US3532917A - Getter ion pump employing high frequency electric field between two electrodes one of which is secondary emissive - Google Patents

Description

Oct 6 1970 y R. EVRARD --3j,5'32,9l7

GETTERION PUMP EMPLOYING HIGH FREQUENCY ELECTRIC FIELD BETWEEN TWO ELECTRODES ONE OF WHICH IS SECONDARY EMISSIVE Filed. April 2, 1968 United States Patent Oce y 3,532,917 Patented Oct. 6, 1970 3,532,917 GETTER ION PUMP EMPLOYING HIGH FRE- QUENCY ELECTRIC FIELD BETWEEN TWO ELECTRODES ONE OF WHICH IS SECOND- ARY EMISSIVE Robert Evrard, Saint Martin-Paris, France, assignor, by mesne assignments, to U.S. Philips Corporation, New York, N.Y., a corporation of Delaware Filed Apr. 2, 1968, Ser. No. 718,048 Claims priority, application1 1France, Apr. 7, 1967,

1 Inf. cl. Hosh; H011 43/00 U.S. Cl. 313-63 4 Claims ABSTRACT F THE DISCLOSURE Getter ion pump in which a'high-frequency field prevails between two electrodes and one of the electrodes is secondary-emitting and the other is a grid. Formed ions are attracted to a collecting electrode by the grid, this sputtered material being trapped by a grid consisting of parallel plates.

is that said electrode is either sputtered or is coated with sputtered material. In both cases the secondary emission coefficient will be reduced. In this arrangement the electrons are therefore mainly formed by ionisation of the gase. The result of this is that the generation of electrons is considerably reduced when the gas pressure is reduced.

It is the object of the invention to provide a device which has advantages compared with the known device.

In a device employing a getter ion pump in which the electrons, which effect ionisation of the gas present, perform oscillatory movements under the influence of highfrequency electric fields and in which secondary electron multiplication occurs, according to the invention, the pump comprises at least three electrodes, the outermost of which are plate-shaped and the intermediate electrode is gridshaped, the high-frequency electric field being present between the grid-like electrode and one of the plate-like electrodes which has a secondary-emission coefficient exceeding 1 for electrons having the energies which are obtained on an average in the high-frequency field, the amplitude and the frequency of the high-frequency electric field having such values in connection with the distance over which said field extends that the produced secondary electrons return to the secondary-emitting electrode after approximately a whole cycle, an electrostatic field being present between the grid and the other plate-like electrode so that the ions are accelerated to said electrode and produce sputtering there.

With the arrangement according to the invention it is achieved that substantially no ions impinge upon the secondary-emitting layer because these can absorb only very little energy in the high-frequency field as a result of their larger mass and because moreover the electrostatic field can extend somewhat through the grid. Although in this arrangement some sputtering through the grid to the secondary-emitting electrode can occur, the influence thereof can be kept small by making the area of said electrode large relative to the ion collecting electrode, for example, according to the invention, a circular cylindrical arrangement may be chosen in which the secondary-emitting electrode has the larger diameter and the ion collecting electrode has the smaller diameter. According to the invention, the drawback of deposition of sputtered material on the secondary-emitting electrode can be prevented substantially entirely by arranging between the grid-like electrode and the ion collecting electrode, an extra electrode which consists of plate-like parts parallel to the electrostatic field between the two surrounding electrodes, and making the voltage difference between said electrode and the grid-like electrode considerably smaller than between the grid-like electrode and the ion collecting electrode. As a result of the low speed of the ions said extra electrode itself will sputter little and, on the other hand, receive the sputtered material of the ion collecting electrode in favour of the geometry of the arrangement. According to the invention, the ion collecting electrode is preferably cooled to the temperature of liquid air or nitrogen.

In order that the invention may be readily carried into effect, it will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view at right angles to the axis of an ion pump for a device according to the invention, and

FIG. 2 shows a cross-sectional view parallel to the axis.

FIG. 1 shows only one half and FIG. 2 only a short Y portion of the length.

In the figures, reference numeral 3 denotes a sheath of stainless steel coated with a layer 2 of oxidized silver. A grid-like electrode is denoted by 4. An electrode 10 inside the grid 4 consists of radially arranged strips 11 of stainless steel. The innermost electrode consists of a layer 8 of titanium on a stainless steel cylinder in which cooling liquid 14 is provided, in this case liquid nitrogen.

The diameter of electrode 2 is approximately 24 cm., that of electrode 4 is 18 cm., that of electrode 8 is 5 cm. The proportions are approximately to scale so that the dimensions of the electrode 10 can be derived herefrom.

Between the electrodes 2 and 4 a voltage is set up having a frequency of approximately 40 mHz. from source 15 and an amplitude of approximately v. electrons emitted by the electrode 2 describe paths 6 in the figure in the period of one cycle so that multiplication occurs.

After thevacuum has been reduced to approximately 10-2 torr by means of a primary backing pump, ignition of the discharge may occur by setting up the high frequency voltage and hence electron bombardment of the electrode 2. Formed ions are accelerated from the electrode 4 to the axis since the electrodes 10 and 8 have negative potentials from battery 16 with respect to the electrode 4 of 500 volts and 2000 volts, respectively. T itam'um which is sputtered from electrode 8 is received substantially entirely on the electrode 10. When the gas pressure is reduced, the flow of electrons between 2 and 4 reduces until at the pressure of 10-4 torr stabilisation occurs since the formation of electrons in the gas then has substantially no influence and only the secondary emission is decisive.

What is claimed is: l

1. A device comprising a getter ion pump in which the electrons, which effect ionization of the gases present, perform oscillatory movements under the influence of highfrequency electric fields and in which secondary electron multiplication occurs, said pump comprising at least three spaced concentric electrodes, the outermost of which are tubular and the intermediate electrode is grid-shaped, at least one of said tubular electrodes having a secondary emission coefficient exceeding l for electrons having the energies which are obtained on an average in a high-frequency field, means to produce between one of said tubular electrodes and said grid-like electrode an electric eld having an amplitude and frequency such that the secondary electrons produced return to the secondary-emitting electrode after substantially one whole cycle, and means to produce an electrostatic field between the grid and the other tubular electrode so that ions are accelerated to said other electrode and produce sputtering thereon.

2. A device as claimed in claim 1, wherein the electrodes are circular cylindrical, the secondary-emitting electrode having the larger diameter and the ion collecting electrode having the smaller diameter.

3. A device as claimed in claim 2, wherein the ion collecting electrode is cooled by means of a liquied gas.

4. A device as claimed in claim 3, wherein between the grid-like electrode and the ion collecting electrode an extra electrode is arranged which comprises plate-like portions parallel to the electrostatic field between the two surrounding electrodes and means to apply a potential between said electrode and the grid-like electrode which is considerably smaller than between the grid-like electrode and the ion collecting electrode.

References Cited UNITED STATES PATENTS JAMES W. LAWRENCE, Primary Examiner D. OREILLY, Assistant Examiner U.S. Cl. X.R. 313-103

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