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.     

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.     

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.     

68 kbit capacity 16 µm-period magnetic bubble memory chip design with 2 µm minimum features

A bubble propagating structure that operates well on a 14 μm to 18 μm propagate period with a nominal 2 μm minimum feature size has been designed. The structure consists of only 1 discrete permalloy feature per circuit period. Sixty-eight kbit-capacity memory chips based on such structures have been designed, built, characterized, packaged and the packages have been characterized. The chip is organized as a set of minor (storage) loops with separate write and read major lines. The bubble manipulating functions, of which the replicate and transfer gates are the most critical, have also been designed with 2 μm minimum features. The design is adequate to provide a 14 Oe bias field margin range with drive fields of about 35 Oe, using a bubble garnet material with approximately 170 Oe free bubble collapse field. Sixty-eight kbit single loop shift register type chips designed using similar propagating structures, however, provide over 20 Oe bias field margin ranges with drive fields of about 35 Oe.     

High-frequency propagation and failure of asymmetric half-disk field access magnetic bubble device elements

High-frequency propagation characteristics and failure modes in 14-μm period, 1.8-μm gap, asymmetric half-disk field-access device were studied using a high-speed optical sampling technique. Propagation elements as well as normal and hand gun corners and chevron structures were included. The operating bias margin at 1MHz, for a structure that had 1.2 MHz as highest possible frequency, was about half of the margin for frequencies of 200 kHz and below. The phase lag between the bubble leading wall and the instantaneous rotating field direction was nearly 90° as the bubble moved through the center of the element where the lag was the greatest. The peak velocity of the leading wall of 55 m/s and the trailing wall of 46 m/s is attributed to bubble interaction with the Permalloy structure creating a ∼125 Oe in-plane field that greatly increases the free bubble "saturation" velocity.     

New method for routine measurement of coercivity in magnetic bubble films

We describe a new technique for measuring coercivity in magnetic bubble films which consists of placing the film in a weak field gradient (∼1 Oe./μm) in order to obtain a set of finger-like domains. The unconstrained ends of these domains are caused to move back and forth in response to an oscillatory field, and the coercivity is obtained from an extrapolation of the linear portion of the response vs. drive field curve. We present a comparison between coercivity values in materials with 3μm and 1.7μm stripe-widths obtained using the new technique and bubble translation. Good correlation is observed for both types of material, the values obtained with the new technique being somewhat higher than the bubble translation values. The difference is ascribed to material non-uniformities.     

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.     

Electron-beam fabrication of high-density amorphous bubble film devices

A low-temperature, all-vacuum process combined with electron-beam lithography suitable for single-level masking devices using 2-μm diameter amorphous bubble films has been developed. A test vehicle which uses 0.75-μm wide chevrons and 1-μm wide T.I bars in an 8,000- bit chip configuration, resulting in an areal density of 1×10⁷bits/in², was used. Important process features are found to be: (1) laminated NiFe films to obtain low Hcand high magneto-resistive effect when deposited at low substrate temperature, (2) maintenance of low surface temperature during metallization to preserve the integrity of exposed and developed electron-beam resist pattern, and (3) proper resist profile for ease of the lift-off process. Excellent bubble device operating characteristics have been obtained as a result of uniformity in materials and structure resulting from careful control of fabrication parameters.     

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.     

Permalloy disk replicator for amorphous film 2 µm bubble devices

A permalloy disk replicator in an amorphous film 2 μm bubble device has been Studied for different geometrical dimensions, and from quasi-static to 200 kHz operating frequency. Distinct regions of the margin plot are observed, and their dependence on geometry is analyzed. It is found that for reliable replication over a wide margin range, the poles from the replicator disk, from the neighboring propagation channel, and from the previously replicated bubbles must be carefully balanced. A replicator with 20% margins on 2-micron GdCoMo films is shown and methods for further improvements are discussed.     

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.     

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     

Replicate/transfer bubble switch

A replicate/transfer switch compatible with the gap tolerant structure has been developed. The switch characteristics are superior to that of the existing switches both at the the 8-μm and 16-μm periods. The switch offers the advantages of good bias and phase margins, ease of fabrication, and reduced drive field requirements. In the 8-μm version the device requires a substantially lower drive field than the pickax design by virtue of reduced bubble-bubble interaction in the minor loops.     

Computer simulation of the operation of a magnetic bubble domain propagation circuit modeled after a current-access device

The operation of a bubble-domain straight-line propagation circuit has been simulated successfully. This simulation has been achieved by our approximating the motion of an s = 0 frozen-azimuth bubble placed under a drive fieldH_{Z}(X, Y, T)= -H_{p} cdot cos [2pi(X/R_{X} - n(T)/4)] cdot exp [-(Y/R_{Y})^{2}]. The simulation has been generated from a previously developed numerical scheme to simulate the motion of a bubble, whose domain shape and magnetization structure along its domain wall were variable. The drive field has been modeled after a dual conductor-sheet, current-access propagation structure, which has a bit period RXand a transverse width on the order of2R_{Y}. The entire field contour has been advanced stepwise in the positiveXdirection by an increase of the integern(T), which represents the drive-phase number. The bubble motion has been observed during the first six drive phases to produce operating margin diagrams for drive frequencies of 250 KHz, 796 KHz and 1 MHz. The method of calculation and the results of the simulation are given.     

A fast access memory design using 3µm bubble 80K chip

A fast-access, non-volatile memory system using 3- μm bubble 80-kbit chips has been designed for an experimental model and evaluated from a systems viewpoint. The goal of this project is to investigate from both the side of technology and cost if the memories built with major-minor organized 3 μm bubble chips are acceptable in the commercial market. This paper describes the practical design of a bubble memory system, with a capacity of 8-Mbits and an average access time of approximately 1 ms at drive frequencies of up to 500 kHz, which involves memory system organization, redundancy design using chips with excess minor loops, packaging, electronic circuits scheme and other considerations. The results of the experiment and the system cost estimate based on this design are also described.     

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.     

Processing a conventional magnetic bubble circuit with a 6-µm period

A conventional magnetic bubble memory with a 6-μm period and submicron details has been made. The memory is an 8-kbit shift register single-mask design with field access NiFe propagation elements. The transfer gates and detector area have an 8-μm period, while the major part of the storage loop has an enhanced density with a 6-μm period. The processing is done with a 1:1 electron image projector, which is capable of making the 0.75-μm smallest features necessary for this circuit. The fabrication uses a lift-off technology with Ti followed by a reactive sputter-etch procedure for the structuring of the NiFe elements.     

Effect of conductor crossings on propagation margins

The effects of conductor delineation technique on magnetic bubble propagation across the conductor edge are described. Propagation margins are obtained for bubble circulation around 18-μm diameter Permalloy discs which cross four edges of an Al-Cu feature. Specifically investigated are isotropic wet etching, anisottopic wet etching to achieve a uniform taper, ion beam milling, and metal lift-off to provide a planar structure. Margins are obtained at ± 40°C, with the most significant degradation observed at the lower temperature. Permalloy magnetic continuity in the crossings can be inferred from hysteresis loop measurements of a Permalloy sheet deposited over a grating pattern formed by the above processing techniques. Although the least anisotropic loops are invariably obtained with smoothly tapered Al-Cu edges under the Permalloy, propagation margins are not maximized with such structures, but rather favor a planar crossing. The results suggest that although patterned stress is still an important concern in functional operation, other geometric effects can be more significant. In particular, poor magnetic step coverage as inferred from loop measurements leads to spurious pole formation from the drive field, while even with adequate step coverage, static bias-field distortions can result because of the component of the field along the step.     

Bubble propagation by disks formed by ion-implantation

The investigation of implantation conditions for bubble propagation points to the necessity for a minimum dose (1.5 times 10^{16}ions cm²) and a sufficiently thick profile (0.4 μ). The use of flat profiles gives uniform implanted layers and allows values of the maximum defect concentration far away from the amorphization threshold. Unlike in permalloy circuits, bubble stability and bias field margins do not increase with the drive field. The propagation is not critically dependent upon disk diameter and spacing.     

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.