Bubble memory

Intel 7110 magnetic-bubble memory module Bubble memory is a type of non-volatile computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles or domains, each storing one bit of data. The material is arranged to form a series of parallel tracks that the bubbles can move along under the action of an external magnetic field. The bubbles are read by moving them to the edge of the material, where they can be read by a conventional magnetic pickup, and then rewritten on the far edge to keep the memory cycling through the material. In operation, bubble memories are similar to magnetic drums and delay-line memory systems. Bubble memory started out as a promising technology in the 1970s, offering performance similar to core memory, memory density similar to hard drives, and no moving parts. This led many to consider it a contender for a "universal memory" that could be used for all storage needs. The introduction of dramatically faster semiconductor memory chips in the early 1970s pushed bubble into the slow end of the scale and it began to be considered mostly as a replacement for disks. The equally dramatic improvements in hard-drive capacity through the early 1980s made it uncompetitive in price terms for mass storage.[1] Bubble memory was used for some time in the 1970s and 1980s in applications where its non-moving nature was desirable for maintenance or shock-proofing reasons. The introduction of flash storage and similar technologies rendered even this niche uncompetitive, and bubble disappeared entirely by the late 1980s. Bubble memory is largely the brainchild of a single person, Andrew Bobeck. Bobeck had worked on many kinds of magnetics-related projects through the 1960s, and two of his projects put him in a particularly good position for the development of bubble memory. The first was the development of the first magnetic-core memory system driven by a transistor-based controller, and the second was the development of twistor memory. Twistor memory is essentially a version of core memory that replaces the "cores" with pieces of magnetic tape. The main advantage of twistor memory is its ability to be assembled by automated machines, as opposed to core memory, which was almost entirely manually assembled. AT&T had great hopes for twistor memory, believing that it would greatly reduce the cost of computer memory and put them in an industry leading position. Instead, DRAM memories came onto the market in the early 1970s and rapidly replaced all previous random-access memory systems. Twistor memory ended up being used only in a few applications, many of them AT&T's own computers. One interesting side effect of the twistor concept was noticed in production: under certain conditions, passing a current through one of the electrical wires running inside the tape would cause the magnetic fields on the tape to move in the direction of the current. If used properly, it allowed the stored bits to be pushed down the tape and pop off the end, forming a type of delay-line memory, but one where the propagation of the fields was under computer control, as opposed to automatically advancing at a set rate defined by the materials used. However, such a system had few advantages over twistor memory, especially as it did not allow random access. Bubble domain visualization by using CMOS-MagView Bubble memory driver coils (windings, or field coils) and guides (T bar guides in this case). The guides, or propagation elements, are on top of a magnetic film, which is on top of a substrate chip. This is mounted to a PCB (not shown) and then surrounded by two windings, shown in yellow and blue. In 1967, Bobeck joined a team at Bell Labs and started work on improving twistor memory. The memory density of twistor memory was a function of the size of the wires; the length of any one wire determined how many bits it held, and many such wires were laid side-by-side to produce a larger memory system. Conventional magnetic materials, like the magnetic tape used in twistor memory, allowed the magnetic signal to be placed at any location and to move in any direction. Paul Charles Michaelis working with permalloy magnetic thin films discovered that it was possible to move magnetic signals in orthogonal directions within the film. This seminal work led to a patent application.[2] The memory device and method of propagation were described in a paper presented at the 13th Annual Conference on Magnetism and Magnetic Materials, Boston, Massachusetts, 15 September 1967. The device used anisotropic thin magnetic films that required different magnetic pulse combinations for orthogonal propagation directions. The propagation velocity was also dependent on the hard and easy magnetic axes. This difference suggested that an isotropic magnetic medium would be desirable. This led to the possibility of making a memory system similar to the moving-domain twistor concept, but using a single block of magnetic material instead of many twistor wires. Starting work extending this concept using orthoferrite, Bobeck noticed an additional interesting effect. With the magnetic tape materials used in twistor memory, the data had to be stored on relatively large patches known as domains. Attempts to magnetize smaller areas would fail. With orthoferrite, if the patch was written and then a magnetic field was applied to the entire material, the patch would shrink down into a tiny circle, which he called a bubble. These bubbles were much smaller than the domains of normal media like tape, which suggested that very high area densities were possible. Five significant discoveries took place at Bell Labs: The controlled two-dimensional motion of single wall domains in permalloy films The application of orthoferrites The discovery of the stable cylindrical domain The invention of the field access mode of operation The discovery of growth-induced uniaxial anisotropy in the garnet system and the realization that garnets would be a practical material The bubble system cannot be described by any single invention, but in terms of the above discoveries. Andy Bobeck was the sole discoverer of (4) and (5) and co-discoverer of (2) and (3); (1) was performed by P. Michaelis in P. Bonyhard's group. At one point, over 60 scientists were working on the project at Bell Labs, many of whom have earned recognition in this field. For instance, in September 1974, H.E.D. Scovil, P.C. Michaelis and Bobeck were awarded the IEEE Morris N. Liebmann Memorial Award by the IEEE with the following citation: For the concept and development of single-walled magnetic domains (magnetic bubbles), and for recognition of their importance to memory technology. It took some time to find the perfect material, but it was discovered that some garnets had the correct properties. Bubbles would easily form in the material and could be pushed along it fairly easily. The next problem was to make them move to the proper location where they could be read back out: a twistor was a wire and there was only one place to go, but in a 2D sheet things would not be so easy. Unlike the original experiments, the garnet did not constrain the bubbles to move only in one direction, but its bubble properties were too advantageous to ignore. The solution was to imprint a pattern of tiny magnetic bars onto the surface of the garnet, called propagation elements. When a small magnetic field was applied, they would become magnetized, and the bubbles would "stick" to one end. By then reversing the field they would be attracted to the far end, moving down the surface. Another reversal would pop them off the end of the bar to the next bar in the line, and so on, controlling or guiding the direction of travel of the bubbles. T and I bars/guides, shaped like the letters, were used in early bubble memory designs, but were later replaced by other shapes such as asymmetrical chevrons, the half disk and the c-bar configurations.[3][4] In practice the magnetic field rotates and is provided by a pair of coils, that produce a rotating magnetic field in the X and Z axes, it is this rotating magnetic field that moves the bubbles in the memory. The half-disk and assymetric chevron configurations are gap tolerant, so they have a minimum feature size, which is the gap between opposing legs of adjacent elements, taht is 2/3 the bubble diameter and reduces demands on lithography resolution.[5][6] X bar propagation elements, shaped like the letter X, as well as Y-I elements shaped like the letters, continuous disks which are plain disks, and asymmetric half-disk elements were also used.[7][8][9][10][11] The photolithography masks are created using electron beam lithography.[12] Amorphous magnetic films were also considered as they had greater potential for improvement of bubble memories vs garnet magnetic films, however the existing experience with garnet films meant that they did not gain a foothold. Garnet films have the same or better magnetic properties than orthoferrite films which were considered less promising by comparison. Garnet materials (as films on top of a substrate) could allow for higher propagation speeds of the bubbles (bubble speed) than orthoferrites. Hard bubbles are slower and more erratic than normal bubbles, a problem that is often overcome by ion-implantation of the garnet magnetic film with neon,[13] and can also be done by coating the garnet magnetic film with permalloy.[14] A memory device is formed by lining up tiny electromagnets at one end with detectors at the other end. Bubbles written in would be slowly pushed to the other, forming a sheet of twistors lined up beside each other. Attaching the output from the detector back to the electromagnets turns the sheet into a series of loops, which can hold the information as long as needed.[4]