JP2000077025A - Quadrupole mass spectrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain a measurement for a wide mass range and a supersensitive measurement for a light mass area. SOLUTION: In this quadrupole mass spectrometer, RF outputs of a lower frequency f1 and a higher frequency f2 are obtained by two oscillation circuits 35 and 36. These RF outputs are switched by a switch 41 and sent to an RF amplifier 31 for amplification. Switches 42 and 43 are switched in association with the switching of the switch 41. Thereby an f1 resonance circuit 33 side or an f2 resonance circuit 34 side is selected to apply a superimposed voltage of a high frequency voltage V and a dc voltage U to a quadrupole electrode 17 accommodated in a vacuum container 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quadrupole mass spectrometer, and more particularly to an improvement of a quadrupole mass spectrometer suitable as a gas monitor or a leak detector for analyzing and detecting process gas components in a vacuum chamber. About.

[0002]

2. Description of the Related Art In a quadrupole mass spectrometer, two pairs of elongated counter electrodes (quadrupole electrodes) whose surfaces are hyperbolic are arranged at 90 ° intervals, and the opposing electrodes are the same. A voltage in which a DC voltage U and a high-frequency voltage Vcosωt are superimposed is applied between each pair which is different from the potential by 90 °, and the U / V ratio is kept constant and V is changed so that the voltage in the counter electrode is changed. Are selectively passed according to the ratio of (mass / charge number).

In a semiconductor process, this quadrupole mass spectrometer is generally used for online monitoring of gas components during the process. In other words, a gas in a vacuum chamber for semiconductor processing is introduced, ionized by electron bombardment, and the generated molecular ions are accelerated / decelerated by a coaxial lens to which a DC voltage is applied, and the beam shape is adjusted and incident on the quadrupole electrode. I do. The incident ions are separated according to a specific (mass / charge number) ratio while passing through a quadrupole electrode to which a DC voltage U and a high-frequency voltage V of about several MHz are superimposed and applied. . The separated ions are collected by a drawing lens after passing through a quadrupole electrode and detected by a current detector. This allows
Obtain a mass spectrum, perform quantitative analysis of the gas inside the vacuum chamber from the peak intensity of the mass spectrum,
Thus, the state inside the vacuum chamber is monitored.
It is also used to detect a leak in the vacuum chamber after maintenance of the vacuum chamber.

[0004] This quadrupole mass spectrometer has the property that adjacent masses cannot be analyzed unless ions vibrate a certain number of times while passing through the quadrupole electrode. Therefore,
In designing a quadrupole mass spectrometer, determine the ion energy, frequency, and length of the quadrupole electrode, etc., so that the number of oscillations of all ions in a wide mass range required for measurement can be sufficiently secured. I have to.

At the time of analysis, the high frequency voltage V applied to the quadrupole electrode is represented by the following equation, where M is the mass, r is the bore diameter of the quadrupole electrode, and f is the frequency of the high frequency. V (volt) = 7.219 * M (amu) * f ² (MHz) * r ² (cm) Similarly, the DC voltage U applied to the quadrupole electrode is: U (volt) = 1.212 * M (amu) * F ² (MHz) * r ² (cm) ...

From the above equations, it can be seen that as the frequency is increased to ensure sufficient mass separation, the required high frequency voltage increases with the square of the frequency. Therefore, if the frequency of the ions is set too high, the required high-frequency voltage becomes too large, and as a result, a huge high-frequency power supply is required, and the insulation required for the vacuum feedthrough that applies a potential to the quadrupole electrode is required. There is a problem that the creeping distance for ensuring the withstand voltage becomes long and the vacuum vessel including the quadrupole electrode becomes too large. Therefore, in practice, the size of the apparatus is set to a size that does not pose a problem in practical use. Therefore, the frequency f is set so that the maximum frequency voltage V (voltage at the maximum mass) required for analysis is about several kV. Often do.

Assuming that the frequency of the high-frequency voltage V is determined in this way and the length of the quadrupole electrode is predetermined, the incident energy of the ion can be obtained at the required number of vibrations over all mass numbers. It is set to pass through the quadrupole electrode with a certain amount of energy.

[0008]

However, the conventional quadrupole mass spectrometer has a problem in analyzing only a gas having a low mass (for example, He gas), particularly when used as a gas monitor or a leak detector. There is. That is, the incident energy and the frequency of the high-frequency voltage V determined as described above are not optimal values for a gas having a low mass. In a quadrupole mass spectrometer, there is an incomplete quadrupole electric field at the entrance of the quadrupole electrode (hereinafter simply referred to as "edge electric field"), and therefore, as described above, the necessary vibration over the entire mass range If ions are made to enter the quadrupole electrode with such a slow energy that the number of times can be obtained, the loss of ions in the end electric field region increases.

[0009] In view of the above, the present invention provides, in addition to the measurement in a wide mass range, a low mass region, in particular, He or the like.
It is an object of the present invention to provide a quadrupole mass spectrometer improved so that ions can be detected with higher sensitivity.

[0010]

According to the present invention, there is provided a vacuum vessel having a gas inlet, and an ion source for ionizing a gas component introduced from the gas inlet by electron impact. A lens for accelerating or decelerating the ions and adjusting the beam shape thereof, a quadrupole electrode through which the ions are incident, and collecting the ions passing through the quadrupole electrode to detect the current. In a quadrupole mass spectrometer equipped with a current detector, either a high-frequency voltage having a frequency corresponding to the entire measurement mass range or a high-frequency voltage having a higher frequency dedicated to low-mass measurement is selected and superimposed on a DC voltage. Then, a voltage application circuit for applying the voltage to the quadrupole electrode is provided.

In the voltage in which the high frequency voltage applied to the quadrupole electrode and the DC voltage are superimposed, the frequency of the high frequency voltage is
It is possible to select between a frequency corresponding to the entire measured mass range and a higher frequency dedicated to low mass measurement. So by choosing a normal (lower) frequency,
Measurement can be performed over a wide mass range. With it,
By selecting a higher frequency, it is possible to secure the number of oscillations and maintain the resolution even for ions having a low mass, so that when analyzing a gas such as He with a low mass, the incident energy of the ions is increased and the end electric field region is increased. Measurement at higher sensitivity by reducing the loss in Therefore, there are two modes: a gas analysis mode for the entire mass range at a normal frequency, and a high-sensitivity gas analysis mode dedicated to a low mass at a higher frequency.
One measurement can be measured by one analyzer.

[0012]

Next, embodiments of the present invention will be described in detail with reference to the drawings. In FIG. 1, a quadrupole electrode 17 and the like are sealed in a vacuum vessel 10, and the vacuum vessel 10 is provided with a side wall 6 of a vacuum chamber to be measured.
1, the gas in the vacuum chamber is introduced into the vacuum vessel 10 through the gas introduction orifice 11. The vacuum vessel 10 is a vacuum pump (turbo molecular pump) driven by a rotary pump 22.
21 is evacuated to a vacuum.

The gas introduced from the orifice 11 is ionized in an ion source 12. This ion source 12
Has a filament 13 of about -100 V,
A repeller 14 having a voltage of 100 V or less is provided.
The ion source 12 has a potential of about 0 to 20V. The ionized molecules are accelerated or decelerated via the coaxial lenses 15 and 16, the beam shape is adjusted, and the ions are incident on the quadrupole electrode 17. The lenses 15 and 16 are set to potentials of 0 to -100 V and 0 to -30 V, respectively. A superimposed voltage of the high frequency voltage V and the DC voltage U is applied to the quadrupole electrode 17 as described later. In addition, the introduction of the voltage into the vacuum vessel 10 is performed by the vacuum feedthrough 2.
3 is performed.

Since the central potential of the quadrupole electrode 17 is kept at 0 V, the ions from the ion source 12
Due to the potential difference from 2, the light enters the quadrupole electrode 17. The ions incident on the quadrupole electrode 17 are separated according to a specific (mass / charge number) ratio while passing through the quadrupole electrode to which the DC voltage U and the high-frequency voltage V are applied in a superimposed manner. Is done.
After passing through the quadrupole electrode 17, the separated ions are collected by a pull-in lens 18 (having a potential of about -20 V) and detected by a current detector 19 such as a Faraday cup or a secondary electron multiplier. (In the secondary electron multiplier,
A voltage of 1 kV to -3 kV is applied). In this way, a mass spectrum is obtained, and quantitative analysis of the gas inside the vacuum chamber is performed from the peak intensity of the mass spectrum, thereby monitoring the state inside the vacuum chamber. In addition, a leak in the vacuum chamber after maintenance of the vacuum chamber can be detected.

The circuit for applying the voltage (U + V) to the quadrupole electrode 17 includes an RF amplifier 31 and a DC amplifier 32 as shown in the figure. The RF amplifier 31 has frequencies f 1 and f of several MHz from two high-frequency oscillation circuits 35 and 36.
One of the two outputs (f1 <f2) is selected by the switch 41 and input. The output of the RF amplifier 31 and the output of the DC amplifier 32 are passed through a switch 42 to the f1 resonance circuit 33 or f2
To the resonance circuit 34, and these resonance circuits 33, 34
Is output to the quadrupole electrode 17 via the switch 43. The switches 41, 42, and 43 are interlocked, and when the switch 41 selects the high frequency of f1, the switches 42 and 43 are switched to the f1 resonance circuit 33 side.
When a high frequency of f2 is selected in step 1, the switches 42 and 43 are switched to the f2 resonance circuit 34 side.

The frequency f amplified by the RF amplifier 31
The high frequency voltage V of 1 or f2 is set by the mass setting voltage, and the feedback control is performed so that the high frequency voltage actually applied to the quadrupole electrode 17 becomes the set voltage. The resonance circuits 33 and 34 are composed of a high-frequency transformer for boosting a high-frequency voltage, and superimpose the high-frequency voltage V and the DC voltage U on the secondary coil side. The detection circuits 51 and 52 detect the voltage after the superimposition, and the detection output corresponds to the high-frequency voltage V. This detection output is sent to a comparator 54 via a switch 44 interlocked with the above-mentioned switches 41, 42, 43, and is compared with a mass setting voltage. The gain of the RF amplifier 31 is determined based on the comparison result. As a result, feedback control is performed so that the high-frequency voltage V actually applied to the quadrupole electrode 17 becomes the mass setting voltage.

The detection circuits 51 and 52 comprise, for example, a capacitor voltage dividing circuit and a diode bridge rectifying circuit.
DC that corresponds to RF voltage by rectifying the divided RF voltage
This is the circuit that creates the voltage. As a result, conversion is performed such that the detection output becomes a controllable voltage of, for example, about ± 15 V at the maximum. Since the efficiency of the detection circuits 51 and 52 differs depending on the frequency, the output of the detection circuit 52 for detecting the RF voltage of the frequency f2 is supplied to the DC gain adjuster 5.
The adjustment is made at 3.

DC voltage U output from DC amplifier 32
Is a constant ratio with respect to the high frequency voltage V thus determined. That is, the U / V ratio is always constant according to the above equation. Therefore, the detection output corresponding to the high frequency voltage V is used as a gain setting signal of the DC amplifier 32.

For example, the bore diameter r of the quadrupole electrode 17 =
Assuming that analysis is performed at a frequency of 2 MHz up to a maximum mass number of 100 at 1 cm, the maximum high-frequency voltage V is 2.88 kV according to the above equation. That is, in this case, the maximum high-frequency voltage V that can be applied to the quadrupole electrode 17 can be up to about 2.88 kV. So, He
(Mass number: 4) is separated so that the peak value comes just at half of this voltage.
Can be set to about 7 MHz.

That is, the oscillation frequency f1 of the oscillation circuit 35
Is 2 MHz, and the oscillation frequency f2 of the oscillation circuit 36 is 7 MHz.
By setting the switches 41 to 44, measurement can be performed at a lower frequency f1 over a wide mass range (up to a mass number of 100), and at a higher frequency f
2 enables measurement with low mass. In this case, f2 is f1
Is about 3.5 times higher, so even if the ion injection speed is increased to 3.5 times (about 12 times in energy),
The number of vibrations of the ions in the quadrupole electrode 17 is the same as that of f1. Therefore, when the frequency is switched to f2, the energy of the ion source 12 is increased to increase the incident speed of the ions, thereby increasing the speed of passing through the end electric field and increasing the sensitivity. And maintain the resolution.

It should be noted that the above is merely an example of the embodiment, and it goes without saying that the specific configuration and the like can be variously changed without departing from the spirit of the present invention.
For example, in the above description, two oscillation circuits 35 and 36 are used to obtain RF outputs of two frequencies f1 and f2, and the outputs are switched by the switch 41. However, one voltage-controlled oscillation circuit is used. Alternatively, the frequency can be switched by an external control voltage. In addition, as for the resonance circuit, in FIG. 1, the two resonance circuits 33 and 34 are switched and used, but the resonance frequency may be changed by using a variable capacitor as the resonance capacitor in one resonance circuit. The fixed frequency can be switched (disconnected) by a relay or the like to change the resonance frequency.

This resonance circuit can be further configured as shown in FIG. In this FIG.
The resonance circuit 71 includes a high-frequency transformer having a secondary coil 73 for f1 and a secondary coil 74 for f2 having a common primary coil 72, and an additional resonance capacitor 76 that can be inserted or disconnected by a switch 75. Is included. The switch 75 is operated in conjunction with the switches 41 to 44 (FIG. 1), and the output of the secondary coil 73 or 74 is selected according to the ON / OFF of the switch 75 (the switch 4 of FIG. 1).
3).

In the above, a pressure difference is provided between the vacuum chamber containing the gas to be analyzed and the vacuum vessel 10 containing the quadrupole electrode 17 by the orifice 11. Although the pulling is performed, it goes without saying that the present invention can be applied to a case where the orifice 11 and the vacuum exhaust unit 21 are not provided.

[0024]

As described above, according to the quadrupole mass spectrometer of the present invention, the gas analysis mode for the entire mass range at a lower frequency and the high-sensitivity gas analysis mode dedicated to a low mass at a higher frequency Can be measured by one analyzer, which is suitable as a highly sensitive process gas monitor or leak gas monitor.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing another embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Vacuum container 11 Orifice 12 Ion source 13 Filament 14 Repeller 15, 16 Lens 17 Quadrupole electrode 18 Retraction lens 19 Current detector 21 Vacuum exhaust (turbo molecular pump) 22 Rotary pump 31 RF amplifier 32 DC amplifier 33 Resonance for f1 Circuit 34 Resonance circuit for f2 35 Oscillation circuit for f1 36 Oscillation circuit for f2 41 to 44 Switch 51, 52 Detection circuit 53 DC gain adjuster 54 Comparator 61 Side wall of vacuum chamber 71 f1 · f2 common resonance circuit 72 Primary coil 73, 74 Secondary coil 75 Switch 76 Additional resonance capacitor

Claims (1)

[Claims] A vacuum vessel having a gas inlet, an ion source for ionizing a gas component introduced from the gas inlet by electron impact, a lens for accelerating or decelerating the ions and adjusting the beam shape thereof, A quadrupole electrode into which ions are incident via the lens, and a quadrupole mass spectrometer including a current detector that collects ions passing through the quadrupole electrode and detects the current, A voltage application circuit for selecting one of a high-frequency voltage of a frequency corresponding to the whole and a high-frequency voltage of a higher frequency dedicated to low-mass measurement, superimposing the DC voltage on the DC voltage, and applying the voltage to the quadrupole electrode. A quadrupole mass spectrometer.