Permanent magnet magnetic circuit

A permanent magnet magnetic circuit is provided capable of producing a unidirectional magnetic field substantially free of unnecessary perpendicular magnetic field components. A pair of main magnetic poles of opposite polarities are situated at a pair of opposite edge surfaces of a permanent magnet block for producing lines of magnetic force. One major surface of the block between the edge surfaces is provided with a channel where a pair of auxiliary magnetic poles are situated at opposed inner surface portions of the channel for controlling a middle portion of the lines of magnetic force to be linear.

This invention relates to a permanent magnet magnetic circuit. Such 
permanent magnet magnetic circuits are used in semiconductor manufacturing 
apparatus and similar apparatus requiring a unidirectional magnetic field 
having linear lines of magnetic force in a desired region and are 
especially useful when restrictions are imposed on their attachment or 
arrangement. 
BACKGROUND OF THE INVENTION 
Referring to FIG. 11, there is illustrated a first example of prior art 
permanent magnet magnetic circuits. The circuit includes a U-shaped yoke 2 
having opposed legs. A pair of permanent magnets 1A and 1B are attached to 
the inside surfaces of the yoke legs such that the N pole of one magnet 1A 
is opposed to the S pole of the other magnet 1B. This arrangement creates 
a unidirectional magnetic field containing the least number of unnecessary 
perpendicular magnetic field components. 
In the first prior art circuit of FIG. 11, however, a region E1 where the 
unidirectional magnetic field is produced is surrounded at three sides by 
the magnetic circuit. This magnetic circuit is not useful where 
restrictions are imposed on its attachment or location. For example, this 
first prior art circuit cannot be applied under the requirement that the 
magnetic circuit should not protrude beyond the lower boundary line F1 of 
the unidirectional magnetic field generating region E1. Such restrictions 
arise, for example, when it is desired for a magnetic circuit located 
outside a vacuum vessel to provide a region E1 of unidirectional magnetic 
field within the vacuum container interior. 
FIGS. 12 and 13 show a second example of the prior art, illustrating a most 
fundamental magnetic circuit. A plate-shaped permanent magnet 5 has N and 
S poles disposed at opposite edges. A region E2 of approximate 
unidirectional magnetic field is available above (or below) one major 
surface of the plate shaped permanent magnet 5 as seen from FIG. 13. The 
magnetic circuit can be located only below (or above) the boundary line F2 
of the approximate unidirectional magnetic field region E2. 
FIG. 14 shows the distribution of lines of magnetic force generated in the 
second prior art circuit of FIGS. 12 and 13. In this example, the plate 
shaped permanent magnet 5 is a ferrite permanent magnet having a lateral 
dimension Lx of 250 nm, a transverse dimension Ly of 300 mm and a 
thickness Lz of 24 mm. 
Assume that X is a horizontal distance from the center of the plate-shaped 
permanent magnet 5 and the vertical distance H from the major surface of 
the plate-shaped permanent magnet 5 is fixed to 40 mm. In FIG. 6, broken 
line curves show horizontal and vertical (or perpendicular) components Bx 
and Bz of the magnetic flux density as a function of distance X in the 
range of from 0 to W/2=80 mm. It is understood from the broken line curves 
in FIG. 6 and FIG. 14 that the second prior art circuit of FIGS. 12 and 13 
produces a magnetic field which is far from an ideal unidirectional 
magnetic field free of perpendicular components since the unnecessary 
vertical component Bz drastically increases with the increasing distance X 
from the center of the plate-shaped permanent magnet 5. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a permanent 
magnet magnetic circuit capable of producing a unidirectional magnetic 
field free of perpendicular components and of a simple structure which 
does not surround the unidirectional magnetic field producing region. 
In a first form of the present invention, the permanent magnet magnetic 
circuit includes a magnet block having a pair of opposite edge surfaces 
and a pair of major surfaces between the opposite edge surfaces. A pair of 
main magnetic poles of opposite polarities are situated at the opposite 
edge surfaces. A channel is defined in one major surface. A pair of 
auxiliary magnetic poles are situated at a pair of opposed inner surface 
portions of the channel disposed inside the main magnetic poles. Each 
auxiliary magnetic pole has opposite polarity to that of the corresponding 
main magnetic pole. The channel may be recessed stepwise or continuously 
to define the opposed inner surface portions. 
In a second form of the invention, the permanent magnet magnetic circuit 
includes a magnet block having a pair of opposite edge surfaces, with a 
magnetic pole of one polarity at one edge surface and a magnetic pole of 
opposite polarity at the other edge surface for creating a magnetic field 
having lines of magnetic force extending from the one edge surface to the 
other edge surface. Compensating magnetic poles are situated between the 
edge surfaces for controlling the vectorial direction of the magnetic 
field. 
In a third form of the invention, the permanent magnet magnetic circuit 
includes a magnet block having a pair of opposite edge surfaces. Main 
magnetic means is provided for creating a loop pattern of lines of 
magnetic force extending from the one edge surface to the other edge 
surface. Compensating magnetic means is provided for controlling a middle 
portion of the lines of magnetic force to be linear. Preferably, the main 
magnetic means is comprised of a magnetic pole of one polarity at one edge 
surface and a magnetic pole of opposite polarity at the other edge 
surface, and the compensating magnetic means is comprised of auxiliary 
magnetic poles. 
In the permanent magnet magnetic circuit of the present invention, a 
vertical component contained in the magnetic field produced between the 
main magnetic poles (a magnetic field component perpendicular to the 
desired unidirectional magnetic field component) can be offset by a 
vertical component of opposite orientation contained in the magnetic field 
produced between the main and auxiliary (or compensating) magnetic poles. 
There can be formed a unidirectional magnetic field free of unnecessary 
vertical component over a wide area. The unidirectional magnetic field 
region is spaced from the magnet block so that the magnetic circuit does 
not surround the unidirectional magnetic field region. The circuit is thus 
best suited as the unidirectional magnetic field producing means 
associated with semiconductor manufacturing apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2, there is illustrated a permanent magnet 
magnetic circuit according to a first embodiment of the present invention. 
The circuit includes a rectangular magnet block 20 of a suitable permanent 
magnet material such as ferrite or rare earth permanent magnet. The block 
20 has a pair of first and second opposite edge or outer surfaces and a 
pair of major surfaces joining the first and second opposite edge surfaces 
and extending substantially perpendicular to the edge surfaces. Main 
magnetic poles 21A and 21B are formed in the first and second opposite 
edge surfaces by magnetization. In the illustrated embodiment, the main 
magnetic poles 21A and 21B are N and S poles, respectively. A channel 22 
is defined in one (upper in the illustrated embodiment) major surface. The 
channel 22 illustrated herein is a transverse trough extending throughout 
the block 20 in a transverse direction and recessed stepwise at the right 
and left sides in a symmetrical manner as viewed in the front elevation of 
FIG. 2. A flat bottom is connected to the stepwise side walls. The channel 
includes at least one pair, three pairs in the illustrated embodiment, of 
steps defining opposed inner surface portions disposed inward of the 
opposite edge surfaces (21A, 21B). Auxiliary magnetic poles in the form of 
compensating magnetic poles 23A.sub.1, 23A.sub.2 and 23A.sub.3 are formed 
by magnetization in the inner surface portions on the left side disposed 
inward of the main magnetic pole 21A. Similarly, auxiliary magnetic poles 
in the form of compensating magnetic poles 23B.sub.1, 23B.sub.2 and 
23B.sub.3 are formed by magnetization in the inner surface portions on the 
right side disposed inward of the main magnetic pole 21B. The auxiliary 
magnetic poles have opposite polarity to that of the main magnetic poles. 
The compensating magnetic poles 23A.sub.1, 23A.sub.2 and 23A.sub.3 are S 
poles and the compensating magnetic poles 23B.sub.1, 23B.sub.2 and 
23B.sub.3 are N poles. 
If the magnet block 20 is of uniform material, the main and auxiliary 
magnetic poles 21 and 23 have equal surface magnetic flux density, but the 
main magnetic poles have a substantially larger total quantity of magnetic 
flux. The compensating magnetic poles 23 function to compensate for 
magnetic force for tailoring the lines of magnetic force produced by the 
main magnetic poles 21 to be linear. 
FIG. 3 shows the distribution of lines of magnetic force generated in the 
magnetic circuit embodiment of FIGS. 1 and 2. FIG. 4 is an enlarged view 
of a portion of FIG. 3. In this embodiment, the magnet block 20 is a 
ferrite permanent magnet having a lateral dimension Lx of 250 mm, a 
transverse dimension Ly of 300 mm and a thickness Lz of 60 mm and the 
channel 22 defined therein includes three pairs of opposed steps each 
having a lateral width of 20 mm and a height of 10 mm. 
Assume that X is a horizontal distance from the center (depicted by a 
phantom line) of the magnet block 20 and the vertical distance H from the 
upper major surface of the magnet block 20 is fixed to 40 mm. In FIG. 6, 
solid line curves show horizontal and vertical (or perpendicular) 
components Bx and Bz of the magnetic flux density as a function of 
distance X in the range of from 0 to W/2=80 mm. 
It is understood from FIGS. 3 and 4 and the solid line curves in FIG. 6 
that the first embodiment of FIGS. 1 and 2 produces a magnetic field whose 
unnecessary vertical component Bz is substantially equal to zero over a 
substantial portion of the X range. The reason is given below. As shown in 
FIG. 5 taken together with FIG. 2, at a point P in the region where a 
unidirectional magnetic field is to be produced, the magnetic field has a 
vector V1 directing from the main N pole toward the main S pole and a 
vector V2 extending between each main magnetic pole (N or S pole) and each 
compensating magnetic pole (S or N pole). The magnetic poles are arranged 
such that the resultant magnetic field vector V0 may become substantially 
horizontal. 
Consequently, the first embodiment of FIGS. 1 and 2 produces a nearly ideal 
unidirectional magnetic field substantially free of unnecessary 
perpendicular components over a wide range. The magnetic circuit may be 
located on one side of the unidirectional magnetic field region because 
the magnetic circuit does not enclose the unidirectional magnetic field. 
FIGS. 7 and 8 show a second embodiment of the present invention. This 
embodiment is similar to the first embodiment except for the channel 
geometry. Instead of the stepwise side wall channel, the magnet block 20 
includes a continuously or smoothly recessed channel 30 in one major 
surface between the opposite edge surfaces. The continuous channel used 
herein means that the opposed inner side walls are curvilinear or 
rectilinear slant walls joining to a flat bottom. The inner side walls are 
symmetrical as viewed in the elevation of FIG. 8. Compensating magnetic 
poles 23A and 23B are formed by magnetization in the opposed inner side 
walls disposed inward of the main magnetic poles 21A and 21B, 
respectively. In the illustrated embodiment, the main magnetic poles 21A 
and 21B are N and S, and the compensating magnetic poles 23A and 23B are S 
and N, respectively. 
In this embodiment, the magnet block 20 is a ferrite permanent magnet 
having a lateral dimension Lx of 250 mm, a transverse dimension Ly of 300 
mm and a thickness Lz of 60 mm and the channel 30 defined therein includes 
a flat bottom having a lateral dimension of 130 mm and a thickness of 30 
mm. 
Also in the second embodiment of FIGS. 7 and 8, the magnetic poles are 
arranged such that the resultant magnetic field vector V0 between a vector 
V1 directing from the main N pole toward the main S pole and a vector V2 
directing from each main magnetic pole (N or S pole) toward each 
compensating magnetic pole (S or N pole) in the region where a 
unidirectional magnetic field is to be produced as shown in FIG. 5 becomes 
substantially horizontal. 
FIGS. 9 and 10 show a third embodiment of the present invention. This 
embodiment is similar to the first embodiment except for the block 
geometry. Used in this embodiment is a disk-shaped magnet block 40 having 
a pair of opposite edge surface segments, that is, arcuate segments 
included within angles .theta.1 and .theta.2 where main magnetic poles 41A 
and 41B are formed by magnetization. The main magnetic poles 41A and 41B 
are N and S poles, respectively, in the illustrated embodiment. The magnet 
block 40 has a pair of major surfaces joining the opposite edge surface 
segments (41A and 41B) and extending substantially perpendicular to the 
segments (in the cross section of FIG. 10). A transverse channel 42 is 
defined in one major surface. The channel 42 has symmetric steps defining 
pairs of opposed inner surface portions disposed inward of the opposite 
edge surface segments. As seen from FIG. 10, in the opposed inner surface 
portions disposed inward of the main magnetic pole 41B of one polarity are 
formed compensating magnetic poles 43B.sub.1, 43B.sub.2 and 43B.sub.3 of 
opposite polarity by magnetization. Similarly, in the opposed inner 
surface portions disposed inward of the main magnetic pole 41A of opposite 
polarity are formed compensating magnetic poles of one polarity. 
The results of the third embodiment are equivalent to those of the first 
embodiment. 
In all the embodiments, the magnet block may be either a unitary permanent 
magnet block or an assembly of a plurality of permanent magnet pieces. 
There has been described a permanent magnet magnetic circuit capable of 
producing a unidirectional magnetic field substantially free of 
unnecessary perpendicular magnetic force components without surrounding 
the region where the unidirectional magnetic field is to be produced. The 
magnetic circuit is advantageously applicable to semiconductor fabricating 
apparatus and similar apparatus where restrictions are imposed on the 
attachment or location of the circuit. 
While we have shown and described particular embodiments of our invention, 
it will be obvious to those skilled in the art that various changes and 
modifications may be made without departing from the invention in its 
broader aspects.