JPH0634616A - Analysis of a trace of impurities - Google Patents

Abstract

PURPOSE:To analyze impurities in oxygen, arsin gas, etc., and to fractionate/ analyze impurity components of equal quantity with very high sensitivity by using an atmospheric pressure ionization mass spectrometer after the process of components fractionation with gas chromatography. CONSTITUTION:A carrier gas passes through a cylinder I and a pressure reducing valve 2, and then removes impurities at a refinery 3, and further, comes into a switching cock 5 after the flow rate adjustment at a mass float controller 4. Meanwhile, a sample gas flows through a sample gas introducing mouth 11, a cock 5 and a measurement tube 6, to fill the tube 6 with the gas. Then, the sample gas in the measurement tube 6 comes into a column 7 of a gas chromatograph, and respective impurity components are separated there. Then dilution gas is added to the carrier gas, and they are fed to an atmospheric pressure ionization mass spectrometer 9 for detecting respective impurity components. The analyzing result is recorded in a recorder 10, and is compared with the peak magnitude of the calibration gas of the known condensation, to obtain condensations of respective impurities in the sample gas.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for analyzing trace impurities, and more particularly to a method for analyzing trace amounts of impurities contained in a high-purity gas.

[0002]

2. Description of the Related Art In the semiconductor manufacturing industry, which has been developing remarkably in recent years, demands for improving the purity of gases used have become stronger and stronger, and various kinds of gas manufacturing plants and gas refining equipment are improved day and night. ing. On the other hand, in order to obtain these high-purity gases, it is an important issue to improve the analysis technique for trace impurities, which is the evaluation criterion. Gases used in the semiconductor manufacturing industry are diverse, including bulk gases such as nitrogen, hydrogen, oxygen, argon, helium and dry air, material gases such as arsine and phosphine used as dopants, CFCs used for cleaning, There are gases such as nitrogen trifluoride and etching gases. Since these gases have different physical and chemical properties, it is necessary to select an impurity analysis method according to the properties. . Therefore, it is strongly desired to establish a technique for analyzing the impurity components in each gas to a level of ppb or lower.

As a conventional method for analyzing trace impurities in gas, hydrogen flame ionization gas chromatography (FI) is used.
D), GC-MS method, reducing gas analyzer method, atmospheric pressure ionization mass spectrometer, galvanic cell type oxygen analyzer, PID gas chromatograph method It is used by appropriately selecting it according to the impurity component to be detected.

Although any of these analysis methods can analyze up to the ppb level, except for the method using an atmospheric pressure ionization mass spectrometer, it is difficult to carry out analysis with ultrahigh sensitivity at a level of ppb or less. However, each analysis method has the following drawbacks. First, the hydrogen flame ionization gas chromatographic method can analyze hydrocarbons and carbon monoxide and carbon dioxide which can be reduced to methane by hydrogen reduction on a catalyst, but cannot analyze other impurities. The GC-MS method is a method in which impurities are separated into components by gas chromatography and then the impurities are analyzed by an analysis method using a mass spectrometer as a detector. Since it is caused by collision with electrons, it has disadvantages that it has relatively low sensitivity and that it is difficult to analyze impurities in oxygen. Furthermore, a reducing gas analyzer can selectively analyze reducing gases such as hydrogen and carbon monoxide, but cannot detect other components. In addition, the galvanic cell type trace oxygen analyzer can analyze only the oxygen component, and the PID gas chromatography method has a relatively low sensitivity. Compared with these, the method has a relatively wide range of application and can analyze at a high level of impurity concentration of ppb or less. Therefore, in recent years, a method using an atmospheric pressure ionization mass spectrometer has rapidly spread.

[0005]

However, in the atmospheric pressure ionization mass spectrometer, since the principle of ionizing the impurity components contained in the gas depends on the ion molecule reaction under atmospheric pressure, it is more important than the analytical main component gas. There is a drawback that impurity components that are difficult to ionize cannot be detected. Therefore, for nitrogen, hydrogen, argon, helium, etc., most impurities contained in these gases can be analyzed with extremely high sensitivity, but in the case of impurities in oxygen, for example, methane, carbon monoxide, carbon dioxide, Most components such as nitrogen cannot be detected. Similarly, most impurities cannot be detected in special gases such as arsine, phosphine, silane, and diborane, which are often used in semiconductor manufacturing processes. In addition, even if impurities contained in nitrogen, hydrogen, argon, helium, etc., they are separately detected for impurity components having the same mass number, such as nitrogen and carbon monoxide, carbon dioxide, nitrous oxide, and propane. There is also the drawback that you cannot do it. In addition, there is a problem that it is not possible to analyze an impurity component having the same mass number as the gas as the main component, such as carbon monoxide and ethylene in nitrogen.

[0006]

As a result of intensive studies aimed at improving these drawbacks of the prior art, the inventors of the present invention have found that the sample gas is preliminarily separated by gas chromatography and atmospheric pressure ionization is carried out together with the carrier gas. The present invention has been completed by discovering that the introduction into a mass spectrometer can analyze various impurity components in a region that could not be analyzed only by an atmospheric pressure ionization mass spectrometer with high sensitivity below a ppb level. That is, the present invention is a method of analyzing trace impurities, characterized in that after the sample gas is separated into components by gas chromatography, it is introduced into an atmospheric pressure ionization mass spectrometer together with a carrier gas, and a trace impurity component in the gas is detected. is there. In the present invention, various impurities such as nitrogen, oxygen, methane, carbon monoxide, carbon dioxide, and nitrous oxide existing in various gases, particularly oxygen and gases such as arsine, phosphine, silane, and civolane are contained in ppb or less. It is applied when analyzing with high sensitivity up to the level.

First, the gas chromatographic method according to the present invention will be described. The gas chromatographic method used in the present invention is based on commonly used FID, TCD, G
Similar to the gas chromatographic method such as C-MS, the carrier gas flow rate control unit, the switching cock for introducing the sample gas, the measuring pipe for keeping the introduced sample gas amount constant, the column for separating the components Etc.
The carrier gas used in the gas chromatograph may be any gas that is less likely to be ionized than the impurity component to be analyzed, and its type is not particularly limited, for example,
Gases such as purified, impurity-free nitrogen, hydrogen, argon, helium, etc. are preferred. The amount of carrier gas is preferably at least the amount of gas required by an atmospheric pressure ionization mass spectrometer for detecting various impurities separated by a gas chromatograph. If the amount of carrier gas required by the atmospheric pressure ionization mass spectrometer cannot be flown due to the conditions for separation of components by the gas chromatograph, etc. A gas of the same type as the carrier gas may be injected as a diluent gas to secure a gas amount required by the atmospheric pressure ionization mass spectrometer or more.
The thickness, length, temperature, type of packing, etc. of the column are set in consideration of the separability between the main component gas and the impurity component to be analyzed and the signal strength of the impurity peak.

Next, the atmospheric pressure ionization mass spectrometer used as a detector in the present invention will be described. The principle of the atmospheric pressure ionization mass spectrometer is that the main component gas is ionized by corona discharge under atmospheric pressure, and the impurity component is ionized by the ion molecule reaction with this ion, and then it is placed in a quadruple vacuum. The polar mass filter unit separates and detects the components for each mass. With an atmospheric pressure ionization mass spectrometer, only the output signal corresponding to the mass number of one analysis target component is continuously examined, or the output signals corresponding to the mass numbers of multiple analysis target components are examined simultaneously in a time-division method. It is possible. Recently, as such an atmospheric pressure ionization mass spectrometer, for example, UG-240A type (manufactured by Hitachi Tokyo Electronics Co., Ltd.) and the like are well known and suitable in this field.

Next, the present invention will be illustrated in more detail with reference to the drawings. FIG. 1 is a flow sheet showing the method of the present invention. In FIG. 1, impurities are removed from the carrier gas that has passed through the pressure reducing valve 2 from the cylinder 1 containing the carrier gas by passing through the refiner 3, and the flow rate is adjusted by the mass flow controller 4 to reach the switching cock 5. The switching cock 5 passes the gas through the measuring pipe 6,
Alternatively, it can be switched so as to lead directly to the column 7 without passing through. On the other hand, the sample gas or calibration gas to be analyzed flows from the sample gas inlet 11 via the switching cock 5 and the measuring pipe 6, so that the inside of the measuring pipe is filled with the analysis control gas. Next, by operating the switching cock 5 to change the course, a certain amount of the gas to be analyzed contained in the measuring pipe is pushed out by the carrier gas and introduced into the column. During this period, the gas to be analyzed that enters through the sample gas inlet 11 is exhausted by the sample gas directly after passing through the switching cock 5 and not through the measuring pipe 6. When analyzing impurities, by switching the cock 5, the sample gas or the calibration gas in the measuring pipe 6 is added to the carrier gas in a pulsed manner and introduced into the column 7 of the gas chromatograph. The sample gas or calibration gas from which each impurity component has been separated in the column 7 is added with a diluent gas through a diluent gas inlet 8 and then introduced into an atmospheric pressure ionization mass spectrometer 9 which is a detector. Each impurity component is detected at. The analysis result of the atmospheric pressure ionization mass spectrometer is recorded in the recorder 10 as a change with time of the output signal corresponding to the mass number of the component to be analyzed, and the peak size is proportional to the amount of the component, so The concentration of each impurity contained in the sample gas can be obtained by comparing the size of the peak when the calibration gas is measured.

[0010]

【Example】

Example 1 An atmospheric pressure ionization mass spectrometer (UG-240APS, manufactured by Hitachi Tokyo Electronics Co., Ltd.) was used as a detector in an apparatus having the same configuration as the flow sheet shown in FIG. A 1/2 inch stainless steel tube was used as the measuring tube, and the volume was 300 cc. The column used was a stainless steel tube having a length of 2 m and an outer diameter of 1/2 inch, which was filled with spherical silica gel having a particle size of 80 to 100 mesh as a packing material. Further, as a carrier gas, purified argon having a concentration of each impurity of 1 ppb or less by a getter-type gas purifier was used, and a calibration curve for analysis of impurities in oxygen gas was created while supplying a fixed amount of 420 cc / min with a mass flow controller. Was done. First, the column was activated at 200 ° C. for 5 hours while flowing a carrier gas, and then the column was installed in a thermostat at 20 ° C.

Oxygen gas containing 2 ppm of carbon dioxide as an impurity and oxygen gas from which impurities have been removed by an oxygen gas purifier are mixed with a mass flow controller so as to have a predetermined concentration, and this is used as a calibration gas to adjust the concentration. The analysis was performed by sequentially changing it, and a calibration curve showing the relationship between the detector output and the impurity concentration at that time was created. The setting conditions such as the discharge condition of the atmospheric pressure ionization mass spectrometer, the voltage of the electron multiplier, the voltage of the lens system and the like were the same as in the usual analysis method of argon gas by the atmospheric pressure ionization mass spectrometer. The signal from the atmospheric pressure ionization mass spectrometer was analyzed after the mass number 44 of carbon dioxide was continuously output, and this signal was recorded by the recorder. The analysis results when creating the calibration curve are as shown in Table 1, and the S / N
The lower limit of detection of carbon dioxide in oxygen calculated with = 1 was 0.3 ppb. Based on the calibration curve thus obtained, the purified oxygen gas to which carbon dioxide was not added, that is, the carbon dioxide contained in the outlet gas of the oxygen gas purifier was analyzed, and the result was 0.7 ppb.

[0012]

[Table 1]

Example 2 An apparatus having the same configuration as that of Example 1 was used to prepare an oxygen gas containing 1 ppb of carbon dioxide and 1 ppb of nitrous oxide as impurities and analyze it to examine the signal change of mass number 44. A peak corresponding to 1 ppb of nitrous oxide was detected 20 minutes after the introduction of the gas, and a peak corresponding to 1 ppb of carbon dioxide was separately detected at 24 minutes.

[0014]

According to the present invention, in addition to general gases such as nitrogen, argon, hydrogen, etc., impurities in oxygen and arsine, phosphine, silane, diborane, which could not be analyzed by only conventional atmospheric ionization mass spectrometers, were used. It has become possible to analyze impurities in gases such as special gases with high sensitivity up to the ppb level or lower. Further, it has become possible to accurately separate impurity components having the same mass number, such as carbon monoxide and ethylene, carbon dioxide and nitrous oxide and propane, and analyze them with high sensitivity up to the ppb level or lower.

[0015]

[Brief description of drawings]

FIG. 1 is a flow sheet of an analysis method of the present invention.

[Explanation of symbols]

 1 Cylinder 2 Pressure reducing valve 3 Purifier 4 Mass flow controller 5 Switching cock 6 Measuring tube 7 Column 8 Diluting gas inlet 9 Atmospheric pressure mass spectrometer 10 Recorder 11 Sample gas inlet 12 Sample gas outlet

Claims (5)

[Claims] 1. A method for analyzing a trace amount of impurities, which comprises separating a sample gas into components by gas chromatography and introducing the carrier gas together with an atmospheric pressure ionization mass spectrometer to detect trace impurities in the gas. 2. The analysis method according to claim 1, wherein the detection sensitivity of each impurity component is 1 ppb or less. 3. The analysis method according to claim 1, wherein the gas to be analyzed is oxygen. 4. The analysis method according to claim 1, wherein the gas to be analyzed is arsine, phosphine, silane, disilane, diborane or hydrogen selenide. 5. The analysis method according to claim 1, wherein the impurity component is one or more of nitrogen, oxygen, hydrogen, carbon monoxide, carbon dioxide, hydrocarbon and nitrous oxide.