JPS61140047A - Ion source of type of closed semispherical positive electrode electron bombardment - Google Patents

Abstract

PURPOSE:To obtain a highly sensitive miniaturized ion source facilitated in degassing and sharply reduced in ion energy of unevenness of space charges produced by oscillating electrons. CONSTITUTION:A positive electrode 1 is constructed into a semispherical shape with use of a metal qauze capable of transmitting electrons therethrough, and further integrally finished by backing welding a substantially flat mesh gauze on the side of a semisphere cross section. In addition, a hot negative electrode 4 is arranged on the outer periphery of the positive electrode 1 on the semisphere side while an ion delivery electrode 3 arranged on the opposite side. Hereby, gas molecules are ionized by oscillating electrons in the vicinity of the positive electrode of closed type. Only ions thereamong produced within the positive electrode are taken out to the flat mesh side by making use of unevenness of space charges produced by oscillating electrons, whereby energy dissipation of the produced ions is made minimum while facilitated in degassing with a simple structure for improving the resolution of a mass analysis. Hereby, any analysis for residual gas of about 10<-8>Torr can be assured without employing a secondary-electron multiplier.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is an ion source used in a mass spectrometry residual gas analyzer that can handle an ultra-high vacuum region. , a closed hemispherical anode electron impact ion source trap with high sensitivity and very small energy dispersion of ion current.

(Conventional technology and problems) Recent advances in vacuum technology have been remarkable. Ultra-high vacuums on the order of 10"Pa can now be easily obtained in large equipment such as accelerators and nuclear fusion reactors, and vacuum quality, i.e. Residual gas analysis has become important. For this reason, a small quadrupole mass spectrometer with a resolution of 100 or less is preferably used as this residual gas analyzer. This type of non-magnetic field type is said to be particularly suitable for surface analysis devices where leakage magnetic fields are suppressed. This is because it is well known that the residual gas composition in the ultra-high vacuum region is low-mass gas molecules below carbon dioxide with a mass number of 44, and also because the resolution does not deteriorate even if the energy dispersion of the generated ions is large to some extent. However, there are still various problems to be solved, many of which are in the ion source. The ion sources used in this type of analyzer include N1er type, hot cathode magnetron type,
There are a wide variety of types, including cold cathode magnetron types, BA gauge types, and axial radiation types. When used in ultra-high vacuum areas, most of them are BA type. This is because relatively high sensitivity can be obtained and degassing is easy, but when actually used in an ultra-high vacuum, the following problems arise.

First, the sensitivity is still insufficient, and is more than two orders of magnitude lower than the BA type total pressure gauge sensitivity. Therefore, IQ-'P
In order to perform residual gas analysis in an ultra-high vacuum of less than a, the assistance of a secondary electron multiplier becomes necessary. However, when a secondary electron multiplier tube is used, the sensitivity inevitably changes over time and the reproducibility deteriorates, so calibration using a total pressure vacuum gauge is essential. In addition, a high-voltage stabilized power supply is required to drive it, making the device large and expensive, making it difficult to use it casually like a total pressure vacuum gauge.

Second, there is significant outgassing from the ion source itself during operation. Ion sources for ultra-high vacuum are generally capable of degassing by electron bombardment, etc.
The analysis tube is also made of a material that releases little gas and can be baked at temperatures of 400°C or higher. However, because the ion source of a mass spectrometer takes a contradictory method of increasing sensitivity while reducing the energy dispersion of a single ion, even if it is a BA type, which is said to be easy to degas, it is not as simple as a total pressure vacuum gauge. However, the structure is inevitably complicated, and degassing at a high temperature comparable to that of a total pressure gauge is difficult. Therefore, the ultra-high vacuum residual gas analyzers currently in use often end up being the largest source of gas emissions within the vacuum apparatus, and the analyzer itself is subjected to gas analysis.

A major feature of a quadrupole mass spectrometer is that even though there is some degree of energy dispersion in the incident ions, there is no risk of a decrease in resolution as long as the upper limit is determined. However, the length of the quadrupole rond used in ultra-high vacuum is 100
The upper limit of the energy of the incident ions is about 1 QeV, since the energy of the incident ions is usually less than 1 QeV. In other words, if it exceeds this range, it will pass through the analysis ronds before sufficient mass spectrometry is performed, resulting in a decrease in resolution. For this reason, BA-type ion sources, whose energy dispersion is spread over 3 Q eV, are unable to guide all of the generated ions into the analysis tube, and cannot achieve the same high sensitivity as a total pressure vacuum gauge. .

In other words, in the BA type ion source, 6 (
Ions are created by applying a potential difference of 1' or more to cause electrons to oscillate in and out of the cage, and the generated ions are transferred from the cage through the small hole below the pole to the ion extraction electrode, which is placed at a lower potential than the hot cathode. Ions are pulled out by the incoming electric field, but since the vibration direction of the electrons that vibrate inside and outside the cage occurs not only horizontally but also vertically, the ions are drawn out by the invading electric field from the ion extraction port of the cage to the entire inside of the cage. It is generated uniformly due to the potential gradient. However, the ion extraction efficiency is higher the closer to the ion extraction port of the small hole in the cage, so the energy range of the resulting ion escape is very wide.
This will reach over 306V. Furthermore, the ions that have passed through the ion extraction electrode have a kinetic energy of 8 QeV or more, and must be decelerated to 1 QeV at the entrance of the analysis tube. However, since the energy dispersion of ions is large, the ion beam diverges when decelerating.
Only some ions with high kinetic energy can be used. Even if type BA is obtained in this way, a decrease in sensitivity cannot be avoided.

Furthermore, the situation is the same for other analyzers other than the quadrupole mass spectrometer type and magnetic field deflection type, and the problem of the ion source is the main problem of the residual gas analyzer. In the case of a magnetic field type, the allowable range of energy dispersion of ions is very small, making it difficult to use a BA type ion source, and the situation is even more serious.

(Objects and Means of the Invention) The present invention has been made in view of the above-mentioned circumstances, and its object is to provide a wire mesh that allows electrons to pass through the anode shape without changing the number of three-electrode configurations. The anode is formed into a hemispherical shape by lining and welding a substantially flat wire mesh on the hemispherical cross-sectional side, and a hot cathode is placed on the outer periphery of the hemispherical side of this anode, and a hot cathode is placed on the opposite, substantially flat cross-sectional side. In this method, an ion extraction electrode is placed, and the gas molecules are ionized by oscillating electrons in and out of this closed anode, and only the ions generated within the anode are generated by utilizing the non-uniformity of the space charge created by the oscillating electrons. By pulling out the ion to the flat screen side, the simple structure allows for easy degassing, but it also minimizes the energy dispersion of the generated ions and improves the resolution of mass spectrometry. The aim is to provide an ultra-sensitive electron impact ion source that enables residual gas analysis.

(Example) Hereinafter, the present invention will be described in detail according to an illustrated example.

FIG. 1 is a schematic diagram of a quadrupole mass spectrometer equipped with a closed hemispherical anode electron impact ion source according to the present invention.

The closed hemispherical anode 1 is made by press-molding a molybdenum wire mesh with a wire diameter of 0.05 am and 30 meshes into a hemispherical shape with a diameter of 12w, and welding a molybdenum ring to prevent the mesh from spreading. At the same time, on the back side of the ring, a wire mesh of the same type is cut into a circle and welded to the ring to form an integral structure. The wire mesh used as the lining need not be limited to the same material as that used to form one hemisphere, and may be any material as long as it is made of a high melting point electrode material and allows electrons to pass therethrough. In addition, this backing wire mesh or metal lattice can be modified as shown in Figure 4.5.6,
Even if the transmittance of electrons and ions is made larger in the central part than in the peripheral part, so that the opening appears to be open, it is still compatible with the present invention and is considered to be a closed type. Further, cases such as those shown in FIGS. 7 and 8, in which the central portion of the wire mesh or metal lattice lining is partially deformed to facilitate concentration of electrons and ions there, can also be considered as closed hemispherical anodes.

The hot cathode filament 2 is placed so as to cover the closed hemispherical anode 1 placed in the state shown in Fig. 2.Thorium oxide powder is applied to an iridium wire with a wire diameter of 0.125 #Ij. Electrodeposited and sintered oxide cathode with a ring diameter of 13 mm and 0.0 mm from this filament. It is possible to extract more emission current than I.

This filament may be made of pure tungsten wire or other materials, and its shape is not limited to a circular ring, but may also be a semi-circular shape, a straight hairpin shape, or other shapes that emit electrons. It may be of any shape or composition as long as it has any shape or composition.

The ion extraction electrode 3 accelerates ions coming out of the flat section of the closed hemispherical anode toward the ion extraction electrode and sends them out in the opposite direction of the ion extraction electrode. As shown in Fig. 10, the ion extraction electrode 3 is a simple one consisting of a circular ring covered with a plain-woven wire mesh, but the ion extraction electrode 3 is not limited to this shape; , it may be a metal disk with a hole in the center as shown in FIG. 12, or it may be of any shape as long as the ions in the center of the closed hemispherical anode 1 can be drawn out. The ion extraction electrode 3 also has the role of reflecting the oscillating electrons and pushing them toward the flat cross-section side of the closed hemispherical anode 1. The shield electrode 4 is an electrode to prevent photographed electrons from flying out from the ion source, and has a wire diameter of 0.
．． A molybdenum wire mesh of 1 ax and 20 meshes is press-formed onto a substantially hemispherical surface, and a molybdenum ring is used to prevent the mesh from expanding. It is integrated by fitting and welding.

This shield electrode 4 is not limited to a hemispherical shape, but may have any shape as long as it can shield electrons. 5 is an insulating plate made of alumina ceramic.
The aforementioned closed hemispherical anode 1, hot cathode filament 2, ion extraction electrode 3, and shield electrode 4 are assembled on this insulating plate 5.

Reference numeral 6 denotes an outer tube of the quadrupole mass spectrometer, and the diameter of the ion injection port located at the center of the outer tube is 2.5 u.

7. The analysis rod of the quadrupole mass spectrometer has a rod diameter of 6u and a length of 50m.

FIG. 13 is a schematic diagram of the power supply relationship when driving a quadrupole mass spectrometer equipped with this closed hemispherical anode electron impact ion source. Reference numeral 8 denotes an ion collector whose output is connected to a DC ion current amplifier 9 via a vacuum current introduction terminal (not shown) using a coaxial cable. 11#i This is a power source for heating the hot cathode filament 2, and this power source 11 is equipped with an automatic stabilization circuit that controls the electron current from the hot cathode filament 2 to always be constant. Reference numeral 12 denotes a bias power source for shielding electrons, which is also connected to the ion extraction electrode 3 on the inside of the vacuum. 13 is a bias power source for accelerating electrons emitted from the hot cathode filament 2; In this state, the entire ion source is set to 70-Ting, and a variable power source that determines the energy of ions entering the quadrupole analysis section A from the ground is connected to the anode 1. Further, the electrical conditions of the high frequency power supply 10 applied to the quadrupole analysis section A are determined so that all the ions incident on the quadrupole analysis section A can be collected by the collector 8. In other words, the total ion current 1 when the total pressure is measured
When the relationship between the anode potential Va and the anode potential Va is investigated, the results shown in FIG. 14 can be obtained.

According to this, the ionic current xtFiva? It increases rapidly from around sv, and the increase stops once at around Va=13V, and Va=
For numbers 13 and above, the changes are complex.

This can be said to indicate that ions are mostly concentrated between 8 and 13V. Follow C'Va
If J is set to 13v, the energy of the incident ions will be distributed between 0 and 5eV, and very high resolution will be obtained. Also, when Va = 13V is set, the ion current is If = 3 x 10'''9A, the electron current at this time is 2mA, and the pressure is I x 1O-1sPa, so the sensitivity S of this ion source is come.

The sensitivity S when a conventional BA gauge ion source is found under the same electrical conditions as the power supply used in the ion source of the present invention is S
The difference is obvious, and the sensitivity of a quadrupole mass spectrometer equipped with the ion source according to the present invention is about 50 times higher. become.

The reason why the ion source according to the present invention is able to have extremely high sensitivity and reduce energy dispersion is that the anode has a double structure made by joining a hemispherical wire mesh and a circular wire mesh. This is due to the fact that only the ions generated therein could be efficiently collected. In the figure, 14 is a bias power supply for determining the incident energy of ions.

(Operating principle of the invention) Electrons ejected from the hot cathode filament 2 are attracted to the closed hemispherical anode 1 and fly into the anode 1. Since the inside of the anode 1 is a completely closed system, the electrons move straight with the kinetic energy they had when they jumped in, and pass through this flat wire mesh, that is, the closed hemispherical anode, toward the center of the cross-section of the opposite hemisphere. However, immediately after that, there is an ion extraction electrode 3 placed at a potential lower than the KFi hot cathode filament potential, so these electrons are immediately repelled by this electrode 3 and are accelerated and attracted into the occluding hemispherical anode 1 again, causing the occlusion to occur at that speed. Hemisphere anode 10 faces toward the hemisphere side and penetrates through it. However, since there is also a shield electrode 4 placed at a lower potential than the hot cathode filament 2, the electrons are further repelled by this electrode 4 and are accelerated and attracted toward the closed hemispherical anode 1 again. In this way, electrons are transferred to the shield electrode 4
and the ion extraction electrode 3. After jumping out of the hot cathode filament 2, the electrons move back and forth while repeating this movement, but each time the electrons are repelled, some of them are captured by the wire mesh of the anode 1 and decrease.

Therefore, electrons are emitted from the hot cathode filament 2 to compensate for this decrease (emission current), and a steady state is maintained as a system. During this time, the oscillating electrons are generated between the shield electrode 4 and the closed hemispherical anode 1, inside the closed hemispherical anode 1, between the flat cross section of the closed hemisphere, and the ion extraction electrode 3.
Ions are generated in the three areas in between. Among these three spaces, the inside of the closed hemispherical anode 1 is the largest space, and many ions are generated. However, since the anode 1 is completely closed by the wire mesh, these ions cannot be extracted unless a potential gradient exists within the anode 1. However, theoretically there is no electric field inside this closed hemispherical anode 1, so if there is some way to extract it,
The energy dispersion of the resulting ions is theoretically zero, making it an ideal ion source.

What should be noted here is that this ion extraction is performed by electrons vibrating inside and outside the closed hemispherical anode 1. In other words, this is precisely where the present invention lies.Ions are extracted from a closed system by utilizing the non-uniformity of the space charge created by oscillating electrons. This is a fundamentally different idea, as opposed to what is done through action, in that the electrons emitted by atoms and molecules themselves are used to attract electrons to the electron population.

That is, when electrons oscillate in and out of the closed hemispherical anode 1, ions incident from the hemisphere side try to focus on the center of the sphere, but they pass through this center and are reflected by the ion extraction electrode 3, leaving the center of the flat cross section. Incoming electrons try to diverge. Therefore, the electron density near the central flat mesh of the closed hemispherical anode 1 is very high, that is, the negative gradient formed by the space charge of electrons is very strong near the center; On the contrary, the density becomes very weak.

Therefore, even in a closed system, such space charge **−
g"1044"'DCm@N(DEWyF.

The electrons are drawn toward the center of the electrode where the electron density is highest,
Even without an accelerating voltage, the energy of the potential difference due to this space charge is collected at the center of the electrode and ejected from between the wire meshes. Therefore, the way the ions are accelerated depends on where they start from within the closed hemispherical bulb, and there is also a directional disturbance in the way they are emitted from the center, so the ionic current obtained from the closed system is Even if there is, it will still have a certain energy range. However, its energy range is about 0 to 4 eV at an electron current of 2 mA, which is more than an order of magnitude smaller than that of other electron impact ion sources.

Also, since the ions are focused towards the center of the electrode,
Even if the ion extraction electrode is the simplest plain-woven wire mesh, the ion beam can be easily collimated and the probability of passing through the quadrupole region can be increased, making it possible to achieve ultra-high sensitivity ions despite its small size. This means that we can now provide a source.

Furthermore, because the anode and ion extraction electrode of a three-electrode ion source can be made of wire mesh with high transmittance for the reasons mentioned above, it is easy to degas through electron bombardment. It is possible to degas at a high temperature similar to the sound of a BA type vacuum gauge with a small power consumption of less than watts, and there is no problem at all even when using it in an ultra-high vacuum region.

Although the present invention has been described above based on one embodiment installed in a quadrupole mass spectrometer, this ion source is not limited to this, and can be applied to other types of analyzers, such as magnetic field deflection type. mass spectrometer, helium leak detector,
It is clear that it can be used for all kinds of vertebral ion sources, such as ionization vacuum gauges and ion beam monitors.

(Effects) As described above, the present invention uses a grid, wire mesh, etc. that allows electrons to pass through the anode in an electron impact ion source based on a three-pole structure of an anode, a hot cathode, and an ion extraction electrode. At the same time, a similar grid or wire mesh is placed on the cross-section side of the hemisphere according to the cross-section and joined to the hemisphere to form an integral structure, and a hot cathode filament is placed on the outer periphery of the hemisphere. In addition, as a result of placing an ion extraction electrode on the cross-section side of the anode, we were able to obtain an ion source that is small and very easy to degas, but has extremely low ion energy dispersion and is extremely sensitive. . As a result, not only has it become possible to perform residual gas analysis on the io-'Pa level without using a secondary electron multiplier, but the gas emitted from the ion source is also extremely small, making it possible to perform highly reliable residual gas analysis. We have now been able to provide a mass spectrometer that can perform these functions.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of a quadrupole mass spectrometer equipped with the present invention.
, 2 and 3 are perspective views, respectively, of a closed hemispherical anode according to the present invention, FIGS. 4 to 8 are perspective views, respectively, showing other embodiments of the closed hemispherical anode, and FIGS. Figure 12 is a perspective view showing the structure of the ion extraction electrode, Figure 13 is a schematic diagram of the power supply of a quadrupole mass spectrometer equipped with the present invention,
FIG. 14 is a graph showing the relationship between the potential of the anode with respect to the ground and the current of the ion collector when the ion source of the present invention is used. In the figure, 1 is a closed hemispherical anode, 2 is a hot cathode filament,
3 is ion extraction' polarization. Figure 2 Figure 5 Figure 6 Figure 3 Figure 4 Figure 7 Figure 8 Figure 11 Figure 12

Claims (3)

[Claims] (1) In an electron impact ion source having at least a three-pole structure of a hot cathode, an anode, and an ion extraction electrode, the anode is formed into a substantially hemispherical shape using an electron-transmissive member such as a metal grid or wire mesh through which electrons can pass. a closed hemispherical anode formed integrally by bonding an electron-transmissive member such as a metal grid or wire mesh to the open edge side of the substantially hemispherical anode; and a hot cathode disposed around the outer periphery of the hemispherical side of the anode. A closed hemispherical anode electron impact ion source is constructed by arranging an ion extraction electrode on the cross-sectional side of the anode. (2) In an electron transmitting member such as a metal grid or wire mesh that is bonded to the open edge side of the approximately hemispherical anode, the shape of the central part is made different from the shape of the other parts to increase the transmittance of electrons or ions. A closed hemispherical anode electron impact type ion source according to claim 1, wherein the closed hemispherical anode electron impact ion source is higher than other parts. (3) In an electron-transmissive member such as a metal grid or wire mesh that is bonded to the open edge side of the approximately hemispherical anode, the shape of the central part is made different from the shape of the other parts, so that the direction of movement of electrons or ions is controlled. The closed hemispherical anode electron impact ion source according to claim 1, wherein the change in the central portion is significantly different from that in other portions.