IONIZATION DEVICE AND MASS SPECTROMETER

Abstract

The invention relates to an ionization device with an ionization space formed in a container, an inlet system for supplying a gas to be ionized to the ionization space, an electron source having at least one filament for supply of an electron beam to the ionization space, and an outlet system for letting the ionized gas out of the ionization space. Electron optics having at least two electrodes are disposed between the filament and the ionization space

Claims

1. An ionization device, comprising: an ionization space formed in a chamber, an inlet system for supplying a gas to be ionized to the ionization space, an electron source having at least one filament for supply of an electron beam to the ionization space, and an outlet system for letting the ionized gas out of the ionization space, characterized in that an electron optics comprising at least two electrodes is installed between the filament and the ionization space.

2. The ionization device according to claim 1, in which the electron optics is designed to focus the electron beam into the ionization space.

3. The ionization device according to claim 1, in which the electron optics is configured to measure an emission current of the filament at at least one electrode.

4. The ionization device according to claim 3, further comprising: a control device for controlling the emission current of the filament to a target emission current.

5. The ionization device according to claim 1, in which the electron optics has at least one switchable electrode for deflecting the electron beam away from an opening in the container.

6. The ionization device according to claim 1, in which the filament is disposed at a distance of at least 0.5 cm, preferably of at least 3 cm, in particular of at least 5 cm, from the chamber.

7. The ionization device according to claim 1, in which the electron source comprises two or more filaments that preferably each serve to supply one electron beam through opposing openings of the chamber.

8. The ionization device according to claim 1, designed to generate a pressure of more than 10.sup.−4 mbar and not more than 1 mbar in the ionization space.

9. The ionization device according to claim 1, in which a flow conductance of the inlet system is greater than a flow conductance of the outlet system.

10. The ionization device according to claim 1, having a vacuum generation device configured to generate a pressure (p.sub.F) at the filament of the electron source that is lower than a pressure (p) in the ionization space.

11. The ionization device according to claim 10, configured to generate a pressure (p.sub.F) between 10.sup.−8 mbar and 10.sup.−4 mbar at the filament.

12. A mass spectrometer for mass-spectrometric analysis of a gas comprising: an ionization device according to claim 1, and a detector for detection of the gas to be analysed that has been ionized in the ionization device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 is a schematic diagram of a mass spectrometer with an ionization device for ionization of a gas that has an electron source with an electron optics.

DETAILED DESCRIPTION

[0028] In the description of the drawings that follows, identical reference numerals are used for components that are the same or have the same function.

[0029] FIG. 1 shows, in schematic form, a mass spectrometer 1 for mass-spectrometric analysis of a gas 2 to be ionized. The gas 2 includes a gas constituent in the form of a matrix gas 3, and further gas constituents, for example an etching product formed in the etching of a substrate. The gas 2 is present in a process space 4 outside the mass spectrometer 1 that forms the interior of a process chamber 5, of which FIG. 1 shows just part. The mass spectrometer 1 is connected to the process chamber 5 a via an inlet system 6. The connection may be formed, for example, by means of a flange. Instead of a gas 2 which is generated in an etching process, it is also possible by means of the mass spectrometer 1 to analyse a gas 2 which is formed in a coating process, in the cleaning of the process chamber 5, etc.

[0030] The inlet system 6 is controllable, meaning that the inlet system 6, in the example shown, has a fast-switching valve 7 by means of which the inlet system 6 can be opened or closed. The valve 7 can be actuated with the aid of a control device 8. The control device 8 may, for example, be a data processing system (hardware, software, etc.) suitably programmed to enable the control of the inlet system 6 and further functions of the mass spectrometer 1 (see below).

[0031] The inlet system 6 has a tubular component 9, in the form of a corrugated stainless steel hose in the example shown. The tubular component 9 is detachably connected, for example via a screw connection, to the mass spectrometer 1. By means of the controllable inlet system 6 with the tubular component 9 in the form of the corrugated hose, the gas 2 enters an ionization space 10 that forms the interior of a metallic heatable container 11 (“source block”) of an ionization device 12 of the mass spectrometer 1. The corrugated hose 9 ends on one side of the ionization space 10 that is open on two opposite sides. The ionization device 12 has an outlet system which, in the example shown, takes the form of an exit opening 13 for exiting of the ionized gas 2a from the ionization space 10 of the container 11. The exit opening 13 is formed on the side of the container 11 opposite from the corrugated hose 9.

[0032] In the example shown in the FIGURE, the ionization device 12 has an electron source 14 with a first and second filament (heating wire) 15a, 15b. The ionization device 12 is connected for signalling purposes to the control device 8, in order to adjust a heat flow through the respective filament 15a, 15b. The control device 8 is also connected for signalling purposes to a first and second electron optics 16a, 16b. The first electron optics 16a is disposed between the first filament 15a and the ionization space 10, more specifically between the first filament 15a and a first opening 20a for entry of a (first) electron beam 19a into the ionization space 10. Correspondingly, the second electron optics 16b is disposed between the second filament 15b and the ionization space 10, more specifically between the second filament 15b and an opening 20b for entry of a second electron beam (not shown in the FIGURE) into the ionization space 10. The first electron optics 16a and the second electron optics 16b each have three electrodes 17a-c, 18a-c, which, in the example shown, can each be controlled individually by the control device 8. It will be apparent that the respective electron optics 16a, 16b has three electrodes 17a-c, 18a-c merely by way of example, and may also comprise more or fewer electrodes.

[0033] As apparent in the FIGURE, two filaments 15a, 15b are provided in the electron source 14, but only the first filament 15a generates an electron beam 19a in operation of the ionization device 12, which is supplied to the ionization space 10 via the opening 20a. The second filament 15a, by contrast, is inactive in operation of the ionization device 12. If the first filament 15a is damaged or fails entirely in the operation of the ionization device 12, the providing of the two filaments 15a, 15b enables continued operation of the ionization device 12 with the second filament 15b while the defective first filament 15a is changed, or vice versa. In the example shown, the openings 20a, 20b are disposed opposite one another in the heatable container 11, such that the filaments 15a, 15b are opposite one another along a line of sight (a straight line).

[0034] The electron source 14, more specifically the cylindrical interior thereof with the two filaments 15a, 15b in the example shown, is connected to the ionization space 10 in the container 11 only via the respective opening 20a,b. The respective filament 15a, 15b is disposed at a distance A from the container 11, which is more than 0.5 centimetre, about 3 cm in the example shown, but may optionally even be more than 5 cm. The comparatively large distance A of the filament 15a, 15b from the container 11 is enabled by the electron optics 16a, 16b and serves to reduce degradation of the metallic material of the filament 15a, 15b, for example tungsten or rhenium, by reactions with the matrix gases 3 or matrix gas ions present in the gas 2 to be ionized or in the ionized gas 2a.

[0035] This is advantageous especially in the case of the ionization device 12 shown in the FIGURE, which is designed to generate a comparatively high (static) pressure p in the ionization space 10, which may be between about 10.sup.−4 mbar and about 1 mbar and is about 0.01 mbar in the example shown. For generation of the comparatively high pressure p in the ionization space 10, a flow conductance C.sub.E of the inlet system 6 is greater than a flow conductance C.sub.A of the outlet system 13. In the example shown, the flow conductance C.sub.E of the inlet system 6 is predefined by the tubular component 9, more specifically by the diameter D.sub.E of the tubular component 9. The flow conductance C.sub.A of the outlet system 13 is predefined by the diameter D.sub.A of the outlet opening. The ratio of the flow conductances C.sub.E/C.sub.A determines the (average) pressure p in the ionization space 10, which should typically be maximized.

[0036] The effect of the high pressure p in the ionization space 10 is generally that a comparatively large number of atoms or molecules of the matrix gas 3 passes through the respective openings 20a, 20b from the container 10 into the interior of the electron source 14 and reaches the respective filament 15a, 15b.

[0037] In the example shown, the ionization device 12 has a vacuum generation device 21 in the form of a turbomolecular pump in order to generate a pressure p.sub.F less than the pressure p in the ionization space 10 in the interior of the electron source 14 and hence at the respective filament 15a, 15b. The pressure p.sub.F in the region of the respective filament 15a, 15b may lie, for example, within an interval between about 10.sup.−8 mbar and 10.sup.−4 mbar. The lower pressure p.sub.F distinctly reduces the number of particles of the matrix gas 3 that can react with the material of the filament 15a, 15b. In this way, it is possible to increase the lifetime of the filaments 15a, 15b.

[0038] In the example shown, the three electrodes 17a-c, 18a-c of the respective electron optics 16a, 16b are designed to focus the electron beam 19a to a focus position F within the ionization space 10. For this purpose, the electrodes 17a-c, 18a-c each have a central aperture, with decreasing diameter of the apertures with increasing distance from the respective filament 15a, 15b. Since the focus of the ions of the matrix gas 3 that leave the ionization space 10 via the opening 20b and enter the electron source 14, owing to their distinctly greater mass, differs significantly from the focus position F of the electron beam 19a, the ions of the matrix gas 3 are defocused by the electron optics 16a, 16b on exit from the ionization space 10 before they hit the filament 15a, 15b. This reduces the probability of a reaction with the material of the respective filament 15a, 15b and increases its lifetime.

[0039] In the example shown in the FIGURE, the electron optics 16a, more specifically the second electrode 17b, serves to measure the emission current I.sub.E of the first filament 15a. The emission current I.sub.E is understood to mean the number of electrons that exit from the first filament 15a per unit time. A measure of the emission current I.sub.F is the number of electrons that strike the second electrode 17b within a given time interval. This exploits the fact that a generally essentially constant proportion of the electrons exiting from the first filament 15a hits the second electrode 17b, and so this can serve as measurement electrode or as sensor for measurement of the (proportional) emission current I.sub.F. The number of charges or electrons that hit the second electrode 17b per unit time may be measured, for example, with a current measurement device (not shown), for example in the form of a charge amplifier or the like, that forms part of the electron optics 16a. The control device 8 is in contact with the electron optics 16a and is designed to control the emission current I.sub.F of the filament 15a to a constant target emission current I.sub.F,S which is recorded in a memory device of the control device 8 and is typically determined depending on the gas 2 to be analysed. For the control of the emission current I.sub.F, the control device 8 may act on a current source, for example, in order to vary the current through the first filament 15a and hence its temperature.

[0040] The third electrode 17c of the electron optics 16a is switchable in the example shown, meaning that its electrical potential can be switched between at least two different potential values. If, in a switching state, the electrical potential applied to the third electrode 17c or the difference to the electrical potential of the first filament 15a is sufficiently large, the electron beam 19a is deflected away from the opening 20a either back in the direction of the filament or toward the third electrode 17c and does not enter the ionization space 10 through the opening 20a. This is favourable, for example, if an already ionized gas enters the ionization device 12, or if it is the case that blank samples are to be taken. The third electrode 18c of the second electron optics 16b is designed correspondingly. By virtue of the switchable third electrode 17c, 18c, it is unnecessary to switch off or cool down the filament 15a, 15b if no electron beam 19a is to enter the ionization space 10, so that the temperature of the filament 15a, 15b remains constant. The electron source 14 can thus be operated in a pulsed manner, so that an electron beam 19a enters the ionization space 10 only if this is useful for the mass-spectrometric analysis of the gas 2.

[0041] The outlet system in the form of the exit opening 13 is followed, in the mass spectrometer 1, by an ion transfer device 22 for transfer of the ionized gas 2a from the ionization space 10 into a detector 24 in which the ionized gas 2a is analysed by mass spectrometry. The ion transfer device 22, in the example shown, has an extraction device 23 in the form of an electrode arrangement in order to extract the ionized gas 2a from the ionization space 10 and accelerate it in the direction of the ion transfer device and optionally to focus it, in order then to separate it by mass in the detector 24.

[0042] By means of the measures described further above, it is possible to distinctly increase the lifetime of the filament(s) 15a, 15b in the mass spectrometer 1 designed for ionization of the gas 2 to be analysed at high pressures p. In addition, it is possible to set a stable emission current I.sub.E,S of the respective filament 15a, 15b. It will be apparent that the ionization device 12 described further above can be used not just in a mass spectrometer 1 but also in many other fields of use in which a gas is to be ionized at comparatively high pressures.

[0043] 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.

[0044] 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.