U.S. patent application number 11/807644 was filed with the patent office on 2007-12-06 for sputter ion pump having an improved magnet assembly.
This patent application is currently assigned to Varian, S.p.A.. Invention is credited to Luca Bonmassar, Michele Mura.
Application Number | 20070280834 11/807644 |
Document ID | / |
Family ID | 37188921 |
Filed Date | 2007-12-06 |
United States Patent
Application |
20070280834 |
Kind Code |
A1 |
Bonmassar; Luca ; et
al. |
December 6, 2007 |
Sputter ion pump having an improved magnet assembly
Abstract
A sputter ion pump (1) has an improved magnet assembly
comprising primary magnets (9a, 9b), 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.
Inventors: |
Bonmassar; Luca; (Cirie
(TO), IT) ; Mura; Michele; (Torino, IT) |
Correspondence
Address: |
Varian Inc.;Legal Department
3120 Hansen Way D-102
Palo Alto
CA
94304
US
|
Assignee: |
Varian, S.p.A.
|
Family ID: |
37188921 |
Appl. No.: |
11/807644 |
Filed: |
May 30, 2007 |
Current U.S.
Class: |
417/49 |
Current CPC
Class: |
H01J 41/18 20130101 |
Class at
Publication: |
417/49 |
International
Class: |
F04B 37/02 20060101
F04B037/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 1, 2006 |
EP |
06425377.6 |
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 (9a, 9b)
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 (9a, 9b) 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 (9a, 9b) 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 (9a, 9b) 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 (9a, 9b) 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 (9a, 9b) 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 (9a, 9b) 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 (9a, 9b) 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.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] 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
[0002] The present invention concerns a sputter ion pump having an
improved magnet assembly.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] It is another object of the present invention to provide a
sputter ion pump that is simple and cheap to manufacture.
[0012] 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.
[0013] 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.
[0014] 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
[0015] FIG. 1 is a cross sectional schematic view of a prior art
ion pump;
[0016] FIG. 2 is a perspective schematic view of a ion pump
according to a first embodiment of the invention;
[0017] FIG. 3 is a schematic side view of the ion pump of FIG.
2;
[0018] 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;
[0019] 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;
[0020] FIG. 6 is a perspective schematic view of a ion pump
according to a second embodiment of the invention;
[0021] FIG. 7 is a schematic side view of the ion pump of FIG.
6.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0022] 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.
[0023] 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.
[0024] Primary magnets 9a, 9b 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.
[0025] 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.
[0026] 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 9a,
9b, thereby reducing the edge effects of the primary magnets.
Preferably, secondary magnets 11 are permanent magnets.
[0027] 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 9a, 9b, 11 are secured
to that structure by means of screws 15.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.sup.-6 to 10.sup.-8 mbars (10.sup.-4 to 10.sup.-6 Pa),
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.
[0034] The main difference can however be appreciated in the
pressure range 10.sup.-8 to 10.sup.-9 mbars (10.sup.-6 to 10.sup.-7
Pa). 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.sup.-9 mbars
(10.sup.-7 Pa), 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
* * * * *