MASS SPECTROMETER AND METHOD FOR CALIBRATING A MASS SPECTROMETER

Abstract

The invention relates to a mass spectrometer, having: a gas inlet adapted to supply a sample gas to be ionized to an ionization region of the mass spectrometer, a calibration unit adapted to supply a calibration gas to be ionized to the ionization region, and an ionization unit adapted to ionize the sample gas and/or the calibration gas in the ionization region. The calibration unit includes at least one evaporation source for generating the calibration gas by evaporating a source material. The invention also relates to a method for calibrating a mass spectrometer.

Claims

1. A mass spectrometer, comprising: a gas inlet adapted to supply a sample gas to be ionized to an ionization region of the mass spectrometer, a calibration unit adapted to supply a calibration gas to be ionized to the ionization region, an ionization unit adapted to ionize the sample gas and/or the calibration gas in the ionization region, wherein the calibration unit comprises at least one evaporation source for generating the calibration gas by evaporating a source material.

2. The mass spectrometer according to claim 1, wherein the source material and the ionization region are arranged along a line of sight.

3. The mass spectrometer according to claim 1, wherein the evaporation source is a thermal evaporation source, preferably a resistive evaporation source, an electron beam evaporation source or an effusion evaporation source.

4. The mass spectrometer according to claim 3, wherein the resistive evaporation source comprises a heated filament that is at least partially coated with the source material.

5. The mass spectrometer according to claim 1, wherein the evaporation source is a pulsed laser deposition, PLD, evaporation source.

6. The mass spectrometer according to claim 1, wherein the source material is a metal, preferably selected from the group consisting of: Al, Co, Mn, Bi, Ni, Fe, Cu and precious metals, in particular Au.

7. The mass spectrometer according to claim 1, wherein the source material is selected from the group consisting of: metal nitrides and metal oxides, in particular of Tantalum, Vanadium, Tungsten, Rhenium, or Yttrium.

8. The mass spectrometer according to claim 1, further comprising: at least one sensor, preferably for determining a pressure of the calibration gas, wherein the sensor is preferably arranged along a line of sight to the ionization region and/or along a line of sight to the source material.

9. The mass spectrometer according to claim 8, wherein the sensor is a pressure sensor, preferably an ionization vacuum gauge, more preferably a cold cathode vacuum gauge, in particular a Penning vacuum gauge, or a hot cathode vacuum gauge, in particular a Bayard-Alpert vacuum gauge or an extractor ionization gauge.

10. The mass spectrometer according to claim 9, wherein the pressure sensor or a control unit of the mass spectrometer is adapted for determining a flow rate of the calibration gas based on the pressure of the calibration gas determined by the pressure sensor.

11. The mass spectrometer according to claim 8, wherein the sensor is a quartz crystal microbalance, preferably for determining a flow rate of the calibration gas.

12. The mass spectrometer according to claim 8, further comprising: a movable cover for blocking a line of sight between the source material and the ionization region and/or a line of sight between the source material and the pressure sensor.

13. The mass spectrometer according to claim 1, wherein the ionization unit is an electron ionization source.

14. The mass spectrometer according to claim 1, further comprising: an ion trap for storing ions of the sample gas and/or of the calibration gas, wherein the ionization region is formed inside of the ion trap.

15. A method for calibrating a mass spectrometer, comprising: generating a calibration gas by evaporating a source material in at least one evaporation source of the mass spectrometer, supplying the calibration gas to an ionization region and ionizing the calibration gas in the ionization region, detecting the ionized calibration gas in a detector of the mass spectrometer, and calibrating the mass spectrometer based on the detected ionized calibration gas.

16. The method according to claim 15, wherein the step of calibrating the mass spectrometer comprises: determining a sensitivity of the mass spectrometer based on a signal intensity of the detector when detecting the ionized calibration gas and based on a pressure detected by at least one pressure sensor when supplying the calibration gas to the ionization region.

17. The method according to claim 15, further comprising: before and/or after supplying the calibration gas to the ionization region: coating surfaces of vacuum components in the mass spectrometer with a getter material for the source material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Exemplary embodiments are shown in the diagrammatic drawing and are explained in the description below. The following are shown:

[0037] FIG. 1 a schematic illustration of an example of a mass spectrometer having a calibration unit with an evaporation source for generating a calibration gas by evaporation of a source material,

[0038] FIG. 2 a schematic illustration of an ion trap mass spectrometer having a calibration unit similar to the one shown in FIG. 1,

[0039] FIG. 3a-c schematic illustrations of a resistive evaporation source and of a filament being partially coated with the source material.

DETAILED DESCRIPTION

[0040] FIG. 1 schematically shows a mass spectrometer 1 having a gas inlet 2 (more precisely, a gas inlet system) for supplying a sample gas 4 from a process chamber outside of a (vacuum) housing 3 of the mass spectrometer 1 to an ionization region 5 inside of the housing 3 of the mass spectrometer 1. The mass spectrometer 1 has a calibration unit 6 adapted to supply a calibration gas 7 to the ionization region 5 of the mass spectrometer 1. The calibration unit 6 is arranged inside of the housing 3 of the mass spectrometer 1 (i.e. in-situ). An ionization unit 8 is also provided in the housing 3 and is adapted to ionize both the sample gas 4 (the analyte) and the calibration gas 7 in the ionization region 5.

[0041] In the present example, the ionization unit 8 is an electron ionization source in the form of an electron gun and generates an electron beam 8a that is directed to the ionization region 5 for ionizing the respective gases 4, 7 by electron impact ionization. The sample gas 4 and the calibration gas 7 are provided to the ionization region 5, i.e. the sample gas 4 and the calibration gas 7 may be provided to the ionization region 5 at the same time, but are typically not provided to the ionization region 5 at the same time. The sample gas 4 having typically unknown constituents and/or unknown amounts of constituents is provided to the ionization unit 5 for mass-spectrometric analysis thereof. The calibration gas 7 is provided to the ionization region 5 for calibration of the mass spectrometer 1.

[0042] After (partial) ionization in the ionization region 5, both the sample gas 4 and the calibration gas 7 are provided to an analysing section of the mass spectrometer 1. The analysing section has an analyzer 11, in the present example in the form of a quadrupole mass filter, for selecting a suitable range of mass-to-charge ratios of the constituents of the sample gas 4 or of the calibration gas 7. The analysing section also has a detector 12 for performing a mass spectrometric measurement of the ionized gases 4, 7. It will be understood that other types of analyzers, such as Time-of-Flight analyzers, sector field analyzers, etc. may be used in the mass spectrometer 1. The detector 12 may comprise a plurality of detector elements such as Faraday cups or the like.

[0043] For the purpose of selectively supplying the sample gas 4 or the calibration gas 7 to the ionization region 5, a control unit 13 is provided in the mass spectrometer 1. The control unit 13 may be adapted to control the gas inlet 2, e.g. a controllable valve or the like, to either supply the sample gas 4 to the ionization region 5 or to block the flow of the sample gas 4 to the ionization region 5. One skilled in the art will appreciate that the gas inlet 2 does not necessarily has a controllable valve. In this case, the sample gas 4 may be provided to the ionization region 5 in a continuous manner. One skilled in the art will also appreciate that the housing 3 may possibly be dispensed with. The control unit 13 is also adapted to control the calibration unit 6 to supply the calibration gas 7 to the ionization region 5 or to avoid generation of the calibration gas 7. In the present example, the calibration unit 6 has a single evaporation source 9 for generating the calibration gas 7 by evaporating a source material 10. In the example shown in FIG. 1, the evaporation source 9 is a thermal evaporation source in the form of a resistive evaporation source, a current being passed through a resistive element, e.g. a filament, where the source material 10 is placed, as will be described in detail below. The calibration unit 6 may also comprise other types of thermal evaporation sources e.g. an electron beam evaporation source, an effusion evaporation source, etc.

[0044] As can be gathered from FIG. 1, the source material 10 and the ionization region 5 (or the ionization volume) are arranged along a line of sight 14a. More precisely, the line of sight 14a extends from the source material 10 in a straight line that corresponds to the main flow direction of the calibration gas 7 and intersects the electron beam 8a generated by the ionization unit 8 in the ionization region 5.

[0045] The source material 10 is typically a non-volatile material, in particular a metal. Suitable metals are precious metals, in particular gold (Au), but other metals may be used as well as the source material 10, e.g. Al, Co, Mn, Bi, Ni, Fe, Cu, etc.

[0046] By evaporating a source material 10 in the form of a metal, a calibration gas 7 comprising atoms of the source material 10 is provided. A calibration gas 7 in the form of atoms of a metal vapour is not fragmented during ionization, simplifying the calibration process. However, the choice of the source material 10 is not limited to metals. For instance, chemical compounds such as metal nitrides or metal oxides, e.g. nitrides or oxides of Vanadium, Rhenium or Tantalum, Tungsten or Yttrium may be provided as the source material as well. Moreover, the calibration unit 6 may have more than one evaporation source 9 for evaporating different source materials 10. The calibration gases 7 associated with these evaporation sources 9 may be provided to the ionization region 5 simultaneously, possibly together with the sample gas 2.

[0047] Preferred source materials 10 for the calibration unit 6 have a high sticking probability for the surfaces of vacuum components of the mass spectrometer 1 that come into contact with the calibration gas 7, e.g. for the surface 3a at the interior of vacuum housing 3 of the mass spectrometer 1 that is typically made of stainless steel. In this way, deposits of the source material 10 on a respective surface 3a stick to that surface 3a and do not contaminate the mass spectrometer 1. In order to avoid a peeling off of the source material 10 from the affected surfaces 3a, these surfaces 3a may be coated with a getter material 17 for the source material 10, e.g. Al or Ti, either before or after supplying the calibration gas 7 to the ionization region 5.

[0048] In the example shown in FIG. 1, the mass spectrometer 1 comprises two sensors 15a, 15b that are not required, but helpful for operation of the mass spectrometer 1 with the calibration unit 6. The first sensor 15a is a pressure sensor arranged along the line of sight 14a with the ionization region 5 and with the source material 10. The pressure values p.sub.1 and p.sub.0 mentioned above can be determined with the first sensor 15a. Additionally, the pressure of the sample gas 4 can be measured with the first sensor 15a as well, provided that the first sensor 15a is arranged at a suitable position in the gas flow of the sample gas 4.

[0049] The second sensor 15b is arranged along a (further) line of sight 14b to the source material 10. The second sensor 15b allows for direct control/measurement of the flow rate Q.sub.c of the calibration gas 7. For this purpose, the second sensor 15b is a quartz crystal microbalance. Alternatively, a pressure sensor like a Bayard-Alpert vacuum gauge may be used for this purpose as well. Other types of vacuum gauges, e.g. cold cathode vacuum gauges such as Penning vacuum gauges or extractor ionization gauges, may be used as first/second sensors 15a, 15b as well.

[0050] The pressure p.sub.c of the calibration gas 7 determined by the first pressure sensor 15a may be used in the control unit 13 for determining a flow rate Q.sub.c of the calibration gas 7 (provided that the flow rate Q.sub.c of the calibration gas 7 is not determined directly by the quartz crystal microbalance 15b). In general, the flow rate Q.sub.c of the calibration gas 7 should be as constant as possible during the calibration process for quantitative mass spectra. The control unit 13 may be adapted to control or to regulate (in closed-loop control) the flow rate Q.sub.c of the calibration gas 7. The flow rate Q.sub.c of the calibration gas 7 or the pressure p.sub.c of the calibration gas 7 may be used in the calibration of the mass spectrometer 1, as will be explained in detail further below.

[0051] In the calibration process, a calibration of the mass scale of the mass spectrometer 1 is performed. In the present example, the calibration involves a correlation between the quadrupole voltages applied to the quadrupole analyzer 10 and the mass-to-charge ratios of the known atomic mass(es) of the constituents of the calibration gas 7 that are detected by the detector 12. The known masses, resp., mass-to-charge ratios, of the peaks of the constituent(s) of the calibration gas 7 in the mass spectrum of the calibration gas 7 serve as a mass scale by which the peaks of the (unknown) constituents of the sample gas 4 that are present in the mass spectrum of the sample gas 4 may be assigned to their correct mass-to-charge ratios.

[0052] In addition to the identification of specific constituents of the sample gas 4, for quantitative measurements, the sensitivity/signal intensity of the mass spectrometer 1 should be calibrated as well.

[0053] For this purpose, in a first step, for a given mass-to-charge ratio k of the source material 10, in the present example gold (.sup.197Au, k=197), a background pressure p.sub.0 in the mass spectrometer 1 (i.e. without the calibration gas 7 or the sample gas 4 being present) is determined using the first and/or the second pressure sensor 15a,b. In addition to the background pressure p.sub.o, a background signal intensity B.sub.k measured by the detector 12 at the mass-to-charge ratio k=197 a.m.u. is determined. In a subsequent step, the calibration gas 7 is introduced into the ionization region 5 and the pressure p.sub.1 (or equivalently, p.sub.c) is measured by the pressure sensors 15a,b. The signal intensity S.sub.k of the ionized calibration gas 7 at a mass-to-charge ratio or a.m.u. of k=197 is determined by the detector 12.

[0054] In a subsequent step, the sensitivity K.sub.k of the mass spectrometer 1 for the mass-to-charge ratio k=197 is determined by calculating the ratio of the difference between the signal intensities in the first and second step S.sub.k−B.sub.k at the mass-to-charge ratio k and the difference between the pressure values p.sub.1−p.sub.0 in the second and in the first step (see also the article by Robert E. Ellefson cited above):

K.sub.k=(S.sub.k−B.sub.k)/(p.sub.1−p.sub.0)   (1)

[0055] In this way, the sensitivity K.sub.k for the mass-to-charge ratio k=197 (i.e. for Au) is determined. It is advantageous to calibrate the mass spectrometer 1 for at least one further value of the a.m.u. (or, equivalent, m/z-rato) that is comparatively small, e.g. for k=27 (i.e. Al). The sensitivity of the mass spectrometer 1 for k=27 can be determined in the way indicated above by using a further evaporation unit for evaporating Al as a source material 10.

[0056] In order to determine a pressure increase in the ionization region 5 or in the mass spectrometer 1 when the thermal evaporation source 9 of the calibration unit 6 is heated up, the mass spectrometer 1 of FIG. 1 has a moveable cover 16. The moveable cover 16 is arranged close to the calibration unit 6 and can be moved from a first position in which the cover 16 does not block the line of sight 14a between the source material 10 and the ionization region 5 and to the pressure sensors 15a,b and a second position in which the movable cover 16 blocks the line of sight 14a. In the present example, the moveable cover 16 can be moved between the two positions in a translational movement, as is indicated by a double-headed arrow in FIG. 1. By blocking the respective line of sight 14a,b, the at least one pressure sensor 15a,b can be used to determine a pressure increase in the vacuum system of the mass spectrometer 1 when the evaporation source 6 is heated up, without at the same time measuring the pressure increase due to the calibration gas 7. The pressure increase due to the temperature increase of the evaporation source 9 may possibly be taken into account for the calibration of the mass spectrometer 1.

[0057] The calibration described above with respect to FIG. 1 may also be performed in a mass spectrometer 1 having an electrical Fourier-Transform ion trap 18 shown in FIG. 2. The mass spectrometer 1 of FIG. 2 has an inlet (not shown) for supplying the sample gas 4 to the ionization region 5 via a line of sight 14b. The ionization region 5 essentially corresponds to the center of the ion trap 18. FIG. 2 shows the mass spectrometer 1 in a state where a calibration unit 6, more precisely an evaporation unit 9 thereof, is activated for evaporating a source material 10, being gold in the present example. In FIG. 2, the calibration gas 7 is shown in the ionization region 5 together with the line of sight 14a leading from the calibration unit 6 to the ionization region 5. In the example of FIG. 2, the evaporation source 9 is a pulsed layer deposition, PLD, source. However, rather than using a PLD source 9, a thermal evaporation source as shown in FIG. 1 may be used as well. Moreover, in the example of FIG. 1, rather than using a thermal evaporation source 9, a PLD source or another type of ionization source may be used as well.

[0058] In the electrical FT ion trap 18 of FIG. 2, ions 7a, 7b of the calibration gas 7 are trapped between a ring electrode 19 and a first and second cap electrode 20a, 20b. For storing the ions 7a, 7b in the ion trap 18, an RF signal generation unit 21 generates a radio frequency signal V.sub.RF that is provided to the ring electrode 19. Two excitation units 22a, 22b each generate an excitation signal S1, S2 provided to a respective cap electrode 20a, 20b to excite the ions 7a, 7b to effect oscillations. An oscillation frequency of the ions 7a, 7b in the ion trap 18 depends on a mass-to-charge ratio of the ions 7a, 7b. Two measurement amplifiers 23a, 23b amplify a respective measurement current caused by the oscillations. An ion signal u.sub.ion(t) is generated from a difference between the two measurement currents. A detector 12 that comprises a FFT (“fast Fourier transform”) spectrometer serves to perform a Fourier transformation of the ion signal u.sub.ion(t) and for determining mass spectrometric data in the form of mass spectra 25. The mass spectra 25 are indicative of the number of excited ions 7a, 7b in dependence of their mass-to-charge ratio m/z. In other words, the mass spectra 25, resp., the mass-spectrometric data 25 is indicative of the mass-to-charge distribution of the ions 7a, 7b in the calibration gas 7.

[0059] In the example of FIG. 2, the calibration gas 7 is introduced into the ion trap 18 in an electrically neutral state. The mass spectrometer 1 has an ionization unit 8 to ionize at least part of the neutral calibration gas 7 introduced into the ion trap 18 in the ionization region 5. In the present example, the ionization unit 8 comprises an electron gun (e.g. 70 eV or another suitable ionization-energy) for electron beam ionization of the neutral calibration gas 7 introduced into the ion trap 18. As in the example of FIG. 1, the electron beam 8a intersects the line of sight 14a that leads from the calibration unit 6 to the ionization region 5. It will be understood that other types of ionization units 8 may be used in the mass spectrometers 1 of FIG. 1 and of FIG. 2 as well, using e.g. an inductively coupled plasma, a glow discharge ionization, etc.

[0060] Previous to the detection, the ions 7a, 7b may be at least once selectively excited according to their mass-to-charge ratio m/z, for instance, by means of a SWIFT (stored waveform inverse Fourier transform) excitation. The SWIFT excitation may in particular serve to eliminate ions 7a, 7b having specific mass-to-charge ratios from the ion trap 18. In particular, ions 7a, 7b of a buffer or background gas may be eliminated from the ion trap 18, thus allowing the detection of minute traces of ions 7a, 7b of gaseous species of the calibration gas 7. The mass spectrometer 1 shown in FIG. 2 also comprises an evaluation unit 13 that controls the mass spectrometer 1, in particular the calibration process, as explained above with reference to the mass spectrometer 1 of FIG. 1.

[0061] FIG. 3a shows the evaporation source 9 of FIG. 1 in greater detail. The evaporation source 9 has a filament 26 and a voltage source 27. The voltage source generates an (adjustable) voltage for passing a current through the filament 26 to heat up the filament 26 to temperatures of 1000° C. or more. In the present example, the filament 26 is made of tungsten (W) and has a diameter of about 0,3 mm to 0,5 mm. As can be gathered from FIG. 3b,c, a gold wire 28 is hooked to the filament 26. By heating the filament 26 to temperatures above the melting point of the gold wire 28, the latter melts and flows along the filament 26, thus providing a coating, e.g. in the form of droplets of the source material 10 as shown in FIG. 3c. It will be understood that wires made of other (metallic) materials such as copper or the like may be used instead of gold to provide a source material 10 that can be evaporated when passing a current through the filament 26.

[0062] 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.

[0063] 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.