Control function margin degradation in a bubble memory chip

Chip control function and propagation circuit margin degradation due to long-term memory operation, was observed, using the bias field switching technique. 16 kbit major-minor loop organized bubble memory chips with 28 μm bit period, which had an average access time of 2.7 ms for a 100-kHz rotating field, were used. It was seen that degradations in the lower side of the bias field range were independent of chip functional elements. However, at the upper side of the bias field range, degradations in the performance can be classified by dividing the elements into two categories. These were propagation circuits (Permalloy patterns only) such as H-bars, chevrons, etc., and control functions (Permalloy and conductor patterns), such as generators, replicators, etc. Also, it was found that the degradation in the performance of propagation circuits is small compared with that of the control functions. These differences were considered to be caused by a failure in the Permalloy steps over conductors and/or by the magnetic interaction of the bubble and the conductor current.     

The propagation of magnetic bubbles on permalloy disks

A magnetic bubble generator consisting of a Permalloy disk and a current conductor loop has been used recently in a mass memory design utilizing magnetic bubble technology. The bias field range in which the disk can hold the seed bubble is measured in this report as a function of of the rotating field frequency. Above a critical frequency fc, the bias field margins begin to decrease. The dependence of fcon disk size is obtained for disks with diameters from 16 μm up to 43 μm at rotating fields of 20 and 30 Oe. The separation between Permalloy disks and the garnet film is kept at 0.8 μm or 1.6 μm. Results show that at a fixed rotating field, a smaller disk is preferable at higher frequency for a magnetic bubble material with a given mobility. The critical frequency fcobtained is in good agreement with a theoretical calculation using the viscous damping model by Rossol et al. For frequencies below fc, the bias field margin on the disk is equal to that of the propagating channel and circuit failure due to the loss of the generator seed bubble can be eliminated.     

Characterization of magnetic bubble generators

Two types of magnetic bubble generators suitable for a field-access bubble memory have been tested at 1.00 kHz bit rate at in-plane rotating fields above 15 Oe. The bias field margins of the generators at 30 Oe rotating field are equal to or greater than those of loop propagation. Both designs are based on the principle of stretching and cutting seed bubbles circulating around a Permalloy disk. Functions of stretching, cutting and transferring in the generator sequence are accomplished either with Permalloy elements or pulsed current conductors. The operating conditions of the generators in terms of current pulse amplitudes, widths and phase angles are presented. Among the two designs, namely Permalloy-stretch and conductor-stretch generators, the latter has a wider phase-angle margin.     

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     

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.     

New single-mask approach to bubble device fabrication

Permalloy devices with two separate levels of fine permalloy and conductor patterns have been fabricated by the use of a single-mask process. This process makes possible the production of devices having an essentially conventional design in addition to a completely planar structure. The features of this new process include 1) preparation of a reticle on which patterns for both permalloy and conductor layers are superimposed, 2) use of SiO2, Mo/Au/Mo, SiO2, permalloy, and TiO2thin films deposited sequentially on the bubble materials, and 3) simultaneous delineation of both photoresist patterns with two different thicknesses and of the desired patterns by CF4plasma etching and ion-milling.     

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.     

Optimum design considerations for dual conductor bubble devices

A design for dual conductor, current-access bubble devices with 8-μm periods has been optimized with a numerical calculation method for bubble motion in a propagating magnetic field, generated around hole patterns in conductor layers. Magnetic bias field distributions are calculated for an oval hole chain in the conductor layers. Bubble motion equations are obtained with analytical field distribution functions approximating the calculated field distributions. Minimum drive current density Jminfor normal bubble propagation is determined by a solution to the equations. The hole shape has been optimized by the minimization of the drive power Pmin, the product of Jminand conductor resistance, which is calculated from current distributions around the hole pattern. Optimum layer thickness have also been obtained for 8-μm period bubble devices. Both registration tolerance between the two conductor layers and bubble skew effects have been studied semiquantitatively on the basis of the equations of motion. The numerical calculation method developed here is found to be a highly effective means to optimize pattern design for smaller period devices.     

New fabrication technique for magnetic bubble circuits with submicron dimensions

A new technique which permits the fabrication of submicrometer bubble propagation circuits has been described. Straight line patterns and contiguous zigzag patterns are combined with an appropriate registration to form bubble propagation patterns. The straight line pattern width corresponds to the gap width in the Permalloy bubble propagation circuits. By controlling the exposure time in fabricating straight line photoresist patterns, submicrometer pattern gaps are easily obtained using photomasks with 1 μm minimum features. The 4 μm period and 0.5 μm gap width permalloy circuits fabricated using this technique provide promising propagation characteristics for 1 μm bubbles: 60 Oe bias field margin at 60 Oe drive field and 25 Oe minimum propagation drive field.     

Gap-tolerant half-disk bubble device margins

A simple model is presented which allows accurate prediction of bias margins of gap-tolerant half-disk propagation tracks for bubble domains. After this is verified by comparison with experimental margin data, an "isomargin" plot is derived to show how the margin varies as a function ofWandG, whereWis the minimum linewidth andGis the inter-bar gap. The bias margin is shown to decrease along a fairly straight line which goes to zero whenW + Gequals the runout diameter, i.e., whenW+G approx 1.5 W_{s}, where Wsis the bubble stripwidth or average bubble diameter. This agrees with experiment, and means that the minimum resolvable feature for half-disk type patterns must be less than0.75W_{s}, and probably will not be much larger than0.5W_{s}to0.6W_{s}. It is concluded that, if made with perfect Permalloy, T-bars and half-disks should propagate isolated bubbles equally well. The advantages of half-disks over T-bars are 1) the fatal bar-crossing problem of T-bars with multiple bubbles is avoided, 2) the minimum propagation field is lower than for T-bars, and 3) half-disks seem more tolerant of "bad" (e.g., high-coercivity) Permalloy. Also tabulated are the effects on margins of variations in the device parameters of a representative design, as might be encountered in a fabrication process with finite tolerances. A brief discussion of stop-start margins is given in conclusion.     

Some characteristics of ion-implanted bubble chips

The relations between the position of charged walls and the bubble motion around propagation circuits are discussed. Long walls which extend between adjacent propagation loops are revealed by the Bitter technique. The examination of the domain structure in the implanted layer shows the existence of a magnetic gradient which is a function of the distance from the propagation circuits. The switching of magnetization in particular directions of the in-plane field is reported and correlated with the bubble movement. An additional easy axis is observed along the circuits due to shape anisotropy. Propagation margins are very similar to those obtained with permalloy circuits. Fabrication technology as well as design of 16 μm period circuits is discussed. Nucleation and transfer have been achieved with currents in the range of 50 mA to 200 mA. Phase margins of about a quarter of a period are found, and bias field margins fall between 10 and 15 Oe.     

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.     

Stroboscopic observation of cylindrical domain propagation in a T-bar structure

A technique of stroboscopic observation has been applied to the examination of the dynamic behavior of cylindrical magnetic domains, in a single crystal platelet of Sm0.55Tb0.45FeO3, propagating around a closed path Permalloy film T-bar structured overlay. Domain propagation is achieved by means of the time-varying magnetic pole configuration induced by a rotating magnetic field in the plane of the Permalloy film. Semiquantitative measurements under conditions of continuous propagation show instantaneous wall velocities varying from almost zero to greater than six times the time-averaged domain velocity. The speed of propagation in this T-bar structure is limited by the design of the comer where domains become trapped. The stroboscopic technique used here should find important application in the dynamic evaluation of designs for domain-propagation structures.     

Magnetic bubble device scaling and density limits

Scaling of magnetic bubble devices to smaller bubble sizes and higher density is considered. Drive field requirements, materials requirements, fabrication requirements, current requirements, and detector signal-to-noise ratio are all calculated as a function of bubble size and related to practical limits imposed by bubble materials, fabrication techniques, and electromigration limits. It is concluded that "conventional" bubble devices using Permalloy bars can be made practical with 1-μm bubble domains (storage densitysim6 times 10^{6}bits/cm²). Although it may be possible to extend these Permalloy bar devices to even smaller bubbles, it seems more likely that other bubble devices such as contiguous disk devices or bubble lattice devices will in fact be used for densities greater than 6 × 10⁶bits/cm².     

Models for I and T pattern permalloy bars based upon bitter pattern observation of domain wall movements

The magnetostatic fields of the I and T pattern Permalloy overlay bars are analyzed by proposing a model based on the Bitter pattern observation of the domain wall structure in Permalloy bars. The magnetic charges that appear on the 90° domain walls are assumed to be the sources of the magnetic fields of the bars. The model has a two-dimensional reaction to an applied rotating in-plane field due to its two-dimensional domain wall movement and the consequent two-dimensional change of magnetic domain pattern inside the bar. The magnetization of the bar is equal to Msthe saturation magnetization of the bar at every section of the bar except on the domain walls. The magnetization curve and the magnetic field well Bz(bubble drive field) under the overlay bars are calculated and compared to that of the previous models. A qualitative explanation of the rotation of the bubble around the bars is given by the three-dimensional plots of the field well obtained for different orientations of the in-plane field.     

The properties of X-bar bubble memory chips

The properties of a 16-kbit bubble memory chip having a conventional major-minor loop organization are described. The chip can be operated up to a shift rate of 300 kHz which corresponds to a mean access time of 0.5 ms to blocks of 128 bits. Overall bias field margins of 13 percent were obtained, especially as the result of an appropriately selected layout of the transfer gates which is described in detail. Additional data are given concerning the influence of ambient temperature and of the number of propagation steps. The possibility of fabricating X-bar transfer gates using a single photomask step is demonstrated.     

Bubble dynamics in T-bar-type circuits

A theoretical model for bubble propagation under T-bar type overlays due to an in-plane rotating field is developed. By making certain assumptions about the nature of the overlay magnetisation, it is shown that bubble driving forces can be readily calculated for a circular bubble of varying diameter moving freely in 2-dimensions. Expressions for the frictional forces acting on a bubble are derived and the resulting equations are solved to yield the bubble centre trajectory, the radius, velocity and phase variations. This analysis is applied to examine bubble propagation along a straight portion and around a corner of a T-bar circuit. Cases of successful propagation of an isolated bubble, as well as of failure to propagate, are given. Bias margins for a straight T-bar track at 100 kHz are obtained theoretically and compared with experimental results for a chip having the same parameters.     

A practical magnetic bubble AND-OR gate

The design and operation of a magnetic bubble AND-OR gate are reported. Operation at 100 kHz in a 25 Oe rotating field with 28.2 μm circuit periodicity was achieved with about 50 percent of the free bubble bias field margins. A transfer pulse is used to divert bubbles from a propagation path which delivers the AND output tO one which delivers the OR output. The transfer is defeated by the presence of a bubble in the appropriate cycle of the OR path. The AND bubble is then delayed by one cycle instead of being transferred. This frastrated transfer strategy was devised to circumvent the restrictive bias field limitations in the operation of previous logic circuits.     

Characteristics of a 1-Mbit bubble memory chip

The design and characterization of a 1-Mbit block-replicate (swap) chip organized as 584 loops of 2048 bits each is described. Double period propagation elements are employed in both input and output tracks. All propagation elements employed outside of the storage area have a period in excess of 14 μm to minimize step coverage and conductor induced stress problems. A semi-planar polyimide based process is used to fabricate the chips. The replicate gate is subdivided to reduce the voltage requirements. By using copper conductors and simple voltage boosting techniques, the chip can be operated from a 12-V supply. The swap gate is a slight modification of an existing double period design and the replicate gate is based upon a modification of Bonyhard's sideways replicator design. A half-shorted (backside) chevron stretcher detector is employed. Operating characteristics of these components are given over temperature for both chip and package testing. Two programmable map loops are included on-chip to enhance yield, either one of which may be loaded by selecting the transfer-in pulse parity with respect to an alternate bit data stream. The write pulses are multiplexed on the same control line as the map loop read pulses thus saving pins. The design details and the operating margins for the map loops are given.     

New ion-implantation method for 4-µm period bubble device

It is well-known that bubble propagation margins for ion-implanted bubble devices depend strongly on ion-implantation conditions. A new ion-implantation method is reported that can significantly improve bubble propagation margins for minor loops with 4 × 4 μm bit cell size. The implantation was done through a Mo thin film layer so that the lattice strain and the anisotropy field change would be more uniform through the depth of the implanted layer. With this method, minor loops can be formed by hydrogen single implantation. Consequently simplification of the implantation process is achieved.