JPS59105257A - Plasma sampling device and method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for sampling inductively generated plasma in a vacuum chamber via an oriswiss, and a method and apparatus for performing mass spectrometry using such sampling. . This invention will be explained below regarding mass spectrometry.

Detecting and analyzing trace substances requires introducing ions of the substance to be analyzed into a vacuum chamber containing a mass spectrometer. This often requires performing elemental analysis, ie detecting and measuring the relative amounts of individual elements in the trace material. Theoretically, the trace material is separated into its individual elements by introducing the trace material into a high temperature plasma that generates ions of primarily individually charged elements.

The use of a high-temperature plasma as an ion source has the advantage of generating mostly individually charged ions, less interference between the element to be detected and other elements, the availability of isotonic information, and the ionization efficiency of the ion source. It is known that this method has the advantage of being able to generate a large number of ions for analysis since it is very high.

Traditionally, however, plasmas are usually operated at atmospheric pressure and mass spectrometers are operated at 0. Since it is placed in a vacuum chamber,
The problem was that a sample of the plasma had to be extracted from the plasma and directed into the vacuum chamber through a small orifice. Plasma has extremely high temperatures (e.g. 4
, 000° to ~10,000°K) and is a relatively good conductor. When a portion of the hot plasma enters a small orifice, an arc-like breakdown occurs between the plasma and the end of the orifice, damaging the orifice and producing ultraviolet noise that enters the mass spectrometer and prevents ion detection. I know that. This effect, referred to by others as the "pinch" effect, is a major impediment to the use of plasma ion sources.

Boundary layer sampling was attempted to overcome this pinch effect. In this case, the orifice for the plasma to be sampled is placed in a plate with a flat surface that is kept relatively cool. When the plasma contacts this cold plate, a cold boundary layer forms in the immediate vicinity of the plate. The ions are not directly extracted from the plasma (but rather are extracted from this boundary layer. Since the boundary layer is cold (and is therefore a relatively good electrical insulator), the arcing effect is likely to be small. However, ions in the plasma tend to recombine or react in the low-temperature boundary layer, and they also tend to form oxides. , and the formation of oxides greatly complicates the analysis. Sampling with a low-temperature boundary layer is therefore extremely unmarketable.

"Plasma Spectrochemistry 1982 Winter Conference" in Orlando, Florida, USA
In January 1982, Alan G. Gray announced the use of a staged vacuum chamber and a relatively large orifice (which does not create a cold boundary layer), which he said eliminated the drawbacks of the conventional method. However, the published results appear to be applicable only to a limited number of specific conditions. Applicants have experimented with similar sampling systems and staged vacuum chambers, but the results described above were not reproduced.

This invention provides a method and apparatus for sampling plasma in a vacuum chamber through an orifice that minimizes the generation of arcs and ultraviolet noise in the orifice, and also suppresses ion recombination and reactions near the orifice. This is what we are trying to provide. According to the present invention, the device for sampling plasma in a vacuum chamber has at least one winding between the first terminal and the second terminal, and a coil gap necessary for plasma generation is provided between the windings. It consists of means for plasma generation including an electric induction coil and a vacuum chamber having an orifice plate in the wall, the orifice plate having an orifice near the gap,
A part of the plasma entering the vacuum chamber is sampled through this orifice, and the circuit is connected between the terminals to reduce peak-to-peak oscillations of the voltage in the plasma.

The term "vacuum chamber" as used in this specification and claims shall refer to a chamber in which the pressure within the chamber is less than atmospheric pressure. Plasma is generated in the coil by passing a high-frequency current through the coil, and peak-to-peak voltage fluctuations in the plasma are reduced by limiting the fluctuations in voltage across the coil between the ends of the coil, and through an orifice that is part of the plasma. It is constructed by guiding it into a vacuum chamber.

Hereinafter, the present invention will be described in detail based on embodiments shown in the accompanying drawings.

In FIG. 1, an electrical induction coil 12 is wound around a plasma tube 10 (X).

carrier gas (e.g. argon) for plasma formation
is conducted from source 13 via conduit 14 to plasma tube 10 . Another flow of carrier gas is directed into the plasma tube 10 via the inner tube 15 and is present at the flared end 16 just upstream of the coil 12. A sample gas containing trace substances to be analyzed is fed into argon gas from a source 17 and led to the plasma tube 10 in a tube 15 and via a thin tube 18 coaxial therewith. (The sample gas will then be ejected into the center of the plasma to be formed.

Coil 12 is typically powered by an RF power source via impedance matching circuit 22, which has only a small number of turns (four turns in the tested embodiment). coil 12
The power for the plasma varies depending on the nature of the plasma required and is between 200 and 10,000 watts. The energy supplied is at high frequency, in particular 27 MHz. The voltage across coil 12 may be several thousand volts depending on operating conditions.

As shown in FIG. 1, the plasma 24 generated by this configuration
For example, it is under atmospheric pressure.

Plasma tube 10 is positioned adjacent to a first orifice plate 26 forming one wall of vacuum chamber 28 . Although not shown, the orifice plate 26 is cooled by water. Gas from plasma 24 is sampled through an orifice in plate 26 into a first vacuum chamber section 32, which section 32 is evacuated through duct 34 by pump 36. Other gases from the plasma are exhausted through the gap 38 between the plasma tube 10 and the plate 26.

The first vacuum chamber section 32 is connected to the second vacuum chamber section 4 by a second orifice plate 42 having a second orifice 44.
Separated from 0. The second vacuum chamber section 40 is evacuated by a vacuum pump 46. A mass spectrometer 48 is arranged within this second vacuum chamber section 40 . The mass spectrometer is a quadrupole mass spectrometer with rods 50'&. For clarity, the plasma tube 10 in FIG. 1 is shown greatly enlarged compared to the vacuum chamber.

In use, the first vacuum chamber section 32 is maintained at a pressure of approximately 1 Torr and the second vacuum chamber section 40 is maintained at a pressure of about 10 Torr. A portion of the plasma 24 is sampled into the first vacuum chamber section 32 via the first orifice plate 0 .

Ions in the plasma are drawn from the first orifice 30 into the first vacuum chamber section 30 by the gas flowing through the first orifice 30 . These ions also enter the second orifice 44 by the gas flowing into the second orifice.

As previously mentioned, when the system shown in FIG. There are even things.

The arc damages these orifices and the mass spectrometer 4
This generates ultraviolet noise that interferes with the analysis of ions entering the sample. In addition, the ionic properties of the orifice material appear in the mass spectrum and impede analysis.

Undesirable arcing, in this case plasma 24
is concentrated on the first orifice plate 26 of the vacuum chamber 28 remote from the . The large gas flow created by the vacuum through the orifice 30 tends to concentrate on the outside of the orifice plate 26 and removes the cooling layer of gas that attempts to electrically insulate the arc to some extent, thereby increasing arcing. do. If the first orifice 30 is small enough, a cooling layer 51 of gas covering the first orifice plate 26 at the first orifice will tend to exist despite the vacuum suction, but since the orifice 30 is very small, plasma 2
4 less sample is aspirated into the first vacuum chamber section 32 and therefore the ion information is smaller. Also, when the first orifice 30 is very small, it easily melts and gets clogged; when the first orifice 30 is made larger, the cooling layer 51 of gas covering the orifice plate 26
The arc as described above will occur, although it will either fade or disappear.

It has been found that this arcing is caused by large peak-to-peak voltage fluctuations within the plasma itself.

Although it is difficult to measure the voltage in the plasma generated by the high-frequency electric field (because the probe used for the measurement is melted by the plasma and because of undesired RF pickup from the generated electric field), the voltage in the plasma in the configuration shown is The peak-to-peak voltage fluctuations were found to be very large (eg, on the order of 1000 volts or more). With this measurement, the next question becomes how this voltage variation occurs.

Tests were conducted to find the cause of large voltage fluctuations within the plasma, which will be explained with reference to FIG. FIG. 2 shows a circuit for a tuning and impedance matching device 22 that provides RF power to the plasma. This impedance matching device 22 has a terminal 52
It has two variable capacitors C1 and C2 connected in series at terminals 52 and 54, and a power supply 20 connected in parallel to capacitor C1 at terminals 52 and 54. Free terminal 56 of capacitor C2 is connected to terminal 58 at the upstream end of coil 12, and the other end 60 of coil 12 is connected to terminal 54. The direction of gas flow through coil 12 is indicated by arrow 62. The f configuration shown in FIG. 2 exhibited very large voltage fluctuations in the plasma 24.

The first test was to test the terminals immediately downstream of the coil because the long leads from terminals 60 to 54 have an inductance that causes voltage fluctuations at terminal 60 and contributes to voltage fluctuations in the plasma. 60 was to be grounded. This additional grounding produced voltage fluctuations that were less than half of the values originally measured, but large voltage fluctuations remained in the plasma and arcing still occurred.

The impedance matching circuit was then modified as shown in FIG. 3, and the ground at terminal 54 was removed.
Coil 12 was tapped at 64 and tap 64 was grounded. The tap 64 is moved back and forth along the coil, and voltage fluctuations in the plasma 24 are caused by the tap 64 along the coil 12.
Measurements were taken at various locations. The measured values are shown in curve 66 in Figure 6.
Indicated by The absolute value of the plasma peak-to-peak voltage fluctuation value is shown on the vertical axis, and the position of the tap 64 is shown on the horizontal axis. On the horizontal axis, the number "0" indicates that the tap 60 is located downstream or at the end of the coil 12, and the number "4" indicates the location of the terminal 58 upstream of the coil 12. number rl
J, r2J, and "3" indicate the number of different wires in each of the coils 12. The center of the coil 12 is located at position "2" on the horizontal axis in FIG.

In the curve of Figure 4, at point 68vc, tap 6
4 is downstream of terminal 60 between terminals 54 and 60. In FIG. 4, the absolute value of the peak-to-peak voltage fluctuation of the plasma decreases as the tap 64 moves from the downstream end of the coil at ``0'' toward the center of the coil at ``2,'' and reaches its minimum value at the second winding. . The voltage variation then increases as the tap 64 moves toward the upstream end of the coil "4". The voltage at zero point 70 is approximately 13 volts, but RF pickup problems make it difficult to measure the voltage with accuracy to within less than 5 volts absolute. Additionally, a small voltage (on the order of 10 volts) flows within the plasma due to the heating current flowing through the plasma;
Moving the tap 64 obviously does not remove it.

It will be appreciated that the illustrated voltage measurement is an absolute value measurement of voltage fluctuations within the plasma, since the polarity of this voltage cannot be measured. However, it is theoretically expected that the voltage variation measured would be such that the tap 64 reverses phase as it passes through the center of the coil 12.

When the tap 64 is located near the center of the coil (e.g., within about 1/4 turn or less of the center of the coil for a 4-turn coil), arcing at the orifice 30° 44 is eliminated and further energy and energy are passed through the orifice. The energy spread of the ions has become very small.

In particular, as shown in FIG.
The number of ions passing through zero and entering the mass spectrometer 48 is shown, and the energy of these ions in electron volts is shown on the horizontal axis.

A curve 72 shown with a solid line and a black dot shows the measured value when the tap 64 is located 1/4 turn from the end "0" of the coil, and a curve 74 shown with a dashed line and a white dot shows the measured value when the tap 64 is at a position of 1/4 turn from the "0" end of the coil. The measured value is shown at the - position of 13/turn (near the center of the coil) from the end "0" of the coil. For curve 72, a large arc was created through the orifice, resulting in a large observational spread, so that a smooth line connected the points. The energy spread is about 44 electron volts,
The 50% height was about 17 electron volts. The maximum energy of many of the ions exceeded 30 electron volts. High energy and energy~td
(of the ions) greatly reduced the ability of the quadrupole mass spectrometer to analyze the trace material to be sampled. Conversely, curve 74 shows that the energy spread of the ions passing through the orifice is small; At 10% height it was about 11 electron volts and at 50% height it was about 5 electron volts. .

FIG. 6 is a diagram similar to FIG. 5, except that curve 76 is the measured value when tap 64 is 3/4 turn from the "0" end of the coil, and curve 78 is the measurement when tap 64 is 3/4 turn from end "0" of the coil. The measured values are respectively shown when the coil is at the position of 1/4 turn from the end "0" of the coil as before. The results are similar to those described above, but when the tap is near the center of the coil,
The energy diffusion of ions and the average energy of ions are
It has decreased significantly.

The effects of reduced ion energy and spread of ion energy will be explained using FIGS. 7 and 8. This diagram is
The mass spectrum of a 10 millionth solution of the element strontium is shown. The detected ion counts are shown on the vertical axis and the mass in atomic mass units (amu) is shown on the horizontal axis. FIG. 7 shows the mass spectrum when the tap 64 is located 3/4 turns from the bottom end of the coil "0" (as shown by curve 76 in FIG. 6). Figure 8 shows Tatsupro 4 at the 1/4 turn position from the negative end of the coil.
(shown by curve 78 in FIG. 6). In both cases, the full scale value on the vertical axis is
There were 3 x 10' counts per second. In Figure 8, three strontium peaks 80a, 82a and 8
4a (corresponding to 86.87 and 88 atomic mass units, respectively) are clearly identified, and in FIG. 7 the same peaks 80b, 82b and 84b are slightly identified,
Therefore, the maximum level of peak 8''4b is lower than that of 84a. As expected, the small ion energy and ion spread gave large resolution and increased ion information for analysis.

Another feature of the invention is that since there is no need to sample from a cold boundary layer to protect the orifice, the orifice sampling plate can be arranged to reduce or eliminate such a cooling boundary layer. That's the point. This will be explained using FIGS. 9 and 10.

FIG. 9 shows the first orifice plate 26a as a frustoconical orifice structure 88 having a conical side wall 89, a flat top wall 90, and a concave orifice 30a. In use, the top wall 90 tends to form a cooling boundary layer (as shown at 51 in FIG. 1) of gas above the orifice 30a, which insulates the orifice from the plasma to reduce arcing. do.

Unfortunately, because the glasmas are at atmospheric pressure, the ions recombine and react with oxygen (the rate of recombination varies in proportion to the cube of the pressure; The velocity of the pressure changes in proportion to the power cubed). This not only results in a loss of ion signal that can be used for analysis;
Oxides enter the mass spectrometer and complicate analysis.

FIG. 9 shows another first orifice plate 26b with an orifice structure 92 having a sharp edge;
has a conical side wall 94 that reaches a sharp end 96 . End 96 forms first orifice 30b. Because the orifice structure of FIG. 9 does not have a flat surface adjacent the orifice on which a cooling boundary layer can easily form, there is a small or no cooling boundary layer over orifice 30b (for example, plate 26b is even if cooled).

In this way, the plasma to be sampled from orifice 30b is not significantly cooled until it enters vacuum chamber section 32. The pressure within the vacuum chamber section 32 is approximately 1
(76 outside the orifice plate 26b)
0 torr), so the antibonding velocity is about 7
603 vc, resulting in a reaction rate of approximately 760 vc).

An improvement through the use of a sharp-ended orifice structure 92 (taps 64 located near the center of the coil for use without arcing) is shown in FIGS.
Figure 2 shows the mass spectrum obtained for a solution of 10 parts per million of cerium. 11 shows a mass spectrum 98 obtained using the frustoconical orifice structure 88 of FIG. 9, and FIG.
100 mass spectra obtained using the sharp-edged orifice structure 92 shown are shown. The full scale of the vertical axis was 106 counts per second. In Figure 11, the peak at 140 atomic mass units (this is the mass of cerium) is very small, whereas the large peak has a mass of 15 atomic mass units.
6 (cerium oxide) VC present and a smaller peak (
(However, it is larger than the peak of cerium) is located at mass 158 (oxide of cerium isotope).

In contrast, Figure 12 shows a large peak with mass 140 (cerium) and mass 142 (cerium isotope).
shows a considerable peak. Here, the mass is 156
Only a small peak of (cerium oxide) appears, mass 1
58, almost no peak appears. The strong increase in the ion signal of the elemental ions and the corresponding decrease in the amount of oxides generated significantly improves the discrimination of complex spectra obtained when many elements are mixed together (see For Figure 12, the resolution was carefully reduced so that no mass discrimination was made for higher mass oxides).

A further feature of the invention is improved response to elements with high ionization potentials. In the past, it was common practice to place a water-cooled orifice plate between the first orifice 3o and the plasma 240. Therefore, the small-scale, rapidly cooled plasma could pass through the first orifice 3o. Air was rapidly mixed with this plasma and reacted to produce nitric oxide (NO). The ionization potential of NO is 9.25 electron volts. The higher ionization potential in the plasma The metal ions undergo a change transfer reaction with No to produce NO+ and neutral metal atoms.The neutral metal atoms cannot be detected by a mass spectrometer.

When the invention is practiced, sampling is done closer to the hot plasma (as there is less arcing) and the air has little opportunity to mix with the plasma sample. Therefore, less oxides of nitrogen are formed. Ions with a high ionization potential do not lose their charge (and are therefore detectable by a mass spectrometer).
This is illustrated in Figure 3, where the vertical axis shows the relative number of ions on a logarithmic scale, and the ionization potential of the element is shown in electron volts on the horizontal axis. The curve obtained by the conventional method not using the configuration of this invention is 11
0, the curve obtained by the method of this invention is
120. For elements with high ionization potential, such as zinc, the ion signal is 50 times greater.

As for the wood tone, the difference is quite large.

Although tap 64 is shown grounded, it may be fixed at other fixed potentials depending on the circuit configuration used. Alternatively, a variable voltage may be applied to tap 64 and operated such that peak-to-peak voltage fluctuations are sufficiently small.

As another modification, the tap 64 may be completely removed and the circuit shown in FIG. 14 may be used. In the circuit of FIG. 14, the power supply 2o is connected to two capacitors (:1/,
It is connected to C2/IC through terminals 54 and 56, and the terminal between capacitors CI' and C2/ is grounded. Terminals 56.58 are connected in the same manner as described above.

If the circuit is carefully balanced, then the capacitance of C1/ and its leads is equal to the capacitance of 02' and its leads.

The circuit is symmetrical and is cyclically equivalent to having a grounded center tap on coil 12. The RF voltage at the center of coil 12 is maintained near zero.

If necessary for the circuit of FIG. 14, impedance matching is performed by placing a transformer or the like between the RF power supply 2o and the position representing the power supply 2°.

Although a four-turn coil is shown, more or less turns may be used as desired.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional mass spectrometer to which the present invention can be applied. FIG. 2 is a circuit diagram of an impedance matching circuit and an induction coil used in the device of FIG. 1. FIG. , a schematic diagram of the impedance matching circuit and induction coil similar to FIG. 2 but modified for the present invention, FIG. 4 plotted against the position of the tap as it moves along the coil. FIG. 5 is a diagram showing the absolute values between the plasma 240 and the energy spread of ions entering the mass spectrometer from the plasma for the two positions of the ground tap in FIG. Figure 7 shows the energy and energy spread of ions entering the mass spectrometer from the plasma for the two positions of the grounding tap in Figure 3; Figure 7 shows the grounding tap in Figure 3 in the first position; Figure 8 is a mass spectrum diagram of strontium taken for the ground tap of Figure 3 in the second position; Figure 9 is a cross-sectional diagram of an orifice plate with a flat orifice surface structure. , FIG. 10 is a cross-sectional view of an orifice plate with an orifice structure having a sharp end, FIG. 11 is a mass spectrum diagram for cerium using the orifice structure shown in FIG. 9, and FIG. A mass spectrum diagram for cerium using an orifice structure, FIG. 13 is a graph showing the relationship between the relative number of ions and ionization potential, and FIG. 14 is a diagram of another embodiment of the circuit of the present invention. 10・・・・・・Plasma tube 12・・・・・・
・・・・・・・・・Coil 20 ・・・・・・・・・
- Power supply 22... Impedance matching circuit 26, 26a, 26b... Orifice plate 32... First vacuum chamber portion 28... Vacuum 'M 30,3
0a, 30b...first orifice 40...
Second vacuum chamber portion 42...Second orifice plate 44...Second orifice 48...
・・・・・・Mass analysis Total C1, C2, CI'
, C2'... Capacitor 64...
・・・・・・Tap IG 1 day G 2 FIG, 3
FIG, 9 FIG, 10F
IG, 4 tcyt→ FIG, 13 -20020&0 6G (・v)-1- FIG, 5 FIG, 6

Claims (22)

[Claims] (1) In an apparatus for sampling plasma in a vacuum chamber, a plasma is generated, and there is a gap between the windings in order to generate the plasma by having a few co-wound windings between the first and second terminals. and a vacuum chamber having an orifice plate in one of its walls, the orifice plate having an orifice adjacent to the gap for directing plasma into the vacuum chamber from the orifice. an apparatus for guiding a portion of the plasma for sampling, and further comprising a circuit connected to the coil between the terminals to reduce peak-to-peak voltage fluctuations of the plasma. (2) The device according to claim 1, wherein the coil has a plurality of turns. (3) The device according to claim 2, wherein said circuit includes means for maintaining a potential at a substantially constant value at one point of the coil between said terminals. (4) The value can be obtained by grounding.Claim 3
Apparatus described in section. (5) The device according to claim 3, wherein the point is located at or near the center of the coil. 6. The apparatus of claim 2, wherein said circuit includes a tap connected to said coil between said terminals, said tap being located at or near the center of said coil. (7) The device according to claim 6, wherein the tap is fixed at a substantially fixed potential. (8) The device according to claim 7, wherein the potential is grounded. (9) The mass spectrometer according to any one of claims 1 to 3, wherein the mass spectrometer is disposed within the vacuum chamber to analyze ions sampled from the plasma into the vacuum chamber. Device. (10) The apparatus according to any one of claims 6 to 8, wherein a mass spectrometer is disposed within the vacuum chamber to analyze ions sampled from plasma into the vacuum chamber. . (11) The orifice plate has a conical wall projecting outwardly from the plate toward the gap, and the vaginal wall has a sharp end forming the orifice. The apparatus according to any one of Items 1 to 3. (12) The orifice plate has a conical wall projecting outwardly from the plate toward the gap, and the vaginal wall has a sharp end forming the orifice. The apparatus according to any one of Items 1 to 8. (13) the orifice plate has a conical wall projecting outwardly from the plate toward the gap, and the vaginal wall has a sharp end forming the orifice; 4. An apparatus according to claim 1, further comprising mass spectrometry means arranged in the vacuum chamber for analyzing ions entering the vacuum chamber. (14) the orifice plate has a conical wall projecting outwardly from the plate toward the gap, and the vaginal wall has a sharp end forming the orifice; and further from the plasma to the vacuum chamber. 9. Apparatus according to any one of claims 6 to 8, further comprising mass spectrometry means arranged in the vacuum chamber for analyzing the ions entering for sampling. (15) In a method of sampling plasma in a vacuum chamber, a high frequency current is passed through the coil to generate plasma in the coil, and the peak-to-peak voltage fluctuation of the plasma is limited by limiting the voltage fluctuation at a predetermined position between the coil terminals. , and directing a portion of the front plasma from an orifice into the vacuum chamber. 16. The method of claim 15, wherein the potential of the coil at a point between the terminals is maintained at a substantially fixed value in order to reduce the voltage fluctuations. (17) The method according to claim 16, wherein the value is obtained by grounding. (18) The method according to any one of claims 15 to 17, wherein the position is at or near the center of the coil. (19) The method according to any one of claims 15 to 17, wherein the position is within 1/4 turn from the center of the coil. (20) The position according to any one of claims 15 to 17, wherein the position is at or near the center of the coil, and the ions are analyzed in the portion of the plasma. Method. (21) Any one of claims 15 to 17, wherein the position is at or near the center of the coil, and the ions in the part of the plasma are analyzed by mass spectrometry means. The method described in section. (22) the location is at or near the center of the coil and analyzes the ions in the portion of the plasma and prevents cooling of the plasma onto the orifice to eliminate the ions in the plasma; 18. A method according to any one of claims 15 to 17, wherein recombination and reaction with oxygen are reduced.