JPH0354828B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an AC detection type mass spectrometer for determining the partial pressure of a gas phase in a vacuum. Conventional mass spectrometers (residual gas analyzers), regardless of type, ionize gas molecules by electron bombardment, and ionize the generated ions electrically or magnetically using the mass of the ions. The residual gas pressure is 10 ^-12 Torr (10 ^-10 Pa).
The following residual gas analyzers used in the ultra-high vacuum range: 1) are small and lightweight and can be used simultaneously with full-pressure ionization vacuum gauges, 2) can be repeatedly heated and degassed at temperatures above and below 400°C, and 3) analyzer
⁴ ) The ion source must be easily degassed by electron impact. 5) Gas release can be minimized. 6) High sensitivity. 7) Resolution M/AM is 50~ 100, 8) The minimum detectable partial pressure is 10 ^-10 Torr or less, 9) There is no component of electron-impact desorption ions, 10) The pumping speed of the ion source is low, etc. It is necessary to meet the following conditions.

However, in order to measure residual gas pressure, etc. in an ultra-high vacuum, the sensitivity of the ion source becomes a problem, and it is difficult to simultaneously satisfy the conditions in item 4, 5, and 10 above. This is because if the electron current of the ion source is amplified to increase the sensitivity, both the gas release rate and the pumping rate will increase, disturbing the measurement target. Further, although ultrahigh sensitivity can be obtained with a magnetron type ion source, the conditions of items 2, 4, and 10 cannot be satisfied.

The mass filter type (quadrupole type) that has variable resolution and can be used only by changes in the electric field has become the most popular mass spectrometer these days, but not only does this mass filter type mass spectrometer not require an analysis magnet, It has the characteristic that even if the energy dispersion of ions is large, it does not affect the analysis accuracy, and the electron impact type BA gauge ion source has become the mainstream ion source that relatively meets the conditions in items 1 to 10 above. There is.
This ion source utilizes the fact that thermionic electrons ejected from the cathode are attracted to a cylindrical cage-shaped anode, and when they vibrate in and out, they collide with gas molecules inside the cage and ionize the gas molecules. A part of these ionized ions is made to escape to the outside through a small hole in the extraction electrode. This ion flow is then passed through an analyzer to analyze the mass of the ions.

There are a wide variety of analytical methods for analyzers, such as magnetic field deflection type, stable orbit type, time-of-flight type, and high frequency resonance acceleration type, but currently mass filter type is the most popular for the reasons mentioned above. It has become mainstream. In the mass filter type analysis method, ions from an ion source are incident on an analysis space surrounded by four analysis rods. (DC voltage and high frequency voltage are applied to this analysis rod in parallel.) When ions fly through the analysis space, they oscillate periodically according to their mass, but ions that do not meet the electrical conditions As time passes, it diverges and continues to increase in amplitude, colliding with the rod and being blocked from passing. That is, by changing the electrical conditions, only ions having masses that meet the conditions can be taken out from the opposite side of the ion entrance.
This gave rise to the name mass filter. The ion flow after passing through the mass filter is captured by an ion collection electrode, read by a DC microcurrent meter, and converted to pressure to determine the partial pressure. In cases such as residual gas analysis in the ultra-high vacuum region, this ion flow is often below 10 ^-12 A or about 10 ^-14 to ^{10 -15} A, so a secondary electron intensifier is installed in the ion flow detector. Often used.

However, although this mass filter type mass spectrometer uses a BA type ion source that can degas with relatively high sensitivity as an ion source, the amount of ions incident on the analyzer is limited to all of the ions generated in the ion source. 10 to 20% of the ion amount, and since the ion source is often surrounded by a metal cylinder,
Electron impact degassing is not sufficient, and a large amount of gas is released, so even the gas released by the ion source itself must be analyzed. Furthermore, when using an electron impact ion source for residual gas mass spectra in ultra-high vacuum, electron impact desorption ions generated when molecules adsorbed on the surface of the anode of the ion source are bombarded with electrons are mixed into gas phase ions. There is also a major problem that makes accurate partial pressure measurement difficult.Therefore, it can be said that the main problem with current mass spectrometers lies in their ion sources.

Also, the ion current after passing through analysis is 10 ^-13 to ^{10 -15} A.
This will amplify a very small current. If such a small current is amplified by a DC amplifier, problems of drift and offset will occur, and it will also be susceptible to external noise. It is nearly impossible to measure it, and therefore not only does a high-grade minute DC amplifier become necessary, but its measurement also requires advanced technology.

In addition, for measurements below 10 ^-14 Torr, a counting method is used in which a secondary electron multiplier is used to amplify and the incident charged electrons are measured in a pulse manner. however,
No matter what method is used, the pressure (partial pressure) must be calibrated at a high pressure of 10 ^-8 or higher; therefore, in the counting method, calibrating the absolute value at a pressure of 10 ^-8 or lower requires a large number of counts. The current situation is that it is becoming increasingly difficult to count as the numbers get older.

Therefore, the present invention has been made in view of the above-mentioned conventional situation, and when determining the partial pressure of a gas phase in a vacuum, the electron impact desorption ions generated from the solid surface are mixed into the gas phase ions, causing an error. Even if it occurs, it can be removed, suppress gas release using a highly sensitive and easily degassed electron impact ion source, and use AC amplification and phase detection to eliminate the need for an electron multiplier.
This makes it possible to measure partial pressures below 10 ^-11 Torr.

Therefore, in the present invention, the electron impact ion source
A hot cathode is disposed along the outer periphery of an anode formed in a substantially hemispherical shape, and is opposed to the open end of the anode,
By providing an alternating current detection type mass spectrometer in which a lattice-shaped ion extraction electrode is provided that gradually expands toward the open end, and a modulation electrode is arranged between the ion source and the analysis section of the analyzer. It is designed to achieve the intended purpose.

The present invention will be described below with reference to an illustrative embodiment.

Reference character a is an ion source of an AC detection type mass spectrometer A, which is composed of an anode 1, a hot cathode 2, an electron reflector 3, and a shield electrode 4. Anode 1
is formed into a substantially hemispherical shape by pressing a molybdenum wire mesh, the diameter of which is 14 mm, and the transmittance of the wire mesh is 90% or more. The hot cathode 2 is formed in an annular shape by an iridium wire coated with sodium oxide, and the cathode heating power is very small at 3 watts or less when an electron current of 2 mA is applied, and is arranged along the outer periphery of the anode 1. There is. The electron reflector 3, which is an ion extraction electrode, is formed of molybdenum in the shape of a trumpet that gradually expands, and has a central hole diameter of about 4 mm. The electron reflector 3 is provided with its enlarged open end facing the open end of the anode 1. Like the anode 1, the shield electrode 4 is formed into a substantially hemispherical shape by pressing a molybdenum wire mesh, and is attached to cover the anode 1 and the hot cathode 2. This allows the ion source a
is electrostatically shielded and becomes a highly sensitive ion source with stabilized electron vibrations.

To explain the operation of this ion source a, electrons flying out of the hot cathode 2 placed at a negative potential are
The electrons are attracted to the anode and enter the inside of the hemisphere, but are reflected by the electron reflector 3, which is placed at a more negative potential than the filament, and are again attracted to the anode 1, accelerated, and fly out of the anode 1. This time, the electrons reflector 3 By being reflected by the shield electrode 4 placed at the same potential as the anode, vibrations are repeated inside and outside the anode. However, the electric field generated by the combination of these electrodes has the effect of concentrating these electron oscillations on the central axes of these electrodes, so most of the oscillating electrons are concentrated on the central axes, that is, the first
Since it vibrates in the x-y direction shown in the figure, ion generation is also concentrated on this axis. Among the ions generated on this axis, those generated between the anode 1 and the electron reflector 3 are accelerated by the electron reflector 3 and can easily fly out of the ion outlet of the electron reflector 3, but the wire mesh of the anode 1 Since it is a pressed mesh, the on-axis electron transmittance is very high and the sensitivity can be significantly increased. Therefore, although the anode is extremely small with a radius of 14 mm, the sensitivity can be reduced to 13 Torr ^-1 at the ion exit of ion source a, which is 1/10 to 1/20 of the volume of a conventional ion source of this type. In addition, because of its small size, even when the three electrodes of cathode 1, electron reflector 3, and shield electrode 4 are electrically coupled to perform electron impact degassing,
1200℃~1300℃ with relatively low power of 600V, 100mA
High-temperature degassing is possible, and compared to conventional ion sources, the emitted gas of this ion source a is negligibly small.
Even pressure in the 10 ^-12 Torr range did not interfere.
Furthermore, since the ion source a opens toward the vacuum side P with a solid angle of 180 degrees, it is very suitable for measuring the partial pressure of gas.

What is further noteworthy about this ion source a is that because the ions are concentrated on the x-y line, gas phase ions and electron-impact desorbed ions can be completely separated, which was nearly impossible in the past. In other words, the ions released by electron impact are captured by the anode 1 after the oscillating electrons vibrate many times, but at that time, if there are gas molecules chemically adsorbed to the electrode 1, they are ionized by the electron impact. Ions are ejected from the solid surface, and this is called an electron-impact desorption ion. However, with this electrode arrangement, the desorbed ions are separated from the anode 1 and the electron reflector 3.
The ions are accelerated from the highest potential formed by the two electrodes , and fly out of the exit of the ion source a. At this time, the desorbed ions have already ejected from the two electrodes with a kinetic energy of 7 eV, so their average energy is much higher than that of the gas phase. is created between the two electrodes of the anode 1 and the electron reflector 3 on the x-y line, so the average kinetic energy of the ions accelerated towards the electron reflector 3 is smaller than that mentioned above.

For this purpose, a modulation electrode 7 is arranged via a focus electrode 6 between the ion source a and a mass filter analyzer 5 which is the analysis section of the mass spectrometer A, and the potential of the modulation electrode 7 is made positive. When the temperature is raised from a low level to a high level, gas phase ions with low kinetic energy are first reflected by the modulation electrode 7 and cannot pass through, and then electron-impact desorbed ions (which act as an ion filter) becomes unable to pass. Further, if the modulation electrode 7 is formed into a cylindrical shape, the ion beam becomes narrower due to the effect of this electrode, and becomes easier to enter the mass filter analyzer 5. 5' is an analysis rod. For this reason, if the center potential of the modulation electrode 7 is set to be slightly lower than the blocking potential of gas phase ions, and modulation is performed with a small amplitude around this point, only gas phase ions will be modulated. Note that since the desorbed ions pass through, they are not modulated. Furthermore, by setting the center at a potential slightly lower than the blocking voltage of ions desorbed by electron impact, the adsorption state of gas molecules on the surface of the anode 1 can also be determined.

The ions entering the mass filter analyzer 5 are separated into ion masses corresponding to the charge ratio of the ions, exit on the other side, are collected at the ion collection electrode 8, and are measured as an ion current through a conductive wire. This ionic current is converted into voltage by a minute alternating current amplifier 9, and further, only the modulation component is transferred to an alternating current amplifier 10.
is amplified by The amplified signal is sent to modulator 1
The DC component of the signal obtained by dividing the signal from 1 is cut off by a DC component cut-off capacitor 12,
3 to be in the same phase as the amplified signal and input to an analog multiplier 14, and its output is passed through a low-pass filter 15 and output. The phase shifter 13, the analog multiplier 14, and the low frequency filter 15 constitute a phase discrimination detector. 16 is a battery that determines the center potential of the modulation electrode 7;

Figure 2 shows the separation effect of gas phase ions and desorbed ions at the exit of the modulation electrode 7 using an X-Y recorder.
It was performed using a sine wave with a modulation frequency of 12Hz and an electron current of 2mA, and the vertical axis is the output of only the modulation component of the ion current exiting the modulation electrode amplified and rectified into direct current.

In the above embodiments, a mass filter type was used as the ion analyzer, but the analyzer is not limited to the mass filter type, and may be a magnetic field deflection type, a stable orbit type, a time-of-flight type, or a high frequency type. It can be adapted to all types such as resonance acceleration type. Further, although it is preferable that the modulation electrode 7 has a cylindrical shape as described above, it is not limited to the cylindrical shape, and may be mesh-shaped. The ion source a is not limited to a hemispherical one as in the above embodiment, but may also be a BA type cylindrical wire mesh.

As described above, according to the present invention, a modulation electrode that modulates the ion flow is arranged at the exit of the ion source,
By changing the modulation range, the electron-impact desorbed ions from the ion source and the gas phase ions can be completely separated before being sent to the analyzer.
In addition, the ion current after passing through the analyzer is amplified by an AC microcurrent amplifier, then synchronized with the modulation signal and applied to a phase discrimination detector.The detected output is smoothed by a low frequency detector, and then the ion current is detected by the output. Since the partial pressure of gas caused by Even small signals can be detected with high accuracy and reliability, making it possible to measure during operation even in severe noise from accelerators and nuclear fusion reactors, and remote measurement over 100 meters is also possible. .

Furthermore, according to the present invention, since the phase-discriminative detection method is used, by increasing the time constant of the low-pass filter, the amplification limit of minute currents can be up to 10 ^-13 Torr, which is 2.
Partial pressure measurement below 10 ^-10 Torr is fully possible without using a secondary electron multiplier.

Further, according to the present invention, the shield electrode of the ion source not only allows the ion source to be made small and easy to degas, but also allows the sensitivity to be constant no matter where the analyzer is installed. This has the effect of significantly increasing reliability.

Further, according to the present invention, gas phase ions (partial pressure) and electron impact desorption ions can be separated, which has been nearly impossible until now, and both ions can be subjected to mass spectrometry, which is extremely innovative.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the relationship between the mass spectrometer and the ion source, and FIG. 2 is a graph showing the separation effect between gas phase ions and desorbed ions at the exit of the modulation electrode. A is a mass spectrometer, a is an ion source, 1 is an anode, 2
is a hot cathode, 3 is an electron reflector, 4 is a shield electrode, 5 is an analyzer, and 7 is a modulation electrode.

Claims (1)

[Claims] 1. In order to find out the molecular density of a specific type of gas molecule in a vacuum device, the molecules are ionized by electron bombardment, and the ions obtained by the ionization are sent to an appropriate analysis section. In a mass spectrometer, ions are separated according to their charge-to-mass ratio by the action of an electric or magnetic field placed in the field, and the type of molecule and its molecular density are determined from the magnitude of the ion current. In the step, the ion current from the ion source is modulated, only the modulation component of the separated ion current after passing through the analysis section is AC amplified, and the amplified signal and the modulation signal are synchronized so that phase detection can be performed. a modulation electrode is disposed between the analysis section and the electron impact ion source, and a hot cathode is disposed along the outer periphery of an anode formed in a substantially hemispherical shape of the ion source, and
An alternating current detection type mass spectrometer, characterized in that a trumpet-shaped ion extraction electrode is provided facing the open end of the anode and gradually expanding toward the open end. 2. The ion source according to claim 1, characterized in that the entire ion source is electrostatically shielded with a grid-shaped shield electrode made of wire mesh in order to stabilize ion generation and improve reliability of the ion source. AC detection mass spectrometer.