EP4187574A1 - Inductively coupled plasma source mass spectrometry for silicon measurement - Google Patents

Abstract

The invention relates to a method for measuring a sample comprising silicon by mass spectrometry, said method being implemented using a mass spectrometer (1) with a tandem-coupled plasma source, or ICP-MS/MS . The measurement method comprising a measurement step by mass spectrometry using a reactive gas, the method being characterized in that the reactive gas comprises nitrous oxide.

Description

Technical area

The invention relates to the field of inductively coupled plasma mass spectrometry measurement, better known by its acronym ICP-MS for “Inductively Coupled Plasma Mass Spectrometry”.

Thus the subject of the invention is more precisely a mass spectrometry measurement method and a use of nitrous oxide N ₂ O in the context of such a mass spectrometry measurement.

State of the prior art

Inductively coupled plasma source mass spectrometry measurements, hereafter ICP-MS, and in particular when it comes to measuring silicon isotopes, can be disturbed by the presence of isobaric interferences. Indeed, silicon comprises three isotopes: the 28 ²⁸ Si isotope, the 29 ²⁹ Si isotope and the 30 ³⁰ Si isotope for which a large number of interferences are observed, presented in the following table: [Table 1] Si isotopes interference ²⁸ If ^{+ 14N 14N +} ; ¹² C ¹⁶ W ^{+ 29} If ^{+ 14N 14N 1H +} ; ^{14N 15N +} ; ²⁸ If ^{1H +} ; ^{13C16O + ; 12} C ¹⁶ O ¹ H ⁺ . ¹² C ¹⁷ W ^{+ 30} If ^{+ 14N 16W +} ; ^{14N 15N 1H +} ; ^{13C17O + ; 12C17O1H + ; _ 15N 15N +}

Because of these interferences, obtaining a measurement of silicon and these isotopes by ICP-MS with good precision is made particularly difficult. However, for certain applications, in particular related to the semiconductor industry, such a measurement is necessary. It will be noted that these needs are particularly present for samples with a high enrichment in the 28 ²⁸ Si isotope of silicon where, in addition, high sensitivity is necessary in order to be able to detect the minority isotopes of silicon in order to quantify the enrichment.

To obtain such precision, the use of multi-collector inductively coupled plasma source mass spectrometry has been proposed [1, 2]. This technique, if it makes it possible to solve the problem of the interferents, remains complicated to implement and is expensive.

Therefore, there could be an interest in developing a simpler inductively coupled plasma source mass spectrometry technique, such as quadrupole inductively coupled plasma source mass spectrometry, better known by the acronym ICP-QMS. , or tandem inductively coupled plasma source mass spectrometry, better known by the acronym ICP-MS/MS, in order to allow a measurement of silicon and its isotopes which is compatible with samples with high enrichment in the isotope 28 ²⁸ If silicon. Nevertheless, the attempts [2, 3, 4, 5] carried out with these techniques, generally based on the use of oxygen [4, 5] as reactive gas, do not make it possible to obtain suitable sensitivity.

Disclosure of Invention

The object of the invention is thus to provide a method for measuring mass spectrometry with an inductively coupled plasma source which is suitable for samples with a high enrichment in the 28 ²⁸ Si isotope of silicon and which is simpler to implement than multi-collector inductively coupled plasma source mass spectrometry.

The invention relates for this purpose to a method for measuring by mass spectrometry a sample comprising silicon, said method being implemented from a mass spectrometer with a tandem coupled plasma source, or ICP-MS/MS ,

Said method of measurement comprising a step of measurement by mass spectrometry by means of a reactive gas, the method being characterized in that the reactive gas comprises nitrous oxide.

The inventors have discovered that nitrous oxide N ₂ 0 as a reactive gas has a significantly greater reactivity than dioxygen which is generally used as a reactive gas in the prior art for such measurements. As a result, when using nitrous oxide N ₂ O as a reactive gas in the ICP-MS/MS measurement framework, there results a significantly higher sensitivity than that obtained with the dioxygen. Thus such a method, as shown by the inventors and as described below, makes it possible to obtain precision in the measurement of the molar proportion of the 28 isotope of silicon with respect to all of the silicon atoms less than 0.01%.

The reactive gas can consist of nitrous oxide.

The measurement step can make it possible to determine a value relating to a quantity of an isotope 28 of silicon.

It will be noted that in a usual configuration of the invention, the measurement can also make it possible to obtain a value relating to the quantities of the isotopes 29 and 30 of silicon.

The value relating to the amount of the silicon 28 isotope can be a molar proportion of the silicon 28 isotope with respect to all the silicon atoms.

Such a value makes it possible to estimate the enrichment of the sample in the isotope 28 of silicon. This value is particularly relevant in the context of certain applications such as those in the semiconductor industry.

The sample may have a molar proportion of the 28 isotope of silicon with respect to all the silicon atoms greater than 99%.

The method according to the invention is particularly suitable for such samples since it makes it possible, as shown in the remainder of this document, to obtain a sensitivity compatible with such proportions of the 28 isotope of silicon.

During the mass spectrometry measurement step, the reactive gas can be introduced into a reaction cell of the mass spectrometer at a flow rate of between 0.03 and 0.28 mL.min ^-1 and preferably between 0.06 and 0.15 mL.min ^-1 .

The inventors have discovered that with such a flow rate, the sensitivity to silicon isotopes is maximized.

The invention further relates to a use of a reactive gas comprising nitrous oxide for a measurement by mass spectrometry of a sample comprising silicon from a mass spectrometer with a tandem coupled plasma source, or ICP -MS/MS.

The reactive gas can consist of nitrous oxide.

The sample may have a molar proportion of the 28 isotope of silicon with respect to all the silicon atoms greater than 99%.

As demonstrated following this document, such a use is particularly advantageous since it allows obtaining a particularly high sensitivity to silicon isotopes with respect to the prior art.

Brief description of the drawings

• Fig. 1 illustre schématiquement les différents étages d'un spectromètre de masse tel qu'utilisé dans le cadre de la mise en œuvre de l'invention.
• Fig.2A illustre graphiquement le profil de réactivité pour l'isotope 28 du silicium en fonction du débit de dioxygène introduit dans une cellule de réaction du spectromètre en tant que gaz réactif pour un échantillon de silicium de référence.
• Fig.2B illustre graphiquement le profil de réactivité pour l'isotope 28 du silicium en fonction du débit de protoxyde d'azote introduit dans la cellule de réaction du spectromètre en tant que gaz réactif pour ce même échantillon de silicium de référence.
• Fig. 3 illustre graphiquement la sensibilité obtenue pour respectivement le dioxygène et le protoxyde d'azote ceci pour les différents isotopes du silicium.

The present invention will be better understood on reading the description of exemplary embodiments, given purely for information and in no way limiting, with reference to the appended drawings in which:

• Fig. 1 schematically illustrates the different stages of a mass spectrometer as used in the context of the implementation of the invention.
• Fig.2A graphically illustrates the reactivity profile for the isotope 28 of silicon as a function of the flow of oxygen introduced into a spectrometer reaction cell as a reactive gas for a reference silicon sample.
• Fig.2B graphically illustrates the reactivity profile for the isotope 28 of silicon as a function of the flow rate of nitrous oxide introduced into the reaction cell of the spectrometer as the reactive gas for this same reference silicon sample.
• Fig. 3 graphically illustrates the sensitivity obtained for dioxygen and nitrous oxide respectively, this for the different isotopes of silicon.

Identical, similar or equivalent parts of the various figures bear the same reference numerals so as to facilitate passage from one figure to another.

The different parts shown in the figures are not necessarily shown on a uniform scale, to make the figures more readable.

The different possibilities (variants and embodiments) must be understood as not mutually exclusive and can be combined with each other.

Description of embodiments

As described below in connection with the figure 1 , the inventors have discovered that tandem coupled plasma source mass spectrometry 1, or

ICP-MS/MS when implemented with nitrous oxide as the reactant gas, provides sample measurement comprising silicon with improved sensitivity to measurements obtained according to the prior art.

• un système de générateur de plasma 10 apte à atomiser et ioniser les espèces de l'échantillon E en un plasma,
• une optique ionique 20 afin de focaliser les ions créé lors de l'ionisation des atomes de l'échantillon,
• un premier filtre électromagnétique 31, tel qu'un analyseur quadripolaire, afin de réaliser un premier filtrage parmi les ions du plasma après que ceux-ci ont été focalisés par l'optique ionique 20, le premier filtre permettant de sélectionner les ions correspondant à l'atome cible et ses interférents,
• une cellule de réaction 40 afin de faire réagir avec un gaz réactif les ions sélectionnés lors du filtrage effectué par le premier filtre quadripolaire 30 et ainsi produire des molécules ionisées dont font partie des molécules produit formées par réaction de l'atome ou molécule cible ionisé avec le gaz réactif,
• un deuxième filtre électromagnétique 32, tel qu'un analyseur quadripolaire, afin de réaliser un deuxième filtrage parmi les molécules ionisées, le deuxième filtre permettant de sélectionner uniquement les molécules produit, qui comprennent donc l'atome cible, formées dans la cellule de réaction,
• un détecteur apte à intercepter les molécules produit après le deuxième filtrage réalisé par le deuxième filtre et fournir un signal relatif auxdites interceptions.

As a reminder, a tandem coupled plasma source mass spectrometer 1 comprises, as illustrated in the figure 1 the following elements :

• a plasma generator system 10 capable of atomizing and ionizing the species of the sample E into a plasma,
• an ion optic 20 in order to focus the ions created during the ionization of the atoms of the sample,
• a first electromagnetic filter 31, such as a quadrupole analyzer, in order to carry out a first filtering among the ions of the plasma after these have been focused by the ion optics 20, the first filter making it possible to select the ions corresponding to the target atom and its interferents,
• a reaction cell 40 in order to cause the ions selected during the filtering carried out by the first quadripolar filter 30 to react with a reactive gas and thus produce ionized molecules of which the product molecules formed by reaction of the ionized target atom or molecule with reactive gas,
• a second electromagnetic filter 32, such as a quadrupole analyzer, in order to carry out a second filtering among the ionized molecules, the second filter making it possible to select only the product molecules, which therefore include the target atom, formed in the reaction cell,
• a detector capable of intercepting the molecules produced after the second filtering carried out by the second filter and supplying a signal relating to said interceptions.

It will be noted that by “the target atom or molecule”, it is understood, here and in the rest of this document, the isotope or the molecule comprising the isotope which is the target of the measurement by spectrometry, that is that is, which is the subject of the quantification in the sample to be measured. In the present example concerning the silicon isotopes, said target atom corresponds in turn to each of the silicon isotopes as described below in connection with the picture 3 .

Similarly, as indicated above, by molecule-product(s) is meant here and in the rest of this document, the molecule-product obtained during the reaction between the ionized target atom or molecule and the reactive gas. In the context of the present invention, namely the measurement of the natural isotopes ²⁸ Si, ²⁹ Si and ³⁰ Si of silicon by the use of nitrous oxide N ₂ O, the molecules produced are respectively ²⁸ SiO ₂ ⁺ , ²⁹ SiO ₂ ⁺ and ³⁰ SiO ₂ ⁺ . It should be noted that for the use of dioxygen O ₂ , the product molecules are identical.

When implementing a mass spectrometry measurement with an ICP-MS/MS mass spectrometer, the choice of a reactive gas must be adapted to the atoms that will be targeted for the measurement. Thus in the context of silicon isotopes, the reactive gases [2, 3, 4, 5] used in the prior art, whether in the context of ICP-MS/MS mass spectrometry measurement or spectrometry measurement of ICP-QMS mass, comprise either ammonia NH ₃ , or methane CH ₄ , or dioxygen O ₂ . More specifically, with regard to ICP-MS/MS mass spectrometry [4,5], oxygen O ₂ is generally used for the reactive gas. Therefore, in this document, the inventors have chosen to compare the results obtained in the context of the invention, that is to say when the reactive gas is based on nitrous oxide N ₂ 0, with those obtained for a reactive gas based on oxygen O ₂ .

The inventors have discovered, on the basis of calculation of reaction enthalpy (known under the reference ΔH _r ), that the reaction of silicon cations Si ⁺ with nitrous oxide N ₂ O is significantly exothermic whereas the reaction of cations of silicon Si ⁺ with dioxygen O ₂ is endothermic. It follows that the inventors have estimated that the use of a reactive gas based on nitrous oxide N ₂ O should make it possible to obtain an optimized reactivity and therefore the obtaining of a mass spectrometry measurement ICP- MS/MS particularly sensitive.

1. (1)
   
            Si⁺ + O₂→SiO⁺+O : ΔH_r= 5,2 kcal.mol^-1
   
   
2. (2)
   
            SiO⁺ + O₂→SiO₂ ⁺+O : ΔH_r = 8,2 kcal.mol^-1
   
   
3. (3)
   
            Si⁺ + N₂O→SiO⁺+N₂ : ΔH_r = -40,7 kcal.mol^-1
   
   
4. (4)
   
            SiO⁺ + N₂O→SiO₂ ⁺+ N₂ : ΔH_r = -37,7kcal.mol^-1
   
   

Indeed, here, in parallel with the enthalpy of reaction calculated for the latter, the reactions likely to be obtained for the isotopes of silicon in cationic form in the reaction cell 40 with respectively oxygen and nitrous oxide N ₂ O:

1. (1)
   
   Si ⁺ + O ₂ →SiO ⁺ +O: ΔH _r = 5.2 kcal.mol ^-1
   
   
2. (2)
   
   SiO ⁺ + O ₂ →SiO ₂ ⁺ +O: ΔH _r = 8.2 kcal.mol ^-1
   
   
3. (3)
   
   Si ⁺ + N ₂ O→SiO ⁺ +N ₂ : ΔH _r = -40.7 kcal.mol ^-1
   
   
4. (4)
   
   SiO ⁺ + N ₂ O→SiO ₂ ⁺ + N ₂ : ΔH _r = -37.7kcal.mol ^-1
   
   

The reactions with nitrous oxide are therefore favorable and must have a particularly high yield with respect to the reactions with dioxygen. As a result, the inventors considered that nitrous oxide should make it possible, in the context of measurement by ICP-MS/MS mass spectrometry of silicon and its various isotopes, to achieve a significantly improved sensitivity with respect to from that obtained with dioxygen.

In order to verify this, the inventors carried out sensitivity measurements in ICP-MS/MS mass spectrometry from the same sample of silicon as a source of isotope 28 of silicon ²⁸ Si for respectively reactive gases based on dioxygen (oxygen flow D _O2 between 0 and 0.78 mL.min ^-1 ), corresponding to the measurements of the prior art, and for reactive gases based on nitrous oxide (flow of nitrous oxide nitrogen D _N2O between 0 and 0.28 mL.min ^-1 ). There figure 2A shows the intensity I(Si ²⁸ ) of the signal in counts per second obtained for this sample from reactive gas based on dioxygen O ₂ as a function of the flow rate of dioxygen D _O2 . We can see on this graph that the maximum is obtained around 0.19 mL.min ^-1 of oxygen for which we obtain about 20,000 cps.

There figure 2B shows the intensity of the signal in counts per second obtained for this sample from reactive gas based on nitrous oxide N ₂ O this as a function of the flow rate of nitrous oxide D _N2O . It is observed that for nitrous oxide, the maximum is obtained around 0.09 mL.min ^-1 of nitrous oxide for which approximately 132,000 cps are obtained, i.e. more than 6 times the value obtained from the dioxygen. It will also be noted that this maximum is obtained for a flow rate of nitrous oxide N ₂ O less than that of dioxygen O ₂ , confirming that nitrous oxide N ₂ O has a higher reaction efficiency than that of dioxygen O ₂ .

Thus in the context of the invention, the flow rate of reactive gas used for the measurement by mass spectrometry is advantageously between 0.03 and 0.28 mL.min ^-1 and preferably between 0.06 and 0.15 mL .min ^-1 .

On the basis of this study, the inventors were able to optimize the instrumental parameters in order to maximize the sensitivity for the silicon isotopes ²⁸ Si, ²⁹ Si, ³⁰ Si for the reactive gases based on dioxygen O ₂ and based on nitrous oxide. nitrogen N ₂ O and to estimate the sensitivity and the background equivalent concentration, better known by its acronym BEC for “Background Equivalent Concentration”. These results are summarized in the following table.

Note that this document does not describe such parameter optimization. This optimization is in fact part of the usual practice of those skilled in the art and being dependent on the apparatus used for the mass spectrometry measurement. The description of such an optimization is therefore of no interest in this document. [Table 2] Reactive gas N ₂ 0 _O2 Sensitivity ²⁸ SiO ₂ (cps ppb-1) 4.0.10 ³ 4.5.10 ² BEC (ppb Si) 2 3

It can be seen that nitrous oxide N ₂ O makes it possible to achieve a sensitivity almost 10 times greater than that of dioxygen O ₂ with a signal to noise ratio reduced by 50%. With a reactive gas based on nitrous oxide N ₂ O, the inventors therefore felt able to carry out measurements on samples comprising a high proportion of a silicon isotope with optimized sensitivity. There picture 3 shows that this improved sensitivity obtained within the framework of such a method of measurement is so as well for the majority isotope of silicon Si, which is the isotope 28 ²⁸ Si, as for the minority isotopes of silicon Si which are the isotopes 29 ²⁹ Si and 30 ³⁰ Si.

In order to demonstrate this, the inventors carried out 3 campaigns (indicated as a session) of 6 ICP-MS/MS mass spectrometry measurements using a reactive gas based on nitrous oxide N ₂ O from a Silicon sample showing an enrichment in the 28 isotope of silicon ²⁸ Si. The results of these measurement campaigns are summarized in the following table with for each of the measurements an estimate of the proportion of the 28 isotope of silicon ²⁸ Si (% mol ²⁸ Si) and the estimated uncertainty U with a coverage factor k equal to 2 (such a coverage factor corresponds to a confidence level of 95%). On the “mean” and “standard deviation” lines are shown respectively the mean and the standard deviation obtained for each of the campaigns (therefore encompassing the 6 measurements) and, for the overall column, obtained for all of these measurements. [Table 3] Session 1 Session 2 Session 3 Overall Replica %mol ²⁸ Si U(k=2) %mol ²⁸ Si U(k=2) %mol ²⁸ Si U(k=2) 1 99.836 0.002 99.843 0.003 99.841 0.003 2 99,839 0.003 99.844 0.003 99.842 0.003 3 99,840 0.002 99.843 0.004 99.842 0.003 4 99.841 0.003 99.844 0.003 99.843 0.003 5 99.842 0.002 99.844 0.003 99.842 0.003 6 99.842 0.003 99.845 0.003 99.841 0.003 mean 99,840 99.844 99.842 99.842 standard deviation 0.0024 0.0008 0.0007 0.002

It can be seen that the values measured during these different campaigns and these different measurements do not differ significantly and that the standard deviation is less than 0.004%. The method according to the invention therefore makes it possible to obtain a more sensitive measurement than those of the prior art and is perfectly suited for the measurement of samples with a high enrichment in the 28 ²⁸ Si isotope of silicon.

It will be noted that the present description, if it focuses on the process, of course also covers the use of a reactive gas comprising nitrous oxide in the context of measurements by mass spectrometry of samples comprising silicon.

Bibliographic references

1. [1] P. Becker (2003) "Metrologia" volume 40 number 6 pages 366 to -375
2. [2] A. Pramann et al. (2011) "International Journal of Mass Spectrometry" volume 299 number 2-3 pages 78-86, 2011
3. [3] HT Liu et al. (2003) “Spectrochimica Acta Part B: Atomic Spectroscopy” volume 58, number 1, pages 153-157 ,
4. [4] F. Aureli et al. (2012) “Journal of Analytical Spectroscopy” volume 27, pages 1540-1548
5. [5] A.Virgilio et al. (2016) “Spectrochimica Acta Part B: Atomic Spectroscopy” Volume 116 pages 31-36

Claims (9)

Method for measuring by mass spectrometry a sample comprising silicon, said method being implemented using a mass spectrometer (1) with a tandem-coupled plasma source, or ICP-MS/MS,
said measurement method comprising a step of measurement by mass spectrometry using a reactive gas, the method being characterized in that the reactive gas comprises nitrous oxide. Method of measurement by mass spectrometry according to claim 1, in which the reactive gas consists of nitrous oxide. A method of measuring by mass spectrometry according to claim 1 or 2, in which the step of measuring makes it possible to determine a value relating to a quantity of an isotope 28 of silicon. Mass spectrometric measuring method according to claim 3, in which the value relating to the quantity of the silicon-28 isotope is a molar proportion of the silicon-28 isotope with respect to the total of the atoms of silicon. Method of measurement by mass spectrometry according to any one of Claims 1 to 4, in which the sample has a molar proportion of the isotope 28 of silicon with respect to all the silicon atoms greater than 99 %. A method of measuring by mass spectrometry according to any one of Claims 1 to 5, in which during the step of measuring by mass spectrometry, the reactive gas is introduced into a reaction cell of the mass spectrometer at a rate of between 0.03 and 0.28 mL.min ^-1 and preferably between 0.06 and 0.15 mL.min ^-1 . Use of a reactive gas comprising nitrous oxide for measurement by mass spectrometry of a sample comprising silicon from a tandem-coupled plasma source mass spectrometer, or ICP-MS/MS. Use of a reactive gas according to claim 7, wherein the reactive gas consists of nitrous oxide. Use of a reactive gas according to claim 7 or 8, in which the sample has a molar proportion of the isotope 28 of silicon with respect to all the silicon atoms greater than 99%.

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