Μm Bubble Research Articles

Stretcher design study for 1.5 μm bubble devices

Column period dependence of an asymmetrical chevron stretcher for 1.5 μm bubble devices has been studied under some film thickness conditions. The asymmetrical chevron stretchers with large column periods improve upper thresholds of bias margins although the 15 μm period degrades both upper and lower thresholds of the bias margins. A newly designed stretcher called Mt.Fuji has been developed, which has excellent stretching margin even at the period of 15 μm, typically 40 Oe bias margin at 55 Oe with a triangular wave drive. Nelson type detector for 1.5 μm bubble has been studied. Sensitivity of 0.25 % has been obtained.

Influence of NiFe quality on bubble devices

It is anticipated that as the period of bubble devices decreases, undesired effects due to loss of permeability of the NiFe overlay will become increasingly important, even for half-disk elements. The failure mechanism of ’’I-bar crossing ’’ in H-I tracks, which leads to high values of the minimum drive field Hxy(min), was studied for both 8 and 20 μm bubble periods as a means of gaining insight into such effects. Direct Ferrofluid observations, along with measurements of the parameter Hbb1 which characterizes the coercivity of an individual H bar, confirmed that the I-bar crossing phenomenon originated in high coercivity, and hence high remanence and low permeability of the NiFe. This interpretation was supported by a study of the effects on a sample of a series of annealing treatments at increasing temperatures, a process known to increase NiFe coercivity. The annealing studies demonstrated that for a given NiFe thickness Hbbl was better correlated with Hxy(min) than was the sheet-film coercivity Hc, showing that measurements of Hc must be interpreted with caution. Furthermore it was found that Hbb1 and Hxy(min) decreased with increasing NiFe thickness.

Coercivity in 1.7 μm bubble garnet films

Measurements of magnetic bubble coercivity in Ga substituted and in Ca-Ge substituted garnet films capable of supporting 1.7 μm diameter bubbles show that the Ga substituted films have lower coercivity which increases less rapidly with decreasing thickness than in Ca-Ge substituted films. High temperature annealing experiments indicate that the coercivity may be proportional to the material length and the saturation moment.

SmTmGa- and YSmTmGa-garnets for small−bubble applications

SmTmGa-garnet has a high growth-induced anisotropy, typical results being Ku = 1.1×105 ergs/cm3 and q = 7 for a sample with ℓ = 0.20 μm. Thus the anisotropy of this material is high enough so that a high-q, 1 μm-diameter bubble material could readily be produced by properly adjusting the Ga content. Rocking experiments indicate that hard bubbles are successfully suppressed by ion implantation even though the implanted layer does not exert the same keepering effect as in lower anisotropy materials. Temperature dependence data show that (ΔHcol/Hcol)/ΔT at 50°C is only −0.24%/°C in the ℓ = 0.20 μm sample mentioned above. The mobility of this sample is 140 cm sec−1Oe−1. The mobility can be increased (at the expense of reducing Ku and q) by adding yttrium to the composition. We have formulated such YSmTmGa-garnets for use as 2 μm bubble materials and have grown films with μ = 660 cm sec−1Oe−1, Ku = 2.3 × 104 erg/cm3 and μ = 340 cm sec −1Oe−1, Ku = 3.8 ×104 erg/cm3, depending on the yttrium content. An 11 μm-period symmetrical half-disc-type circuit fabricated on the latter film demonstrated an ∼20. Oe margin at 330 kHz with 50. Oe-peak triangular drive field.

Characterization of 1.5 µm Bubble Devices built on low Magnetization Garnet Films for minimizing Drive Field

Minimization of the drive field by using garnet films with low magnetization has been studied for 1.5 µm diameter bubble devices. Test chips designed on 8 µm period half disks with a 1 µm minimum feature have been built on three sets of garnet films with different magnetizations of typically 350, 400 and 490 Gauss (G) by using planar processing. Results for devices made on garnet films of various magnetization show that the normalized bias margins increase with decreasing magnetization and minimum drive fields for these devices decrease from 30 to 20 Oe. Furthermore, when the drive field was defined as the field necessary to produce normalized bias margins of 20% for propagation and 15% for the stretcher, we obtained a drive field value of 30 to 60 Oe for a magnetization of 350 to 490 G.

Liquid phase epitaxy of garnets

A short introduction to the technique of liquid phase epitaxy of single crystal thin films of magnetic garnets is given with the emphasis on the relative merits of vertical and horizontal dipping. For vertical dipping results are given for the thickness profiles from top to bottom of the samples and the inhomogeneities within the film thickness. The thickness profile is explained by fluid motion caused by density segregation. The inhomogeneities are explained by growth rate changes due to transient growth at the beginning and nearly steady state growth when the diffusion boundary layer has been formed. It is shown under special conditions these inhomogeneities in composition in LaGa : YIG films can be an advantage and can increase the domain wall velocity. These effects are compared to films grown by horzontal dipping with rotation. Using spin wave resonance it is shown that this process does not produce as homegeneous films as expected. Explanation of this is given in terms of fluid flow during the growth. Because literature data on the diffusion coefficient D has a wide spread of values, an experiment is described in which D has been measured independent of the model using radioactive tracers. These results, combined with the model that the garnet does not diffuse as a molecule but as species like Y 2O 3 and Fe 2O 3 and at the growing interface react to form garnet, has lead to a new insight into the solvation of these species in the liquid. Magnetic bubble films of the type CaGe substituted iron garnets are used in large numbers for devices. This makes the reproducibility an important factor. To increase the yields of usuable material it has been found necessary to make the magnetic parameter H collapse independent of the growth temperature because of the thermal constants in LPE furnaces. This has been achieved by combining CaGe melts with Ga. Results are given for a 3 μm bubble material. The LPE technique can be used for non-magnetic garnet materials as well as iron garnets. These materials are usually substituted with rare-earths. The incorporation of Ce into Y 3Al 5O 12 is a difficult case. How this can be achieved and difficulties encountered are explained in the last part of the paper.

Observation of a domain drag effect in amorphous GdCoMo films

We report the propagation of magnetic domains in bias-sputtered GdCoMo amorphous films by the domain drag effect. Current was pulsed directly through GdCoMo films which contained stripe domains or an hexagonal array of bubbles or "bubble raft". In both cases, once the current density exceeded a threshold value associated with coercivity, the domains were set in motion at a velocity proportional to current pulse length and density. All experimental observations are consistent with a domain drag effect previously predicted. Characteristic velocities of 2.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> /sec-amp per current density were found in films with 1 μm bubbles, in order of magnitude agreement with theory. To our knowledge, this is the first direct experimental demonstration of the theoretically proposed domain drag effect.

Operation of 3µm bubble serial loop devices designed on single level masking

An all-permalloy single mask level design for a serial loop circuit has been developed and successfully operated using 3 μm bubbles. The design of individual elements for a single level circuit such as generator, replicator and annihilator which were compatible with half-disk propagation patterns was successfully investigated and optimization of permalloy and SiO 2 spacer film thickness was achieved. The small capacity test chips fabricated provided 13-16 Oe window margins with circular drive field ranges of 40 to 50 Oe at 100 kHz. The critical stretching pulse phase margin for replication, which was the minimum phase margin for all functions was found to be 10 degrees. Details of design and characteristics are also discussed including operating margin dependence on frequency and temperature.

Properties of 2 µm bubbles garnet films

2 μm bubbles Ga, Al or (Ca,Ge) substituted (EuLu) 3 Fe 5 O 12 and (YSmLuCa) 3 (FeGe) 5 O 12 garnet films were grown by liquid phase epitaxy on Gd 3 Ga 5 O 12 substrates. The compositions of the films were varied by modifying the composition of the growth solution. The magnetic properties of the films with 1 = 0,27 ± 0,03μm were discussed. The two compositions (EuLuCa) 3 (FeGe) 5 O 12 and (YSmLuCa) 3 (FeGe) 5 O 12 present a sufficient temperature stability from - 20 to + 80°C to be used in memory devices.

Growth and temperature characteristics of small bubble YSmLuCaFeGe garnets

Several hundred YSmLuCaFeGe garnet films supporting bubbles 1–3 μm in diameter were grown by LPE, and their static bubble properties were measured from −50 to 100°C and compared with those of similar films supporting 5 μm bubbles. Bubble mobilities in the range ∼ 300–1000 cm/sec-0e and q values as high as 4.7 were achieved, along with saturation magnetizations from 330 to 550 G and collapse-field temperature coefficients in the range −0.20 to −0.27%/°C. Static coercivities were generally less than 0.3 0e at 25°C and 1 0e at −25°C. The growth-induced component of the uniaxial anisotropy field, when scaled by bubble diameter, was found to vary in proportion to the relative concentrations of Sm 3+ and Y 3+ in the film, i.e., H K/d ∝ Sm/Y.

Conductor and transfer-gate performance of single-mask field-current access magnetic-bubble devices

Bubble propagation has been investigated in conductor structures on 5 μm bubble (Y,Sm,Lu,Ca)3(Fe,Ge)5O12. The conductors consist either entirely of Permalloy or of a sandwich of Permalloy on gold. A static in-plane field is used for unidirectional bubble propagation. The dependence of bubble motion on current amplitude and in-plane field magnitude has been measured at low and high frequencies. The bubble motion was observed to consist of a first slow step where the current acts against the in-plane field, and a second quick step where both act together. Attainable frequencies increase with increasing in-plane field and current intensities up to a maximum of 1.1 MHz. The sandwich conductor has a 12% operating margin at 830 kHz frequency and 30 mA current. It performs better than the all-Permalloy conductor at the same overall spacing of the Permalloy to the epifilm. An improved transfer gate from current to field-access has an 18% margin, from field to current-access 16%, at 100 kHz. The overlap with the conductor margins is 7% at in-plane fields from 30 Oe. The gates can be combined to obtain a field-current swap gate.

Design and characterization of 3 μm bubble memory chips utilizing Y-Y patterns

Major-minor organized bubble memory chips have been developed utilizing 14 μm period Y-Y propagation circuits. First type is a 75 kbit memory chip consisting of four major-minor units for high performance demands. Designed access time and data rate for 300 kHz drive are 0.3 nsec and 2×600 kbits/sec, respectively. Second type is a single unit major-minor 78 kbit memory chip for medium access time use. Main chip design features are as follows: (1) 14 μm Y shaped Permalloy circuits are used throughout the chip, except for the 18 μm chevron stretcher. (2) Y-Y transfer gates have been tailored in order to adapt to Y patterns. (3) A thin film Permalloy detector is used for the sake of large output (15 mV/2 mA) amd O-π phase detection. The 3 μm bubble chips are fabricated on (YSmTmCa)3(FeGe)5O12 garnet films. Wafers are processed through 3 evaporation steps and 5 masks. Total 20 Oe operating bias margin is obtained at 300 kHz for 50 Oe drive field.

A 300 kbit bubble memory chip with planar structure

A 300 kbit bubble memory chip has been designed based on 3 μm bubble technology. Nominal circuit period is 14 μm and chip size is 9.8 mm×9.6 mm. A gap-tolerant design is fully employed, such as, asymmetric half-disk propagation pattern, half-disk transfer gate, a novel asymmetric-chevron stretcher, etc. The chip is fabricated using a new planar process solving step-coverage problems to achieve good process yield. A typical bias field margin of 18 Oe is obtained at 55 Oe with a triangular wave drive.

Bubble Stretch Propagation Properties on Chevron Patterns

Propagation dependences on chevron pitch and apex angle of chevron stretcher for 3.5 µm bubble devices are investigated by measuring the quasistatic stretched domain propagation margin and static stretch margin. Stretched domain propagation margin in chevron stretcher is found to decrease, when the pattern pitch is reduced to 14 µm for 3.5 µm bubble devices from 28 µm for 7 µm bubbles, where the apex is kept at 107°. In this study, it is clarified that the stretched domain propagation margin increases with increasing the chevron pitch from 14 µm to 18 µm, as well as decreasing the apex angle from 130° to 86°. This is concluded to be due to the change in magnetic charges in the chevron patterns, especially at the apex, induced by a rotating driving field.

Liquid phase epitaxial growth of Ca, Ge-Substituted garnet films having magnetic bubbles 1.0–3.0 μm in diameter

Ca, Ge-substituted, magnetic garnet films which would support bubbles of 1.0–3.0 μm diameter were grown by isothermal deposition frm supersaturated PbO-B 2O 3 fluxes upon garnet films were yttriumm iron garnet Y 3Fe 5O 12, in which rare earths substituted for yttrium and Ge substituted for Fe. Ca was added to charge compensate the Ge. A typical composition of a 2 μm bubble film might be Y 1.35Lu 0.3Sm 0.6Ca 0.75Fe 4.25O 12 , but the compositions of the films were varied by modifying the composition of the growth solution and the growth conditions. The magnetic properties of the films were dependent upon film perfection and composition. Bubble mobilities ranged ≈300–1000 cm/sec-Oe. The films, whose thickness varied from 1.0–3.0 μm to match buble diameters, were uniform in thickness to less than 0.30 μm across a 2.54 cm diameter slice. The properties of the films were sufficiently stable with temperature from 0–100°C to indicate that these materials could be used in memory devices.

Characterization and test results for a 272K bubble memory package

A magnetic bubble memory package has been designed specifically as a serial data store for use in a voice synthesis system. The memory package contains four 68,121 bit serial shift-register chips for a total capacity of 272,484 bits. One such package, operating at a 24 kHz data rate, can provide 12 seconds of speech. Each 5 mm by 6 mm bubble chip consists of a single 68,121 bit shift register, data replicator, expander active and dummy detector, and nucleate generator. A garnet composition with nominally 3 μm bubbles was selected because its bubble-collapse-field temperature coefficient -0.2%/°C accurately tracks that of Indox over the required 10°C to 60°C temperature range. The nominal in-plane fields are 40 Oe amplitude at a frequency of 48 kHz. The bubble chips and the packages used in this study were fabricated in a Device Capability Line at the Reading Western Electric Facility.

Margin degradation in the long-term testing of 3 µm bubble devices

Considerable degradation of the bias threshold ranging from 0.2 to 0.6 Oe/decade in its collapse side was found in the long-term testing of 3 μm bubble 80 kbit memory chips using (YEuYbCa) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (FeGe) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> films. Investigating this problem in all its aspects, it was concluded that this margin degradation was caused by the fluctuation of the bias threshold, which closely corresponded to the fluctuation in the collapse field of bubbles statically trapped under the permalloy bars. To clarify the cause of this fluctuation, the permalloy film, the garnet film ion-implanted layer etc. were investigated, and it was found that this fluctuation depended on the garnet film thickness, the garnet film composition and on the arrangement of the surrounding permalloy patterns. The instability of the in-plane domains in the ion-implanted layers was proposed as a possible explanation of this fluctuation. Satisfactory results were obtained in long-term operation of the 80 kbit chips fabricated using the (YEuYbCa) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (FeGe) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> and (YSmLuCa) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (FeGe) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> films with the increased thickness of 3.4 to 3.6 μm, and it was found that the (YSmLuCa) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (FeGe) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> films exhibited better characteristics compared to (YEuYbCa) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (FeGe) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> films.

Fabrication of 3 µm Bubble 80 kbit Chips

A 3 µm bubble 80 kbit chip was designed by 1/2 scaling from the pattern for 6 µm bubble chip. To obtain a 1 µm minimum feature over a 5 mm square, projection printing was employed. By reducing the reflectance of the surface to be exposed below 33%, standing wave effects with monochromatic illumination could be minimized, and uniform 2 µm width and 1 µm gap pattern could be obtained, notwithstanding the variation in photoresist thickness and the unevenness of substrate. A standard deviation of this distribution in the pattern gap width in the fabricated 80 kbit chips was 0.067 µm when the flatness of the wafer is less than 0.5 µm/chip size. The effects of the variation in the pattern gap on the propagate margins could be ignored, if it fell in the range of 1.03 µm to 1.38 µm. Consequently, the resolution of 1±0.3 µm and uniform bias margin over 80 kbit chip area could be achieved with high yield.

A fast access memory design using 3&amp;#181;m 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.

Magnetic bubble device processing and pickax circuit design

The processing of Permalioy field-access magnetic bubble devices will be discussed for lead-first, double level circuits emphasizing 5 μm diameter bubble circuits, but with reference to scaling down the process to 2 μm bubble diameter circuits. Device structures will be presented with typical material thicknesses and processes used for each level. A new bubble circuit based on the pickax replicator and the pickax propagation element will be introduced with emphasis on the relaxation of processing constraints that this circuit provides. Also quasi-static propagation margins for several of the key pickax elements will be presented.