Patent Application
20230170197
This application claims priority from French Patent Application No. 2112664 filed on Nov. 29, 2021. The content of this application is incorporated herein by reference in its entirety.
The invention relates to the field of inductively coupled plasma source mass spectrometry measurement, better known by its acronym ICP-MS.
Thus, the invention more specifically relates to a mass spectrometry measurement method and a use of nitrous oxide N2O in the context of such a mass spectrometry measurement.
The inductively coupled plasma source mass spectrometry measurements, hereinafter 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 Si-28 isotope 28Si, the Si-29 isotope 29Si and the Si-30 isotope 30Si for which a large number of interferences are observed, presented in the following table:
28Si+
14N14N+; 12C16O+
29Si+
14N14N1H+; 14N15N+; 28Si1H+; 13C16O+; 12C16O1H+; 12C17O+
30Si+
14N16O+; 14N15N1H+; 13C17O+; 12C17O1H+; 15N15N+;
Due to these interferences, obtaining a measurement of silicon and these isotopes by ICP-MS with a good accuracy is made particularly difficult. However, for certain applications, in particular related to the semiconductor industry, such a measurement is required. It will be noted that these needs are in particular present for the samples with a high enrichment in the silicon-28 isotope 28Si of silicon where, in addition, a high sensitivity is required to be able to detect the minority isotopes of silicon in order to quantify the enrichment.
In order to obtain such an accuracy, the use of multi-collector inductively coupled plasma source mass spectrometry has been proposed [1, 2]. This technique, if it allows solving the problem of the interferents, remains complicated to implement and is expensive.
As a result, there could be an interest in developing a simpler inductively coupled plasma source mass spectrometry technique, such as the quadrupole inductively coupled plasma source mass spectrometry, better known under the acronym ICP-QMS, or inductively coupled plasma-tandem mass spectrometry, better known under the acronym ICP-MS/MS, in order to allow a measurement of silicon and the isotopes thereof which is compatible with the samples with a high enrichment in the silicon-28 isotope 28Si of silicon. Nevertheless, the attempts [2, 3, 4, 5] carried out with these techniques, generally based on the use of oxygen [4, 5] as a reactive gas, do not allow obtaining an adapted sensitivity.
The aim of the invention is thus to provide an inductively coupled plasma source mass spectrometry measurement method which is suitable for the samples with a high enrichment in the silicon-28 isotope 28Si of silicon and which is simpler to implement than the multi-collector inductively coupled plasma source mass spectrometry.
The invention relates for this purpose to a method for measuring a sample comprising silicon by mass spectrometry, said method being implemented from an inductively coupled plasma-tandem mass spectrometer, or ICP-MS/MS, said measurement method comprising a step of measuring by mass spectrometry by means of a reactive gas, the method being characterised in that the reactive gas comprises nitrous oxide.
The inventors have discovered that nitrous oxide N2O as a reactive gas has a reactivity which is significantly higher than dioxygen which is generally used as a reactive gas in the prior art for such measurements. As a result, when using nitrous oxide N2O as a reactive gas in the ICP-MS/MS measurement framework, a sensitivity which is significantly greater than that obtained with dioxygen results therefrom. Thus such a method, as shown by the inventors and as described below, allows obtaining an accuracy in the measurement of the molar proportion of the silicon-28 isotope relative to all silicon atoms which is less than 0.01%.
The reactive gas can consist of nitrous oxide.
The measurement step can allow determining a value relating to the amount of a silicon-28 isotope.
It will be noted that in a usual configuration of the invention, the measurement can also allow obtaining a value relating to the amounts of the silicon-28 and silicon-29 isotopes.
The value relating to the amount of the silicon-28 isotope can be a molar proportion of the silicon-28 isotope relative to all silicon atoms.
Such a value allows estimating the enrichment of the sample in the silicon-28 isotope. 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 silicon-28 isotope relative to all silicon atoms which is greater than 99%.
The method according to the invention is particularly adapted for such samples since it allows, as shown later in this document, obtaining a sensitivity which is compatible with such proportions of the silicon-28 isotope.
During the mass spectrometric measurement step, the reactive gas is introduced into a reaction cell of the mass spectrometer at a flow rate comprised between 0.03 and 0.28 mL.min-1 and preferably comprised 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 maximised.
The invention further relates to a use of a reactive gas comprising nitrous oxide for a mass spectrometric measurement of a sample comprising silicon from an inductively coupled plasma-tandem mass spectrometer, or ICP-MS/MS.
The reactive gas may consist of nitrous oxide.
The sample may have a molar proportion of the silicon-28 isotope relative to all silicon atoms which is greater than 99%.
As demonstrated later in this document, such a use is particularly advantageous since it enables obtaining a particularly significant sensitivity to the silicon isotopes relative to the prior art.
The present invention will be better understood on reading the description of exemplary embodiments, given for purely illustrative and non-limiting purposes, with reference to the appended drawings in which:
Identical, similar or equivalent portions of the different figures bear the same reference numerals so as to facilitate the passage from one figure to the other.
The different portions represented in the figures are not necessarily represented according to a uniform scale, to make the figures more readable.
The different possibilities (variants and embodiments) should be understood as not being mutually exclusive and can be combined with each other.
As described below in connection with
As a reminder, an inductively coupled plasma-tandem mass spectrometer 1 comprises, as illustrated in
It will be noted that the term “the target atom or molecule” means, herein, 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 to say which is the object 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
Similarly, as indicated above, the term “product molecule” means herein and in the rest of this document, the product molecule obtained during the reaction between the ionised target atom or molecule and the reactive gas. Within the scope of the present invention, namely the measurement of the natural silicon isotopes 28Si, 29Si and 30Si by the use of nitrous oxide N2O, the product molecules are respectively 28SiO2+, 29SiO2+ and 30SiO2+. It should be noted that for the use of dioxygen O2, the product molecules are identical.
During the implementation of a mass spectrometric measurement with an ICP-MS/MS mass spectrometer, the choice of a reactive gas must be adapted to the atoms which 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 ICP-QMS mass spectrometry measurement, comprise either ammonia NH3, or methane CH4, or dioxygen O2. More specifically, concerning the ICP-MS/MS mass spectrometry [4, 5], it is dioxygen O2 which is generally used for the reactive gas. As a result, in this document, the inventors have chosen to compare the results obtained within the scope of the invention, that is to say when the reactive gas is based on nitrous oxide N2O, with those obtained for a reactive gas based on dioxygen O2.
The inventors have discovered, based on the reaction enthalpy calculation (known under the reference ΔHr), that the reaction of the silicon cations Si+ with nitrous oxide N2O is significantly exothermic whereas the reaction of silicon cations Si+ with dioxygen O2 is endothermic. As a result, the inventors have estimated that the use of a reactive gas based on nitrous oxide N2O should allow obtaining an optimised reactivity and therefore obtaining an ICP-MS/MS mass spectrometry measurement which is particularly sensitive.
Indeed, here, in parallel with the reaction enthalpy calculated for the latter, are the reactions likely to be obtained for the silicon isotopes in cationic form in the reaction cell 40 with respectively dioxygen and nitrous oxide N2O:
The reactions with nitrous oxide are therefore favourable and must have a particularly significant yield relative to the reactions with dioxygen. As a result, the inventors have considered that nitrous oxide should allow, in the context of measurement of silicon and the different isotopes thereof by ICP-MS/MS mass spectrometry, reaching a significantly improved sensitivity relative to that obtained with dioxygen.
In order to verify this, the inventors carried out sensitivity measurements in ICP-MS/MS mass spectrometry from the same silicon sample as a source of silicon-28 isotope 28Si for respectively reactive gases based on dioxygen (oxygen flow rate DO2 comprised between 0 and 0.78 mL.min-1), corresponding to the measurements of the prior art, and for reactive gases based on nitrous oxide (nitrous oxide flow rate DN2O comprised between 0 and 0.28 mL.min-1).
Thus, within the scope of the invention, the reactive gas flow rate used for the mass spectrometric measurement is advantageously comprised between 0.03 and 0.28 mL.min-1 and preferably between 0.06 and 0.15 mL.min-1.
Based on this study, the inventors were able to optimise the instrumental parameters in order to maximise the sensitivity for the silicon isotopes 28Si, 29Si and 30Si for the reactive gases based on dioxygen O2 and based on nitrous oxide N2O and to estimate the sensitivity and the background equivalent concentration, better known by its acronym BEC. These results are summarised in the following table.
It will be noted that the present document does not describe such a parameter optimisation. This optimisation is indeed part of the usual practice of the person skilled in the art and being dependent on the apparatus used for the mass spectrometry measurement. The description of such an optimisation therefore has no interest in the present document.
It can be seen that nitrous oxide N2O allows achieving a sensitivity which is almost 10 times greater than that of dioxygen O2, with a signal to noise ratio reduced by 50%. With a reactive gas based on nitrous oxide N2O, the inventors have therefore estimated being able to carry out measurements on samples comprising a high proportion in a silicon isotope with an optimised sensitivity.
In order to demonstrate this, the inventors have performed 3 campaigns (indicated as session) of 6 ICP-MS/MS mass spectrometry measurements from a reactive gas based on nitrous oxide N2O from a silicon sample showing an enrichment in the silicon-28 isotope 28Si. The results of these measurement campaigns are summarised in the following table with, for each of the measurements, an estimate of the proportion of the silicon-28 isotope 28Si (%mol 28Si) 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, the mean and the standard deviation obtained for each of the campaigns (therefore encompassing the 6 measurements) and, for the global column, obtained for all of these measurements, are shown respectively.
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 allows obtaining a measurement which is more sensitive than those of the prior art and is perfectly adapted for the measurement of samples with a high enrichment in the silicon-28 isotope 28Si.
It will be noted that the present description, if it is focused on the method, of course also covers the use of a reactive gas comprising nitrous oxide in the context of mass spectrometric measurements of samples comprising silicon.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2112664 | Nov 2021 | FR | national |
