Apparatus for high density bubble storage

A magnetic bubble storage system and a method for making it using only two masking steps, one of which is critical. In a preferred embodiment, the storage regions are comprised of ion implanted propagation elements which can be contiguous with one another. The functions of write, read, storage, transfer between storage elements in different shift registers, and annihilation are provided by the method in which the same mask is used to define ion implanted regions and for formation of conductor metallurgy. Permalloy bridges over ion implanted regions are used to provide transfer of information between one storage element and another. In a preferred embodiment, NiFe is used for sensing, annihilation, and transfer of information, while the storage registers are comprised of ion implanted regions defining contiguous propagation elements of generally circular geometry.

CROSS REFERENCE TO RELATED APPLICATIONS 
Ser. No. 537,797 in the name of G. S. Almasi et al, filed Dec. 31, 1974, 
now U.S. Pat. No. 3,967,002, describes a method for making a high density 
magnetic bubble domain storage system in which three masking steps are 
required, one of which requires critical alignment. In that process, 
magnetic disks of soft magnetic material, such as NiFe, are part of the 
ion implantation mask and will not interfere with propagation by ion 
implanted regions. Thus, the magnetic disks define ion implantation masks 
as well as providing functions such as generation, propagation, reading 
and annihilation. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a high density bubble domain memory and a method 
for making it, and in particular to an improved method for making such a 
memory using only two masking steps, one of which requires critical 
alignment. 
2. Description of the Prior Art 
Various systems using magnetic bubble domains are known in the art. For 
example, self-contained magnetic bubble domain memory chip using a decoder 
is shown in U.S. Pat. No. 3,701,125. Additionally, a major/minor loop 
memory configuration is shown in U.S. Pat. No. 3,618,054. In these memory 
systems, components are required which provide the functions of read, 
write, bubble domain propagation, transfer between storage elements, and 
annihilation. That is, bubble domains are generated for representation of 
information, and these bubble domains are generally propagated in the 
memory. After propagation, they are read and then annihilated or returned 
to their storage locations. Furthermore, these memories often require 
transfer functions where bubble domains are transferred from one 
propagation path to another, usually by the use of current carrying loops 
that produce magnetic field gradients for implementing the transfer. 
Many components are known in the art for generating magnetic bubble domains 
and for detecting magnetic bubble domains. For example, a magnetoresistive 
sensing technique is shown in U.S. Pat. No. 3,691,540. For the function of 
storage, bubble domains are generally propagated using any of many well 
known structures. In particular, ion implanted propagation elements having 
generally curved paths are useful for bubble domain storage, since the 
line widths of these elements are generally about four bubble diameters, 
thereby leading to relaxed lithography requirements. Such ion implanted 
structures are described by R. Wolfe et al in the AIP Conference 
Proceedings, No. 10, Part 1, p. 339 (1973). These proceedings contain the 
text of the papers delivered at the 18th Annual Conference on Magnetism 
and Magnetic Materials, held in Denver, Colo., in 1972. Furthermore, U.S. 
Pat. No. 3,828,329 describes propagation structures using ion implanted 
regions. 
The processes used for making magnetic bubble domain chips have developed 
through the years so that single level masking techniques are now 
described for making bubble domain memories in which the propagation 
elements are separated from one another (gapped propagation elements). In 
such techniques, magnetic sensors are deposited using the same mask that 
is used for depositing the magnetic propagation elements. Also, since the 
propagation elements are not in contact with one another, conductors can 
be placed directly over the propagation elements without shorting any 
electrical currents. This means that the bubble domain chip can be 
fabricated using only a single critical masking step. 
However, with the exception of aforementioned Ser. No. 537,797, (U.S. Pat. 
No. 3,967,002) the prior art does not address the problem of making high 
density magnetic bubble domain chips where the propagation elements are 
contiguous to one another. In such systems it is difficult to place 
conductors directly on the propagation elements, since electrical shorting 
may occur. Additionally, several critical masking steps are usually 
required in order to define the sensors, propagation elements, and 
conductors used for bubble domain transfer and sensor current. 
Furthermore, it is usually necessary to provide a "protect" mask to 
protect the magnetoresistive sensor when the electrical conductors are 
formed. Because these are critical problems when bubble domain technology 
is to be used to provide economical memory structures having high density, 
the present invention seeks to provide an improved bubble domain memory in 
which all necessary functions are provided, and which can be made by a 
process using a minimum number (two) of masking steps, only one of which 
requires critical alignment. 
Accordingly, it is a primary object of this invention to provide an 
improved process for fabricating high density magnetic bubble domain chips 
in which only one critical masking step is required. 
It is another object of this invention to provide a process for fabricating 
a magnetic bubble domain chip having contiguous propagation elements, 
requiring a minimum number of masking steps. 
It is still another object of the present invention to provide a high 
density magnetic bubble domain chip having components for generation, 
reading, propagation, transfer, and annihilation, all of which components 
do not require a resolution less than about 4d, where d is the bubble 
domain diameter. 
It is a further object of this invention to provide an improved process for 
fabricating a high density magnetic bubble domain chip using ion implanted 
propagation elements which are contiguous to one another, magnetic sensors 
and annihilators, and current carrying lines for generation, sensing, and 
transfer. 
It is another object of this invention to provide a bubble domain memory 
having improved means for transfer of information from one storage 
register to another. 
BRIEF SUMMARY OF THE INVENTION 
This magnetic bubble domain memory system is characterized by the use of 
metallurgy that serves a dual function and by an improved gate for 
transferring information from one storage register to another. 
Furthermore, this memory is characterized by a high density 
stretcher-replicator-sensor design which is especially useful for reading 
very small (submicron) magnetic bubble domains. 
In a preferred embodiment, the memory is comprised of ion implanted 
propagation regions which serve to define propagation elements in a 
major/minor loop memory organization. The conductor metallurgy (typically 
gold) serves not only conductor functions but also serves as a mask for 
protecting regions of the underlying bubble domain medium which are not to 
be ion implanted. Thus, the gold provides current carrying conductor 
functions and also is an ion implantation mask. 
The improved transfer gate is used to transfer information from the major 
loop into the minor loops, or vice versa. It is comprised of permalloy 
bridges which provide guides for the transfer of information between the 
major and minor loops in response to current in a conductor. Because the 
permalloy bridges aid the transfer operation, the transfer conductors can 
be defined from a continuous conducting layer using the same mask that is 
used to define the regions which are to be ion implanted. Because of this, 
the conductor metallurgy can be provided early in the fabrication process, 
rather than having to be provided through the use of a critically aligned 
mask after ion implantation has been completed. 
Generally, the bubble domain medium is any magnetic medium which will 
support bubble domains, and is preferably a material having garnet 
structure. After this, a continuous layer of the magnetic material used 
for functions such as sensing and transfer bridging is deposited as a 
continuous layer over the entire substrate. This layer is typically NiFe 
which also serves as a conductor plating base. A resist material is used 
to protect regions of the NiFe layer which are not to have a conductor 
deposited on them. A conductor (Au) is then plated over the NiFe layer. 
The Au serves as the current carrying conductors and also as an ion 
implantation mask. 
The bubble domain material is then ion implanted through the gold mask to 
define propagation elements in the major loop and minor loops. Then, a 
second masking step is used to define the conductor metallurgy, that is, 
to define and isolate the separate current carrying conductors from one 
another. Sputter etching is used to remove the gold and NiFe in order to 
leave the desired conductors and to remove conducting material from those 
regions of the magnetic chip where it is not desired. Thus, the 
magnetoresistive sensor (NiFe), the annihilators, and the magnetic bridges 
have no conductors deposited on them. 
These and other objects, features, and advantages of the present invention 
will be more apparent from the following more particular description of 
the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 
FIG. 1 shows a major/minor loop memory organization which can be made using 
two masking steps. In this embodiment, ion implantation is used to make 
contiguous propagation elements along which the bubble domains are 
propagated in response to a magnetic field H which reorients in the plane 
of magnetic medium 10. 
In more detail, a magnetic medium 10 capable of supporting magnetic bubble 
domains therein has a non-magnetic spacer layer 12 located thereover. 
Magnetic medium 10 can be comprised of any known bubble material, such as 
a rare earth iron garnet. Layer 12 is a non-magnetic spacer layer used to 
prevent etching of the bubble domain material during subsequent processing 
steps, such as sputter etching. Also, spacer 12 prevents spontaneous 
nucleation of bubbles in layer 10. A typical thickness of layer 12 is 
4,000 Angstroms. 
The memory of FIG. 1 provides the functions of write, read, propagate 
(storage), transfer between storage elements, and annihilation. All 
portions of magnetic medium 10 which are ion implanted are shown by 
cross-hatched regions 14. All other regions of magnetic layer 10 are not 
ion implanted. 
Functionally, the write circuit W used to nucleate bubble domains in 
magnetic layer 10 is comprised of a conductor 16 which returns to ground 
along a portion 16A thereof. A current pulse I.sub.W in conductor 16 will 
nucleate a bubble domain B1 in the U-shaped portion of this conductor. 
The storage area of the memory is comprised of the major loop 18 and the 
various minor loops 20. Bubble domains B move along the major loop 18 and 
minor loops 20 in response to the reorientation of a magnetic field H in 
the plane of magnetic medium 10. These domains B move in contact with the 
edges of the ion implanted regions of bubble material 10. In FIG. 1, the 
major loop does not move magnetic domains along a continuous path defining 
a closed loop, but rather moves the domains from write circuit W to a read 
circuit generally designated R. 
In order to transfer bubble domains B between the major loop 18 and the 
minor loops 20, a transfer conductor 22 is provided. Conductor 22 overlays 
a portion of the major loop 18 and then returns to ground along conductor 
16A. That is, write conductor 16 and transfer conductor 22 share a common 
electrical path along portion 16A. Located between major loop 18 and each 
of the minor loops 20 are magnetic bridges 24. Typically these bridges are 
comprised of the same material, such as NiFe, that is used for other 
functions in the memory. In response to a current pulse I.sub.T in 
conductor 22, bubble domains will be transferred between the major loop 
and the various minor loops. The direction of transfer depends upon the 
direction of current in conductor 22. 
Prior to entering the read circuit R, bubble domains traveling along major 
loop 18 are stretched to an elongated shape, illustrated by domain B2, in 
response to a current I.sub.ST in stretcher conductor 26. As the elongated 
domain B2 continues to move toward the right to read circuit R, it will be 
split into a plurality of bubble domains such as B3, which travel along 
the edges of ion implanted regions 14 to read circuit R and are then 
cumulatively sensed to provide an amplified output signal. 
Read circuit R is comprised of a plurality of sense elements 28 which are 
connected in a manner to provide a cumulative output signal representing 
the combined effects of the bubble domains B3. In a preferred embodiment, 
sense elements 28 are comprised of a magnetoresistive material such as 
permalloy (a trademark of Allegheny Ludlum Corp.). Electrical conductor 29 
provides a sense current I.sub.S through the series connected sense 
elements 28. Thus, a bubble domain moving to the area of the stretcher 
line 26 will be elongated and then split to provide a plurality of domains 
which individually are detected by read circuit R. This provides an 
amplified output and is therefore suitable for detection of very small 
magnetic bubble domains, such as submicron magnetic bubble domains. 
After being detected, the domains B3 continue to move to the right where 
they are trapped by the annihilators 30. These annihilators are typically 
comprised of a soft magnetic material, such as NiFe, which traps the 
domains. Because this is a destructive read-out memory, the write circuit 
W is then activated to provide new bubble domain data corresponding to the 
data just read. 
As will be apparent from the fabrication steps illustrated in FIGS. 2A-2F, 
certain regions of the magnetic bubble domain system will have overlying 
layers of magnetic material and conductive material, while other areas 
will not have these overlying layers. In particular, the bridges 24, sense 
elements 28, and annihilators 30, indicated by speckled regions, will have 
no conductive layer over them. In a typical embodiment, bridges 24, sense 
elements 28, and annihilators 30 are comprised of NiFe which is initially 
deposited as a continuous layer over the entire substrate 12. It should be 
noted that the cross-hatching for the ion implanted regions 14 extends 
under the bridges 24, sense elements 28, and annihilators 30. The presence 
of this ion implantation does not impair the performance of the functions 
achieved by the bridges 24, sense elements 28, and annihilators 30. 
The portion 32 of the major loop 18 (that is, the region of major loop 18 
between transfer conductor 22 and stretcher conductor 26) has no overlying 
magnetic layer and conductive layer in the final memory organization. This 
is also true in regions 34 and 36. That is, in regions 34 located between 
stretcher line 26 and sense conductor 30, there is no overlying layer of 
magnetic material or conductive material. Further, in regions 36 of the 
propagation elements located between sense conductor 29 and annihilators 
30, there is no overlying magnetic or conductive layer. 
The bias field H.sub.z used to stabilize the size of domains in magnetic 
medium 10 is provided by the bias field source 38. This could be any of a 
number of well known components, such as current carrying coils or 
permanent magnets. The magnetic drive field H used to move domains along 
the edges of the ion implanted regions is provided by the drive field 
source 40. Generally this is a combination of X and Y current carrying 
coils for establishing magnetic fields that reorient in the plane of 
magnetic medium 10. 
The write current I.sub.W is produced by a write current source 42, while 
the transfer current I.sub.T is provided by a transfer current source 44. 
The stretcher current I.sub.ST is provided by a stretcher current source 
46 while the sense current I.sub.S is provided by a sense current source 
48. The magnetic field sources 38 and 40, as well as all of the current 
sources 42, 44, 46, and 48 are activated under control of a circuit 50, 
which is any type of well known electronic circuitry for providing timing 
pulses to synchronize the operation of the various current sources. For 
example, after a bubble domain (or absence of a domain) is sensed by read 
circuit R, a signal is provided to the write current source 42 to either 
provide a nucleating current I.sub.W in conductor 16, or not, depending 
upon whether or not a bubble was present at read circuit R. 
In operation, data is written into major loop 18 by the presence or absence 
of the current I.sub.W in nucleating write conductor 16. If current 
I.sub.W is present, a domain B1 will be nucleated at the location shown 
and will move to major loop 18 as field H reorients. The data thus 
generated propagates to the right in response to the reorientation of 
field H. When the desired data is written into loop 18, it can be 
transferred to the minor loops 20 by a current I.sub.T in conductor 22. 
Depending upon the direction of this current, transfer occurs from the 
major loop 18 to the minor loops 20 or vice versa. Nucleation and transfer 
between major loop 18 and minor loops 20 can occur at the same time. 
When information is to be read from the minor loops 20, a current pulse 
I.sub.T is provided in conductor 22. This transfers the bubble domain 
pattern to the major loop 18 after which it propagates to the right as 
field H reorients. When this information reaches the stretcher conductor 
26, a current I.sub.ST is provided in conductor 26 which elongates any 
bubble domain in the data pattern. The elongated domain then moves to the 
right along the edges of ion implanted regions 14 surrounding areas 34. 
The elongated domain is split into the domains B3 which then pass under 
the sense elements 28. A signal is produced in an associated sense 
amplifier (not shown) which indicates whether the data was the presence or 
absence of a bubble domain. If the data were a bubble domain, the split 
domains B3 would be transferred along the edges of regions 36 to the 
annihilators 30 where they would be trapped. 
The sense elements 28 can typically be magnetoresistive sense elements 
electrically connected in series. As is known by referring to U.S. Pat. 
No. 3,691,540, these elements will undergo a resistance change when the 
stray magnetic field of a bubble domain is coupled to them. This 
resistance change can be detected as a voltage change across all of the 
series connected sense elements, in order to provide an indication of the 
presence and absence of bubble domains in flux coupling proximity to the 
sense elements. 
Depending upon whether or not the data consists of bubble domains, a signal 
will be provided by the control circuit 50 to the write current source 42. 
This signal will activate source 42 if it is desired to nucleate a new 
bubble domain to take the place of a bubble domain just detected in the 
output data. 
Thus, the major/minor loop memory of FIG. 1 is characterized by the use of 
conductors 16, 22, 26, and 29 which serve as ion implantation masks in 
addition to their functions as current carrying elements. This memory is 
also characterized by the use of magnetic bridges between the major loop 
18 and the various minor loops 20. Still further, a 
stretcher-replicator-sensor arrangement is provided which can be 
fabricated in a minimum number of masking steps. 
FABRICATION PROCESS (FIGS. 2A-2F) 
These Figures illustrate typical fabrication steps used to provide the 
memory of FIG. 1. In particular, FIG. 2F is a side elevational view of the 
major/minor loop memory of FIG. 1 taken along line 2F--2F. 
FIG. 2A shows the magnetic bubble material 10 having a non-magnetic spacer 
layer 12 thereover. A continuous layer 52 of magnetic material is 
deposited over the entire underlying layer 12. Layer 52 is a magnetic 
material such as NiFe. It is used as a conductive plating base and also 
for the magnetic elements, such as the magnetic bridges 24, sense elements 
28, and annihilators 30. 
In FIG. 2B, a patterned resist layer 54 is formed on layer 52, in order to 
protect those areas which are not to be covered by a conductive layer. 
More specifically, resist 54 protects the magnetic bridges 24, sense 
elements 28, and annihilators 30, and all areas 14 of the magnetic medium 
which are to be ion implanted. 
In FIG. 2C, an ion implantation mask is provided by plating a metal 56, 
such as gold, over the underlying magnetic layer 52. The plated gold is 
the conductors 16, 22, 26, and 29. Thus, layer 52 provides certain device 
functions and also serves as a plating base when forming the ion 
implantation mask. The various conductors 16, 22, 26, and 29 are later 
isolated from one another in an etching step. 
In FIG. 2D, resist layer 54 is removed and the magnetic material 10 is ion 
implanted using masking layer 56. The ion implantation is indicated by the 
arrows 58. This step can be provided using well known techniques employing 
protons or boron ions. During ion implantation, regions 14 of magnetic 
layer 10 will be implanted. 
In FIG. 2E, a second masking step is shown. A patterned resist layer 60 is 
used to electrically isolate the write conductor 16, transfer conductor 
22, stretcher conductor 26, and sense conductor 29. Therefore, portions of 
the magnetic layer 52 and conductive layer 56 are removed in regions 32, 
34, and 36 (FIG. 1) in order to electrically isolate and define conductors 
22, 26, and 29. 
FIG. 2F is a side view of the completed structure. As is apparent, only two 
masking steps have been used and only the second of these requires any 
kind of alignment. Thus, a major/minor loop memory having functions of 
write, read, storage, transfer and annihilation has been provided using 
contiguous propagation elements by a process involving only two masking 
steps. 
It will be appreciated by those of skill in the art that various 
alternatives exist in this process and that the order of the processing 
steps can be interchanged. For instance, the gold layer 56 can be 
evaporated or sputtered rather than being electroplated. Further, the gold 
layer can be deposited prior to deposition of the NiFe layer 52. As an 
example, a layer of gold can be deposited over SiO.sub.2 layer 12, after 
which the gold layer is etched to form the conductors which are part of 
the ion implantation mask. The gold is also removed from the areas of the 
underlying SiO.sub.2 layer where the sensors, magnetic bridges, and 
annihilators are to be formed. After this, the magnetic medium is ion 
implanted and a continuous NiFe layer is deposited over the entire 
substrate. A mask is then formed to define the magnetic bridges, sensors, 
and annihilators and also to electrically isolate the conductors 16, 22, 
26, and 29. 
Thus, it will be appreciated that many variations of the basic process can 
be envisioned by those of skill in the art. Whatever the sequence of the 
processing steps, the process is characterized in that the conductors are 
used for both the ion implantation mask and for current carrying 
functions, and that the transfer means for transferring information 
between the minor loops 20 and the major loop 18 are comprised of magnetic 
bridges. 
While the principles of the present invention provide advantages which are 
even more apparent when contiguous propagation elements defined by ion 
implanted regions are utilized, it should be evident that the propagation 
elements need not be contiguous with one another in order to practice this 
invention. Further, the materials used for the various regions of the 
bubble domain memory can be different than those illustrated, and other 
geometries can be used for the propagation elements, whether or not they 
are contiguous.