Abstract
A mass spectrometer includes an ion trap, which has an interior for storing ions, a signal generator, which is connected to an electrode of the ion trap, which delimits the interior, for coupling in a voltage signal, in particular a radiofrequency voltage signal, and an ionization device for ionizing a gas to be ionized and supplied to the interior. The ionization device is connected to the signal generator in order to use the voltage signal (U.sub.RF, U.sub.Stim1, U.sub.stim2) of the signal generator, which is coupled into the electrode, for generating ions.
Claims
1. A mass spectrometer, comprising: an ion trap, which has an interior for storing ions, a signal generator, which is connected to an electrode of the ion trap, which delimits the interior, for coupling in a voltage signal, in particular a radiofrequency voltage signal, an ionization device, in particular a plasma ionization device, for ionizing a gas to be ionized and supplied to the interior, characterized in that the ionization device is connected to the signal generator in order to use the voltage signal of the signal generator, which is coupled into the electrode, for generating ions.
2. The mass spectrometer according to claim 1, wherein the electrode, which is connected to the signal generator, has a passage opening for the supply of the gas into the interior.
3. The mass spectrometer according to claim 1, further comprising: a gas supply, which is embodied to supply a gas in the form of a gas to be analysed or an ionization gas to the ionization device.
4. The mass spectrometer according to claim 3, wherein the gas supply has at least one valve, which is controllable by means of a control device, for the pulsed supply of the gas to the ionization device.
5. The mass spectrometer according to claim 1, wherein the electrode of the ion trap, which is connected to the signal generator, forms a first of at least two electrodes of the ionization device, between which the ions are generated.
6. The mass spectrometer according to claim 5, wherein the electrode has a tubular electrode portion, in particular a tubular electrode portion that tapers to a tip, on its side facing away from the interior, in particular in the region of the passage opening.
7. The mass spectrometer according to claim 5, wherein the ionization device has an electrically conductive supply line, in particular an electrically conductive tubular supply line, which is intended for supplying the gas to the ion trap and which forms the second electrode of the ionization device.
8. The mass spectrometer according to claim 5, wherein the ionization device has a supply line, in particular a tubular supply line, made of an electrically insulating material for supplying the gas to the ion trap and wherein the second electrode of the ionization device is arranged on the outer side of the supply line.
9. The mass spectrometer according to claim 5, wherein the ionization device has a supply line, in particular a tubular supply line, made of an electrically insulating material and wherein the second electrode of the ionization device is arranged within the supply line.
10. The mass spectrometer according to claim 9, wherein the second electrode, which is disposed in the supply line, has a tip that faces the first electrode of the ionization device.
11. The mass spectrometer according to claim 1, wherein the signal generator is embodied to couple the voltage signal into a ring electrode of the ion trap for storing the ions in the interior.
12. The mass spectrometer according to claim 1, wherein the signal generator is embodied to couple the voltage signal into at least one cap electrode of the ion trap for exciting the ions in the interior.
13. The mass spectrometer according to claim 1, further comprising: a detector for detecting ions removed from the ion trap or an ion signal generated by the ions stored in the ion trap.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the figures:
[0033] FIG. 1 shows a schematic illustration of a mass spectrometer, which has an ion trap and an ionization device for ionizing a gas which is supplied to the ion trap via a passage opening in an electrode,
[0034] FIG. 2a,b show schematic illustrations of a detail of the mass spectrometer of FIG. 1, in which the electrode has a sharply protruding electrode portion at the passage opening,
[0035] FIG. 3a,b show schematic illustrations analogous to FIGS. 2a,b, in which the ionization device is embodied to generate a dielectric barrier discharge or a tip discharge, and
[0036] FIG. 4 shows an illustration of the Paschen curve of the ignition voltage of a plasma as a function of the product of gas pressure and electrode spacing.
DETAILED DESCRIPTION
[0037] In the following description of the drawings, identical reference signs are used for identical or functionally identical components, respectively.
[0038] FIG. 1 schematically shows a mass spectrometer 1 for examining ions 4a, 4b, which are stored in an ion trap 2 of the mass spectrometer 1, by mass spectrometry. In the example shown, the ion trap 2 is embodied as an electric ion trap (Paul trap) and has a first electrode in the form of a ring electrode 3. A radiofrequency storage voltage signal in the form of an AC voltage U.sub.RF is applied to the ring electrode 3, which signal generates in the ion trap 2 an electric storage field E in the form of a radiofrequency alternating field, in which ions 4a, 4b of a gas 4 to be analysed are dynamically stored. The mass spectrometer 1 has a storage signal generator 5 for generating the radiofrequency storage voltage signal U.sub.RF. In the example shown, the storage signal generator 5 is embodied to generate a storage voltage signal U.sub.RF at a constant frequency of the order of kHz to MHz, e.g., 1 MHz, and a constant (maximum) amplitude of several hundred volts. Alternatively, the storage signal generator 5 can be embodied to set or change the frequency and/or amplitude of the storage voltage signal U.sub.RF. To this end, the storage signal generator 5 can be embodied, e.g., as illustrated in WO 2017/194333 A1, cited at the outset.
[0039] From the electric storage field E there results an average restoring force that acts on the ions 4a, 4b to a greater extent, the further away the ions 4a, 4b are from the middle or centre of the ion trap 2. In order to measure the mass-to-charge ratio (m/z) of the ions 4a, 4b, the latter are excited by an excitation signal U.sub.Stim1, U.sub.Stim2 (stimulus) to oscillate, wherein the frequency of the oscillations depends on the ion mass and the ion charge and is typically in the frequency range of kHz to MHz orders of magnitude, e.g. from approximately 1 kHz to 200 kHz. The respective excitation signal U.sub.Stim1, U.sub.Stim2 is generated by a first and a second excitation signal generator 6a, 6b, downstream of which an amplifier is connected in each case.
[0040] For reactionless, non-destructive detection (i.e., the ions 4a, 4b are still present in the ion trap 2 following the detection), the oscillation signals of the excited ions 4a, 4b are tapped off in the form of induced mirror charges at two measurement electrodes, which form the cap electrodes 7a, 7b of the ion trap 2. The two cap electrodes 7a, 7b are connected to a respective low-noise charge amplifier 8a, 8b via a respective filter.
[0041] The charge amplifiers 8a, 8b firstly capture and amplify in each case one of two ion currents I.sub.Ion1, I.sub.Ion2 that are generated at the cap electrodes 7a, 7b on account of the excitation, and secondly keep them at virtual earth potential. From the ion currents I.sub.Ion1, I.sub.Ion2 converted into voltage signals by the charge amplifiers 8a, 8b, an ion signal u.sub.ion(t) is generated by subtraction, the temporal profile of said ion signal being illustrated at the bottom right in FIG. 1.
[0042] The ion signal u.sub.ion(t) is supplied to a detector 9, which, in the example shown, has an analogue-to-digital converter 9a and a spectrometer 9b for fast Fourier analysis (FFT) in order to produce a mass spectrum, which is illustrated at the top right in FIG. 1. In this case, the detector 9 or the spectrometer 9b firstly generates a frequency spectrum of the characteristic ion resonant frequencies f.sub.ion of the ions 4a, 4b stored in the ion trap 2, which frequency spectrum is converted into a mass spectrum on the basis of the dependence of the ion resonance frequencies f.sub.ion on the mass and charge of the respective ions 4a, 4b. In the mass spectrum, the number of detected particles or charges in dependence on the mass-to-charge ratio m/z is shown.
[0043] In the example shown in FIG. 1, the gas 4 to be analysed is taken from a chamber 10 by means of a gas supply 11, said chamber being a process chamber forming part of an industrial apparatus, in which an industrial process, for example a coating process, is carried out. Alternatively, the chamber 10 can be, e.g., a (vacuum) housing of a lithography apparatus or any other type of chamber. The gas supply 11 has a gas outlet 12 to allow the gas 4 to emerge from the chamber 10, and a valve 13 that is controllable by means of a control device 14 in order to feed the gas 4 to be analysed to an ionization device 15, which ionizes the gas 4 to be analysed, in pulsed fashion. In the example shown in FIG. 1, the ionization device 15 is disposed adjacent to the ring electrode 3. A passage opening 16, through which the ionized gas 4 to be analysed, i.e., the ions 4a, 4b, is/are introduced into the interior 2a of the ion trap 2, is formed in the ring electrode 3. In the example shown, the passage bore 16 extends in a central plane of the ion trap 2, in respect of which the cap electrodes 7a, 7b and the ring electrode 3 are disposed in mirror symmetric fashion.
[0044] In the mass spectrometer 1 shown in FIG. 1, the ionization device 15 is disposed directly adjacent to the ion trap 2, more precisely immediately adjacent to the region of the ring electrode 3, in which the passage opening 16 for the supply of the gas 4 to be analysed is formed. The storage voltage signal U.sub.RF, which is generated by the storage signal generator 5 and supplied to the ring electrode 3 via a first electric connection line 20a, is consequently also available in the ionization device 15 and can be used to generate ions 4a, 4b, 17 or a plasma, as explained below on the basis of FIGS. 2a,b.
[0045] In the ionization device 15 illustrated in FIG. 2a, the ring electrode 3 of the ion trap 2, which delimits the interior 2a, at the same time forms a first electrode 3 of the ionization device 15 which, together with a second electrode 18, is used to generate ions 4a, 4b in the space between the two electrodes 3, 18. The fact that the RF storage voltage signal U.sub.RF, which is applied to the electrode 3, can be used to generate an RF plasma in the gas 4 flowing through the ionization device 15 is exploited. Here, a constant potential (e.g., earth potential) is applied to the second electrode 18.
[0046] It is understood that the second electrode 18 need not necessarily be connected to the storage signal generator 5 in order to generate a constant potential at said electrode.
[0047] In the ionization device 15 shown in FIG. 2a, the second electrode is embodied as a metallic supply line 18, through which the gas 4 to be analysed flows in the direction of the ion trap 2. On its outer side facing away from the interior 2a, the ring electrode 3 has a tubular electrode portion 3a, which tapers to a tip and surrounds the passage opening 16 or extends the latter in the direction of the second electrode 18. The second electrode 18 is disposed at a predetermined distance d from the end of the electrode portion 3a which tapers to a tip. In order to bridge the interstice between the ring electrode 3 or the electrode portion 3a which tapers to a tip and the end of the supply line, which is used as second electrode 18, the ionization device 15 has a tubular supply line portion 19, which consists of an electrically insulating material, a ceramic in the example shown. The electrically insulating supply line portion 19 extends along the outer side of the supply line 18 which forms the second electrode and bridges the interstice between the end thereof facing the ring electrode 3 and the ring electrode 3. The supply line portion 19 prevents the gas 4 to be analysed from being able to escape to the surroundings.
[0048] An ignition path is available for igniting a plasma or for generating ions 4a, 4b in the space between the two electrodes 3, 18, said ignition path corresponding to the distance d between the two electrodes 3, 18 in the flow direction of the gas 4 to be analysed and being able to have a length of between approximately 100 μm and 50 mm, for example.
[0049] Since the control device 14 must drive the controllable valve 13 in any case in order to supply the gas 4 to be analysed to the interior 2a of the ion trap 2 in pulsed fashion, the plasma is automatically ignited in the case of a suitable choice of the parameters of the pulsed supply of the gas 4 to be analysed and said plasma is quenched again when the gas pressure drops, without this requiring closed-loop control. Quenching the plasma while storing and analysing the ions 4a, 4b, which were supplied in pulsed fashion, in the ion trap 2 is advantageous for avoiding interference in the electric storage field E in the ion trap 2 by the plasma, for example for minimizing space charging effects.
[0050] The ionization device 15 shown in FIG. 2b substantially differs from the ionization device 15 shown in FIG. 2a in that the former has a supply line 19 made of an electrically insulating material, in which the second electrode 18 is disposed. In the example shown in FIG. 2b, the second electrode 18 has an end 18a, which tapers to a tip and which protrudes into the passage opening 16 of the ring electrode 3 at the protruding electrode portion 3a. In this way, it is possible to generate the ions 17 in the passage opening 16, directly adjacent to the interior 2a of the ion trap 2.
[0051] In the example shown in FIG. 2b, the gas to be ionized and to be supplied to the interior 2a of the ion trap 2 is an ionization gas 22, typically a noble gas, for example helium. The ionization gas 22 is kept in a gas reservoir 21 of the gas supply 11 and supplied to the supply line 19 of the ionization device 15 via a gas outlet 12 and the controllable valve 13. The ionization gas 22 is used to ionize a gas 4 to be analysed in the interior 2a of the ion trap 2. In this case, the gas 4 to be analysed is introduced into the interior 2a of the ion trap 2 through a passage opening 26 in the first cap electrode 7a and aligned approximately on the centre of the ion trap 2. The ion trap 2 has an axis of symmetry 23, in respect of which the electrodes 3, 7a, 7b of the ion trap 2, more precisely the inner sides thereof which delimit the interior 2a, have rotational symmetry. The gas 4 to be analysed is ionized by way of impact and/or charge exchange ionization in the interior 2a of the ion trap 2 by means of the ions 17 of the ionization gas 22 generated in the ionization device 15. The number of impacts between the gas 4 to be analysed or the ions 4a, 4b of the gas 4 to be analysed and the ions 17 of the ionization gas 22 can be increased in targeted fashion if the ions 17 of the ionization gas 22 are stored in the storage field E of the ion trap 2 or at least forced into comparatively long trajectories. The use of neon or argon as ionization gas 22 is advantageous to this end.
[0052] Should—unlike what is illustrated in FIG. 2b—the gas 4 to be analysed be ionized outside of the ion trap 2, i.e., should the use of an ionization gas 22 be dispensed with, the gas 4 to be analysed, which enters into the interior 2a of the ion trap 2 through the first cap electrode 7a, can likewise be ionized with the aid of a (plasma) ionization device 15, which may be constructed as illustrated in FIG. 2a, for example. In this case, the first cap electrode 7a and not the ring electrode 3 forms part of the ionization device 15. In this case, the excitation voltage signal U.sub.Stim1 generated by the first excitation signal generator 6a is used to generate a plasma 17 in the plasma generating device 15. It is understood that the second cap electrode 7b or the second excitation signal generator 6b can be used accordingly in order to ionize the gas 4 to be analysed or the ionization gas 22.
[0053] FIGS. 3a,b show two further options for generating ions 4a, 4b or a plasma in the (plasma) ionization device 15, which differ from the examples shown in FIGS. 2a,b by the configuration of the second electrode 18.
[0054] In the example shown in FIG. 3a, the supply line 19, like in FIG. 2b, is formed from an electrically insulating material. A ring-shaped, metallic cuff 18 (or a pipe portion) is attached to the outer side of the supply line 19 and forms the second electrode of the ionization device 15. Since the second electrode or the cuff 18 is shielded by the supply line 19, the plasma 17 is generated within the supply line 19 in a region directly adjacent to the ring electrode 3 by way of a dielectric barrier discharge.
[0055] In the example shown in FIG. 3b, the second electrode 18 is disposed within the electrically insulating supply line 19, like in the example shown in FIG. 2b. The second electrode 18 has a rod-shaped embodiment and also has a tip 18a, which faces the ring electrode 3 or the passage opening 16. In addition to a first protruding, cylindrical electrode portion 3a, which is used like in FIG. 3a for receiving or fastening the cylindrical supply line 19, a second protruding electrode portion 3b which tapers to a tip is formed on the ring electrode 3. The second electrode portion 3b is attached to the outer side of the ring electrode 3 with a lateral offset from the passage opening 16 and, with its end which tapers to a tip, extends in the direction of the tip 18a of the second electrode 18 in order to generate ions 4a, 4b or a plasma in an interstice to the tip 18a of the second electrode 18.
[0056] In summary, the voltage signal(s) or potential(s) applied to the electrodes 3, 7a, 7b of the ion trap 2 can be used to generate ions 4a, 4b, 17 or a plasma in the region of the inlet of the gas 4 to be analysed or of the ionization gas 22 into the interior 2a of the ion trap 2 in the manner described above, i.e., by the specific geometry of the electrode 3 or a suitable embodiment of the ionization device 15. Since the electrodes 3, 7a, 7b are supplied with a respective voltage signal U.sub.RF, U.sub.Stim1, U.sub.Stim2 by the signal generators 5, 6a, 6b, no additional voltage supply is required for the ionization device 15. Moreover, the respective electrode 3, 7a, 7b could be used as a (first) electrode of the ionization device 15, where appropriate.
[0057] It is understood that the procedure described above can be advantageously applied not only in the mass spectrometer 1 with an ion trap 2 in the form of an electric resonance trap, as shown in FIG. 1, but also to different types of ion traps 2. The voltage signal which is used to generate the ions 4a, 4, 17 or the plasma could, where applicable, be not a (radiofrequency) AC voltage but a DC voltage in this case.
[0058] Nor is it mandatory to carry out a non-destructive analysis of the ions 4a, 4b stored in the ion trap 2, as is the case in the mass spectrometer 1 illustrated in FIG. 1. Rather, the ions 4a, 4b or, in targeted fashion, individual ion species could be removed from the ion trap 2 for detection purposes. In this case, the ions 4a, 4b removed from the ion trap 2 are detected in a detector 9 which is disposed outside of the ion trap 2.
[0059] 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.
[0060] 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.
