US20070280834A1

US20070280834A1 - Sputter ion pump having an improved magnet assembly

Info

Publication number: US20070280834A1
Application number: US11/807,644
Authority: US
Inventors: Luca Bonmassar; Michele Mura
Original assignee: Varian SpA
Current assignee: Varian SpA
Priority date: 2006-06-01
Filing date: 2007-05-30
Publication date: 2007-12-06
Also published as: EP1863068A1; JP2007324134A; EP1863068B1; DE602006002264D1

Abstract

A sputter ion pump (1) has an improved magnet assembly comprising primary magnets (9 a, 9 b), disposed on opposite ends of the pump cells of an anode, and secondary magnets (11; 11′, 11″) disposed on one side only of the pump cells, whereby the assembly exhibits an asymmetrical configuration. The sputter ion pump with the improved magnet assembly allows for attaining high pumping speeds even at low pressures with reduced size, weight and manufacturing cost of the pump itself.

Description

CROSS REFERENCE TO RELATED APPLICATION

The subject patent application is claiming a priority of European Patent Application No. 06425377.6 filed in European Patent Office on Jun. 1, 2006.

BACKGROUND OF THE INVENTION

The present invention concerns a sputter ion pump having an improved magnet assembly.
A conventional sputter ion pump 10 shown in FIG. 1 is a device for producing high vacuum conditions. It comprises a vacuum enclosure 20 housing at least one anode 30 consisting of a plurality of hollow cylindrical pump cells 40, and a cathode 50 consisting of plates, e.g. made of titanium, located on opposite ends of cells 40. Pump 10 comprises means 60 for applying to the anode a higher potential than the cathode potential. Magnets 70 are located external to enclosure 20, at opposite ends of pump cells 40, for producing a magnetic field oriented parallel to the axes of said pump cell.
During operation, when a potential difference is applied between anode 30 and cathode 50 (typically, 3 to 9 kV), a strong electric field region is generated between anode cells 40 and cathode 50, resulting in electron emission from the cathode. The electrons are then captured in the anode cells. Electrons are colliding with and ionising gas molecules inside pump cells 40. Due to the electric field, the resulting positive ions are attracted by cathode 50 and collide with the surface thereof. Ion collision with the titanium plates forming cathode 50 results in the “sputtering” phenomenon, that is, the emission of titanium atoms from the cathode surface.
The presence of magnets 70 for generating a magnetic field B allows for imparting helical trajectories to electrons, so as to increase the lengths of their paths between the cathode and the anode and, consequently, the chances of colliding with gas molecules inside the pump cells and ionising such molecules.
The conventional ion pumps are characterised by considerable decrease in the pumping speed at low pressures. A number of different parameters affect the pumping speed of an ion pump and that can be acted upon. One such parameter is the magnetic field strength.
In this respect, the U.S. Pat. No. 6,835,048 discloses an ion pump in which the magnetic field strength is changed by providing additional magnets. More particularly, the ion pump disclosed in this patent comprises primary magnets of opposite polarities disposed on opposite ends of the pump cells, and secondary magnets disposed on two opposite sides of the pump cells, perpendicularly to the primary magnets. Possibly, additional secondary magnets can be provided on two other opposite sides of the pump cells, perpendicularly to both the primary magnets and the other secondary magnets.
Use of magnetic assemblies including perpendicular pairs of primary magnets and secondary magnets in order to achieve a high strength magnetic field was already known for example from the teaching of U.S. Pat. No. 4,937,545.
Though the solution given in the U.S. Pat. No. 6,835,048 demonstrates an improved performance by providing relatively constant filed quality over the full width of the primary magnets to maintain a high speed in the pump cells, it also gives considerable size and weight increase due to the provision of secondary magnets along two, or even four sides of the pump cells.
It is therefore an object of the present invention to overcome the drawbacks of the prior art, by providing a sputter ion pump capable of providing satisfactory pumping speeds even at low pressures, while having limited overall size and weight.
It is another object of the present invention to provide a sputter ion pump that is simple and cheap to manufacture.
Experimental studies carried out by the Applicant demonstrated that providing secondary magnets disposed on only one side of the pump cells, even though it leads to an asymmetric configuration of the magnetic assembly, is sufficient to ensure an increase in the magnetic field strength and a corresponding increase in the pumping speed, even at low pressures.
Thanks to the above asymmetric configuration of the pumping assembly, an ion pump can be obtained that has reduced size, weight and manufacturing costs as compared to the pump disclosed in the U.S. Pat. No. 6,835,048, which has a symmetric configuration of the magnetic assembly.
Further features and advantages of the sputter ion pump in accordance with the invention will become more apparent from the detailed description of some preferred embodiments of the invention, given by way of non limiting examples, with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional schematic view of a prior art ion pump;

FIG. 2 is a perspective schematic view of a ion pump according to a first embodiment of the invention;

FIG. 3 is a schematic side view of the ion pump of FIG. 2;

FIGS. 4A and 4B are graphs showing the behaviour of the transversal magnetic field component in a longitudinal cross-section of a prior art pump and of the pump of FIG. 2, respectively;

FIG. 5 is a graph showing the behaviour of the pumping speed as a function of pressure for a prior art ion pump and for the ion pump of FIG. 2;

FIG. 6 is a perspective schematic view of a ion pump according to a second embodiment of the invention;

FIG. 7 is a schematic side view of the ion pump of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 2 and. 3, there is shown a sputter ion pump according to a first embodiment of the invention. Ion pump 1 comprises a vacuum enclosure 3 housing the plates forming the cathode and the pump cells forming the anode. Vacuum enclosure 3 and the components housed therein, which are made in accordance with the prior art shown in FIG. 1, will not be further described.
Vacuum enclosure 3 is connected to a connecting flange 5 for connecting pump 1 with a chamber to be evacuated and is provided with a high voltage electric feedthrough 7 for pump connection to a power supply.
Primary magnets 9 a, 9 b are located external to vacuum enclosure 3, at opposite ends of the cylindrical anode pump cells, for producing a magnetic field parallel to the pump cell axes.
In accordance with the invention, in order to achieve a high pumping speed even at low pressures, a secondary magnet assembly 11, comprising one or more magnets, is provided on one side only of pump cells housed within enclosure 3. More particularly, in the illustrated example, secondary magnet assembly 11 is provided only on the bottom side of enclosure 3, opposite to connecting flange 5.
The magnets in secondary magnet assembly 11 (or secondary magnets) are arranged so as to produce a magnetic field in orthogonal direction to the field produced by primary magnets 9 a, 9 b, thereby reducing the edge effects of the primary magnets. Preferably, secondary magnets 11 are permanent magnets.
As shown in FIGS. 2 and 3, pump 1 is equipped with a substantially U-shaped bearing structure 13 associated with enclosure 3, primary and secondary magnets 9 a, 9 b, 11 are secured to that structure by means of screws 15.
Referring in particular to FIG. 3, in the illustrated embodiment, secondary magnet assembly 11 includes two secondary magnets 11′, 11″arranged side by side and having opposite polarities.
Referring to FIGS. 4A and 4B, there is shown respectively the strength of the transversal magnetic field component (in Tesla) in a longitudinal cross-section of the pump made in accordance with the layout of FIG. 1 (prior art) and of the pump made in accordance with the embodiment shown in FIGS. 2 and 3. The dotted-line rectangle corresponds to the region occupied by the pump cells forming the pump anode.
As it is clearly apparent, the provision of secondary magnets 11 results in a considerable increase in magnetic field strength. More particularly, due to secondary magnets 11, there is a considerable increase in the region where the transversal magnetic field component exceeds a critical value (0.14 Tesla in the illustrated example), above which the maximum efficiency of the pump cells is achieved.
It is known that two different pumping modes are associated with sputter ion pumps, namely a high magnetic field (HMF) mode and a low magnetic field (LMF) mode. If the magnetic field inside the ion pump falls below a critical value, the transition from HMF pumping mode to LMF pumping mode occurs, with a consequent reduction in the pumping speed. The critical value of the magnetic field is a function of pressure and, more particularly, it is increases as pressure decreases, so that remaining above the critical value as pressure decreases is progressively more difficult.
Thus, a stronger magnetic field (in particular above 0.14 Tesla, in the illustrated example) results in maintaining HMF pumping mode also at very low pressures, consequently improving the pumping speed.
In this respect, in FIG. 5, the behaviour of the pumping speed versus pressure for the ion pump of FIGS. 2 and 3 is shown and compared to the behaviour of a prior art pump made in accordance with the layout of FIG. 1. It can be appreciated that both curves have substantially the same behaviour in the pressure range 10⁻⁶to 10⁻⁸mbars (10⁻⁴to 10⁻⁶Pa), even if the ion pump in accordance with the invention allows attaining pumping speeds exceeding by about 20% those of a pump without secondary magnets.
The main difference can however be appreciated in the pressure range 10⁻⁸to 10⁻⁹mbars (10⁻⁶to 10⁻⁷Pa). In the case of the pump in accordance with the invention, the pumping speed decreases as pressure decreases, but the pumping speed loss keeps limited. On the contrary, without secondary magnets, the pumping speed suffers from an extremely strong reduction. Consequently, at pressures close to 10⁻⁹mbars (10⁻⁷Pa), the pumping speed of a pump in accordance with the invention is about twice the pumping speed of a pump lacking secondary magnets, but otherwise identical.
Turning now back to FIG. 4B, it should be noted that the strength of the transversal magnetic field component exceeds the critical value in a larger portion of the region occupied by the pump cells as compared to the prior art solutions, and, in particular, that, notwithstanding the asymmetric arrangement of the secondary magnets in accordance with the invention, such a strength exceeds the critical value over the whole central area of the region and not only on the side closest to secondary magnet assembly 11.
Thus, as stated above, a sputter ion pump with satisfactory pumping speed even at low pressures can be obtained by using a reduced number of secondary magnets disposed on a single side of the pump cells and, consequently, by keeping the size, the weight and the manufacturing costs limited as compared to the ion pump disclosed in the U.S. Pat. No. 6,835,048.
Turning now to FIGS. 6 and 7, there is shown a second preferred embodiment of pump 1. In accordance with that second embodiment, a plate 17 is provided on the side of vacuum enclosure 3 opposite to secondary magnets 11 in order to confine inside the pump the magnetic field due to the provision of secondary magnets 11. The plate 17 is made of a ferromagnetic material.
In FIGS. 6 and 7, secondary magnets 11 are disposed on the bottom side of pump 1, plate 17 is located at the top side of the pump and is secured to bearing structure 13 through screws 19 and is so shaped as to allow the neck of connecting flange 5 to pass.
It is clear that the above description has been given by way of non-limiting example and that several changes and modifications can be included within the inventive principle upon which the present invention is based. By way of example, a number of secondary magnets other than two could be provided, or the secondary magnets could be disposed on a different side of the vacuum enclosure, without departing from the scope of the invention.

Claims

1. A sputter ion pump (1) comprising:

a vacuum enclosure (3);

an anode located inside said vacuum enclosure and consisting of a plurality of pump cells;

a cathode located inside said vacuum enclosure and consisting of plates positioned at and spaced apart from opposite ends of said pump cells;

primary magnets (9 a, 9 b) positioned at opposite ends of said pump cells for producing a magnetic field coaxial with said pump cells; and

secondary magnets (11; 11′, 11″) disposed on one side only of said pump cells for providing an asymmetrical configuration to a magnetic assembly being formed by said primary magnets (9 a, 9 b) and said secondary magnets (11; 11′, 11″).

2. The sputter ion pump (1) as claimed in claim 1, wherein said secondary magnets (11; 11′, 11″) are disposed on a bottom side of said vacuum enclosure (3).

3. The sputter ion pump (1) as claimed in claim 1, wherein two said secondary magnets (11; 11′, 11″) are provided and are arranged with opposite polarities.

4. The sputter ion pump (1) as claimed in claim 2, wherein said secondary magnets (11; 11′, 11″) are permanent magnets.

5. The sputter ion pump (1) as claimed in claim 3, wherein said secondary magnets (11; 11′, 11″) are permanent magnets.

6. The sputter ion pump (1) as claimed in claim 1, wherein said primary magnets (9 a, 9 b) and said secondary magnets (11; 11′, 11″) are housed within a substantially U-shaped bearing structure (13), which is secured to said vacuum enclosure (3).

7. The sputter ion pump (1) as claimed in claim 4, wherein said primary magnets (9 a, 9 b) and said secondary magnets (11; 11′, 11″) are housed within a substantially U-shaped bearing structure (13), which is secured to said vacuum enclosure (3).

8. The sputter ion pump (1) as claimed in claim 5, wherein said primary magnets (9 a, 9 b) and said secondary magnets (11; 11′, 11″) are housed within a substantially U-shaped bearing structure (13), which is secured to said vacuum enclosure (3).

9. The sputter ion pump (1) as claimed in claim 1, wherein said pump (1) further comprises a plate (17) located on the side of said vacuum enclosure (3) opposite to said secondary magnets (11; 11′, 11″).

10. The sputter ion pump (1) as claimed in claim 9, wherein said plate (17) is made of ferromagnetic material for confining the magnetic field generated by said secondary magnets (11; 11′, 11″).

11. The sputter ion pump (1) as claimed claim 2, wherein said pump (1) further comprises a plate of ferromagnetic material (17), which is located on the side of said enclosure (3) opposite to said secondary magnets (11; 11′, 11″).

12. The sputter ion pump (1) as claimed in claim 3, wherein said pump (1) further comprises a plate (17) of ferromagnetic material (17), which is located on the side of said vacuum enclosure (3) opposite to said secondary magnets (11; 11′, 11″).

13. The sputter ion pump (1) as claimed in claim 10, wherein said primary magnets (9 a, 9 b) and said secondary magnets (11; 11′, 11″) are housed within a substantially U-shaped bearing structure (13), which is secured to said vacuum enclosure (3).

14. The sputter ion pump (1) as claimed in claim 11, wherein said primary magnets (9 a, 9 b) and said secondary magnets (11; 11′, 11″) are housed within a substantially U-shaped bearing structure (13), which is secured to said vacuum enclosure (3).

15. The sputter ion pump (1) as claimed in claim 12, wherein said primary magnets (9 a, 9 b) and said secondary magnets (11; 11′, 11″) are housed within a substantially U-shaped bearing structure (13), which is secured to said vacuum enclosure (3).

16. The sputter ion pump (1) as claimed in claim 13, wherein said secondary magnets (11; 11′, 11″) are permanent magnets.

17. The sputter ion pump (1) as claimed in claim 14, wherein said secondary magnets (11; 11′, 11″) are permanent magnets.

18. The sputter ion pump (1) as claimed in claim 15, wherein said secondary magnets (11; 11′, 11″) are permanent magnets.