JPH0354829B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention modulates an ion current representing a partial pressure of a gas phase in a vacuum, and obtains an AC ion current as an AC amplification,
This invention relates to an AC detection type mass spectrometer that uses phase detection to improve the signal-to-noise ratio, improve the amplification limit of minute currents, and improve stability.

Conventional mass spectrometers (residual gas analyzers), regardless of type, ionize gas molecules by electron bombardment and separate the produced ions electrically or magnetically by their mass. However, the sensitivity of the ion source used as a mass spectrometer varies slightly depending on the type, but is approximately 0.03.
The ion current is between ~0.3×10 ^-3 A/Torr, so when measuring partial pressure in an ultra-high vacuum below 10 ^-8 Torr, the ion current will be very weak, below 10 ^-12 A. I end up. Conventionally, the method of amplifying this weak current has been to use a DC amplifier using a varactor, diode, bridge input operational amplifier, etc., but such DC amplifiers not only cause offsets and drifts, but also When there are various electromagnetic devices nearby, or when remote measurements are being performed, the true signal is disturbed by noise from the peripheral devices, making accurate partial pressure measurement difficult. In particular, it is nearly impossible to measure partial pressure during motion in acceleration or nuclear fusion devices that are subject to strong electric or magnetic fields.

In addition, for measurements below 10 ^-10 Torr, a counting measurement method is used that uses a secondary electron multiplier to amplify and obtain the incident charged particles in a pulsed manner, but with this method, the absolute value effect of counting is In addition to being difficult, it requires a high-voltage power supply and high-grade counting circuit, and as a result, many of the models are large, making the device expensive.

Therefore, the present invention has been made in view of the above-mentioned conventional drawbacks, and its purpose is to:
The ion current (ion beam) from the ion source, which is obtained in vacuum in proportion to the pressure by electron bombardment, is applied to an AC electric field or AC magnetic field ( The ionic current is modulated by the action of (modulation), and the ionic current obtained through the ion collecting electrode is extracted as an alternating current signal, and only the modulated component of the ionic current is amplified, and the amplified alternating current signal (modulated signal) By synchronizing the detection and smoothing the detection output with a low-pass filter, the electrical noise is removed, and offsets and drifts that occur in the amplifier are eliminated, making accurate measurement possible. with and without secondary electron amplifier tube
The object of the present invention is to provide an AC detection type mass spectrometer that makes it possible to measure partial pressures below 10 ^-10 Torr.

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

First Embodiment (See FIG. 1) In this embodiment, a grid-shaped modulation electrode is arranged between the mass spectrometer and the ion collection electrode to modulate the ion electrode.

1 is a hot cathode electron impact ion source, and the ion source 1 may be of any type such as Nier type, BA gauge type, cold cathode magnetron type, etc., and the type is not limited and can be freely selected. I can do it. Note that the ion current obtained from the ion source 1 is proportional to the gas pressure in vacuum.

2 is a mass spectrometer, and the analyzer used in the mass spectrometer 2 is a magnetic field deflection type, a high frequency resonance acceleration type,
Any type such as a quadrupole type (mass filter) may be used as long as the ions are separated by mass, and the type is not limited and can be freely selected.

Reference numeral 3 denotes an ion collection electrode that collects ions that have passed through the mass spectrometry section 2, and the ion collection electrode 3 may be a flat disk, a Faraday box, an electrode with a secondary electron multiplier electrode, etc. Any material that can collect the ions that pass through can be used, and can be freely selected.

A modulation electrode 4 is arranged between the mass spectrometer 2 and the ion collection electrode 3. The modulation electrode 4 is a donut-shaped metal disk 5 with an inner diameter of about 5 mm and a transmittance of 98.
A tungsten mesh 6 of 100% mesh is stretched, and a voltage in which a DC component and an AC component (modulation) are superimposed is applied to the modulation electrode 4, and the mass components are detected in the mass spectrometer 2. Converts the divided ionic current into an alternating current signal. Here, by changing the bias power supply 8 that supplies the DC component via the modulator 7, the region to be modulated can be selected. Furthermore, by changing the modulation amplitude of the modulator 7, the modulation rate can also be changed. Note that the shape of the modulation electrode 4 is not limited to that of the embodiment, and may be any shape as long as it can modulate the ionic current by changing the voltage applied to the modulation electrode.
Its shape and configuration, such as a cylindrical shape, can be freely selected.

Reference numeral 9 denotes a shield electrode attached to the ion collecting electrode 3. The shield electrode 9 is a protective electrode for reducing the induced current induced in the ion collecting electrode due to modulation. It is a sieve-type electrode covered with a tungsten mesh 10 of the same type as the tungsten mesh used, and is attached so as to wrap around the ion collecting electrode 3, giving a potential of 0 to -50V. The ion current collected at the ion collection electrode 3 is led out of the vacuum wall through a coaxial vacuum terminal (not shown) and converted into a voltage by a minute alternating current amplification 11. Furthermore, only the modulation component of the output is amplified by the AC amplifier 12.

The amplified signal is obtained by dividing the signal from the modulator 7, removing the DC component with a cut-off capacitor 13, passing it through a phase shifter 14, making it in phase with the amplified signal, and inputting it into an analog multiplier 15. The output is passed through a low-pass filter 16 and output. The phase shifter 14, the analog multiplier 15, and the low frequency shifter 16 constitute a phase discrimination detector.

Second Embodiment (See Figure 2) In this embodiment, a modulation electrode 4 similar to the first embodiment was placed between the ion source 1 and the mass spectrometer 2 to modulate the ion current. In this embodiment, since no induced current due to modulation is generated in the ion collecting electrode, there is no need to attach a shield electrode to the ion collecting electrode 9.

Other structures and functions are the same as those of the first embodiment.

Third Embodiment (See FIG. 3) In this embodiment, the ion current is changed to alternating current by changing the electric field within the mass spectrometer without using a modulation electrode.

In this embodiment, a quadrupole mass spectrometer 20 is used as a mass spectrometer, and the analyzer 20 and modulator 7 are connected to a power source 21 that provides a DC component of 0 and an AC component.
It is connected via a frequency-voltage converter 22 which provides vcoswt.

The analyzer 20 from the ion entrance port 23 of the analysis section
The ion current Ii (sensitivity) from which ions incident on the quadrupole field surrounded by the analysis rod 24 reach the ion collection electrode 3 depends on the frequency (usually several MHz) of the high frequency applied to the analysis rod 24. proportionally,
It is known that it is inversely proportional to resolution. Therefore, the frequency of the high frequency is changed according to the voltage change of the modulation (low frequency). That is, if FM modulation is performed, the ion current obtained through the collector becomes an alternating current signal.

In addition to the circuit of the power supply 21, the analysis rod 24
A separate circuit connecting the voltage converter 22 and the voltage converter 22 is provided with a power source 25 for providing a high frequency component, and the frequency of the high frequency is modulated through the voltage converter 22 by a modulator 7 which is a low frequency oscillator. 26 is an ion outlet of the analysis section.

Other configurations and operations are similar to those of the first embodiment.

Fourth Embodiment (See Figure 4) This embodiment shows another embodiment in which the ion current is changed to alternating current by changing the electric field within the mass spectrometer without using a modulation electrode. In place of the power source that provides a high frequency component used in the third embodiment, a high frequency oscillator 27 that provides an AC vcoswt is used. Therefore, a voltage converter is not required in this embodiment.

As mentioned above, it is known that the ion current from the ions incident on the quadrupole field reaching the ion collection electrode 3 is inversely proportional to the resolution, but the resolution is also given by 0.126/0.1678-U/V. It has been known. Here, U is the DC component applied to the two analysis rods 24, and the high frequency oscillator indicates the magnitude of the amplitude of the high frequency wave applied to the analysis rods. Thus, by modulating U or V, the resolution will be modulated and thus the ion current reaching the ion collection electrode will be modulated.

Although a quadrupole mass spectrometer is used as the mass spectrometer in both the third and fourth embodiments, a magnetic field deflection type mass spectrometer may also be used instead.

As described above, according to the present invention, the ion current is modulated by the action of an alternating current electric field or an alternating magnetic field within the interval until the ion beam emitted from the ion source is attracted to the ion collecting electrode, and the ion collecting electrode The ionic current is extracted as an alternating current signal through the ionizer, and the alternating current is amplified by an alternating current microcurrent amplifier, then applied to a phase discrimination detector in synchronization with the modulation signal, and the partial pressure of the gas caused by the ions is measured from the output. This not only allows the amplifier to operate for long periods of time without causing drift or offset, but it is also resistant to external noise, ensuring high accuracy and reliability even with minute signals buried in noise. Because it can be detected with high sensitivity, it can be measured during operation even in severe noise from accelerators and nuclear fusion reactors, and remote measurement over 100 meters is also possible.
It is now possible to measure partial pressures in fields that were previously impossible to measure.

Furthermore, according to the present invention, since the phase-discriminative detection method is used, if the time constant of the low-pass filter is increased, the amplification limit of minute currents can be up to about 10 ^-13 Torr, so secondary electron multiplication is possible. This makes it possible to measure partial pressures of 10 ^-10 Torr or less even without a tube.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an analyzer showing a first embodiment of the present invention, FIG. 2 is a block diagram of an analyzer showing a second embodiment of the present invention, and FIG. 3 is a block diagram of an analyzer showing a third embodiment of the present invention. FIG. 4 is a block diagram of an analyzer showing a fourth embodiment of the present invention. 1 is an ion source, 2 is a mass spectrometer, 3 is an ion collection electrode, 4 is a modulation electrode, 7 is a modulator, 9 is a shield electrode, 13 is a cut-off capacitor, 20 is a quadrupole analyzer, 21 is an electrode, 22 is a voltage converter;
27 is a high frequency oscillator.

Claims (1)

[Claims] 1. In order to find out the molecular density of a specific type of gas molecule in a vacuum device, the molecule is ionized by electron bombardment,
The ions obtained by the ionization are separated according to the charge-to-mass ratio of the ions by the action of an electric field or magnetic field appropriately placed in the analysis section, and the type of molecule is determined from the magnitude of the ion current. In a mass spectrometer designed to determine molecular density, the ion current emitted from the ion source is modulated by the action of an alternating current electric field or alternating magnetic field before reaching the ion collecting electrode, and the ion current is obtained through the ion collecting electrode. The ion source and ion collecting electrode are designed so that the current can be extracted as an AC signal, only the modulation component of the ion current can be AC amplified, and the amplified signal and the AC signal used for modulation can be synchronized and detected. An alternating current detection type mass spectrometer characterized in that a modulation electrode is arranged at an appropriate position between the two. 2. The alternating current detection type mass spectrometer according to claim 1, wherein the modulation electrode is disposed between a mass spectrometer and an ion collection electrode. 3. Claim 1, characterized in that the modulation electrode is disposed between a mass spectrometer and an ion source.
The AC detection method mass spectrometer described in . 4. 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.
In a mass spectrometer, the ions obtained by ionization are separated according to the charge-to-mass ratio of the ions by the action of an electric or magnetic field appropriately placed in the analysis section, and the molecular density of the ions is determined. , the ion current emitted from the ion source is modulated by the action of an alternating current electric field or alternating magnetic field before reaching the ion collecting electrode, and the electrode obtained through the ion collecting electrode is extracted as an alternating signal, and the ion current is In order to amplify only the modulation component and synchronize and detect the amplified signal and the AC signal used for modulation, a power source that supplies the DC component U and an AC power source are connected between the mass spectrometer and the modulator. component vcoswt
An AC detection mass spectrometer characterized by having a frequency-voltage converter or a high-frequency oscillator arranged to give 5. The AC detection type mass spectrometer according to claim 4, wherein the mass spectrometer is a quadrupole type mass spectrometer. 6. The AC detection type mass spectrometer according to claim 4, wherein the mass spectrometer is a magnetic field deflection type analyzer.