JP3303587B2 - Mass spectrometer and ion source - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mass spectrometer, and more particularly to an ion source having a beam extraction electrode system suitable for extracting an ion beam having a small angular dispersion and a high transmittance from an ion source, and a mass spectrometer. .

[0002]

2. Description of the Related Art FIG. 7 is a sectional view of an ion source and a beam extraction electrode system of a conventional mass spectrometer. In the case of an electron impact ionization ion source, ions generated by irradiation of electrons in the ionization chamber 1a are repelled by a repeller electrode 1f to which a plus potential is applied by a few volts from the ionization chamber 1a, and a drawing slit of an acceleration electrode 1b. Extruded from 15. Since the extruded ion beam 2 spreads after crossing at the crossover point 2c, a focusing electrode 1c is provided to converge the ion beam 2, and the acceleration electrode 1b and the ground electrode 1d are provided.
Is finally accelerated to about 6 kV. At that time, the width of the ion beam 2 and the divergence angle are controlled by adjusting the voltage of the focusing electrode 1c in order to efficiently guide the ion beam 2 to the analysis unit, that is, the sector magnetic field and the sector electric field. Specifically, the Faraday cup,
An ion current monitor 8a using a multiplier or a channel plate is inserted into a region where the ion beam 2 passes, and the amount of ion current passing through the exit slit 1e is measured by the ion current monitor 8a. Thus, the voltage 12f applied to the focusing electrode 1c was adjusted.

[0003] Japanese Patent Laid-Open No. 57-27553 is related to the prior art.

[0004]

In the above prior art, the distance between the accelerating electrode 1b and the converging electrode 1c is generally equal to the distance between the slit 1 and the focusing electrode 1c.
5, the change in the divergence angle of the beam 2 with respect to the change in the electrode voltage, that is, the change in the position of the crossover point 2c is extremely small. However, since the beam width of the optical system of the mass spectrometer is required to be within a certain value at the same time as the divergence angle, the divergence angle and the beam width are restricted by the conventional electrode structure that can adjust only one electrode voltage 12f. It is difficult to make adjustments that simultaneously satisfy the following conditions.

An object of the present invention is to provide a high-sensitivity mass spectrometer having an extraction electrode system which can be easily adjusted so as to simultaneously satisfy the restrictions on the divergence angle and the beam width, and an ion source for the mass spectrometer.

[0006]

An object of the present invention is to provide an ion source in which ions generated in an ion generation region are extracted from a slit of an acceleration electrode to form an ion beam, and the ion beam is converged by a focusing electrode and emitted from an emission slit. This is achieved by providing an extraction electrode for separating the crossover point of the ion beam extracted from the slit of the acceleration electrode by a required distance from the acceleration electrode, in the vicinity of the downstream of the acceleration electrode.

Another object of the present invention is to provide an extraction electrode having a slit having a width smaller than that of the acceleration electrode at a position on the downstream side of the acceleration electrode by the same distance as the distance between the acceleration electrode and the ion generation region. Is achieved by

Further, the above object is achieved by setting voltage conditions applied to the extraction electrode based on the state of the ion source.

[0009]

In the conventional structure, the ion beam extracted from the accelerating electrode crosses over immediately after being extracted, so that the divergence angle becomes large. However, in the present invention, the crossover point is increased by providing the extracting electrode. Can be separated from the accelerating electrode, the divergence angle can be suppressed, and a high-quality ion beam with excellent convergence can be obtained. Further, since the voltage applied to the extraction electrode can be adjusted, it is possible to simultaneously satisfy the restrictions on the divergence angle and the beam width.

Hereinafter, the principle of the present invention will be described with reference to the drawings. FIG. 3A is an electrode configuration diagram of the electron impact ionization ion source, and FIG. 3B is an electrode configuration diagram of the plasma ion source, in which the movement of the ion beam is schematically shown.
In the present invention, at a position separated from the ionization chamber 1a on the downstream side by a distance substantially equal to the width of the slit 15 of the acceleration electrode 1b,
An extraction electrode 1g is provided as an auxiliary electrode, and the voltage applied to the extraction electrode 1g is determined based on the state of the ion source.

Ions generated in the ion generation region 2a substantially at the center of the ionization chamber 1a are repelled by the repeller electrode 1f to which a plus voltage is applied by several volts from the acceleration electrode 1b, and are pushed out to the acceleration electrode 1b side. . However, the initial velocity of the generated ions is disturbed by thermal motion and collision of electrons, and the electric field created by the repeller electrode 1f alone cannot efficiently transmit through the slit 15 of the acceleration electrode 1b.

Therefore, in the present invention, the width of the slit 15 of the acceleration electrode 1b is made larger than the width of the slit 16 of the extraction electrode 1g, and the distance between the extraction electrode 1g provided downstream of the acceleration electrode 1b and the acceleration electrode 1b is increased. Is set to be substantially equal to the distance between the acceleration electrode 1b and the ion generation region 2a.

Therefore, the electric field leaking from the slit 15 of the accelerating electrode 1b into the ionization chamber 1a is generated by the ion generation region 2a.
The ion beam 2 spreads to the vicinity, and the position of the crossover point of the ion beam 2 is on the downstream side of the extraction electrode 1g, and the ion beam 2 is efficiently extracted from the slit 15 by the leakage electric field (penetration electric field) and passes through the slit 16 of the extraction electrode 1g. Can be done. Since such a penetrating electric field is rapidly attenuated as the distance from the slit increases, the ion generation region 2a
The increase in the electric field inside is slight, and the increase in the energy dispersion of the ion beam 2 does not matter.

On the other hand, in the case of the plasma ion source shown in FIG. 3B, a sheath region several times the Debye length is formed between the acceleration electrode 1b and the plasma 2b. Plasma 2b
Increases the potential by several volts to several tens of volts compared to the acceleration electrode 1b, and efficiently accelerates the ion beam 2 toward the acceleration electrode 1b. The voltage at this time is called a plasma floating potential, which is about six times the electron temperature which is one of the most important parameters of the plasma.

Since ions are emitted from the entire surface of the plasma, an ion current amount substantially proportional to the width of the slit 15 of the acceleration electrode 1b is obtained. However, since the spherical aberration at the end of the slit 15 is larger than that near the center, the slit 1
The ions transmitted through the end of 5 are excessively focused and increase the angular dispersion. Therefore, in order to offset the excessive convergence, the width of the slit 16 of the extraction electrode 1g is reduced similarly to the width of the ion beam 2. That is, by setting the cut angle of the slit 16 to π−θ with respect to the cut angle θ of the slit 15, the spherical aberration of the slit 15 is canceled by the spherical aberration of the slit 16, thereby reducing the overall spherical aberration and reducing the angular dispersion. Can be obtained.

FIG. 4 is a diagram showing the relationship between the ratio f / d of the focal length f to the electrode distance d between two electrodes and the ratio Φ2 / Φ1 of the outgoing energy Φ2 to the incident energy Φ1. When this is applied to the case of FIG. 3A, Φ1 is に of the potential difference between the repeller electrode 1f and the acceleration electrode 1b, and Φ1 is
2-Φ1 corresponds to the potential difference between the acceleration electrode 1b and the extraction electrode 1g. If the focal length f is short, the angular dispersion of the beam increases, which is not preferable. In order to reduce the angular dispersion, it is necessary to increase the focal length f and form a crossover in which the beam converges most narrowly on the downstream side of the extraction electrode 1g. Quantitatively, it can be seen that the voltage applied to the extraction electrode 1g should be set so as to satisfy Φ2 / Φ1 <5.

[0017]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 2 is an overall configuration diagram of a mass spectrometer according to one embodiment of the present invention, FIG. 1 is a configuration diagram of an ion source according to the present invention, (a) is a diagram showing a longitudinal section, and (b) FIG. 3 is a view taken along the line AA in FIG. The mass spectrometer of this embodiment is of a double focusing type. The ions generated by the ion source 1 are extracted and become a high-quality ion beam 2 having excellent convergence.
After being diffused by the quadrupole lens 7a, the ion beam 2 is mass-separated by the sector magnetic field 3. After the ion beam 2 is mass-separated, it is converged by the quadrupole lens 7b, and the sector electric field 4
Energy separation, and finally the mass separation slit 5
The ion current that has passed through is measured by the detector 6. The measurement result is stored in the operation control / measurement data processing unit 10 as a mass spectrum.

In this embodiment, in order to automatically set the approximate voltage condition of the ion beam extraction electrode system, for example, in the case of an electron impact ionization ion source, a repeller electrode voltage 12d is input to the ion source state monitoring unit 11, Lead electrode voltage 12g
Is output to the extraction power supply 9. In the case of a plasma ion source, it is difficult to directly measure the plasma floating potential, but in a sector electric field, the sum Vacc of the accelerating electrode voltage Vacc, which is the actual acceleration voltage, and the plasma floating potential Vp is used.
Paying attention to the relationship of the orbital radius R to + Vp can be expressed by Equation 1, the plasma floating potential Vp can be estimated.

[0020]

R = 2 (Vacc + Vp) / E (Equation 1) where E is the electric field strength of the sector electric field.

The voltage of the extraction electrode 1g thus obtained is a good predicted value, but is not always an optimum value. Therefore, the ion source state monitoring unit 11 inputs the current signal 12a of the ion current monitor 8a provided on the beam line, and adjusts the voltage of the extraction electrode system so that the current value becomes maximum.

The detailed structure of the ion source and the simplest method of setting the extraction electrode voltage will be described with reference to FIG. The entire ion source is placed in the vacuum vessel 18 on the beam line of the analysis unit, and the electrode support insulator 1
7, and the inside of the ion source is kept in a vacuum by a vacuum vessel 14. AA in FIG. 1 (b)
As shown in the cross section, the extraction electrode 1g is composed of two upper and lower plate electrodes, and the acceleration electrode 1b is a plate electrode having a rectangular opening. The ion source state monitoring unit 11 sets the voltage 12g of the extraction electrode 1g based on the voltage 12d of the repeller electrode 1f. Next, the amount of current transmitted through the slits 15 and 16 is measured by the ion current monitor 8a, and the voltage 12f of the focusing electrode 1c is adjusted so that the current is maximized. In the example of FIG. 1, the extraction power supply 9 is shared as a power supply for the focusing electrode 1c and a power supply for the extraction electrode 1g. In this case, electrodes 1c and 1
The applied voltage of g may be the same voltage. This makes it possible to save power, but it goes without saying that separate power supplies may be prepared.

FIGS. 5 and 6 show the results of confirming the effect of this embodiment by numerical simulation. FIG. 5 shows a simulation result of the electron impact ionization ion source, and FIG. 5A shows a simulation result of the conventional ion source. Here, the accelerating electrode voltage Vacc is 6.000 k
V, the repeller electrode voltage Vr was 6.003 kV, and the slit width of the accelerating electrode was 0.5 mm. The ions have an isotropic initial velocity equivalent to 0.01 eV, and the beam width increases as approaching the accelerating electrode. Therefore, there are components that cannot pass through the slit of the accelerating electrode. Moreover, since the ion beam is sharply converged in the slit, it has a large angular dispersion. The beam width spreads to 1mm or more near the focusing electrode,
This makes it difficult to converge the beam thereafter. Under such circumstances, it is not effective to increase the slit width of the accelerating electrode in order to increase the current amount of the ion beam,
It is simply increasing the ineffective current.

On the other hand, in this embodiment shown in FIG. 5B, the slit width of the accelerating electrode is 1 mm, and the slit width of the extraction electrode is 0.5 m.
m, and Vex = 5990 V is applied to the extraction electrode. The electric field exudes from the wide slit of the accelerating electrode to the vicinity of the ion generation region, and succeeds in extracting all the isotropically emitted ions. The focal length at the extraction electrode 1g changes greatly only by adjusting Vex by a few volts, and it is easy to control the angular dispersion of the beam. For these reasons, it has been confirmed that in the present embodiment, the current amount is about twice and the angular dispersion is about 1/2 as compared with the related art. Further, since the width of the slit 16 of the extraction electrode 1g is narrow, the ionization chamber 1a
There is an additional effect that the gas pressure in the inside can be increased and the degree of vacuum on the beam line can be kept low.

FIGS. 6A and 6B show simulation results of the plasma ion source. FIG. 6A shows a case where the plasma floating potential Vp is 30 V, FIG. 6B shows a case where Vp is 60 V, and FIG.
The case where the voltage condition is optimally controlled by V is shown. In FIG. 6, the reference accelerating electrode voltage Vacc is constant at 6.00 kV, and the floating potential of the plasma with respect to Vacc is represented by Vp. The optimum extraction electrode voltage Vex at (a) where Vp = 30 V was 5.88 kV. In this case, Vacc
And Vex is 120V. Next, FIG.
If Vp is raised to 60 V with Vex fixed, the ion beam will not converge. Then, in response to the double increase of Vp from 30V to 60V, Vp
By doubling the potential difference between acc and Vex from 120 V to 240 V, that is, by adjusting Vex to 5.760 kV, an almost optimally focused ion beam is obtained as shown in FIG. 6C. be able to.

[0026]

As described above, according to the present invention,
By measuring the state of the ion source, it is possible to easily obtain a large current ion beam with small angular dispersion, and to increase the sensitivity of the mass spectrometer.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an ion source according to an embodiment of the present invention.
(a) is a figure which shows a longitudinal cross section, (b) is the AA arrow line view of (a).

FIG. 2 is an overall configuration diagram of a mass spectrometer according to one embodiment of the present invention.

FIGS. 3A and 3B are explanatory diagrams illustrating the principle of reducing the angular dispersion by the ion source according to the present invention. FIG.
(B) shows the case of a plasma ion source.

FIG. 4 is a diagram showing a relationship between an extraction electrode voltage and a focal length.

5A and 5B are diagrams showing numerical simulation results of an electron impact ionization ion source, where FIG. 5A shows a case of a conventional ion source;
(B) is a diagram showing the case of the ion source of the present embodiment.

6A and 6B are diagrams showing numerical simulation results of a plasma ion source. FIG. 6A shows a case where the plasma floating potential Vp is 30 V, FIG. 6B shows a case where Vp is 60 V, and FIG.
It is a figure showing the case where voltage conditions are controlled optimally at 0V.

FIG. 7 is a sectional view of a conventional ion source and beam extraction electrode system.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Ion source, 1a ... Ionization chamber, 1b ... Acceleration electrode, 1
c: focusing electrode, 1d: ground electrode, 1e: exit slit,
1f repeller electrode, 1g extraction electrode, 2 ion beam, 2a ion generation region, 2b plasma, 2c crossover, 3 sector magnetic field, 4 sector electric field, 5 ...
Mass separation slit, 6: detector, 7a, 7b: quadrupole lens, 8a: ion current monitor, 9: extraction power supply, 10:
Operation control / measurement data processing unit, 11: ion source state monitoring unit, 13: analysis unit power supply.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masayoshi Yano 882-Chair, Ichimo, Hitachinaka-shi, Ibaraki Prefecture Within the Measurement Instruments Division, Hitachi, Ltd. (72) Inventor Tadao Mimura 882-Chair, Oaza-City, Hitachinaka-shi, Ibaraki Hitachi, Ltd. (56) References JP-A-1-265446 (JP, A) JP-A-1-304650 (JP, A) JP-A-57-27553 (JP, A) JP-A-62-161047 (JP JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01J 49/12 H01J 49/14 H01J 37/08 G01N 27/62

Claims (6)

(57) [Claims] 1. An ion source in which ions generated in an ion generation region are extracted from a slit of an acceleration electrode to form an ion beam, and the ion beam is converged by a focusing electrode and emitted from an emission slit. An extraction source having an extraction electrode having a slit having a small diameter at a position on the downstream side of the acceleration electrode by a distance substantially equal to the distance between the acceleration electrode and the ion generation region. 2. The method according to claim 1 , wherein the cut angle of the slit of the extraction electrode is π with respect to the cut angle of the slit of the acceleration electrode.
An ion source characterized in that -θ is set to cancel spherical aberration. 3. The ion source according to claim 2 , wherein the ion source is an electron impact ionization ion source, and the ions generated by the ion source are applied to an extraction electrode based on a voltage of a repeller electrode that pushes the ions toward the extraction electrode. An ion source characterized by setting voltage conditions. 4. The ion source according to claim 1 , wherein said ion source is a plasma ion source, and a voltage condition applied to the extraction electrode is set based on a plasma floating potential. 5. An extraction electrode system for generating ions generated by an ion source.
To generate an ion beam,
Of specific mass by separating the
5. A mass spectrometer for measuring an amount of on-current , wherein the ion source according to claim 4 is used as an ion source, and the plasma floating potential is obtained using a radius of gyration of ions in an electromagnetic field. 6. A mass spectrometer for extracting ions generated in an ion generation region by an extraction electrode system to generate an ion beam, and passing the ion beam through an electromagnetic field to detect ions of a specific mass. The electrode system includes an accelerating electrode having a slit for extracting ions from the ion generation region, and an extraction electrode having a slit smaller in width than the slit of the acceleration electrode. A mass spectrometer characterized in that the extraction electrode is provided at a position on the downstream side of the acceleration electrode by about the same distance.