Residual gas analysis methods and residual gas analysis systems

Abstract

A residual gas analysis method in which a high vacuum is applied in a vacuum chamber (40). In a first phase (T1), a first residual gas composition contained in the vacuum chamber (40) is analyzed by mass spectrometry, and a first pressure measurement (M1) of the total pressure (P1) of the first residual gas composition is recorded using a pressure sensor (45). In a second phase (T3), an auxiliary gas is introduced into the vacuum chamber (40), and a second residual gas composition resulting from the introduction of the auxiliary gas is analyzed by mass spectrometry. A second pressure measurement (M2) of the total pressure (P2) of the second residual gas composition is recorded using the pressure sensor (45). The difference between the second pressure measurement (M2) and the first pressure measurement (M1) is compared to a mass spectrometrically determined change in ion current (ΔIauxiliary gas) corresponding to the auxiliary gas in order to obtain a sensitivity factor (S).A mass spectrometrically determined ion current (Ii) of another residual gas component (i) is multiplied by the sensitivity factor (S) to obtain a normalized partial pressure (pi) for the residual gas component (i). The invention also relates to a residual gas analysis system.

Description

The invention relates to a residual gas analysis method and a residual gas analysis system.Residual gas analyses are performed to examine the composition of a residual gas contained in a vacuum chamber. A mass spectrometric measurement can be carried out in order to determine an ion current for different residual gas components, which ion current represents the number of corresponding particles in the residual gas composition. Assuming that the mass spectrometer is suitably calibrated, it is possible to read from the recorded mass spectra in which relationship to one another the various residual gas components in the residual gas are represented. In order to be able to draw conclusions about the partial pressure of a residual gas component from the ion current, it is necessary to establish a relationship between the total pressure in the vacuum chamber and the measured ion current. Without a correct normalization between the total pressure and the ion current, the partial pressures determined are subject to a systematic error. In general, it has not proven quite simple to calibrate and normalize mass spectrometric measurement values correctly, DE 10 2016 209 878 A1, DE 10 2020 209 482 A1, DE 10 2007 057 252 A1.The object of the invention is to provide a residual gas analysis method and a residual gas analysis system, with which the disadvantages mentioned are reduced. The object is achieved with the features of the independent claims. Advantageous embodiments are specified in the dependent claims.In the residual gas analysis method according to the invention, a high vacuum is applied in a vacuum chamber. In a first phase, a first residual gas composition contained in the vacuum chamber is analyzed by mass spectrometry and a first pressure measured value is recorded using a pressure sensor over the total pressure of the first residual gas composition. In a second phase, an auxiliary gas is introduced into the vacuum chamber and a second residual gas composition formed by the introduction of the auxiliary gas is analyzed by mass spectrometry and a second pressure measurement value over the total pressure of the second residual gas composition is recorded with the pressure sensor. The difference between the second pressure measurement value and the first pressure measurement value is related to a mass spectrometrically determined ion current change corresponding to the auxiliary gas in order to obtain a sensitivity factor for the mass spectrometric measurement. A mass spectrometrically determined ion current of another residual gas component is multiplied by the senvity factor in order to obtain a normalized partial pressure for the residual gas component. The partial pressure of the other residual gas component can be determined in a uniform process using the sensitivity factor or in a process separate from the determination of the sensitivity factor. The first phase and second phase details do not include any limitation with regard to the temporal sequence of the phases. The method can be carried out in such a way that the first phase is temporally before the second phase or temporally after the second phase.The invention is based on the finding that a measured value about the total pressure of a residual gas does not always offer a suitable reference point, on the basis of which partial pressures can be correctly normalized. The pressure sensor used to measure the total pressure may be an ionization vacuum meter, for example a glow cathode vacuum meter, in particular a Bayard-Alpert vacuum meter. In such a pressure sensor, in which the number of particles in the residual gas is deduced from electrical variables, not all components of the residual gas contribute to the measured pressure value to the same extent. Ionization vacuum gauges are generally calibrated to the components of the natural atmosphere, so that a correct pressure measurement results if the composition of the residual gas is similar to the composition of the natural atmosphere. If the residual gas contains a high proportion of other substances, such as hydrocarbons, for example, this results in distorted pressure measurement values. Partial pressures normalized to corrupted pressure measurements are also corrupted.The invention proposes that the normalization of the partial pressures of residual gas components be attributed to a difference between two pressure measurement values of the total pressure. The two pressure measurement values are obtained by changing the composition of a residual gas by supplying an auxiliary gas. The total pressure changes due to the auxiliary gas, which results in a change in the pressure measurement value. In addition, the distribution of the ion currents determined with the mass spectrometer changes. The two changes correspond to each other because they are due to the same change in the composition of the residual gas. A sensitivity factor for the mass spectrometric measurement can be determined, with which the measured ion current change is set in a ratio to the corresponding change in the total pressure. This sensitivity factor can be applied to other residual gas components in order to calculate a normalized partial pressure from the ion current of the residual gas component. A side effect of this procedure is that the sum of the partial pressures calculated according to the invention generally does not correspond to the pressure measurement value of the total pressure.The method is preferably carried out in such a way that the first pressure measurement value and the second pressure measurement value are obtained with a pressure sensor calibrated to the auxiliary gas. This means that in the case of a residual gas whose composition corresponds to the composition of the auxiliary gas, either the pressure sensor directly supplies a correct information about the total pressure or a correct information about the total pressure can be obtained by applying a known correction factor to the measured value. If the feeding of the auxiliary gas is the only change in the composition of the residual gas, then a direct relationship results between the change in the auxiliary gas partial pressure and the difference between the two pressure measurement values.The auxiliary gas can be introduced into the vacuum chamber at a constant flow rate. A constant total pressure is then established, which forms as an equilibrium over the flow rate of the auxiliary gas and the suction rate of the vacuum system. A total pressure is referred to as constant in the context of the invention if unavoidable pressure changes are small in relation to the total pressure and to pressure changes between the first phase and the second phase. After the time the introduction of the assist gas is started, it may take time until a new equilibrium is established. The second pressure measurement value over the total pressure of the second residual gas composition is preferably recorded after a constant total pressure of the second residual gas composition has been established.Since relative quantities are related to one another in the method according to the invention, the method can be carried out without the flow rate of the auxiliary gas being calibrated to an absolute value. If, in one possible embodiment of the method, the flow rate is calibrated to an absolute value, this opens up the additional possibility of monitoring the sensitivity of the total pressure measurement.The introduction of assist gas may be ended after the second pressure measurement is taken. Subsequently, it may take time until a new equilibrium has been established after the termination of the auxiliary gas supply, in which the residual gas composition in the vacuum chamber is no longer enriched with the auxiliary gas. The residual gas composition that is established after the termination of the auxiliary gas introduction may correspond to the first residual gas composition that was applied in the vacuum chamber before the auxiliary gas was supplied. By switching on and switching off the auxiliary gas introduction, a repeated change between the first residual gas composition and the second residual gas composition can be carried out. The first pressure measurement value associated with the first residual gas composition and the second pressure measurement value associated with the second residual gas composition can each be recorded after transient states resulting from the switching on or switching off of the auxiliary gas feed have decayed.The method can be carried out in such a way that the sensitivity factor is determined in a measurement carried out in advance and that the sensitivity factor determined in advance is applied to mass spectra which are recorded at a later point in time. Alternatively, the determination of the sensitivity factor and the recording of the mass spectra to which the sensitivity factor is applied can take place within a uniform process. If a multiple change is made between the first residual gas composition and the second residual gas composition, current sensitivity factors can be obtained continuously. This increases the probability that the sensitivity factor corresponds well to the actual conditions within the residual gas analysis system.The auxiliary gas can be supplied from an auxiliary gas source in the form of an auxiliary gas supply. The auxiliary gas supply can be connected to the vacuum chamber via a connecting line. Between the auxiliary gas supply, in which the pressure can be higher than in the vacuum chamber, and the vacuum chamber, a switchable valve can be formed. The switchable valve can comprise a first switching state in which the connection between the auxiliary gas supply and the vacuum chamber is interrupted, so that no auxiliary gas can pass from the auxiliary gas supply into the vacuum chamber. The switchable valve can comprise a second switching state in which the connection between the auxiliary gas supply and the vacuum chamber is open. A throttle can be arranged between the auxiliary gas supply and the vacuum chamber, so that the auxiliary gas flows into the vacuum chamber at a constant flow rate in the second switching state. The throttle may be an adjustable throttle.The auxiliary gas may be a pure gas. The atomic mass unit (amu) of the auxiliary gas can be, for example, between 10 and 70, preferably between 20 and 50. The pressure measurement values recorded with a pressure sensor calibrated for atmospheric gas can then frequently be used directly, without additional calibration being required. In one embodiment, the auxiliary gas is nitrogen, in particular N 2.In general, it is advantageous if the auxiliary gas has a low tendency to fragment. There is then a high probability that the molecular structure will survive the ionization process in the mass spectrometer unchanged, and the need to identify both the auxiliary gas itself and the fragments in the mass spectrometric measurement is eliminated. Against this background, a noble gas, in particular argon, can be used as auxiliary gas. Noble gases sometimes have mass numbers which deviate significantly from the mass numbers of the predominant components of the atmospheric gas. In order to be able to use measured values recorded with a pressure sensor calibrated for atmospheric gas, a correction factor can nevertheless be used, by means of which the partial pressure change of the auxiliary gas is set in a ratio to the change of the total pressure. Isotopes of a noble gas can be taken into account by applying a compensation factor. The same applies to molecular fragments of an auxiliary gas.The vacuum chamber can be evacuated to a total pressure between 10 -5 mbar and 10 -9 mbar, preferably to a total pressure between 10 -6 mbar and 10 -8 mbar. The pressure difference between the total pressure of the first residual gas composition and the total pressure of the second residual gas composition can be, for example, between 10 -8 mbar and 10 -4 mbar, preferably between 10 -8 mbar and 10 -6 mbar.In order to generate and / or maintain the vacuum, a vacuum pump may be connected to the vacuum chamber. If the vacuum pump has a calibrated suction power, then the associated outgassing rate of a component arranged in the vacuum chamber can be deduced from a determined partial pressure.For mass spectrometric examination of the residual gas compositions in the vacuum chamber, a quadrupole mass spectrometer can be used. In one embodiment, the range of mass-to-charge ratios detected by the mass spectrometer extends from 0 to 200.In the method according to the invention, at least the ion current determined for a mass-charge ratio is multiplied by the sensitivity factor in order to obtain a normalized partial pressure for the relevant residual gas component. The method can be carried out in such a way that for a plurality of mass-charge ratios, the determined ion currents are multiplied by the sensitivity factor in order to obtain a normalized partial pressure for a plurality of residual gas components. In one embodiment, the mass spectrometrically detected range of mass-charge ratios is divided into a plurality of bins, for example bins to integer mass numbers, wherein an ion current is determined for each of the bins. By applying the sensitivity factor to the ion current, a normalized partial pressure can be determined for each of the bins.If a partial pressure calculated according to the invention is present for all the residual gas components determined, a calculated total pressure can be obtained by adding the calculated partial pressures. The calculated total pressure can be compared to a pressure reading of the total pressure. Assuming that the pressure measurement value is calibrated to the natural atmosphere, a statement can be derived from the deviation between the calculated total pressure and the measured total pressure whether the composition of the residual gas is similar to the natural atmosphere or deviates significantly therefrom.The method according to the invention can be carried out for the purpose of qualification of a component, in particular for ascertaining the outgassing rate of a component. The component can be a component of a microlithographically projected exposure apparatus. Microlithographically projecting exposure apparatuses are frequently operated under vacuum. This applies in particular when the wavelength of the radiation used is in the extreme ultraviolet spectral range, in particular between 5 nm and 30 nm. For the quality of the imaging, it is important that the composition of the residual gas in the interior of the projection exposure apparatus is within the predefined specifications. This presupposes that the components used there do not separate any excessive outgassing.In order to determine whether the outgassings of a component provided for a microlithographically projected exposure apparatus are within the predefined limits, the component can be introduced into the vacuum chamber according to the invention and exposed to the high vacuum. The composition of the residual gas in the vacuum chamber is substantially determined by the outgassing of the component. From the composition of the residual gas, it is possible to draw a conclusion as to whether the component meets the specification with regard to its outgassing rate.It is also possible that the method according to the invention is carried out for the purpose of process monitoring. The vacuum chamber can be a component of a microlithographically produced projection exposure apparatus operated with EUV radiation. The optical components of the projection exposure apparatus which guide or form the EUV beam path can be arranged within the vacuum chamber. The method according to the invention can be used to determine whether the composition of the residual gas in the vacuum chamber meets the requirements.The invention also relates to a residual gas analysis system comprising a vacuum chamber, a vacuum pump, an auxiliary gas source, a pressure sensor, a mass spectrometer and a control unit. The vacuum pump is designed to apply a high vacuum in the vacuum chamber. The control unit is designed to actuate the components of the residual gas analysis system such that, in a first phase, a first residual gas composition contained in the vacuum chamber is examined with the mass spectrometer and a first pressure measurement value is recorded with the pressure sensor over the total pressure of the first residual gas composition, such that, in a second phase, an auxiliary gas is introduced from the auxiliary gas source into the vacuum chamber and a second residual gas composition formed by the introduction of the auxiliary gas is examined with the mass spectrometer, wherein a second pressure measurement value is recorded with the pressure sensor over the total pressure of the second residual gas composition. The control unit is further designed to put the difference between the second pressure measurement value and the first pressure measurement value in a ratio to a mass spectrometrically determined ion current change corresponding to the auxiliary gas in order to obtain a sensitivity factor, and to multiply a mass spectrometrically determined ion current of another residual gas component by the sensitivity factor in order to obtain a normalized partial pressure for the residual gas component.The disclosure comprises developments of the method with features which are described in connection with the system according to the invention. The disclosure comprises developments of the system having features which are described in connection with the method according to the invention.The invention is described below by way of example with reference to the attached drawings on the basis of advantageous embodiments. The following are shown: FIG. 1 : an embodiment of a microlithographically projected exposure apparatus; FIG. 2 : an embodiment of a residual gas analysis system according to the invention; FIG. 3 : shows a schematic illustration of a quadrupole mass spectrometer; FIG. 4 : shows a schematic illustration of the pressure curve during the execution of the method according to the invention; FIG. 5 shows an alternative embodiment of a residual gas analysis system according to the invention.FIG. 1 schematically illustrates a microlithographically EUV projection exposure apparatus. The projection exposure apparatus comprises an exposure beam source 14, an illumination system 10 and a projection objective 22, which are operated together in a vacuum housing 23.The exposure beam source 14 generates electromagnetic radiation in the EUV range, i.e. in particular with a wavelength between 5 nm and 30 nm. The exposure radiation emanating from the exposure beam source 14 is focused by a collector 15 into an intermediate focal plane 16. Exposure radiation passing out of the intermediate focus plane 16 is guided by the illumination system 10 into an object plane 12, so that an object field in the object plane 12 is illuminated with uniform radiation intensity.The illumination system 10 comprises a deflection mirror 17 with which the exposure radiation is deflected onto a first facet mirror 18. A second facet mirror 19 is arranged downstream of the first facet mirror 18. The facets of the first facet mirror 18 are imaged into the object plane 12 by the second facet mirror 19.A photomask 13 is arranged in the object plane 12, which is imaged into an image plane 21 via a plurality of mirrors 11 of the projection objective 22. A pattern formed on the photomask 13 is transferred to a radiation sensitive layer of a wafer 20 disposed in the image plane 21. The photomask 13 hangs on a first scanning device 24, the wafer 20 rests on a second scanning device 25, so that the wafer 20 can be exposed in a scanning process in which the photomask 13 and the wafer 20 are moved synchronously with one another.The various mirrors of the projection exposure apparatus at which the illumination radiation is reflected are designed as EUV mirrors. The EUV mirrors are provided with highly reflective coatings. These may be multilayer coatings, in particular multilayer coatings with alternating layers of molybdenum and silicon.Components arranged in the vacuum housing 23 of the projection exposure apparatus are subject to stringent requirements for outgassing. Excessive outgassing can lead to the composition of the residual gas in the vacuum housing 23 being changed in an undesired manner, which can in turn have a disadvantageous effect on the operation of the projection exposure apparatus. Prior to the use of a component in the projection exposure apparatus, therefore, an examination can be carried out in order to determine how high the outgassings of the component are.As schematically shown in FIG. 2, the examination can be carried out in a vacuum chamber 40. Connected to the vacuum chamber 40 is a vacuum pump 47, which is designed to apply a high vacuum with a pressure in the order of magnitude of, for example, 10 -7 mbar in the interior of the vacuum chamber 40. A pressure sensor 45 in the form of an ionization vacuum meter is connected to the vacuum chamber 40 via a connection 46. A quadrupole mass spectrometer 43 communicates with the interior of the vacuum chamber 40 via a connecting tube 44. A support surface 41 is formed in the vacuum chamber 40, on which a component 42 to be examined rests.The component 42 may have one or more fabrication steps behind it, which may have taken place, for example, in a process chamber under vacuum. The component 42 may have been transported from the process chamber into the vacuum chamber 40 after the production steps have been completed. It is also possible that one or more manufacturing steps have taken place directly in the vacuum chamber 40. In one embodiment, the vacuum is applied during the manufacturing step and maintained until the inspection.After passing through such manufacturing steps, the component 42 generally has an outgassing rate which is higher than is permissible for use in the projection exposure apparatus. With the method according to the invention, it is possible to investigate how high the outgassing rate of the component 42 is, and a qualification can be carried out as to whether the outgassing rate of the component 42 meets the relevant specifications. When the vacuum pump 43 is in operation, an equilibrium is established which depends, inter alia, on the outgassing rate of the component 42 and the suction rate of the vacuum pump 43. This equilibrium results in a constant total pressure being established in the interior of the vacuum chamber 40 over time.The ionization vacuum meter 45 provides a measurement of the total pressure in the vacuum chamber 40. It cannot be assumed in this type of measurement that the ionization vacuum meter 45 has uniform sensitivity for different types of particles. For example, it has been observed that the sensitivity decreases with increasing mass number of the particles. An erroneous pressure measurement value is provided, which is usually lower than the actual total pressure. A pressure reading matching the actual total pressure could be provided by the ionization vacuum meter 45 if calibrated to a gas composition corresponding to the residual gas composition. In practice, ionization vacuum meters 45 are generally calibrated to the composition of the atmospheric gas, i.e., air. Due to outgassing of the component 42, however, the residual gas composition in the vacuum chamber 40 has a significantly different composition than air and in particular a significantly higher proportion of particles with a large mass number.The connecting pipe 44 leading to the quadrupole mass spectrometer 43 is connected to the vacuum chamber 40. By free gas exchange, which can be optionally supported by the influence of a pressure gradient acting in the direction of the quadrupole mass spectrometer 43, the residual gas propagates in the direction of the quadrupole mass spectrometer 43, so that the composition of the residual gas can be examined with the quadrupole mass spectrometer 43.According to FIG. 3, the quadrupole mass spectrometer 43 has an inlet opening 34, through which the residual gas enters the interior of the quadrupole mass spectrometer 43. In the quadrupole mass spectrometer 43, an ion source 32 and a mass analyzer 27 are arranged one behind the other. The mass analyzer 27 comprises a quadrupole 31 and a detector 37, the usually still neutral constituents of the residual gas on entering the quadrupole mass spectrometer 43 being ionized in the ion source 32. Species ionized by ion source 32 continue their path through quadrupole 31 and enter the detector area of mass analyzer 27 where they are analyzed by their mass to charge ratio.The mass analyzer 27 includes the detector 37 extending in the radial direction with respect to the moving direction of the particles. The ionic species change their direction of movement under the influence of an electric field before they are detected by the detector 37. For this purpose, an electrical voltage is applied, with which the ionic species are accelerated as electrically charged particles in the direction of the detector 37. The voltage may be on the order of a few kilovolts, for example.In the exemplary embodiment, the detector 37 is designed as a secondary electron multiplier which determines the number of charge carriers flowing on the detector 37 when the ionic species impinges on it. By applying a suitable combination of DC voltage and AC voltage to the rods of the quadrupole 31, the quadrupole 31 can be adjusted such that only species with a specific mass-to-charge ratio can pass through the quadrupole 31. The number of charge carriers counted by the detector 37 corresponds to the number of ionic species that have passed through the quadrupole and all have the same mass-to-charge ratio due to the filtering by the quadrupole 31. The quadrupole 31 is matched over time over different mass-to-charge ratios, so that a number of charge carriers can be assigned to each mass-charge ratio on the basis of a temporal assignment between charge carriers counted by the detector 37 and the state of the quadrupole 31.Each ion current measured by mass spectrometry corresponds to a partial pressure in the residual gas composition. In order to convert the ion current into a partial pressure, it is necessary to put the ion current in a relationship with the total pressure in the vacuum chamber 40. If normalization is effected on the basis of a measurement value for the total pressure which is not correctly determined, the partial pressure is also not correctly determined. According to the invention, it is proposed to bring about a change in the total pressure and to normalize the partial pressure on the basis of the pressure change.According to FIG. 2, an auxiliary gas source is connected to the vacuum chamber 40 in the form of a storage container 48 which is filled with a gas of defined composition. In the present exemplary embodiment, the gas is nitrogen (N 2). However, other gases are also possible in the storage container 48, such as inert gases. A switchable valve 49 and an adjustable throttle 50 are connected in series between the storage container 48 and the vacuum chamber 40. The series shape of the valve 49 and the throttle 50 can also be reversed as shown in FIG. 2.In the method according to the invention, the system is initially in a state in which the switchable valve 49 is closed. The vacuum pump 47 is in operation so that an equilibrium state is established in which the total pressure P 1 of a first residual gas composition in the vacuum chamber 40 is constant. At a time T 1, which lies within a first phase of the method according to the invention, a pressure measurement value M 1 is recorded by the ionization vacuum meter 45, see FIG. 4. the pressure measurement value M 1 correlates with the total pressure P 1 in the vacuum chamber 40, but is subject to a systematic error as explained. At a subsequent time T 2, the switchable valve 49 is opened. The adjustable throttle 50 results in a constant leakage rate at which the gas flows from the storage container 48 as auxiliary gas into the vacuum chamber 40. A rapid increase in pressure results, after which it takes a short time until equilibrium has been established with the new total pressure P 2 of a second residual gas composition. At a time T 3 which lies within a second phase of the method according to the invention, a second pressure measurement value M 2 is recorded which correlates with the higher total pressure P 2 but likewise is subject to a systematic error. At a subsequent time T 4, the switchable valve 49 is closed again, so that the flow of the auxiliary gas is interrupted. It takes a certain time until the vacuum pump 47 has again established an equilibrium corresponding to the lower total pressure P1.In parallel, mass spectra are continuously recorded with the mass spectrometer 43 which extend over mass-charge ratios of, for example, 1 to 200. By supplying the auxiliary gas, the mass spectrum changes by detecting particles of the auxiliary gas in a large number. For the mass-charge ratio associated with the auxiliary gas, a difference results between a first mass spectrum recorded at time T 1 without the auxiliary gas and a second mass spectrum recorded at time T 2 with auxiliary gas.Incidentally, since the conditions remain unchanged, the difference ΔI in the assist gas ion current corresponds to the difference Δp assist gas between the pressure measurement values M 2 and M 1. A sensitivity factor S can be determined by setting the difference ΔpHilfsgas between the second pressure measurement value M 2 and the first pressure measurement value M 1 caused by the auxiliary gas in a ratio to the difference ΔiIfsgas in the ion current of the auxiliary gas.With each mass spectrum, an ion current I i is also determined for each of the other components i of the residual gas composition. By multiplying the ion currents I i by the sensitivity factor S, a partial pressure P i can be determined for each of the components i of the residual gas composition.A partial pressure p i determined in this way does not require any further normalization if the ionization vacuum meter 45 is calibrated to the auxiliary gas, which is generally the case when nitrogen is used as the auxiliary gas.If an auxiliary gas is used to which the ionization vacuum meter 45 is not calibrated, a correction factor k may be applied to measure the proper relationship between the measured pressure differential Δp auxiliary gas and the actual pressure differential ΔpHilfsgas,realdarzustellen.The use of such a correction factor k is frequently necessary if, for example, a noble gas is used as auxiliary gas. The sensitivity factor is then applicableIf, without further compensation, the ion current change ΔIHilfsgasherangezogen determined for the mass-charge ratio of the auxiliary gas is used to determine the sensitivity factor S, it is not taken into account that individual particles of the auxiliary gas may have changed state before they are detected by the detector 37 of the mass spectrometer 43. For example, if the gas is nitrogen, a portion of the N 2- molecules may have split into individual N atoms. As a result, they are detected not with the mass-charge ratio of the assist gas but with a different mass-charge ratio. This effect can be corrected with a compensation factor R.Likewise, compensation factors R may be used to properly account for various isotopes of a noble gas. The compensation factors R applicable to the respective application are available because the behavior of the gas types in question is well examined under the influence of the ionization unit of a mass spectrometer.The residual gas analysis system in Figure 2 comprises a control unit 51 communicating with the quadrupole mass spectrometer 43, the ionization vacuum meter 45, the switchable valve 49 and the adjustable throttle 50. The quadrupole mass spectrometer 43 and the switchable valve 49 are controlled in such a way that the pressure measurement values M 1, M 2 required for the method according to the invention and the associated mass spectra can be recorded. The data are combined and evaluated in the control unit 51 in such a way that normalized partial pressures for the different mass-charge ratios of the mass spectra are obtained.FIG. 5 shows an alternative embodiment in which the method according to the invention is applied to a microlithographically projected exposure apparatus for the purpose of process monitoring. An inner housing 28 is formed in the interior of the vacuum housing 23, surrounding the exposure beam path. In FIG. 1, only a section of the inner housing 28 arranged between the deflecting mirror 17 and the first facet mirror 18 is indicated. In fact, the inner housing 28 surrounds further sections of the beam path. The inner housing 28 is designed such that the exposure beam path can take its path via the various optical elements 17, 18, 19, M 1-M 6 without the inner housing 28 being in the path.A plasma is generated by interaction between the EUV radiation and the hydrogen, wherein, inter alia, ionic plasma species (H +) or radical plasma species (H) are formed. The hydrogen plasma has the effect that contaminants, which can arise, for example, as a result of outgassing from components of the projection exposure apparatus, are removed from surfaces of the optical elements arranged in the inner housing 28. This produces compounds, for example, of hydrogen and carbon or of hydrogen and nitrogen, which are distributed in the residual gas atmosphere in the interior of the inner housing 28.The projection exposure apparatus is equipped with a mass spectrometer 43 which is designed to analyze the composition of the residual gas. FIG. 1 shows a single mass spectrometer 43 which is connected to the inner housing 28 in the region between the deflecting mirror 17 and the first facet mirror 18. The projection exposure apparatus can comprise a plurality of mass spectrometers 43 in order to be able to analyze the composition of the residual gas in different regions of the inner housing 28. The projection exposure apparatus with the mass spectrometer 43 forms a residual gas analysis system according to the invention, which is operated under the control of a control, not shown, in such a way that the method according to the invention is carried out.

Claims

Residual gas analysis method, in which a high vacuum is applied in a vacuum chamber (40), in which a first residual gas composition contained in the vacuum chamber (40) is analyzed by mass spectrometry in a first phase (T1), and a first pressure measured value (M1) over the total pressure (P1) of the first residual gas composition is recorded with a pressure sensor (45), in which an auxiliary gas is introduced into the vacuum chamber (40) in a second phase (T3), and a second residual gas composition formed by the introduction of the auxiliary gas is analyzed by mass spectrometry, wherein a second pressure measured value (M2) over the total pressure (P2) of the second residual gas composition is recorded with the pressure sensor (45), wherein the difference between the second pressure measurement value (M2) and the first pressure measurement value (M1) is set in a ratio to a mass-spectrometrically determined ion current change (ΔIifs-gas) corresponding to the auxiliary gas in order to obtain a sensitivity factor (S), and wherein a mass-spectrometrically determined ion current (I i) of another residual gas component (i) is multiplied by the sensitivity factor (S) in order to obtain a normalized partial pressure (p i) for the residual gas component (i).Method according to claim 1, wherein the first pressure measurement value (M1) and the second pressure measurement value (M2) are recorded with an ionization vacuum meter (45).Method according to claim 1 or 2, wherein the first pressure measurement value (M1) and the second pressure measurement value (M2) are obtained with a pressure sensor (45) calibrated to the auxiliary gas.The method according to any one of claims 1 to 3, wherein the auxiliary gas is introduced into the vacuum chamber (40) at a constant flow rate.The method of claim 4, wherein the flow rate is calibrated to an absolute value.Method according to claim 4 or 5, wherein the second pressure measurement value (M2) is recorded after a constant total pressure (P2) has been established in the vacuum chamber (40).Method according to one of Claims 1 to 6, wherein the introduction of auxiliary gas is ended after the second pressure measurement value (M2) is recorded.Method according to one of Claims 1 to 7, wherein a repeated change between the first residual gas composition and the second residual gas composition is carried out by switching on and switching off the auxiliary gas feed.The method of any one of claims 1 to 8, wherein the auxiliary gas is nitrogen, argon or another inert gas.The method of any one of claims 1 to 9, wherein the vacuum is applied with a vacuum pump (47), and wherein the vacuum pump (47) has a calibrated suction power.Method according to one of Claims 1 to 10, wherein the method is carried out in order to determine the outgassing rate of a component (42) arranged in the vacuum chamber (40).The method of claim 11, wherein the component (42) is for use in a microlithographically projected exposure apparatus.Method according to one of Claims 1 to 10, wherein the method for process monitoring is carried out during operation of a microlithographically projected exposure apparatus.Residual gas analysis system comprising a vacuum chamber (40), a vacuum pump (47), an auxiliary gas source (48), a pressure sensor (45), a mass spectrometer (43) and a control unit (51), wherein the vacuum pump (47) is designed to apply a high vacuum in the vacuum chamber (40), and wherein the control unit is designed to control the components of the residual gas analysis system such that, in a first phase (T1), a first residual gas composition contained in the vacuum chamber (40) is examined with the mass spectrometer (43) and a first pressure measurement value (M1) over the total pressure (P1) of the first residual gas composition is recorded with the pressure sensor (45), in that an auxiliary gas is introduced into the vacuum chamber (40) from the auxiliary gas source (48) in a second phase (T3) and a second residual gas composition arising from the introduction of the auxiliary gas is examined using the mass spectrometer (43), wherein a second pressure measured value (M2) over the total pressure (P2) of the second residual gas composition is recorded using the pressure sensor (45), wherein the control unit (51) is further designed to put the difference between the second pressure measured value (M2) and the first pressure measured value (M1) into a ratio to a mass spectrometrically determined ion current change (ΔIHilfsgas) corresponding to the auxiliary gas in order to acquire a sensitivity factor (S), and multiplying a mass spectrometrically determined ion current (I i) of another residual gas component (i) by the sensitivity factor (S) in order to obtain a normalized partial pressure (p i) for the residual gas component (i).

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