Confinement channels for magnetic bubble memory devices

Confinement channels for magnetic bubbles are formed in a magnetic garnet layer. In the preferred embodiment, the magnetic garnet layer is etched to make it thinner than those regions not directly beneath the permalloy members. This defines channels in the garnet layer having better propagation characteristics (e.g., more garnet). This results in more reliable bubble propagation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the magnetic bubble memory devices. 
2. Prior Art 
Magnetic bubble memory devices for storing digital information are 
commercially available, for instance, from Intel Corporation. Generally, 
these devices employ a garnet substrate on which a magnetic garnet layer 
is formed in an epitaxial process. Permalloy members define localized 
magnetic fields under the influence of a rotating magnetic field causing 
the bubbles to move in the epitaxial layer. The permalloy members are 
fabricated over, and insulated from, the epitaxial layer. An intermediate 
layer of conductors, where needed, are also used for replicate gates, 
detectors, etc. 
One failure mode in magnetic bubble memory devices occurs when a bubble 
slips or jumps, that is, when it does not move as intended. For example, a 
bubble, instead of following along a line of propagators, may jump to 
another line; or, instead of propagating from one chevron to the next, may 
slip backwards. The physical relationship between the permalloy members 
and the epitaxial layer, the magnetic field strength, and the shape of the 
permalloy members themselves, are some of the factors determining the 
reliability of the bubble propagation within the epitaxial layer. It is 
known, for instance, that by increasing the magnitude of the rotating 
magnetic field, more reliable propagation occurs, however, this requires 
additional power. 
Other improvements have been suggested for improving bubble reliability. 
For example, in copending application, Ser. No. 483,914, filed Apr. 11, 
1983, entitled "Method for Selecting Propagation Elements for Magnetic 
Bubble Memory", and assigned to the assignee of the present invention, 
differently shaped propagation elements are used for moving bubbles in 
opposite directions. This compensates for an asymmetry in crystal 
orientation in the epitaxial layer. Another suggestion has been to use 
different thicknesses of insulation between the permalloy elements and 
epitaxial layer to improve the magnetic coupling between the elements and 
the bubbles. 
As will be seen, the present invention is directed to improving the 
reliability of bubble propagation in the epitaxial layer. Confinement 
channels are defined within the epitaxial layer to better confine the 
bubbles to predetermined propagation paths. 
SUMMARY OF THE INVENTION 
An improvement in a magnetic bubble memory device which includes a 
substrate, magnetic garnet layer in which magnetic bubbles are propagated 
and overlying permalloy members (propagation elements) is described. 
Confinement channels are formed in the magnetic garnet layer to confine 
the magnetic bubbles to predetermined propagation paths. The confinement 
channels are formed directly beneath the permalloy members. One of several 
processes is used for improving the magnetic characteristics in the garnet 
layer beneath these members; the other regions of the garnet layer, for 
instance, those between lines of propagation elements have inferior 
magnetic characteristics. In the presently preferred process for forming 
the confinement channels masking members are defined on the garnet layer 
at the locations below the permalloy members. The layer is then subjected 
to a plasma etch which thins the exposed magnetic garnet. In subsequent 
processing the propagation elements are formed over the confinement 
channels (i.e., thicker regions of the garnet layer). A metal alignment 
marker is fabricated on the magnetic garnet layer to allow subsequent 
alignment of the permalloy members to the underlying confinement channels.

DETAILED DESCRIPTION OF THE INVENTION 
Magnetic bubble confinement channels and processes for fabricating these 
channels in a magnetic bubble memory device is described. In the following 
description, numerous specific details are set forth, such as layer 
thicknesses, in order to provide a thorough understanding of the present 
invention. It will be obvious, however, to one skilled in the art that the 
present invention may be practiced without these specific details. In 
other instances, well-known methods, processes and structures have not 
been shown or discussed in detail in order not to unnecessarily obscure 
the present invention. 
Referring to FIG. 1, in prior art magnetic bubble memory devices, the 
devices are generally fabricated on a garnet substrate 10, such as 
gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12). A magnetic garnet 
(epitaxial layer) is formed on the substrate and after ion implantation, 
is used for the magnetic storage layer. Permalloy members such as 
propagation elements 13 are formed above the magnetic garnet layer 11 and 
are insulated from this layer by insulative layer or layers such as a 
silicon dioxide layer 12. Conductors for detectors, replicate gates, and 
the like, are fabricated between the magnetic garnet layer and permalloy 
elements, these are not shown in FIG. 1. The bubbles are moved in the 
layer 11 in a well-known manner by an in-plane rotating magnetic field. A 
fixed magnetic field perpendicular to the rotating magnetic field (or 
slightly skewed to this perpendicular) is used in a well-known manner to 
maintain the bubbles in the magnetic garnet layer. 
Referring to FIG. 2, with the present invention the magnetic bubble device 
is again formed on a garnet substrate 20 such as was the case with the 
prior art device of FIG. 1. An ion implanted magnetic garnet layer 
(epitaxial layer 22) of uniform thickness is formed on the substrate 20. 
Confinement channels, that is, channels in which the magnetic bubbles are 
to be confined are etched within the layer 22. These channels are regions 
having better magnetic characteristics than regions of the layer 22 
surrounding the confinement channels. As is seen in FIG. 2, the 
confinement channels 22a and 22b are thicker than the remainder of the 
layer 22. Thus, even though the entire layer 22 is fabricated from a 
uniform magnetic garnet material and is uniformly ion implanted, improved 
magnetic characteristics exist within the channels 22a and 22b since it is 
known that magnetic bubbles will tend to stay in the thicker regions and 
avoid the thinner regions. 
In the presently preferred embodiment a silicon dioxide layer of 
approximately 2000A thick is formed over the magnetic garnet layer. The 
conductors (where they occur in the device) are fabricated on the silicon 
dioxide layer 24. Then, an additional insulative layer which in the 
presently preferred embodiment is a polyimide layer 25 of approximately 
2000A thick, is formed over the silicon dioxide layer. The permalloy 
members 26 and 27 are fabricated on the polyimide layer. (For a discussion 
of polyimide, see U.S. Pat. No. 3,179,634). 
In FIG. 3, the substrate 20 of FIG. 2 is illustrated prior to the formation 
of the confinement channels 22a and 22b. In the presently preferred 
process, a magnetic garnet layer 22 of uniform thickness (e.g., 2 microns) 
is formed on the substrate 20 employing an ordinary epitaxial process. 
Next, a metal layer is deposited on the garnet layer to allow formation of 
the alignment markers 35. For instance, a 1000A thick layer of permalloy, 
chrome, or other metal is deposited. Then the metal is removed from all 
areas except in the vicinity of areas where a marker will later be 
patterned. Two markers per device are formed on the wafer. The marker 35 
as will be described in more detail is used to assure alignment between 
the confinement channels and the permalloy members. 
Now a photoresist layer is formed on the magnetic garnet layer and is 
patterned to form masking members over the predetermined locations of the 
confinement channels and the alignment markers 35. As mentioned, the 
confinement channels are disposed directly beneath the permalloy member. 
The magnetic garnet layer 22 is etched in alignment with the members 30 
and 31 to form the confinement channels 22a and 22b of FIG. 2. A plasma 
etch is preferred, however, a wet etchant may be employed. In the 
presently preferred process, the layer 22 is etched to a depth of 
500A-1000A, although it is possible to etch the magnetic garnet layer 22 
to a greater depth. (The confinement channels may also be formed using ion 
milling in alignment with the masking members 30 and 31.) 
Next after removal of the masking members, the layer 22 is subjected to ion 
implantation of neon, and the insulative layers 24 and 25 of FIG. 2 are 
formed with intermediate conductors where needed. Then a permalloy layer 
is formed on the insulative layer 25 followed by the formation of masking 
members used to define the elements 26 and 27 of FIG. 2. These masking 
members are formed with a mask which is aligned using the metal marker 35 
of FIG. 3. Since the markers of 35 are used both for the masking of both 
the confinement channels and the permalloy members, alignment is assured 
between the channels and these members. It should be noted that the 
confinement channels 22a and 22b are not clearly visible when masking for 
the permalloy members. The metal marker 35, however, can be used since it 
will be visible through the polyimide and silicon dioxide layers or since 
it will cause a sufficiently large step in these insulative layers. 
In FIG. 4, a plurality of permalloy members, specifically chevron 
propagation elements 40 are illustrated. The outline of the underlying 
confinement channels is shown by outline 41. In general, the confinement 
channels extend slightly beyond the outline of the chevrons. 
Normally, a bubble will propagate, by way of example, from chevron 40a to 
chevron 40b. A failure occurs when a bubble fails to move from one end of 
chevron 40a onto the adjacent end of chevron 40b. Instead, the bubble may 
slip against the direction of propagation back onto the other end of 
chevron 40a, or it may strip out into a longer domain which would extend 
from the desired location to another chevron element. This is shown by 
arrow 44. With the confinement channels as described, this is less likely 
to occur since there is no confinement channel below the path shown by 
arrow 44. (The magnetic characteristics in the magnetic garnet layer 22 
are less favorable below arrow 44 since the layer is thinner in this 
region and consequently the bubble is less likely to move in the path of 
arrow 44. Also, because there is no confinement channel between the lines 
of propagation elements, a bubble is less likely to jump from one line of 
propagators to another as shown by arrow 45. (This jumping, of course, 
constitutes a failure.) Importantly, the confinement channels are 
continuous along the path of propagation, and more specifically, between 
the chevrons in the region shown by arrow 48, thus encouraging the bubbles 
to move from chevron to chevron. 
The outline of the confinement channels is relatively easy to determine for 
the chevron propagators shown in FIG. 4. In more complex structures, such 
as the replicate gates of FIG. 5, layout considerations in some cases 
force the confinement channels into other than the most desirable regions. 
The replicate gates of FIG. 5 receives bubbles on lines 50 and 51 and 
return the bubbles to lines 53 and 54. The bubbles are replicated on lines 
55 and 56 which moves the bubbles into detectors. Well-known hairpin 
control lines 58 and 59 formed in the conductive layer are used to 
replicate the bubbles. The outlines of permalloy members 62 and 63 have 
been shown with cross hatching to identify them from the underlying 
structure. Ideally, the confinement channels should extend at least to the 
edge of the members 62 and 63. To facilitate layout, the confinement 
channels on the inner portion of elements 62 and 63 remain entirely under 
the propagation elements as shown by line 65. 
FIG. 6 illustrates another process by which the confinement channels may be 
formed. An ordinary garnet substrate 67 is illustrated with an overlying 
magnetic garnet layer 68. This layer may be identical to layer 11 of FIG. 
1, that is, an epitaxial magnetic garnet layer of uniform thickness. An 
insulative layer 69 is formed over the magnetic garnet layer and permalloy 
members, (e.g., chevrons 70 and 71) are formed on the insulative layer 69. 
The layer 69, members 70 and 71 are formed in an ordinary manner, such as 
discussed in conjunction with FIG. 1. 
In FIG. 6, the permalloy members themselves are used as masking members to 
define the confinement channels in the layer 68. The substrate is 
subjected to ion implantation of hydrogen ions shown by lines 73 in FIG. 
6. The ions are blockd by the permalloy members. In regions without 
permalloy members the hydrogen ions weaken the magnetic characteristics in 
the magnetic garnet layer 68. As illustrated in FIG. 7, since no permalloy 
is present between chevrons 70 and 71, the ions (but for the masking 
members 72) would be implanted between the chevrons. This would weaken the 
magnetic characteristics along the path of propagation. For the process 
shown in FIGS. 6 and 7, masking members 72 are placed between the 
permalloy elements in the path of propagation. This is best illustrated by 
the masking member 72 of FIG. 7 formed along the line of the bubble 
propagation between the chevrons 70 and 71. The mask 72 prevents the ions 
from weakening the magnetic characteristics of the garnet layer 68 along 
the propagation path. An ordinary masking and etching step is used to form 
masks 72. The masks 72 can be removed following the implant. Thus, by 
using the permalloy members themselves along with selective masking, 
confinement channels are formed in the garnet layer 68. 
FIG. 8 illustrates still another process by which confinement channels can 
be formed. An ordinary substrate 74 is illustrated in FIG. 8 with an 
overlying epitaxial magnetic garnet layer 75. As was the case for the 
embodiment of FIG. 6, this laye may be of uniform thickness. Permalloy 
members 76 and 77 are formed above the magnetic garnet layer 75 on the 
insulative layer 81. The structure of FIG. 8 is subjected to laser 
irradiation. The portion of this light which is not blocked by the 
permalloy members passes through the substrate 74 without causing any 
heating as shown by rays 79. Other of this light, shown by rays 78, 
strikes the permalloy members and is absorbed by these members. This cause 
heating in the permalloy members. The permalloy members transfer heat to 
the magnetic garnet as shown by lines 80. The garnet material closest to 
the permalloy members receives more heat than, for instance, the garnet 
material disposed between lines of chevron elements. The heat causes 
annealing in the garnet layer which improves the magnetic characteristics 
of the garnet. Consequently, those portions of the garnet layer 75 closest 
to the permalloy members (since they receive more heat) have better 
magnetic characteristics, thereby defining the confinement channels. Note 
that unlike the process of FIGS. 6 and 7, masking members such as members 
72 or their equivalent, are not needed. The permalloy members along the 
path of propagation are close enough to one another that the heat from the 
adjacent chevrons, for instance, in region 85 is annealed, making the 
confinement channel continuous along the line of propagation. 
Thus, an improvement in magnetic bubble memory devices has been described. 
Confinement channels are formed beneath the permalloy members in the 
magnetic garnet layer. These channels have better magnetic characteristics 
than the surrounding regions of the layer. The magnetic bubbles tend to 
stay within the channels and do not as readily cause failures by slipping 
or jumping or stripping out. The invention is particularly useful for 
assuring that magnetic bubble memory devices which would otherwise have 
marginal performance, perform satisfactorily.