Planar processing for magnetic bubble devices

A two-mask level, conductor first thin film process is described for fabrication of magnetic bubble devices. The process permits a stepless permalloy level over a conductor that may be two to three times as thick as conventional processing The planar process is attained by anodizing a thick aluminum alloy film in all regions where no conductor is needed. The process described solves problems in conventional processing caused by thin conductor metalization and permalloy step coverage. Replication, nucleation, and annihilation devices made with this process promise superior performance.     

A hybrid decoder bubble memory organization

A hybrid bubble organization is proposed which combines the advantages of both major-minor loop and decoder organizations while eliminating some of the complexity of the decoder organization. The organization consists of multiplexed decoder sections in which the selection of different blocks is performed in part by decoding and in part by a major-minor type time domain selection. The number of decoder steps can be reduced at the expense of a modest increase in access time. This organization allows the use of a low current retarding type decoder element, thus reducing the on-chip power consumption. Also, a number of decoder conductors can share the same driver, reducing the number of external chip connections.     

Logic functions for magnetic bubble devices

An analysis has been made of the number and types of logical functions which can be performed using the interaction of circular magnetic domains in rare earth iron oxides. Multiple logic functions are found to be produced simultaneously at any logical area. These conjunctive output sets have been categorized. Several conjunctive logic gates have been designed, fabricated, and tested successfully in Sm0.55Tb0.45FeO3using Permalloy overlays. Utilizing a circuit in which AND/OR logic gates are coupled to a dynamic memory bank, the total correlation of two data streams has been performed. It is shown how this multiply accessed dynamic memory serves to establish the correlation threshold.     

Bias magnet design for bubble memory devices

A study has been made of a system consisting of a pair of magnets with thin plates of permeable material on their inner faces. The effect of the plates on the profile of the field between the magnets is predicted as a function of the geometry and the permeability and the way in which the uniformity can be improved is demonstrated.The effect of the permeability is very limited.     

Complementary permalloy bubble propagation structure

Field-access bubble propagation has been achieved in a novel Permalloy structure made up of a pattern and its complement. The pattern is defined by a step in a nonmagnetic spacer on top of which the Permalloy is deposited leaving the Permalloy in two levels. The two layers act in concert to provide coherently travelling potential wells for bubble propagation. The stepped structure is fabricated using a lift-off technique (4000-6000 Å) of Schott glass. Permalloy (1500-2500 Å) is then deposited by radio frequency sputtering over the entire device area. Devices of 10-μm period and 2- to 3-μm minimum feature were fabricated on 2-μm bubble garnets. A propagation margin >10 percent was obtained for 35-to 50-Oe drive fields.     

A new junction design on a Permalloy corner pattern forion-implanted and Permalloy hybrid bubble memory devices

A junction has been developed for hybrid bubble memory devices using ion-implanted tracks for high density data storage and Permalloy tracks for write and read functions. An 18-μm diameter Permalloy corner pattern is used. Both the tapered ion-implanted edges and the operating bias field adjustment boundary at the junctions are located under the Permalloy corner pattern edges. Improved junction properties and analysis by visual inspection are reported. The bubble potentials and the phase of the rotating field, when a bubble reaches the junction boundary, were compared for the conventional and the corner-type junctions. Replicate gate performance for the corner-type junction was investigated. The replicate phase margin was greatly improved for the enlarged Permalloy corner pattern. The temperature dependences of the junction performance were measured between 0 and 80°C. In this temperature range, the margins of the junctions were improved, making them suitable for hybrid bubble memory devices     

Theoretical analysis of longevity testing on bubble memory devices

Theoretical results of magnetic bubble device long-term reliability testing are reported. The bubble during propagation along Permalloy tracks is represented by a simple, one-dimensional stochastic model. An equation to describe fluctuation in cylindrical bubble radius is approximated in the Langevin type stochastic differential equation, in which a set of small effects, such as interaction among bubbles and crystal nonuniformity, are considered as a white noise forcing term. Estimating the average time to bubble annihilation or runout (bubble memory mean time to failure) is reduced to a level-crossing problem for a random process. Calculated bias field margin degradation shows a qualitative agreement with experimental results for an actual bubble device. Bubble material parameters for obtaining maximum operation time are suggested.     

Multilayer processing for magnetic film memory devices

Planar processing techniques for producing coupled film storage devices are presented. The discussion centers around a process sequence for a coupled easy-axis structure built on a metal substrate, largely by vacuum deposition techniques. The process results in packing density of about 8000 bits/in². Included is a test of one such element displaying useful memory device characteristics under conditions of a worst case pulse test program.     

A true swap gate for magnetic bubble memory chips

The design of a true swap gate suitable for incorporation into magnetic bubble memory chips with 16-μm to 18-μm circuit periods is reported. The swap operation is true in that the outcoming bubble takes the position vacated by the ingoing bubble, as well as vice versa. Swap gates of this design have been operated successfully at temperatures from 0°C to 70°C, and frequencies up to 100 kHz.     

Repulsive interactions between magnetic bubbles: Consequences for bubble devices

Experiments and calculations are reported about some aspects of the repulsive interactions of cylindrical magnetic bubble domains. Experiments were done on hexagonal lattices of bubbles in which, for one case, the stable positions of the bubbles were determined by a Permalloy overlay. The calculations refer to different structures and Show that from a point of view of bubble interactions the maximum bubble density for a square lattice is almost the same as for a hexagonal lattice. It is also shown that a preferred thickness of the bubble material sheet exists.     

Asymmetric Chevron type junctions for hybrid bubble memory devices with 16-Mbit/cm/sup 2/ storage density

Junctions between Permalloy and ion-implanted tracks for high-density hybrid bubble memory devices (>or=16 Mb/cm/sup 2/) using 0.5- mu m-diameter bubbles have been investigated at a 100-kHz drive field. The conventional junctions developed for 4-Mb/cm/sup 2/ devices have been found not to work at all. A novel type of asymmetric chevron (AC) junction has been proposed and operated. With the large volume of AC Permalloy patterns and the shallow ion-implanted channel, AC junctions show an operating bias-field margin of more than 6%. The feasibility of junctions for hybrid devices with 16-Mb/cm/sup 2/ storage density has been confirmed.<>     

Fabrication process for ion-implanted and Permalloy hybrid magneticbubble memory devices

A fabrication process for ion-implanted and Permalloy hybrid bubble memory devices has been developed. The mask patterns of polyimide (PIQ) for ion-implanted tracks are fabricated using a tri-level resist process where the top imaging resists are delineated by deep UV contact printing. The minimum feature of the PIQ pattern is 0.75 μm and can be controlled precisely. The sidewalls of PIQ patterns are smooth and vertical. At the junctions between ion-implanted and Permalloy tracks, tapered resist patterns are used as masks for implantation. Novolak-based resist is used and is reflowed thermally to obtain a tapered profile. Fabrication conditions, such as type of resist and baking temperature, were investigated to achieve good reproducibility. Tapered resist patterns are also used as masks for iron milling which reduces the thickness of garnet film in the area of the Permalloy tracks. This is necessary to adjust the operating bias field of Permalloy tracks to that of the ion-implanted tracks. A prototype of a 4-Mb memory device was fabricated, confirming the feasibility of the present process     

NiFeCo--An alternative to permalloy for bubble domain detection?

The characteristics of bubble domain sensors fabricated from ternary NiFeCo films have been studied and compared with Permalloy film sensors. In 350 Å thick films, the ternary alloy exhibits a magnetoresistance ratio of 3.5 percent in contrast to 2.8 percent for Permalloy films prepared under similar conditions. Sensor sensitivity in functional bubble chips is correspondingly greater, while the sensor noise level is equal to-or lower than-that obtained with the Permalloy detector. Low coercivity and dispersion in NiFeCo films aid in producing an overall improvement in signal-to-noise ratio. The performance of NiFeCo sensors operating in 1-μm bubble ion-implanted contiguous-disk devices is described.     

Low-stress permalloy for magnetic MEMS switches

The residual stress in the electroplated magnetic films is an important factor that limits the functionality of many micromagnetic devices. We have investigated the stress in electroplated Ni-Fe alloy for MEMS as a function of bath concentration, utilizing the wafer-bending method. Our investigation demonstrated that a low-concentration plating solution decreases the residual stress in the electrodeposits, and the stress is further decreased by increasing the saccharin additive content. We obtained the low-stress Permalloy Ni/sub 81/Fe/sub 19/ in our experimental conditions. We fabricated a bistable electromagnetic RF MEMS switch with deformation-free cantilever beam using the electroplated Permalloy.     

Contiguous-disk bubble domain devices

Contiguous-disk bubble devices are an approach to higher bit density through the use of coarse overlay patterns in manipulating small bubbles to relax device lithography requirements. As a first step towards such an objective, a fully processed chip using ion-implanted devices has been tested, showing the feasibility of all required memory functions with 5-μm bubbles and 25-μm period overlay patterns. A critique of permalloy versus implanted contiguous-disk devices is made, pointing out their basic difference in magnetization reversal processes and explaining the superiority of the latter over the former in achieving a good edge affinity of bubbles. The requirements for a good implanted device are reviewed, including the selection of garnet material parameters (K1, λ111), of implantation parameters (ion energy and dosage) and of device pattern geometry (thickness and shape of implanted layer). An understanding of these requirements has made it possible to demonstrate 1-μm bubble propagation in several contiguous-disk type circuits with 4.5-μm periods, yielding an areal density of over 3 × 10⁷bit/in²made by conventional photolithography.     

Eight-micron period bubble devices

Magnetic bubble shift register devices of 68 121- and 266 473-bit capacity have been fabricated and tested. The epitaxial garnet bubble films were nominally 1.7 μm thick, supported nominally 1.7-μm diameter bubbles, and had collapse fields of about 260 Oe. The storage area per bit was 64 μm², which was realized with a minimum coded feature dimension of 1 μm and contact photolithography using EBES chrome masters. Initial yields obtained in two experimental batches each of the two chip capacities are discussed. Parametric test results are presented for generator current, transfer current and phase, and rotating field intensity. Nominal values have been established to be 130-mA generate current, 21-mA transfer current, and 60-Oe drive. The detector signals were about half as large as normally obtained from 3.3-μm bubble devices with comparable resistance and conventional design.     

Applications of bubble devices

The applications of magnetic bubbles that are currently seen as most attractive and the current status of development of such devices are considered. A major application is thought to occur in solid-state mass memories. An attractive form of organization is possible whereby relatively short access times can be achieved while using only a small number of read and write circuits. A repertory dialer memory, which has been chosen as a test vehicle, has been developed. The design and performance of the propagating circuits, the self-latching magnetic gates and the generator, which comprise the memory, are discussed. A novel area of application may be in pulse code modulation (PCM) switching networks. It is shown how a network operating as a PCM crossbar switch can be designed using bubbles. A circuit is described which can eliminate the delay associated with bubble propagation in such networks.