Method and apparatus for sampling a plasma into a vacuum chamber

Abstract

A plasma is generated within an induction coil and the plasma is sampled through an orifice into a vacuum chamber for mass analysis of trace ions in the plasma. Arcing at the orifice is prevented by grounding the induction coil at or near its center, thus eliminating ultraviolet noise and reducing average ion energies and ion energy spread, as well as preventing destruction of the orifice. The elimination of arcing at the orifice allows the use of a sharp edge orifice structure to prevent formation of a cool boundary layer over the orifice and also permits direct sampling of the plasma. The direct sampling and the lack of cooling prevent recombination and reaction of the ions with oxygen and improve the response to elements of high ionization potential, increasing the desired ion signal and greatly reducing the presence of oxides which would otherwise complicate the spectrum.

Description

This invention relates to method and apparatus for sampling an inductively generated plasma through an orifice into a vacuum chamber, and to method and apparatus for mass analysis using such sampling. The invention will be described with reference to mass analysis.

Mass analyzers for detecting and analyzing trace substances require that ions of the substance to (b) said current in said coil being the sole electrical power means for generating said plasma, sharp edge 96. The edge 96 defines the first orifice 30b. The FIG. 10 orifice structure results in the reduction or elimination of a cool boundary layer over orifice 30b (even though the plate 26b itself may be cooled), because there is not flat surface adjacent the orifice over which a cooled boundary layer can readily form. Thus the plasma being sampled through orifice 30b is not greatly cooled until after it enters vacuum chamber section 32. Since the pressure in vacuum chamber section 32 is only about one torr (as compared with 760 torr on the outside of orifice plate 26b), the recombination rate is reduced by about 760³ and the reaction rate by about 760².

The improvement produced by the use of the sharp edge orifice structure 92 (which can be used without arcing because of the tap 64 located near the center of the coil) is shown in FIGS. 11 and 12, which show mass spectra obtained for a ten parts per million solution of cerium. FIG. 11 shows the mass spectrum 98 obtained using the blunt orifice structure 88 of FIG. 9 and FIG. 12 shows the mass spectrum 100 obtained using the sharp edge orifice structure 92 of FIG. 10. Here full scale on the vertical axis was 10⁶ counts per second. It will be seen that in FIG. 11 the peak at 140 amu (which is the mass of cerium) is extremely small, while a large peak is located at mass 156 (cerium oxide) and a smaller peak (but still larger than the cerium peak) is located at mass 158 (the oxide of an isotope of cerium).

In contrast FIG. 12 shows a large peak at mass 140 (cerium) and a substantial peak at mass 142 (an isotope of cerium). Only a small peak now appears at mass 156 (cerium oxide), and virtually no peak appears at mass 158. The enormous increase in ion signal for the elemental ions and the corresponding reduction in the quantity of oxides produced greatly improve the ability to decipher the complex spectrum obtained when many elements are mixed together. (For FIG. 12 the resolution was deliberately reduced to ensure that there would be no mass discrimation against the higher mass oxides.)

A further advantage of the invention is that it improves the response to elements of high ionization potential. Formerly it was common practice to place an extra water cooled orifice plate between the first orifice 30 and the plasma 24. Thus a reduced scale, rapidly cooling plasma was sampled through the first orifice 30. Air mixed rapidly into this plasma and reacted thereon to produce nitric oxide (NO). The ionization potential of NO is 9.25 electron volts. Metal ions of higher ionization potential in the plasma tended to undergo change transfer reactions with the NO to produce NO+ and neutral metal atoms. The metal atoms, having become neutral, could not be detected by the mass analyzer.

When the invention is used, sampling may be carried out much closer to the hot plasma (since arcing has been essentially eliminated) and air has less opportunity to mix into the plasma sample. Therefore nitrogen oxides are less likely to form. Thus ions of higher ionization potential do not lose their charge and hence can be seen by the mass analyzer. This is illustrated in FIG. 13, which shows relative numbers of ions on the vertical axis on a log scale, and the ionization potential of the elements in electron volts (various elements are marked on the graph) on the horizontal axis. The curve for the prior art method without the use of the invention is shown at 110 and the curve with the invention used is shown at 120. For higher ionization potential elements such as zinc, the improvement in ion signal can be by a factor of fifty. For mercury the improvement is even greater.

It will be realized that although the tap 64 is shown as grounded, it may instead be clamped to a different fixed potential, depending on the circuit arrangements provided. Alternatively a variable voltage may be applied to tap 64, so long as the effect is to reduce sufficiently the peak to peak voltage swing in the plasma.

As a further alternative the tap 64 may be eliminated entirely and a circuit such as that shown in FIG. 14 may be used. In the FIG. 14 circuit the power supply 20 is connected to terminals 54, 56, i.e. across the two capacitors now indicated as C1', C2', and the terminal 52 between capacitors C1', C2' is grounded. Terminals 56, 58 are connected together as are terminals 54, 60, as before. Provided that the circuit is carefully balanced so that the capacitance of C1' and its leads is equal to the capacitance of C2' and its leads, the circuit will be symmetrical and will be equivalent electrically to having a ground centre tap in coil 12. Thus the RF voltage at the centre of coil 12 will remain at or near zero as before.

Impedance matching, if needed for the FIG. 14 circuit, may be effected by a transformer or other means located between the RF power source 20 and the location in the circuit now shown for the source 20.

Although a four turn coil has been shown, more or fewer turns may be used as appropriate for the application in question.

Claims

1. Apparatus for sampling a plasma into a vacuum chamber comprising:

(a) means for generating a plasma, including an electrical induction coil having first and second terminals and at least one turn between said first and second terminals, said turn defining a space within said coil for generation of said plasma, and means for supplying an alternating electrical current to said coil to excite said plasma,

(b) a vacuum chamber including an orifice plate defining a wall of said vacuum chamber,

(c) said orifice plate having an orifice therein located adjacent said space for sampling a portion of said plasma through said orifice into said vacuum chamber,

(d) the electrical current in said coil constituting the sole electrical power means for generating said plasma,

(d) (e) and circuit means connected to said coil between said terminals to reduce the peak to peak voltage swing in said plasmathus to reduce the likelihood of electrical arcing from said plasma.

2. Apparatus according to claim 1 wherein said coil includes a plurality of turns.

3. Apparatus according to claim 2 wherein said circuit means includes means for holding the potential at a point in said coil between said terminals at a substantially constant value.

4. Apparatus according to claim 3 wherein said value is ground.

5. Apparatus according to claim 3 wherein said point is located at or near the centre of said coil.

6. Apparatus according to claim 4 wherein said point is located at or near the centre of said coil.

7. Apparatus according to claim 2 wherein said circuit means includes a tap connected to said coil between said terminals thereof, said tap being located at or near the centre of said coil.

8. Apparatus according to claim 7 wherein said tap is clamped to a substantially fixed potential.

9. Apparatus according to claim 8 wherein said potential is ground.

10. Apparatus according to claim 3 and including mass analyzer means located within said vacuum chamber for analyzing ions sampled into said vacuum chamber from said plasma.

11. Apparatus according to claim 9 and including mass analyzer means located within said vacuum chamber for analyzing ions sampled into said vacuum chamber from said plasma.

12. Apparatus according to claim 3 wherein said orifice plate includes a conical wall extending outwardly from said plate towards said space, said conical wall having a sharp outer edge which defines said orifice.

13. Apparatus according to claim 9 wherein said orifice plate includes a conical wall extending outwardly from said plate towards said space, said conical wall having a sharp outer edge which defines said orifice.

14. Apparatus according to claim 3 wherein said orifice plate includes a conical wall extending outwardly from said plate towards said space, said conical wall having a sharp outer edge which defines said orifice, said apparatus further including mass analyzer means located within said vacuum chamber for analyzing ions sampled into said vacuum chamber from said plasma.

15. Apparatus according to claim 9 wherein said orifice plate includes a conical wall extending outwardly from said plate towards said space, said conical wall having a sharp outer edge which defines said orifice, said apparatus further including mass analyzer means located within said vacuum chamber for analyzing ions sampled into said vacuum chamber from said plasma.

16. A method of sampling a plasma into a vacuum chamber comprising:

(a) applying a high frequency electrical current to a coil to generate a plasma within said coil,

(b) said current in said coil being the sole electrical power means for generating said plasma,

(b) (c) reducing the peak to peak voltage variations is in said plasma by limiting the voltage variations in said coil at a position between the ends thereof, thus to reduce the likelihood of electrical arcing from said plasma, and

(c) (d) directing a portion of said plasma through an orifice into said vacuum chamber.

17. A method according to claim 16 wherein said step (b) comprises holding the potential in said coil at a position between the ends thereof at a substantially constant value.

18. A method according to claim 17 wherein said value is substantially ground.

19. A method according to claim 17 wherein said position is at or near the center of said coil.

20. A method according to claim 17 wherein said position is within one-quarter turn from the center of said coil.

21. A method according to claim 17 wherein said position is at or near the center of said coil and including the step of analyzing ions in said portion of said plasma.

22. A method according to claim 17 wherein said position is at or near the center of said coil and including the step of analyzing with a mass analyzer ions in said portion of said plasma.

23. A method according to claim 17 wherein said position is at or near the center of said coil and including the step of analyzing ions in said portion of said plasma, said method further including the step of preventing substantial cooling of said plasma over said orifice to reduce recombination and reaction of said ions in said plasma with oxygen. 24. Apparatus according to claim 1 and including means for maintaining said plasma at substantially atmospheric pressure. 25. A method according to claim 16 wherein said plasma is operated at substantially atmospheric pressure.