MASS SPECTROMETER AND METHOD FOR ANALYSING A GAS BY MASS SPECTROMETRY

Abstract

The invention relates to a mass spectrometer for analysing a gas by mass spectrometry, comprising: a controllable inlet system for pulsed feeding of the gas to be analysed from a process region outside the mass spectrometer into an ionisation region, an ionisation device for ionising the gas to be analysed in the ionisation region, an ion transfer device for transferring the ionised gas from a ionisation region via an ion transfer region into an analysis region, and an analyser for detecting the ionised gas in the analysis region. The invention further relates to an associated method for mass spectrometrically analysing a gas.

Claims

1. A mass spectrometer for analysing a gas by mass spectrometry, comprising: a controllable inlet system for pulsed feeding of the gas that is to be analysed from a process area outside the mass spectrometer into an ionisation area, an ionisation device for ionising the gas that is to be analysed in the ionisation area, an ion transfer device for transferring the ionised gas from the ionisation area via an ion transfer area into an analysis area, and an analyser for detecting the ionised gas in the analysis area.

2. The mass spectrometer according to claim 1, in which the inlet system has a tubular, preferably temperature-controllable, replaceable and/or coated component for feeding the gas that is to be analysed into the ionisation area.

3. The mass spectrometer according to claim 1, in which the inlet system has a filter device, preferably a tubular component in the form of a corrugated hose, in particular made of stainless steel, for filtering at least one corrosive gas component that is contained in the gas that is to be analysed, in particular for filtering a corrosive gas.

4. The mass spectrometer according to claim 1, in which the inlet system has a controllable component, in particular a controllable valve, which preferably can be switched between a first switching state for pulsed feeding of the gas that is to be analysed into the ionisation area and a second switching state for pulsed feeding of a carrier gas into the ionisation area.

5. The mass spectrometer according to claim 1, in which the ionisation device has an electron source, in particular one that can be operated in a pulsed manner, for ionising the gas that is to be analysed, in the ionisation area.

6. The mass spectrometer according to claim 5, in which the electron source is surrounded by a heat sink that has an opening for the emergence of an electron beam.

7. The mass spectrometer according to claim 5, in which the mass spectrometer is designed to maintain a temperature of less than 100° C. in a temperature-controllable ionisation space of the ionisation device when the electron source is operated.

8. The mass spectrometer according to claim 5, in which the electron source has an exchange device for, in particular, the automated exchange of a filament of the electron source, or in which the electron source is detachably fitted to the mass spectrometer.

9. The mass spectrometer according to claim 1, in which the ionisation device has a plasma ionisation device for producing ions and/or metastable particles of an ionisation gas.

10. The mass spectrometer according to claim 1, in which the ionisation device has a gas feed for the pulsed or continuous addition of CI gas into the ionisation area of the ionisation device.

11. The mass spectrometer according to claim 1, in which the ion transfer device has an ion transfer chamber in which the ion transfer area is formed, wherein the ion transfer chamber is connected via a diaphragm aperture to the ionisation area and preferably via a further diaphragm aperture to the analysis area.

12. The mass spectrometer according to claim 1, further comprising: a pump device for creating a pressure (pA) in the analysis area and for creating a pressure (pI) in the ionisation area, wherein the pump device is preferably designed to set the pressure (pI) in the ionisation area independently of the pressure (pA) in the analysis area.

13. The mass spectrometer according to claim 1, in which a pressure (pI) in the ionisation area is greater, preferably by a factor of between 103 and 106, than a pressure (pA) in the analysis area.

14. The mass spectrometer according to claim 1, in which in the ion transfer area of the ion transfer device, a pressure (pT) prevails which lies between the pressure (pI) in the ionisation area and the pressure (pA) in the analysis area, wherein the pump device is preferably designed to set the pressure (pI) in the ion transfer area independently of the pressure (pI) in the ionisation area and of the pressure (pA) in the analysis area.

15. The mass spectrometer according to claim 1, further comprising: a controllable extraction device for the pulsed extraction of the ionised gas out of the ionisation area into the ion transfer area.

16. The mass spectrometer according to claim 15, in which the extraction device has an electrode arrangement for accelerating and preferably for focusing the ionised gas in the direction towards the ion transfer area, in particular in the direction towards the diaphragm aperture.

17. The mass spectrometer according to claim 15, further comprising: a controller for the synchronised actuation of the controllable inlet system and of the extraction device, such that the extraction device does not extract any ionised gas from the ionisation area when the inlet system is closed.

18. The mass spectrometer according to claim 17, in which for the mass spectrometer analysis of the gas, the analyser is designed to compare a mass spectrum that is recorded in at least one measurement time interval (M1) with an open inlet system to a mass spectrum (MS2) that is recorded in at least one measurement time interval (M2) with a closed inlet system.

19. The mass spectrometer according to claim 17, in which the analyser is designed for the continuous analysis of the ionised gas and in which the controller actuates the extraction device throughout the entire duration (ΔtM1) of a respective measurement time interval (M1) 5 with an open inlet system, for the extraction of the ionised gas from the ionisation area.

20. The mass spectrometer according to claim 19, in which the analyser is designed to be switchable between a signal channel (K1) and a background channel (K2) and, for the analysis of the gas, to form a resulting mass spectrum (MS1) from a number of measurement time intervals (M1) of the signal channel (K1) and to form a resulting mass spectrum (MS2) from a number of measurement time intervals (M2) of the background channel (K2), and to compare the two resulting mass spectra (MS1, MS2) of the signal channel (K1) and the background channel (K2) with one another for mass spectrometry analysis.

21. The mass spectrometer according to claim 17, in which the analyser is designed for the pulsed analysis of the ionised gas and in which the controller is designed to actuate the extraction device in a plurality of sub-intervals (T1) during a measurement time interval (M1) with an open inlet system, for the extraction of the ionised gas from the ionisation area.

22. The mass spectrometer according to claim 21, in which the analyser is designed to form a resulting mass spectrum (MS1) from a plurality of sub-intervals (T1, T2) within a measurement time interval (M1) with an open inlet system, and a resulting mass spectrum (MS2) from a plurality of sub-intervals (T) of a measurement time interval (M2) with a closed inlet system preceding or following the measurement time interval (M1), and to compare the two resulting mass spectra (MS1, MS2) with one another for mass spectrometer analysis.

23. The mass spectrometer according to claim 1, in which the analyser is selected from the group comprising: quadrupole analyser, triple quadrupole analyser, Time-of-Flight (TOF) analyser, in particular orthogonal acceleration TOF analyser, scanning quadrupole ion trap analyser and Fourier transform ion trap analyser.

24. A method for the mass spectrometry analysis of a gas that is to be analysed by means of a mass spectrometer, in particular by means of a mass spectrometer according to claim 1, comprising: pulsed feeding of the gas that is to be analysed out of a process area outside the mass spectrometer into an ionisation area via an inlet system, ionising of the gas that is to be analysed in the ionisation area, preferably pulsed extraction of the ionised gas from the ionisation area into an ion transfer area by means of an extraction device, transfer of the ionised gas out of the ion transfer area into an analysis area, and detection of the ionised gas in the analysis area for its analysis by mass spectrometry.

25. The method according to claim 24, further comprising: actuation of the controllable inlet system and of the extraction device such that the extraction device does not extract any ionised gas from the ionisation area when the inlet system is closed.

26. The method according to claim 24, in which for the mass spectrometry analysis of the gas that is to be analysed, at least one mass spectrum (MS1) that is recorded in at least one measurement time interval (M1) with an open inlet system is compared with at least one mass spectrum (MS2) that is recorded in at least one measurement time interval (M2) with a closed inlet system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0082] Design examples are shown in the schematic drawing, and are explained in the following description.

[0083] FIG. 1 shows a schematic representation of a mass spectrometer which comprises a controllable inlet system with a switchable valve, an ionisation device with an electron source and with an ionisation space that can be temperature-controlled; a controllable extraction device and an analyser,

[0084] FIG. 2 shows a schematic representation analogous to FIG. 1 with an ionisation device which has a plasma ionisation device,

[0085] FIG. 3 shows schematic representations of the time progression of the actuation of the pulsed inlet, of the extraction device, and of a continuously operated analyser, and

[0086] FIG. 4 shows schematic representations analogous to FIG. 3, in the case of an analyser operated in a pulsed manner.

[0087] In the following description of the drawings, identical reference numbers are used for components that are the same or have the same function.

DETAILED DESCRIPTION

[0088] Shown schematically in FIG. 1 is a mass spectrometer 1 for the mass spectrometry analysis of a gas 2. The gas 2 has a corrosive gas component 3a in the form of a reactive corrosive gas and an etching product 3b that is formed when a substrate is etched. The gas 2 is located in a process area 4 outside of the mass spectrometer 1 which forms the interior of a process chamber 5, of which only a part is shown in FIG. 1. The mass spectrometer 1 is connected to the process chamber 5 via an inlet system 6. The connection can be formed for example via a flange.

[0089] Instead of a gas 2 which is produced in an etching process, by means of the mass spectrometer 1 one can also analyse a gas 2 which is formed in a coating process, in the cleaning of the process chamber 5, etc.

[0090] The inlet system 6 is controllable, i.e. the inlet system 6 has a rapid-switching valve 7, via which the inlet system 6 can be opened or closed. The valve 7 can be actuated with the help of a controller 8. The controller 8 can for example be a data processing system (hardware, software etc.) that is suitably programmed to enable control of the inlet system 6 as well as of other functions of the mass spectrometer 1 (see below).

[0091] For the analysis of the gas 2 by mass spectrometry, as a rule it is not necessary to detect the corrosive gas component 3a, i.e. the etching gas, since this is supplied to the etching process at a known concentration. The corrosive gas component 3a can furthermore damage components of the mass spectrometer 1 through its corrosive action. In order to prevent this, the controllable inlet system 6 has a filter device for filtering the corrosive gas component 3a, which in the case of the example shown, is designed in the form of a tubular component, e.g. a stainless-steel corrugated hose 9. The corrugated hose 9 has a comparatively large surface in relation to the volume, and therefore enables the gas 2 to react with the material of the corrugated hose 9 along a large surface.

[0092] The corrugated hose 9 is connected to the mass spectrometer 1 in a detachable manner, e.g. by means of a screw connection, so that it can be replaced easily and economically. Through the reaction of the corrosive gas component 3a with the corrugated hose material 9, subsequent components of the mass spectrometer 1 that cannot be so easily exchanged as the corrugated hose 9 are protected from the action of the corrosive gas component 3a.

[0093] The material of the corrugated hose 9 thus serves as sacrificial material.

[0094] The corrosive gas component 3a can for example be the following gases: Main group VII: halogens (e.g. F.sub.2, Cl.sub.2, Br.sub.2), interhalogens (e.g. FCl, ClF.sub.3), haloalkanes (e.g. CF.sub.4), hydrogen halides (e.g. HF, HCl, HBr).

Main group VI: halogen oxoacids (e.g. HOCl, HClO.sub.x), chalcohalides (e.g. SF.sub.6).
Main group V: oxyhalides (e.g. POCl.sub.3), hydrides (PH.sub.3, AsH.sub.3), halides (e.g. NF.sub.3, PCl.sub.3).
Main group IV: hydrides (e.g. silanes, Si.sub.nH.sub.m), halides (e.g. SiF.sub.4,
Main group III: hydrides (e.g. boranes B.sub.nH.sub.m), halides (e.g. BCl.sub.3).

[0095] The tubular component in the form of the corrugated hose 9 can be designed, besides for the filter action for the corrosive gas component 3a, also to reduce the decomposition or condensation of the gas component 3b that is actually of interest, here in the form of the etching product. To this end, the corrugated hose 9 has a coating 9a on its inner surface. The material of the coating 9a depends on the gas component 3b that is to be analysed. For different types of etching products or for different types of etching processes, different types of materials can be used for the coating 9a. Different types of corrugated hoses 9 with a respectively adapted coating 9a can be kept in reserve, wherein depending on the respective gas component 3b that is to be analysed, a respectively adapted corrugated hose 9 is introduced into the mass spectrometer 1. Assigned to the corrugated hose 9 is a temperature-control device 10 in the form of a heating element which heats the corrugated hose 9 up to a temperature that is suitable for the passage of the gas 2 or of the gas component 3b that is to be analysed.

[0096] The temperature-control device 10 is connected to the controller 8, in order to select the temperature of the corrugated hose 9 to match the type of gas component 3b that is to be analysed.

[0097] In the case of the example shown in FIG. 1, the switchable valve 7 is designed as a 3-way valve, i.e. via an additional inlet, a carrier gas 3c can be supplied to it. The controller 8 is configured to switch the 3-way valve 7 between a first switching state and a second switching state. In the first switching state, the gas 2 that is to be analysed is fed to the ionisation area 11, whereas in the second switching state the carrier gas 3c is fed to the ionisation area 11. To this end, the carrier gas 3c is fed to the switchable valve 7 via an additional supply conduit, and in fact in the pulse pauses in which no gas 2 that is to be analysed is fed to the ionisation area 11. The ionisation area 11 is thus supplied with the gas 2 that is to be analysed and the carrier gas 3c in alternation. In this way, the operating point of the ionisation device that is used (see below) can be kept constant. Moreover, the carrier gas 3c can create a positive, flushing action in the ionisation area 11. As a carrier gas 3c, one can for example use an inert gas.

[0098] Via the controllable inlet system 6 with the tubular component 9 in the form of the corrugated hose, the gas 2, ideally only the gas component 3b that is to be analysed, enters into an ionisation area 11, which forms the interior of an ionising chamber 12 of the mass spectrometer 1. The corrugated hose 9 ends in a schematically indicated, temperature-controllable ionisation space 13 (container) which is open at two sides and which is part of an ionisation device 14 that serves to ionise the gas 2 in the ionisation area 11. In the example shown in FIG. 1, the ionisation device 14 has an electron source 14 with a filament (glow wire) 15.

[0099] The ionisation device 14 is in signalling connection with the controller 8, in order to actuate a deflector device, not shown in the illustration, for example an electrode arrangement to create an electrical field, in order to intermittently deflect an electron beam 14a emerging from the filament 15, so that it cannot pass through an opening 17 in a shield 16 surrounding the filament 15 and into the container 13, in order to ionise the gas 2. The electron source 14 can thus be operated in a pulsed manner, i.e. an electron beam 14a is beamed into the ionisation area 11 only if this is expedient for the mass spectrometry analysis of the gas 2, as described in more detail below.

[0100] In the example shown, the shield 16 of the filament 15 is designed as a heat sink, i.e. it is made of a material with a high coefficient of thermal conduction, for example copper, brass, aluminium or stainless steel (in each case with a coating, if applicable), in order to draw heat away from the vicinity of the filament 15. The heat sink 16 or shield also enable the vicinity of the filament 15 to be separated from the ionisation area 11, i.e. it is connected to the ionisation area 11 only via the opening 17. Through the heat sink 16, it is made possible for the temperature-controllable ionisation space 13 or ionisation container to be kept at a desired temperature T or desired temperature interval even with the electron source 14 switched on. The temperature T or temperature range in the temperature-controllable ionisation space 13 can for example be less than approx. 100° C., but higher temperatures are also possible. For the temperature control, the ionisation device 14 typically has a heating and/or cooling device, not shown.

[0101] The heat sink or shield 16 is advantageous if the filament 15 is to be exchanged with the aid of an exchange device 18 indicated by a double arrow, ideally in an automated manner.

[0102] For the exchange of the filament 15, the exchange device 18 can have a transport device for transporting the filament 15 to a replacement position, at which the filament 15 can be exchanged in an automated manner or manually. In order not to break the vacuum or low pressure in the mass spectrometer 1 while the filament 15 is being exchanged, the exchange device 18 can have a lock. If necessary, a diaphragm can serve as a lock, which seals the opening 17 in the heat sink 16 so that the interior of the heat sink 16 is no longer connected to the ionisation area 11. Alternatively, if necessary the entire electron source 14 including the heat sink 16 can be exchanged, if this is a detachable component of the mass spectrometer 1.

[0103] As can likewise be seen in FIG. 1, the mass spectrometer 1 also has a controllable extraction device 19 for the pulsed extraction of the ionised gas 2a from the ionisation area 11 into an ion transfer area 20 of an ion transfer device 21, this area being formed in an ion transfer chamber 22. In the example shown, the extraction device 19 has an electrode arrangement with three electrodes 23a-c for the pulsed acceleration and, if applicable, focusing of the ionised gas 2a in the direction towards the ion transfer area 20. The extraction device 19 or the three electrodes 23a-c are in signalling connection with the controller 8, in order to apply a desired potential to the electrodes 23a-c and in this way to produce a desired acceleration voltage between adjacent electrodes 23a-c, in order to extract (in a pulsed manner) the ionised gas 2a with a suitable acceleration (dependent on mass-to-charge ratio) from the ionisation area 11 into the ion transfer area 20.

[0104] For the passage of the ionised gas 2a, the electrodes 23a-c each have a central diaphragm aperture. The diameter of the respective openings in the electrodes 23a-c decreases in the direction towards the ion transfer area 20, in order to concentrate the ionised gas 2a on a diaphragm aperture 24 in a chamber wall, which is formed between the ion transfer chamber 22 and the ionisation chamber 12.

[0105] The ion transfer device 21 has an ion lens, not shown, in order to transfer the ionised gas 2a, as far as possible without contact, into an analysis area 25 of an analyser 26. The ion transfer area 20 is connected to the analysis area 25, or more precisely to a wall of an analysis chamber 27, via a further diaphragm aperture 28. The diaphragm aperture 24, the further diaphragm aperture 28 and the openings in the electrodes 23a-c, or more precisely their respective mid-points, lie on a line of sight 29 (i.e. in a straight line).

[0106] The mass spectrometer 1 comprises a pump device in the form of at least two, or three vacuum pumps 30a,b,c as shown in the example, which can be actuated independently of one another, by means of the controller 8. Alternatively, the pump device can be designed as a multi-stage split-flow pump. In this way, a pressure p.sub.I in the ionisation area 11 can be set independently of a pressure p.sub.A in the analysis area 25. This is advantageous, since particularly the pressure p.sub.I in the ionisation area 11 is to be set depending on the gas 2 that is to be analysed, and thus on the process that is to be monitored by means of the mass spectrometer 1. In the example shown, the ion transfer device 21 is likewise pumped differentially by a vacuum pump 30c. In this way, a static pressure p.sub.T forms in the ion transfer area 20, which lies between the pressure p.sub.A in the analysis area 25 and the pressure p.sub.I in the ionisation area 11. Thus the neutrals in the ion transfer area 20 are pumped out as efficiently as possible and the ionised gas 2a is transferred into the analysis area 25 with as little loss as possible.

[0107] It has proved to be advantageous if the pressure p.sub.A in the analysis area 25 and the pressure p.sub.I in the ionisation area 11 have a large difference in pressure. The pressure p.sub.I in the ionisation area 11 can for example—depending on the ionisation method chosen—be greater than the pressure p.sub.A in the analysis area 25 by a factor lying between 10.sup.3 and 10.sup.6.

[0108] The pressure p.sub.I in the ionisation area 11 can be smaller, by a factor of 10.sup.2, possibly 10.sup.3, than a pressure p.sub.U in the process area 4 in the process chamber 5. For example, the pressure p.sub.U in the process area 4 can be approx. 1000 mbar, the pressure p.sub.I in the ionisation area 11 can be approx. 1 mbar, the pressure p.sub.T in the ion transfer area 20 can be approx. 10.sup.−3 mbar and the pressure p.sub.A in the analysis area 25 can be approx. 10.sup.−6 mbar or below. In order to be able to maintain these pressure differences and in order to make the diaphragm apertures between the pressure stages, for example the diaphragm aperture 24 between the ionisation area 11 and the ion transfer chamber 22, or the additional diaphragm aperture 24 between the ion transfer chamber 22 and the analysis area 25, as large as possible and thus to be able to make it as transparent as possible for ions, the ion transfer device 21, or more precisely the ion transfer chamber 22, is in most cases pumped. In particular, field generators, e.g. in the form of multipoles, can be arranged in the ion transfer chamber 22, in order to transport the ions into the analysis area 25 with as little loss as possible, and to pump out neutral particles as efficiently as possible, so that they do not get into the analysis area 25.

[0109] The mass spectrometer 1 shown in FIG. 1 has an additional gas feed 31 for the continuous or pulsed supply of CI gas 32 from a gas reservoir 33 into the ionisation area 11, or more precisely into the temperature-controllable ionisation space 13. The gas reservoir 33 stands as an example for a device for providing the CI gas 32, wherein supply e.g. via a conduit is likewise possible. The additional gas feed 31 has a further valve 34 for controlling the inflow of the CI gas 32, and is controlled via the controller 8. The CI gas 32 serves for the targeted chemical ionisation in the temperature-controllable ionisation space 13. In this case, ionisation does not take place directly via the electron beam 14a; rather, first the CI gas 32 is ionised by the electron beam 14a and then—possibly with a time offset—the charges are transferred from the CI gas 32 to the gas component 3b that is to be analysed.

[0110] FIG. 2 shows a mass spectrometer 1 which is designed like the mass spectrometer 1 shown in FIG. 1 and differs only in the type of ionisation device 14: the ionisation device 14 has a plasma ionisation device 35 for producing ions 36a and/or metastable particles 36b of an ionisation gas 37. The ionisation gas 37, which can for example be a noble gas, e.g. helium, is stored in a gas reservoir 38 and can be supplied via a controllable gas inlet 39 to the plasma ionisation device 35. The ions 36a and the metastable particles 36b of the ionisation gas 37 emerge from the plasma ionisation device 35 and enter the ionisation area 11, in order to ionise the gas 2 that is to be analysed through impact ionisation and/or through charge exchange ionisation. The ionisation device 14 shown in FIG. 2 can also be operated in a pulsed manner, in that the controllable gas inlet 39 is opened or closed with the aid of the controller 8, with the controllable gas inlet 39 typically only being opened when the inlet system 6 for feeding the gas 2 that is to be ionised 2 into the ionisation area 11 is also opened.

[0111] In the case of the ionisation device 14 shown in FIG. 2, the ions 36a and the metastable particles 36b of the ionisation gas 37 are transferred into a reaction space 40 in which a pressure prevails that lies between the pressure in the plasma ionisation device 31 and the pressure in the ionisation space 13. A reactant gas, e.g. hydrogen, is supplied to the reaction space 40. The gas flow into the reaction space 40 and the respective diameters of the entry and exit diaphragms establish the pressure in the reaction space 40.

[0112] The reactant gas is transformed by the ionisation gas 37 through impact ionisation and/or charge exchange into reactant ions, e.g. into H.sub.3.sup.+. Via the exit diaphragm of the reaction space 40, these enter the ionisation space 13 within the ionisation area 11, where through chemical ionisation they produce the analyte ions, e.g. [M+H].sup.+, of the analyte M.

[0113] The analyser 26 that serves for detecting the ionised gas 2a or the components of the ionised gas 2a can be designed in various ways: for example, this can be a quadrupole analyser, a triple quadrupole analyser, a Time-of-Flight (TOF) analyser, e.g. an orthogonal acceleration TOF analyser, a scanning quadrupole ion trap analyser, a Fourier transform ion trap analyser, for example an FT-IT) (ion trap) analyser or another type of conventional analyser 26.

[0114] FIG. 3 shows an example of the time progression for the actuation of the pulsed inlet 6 (“A” in FIG. 3, top), the extraction device 19 (“B” in FIG. 3, in the middle), and the continuously operated analyser 26 (“C” in FIG. 3, bottom). Here, the analyser 26 is synchronised with the pulsed inlet system 6 and the extraction device 19 by means of the controller 8. As can be seen in FIG. 3, at the top, the controllable inlet system 6 is periodically switched between am opened switching state (upper signal level in FIG. 3, top) during the first measurement time interval M1 (duration Δt.sub.M1) and a closed switching state (lower signal level in FIG. 3, top) during the second measurement time interval M2 (duration Δt.sub.M2). The duration of the first and second measurement time intervals M1, M2 can be chosen to be the same or different, with the duration Δt.sub.M1, Δt.sub.M2 of the measurement time intervals M1, M2 typically being in the order of microseconds to seconds.

[0115] The extraction device 19 is actuated synchronised with the controllable inlet system 6, i.e. it is likewise actuated during the duration Δt.sub.M1 of a respective first measurement time interval M1 (upper signal level in FIG. 3, middle) and switched off during the duration Δt.sub.M2 of a respective second measurement time interval M2 (lower signal level in FIG. 3, middle). The analysis area 25 is thus not supplied with any ionised gas 2a from the ionisation area 11 when the controllable inlet system 6 is closed. By contrast, when the inlet system 6 is opened, throughout the entire duration Δt.sub.M1 of a respective first measurement time interval M1, ionised gas 2a is taken or extracted from the ionisation area 11 and supplied to the analysis area 25.

[0116] As can be seen in FIG. 3, at the bottom, the analyser 26 is periodically switched between a first measurement channel K1 (signal channel) with first measurement time intervals M1 and a second measurement channel K2 (background channel) with second measurement time intervals M2, and in fact simultaneously with the switching over of the controllable inlet 6 and the extraction device 19. In the case of the continuously operated analyser 26, a resulting mass spectrum MS1 is formed from measurement signals, or mass spectra are formed from a given number of first measurement time intervals M1. Accordingly, a resulting mass spectrum MS2 is formed from measurement signals, or mass spectra are formed from a given number of second measurement time intervals M2. The resulting mass spectra MS1, MS2 represent the sum of the measurement signals that are continuously recorded in the respective measurement time intervals M1, M2. In the example shown, the resulting mass spectrum MS1, MS2 is in each case formed from two measurement time intervals M1, M2 of the signal channel K1 and the background channel K2, but it is understood that the number of measurement time intervals M1, M2 that are used for the summation is generally greater, and is set depending on the speed of the process carried out in the process chamber 5. If applicable, the summation can also take place over all measurement time intervals M1, M2 from the start of the measurement.

[0117] To form the resulting mass spectra MS1, MS2, in place of a summation one can also form a—possibly weighted—average from the measured values recorded in the respective measurement time intervals M1, M2.

[0118] For the mass spectrometry analysis of the gas 2, the resulting mass spectrum MS1 recorded in the signal channel K1 is compared with the resulting mass spectrum MS2 recorded in the background channel K2, or to put it more precisely, the resulting mass spectrum MS2 recorded in the background channel K2, which is attributable to background noise, is subtracted from the resulting mass spectrum MS1 recorded in the signal channel K1. The mass spectrum MS1 that is recorded in the signal channel K1 has a signal portion that corresponds to the mass-to-charge ratios von ionised gas components contained in the gas 2 that is to be analysed, as well as a noise portion that is not shown in FIG. 3, for simplification. In the simplest case, in the comparison, the mass spectrum MS2 of the background channel K2 is subtracted from the mass spectrum MS1 of the signal channel K1, in order to improve the mass-to-charge ratio. It is understood that the comparison between the two mass spectra MS1, MS2 need not be limited to a mere subtraction, but if applicable, more complex links between the two mass spectra MS1, MS2 can be carried out in order to improve the signal-to-noise ratio.

[0119] Analogously to FIG. 3, FIG. 4 shows the time progression for the actuation of the pulsed inlet 6 (“A” in FIG. 4, top), the extraction device 19 (“B” in FIG. 4, in the middle), and an analyser 26 that is operated in a pulsed manner (“C” in FIG. 4, bottom). Here, the analyser 26 is likewise synchronised with the pulsed inlet system 6 and the extraction device 19 by means of the controller 8. As can be seen in FIG. 4 at the top, the controllable inlet system 6 is periodically switched between an open switching state (upper signal level in FIG. 4, top) during first measurement time intervals M1 (duration Δt.sub.M1) and a closed switching state (lower signal level in FIG. 4, top) during second measurement time intervals M2 (duration Δt.sub.M2).

[0120] The extraction device 19 is actuated synchronised with the controllable inlet system 6, i.e. it is activated only during the duration Δt.sub.T1 of a respective first sub-interval T1 of a first measurement time interval M1 (upper signal level in FIG. 4, middle), whereas the extraction device 19 is inactive during the duration Δt.sub.T2 of a respective second sub-interval T2 of a respective first measurement time interval M1, so that no ionised gas 2a can enter from the ionisation area 11 into the analysis area 25. The number of first/second sub-intervals T1, T2 of a respective first measurement time interval M1 can vary depending on the speed of the analyser 26, and can e.g. be ten or more. As in FIG. 3, in FIG. 4 too no ionised gas 2a is supplied to the analysis area 25 when the controllable inlet system 6 is closed during a corresponding second measurement time interval M2.

[0121] In the pulsed operation, the analyser 26 in each case records a mass spectrum during the duration Δt.sub.T1 of a respective first sub-interval T1 in which the ionised gas 2 is transferred into the analysis area 25, as well as during the duration Δt.sub.T2 of a second sub-interval T2 that follows on. From the mass spectra of the first measurement time interval M1 recorded in the respective sub-intervals T1, T2, a resulting mass spectrum MS1 is formed through summation or averaging, which is shown in FIG. 4, at the bottom on the left. During the duration Δt.sub.T of corresponding sub-intervals T of a second measurement time interval M2 following the first one, likewise a number of mass spectra or a signal intensity are recorded and totalled or averaged via the number of sub-intervals T of the second measurement time interval M2, in order to form a resulting mass spectrum MS2.

[0122] The first measurement time intervals M1 thus form a signal channel K1 and the second measurement time intervals M2 form a background channel K2 of the analyser 26.

[0123] As was described further above in connection with FIG. 3, the resulting mass spectrum MS1 of the first measurement time interval M1 and the resulting mass spectrum MS2 of the second measurement time interval M2 can be compared with one another, for example by the two resulting mass spectra MS1, MS2 being subtracted from one another. In this way, even in the case of the pulsed operation of the analyser 26 which is shown in FIG. 4, the mass-to-charge ratio can be improved. As a rule, it is only expedient to compare the mass spectra MS1, MS2 of adjacent first and second measurement time intervals M1, M2 with one another. In place of the subsequent second measurement time interval M2 it would therefore also be possible to use the second measurement time interval M2 preceding the respective first measurement time interval M1 for carrying out the comparison.

[0124] Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.

[0125] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.