagram showing a sample gas introduction path and its potential distribution when sampling the sample gas from the plasma chamber, and FIG. 4 is an ion source section. It is a schematic block diagram which expands and shows a principal part.

The mass spectrometer 10 is a quadrupole mass spectrometer, and has an ionization chamber 12 for ionizing a sample gas.
And a quadrupole electrode 14 for separating the ions generated in the ionization chamber 12 based on mass, and a conical sampling tube 16. The mass spectrometer 10 is connected to the plasma chamber 18 and
Part of the plasma gas generated in No. 8 sampling tube 16
It is configured to be taken into the ionization chamber 12 via. A parallel plate electrode 20 is provided in the plasma chamber 18.
A and 20B are provided, and high frequency (R
F) The voltage is applied.

A fine hole (not shown) having a diameter of 0.15 mm is formed at the tip of the sampling tube 16 inserted into the plasma chamber 18, and through this fine hole,
A part of the plasma gas can be sampled in the mass spectrometer 10 by the differential pressure generated by the differential exhaust.
During sampling, the gas path from the sampling tube 16 to the ionization chamber 12 has a potential distribution as shown in FIG. Therefore, charged particles such as electrons, negative ions, and positive ions are not introduced into the ionization chamber 12. The neutral particles introduced into the ionization chamber 12 are irradiated with an electron beam and ionized by electron collision. The ions thus generated in the ionization chamber 12 are extracted from the ionization chamber 12 and are mass-separated by the quadrupole electrode 14, and at the same time, only ions to be detected pass through the quadrupole electrode 14,
The signal is incident on the secondary electron multiplier 22, and the signal is amplified and detected.

In the mass spectrometer 10, a filament 32 which emits thermoelectrons to form an electron beam 32a and a plane which is orthogonal to the acceleration direction (electron emission direction) of thermoelectrons emitted from the filament 32 are arranged. Two electron beam shield plates 34 and 36 arranged are provided. The electron beam shield plates 34 and 36 are separated from each other and a gap 38 is formed. The thermoelectrons emitted from the filament 32 pass through the gap 38, so that the energy is narrowed down to a narrow range and an electron beam 32b is formed.
The electron beam shield plates 34 and 36 are thin plates made of tantalum (Ta) having a thickness of 0.5 mm. Tantalum was used as the material because it has low reactivity with the analysis gas. The electron beam shield plate 36 is fixed to the wall (not shown) of the ionization chamber 12. On the other hand, the electron beam shield plate 34 is a piezo stack 4 in which piezo elements are arranged in layers.
0 through a connecting rod 41. At each end of the connecting rod 41, a rod 40 a connected to the piezo stack 40 and a rod 34 connected to the electron beam shield plate 34 are provided.
a is attached. Further, the connecting rod 41 is configured to rotate about the shaft 41a. Therefore, by changing the electric potential applied to the piezo stack 40, the connecting rod 41 rotates and the width of the gap 38 is precisely controlled. The narrower the width of the gap 38, the higher the resolution of the electron beam energy, but the narrower the width, the lower the flux of the electron beam obtained and the lower the detection sensitivity. The detection sensitivity can be maximized by adjusting the width of the gap 38 to the maximum width at which necessary and sufficient energy decomposition can be obtained according to the type of sample gas to be measured.

The width of the gap 38 is optimized by using an energy measuring device 42 for measuring the energy of the electron beam.
The energy measuring device 42 includes electron beam shield plates 34, 36.
Similarly, it is made of a thin plate made of tantalum having a thickness of 0.5 mm, and the potential can be changed by the variable power source 44, whereby the energy distribution of the electron beam 32b can be measured. Therefore, in order to obtain the desired energy distribution, this measurement result is fed back to the gap 3
You can set a width of 8. Normally, the width of the gap 38 and the energy distribution width can be made to correspond one-to-one, but the energy distribution of the electron beam may deviate due to the change over time in the shape and surface condition of the filament 32. Therefore, it is desirable to actually measure the electron beam energy distribution before performing the mass spectrometry measurement. Further, in order to improve the resolution of the electron beam energy, it is better to arrange the electron beam shield plates 34 and 36 so that the gap 38 is formed in the direction orthogonal to the longitudinal direction of the filament 32.

Electron beam energy (curve (II
The value corresponding to the peak of I) is the electron beam shield 3
It is controlled according to the potential difference between 4, 36 and the filament 32. Therefore, this potential difference is controlled by the variable power sources 45a, 45.
It is possible to change the set value of the electron beam energy by changing it according to b. However, since the set value and the effective value do not exactly match, it is necessary to confirm the effective value using the energy measuring device 42.

As described above, according to this embodiment, the energy of the electron beam used for ionizing radicals and molecules introduced into the ionization chamber 12 is set to a necessary and sufficient distribution width according to the object to be measured. The energy value can be changed and adjusted over a wide range. Therefore, for example, the electron beam energy can be set at the position of the energy distribution of the curve (III) in FIG. 1, and in this case, the atomic radical X can be selectively measured. Further, the detection sensitivity can be maximized by setting the width of the gap 38 to the maximum under the condition that the energy distribution width of the electron beam falls between E1 and E2 in FIG.

Next, an example in which radicals in plasma generated in the plasma chamber 18 are detected by using the mass spectrometer 10 will be described. A plasma source gas was introduced into the plasma chamber 18 under a predetermined pressure, and an RF discharge was generated between the parallel plate electrodes 20A and 20B to generate plasma.

In this example, tetrafluoromethane CF ₄ is used as the plasma source gas and the plasma chamber 1
CF ₄ gas is introduced into the chamber 8 for RF discharge. In this case, although the plasma CF ₃ radicals are generated, in order to detect the CF ₃ radical, a CF ₃ ⁺ ions arising from CF ₃ radicals, C resulting from the parent gas CF ₄
It is necessary to distinguish the F ₃ ⁺ ions.

Ions (C) are generated by ionization from CF ₃ radicals and ionization from parent gas CF ₄ by electron collision.
The reaction equation when F ₃ ⁺ ) is generated and the threshold value of the reaction energy are shown in the following equations (3) and (4). CF ₃ + e ⁻ → CF ₃ ⁺ + 2e ⁻ (ionization from radical): 10.4 eV (3) CF ₄ + e ⁻ → CF ₃ ⁺ + F + 2e ⁻ (ionization from parent gas): 16.0 eV (4) (4) From the equations (3) and (4), in order to selectively detect only CF ₃ radicals in CF ₄ plasma, the electron beam energy Ee is set to 10.4 eV <Ee <16.0 eV. You know what you need to do.

Therefore, in order to show how the signal corresponding to the detected amount of ions caused by CF ₄ and the signal corresponding to the detected amount of ions caused by CF ₃ are separated, the energy Ee of the electron beam is set to 9. The measurement changing from 35 to 20.35 eV was performed as follows. First, CF ₄ gas was introduced into the plasma chamber 18 (see FIG. 2) and the pressure was adjusted to 4
The gas flow rate was adjusted to mTorr, the frequency was 13.56 MHz, and the power was 50 W (0.25 W / cm).
^{In 2} ), an RF discharge was generated to generate CF ₄ plasma.

A part of the generated plasma gas is introduced into the mass spectrometer 10 by differential evacuation, and CF ₃ ⁺ (m / e =
69) was detected. At the time of measurement, in order to remove ions and electrons in the plasma gas, a positive bias (150 V) is applied to the ionization chamber 12 and a negative bias (-30 V) is applied to the sampling tube 16 as shown in FIG.
Was applied.

The result of measuring CF ₃ ⁺ under the above conditions is shown by a circle in FIG. For comparison, the results of CF ₃ ⁺ detection under the same conditions except that no discharge is performed are also shown by Δ marks in FIG. The horizontal axis of the graph of FIG. 5 shows the electron beam energy, and the vertical axis shows the output value of the mass spectrometer. Specifically, this output value represents how many pulses having a certain threshold value (here, 1 V) or more are detected within a certain period of time.

It can be seen from FIG. 5 that since no CF ₃ radical is present when no discharge is made, only a signal due to ionization from CF _4, which is the parent gas, is detected. On the other hand, when discharged, since CF ₃ radicals are present, it can be seen that a signal due to ionization from the CF ₃ radicals is also detected. Further, FIG. 5 clearly shows that when the electron beam energy Ee is in the range of 10.4 eV <Ee <16.0 eV, a signal resulting from ionization from CF ₃ radicals is detected. .

FIG. 6 is a summary of the CF ₃ radical data shown in FIG. The data of CF ₃ + e ⁻ → CF ₃ ⁺ + 2e ⁻ (ionization from radicals) shows that the electron beam energy Ee is Ee <1.
This is the difference between the discharge on and discharge off signals at 6.0 eV. The data of CF ₄ + e ⁻ → CF ₃ ⁺ + F + 2e ⁻ (ionization from the parent gas) is the data of the signal when discharge is off. is there.

It can be seen from FIG. 6 that only the signals due to the radicals in the plasma can be selectively detected by utilizing the difference in threshold energy between the direct ionization from the radicals and the ionization from the parent gas. The electron beam energy is set as follows when selectively detecting only the signal caused by radicals. The inside of the plasma chamber 18 and the mass spectrometer 10 are evacuated to ^reduce the pressure in the mass spectrometer to 10 ^−6.
The energy of the electron beam is 14 eV, and the width of the gap 38 is about 0.5 mm without introducing CF ₄ gas to Torr or less.
And an electron beam was generated. Variable power source 4
When the current I flowing through the resistor 46 was measured while changing the potential V of 4 (see FIG. 4) from 0 V to −20 V, a characteristic curve as shown in FIG. 7A was obtained. By differentiating this characteristic curve by the potential V, an energy distribution curve of the electron beam as shown in FIG. 7B was obtained. Since the energy of the electron beam and the energy distribution width can be read from this energy distribution curve, the width of the gap 38 can be set from this reading result. Thus, the electron beam energy Ee is set to 10.4 eV <Ee <16.0.
It was set so as to fall within the range of eV. The operation for this setting may be performed manually, but it is also possible to program it in a personal computer and automate it.

As described above, if the electron beam energy value is kept constant under the condition that only radicals are detected, the radical density changes with time, changes with time, and changes with discharge conditions are output by the mass spectrometer. Can be compared by relative value. Further, by using the mass spectrometer of the present embodiment, the absolute density of radicals can also be calculated when the data of the electron impact ionization cross section are available.

As described above, the electron beam emitted from the filament 32 is shielded by the electron beam shield plates 34 and 36 having the gap 38, so that the energy and the energy distribution of the emitted electron beam. Since the width can be precisely controlled, radicals and stable molecules that are the source of radicals can be distinguished and measured. Furthermore, according to the present embodiment, the energy of the emitted electron beam and the energy distribution width can be precisely controlled, so that different radicals (ground state and metastable state) of the same radical can be determined and measured.

Next, another structure of the electron beam source and the ionization chamber will be described with reference to FIG. FIG. 8 is a schematic configuration diagram showing another structure of the electron beam source and the ionization chamber, and the same elements as those in FIG. 4 are denoted by the same reference numerals. The ion source section including an electron beam source and an ionization chamber is configured to include a filament 32 as a thermoelectron generating means, a differential pressure chamber 50 accommodating the filament 32, and an ionization chamber 12. Among the walls forming the differential pressure chamber 50, the side wall arranged in the acceleration direction (electron emission direction) of the thermoelectrons emitted from the filament 32 is made of a metal shielding plate 50a, and the other side wall 50b is heat resistant. It is composed of an insulator made of quartz glass. An opening 50c is formed in the shielding plate 50a, and the opening 50c has a function of passing the thermoelectrons emitted from the filament 32 and a function of making a pressure difference between the inside and outside of the differential pressure chamber 50. The shield plate 50a is made of tantalum (T
It is made of a thin plate consisting of a). Tantalum was used as the material because it has low reactivity with the analysis gas. Further, a gas inlet 50d is formed in the side wall 50b, and a gas that does not affect the mass spectrometric measurement is introduced from the gas inlet 50d at a very small flow rate.

The energy of the electron beam 32a is the shielding plate 5
It is controlled and set by the potential difference between 0a and the filament 32. However, since this set value and the effective value do not strictly match, calibration is usually required.This calibration requires a gas whose electron impact ionization threshold energy is known, for example, electron impact ionization cross-section is low when the electron beam energy is low. This can be performed using Ar gas (threshold value: 15.75 eV) having good linearity on the energy side.

As described above, according to the present embodiment, by providing a pressure difference between the ionization chamber 12 and the differential pressure chamber 50 accommodating the filament 32 for emitting thermions which is an electron beam source. Therefore, the sample gas in the ionization chamber 12 cannot enter the vicinity of the filament 32.
Therefore, even when the sample gas contains a component that easily causes thermal dissociation, the sample gas can be measured without causing thermal dissociation.

Next, an example in which radicals in the plasma generated in the plasma chamber 18 (see FIG. 2) are detected by using the mass spectrometer equipped with the differential pressure chamber 50 will be described. A plasma source gas is introduced into the plasma chamber 18 under a predetermined pressure, and the parallel plate electrodes 20A, 20
Plasma was generated by causing an RF discharge during B. Here, Cl ₂ was used as the plasma source gas, Cl ₂ gas was introduced into the plasma chamber 18, and RF discharge was performed. At this time, Cl radicals are generated in the plasma. To detect the Cl radicals, Cl ⁺ ions generated from the Cl radicals and Cl ₂ ions are generated.
It is necessary to distinguish it from the same ion generated from.

Ions (Cl) are generated by ionization from Cl radicals and ionization from the parent gas Cl ₂ by electron collision.
The reaction formula and the threshold value of the reaction energy when ⁺ ) is generated are shown in the following (5) and (6). Cl + e ⁻ → Cl ⁺ + 2e ⁻ (ionization from radical): 12.97 ev (5) Cl ₂ + e ⁻ → Cl ⁺ + 2e ⁻ (ionization from parent gas): 16.67 ev (6) (5), ( From the equation (6), it is understood that the electron beam energy Ee should be set to 12.97 eV <Ee <16.67 eV in order to selectively detect only Cl radicals in Cl ₂ plasma. .

Therefore, in order to show how the signal corresponding to the detected amount of ions caused by Cl ₂ and the signal corresponding to the detected amount of ions caused by Cl are separated, the energy Ee of the electron beam is 12.25. Measurements up to -22.25 eV were made as follows. First, Cl ₂ gas was introduced into the plasma chamber 18 (see FIG. 2) and the pressure was adjusted to 4
The gas flow rate was adjusted to mTorr, the frequency was 13.56 MHz, and the power was 50 W (0.25 W / cm).
² ) RF discharge was generated to generate Cl ₂ plasma.

A part of the generated plasma gas was introduced into the mass spectrometer 10 by differential evacuation, and Cl ⁺ (m / e =
35) was detected. At this time, the pressure of the ionization chamber in the mass spectrometer 10 was 5 × 10 ⁻⁷ Torr. Ar so that the pressure inside the differential pressure chamber 50 is lower than the pressure inside the ionization chamber by about three orders of magnitude, so that the Ar gas is introduced from the gas inlet 50d.
The gas was flowed at 0.1 sccm. Here the Ar gas was used, the low reactivity with the Cl _2, also, that even when a part has been ionized, for mass in Ar ⁺ and Cl ⁺ capital differs, the detection of the Cl ⁺ is This is because it has no effect.

In the measurement, in order to remove the ions and electrons in the plasma, a positive bias (150 V) is applied to the ionization chamber and a negative bias (-30 V is applied to the sampling tube, as shown in FIG. ) Was applied. The result of measuring Cl ⁺ under the above conditions is shown by a circle in FIG. Further, for comparison, the results of detecting Cl ⁺ under the same conditions except that no discharge is performed are also shown by Δ in FIG. The horizontal axis of the graph in FIG. 9 represents the electron beam energy, and the vertical axis represents the output value of the mass spectrometer. This output value is specifically a constant threshold value (here, 1 V) within a constant time.
It shows how many of the above pulses were detected.

It can be seen from FIG. 9 that when no discharge is made, Cl radicals do not exist, so that only the signal due to ionization from the parent gas Cl ₂ is detected. On the other hand, when discharged, since Cl radicals are present, it can be seen that a signal due to ionization from these radicals is also detected. 16.67 when not discharged (marked with Δ)
The output value of the mass spectrometer at eV or lower is the value of the background caused by the instrument. For comparison, the same Cl radical analysis as described above was performed using a conventional apparatus configuration without the differential pressure chamber 50. As a result, Cl radicals that were considered to have been generated by thermal decomposition were detected regardless of the presence or absence of discharge. No difference due to the presence or absence of discharge as shown in FIG. 9 was observed.

According to the present embodiment described in detail above, the sample gas can be prevented from entering the vicinity of the filament 32 by providing a pressure difference between the ionization chamber 12 and the differential pressure chamber 50. Even if the component contains a component that easily causes thermal dissociation, the sample gas can be measured without causing thermal dissociation. Next, an example in which the differential pressure chamber 50 is used as a vacuum chamber will be described. In the above example, the gas was introduced into the differential pressure chamber 50 through the gas introduction port 50d, but here, a vacuum pump (not shown) is connected to the gas introduction port 50d to create a vacuum inside the differential pressure chamber 50. As a result, the opening 50c has a function of passing the thermoelectrons emitted from the filament 32 and a function of limiting the ratio of the sample gas in the ionization chamber 12 entering the vicinity of the filament 32.

When the differential pressure chamber 50 is used as a vacuum chamber, since the filament 32 is arranged in the differential pressure chamber 50, the rate at which the sample gas enters the vicinity of the filament 32 is limited. Further, the differential pressure chamber 50 is differentially evacuated so that the sample gas that has entered the differential pressure chamber 50 does not return to the ionization chamber 12 again.
Therefore, even if the sample gas contains a component that easily causes thermal dissociation, the sample gas can be measured without being affected by the thermal dissociation.

Using the differential pressure chamber 50 as a vacuum chamber, an experiment was conducted under the same conditions as the experiment in which the data of FIG. 9 was obtained. However, the pressure in the ionization chamber is 5 × 10 ⁻⁶ Torr, and the pressure in the differential pressure chamber 50 is equal to or lower than the pressure in the ionization chamber by about one digit.
Differential evacuation was performed from d. The experimental results were the same as above.

Next, an embodiment in which the mass spectrometer of the present invention is applied as a neutral beam detector for detecting the beam constituent species of the neutral beam will be described. FIG. 10 is a schematic configuration diagram showing a neutral beam detection device. The mass spectrometer applied to the neutral beam detector 60 of this embodiment is a magnetic field type mass spectrometer, and as shown in FIG. 10, an ionization chamber 62 for ionizing a sample gas and an ionization chamber 62 are provided. A mass spectrometric section 64 for mass-separating the generated ions,
And a secondary electron multiplier 66 for amplifying the detection signal. Further, the neutral beam detection device 60 is configured so that the pressure can be adjusted independently by differential pumping by the pump 67.

The neutral beam detector 60 is connected to a neutral beam generator (not shown), and a part of the neutral beam 68 generated by the neutral beam generator is used for the aperture 7.
It is configured to be taken into the ionization chamber 62 through 0. In the neutral beam taken into the ionization chamber 62,
An electron beam 74 is emitted from the electron beam source 72, and a part thereof is ionized by electron collision. Electron beam source 7
2 is provided in multiple stages in the ionization chamber 62, and the detection sensitivity can be improved by increasing the ionization rate of the neutral beam. Here, a three-stage one was used.

The ions generated in the ionization chamber 62 as described above are extracted from the ionization chamber 62 and are mass-separated by the mass spectrometric section 64. At the same time, only ions to be detected pass through the mass spectrometric section 64. , Enters the secondary electron multiplier 66. As a result, the signal representing the ion to be detected is amplified and detected. Next, an example of actually detecting the constituent species in the neutral beam using the neutral beam detection device 60 of the present embodiment will be described.

Here, N₂ Use neutral beam, N₂ During ~
The N radicals in the sexual beam were separately detected. N radical
In order to detect, N generated from N radicals⁺ ion
And N ₂ It is necessary to distinguish it from the same ion generated from.
Ionization from N radical by electron collision and parent gas N₂ Or
Ionization (N⁺ ) Produces
And the reaction energy threshold are expressed by the following equations (7) and (8)
Shown in.

N + e ⁻ → N ⁺ + 2e ⁻ (ionization from radicals): 14.5 eV (7) N ₂ + e ⁻ → N ⁺ + N + 2e ⁻ (ionization from parent gas): 24.3 eV (8) (7) ), (8), in order to selectively detect only the N radicals in the N ₂ core beam, the electron beam energy Ee should be set to 14.5 eV <Ee <24.3 eV. I understand that it is good.

Therefore, in order to show how the signal corresponding to the detected ion amount due to N ₂ and the signal corresponding to the detected ion amount due to N ₂ are separated, the energy Ee of the electron beam is set to 10.0. Measurements varying from ˜30.0 eV were made as follows. The gas flow rate of the neutral beam generator was adjusted so that the pressure of the neutral beam generator (not shown) was 4 mTorr, and a neutral beam with a beam energy of 300 eV was generated. Part of the beam is introduced into the neutral beam detector 60 through the aperture 70, and N ⁺
(M / e = 14) was detected. At this time, the pressure inside the neutral beam detector 60 is 5 × 10 ⁻⁶ due to the differential exhaust.
It was set to Torr.

The results of measuring N ⁺ under the above conditions are
It is indicated by a circle in FIG. Further, for comparison, the result of detecting N ⁺ under the same conditions except that a neutral beam is not generated only by flowing N ₂ gas is also shown by a triangle mark in FIG. 11. The horizontal axis of the graph of FIG. 11 represents the electron beam energy, and the vertical axis represents the output value of the magnetic field mass spectrometer. This output value indicates that dissociative ionization from N ₂ is occurring 3
It is a relative value standardized with a value of 0.0 eV.

From FIG. 11, since N radicals do not exist when the neutral beam is not generated, the parent gas N ₂
It can be seen that only the signal due to ionization from is detected. On the other hand, when the neutral beam is generated, N radicals are present, so it is understood that a signal due to ionization from the N radicals is also detected at the same time. The output value of the mass spectrometer at 24.3 eV or less when the neutral beam is not generated (marked with Δ) is the background value due to the instrument.

According to this embodiment described in detail above, a mass spectrometer is used as a neutral beam detector, and electron beam sources whose beam energy can be controlled are installed in multiple stages as ion sources of the mass spectrometer. Since a specific species (here, N radical) can be selectively ionized at a high ionization rate, the specific species can be detected with high sensitivity.

Here, the specific device configuration of the neutral beam detection device is not limited to that shown in the above embodiment, and the mass spectrometer is not limited to the magnetic field type mass spectrometer. Further, the neutral beam to which the present invention can be applied is not limited to the N ₂ neutral beam shown in the above embodiment, but can be arbitrarily changed. For example, when a monoatomic molecule neutral beam such as Ar neutral beam is used, a metastable state Ar is used.
It is also possible to separately detect.

The present invention has been specifically described above, but the present invention is not limited to the above-mentioned embodiments, and various modifications can be made without departing from the scope of the invention. For example, the specific device configuration of the mass spectrometer of the present invention is not limited to that shown in the above-mentioned embodiment, and is not limited to the quadrupole mass spectrometer. The thermoelectron generating means is not limited to the filament. Further, the sample gas to which the present invention is applicable is CF ₄ , Cl ₂ shown in the above embodiment.
The present invention is not limited to the above and can be arbitrarily changed, and can be applied to, for example, CH ₄ , F _{2 and the} like.

[0061]

As described above, according to the first mass spectrometer of the present invention, a gap through which a part of thermoelectrons pass is formed by the two electron beam shield plates, and the gap can be freely set. The potential difference between the electron beam source and the electron beam shield plate can be controlled by the potential difference control means. Therefore, the energy of the electron beam that ionizes the sample gas and the energy distribution width can be changed, so that even if there are multiple components as the ion generation source detected by mass spectrometry, the specific component among them can be accurately detected. it can.

Further, according to the second mass spectrometer of the present invention, the electron beam source is hermetically housed in the container, and the container is formed with an opening through which thermoelectrons pass. Therefore, it is possible to substantially prevent the sample gas from entering the vicinity of the electron beam source. As a result, even if the sample gas contains a component that easily causes thermal dissociation, this component cannot enter the vicinity of the electron beam source, so that thermal dissociation of this component does not occur in the sample gas. The specific component of can be accurately detected.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between electron beam flux and electron impact ionization cross section with respect to electron beam energy.

FIG. 2 is a schematic configuration diagram showing a mass spectrometer according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a sample gas introduction path and its potential distribution when the sample gas is sampled from the plasma chamber.

FIG. 4 is a schematic configuration diagram showing an enlarged main part of an ion source unit.

5 is a graph showing the results of measuring CF ₃ ⁺ using the mass spectrometer shown in FIG.

FIG. 6 is a graph in which data of CF ₃ radicals shown in FIG. 5 is arranged.

7A is a graph showing the relationship between the potential V of the variable power supply shown in FIG. 4 and the current I flowing through the resistor, and FIG. 7B is a graph showing the relationship between the potential V of the variable power supply and the electron beam flux. is there.

FIG. 8 is a schematic configuration diagram showing another structure of an electron beam source and an ionization chamber.

9 is a graph showing the results of measuring Cl ⁺ using the mass spectrometer equipped with the electron beam source and the ionization chamber shown in FIG. 8.

FIG. 10 is a schematic configuration diagram showing a neutral beam detection device to which the mass spectrometer of the present invention is applied.

FIG. 11 is a schematic diagram illustrating an example of the neutral beam detection apparatus shown in FIG.
It is a graph which shows the result of having measured ⁺ .

[Explanation of symbols]

 10 Mass Spectrometer 12 Ionization Chamber 14 Quadrupole Electrode 18 Plasma Chamber 20A, 20B Parallel Plate Electrode 22 Secondary Electron Multiplier Tube 32 Filament 34, 36 Electron Beam Shielding Plate 38 Gap 40 Piezo Stack 42 Energy Measuring Device 45a, 45b Variable Power supply 50 Differential pressure chamber 50d Gas inlet

Claims (5)

[Claims] 1. A mass spectrometer comprising an electron beam source that emits thermoelectrons, and an ionization chamber into which the thermoelectrons emitted from the electron beam source are introduced and a sample gas to be analyzed is introduced. Two electron beam shields provided between the electron beam source and the ionization chamber to freely control the distance between them to form a gap through which thermoelectrons pass, the electron beam source and the electrons A mass spectroscope comprising: a potential difference control means for controlling a potential difference between the beam shield plates. 2. The mass according to claim 1, further comprising energy measuring means which is provided in front of the electron beam shielding plate in the thermionic emission direction and which measures the energy of the electron beam passing through the gap. Analysis equipment. 3. A mass spectrometer comprising an electron beam source that emits thermoelectrons, and an ionization chamber into which the thermoelectrons emitted from the electron beam source are introduced and a sample gas to be analyzed is introduced. A mass spectrometer, comprising: a container that has an opening through which thermoelectrons emitted from the electron beam source pass and that hermetically houses the electron beam source. 4. The mass spectrometer according to claim 3, further comprising gas supply means for supplying gas to the container. 5. The mass spectrometer according to claim 3, further comprising exhaust means for exhausting the inside of the container.