US 3353054 A
Description (OCR text may contain errors)
Nov. 14, 1967- L. A. HOLLAND 3,353,054
PENNING TYPE VACUUM PUMPS Filed June 9, 1964 3 Sheets-Sheet 1 H646 I .v I
IHVEMTQE L E A.HOLLAND ATTOENEY- Nov. 14, 1967 L. A. HOLLAND PENNING TYPE VACUUM PUMPS 3 Sheets-Sheet 5 Filed June 9, 1964 55 950 z Tmob mmawwwma 35% ig 200% o om I O Y N m [935/1] oaads smaw'nd Imvanrroe L A Hon/VD BY Mf- ATTORNEY Patented Nov. 14, 1967 3,353,054 PENNING TYPE VACUUM PUMPS Leslie Arthur Holland, Crawley, England, assignor to Edwards High Vacuum International Limited, Crawley, England, a British company Filed June 9, 1964, Ser. No. 373,798
ABSTRACT OF THE DFSCLQSURE A getter ion pump of the Penning type which induces spiralling of electrons between two parallel cathodes through a grid-like anode in which each cathode has raised and depressed portions joined by side walls which are available for redeposition of material and gas removed during sputtering of the cathode. This results in improved stability and increased pumping rates when pumping either active or inert gases.
' follow spiral paths thereby increasing their ionization efficiency. The cathodes are made of chemically active metal, for example titanium, zirconium, etc. Positive ions attracted to the cathodes sputter metal atoms which then deposit onto the anode and envelope wall where they absorb gases.
It is known to construct a pump of this nature provided with a number of individual Penning pumps in parallel enclosed within a single evacuable envelope using a cellular anode of box-elements between two cathodes of large area. Ion bombardment of the cathodes sputters active metal from the cathode surfaces which is then deposited on the surfaces of the multi-cellular anode.
A fundamental problem in such pumps is the etficient sorption of both active and inert gases. When the pump is operative to exhaust gas with a composition of air, for example, then the cathodes are bombarded by both active and inert gas ions which partly penetrate the cathodes saturating their outer surfaces. As ion bombardment continues the gases being ion-pumped are released by the sputtering of the cathode metal and pumping will cease when the rate at which gases are released from the cathodes equals the rate at which they impinge upon the cathodes. Neutral atoms and molecules of certain active gases are still removed by gettering at the anode surfaces but the inert gases cannot be sorbed at the anode by chemical reactions; even though they may be rendered temporarily active by the electrical discharge this does not re- I sult in any measurable sorption rate. Furthermore metal getters do not combine with equal readiness with all active gases and with a mixture of gases one specie may be chemically sorbed at the expense of another.
Thus a normal Penning pump as described above with either a unit or multi-cellular anode will, after a period, exhaust only active gases, and at a rate depending upon the gas composition and the rate at which sputtered material creates a fresh gettering surface on the anode.
The purpose of this invention is to provide a region of sputtered material which can sorb non-ionised active gases and ions of both active and inert gases.
According to the present invention an electrical vacuum getter pump comprises an evacuable envelope containing at least two cathodes, an anode of grid construction, and means providing a magnetic field transverse to the planes of the cathodes, the two cathodes having a surface made up of an alternate array of raised and depressed polygonal sections joined by side walls disposed to receive sputtered material from said depressed section in operation of the pump, the plane in which the raised sections and the plane in which the depressed sections lie being in spaced parallel relation. The area of each raised and depressed portion and the area of each anode grid aperture are approximately equal, the distance between the depressed sections and the raised sections of each cathode measured in a direction at right angles to the cathode planes is not larger than the distance between adjacent raised sections, and the side walls of each of said cathodes are disposed substantially opposite the grid outline of the anode. 1
In one embodiment of the invention both cathodes have surfaces made up of alternate arrays of raised and depressed rectangular sections, the raised portions of one cathode being mounted opposite the depressed portions of the other cathode, each cathode having at least one raised and one depressed rectangular section.
In another embodiment the raised portions of both cathodes are mounted opposite each other.
In any of the constructions embodying the invention some or all of the side portions joining the raised and depressed rectangular sections in either or both cathodes may be mounted inclined to a plane perpendicular to the planes of the rectangular sections by an angle between 0 and 80.
Preferably the rectangular sections in any embodiments of the invention are square.
In a Penning discharge using plane cathodes with a hollow anode construction the central region of the cathode is exposed to the most intense bombardment by positive ions. Consequently, most of the sputtered metal is liberated from the central region of the cathode. Thus in the several embodiments of the present invention the sputtered metal emitted from the intensely bombarded region, namely the depressed sections, is deposited on the sides'of the open cathode boxes. A weak current of positive ions flows to the sides of the boxes, but the sputtering it produces is small compared to that of the intensely bombarded region. Consequently there isa net transfer of cathode material onto the sides of the boxes. The sputtered metal is then placed in a region where it can sorb neutral atoms and molecules of active gases,
and a proportion of the total ions of both active and inert gases. Employing a large number of cathode cells enhances the pumping speed.
The invention will now be described in greater detail with reference to the accompanying drawings in which:
FIGURES 1a and. 1b each show a schematic cross-section of unit cells;
FIGURES 2a and 2b each show a schematic cross-section of a multi-cell construction;
FIGURE 3 shows a perspective view of the cathodes and the anode;
FIGURES 4a and 4b each shows diagrammatically further sections of multi-cell constructions;
FIGURE 5 is an elevation, partly in section of a pump embodying the invention; and
FIGURE 6 shows curves of pumping speeds.
Referring firstly to FIGURE 3 of the drawings an anode 1, is of grid form constructed in two sets of equally spaced parallel conductive wires, the two sets being at right angles positioned in a plane parallel to and midway between planes containing the cathodes 2 and 3. Both cathodes comprise a square array of alternately open and closed boxes. The numeral 4 denotes one such open box in the cathode 3 and 5 denotes a closed box. Each of cathodes 2 and 3 comprises an array of 24 unit cells, 12 closed and 12 open. The two cathodes are mounted with the open boxes in each cathode opposite the open boxes in the other cathode. For example open box 4 in cathode 3 is opposite open box 6 in cathode 2 and closed boxes 5 and 7 are likewise disposed. FIGURE 1a shows a vertical section through a unit cell, the two open box cathode portions disposed either side of the grid anode 1. The direction of the applied magnetic field is shown by the, letter H and associated arrows. FIGURE 2a shows a vertical section through a multiple array of unit cells as illustrated in FIGURE 3 with cathode portions disposed on either side of the anode grid 1.
In operation of the pump the cathodes 2 and 3 are held at the same negative potential with respect to the anode potential. A magnetic field is applied in the direction described. For operating the pump a typical combination of applied Voltage and magnetic field would be 6 kv. at 1,000 gauss. However, the apparatus is not limited to this operating condition, which depends on the cathode/anode dimensions and spacing. Normally the pump will not be operated until a pressure of about 0.1 mm. Hg has been obtained by conventional vacuum techniques. Electrons pass from cathodes to the anode executing spiral paths and ionising gas atoms and molecules in their path. The positive ions thus provided are attracted to the cathode and impinge thereon. The active gases present combine chemically with the cathode whilst ions of inert or chemically active gases partly penetrate the surface of the cathode. Metals atoms sputtered from the cathode surfaces are dislodged preferentially from the centre of the square sections and gas particles trapped in or on the surface of the cathode may become dislodged at the same time. Since however the discharge current of positive ions is greatest at the centre of the square sections there is a net transfer of metal atoms onto the side walls of the boxes. For example sputtered metal from the base 8 of box 4 is deposited onto the walls 9, and the two walls adjacent 9 and 10. These surfaces therefore provide areas in which gaseous ions and neutral atoms or molecules may become trapped by sputtered metal atoms transferred from the base 8. Neutral atoms or molecules may also be gettered by sputtered metal deposited on the walls of the envelope of the pump, not shown, and the anode.
The pump described above is only one embodiment of the invention and it is possible to employ alternative arrangement of the cathodes, or even alternative constructions of the anode.
For example FIGURE 1b and FIGURE 2b show respectively sections of a unit cell and a multicell in which the cathodes are arranged with the raised portions 11 0pposite depressed portions 12.
Alternative arrangements in the construction of the unit cell are illustrated in FIGURE 4a and FIGURE 412. FIGURE 4a illustrates a section of a pair of cathodes 13 and 14 the raised and depressed sections of which, 11, 12 respectively, are joined by side walls 15 inclined to the plane of the raised or depressed portions by an angle greater than FIGURE 2a represents a section along theline AA in FIGURE 3. It is to be understood thatthe walls parallel to wall 9 in FIGURE 3 are at 90 to the plane of the raised or depressed portions.
FIGURE 4b illustrates a similar section in which the cathode boxes are re-entrant the angle 0 being less than 90, the angle in FIGURE 3 being 90.
Referring now to FIGURE 5, the pump shown consists of a body 16 supporting a magnet 17 and provided with a terminal 18 for the operating voltage supply leads for the electrodes. The anode 1 is of the form shown in FIGURE 3 and the cathodes 2 and 3, composed of titanium are of the general form also shown in FIGURE 3. The cells of the cathodes are half an inch square and a quarter of an inch deep, the opposed closed faces of:
the cells being spaced one inch apart. The anode is disposed midway between the cathodes and is composed of wire inch diameter. The magnet 17 provides a field of 1,000 gauss and the operating voltage is 6 kv.
The pumping speeds indicated by the curves shown in FIGURE 6 are in litres per second at a specific pressure and the pressures in torr values. Thus, in the particular example of the pump being described, the pumping speed for air as shown by curve 19 is 38 hIres per second at a pressure of 10" torr measured at 10- torr. The .volumetric speeds may be measured using two chambers one of which is constituted by the pump, the two chambers being separated by an orificed plate. The pumping speeds are determined by measuring the pressure in the two vessels, the conductance of the orifice being known.
Curve 20 shows the pumping speed obtained for argon in the case of the pump described. Thus, the pumping speed for argon may be expected to be of the order of 14 litres per second at a pressure of 10- torr measured at 10- torr.
It will be understood that the invention may be carried out in ways different from the particular embodiment described. For example, the dimensions of the electrodes, their relative spacing and the operating voltages may be selected to suit required conditions.
1. An improved electrical vacuum getter pump of the type provided with (A) an electrode assembly comprising (1) two substantially parallel spaced cathodes and (2) an anode of grid construction disposed between said cathodes, and
(B) means for impressing a magnetic field across said electrode assembly in a direction substantially normal to the cathode planes to cause electrons moving in the space within said assembly to follow spiral paths between the said cathodes before being captured, the improvement residing in the factthat (C) each of said cathodes has a surface made up of an alternate array of raised and depressed polygonal sections joined by side walls disposed to receive sputtered material from said depressed section, the area of each raised and depressed section and the area of each grid aperture of said anode are approximately equal, the distance between said depressed sections and said raised sections measured in a direction at right angles to the cathode planes is not larger than the distance between adjacent raised sections, and said side walls of each of said cathodes are disposed substantially oppositethe grid outline of said anode.
2. An improved pump according to claim 1 wherein said raised and depressed sections are rectangular, the raised portions of one cathode being disposed opposite the depressed portions of the other cathodes.
3. An improved pump according to claim 1 wherein said raised portions of one cathode are disposed opposite said raised portions of the other cathode.
4. An improved pump according to claim 1 wherein said side walls are inclined to a plane perpendicular to the cathode planes by an angle between 0 and 80.
5. An improved pump according to claim 1 in which said distance between said depressed sections and said raised section is one half of the distance between adjacent raised sections.
5 References Cited UNITED STATES PATENTS 3,070,719 12/1962 Jepsen 230-69 X 3,228,589 1/1966 Kearns 3137 X 10 JAMES W. LAWRENCE, Primary Examiner. S. A. SCHNEEBERGER, Assistant Examiner.