GB2344655A

GB2344655A - Analysing organic contaminants in a moisture-containing gas

Info

Publication number: GB2344655A
Application number: GB9926741A
Authority: GB
Inventors: Yasushi Sasaki
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1998-11-11
Filing date: 1999-11-11
Publication date: 2000-06-14
Also published as: TW468044B; JP2000146912A; KR20000035421A; JP3139477B2; GB9926741D0

Abstract

A method of analysing an organic substance in a gas uses an atmospheric pressure ionisation mass spectrometer. A carrier gas is ionised at atmospheric pressure, S1. The ions are caused to collide with particles of a gas sample, ionising an organic contaminant therein, S2. The ionised contaminant particles are examined in a mass spectrometer, S3, to identify the organic substance and determine its concentration. The carrier gas contains a component with an ionisation potential lower than that of water molecules and higher than that of the contaminant to reduce the errors due to the moisture content of the gas. Suitable carrier gases include O<SB>2</SB>, or a mixture of Ar or N<SB>2</SB> with a silane.

Description

METHOD OF ANALYSING CONTAMINANT IN GAS The present invention relates to a method of analyzing a gas. There will be described below, by way of example in illustration of the invention a particular method of analyzing organic substances as a contaminant existing in a gas, such as clean air for a clean room, where it is suitable to apply quantitative analysis in order to know the concentration of the substances, whereby the sensitivity of the analysis may be approved.

Chemical Vapor Deposition (CVD) has been previously used in the process sequence for fabricating semiconductor devices. If some organic substance (especially one which contains amines) exists in the atmosphere in the reaction or the deposition chamber of a CVD apparatus as a chemical contaminant, there is a possibility that a deposited film in the chamber contains the substance and has a bad effect upon the characteristics of silicon devices, such as an increase in contact resistance. Such contaminants have more impact on the performance or reliability of the devices as the miniaturization of the devices progresses. Thus, recently, a trace of an organic contaminant present in the parts pertrillion (ppt) level has been required to be controlled in a cleaned atmosphere, such as a clean room, used in semiconductor device fabrication.

It is known that organic contaminants existing in a purified gas can be analyzed at a high sensitivity of the ppt level by using an Atmospheric Pressure onized Mass Spectrometer (API-MS). Conventionally, to analyze organic contaminants present in a purified gas at a high sensitivity, an argon (Ar) or nitrogen (N2) gas has been used as a carrier gas for a primary ionization. The molecules of argon and nitrogen gases have an ionization potential higher than 15 eV.

In a previously proposed method of analyzing organic substances existing in a purified gas, first, the molecules of the carrier gas (i. e., Ar or N2) were ionized by corona discharge at atmospheric pressure (i. e., 1 atm), which is termed the"primary ionization". Then, the carrier gas thus ionized was contacted with the sample gas to be analyzed (i. e., the purified gas), thereby causing the ionized molecules of the carrier gas to collide with molecules of the sample or purified gas. In this step, particles (i. e., atoms or molecules) of contaminants existing in the sample gas, which had an ionization potential lower than that of the carrier gas, were ionized due to the molecular-ion reaction. This is termed the"secondary ionization". The ionized particles of the contaminants in the secondary ionization were sent to an analyzer and quantitatively analyzed by using mass spectrometry. As a result, the contaminants were identified and their concentrations were found.

When cleaned air prepared for a clean room used in semiconductor device fabrication is analyzed by using atmospheric-pressure mass spectrometry while an Ar or N2 gas is used as the carrier gas according to the above-described previously proposed method, there is a problem that organic contaminants such as amines present in the cleaned air are unable to be detected at a satisfactorily high sensitivity.

Another previously proposed method is disclosed in the Japanese Non Examined Patent Publication No. 6-34616 published in October 1994. In this method, gaseous contaminants contained in a sample gas are separated by using gas chromatography (GC) and then, the contaminants thus separated are subjected to atmospheric-pressure ionization mass spectrometry. Even if the gaseous contaminants have particles having an ionization potential higher than that of the sample gas, they can be analyzed by this method.

Still. another previously proposed method is disclosed in the Japanese Non-Examined Patent Publication No. 9-15207 published in January 1997. In this method, similar to the method disclosed in Japanese Non-Examined Patent Publication No. 6-34616, gaseous contaminants contained in a sample gas are separated by using gas chromatography and then the contaminants, thus separated, are subjected to atmospheric-pressure ionization mass spectrometry. Gaseous contaminants such as SiH4, PH3, AsH3 existing in the sample gas such as He, Ar, N2, and H2 have particles with an ionization potential approximately equal to that of the sample gas itself. Therefore, these contaminants are unable to be detected at a high sensitivity. However, this previously proposed method makes it possible for these contaminants to be analyzed at a high sensitivity of the parts per billion (ppb) level.

A further previously proposed method is disclosed in the Japanese Examined Patent Publication No. 1-15985 published in March 1989. In this method, to distinguish between two substances (e. g., nitrogen and carbon monoxide) that generate ions with equal or similar mass numbers, a specific gaseous substance (e. g., krypton), which has an ionization potential lying between the ionization potentials of the two substances to be distinguished, is added to a carrier gas. In other words, the carrier gas contains a gaseous substance having an lonization potential lying between the ionization potentials of the two substances to be distinguished.

However, none of the above-described previously proposed methods disclosed in the Japanese Patent Publication Nos. 6-34616,915207, and 1-15985 is able to solve the above-described problem caused by an organic contaminant, such as amines, existing in cleaned air at a high sensitivity of the part per trillion (ppt) level.

Features of methods to be described below by way of example in illustration of the present invention for analyzing an organic contaminant existing in a moisture-containing gas at a high sensitivity are the use of atmospheric-pressure mass spectrometry, sensitivity to the ppt level, and analyzing amines existing as a contaminant in clean air for a clean room.

A method of analyzing an organic contaminant existing in a gas to be described below by way of example in illustration of the present invention includes the steps of (a) ionizing particles of a carrier gas at an atmospheric pressure (a primary ionization step) ; (b) causing the ionized particles of the carrier gas to collide with particles of a sample gas, thereby ionizing particles of an organic contaminant existing in the sample gas at an atmospheric pressure (a secondary ionization step); and (c) analyzing the ionized particles of the contaminant existing in the sample gas by mass spectrometry, thereby identifying the contaminant and finding the concentration of the contaminant.

The sample gas contains moisture or water molecules. The carrier gas contains a component whose molecules have an ionization potential lower than that of water molecules and higher than that of the contaminant. The component of the carrier gas is selectively ionized in the step (a).

With a method to be described below in illustration of the present invention, the carrier gas used in the step (a) contains a component whose molecules have an ionization potential lower than that of water molecules and higher than that of the organic contaminant existing in the sample gas.

Therefore, when the ionized particles of the component of the carrier gas, which have been generated in the step (a), are caused to collide with the particles of the sample gas in the step (b), the particles of the organic contaminant existing in the sample gas can be selectively ionized while preventing the water molecules contained in the sample gas from being ionized. If the water molecules are ionized in the step (b), they inhibit the analysis in the step (c).

As a result, the organic contaminant existing in the moisturecontaining sample gas can be quantitatively analyzed at a high sensitivity using atmospheric-pressure mass spectrometry. This sensitivity can be raised or improved to the ppt level.

In a method to be described below, by way of example in illustration of the invention, the carrier gas may be composed of only the component whose molecules have an ionization potential lower than that of water molecules and higher than that of the organic contaminant existing in the sample gas.

As the sample gas, any gas may be used if it contains moisture or water molecules and an organic contaminant.

The organic contaminant means any organic substance present in the sample gas as a contaminant. For example, it inclues amines, phthalate esters, siloxanes, and so on, each of which has a property that easily reacts withsilicon.

In one preferred method to be described below by way of example in illustration of the invention, an oxygen (02) gas is used as the carrier gas.

This is because the advantages of the arrangement are effectively exhibited.

In another preferred method to be described by way of example in illustration of the invention, a mixture of an argon (Ar) or nitrogen (N2) gas and a silane (SiH4) gas is used as the carrier gas, in other words, the component of the carrier gas is SiH4 gas. In this case, the concentration of the silane gas in the mixture is in the range from one-millionth (1/1, 000,000) mole to onethousandth (1/1, 000) mole. This is because the advantages of the arrangement are effectively exhibited.

In yet another preferred method to be described by way of example in illustration of the invention, air is used as the sample gas. This is because the advantages of the invention are effectively applied.

In a further preferred method to be described below by way of example in illustration of the invention, the organic contaminant contains amines. Amines tend to affect silicon devices badly due to their reaction with silicon.

Arrangements illustrative of the invention will now be described by way of example with reference to the accompanying drawings, in which Fig. 1 is a flow chart showing the steps of a method for analyzing an organic contaminant existing in a gas, and Fig. 2 is a block schematic diagram showing an apparatus for carrying out the method described with reference to Fig. 1.

In a first method to be described for analyzing an organic contaminant existing in a gas using atmospheric-pressure mass spectrometry, an organic contaminant, such as amines, existing in cleaned air prepared for a clean room is analyzed. Therefore, this cleaned air is used as a"sample gas" SG. Also, as a carrier gas"CG for carrying the sample gas SG, an oxygen gas (02) is used, the molecules of which have an ionization potential lower than the ionization potential (i. e., 12.06 eV) of water (H2O) molecules and higher than that of amines molecules (e. g., 10 eV).

As shown in Fig. 1, in the first step S1, the 02 gas (i. e., the carrier gas) CG is ionized at atmospheric pressure (i. e., 1 atm) in a first ionization chamber 1 shown in Fig. 2 using the corona discharge. Since this ionization step S1 is carried out at atmospheric pressure, the ionization is realized efficiently. This step S1 is termed the"primary ionization step".

In the second step S2, the ionized carrier gas CG' (i. e., the 02 gas containing the ionized 02 molecules) is contacted or mixed with cleaned air (i. e., the sample gas SG) in order to cause the ionized 02 molecules to collide with the molecules of the components (e. g., N2and 02) of the cleaned air, thereby ionizing the amine molecules existing in the air without ionizing the water molecules existing therein. This step S2 is termed the"secondary ionization step", which is caused by the molecular-ion reaction.

Since this ionization step S2 also is carried out at atmospheric pressure, the ionization is realized efficiently, in a similar way to the primary ionization step S1. The step S2 is carried out in a second ionization chamber 2 shown in Fig. 2, which is separate from the first ionization chamber 1.

In the third step S3, the ionized molecules Cl in the ionized carrier gas CG'and the ionized molecules SI in the sample gas SG are sent to an analyzer 3 and subjected to mass spectrometry in a specific vacuum atmosphere. Thus, data or information D about the sort and concentration of the organic contaminant, such as amines, contained in the sample gas SG is derived from the result of the analysis in the analyzer 3.

The present inventor has been researching the reason why chemical contaminants present in atmospheric air are unable to be analyzed or detected at a desired high sensitivity. As a result, he has concluded that the cause exists in the ionization of moisture (i. e., water molecules) contained in the air, at several tens percent, in the secondary ionization step.

As described in previous proposas, argon or nitrogen gas has been used as the carrier gas CG whose atoms or molecules have an ionization potential as high as 15eV or higher. In this case, molecules having an ionization potential lower than that of argon or nitrogen are ionized in the secondary ionization step. Therefore, any contaminant having such the molecules can be analyzed.

When the sample gas SG is atmospheric air, however, any contaminant having such molecules is unable to be analyzed at a high sensitivity level, because the air usually contains some moisture (i. e., water molecules). Specifically, not only the molecules of the contaminant that has an ionization potential lower than that of an argon or nitrogen, but also the molecules of water, are ionized in the secondary ionization step. As a result, the contaminant is unable to be selectively analyzed at a high sensitivity level.

On the other hand, in the method according to the first arrangement to be described by way of example in illustration of the present invention, when the ionized oxygen molecules of the carrier gas CG are caused to collide with the molecules of the water and an organic contaminant, such as amines, contained in the sample gas SG in the secondary ionization step S2, the molecules of the contaminant are ionized without ionizing the water molecules.

This means that the ion analysis step S3 is not inhibited by the ionized water molecules. As a result, an organic contaminant, such as amines, existing in the moisture-containing sample gas SG can be quantitatively analyzed at a high sensitivity of the ppt level.

In a second described by way of example in illustration of the invention for analyzing an organic contaminant existing in a gas using atmospheric-pressure mass spectrometry, which is similar to the first arrangement, an organic contaminant, such as amines, existing in cleaned air for a clean room is analyzed. Therefore, the cleaned air is used as the "sample gas"SG.

Unlike the first arrangement described, an argon gas (Ar) containing a silane gas (SiH4) is used as the"carrier gas"CG, in other words, a gaseous mixture of argon and silane is used. The concentration of the silane gas in the mixture is in the range from one-millionth (1/1,000,000) mole to onethousandth (1/1, 000) mole. If the concentration of the silane gas in the carrier gas CG is less than one-millionth mole, the molecules of all the organic substances contained in the sample gas SG are unable to be ionized in the secondary ionization step S2, resulting in the possibility that the accuracy is degraded in the analysis. Contrarily, if the concentration of the silane gas in the carrier gas CG is greater than one-thousandth mole (i. e., 0.1 mole %), the silane gas tends to be decomposed in the primary ionization step S1 and accordingly, the resultant silicon (Si) tends to be deposited in the first ionization chamber 1. The deposited silicon may obstruct the analysis itself.

Moreover, if the concentration of the silane gas in the carrier gas CG is greater than 1.1 mole %, the danger arises that the carrier gas may explode.

As shown in Fig. 1, in the first step (primary ionization step) S1, the SiH4 gas contained in the Ar-SiH4 mixture (i. e., the carrier gas CG) is selectively ionized at atmospheric pressure (i. e., 1 atm) in the first ionization chamber 1. The molecules of the Ar gas are not ionized. This is to prevent the ionization of the water molecules contained in the cleaned air as the sample gas SG.

In the second step (secondary ionization step) S2, the ionized carrier gas CG', i. e, the Ar-SiH4 gaseous mixture containing the ionized Si atoms and ionized H4 molecules, are contacted or mixed with the cleaned air (i. e., the sample gas SG) and the ionized Si and H4 particles and caused to collide with the molecules of the air, thereby ionizing the molecules of a contaminant, such as amines, existing in the air without ionizing the water molecules existing therein. This step S2 is carried out at atmospheric pressur in the second ionization chamber 2.

In the third step S3, the ionized particles Cl contained in the carrier gas CG'and the ionized particles Si in the sample gas SG are sent to the analyzer 3 and subjected to mass spectrometry in a specific vacuum atmosphere.

Since the method described in relation to the second arrangement includes the same steps S1 to S3 as those in the first described arrangement it has the same advantages as those in the first arrangement. Furthermore, there is an additional advantage, as will be described below.

Specifically, as explained above, the silane gas contained in the carrier gas CG may be decomposed and the resultant silicon may be deposited in the first ionization chamber 1 in the first ionization step S1.

However, the first and second ionization steps S1 and S2 are respectively performed in the first and second ionization chambers 1 and 2. As a result, even if silicon is deposited in the first ionization chamber 1, due to decomposition of the silane gas, the second ionization chamber 2 is not affected by the deposition of silicon, which ensures that the analysis is performed independently of the use of a silane gas.

In a method of analyzing an organic contaminant existing in a gas using atmospheric-pressure mass spectrometry in accordance with a third illustrative arrangement, similar to the arrangement first described above, an organic contaminant, such as amines, existing in cleaned air for a clean room is analyzed as a contaminant. Therefore, the cleaned air is used as the "sample gas"SG.

Unlike the first described arrangement, as the gcarrier gasX CG, a nitrogen gas containing a silane gas (SiH4), in other words, a gaseous mixture of nitrogen and silane, is used. The concentration of the silane gas in the mixture needs to be in the same range as described above with reference to the second arrangement.

The same steps S1 to S3 as those described with reference to the second arrangement are carried out in the method of the third arrangement.

Therefore, there are the same advantages as those in the second arrangement.

The methods described above with reference to the first to third arrangements can be carried out by using a known Atmospheric-Pressure lonized Mass Spectrometer (API-MS) equipped with two ion sources.

However, this method may be carried out by using a known API-MS equipped with a single ion source.

Moreover, it is needless to say that the carrier gas CG may contain any organic or inorganic substance or substances that are inevitably generated through the actual processes for producing the gas CG.

While particular arrangements illustrative of the present invention have been described by way of example, it is to be understood that variations and modifications thereof as well as other arrangements may be conceived within the scope of the appended claims

Claims

CLAIMS 1. A method of analyzing a contaminant existing in a gas, including the steps of (a) ionizing particles of a carrier gas at an atmospheric pressure, (b) causing the ionized particles of the carrier gas to collide with particles of a sample gas, thereby ionizing particles of an organic contaminant existing in the sample gas at an atmospheric pressure, and (c) analyzing the ionized particles of the contaminant existing in the sample gas by mass spectrometry, thereby identifying the contaminant and finding a concentration of the contaminant, wherein the sample gas contains water molecules, wherein the carrier gas contains a component whose molecules have an ionization potential lower than that of water molecules and higher than that of the contaminant, and wherein the component of the carrier gas is selectively ionized in the step (a).
2. A method as claimed in claim 1, wherein an oxygen gas is used as the carrier gas, the component being the oxygen gas itself.
3. A method as claimed in claim 1, wherein a mixture of an argon gas and a silane gas is used as the carrier gas, the component being silane gas itself.
4. A method as claimed in claim 3, wherein a concentration of the silane gas in the mixture is in a range from one-millionth (1/1, 000,000) mole to onethousandth (1/1, 000) mole.
5. A method as claimed in claim 1, wherein the mixture of a nitrogen gas and a silane gas is used as the carrier gas, the component being silane gas itself.
6. A method as claimed in claim 5, wherein the concentration of the silane gas in the mixture is in the range from one-millionth (1/1,000,000) mole to onethousandth (1/1, 000) mole.
7. A method as claimed in claim 1, wherein air is used as the sample gas.
8. A method as claimed in claim 1, wherein the organic contaminant contains amines.
9. A method as claimed in claim 1 substantially as described herein with reference to Fig. 1 and Fig. 2 of the accompanying drawings.