Methods for the quantitative mass spectrometric analysis of a residual gas

Abstract

The invention relates to a method for the quantitative mass spectrometric analysis of a residual gas (2), preferably a residual gas (2) in an EUV lithography system containing a background gas (2b), in particular hydrogen, comprising: introducing the residual gas (2) from a vacuum environment (3) into a residual gas analyzer (1), ionizing the residual gas (2) in the residual gas analyzer (1) by means of an ionization device (5), and transferring ionized components (2a', 2b') of the residual gas (2) into a mass analyzer (6) for the quantitative mass spectrometric analysis of the residual gas (2). In the method, ionized components (2b') of the background gas (2b) are partially suppressed during transfer into the mass analyzer (6) by means of an ion filter device (10) with a predetermined suppression factor.

Description

Background of the invention

[0001] The invention relates to a method for the quantitative mass spectrometric analysis of a residual gas, preferably a residual gas in an EUV lithography system containing a background gas, in particular hydrogen, the method comprising: introducing the residual gas from a vacuum environment into a residual gas analyzer, ionizing the residual gas in the residual gas analyzer by means of an ionization device, and transferring ionized components of the residual gas into a mass analyzer for the quantitative mass spectrometric analysis of the residual gas.

[0002] The EUV lithography system can be an EUV lithography system (EUV scanner) for exposing a wafer or another optical arrangement that uses EUV radiation, for example an EUV metrology system, e.g. for inspecting masks, wafers or the like used in EUV lithography, an EUV test setup, etc.

[0003] German patent DE 10 2020 209 157 A1 describes a residual gas analyzer designed to detect very small quantities of components of a residual gas at high residual gas pressures, particularly during the operation of an EUV lithography system. For this purpose, the multi-stage residual gas analyzer includes an ion transfer unit comprising an ion filter designed to filter at least one ionized component of the residual gas. Filtering ionized components of the residual gas, such as ions from a background gas present in the residual gas that exhibits a high partial pressure, is intended to increase the dynamic range of the residual gas analyzer, thus improving the accuracy of determining the concentrations of the remaining, unfiltered ionized components of the residual gas.The ion filter device can be configured as a notch filter, an RF-only quadrupole, an RF-only hexapole, or an RF-only octopole.

[0004] In the residual gas analyzer described in DE 10 2020 209 157 A1, the partial pressure determination is based on the measurement of ionized components of the residual gas that are formed in the plasma of the EUV lithography system. Since the total ion distribution at the inlet of the residual gas analyzer on the boundary wall of the plasma chamber does not necessarily correspond to the ion distribution primarily formed in the plasma, because transport processes and secondary chemical reactions can alter this distribution, a conclusion about the actual neutral residual gas composition is only possible with additional effort, e.g., by means of numerical modeling of the chemistry and transport processes in the plasma process.Furthermore, there is the possibility that gas components are not detected as ions due to their physicochemical properties, because they have reacted before reaching the inlet opening in the residual gas analyzer or are not formed at all (selectivity of the plasma process). Object of the invention

[0005] The object of the invention is to provide a method for the quantitative mass spectrometric analysis of a residual gas which has a background gas with a high partial pressure. Subject matter of the invention

[0006] This problem is solved by a method of the type mentioned above, in which ionized components of the background gas are partially suppressed during transfer to the mass analyzer by means of an ion filter device with a predetermined suppression factor.

[0007] The inventors recognized that for a precise determination of the partial pressures of the components of the residual gas, or for determining the gas composition of the residual gas, all intensities of the ion signals detected by the mass analyzer must be considered. The high partial pressure of the background gas, e.g., in the form of molecular hydrogen (H₂), and the resulting high intensity of the corresponding ion signal(s) lead to saturation of the detector in a conventional mass spectrometer with an ionization device, e.g., an electron ionization device. This makes a partial pressure determination of the residual gas impossible because the saturated signal of the hydrogen ions in the mass spectrum is independent of the hydrogen partial pressure.

[0008] To circumvent this problem, the detector sensitivity can be reduced to such an extent that none of the ion signals reach saturation. However, this simultaneously leads to a significant loss of sensitivity. When scanning mass analyzers are used, such as quadrupole mass filters, the detector voltage can be dynamically adjusted during the mass analysis to circumvent this problem (see, for example, US7745781 B2). For this purpose, however, the slew rate of the detector voltage must be many times greater than the scan rate of the mass analyzer to ensure a constant gain factor for a given ion species. Such an approach is not applicable, for example, when using a time-of-flight mass analyzer, as a sufficiently rapid change in the detector voltage on the timescale of the mass analysis is not possible.

[0009] The inventors propose that, when performing partial pressure measurements of trace gases—that is, components of a residual gas with low partial pressures contained within a residual gas that also has a background gas with a high partial pressure—a controlled partial suppression of the dominant ion signal of the background gas is employed. This solves the problems of detector saturation and loss of sensitivity in residual gas analysis. The measurement can be performed at a constant detector voltage, meaning it does not need to be varied as in US7745781 B2. This allows, among other things, the use of a mass analyzer in the form of a time-of-flight analyzer, which is advantageous for performing residual gas analysis.

[0010] The method described here thus enables the quantitative mass spectrometric analysis of the gas composition of a neutral residual gas containing a background gas and trace gases whose concentrations differ by many orders of magnitude. Since the analysis is performed on a neutral residual gas, the inlet system described in DE 10 2020 209 157 A1 for transferring ionized components of the residual gas into the residual gas analyzer is unnecessary.

[0011] This method can be applied particularly to residual gases in a vacuum environment within a process chamber operated in a hydrogen atmosphere, i.e., where hydrogen is the background gas of the residual gas. The method enables the precise determination of partial pressures of trace gases in such a vacuum environment, where the total pressure is typically in the fine vacuum range, i.e., between approximately 0.1 Pa and 10 Pa. The process chamber with the vacuum environment can be, for example, an EUV lithography system, an EUV irradiation system, or any other hydrogen-operated process chamber.

[0012] In one variant of the method, ionized components of the background gas, which is present in the form of hydrogen, are partially suppressed during transfer by an ion filter device in the form of an RF-only quadrupole. The RF-only quadrupole can also serve as an ion transfer device from the ionization unit to the mass analyzer and is operated in such a way that it suppresses ionized components of the residual gas in the form of hydrogen ions. This can be achieved through the inherent high-pass filter property ("low-mass cut-off," LMCO) of an RF-only quadrupole by selecting a suitable amplitude for the applied high-frequency alternating voltage. Utilizing the LMCO property of the RF-only quadrupole for suppression is particularly suitable for the application of hydrogen suppression, since hydrogen ions occupy the lower end of the mass spectrum (H₂). x +, x= 1..3, m / z = 1..3) and their suppression does not affect other relevant signals. For other background or process gases, the use of the LMCO of the RF-only quadrupole is not suitable if their atomic / molecular mass is greater than that of the relevant trace gases, as these would then also be affected by the signal suppression and may no longer be detectable.

[0013] In another variant, ionized components of the background gas are partially suppressed during transfer using an ion filter device in the form of a notch filter. The use of a notch filter is particularly advantageous in the case described above, i.e., when the background gas has an atomic / molecular mass greater than that of relevant trace gases whose partial pressures are to be determined in the quantitative mass spectrometric analysis. In such a case, extending the procedure described in DE 10 2020 209 157 A1 for filtering large signals from plasma-generated ions—also in combination with an electron ionization measurement—the filtered signal can be suppressed not completely, but only partially, whereby the suppression factor and any suppression effect on adjacent ion signals must be precisely known.A suitably operated RF-only quadrupole, an RF-only hexapole or an RF-only octopole can be used as a notch filter, as described in DE 10 2020 209 157 A1, which is incorporated in its entirety by reference into this application.

[0014] In another variant, the predetermined suppression factor is determined by calibrating the residual gas analyzer before the quantitative mass spectrometric analysis of the residual gas is performed. With suitable prior calibration, the suppression factor for the ionized components of the background gas can be precisely determined.

[0015] In another variant, the quantitative mass spectrometric analysis of the residual gas involves summing the intensities of ion signals detected by the mass analyzer, taking into account the predetermined suppression factor of the ionized components of the background gas. As described above, accurate quantitative determination of the partial pressures requires knowledge of the intensities of all ion signals, including those of the ionized components of the background gas. To determine the partial pressure(s) of the ionized components in the residual gas based on their intensities determined by the mass analyzer, the suppression factor is applied.In the simplest case, the ion signal of each partially suppressed ionized component of the residual gas is multiplied by the reciprocal of the suppression factor to determine the partial pressure of the ionized component in the residual gas.

[0016] In a further development of this variant, the partial pressures of ionized components of the residual gas that are not partially suppressed during transfer are determined based on the sum of the intensities of the detected ion signals. In the method described here, the neutral residual gas, which is introduced into the residual gas analyzer, is non-selectively ionized in the ionization unit, for example, by electron bombardment. The concentrations of the ionized components of the residual gas formed in this way are proportional to the partial pressures of the corresponding gas components in the residual gas. The quantitative mass spectrometric analysis of the ion population formed yields the relative ratio of the different ion species, from which the gas composition of the neutral residual gas can be determined, provided the suppression factor is taken into account (so).

[0017] In another variant, a total pressure in the vacuum environment is measured using an absolute pressure reference instrument. Based on the sum of the intensities of all detected ion signals, a total base pressure can be determined, which is then compared to the total pressure of the residual gas, precisely determined using an absolute pressure reference instrument or an absolute pressure sensor.

[0018] In general, the following prerequisites must be met for the quantitative mass spectrometric analysis of the residual gas using the method described here: 1.) All essential sample components must be clearly separated and unambiguously identified by mass spectrometry. Missing components will lead to an overestimation of the proportions of the remaining components. 2.) Signal saturation must not occur (like this). 3.) For all significant signals in the mass spectrum, the relative ionization cross-sections of the respective corresponding neutral substances must be known. This requires a corresponding mass resolution that enables at least the clear separation, unambiguous identification, and quantification of the essential main components of the mass spectrum. 4.) Mass discrimination must not occur during the supply of the neutral gaseous sample or during the transfer of the ionic ensemble into the mass analyzer, or the function of such discrimination depending on the m / z ratio must be known. 5.) No ionic molecular chemistry may be performed downstream of the ionization process. The relative ion population of the ensemble transferred to the mass analyzer must correspond to the product of the relative neutral composition within the ionization volume and the substance-specific ionization cross-sections. 6.) An absolute pressure reference gauge (so) must determine the exact total pressure.

[0019] Preferably, an electron ionization device is used. This type of ionization device is advantageous for fulfilling points 4) and 5): The electron ionization device is non-selective and, at a sufficiently low pressure, requires no downstream ion molecule chemistry, thus ensuring the relative detection of all gas components. For compatibility with an increased process pressure compared to the required high vacuum in the mass analyzer, the ionization device is typically located in a differentially pumped vacuum chamber with an appropriately sized inlet orifice to restrict the gas flow.

[0020] In another variant, a time-of-flight analyzer is used as the mass analyzer. The time-of-flight analyzer improves measurement accuracy: it allows for a short measurement time (< 1 s) to acquire a complete mass spectrum in the relevant mass range, typically with m / z ratios between approximately 1 and 200, thus preventing distortion of the measurement result due to a changing sample composition during the measurement. Furthermore, a time-of-flight analyzer with high mass resolution allows the separation of isobaric ions, e.g., NH3. + (m / z 17.02600) and OH +(m / z 17.00219), thus preventing distortion of the measurement result by superimposed signals, see points 1) and 3) above. Due to insufficient mass resolution, carrying out the procedure with a different type of mass analyzer, such as a quadrupole mass filter, is difficult in most cases.

[0021] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures in the drawing, which show details essential to the invention, and from the claims. The individual features can be implemented individually or in any combination in a variant of the invention. drawing

[0022] Examples of implementation are shown in the schematic drawing and are explained in the following description. It shows Fig. 1 a schematic representation of a residual gas analyzer which has an RF-only quadrupole for partial suppression of hydrogen ions during transfer to a mass analyzer, as well as Fig. 2 a schematic representation of a mass spectrum with a saturated ion signal and with a partially suppressed ion signal of hydrogen ions.

[0023] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0024] Fig. Figure 1 shows a residual gas analyzer 1 used for the quantitative mass spectrometric analysis of a residual gas 2 located in a vacuum environment 3 within a process chamber 4. In this example, the process chamber 4 is a vacuum chamber of an EUV lithography system (not shown). The residual gas 2 is electrically neutral and contains trace gases 2a with low partial pressures and a background gas 2b in the form of molecular hydrogen with a high partial pressure.

[0025] For the quantitative mass spectrometric analysis, the residual gas 2 – more precisely, a sample of the residual gas 2 – is first introduced from the vacuum environment 3 into the residual gas analyzer 1 and enters an ionization unit 5 of the residual gas analyzer 1. The ionization unit 5 is designed as an electron ionization unit and has an electron source for ionizing the residual gas 2. This electron source comprises two filaments (heating wires) to generate an electron beam through the thermionic effect. These electron beams strike the components 2a and 2b of the residual gas 2 to be ionized and ionize them.The ionized residual gas, more precisely the ionized components 2a', 2b' of the residual gas 2, are transferred by the ionization unit 5 into a mass analyzer 6, which in the example shown is configured as a time-of-flight analyzer and has a detector 7 for detecting the ionized components 2a', 2b' of the residual gas 2. To ensure compatibility with an increased process pressure compared to the required high vacuum in the mass analyzer 6, the ionization unit 5 is located in a differentially pumped vacuum chamber 9 with an appropriately sized inlet orifice to restrict the gas flow.

[0026] For the transfer of the ionized components 2a', 2b' of the residual gas 2 into the mass analyzer 6, the residual gas analyzer 1 has an ion optic 8 and an ion filter device 10, which in the example shown is configured as an RF-only quadrupole. The ion filter device 10, in the form of the RF-only quadrupole, is used to partially filter or suppress the ionized components 2b' of the background gas 2b, so that only a portion of the ionized components 2b' of the background gas 2 enters the mass analyzer 6, as shown in Fig. 1 can be seen. The filtering of the ionized components 2b' of the background gas 2b in the RF-only quadrupole 10 occurs in the case of the background gas 2b, which is hydrogen, through its inherent high-pass filter property, whereby ions with small ionic mass such as H + , H2 +etc. are suppressed, the degree of suppression being determined by the amplitude of the high-frequency alternating voltage applied to the RF-only quadrupole 10. If the background gas 2b has a larger atomic or molecular mass than the trace gases 2a to be analyzed, the RF-only quadrupole 10 can also be used as a notch filter by appropriately adjusting the alternating field applied to it to partially suppress the ionized components 2b' of the background gas 2b in this case as well. For details of the use of the RF-only quadrupole 10 as a notch filter, reference is made to US 5,672,870, which is incorporated in its entirety by reference into this application.

[0027] The partial suppression of the ionized components 2b' of the background gas 2b with a previously known suppression factor f – determined by a prior calibration – makes it possible to avoid saturation of the detector 7, which would result in the saturated ion signal of the ionized component 2b', e.g. in the form of H2 + , an intensity IH2+ would exhibit a characteristic that is independent of the partial pressure of hydrogen in the residual gas 2, as is the case with the one in Fig. This is the case as shown in the mass spectrum 2 above.

[0028] To avoid saturation, the following is necessary in the Fig. The mass spectrum shown below – mirrored – has the intensity IH2+ of the ionized component 2b' in the form of H2 +The partial pressure is reduced by the suppression factor f through the ion filter device 10 and lies within the dynamic range of the detector 7. In this way, the partial pressure of the ionized component 2b' in the residual gas 2 can be measured. Accordingly, in the quantitative mass spectrometric analysis of the residual gas 2, a sum of the intensities I of the ion signals detected by the mass analyzer 6 can be calculated. Fig. 2 mass spectrum shown below are formed, taking into account the predetermined suppression factor f of the ionized components 2b' of the background gas 2b by the measured intensity IH2+ The ion signal is multiplied by the reciprocal 1 / f of the suppression factor f. In this way, a total base pressure is determined, which is compared with the total pressure p in the vacuum environment 3, which is precisely determined using an absolute pressure reference instrument 11.

[0029] Since the concentrations of the trace gases 2a contained in the residual gas 2, which are ionized in the ionization device 5, are proportional to their partial pressures in the vacuum environment 3, the relative ratio of the different ion species and, from this, the gas composition of the neutral residual gas 2 can be determined by mass spectrometric analysis of the ion population formed. Using the sum of the intensities I of the detected ion signals, corrected by the suppression factor f, the partial pressures of the ionized components 2a' of the residual gas 2 that are not partially suppressed during transfer can be quantitatively determined.

[0030] The method described above therefore enables the quantitative mass spectrometric analysis of the gas composition of a neutral residual gas 2 containing a background gas 2b and trace gases 2a whose concentrations differ by many orders of magnitude. The method described above solves the dynamics problem arising from the large concentration differences by partially suppressing the excess component or the ionized components 2b' of the background gas 2b. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2020 209 157 A1 [0003, 0004, 0010, 0013] US7745781 B2 [0008, 0009] US 5,672,870

[0026]

Claims

[1] Method for the quantitative mass spectrometric analysis of a residual gas (2), preferably a residual gas (2) in an EUV lithography system containing a background gas (2b), in particular hydrogen, comprising: Introducing the residual gas (2) from a vacuum environment (3) into a residual gas analyzer (1), Ionizing the residual gas (2) in the residual gas analyzer (1) by means of an ionization device (5), and Transferring ionized components (2a', 2b') of the residual gas (2) into a mass analyzer (6) for quantitative mass spectrometric analysis of the residual gas (2) characterized by , that ionized components (2b') of the background gas (2b) are partially suppressed during transfer into the mass analyzer (6) by means of an ion filter device (10) with a predetermined suppression factor (f). [2] Method according to claim 1, wherein ionized components (2b') of the background gas (2b) which is in the form of hydrogen are partially suppressed during transfer by means of an ion filter device in the form of an RF-only quadrupole (10). [3] Method according to claim 1 or 2, wherein ionized components (2b') of the background gas (2b) are partially suppressed during transfer by means of an ion filter device in the form of a notch filter (10). [4] Method according to one of the preceding claims, wherein the predetermined suppression factor (f) is determined by calibrating the residual gas analyzer (1). [5] Method according to one of the preceding claims, in which, in the quantitative mass spectrometric analysis of the residual gas (2), a sum is formed from the intensities (I) of ion signals detected by the mass analyzer (6), wherein the predetermined suppression factor (f) of the ionized components (2b') of the background gas (2) is taken into account when forming the sum. [6] Method according to claim 5, in which the partial pressures of ionized components (2a') of the residual gas (2) which are not partially suppressed during transfer are determined on the basis of the sum of the intensities (I) of the detected ion signals. [7] Method according to one of the preceding claims, in which a total pressure (p) in the vacuum environment (3) is measured using an absolute pressure reference measuring device (11). [8] Method according to one of the preceding claims, in which an electron ionization device (5) is used as the ionization device. [9] Method according to one of the preceding claims, in which a time-of-flight analyzer (6) is used as the mass analyzer.

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