Burkhard Littwin

X-bar, a new propagation pattern for magnetic bubbles

It is possible to improve the margins of propagation in field-access bubble memories considerably by a new arrangement of the Permalloy bars which constitute the propagation path. More slender bars of equal length become possible if the bars form an X-shaped pattern and the bubbles propagate at an angle of 45° with respect to the bars. U-turns and bubble gates have been designed and tested successfully as well. By using a 50 μm platelet of Sm-Tb-orthoferrite and bars 20 μm in width, propagation speeds up to 600 kbit/s could be obtained. This represents an improvement by a factor of two as compared with the propagation along a conventional Y-bar pattern, also tested. Below the critical frequency of propagation the field margins are improved when using the X-bar pattern. Some details of the bar fabrication are given.

The NDRW-characteristic of a plated wire memory element with an interlayer of tin

Plated wire memory elements with good NDRW-characteristics can be produced by a sandwich diffusion technique. A thin layer of tin is placed between a smooth copper and the permalloy film and the wire has to be annealed before and after the plating of the permalloy. The reproducibility and the extrapolated long time stability were good. The parameters of this technology and the influence of the two annealing temperatures on the memory characteristics are described. The two fold diffusion of the tin film into the copper and permalloy layers produces the NDRW-behavior.

Improved flat Ni-Fe-film elements obtained by diffusion of an electroplated nonmagnetic layer

The storage properties of flat thin Ni-Fe films can be improved by diffusing a nonmagnetic metal into them. Usually this metal is deposited on the Ni-Fe film in a vacuum chamber. By this well-known technique one can also electroplate the non-magnetic overlay. Several metals, e.g., Zn, Cd, Cu, Pb, Sn, Bi, and In have been tried. Tin gave the best reproducibility. The dispersion remains almost unchanged up to a H_{c}/H_{k} ratio of 1, so that very good bit current margins are achieved. To avoid demagnetization effects, which could mask the film qualities, large spots were investigated. It was found that the creep threshold is independent of the film thickness (350-1000 A). An explanation for this desirable behavior can be found if one assumes that the nonmagnetic metal fills up and enlarges easy-diffusion paths in the film and thus creates obstacles for a creeping wall. Despite the low diffusion temperature for Sn, 50-100°C, the long-time stability of the magnetic parameters is good, and it is possible to get uniform magnetic parameters on large memory planes.

A 4Kbit bubble memory chip using X-bar propagation patterns

The design and operation margins of a bubble memory chip 2,8 mm × 2,1 mm with 32 minor loops of 128 bits of storage capacity each are described. In a special package, designed for test purposes and containing the drive field coils and a variable bias magnet system, signals up to 10 mV are generated by bubbles of 6 μm diameter in a thin film detector. The X-bar propagation works well up to 500 kHz using garnets having the composition Y 1,6 Sm 0,4 Ca 1 Fe 3,9 Ge 1 Si 0,1 O 12 . Also the transfer is performed by special X-bar transfer gates.

Bubble circuit fabrication by electrodeposition

Bubble domain memory chips with micron dimension patterns have been fabricated by additive electroplating through photoresist windows using the conductor first processing. Etching defines the detector strips and removes the plating base. The various fabrication steps are described and discussed. The uniformity of the plated films is good and gives a reasonable yield. Nonmagnetic underlayers were plated below the NiFe bars. The gap width could be reduced by overplating the photoresist windows. 4 kbit chips have been fabricated with the processes described.

Herstellung magnetischer mikrostrukturen für bubblespeicher

Various subtractive and additive microfabrication techniques for Permalloy circuits for bubble domain memories are described. Bit densities of about 6 Mbit/cm2 are possible with ion milling, electroplating, and lift-off processes. The state of art of the high density lithography gives bit capacities up to 36 kbit on storage areas of 0.5 mm2 to 1 mm2. Storage capacities of 64 kbit can be achieved on chip areas of 36 mm2 by photolithographic processes. Permalloy films are used, which have thicknesses of 0.2 μm to 0.5 μm, a coercivity of less than 4 A/cm, a low magnetostriction | λs | < 1 × 10-6 and, for detector stripes, a fractional change in resistance of 3%. The fabrication of magnetic circuits by using only vacuum techniques has advantages for an automation, whereas the additive technique is inexpensive and offers short runs. The uniformity of electroplated microstructures can be compared with that of vacuum fabricated ones. A 3-mask-level design is suited for the additive electroplating. Subtractive techniques prefer a 2-mask-level design. The magnetostriction of the Permalloy films influences the transport characteristics of the various propagation patterns in a different way and can be adapted to the properties of specific patterns.

Galvanic overlay fabrication for bubble memory chips on 2-in garnets

Two-inch-diameter wafers containing 56 bubble memory chips of 16 kbit capacity each were fabricated by electroplating. A three-mask level design with a thin-film detector and a minimum feature of 1.5 μm were used. The NiFe films, plated out of a NiFe-sulfamate or a NiFe-citrate bath, had a coercivity of 1.5 Oe, a magnetostriction of 0 to -1 × 10-6a relative magnetoresistance of 1.8 percent at maximum and a resistivity of 28 μΩcm. A thickness uniformity of ± 2.5 percent was achieved on a Permalloy plating base of 30 nm by balancing the plating conditions. Statistical defects appearing in the transport pattern are significantly reduced by the preplating of a thin Au underlayer into the photoresist windows before the deposition of the Permalloy. The resistance of electroplated Au conductors scattered by ± 8 percent around the mean value and a similar scattering in the dc burn-out current was observed. For reasons of comparison, 16-kbit chips were fabricated using only subtractive techniques. In these experiments Ti-Pd-Au and Ni-Fe layers of the same thickness as the plated ones were evaporated or deposited by sputtering and the microstructures were delineated by ion milling. Comparing the chips fabricated by both technologies, we found no differences in the transport characteristics and the bias-field margins at a drive-field frequency of 100 kHz and for up to 108propagation steps.