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.     

Control of coercivity in NiFe films for single-level masking bubble devices

Methods of reducing Hcin thick NiFe films deposited on evaporated Au have been studied for applications in conductor-first single-level masking bubble device fabrication. Results obtained using surface modification techniques, such as ion implantation, plasma etching, and semiconducting intermediate layers (TiOx) are described. In addition, the performance of actual 5-μm and 2-μm devices is discussed.     

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.     

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.     

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 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.     

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.     

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.     

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.     

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.     

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.     

Ion-implanted and permalloy hybrid magnetic bubble memory devices

Hybrid bubble memory devices have been proposed and operated with the memory density of 4 Mbit/cm². In the hybrid bubble memory devices, minor loops are composed of ion-implanted tracks with 4-µm period, and major lines and functional parts including block-replicate and swap gates are composed of Permalloy tracks with a longer period of 12 µm. Passive junctions between ion-implanted and Permalloy tracks have been developed, introducing the tapered ion-implantation technique. Improving the characteristics of the functional parts composed of Permalloy tracks, the hybrid bubble memory devices with block-replicate and swap gates have been operated, and the feasibility of the devices has been confirmed. In addition, the possibility of higher memory density has been shown.     

Permalloy film NDRO memory

A thin magnetic film NDRO storage cell has been developed for very high-speed word-organized memories. The storage cell contains two 500-Å, 15-mil-square Permalloy film elements with a read and sense line between them. One film element is deposited on a metallic ground plane, so that the read line and its image in the ground plane are coupled to the readout film and decoupled from the storage film. A 3-ns-wide 300-mA read pulse with a 1-ns rise time yields a 4- to 6-mV output signal. Two methods of coincident current writing were investigated. One with the word and digit lines outside the storage cell required 500-mA word current, 80-mA digit current, and a read after write pulse. Writing takes 50 ns and the storage cell will tolerate coincident word and digit disturb currents of 50 and 100 mA, respectively. The other method uses the read and sense lines between the films as word and digit lines. Word currents of 300 mA and digit currents of 50 mA are required to write. The disturb margins are greater, and read after write pulse is not required. Because the eddy currents in the ground plane must decay, writing takes 3 μs. A 512-word, 40-digit test plane has been built and partially populated to determine problems in building a large array. The characteristic impedances and effects of attenuation and delay have been observed. In particular, the output signals are reduced one half because of attenuation and the difference in signal propagation time in the two directions on the sense line.     

1-µm bubble ion-implanted devices with two-level ion-implanted layers between propagation tracks

A new kind of 1-μm bubble device with two ion-implanted layers between propagation tracks has been studied. The ion-implanted layer in the center between two adjacent propagation tracks is thinner than that in the vicinity of the propagation track edge, where bubbles adhere. The unimplanted area is produced by the use of both photoresist and Mo masks to stop the ions, whereas the thinner implanted layer is produced by the use of the thin Mo mask alone. The ion-implantation mask-making for fabricating two ion-implanted layers is achieved with a gas plasma etch procedure. This structure makes it possible to scale down a loop period and create a circuit of high density. Reliable propagation margins for minor loops with 4 × 3 μm cell sizes can be obtained.     

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.     

Magnetic bubble logic component library

A comprehensive library of magnetic bubble logic components has been established, with emphasis on standardization in terms of dimensions and I/O to facilitate VLSI chip composition. These components were designed in a current-access perforated-sheet configuration and fabricated on magnetic garnet wafers supporting 2-µm bubbles. The operating margin for each component was studied individually on 8-µm period devices at 1 MHZ by using a high-speed magneto-optical sampling-camera system. The bubble-to-bubble interaction force was found to be very reliable in producing successful logic operation with a 12-percent bias-field margin for the XOR/AND gate equal to 80 percent of the bubble-propagation margin and about 60 percent of the free-bubble bias-field margin. The experimental results show 10.8, 10.7, 10.2, 10, 10.6, 8.5, 12, 8.5, 15, and 9.5-percent bias-field margins for the SWITCH, AND/OR, COMPARATOR, SEARCH, LATCH, BIT-PAIR SEPARATOR, MERGER, CROSSOVER, ANNIHILATOR, and SPLITTER gates, respectively. An 8.5-percent overlapped bias margin has been obtained for all of the logic components. Transient pictures taken with a small incremented delay revealed that the bubble motion in perforated-sheet logic devices is uniform both in the velocity excursion and shape distortion. The behavior of various components at different values of the bias field and drive current is all qualitatively interpretable on the basis of experimental observations. The feasibility of these logic components is successfully demonstrated.     

Separately bounded InGaAsP high-power laser heterostructures obtained by VPE of organometallic compounds

InGaAsP/InP laser heterostructures with two stressed quantum wells operating in the wavelength range 1.3–1.55 μm were obtained by VPE of organometallic compounds. An optical emission power of 2.4 W was reached for the laser diodes with 100-μm wide strips operated in the continuous lasing mode at 20°C. A minimum threshold current density was 260 A/cm². A differential quantum efficiency η_d= 40% was obtained with a 1.9-mm-long Fabry-Perot resonator. The internal optical losses of the heterostructures are reduced to 2.6–4.2 cm⁻¹.     

Gap tolerant bubble propagation circuit

A new bubble propagation pattern for field access devices has been developed which has a period to gap ratio of 8:1. In this pattern of semicircular elements (half-disk) the gaps are situated between essentially parallel poles in contrast to the TI pattern where gaps are located between orthogonal poles. The bubble, therefore, comes under the influence of two strong parallel poles causing it to stretch across the gap. The energy barrier that would normally be encountered thus virtually disppears. Devices of 32, 18, and 10 μm periods have been designed and fabricated. The results show a typical margin of at least 20% of the bias field under normal operating conditions. The operating drive field is relatively low for small bubbles. The minimum drive field for a 10 μm period pattern is only 13 Oe.     

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.     

Four-micron period ion-implanted bubble test circuits

Packaged magnetic bubble test circuits made with 4-µm period ion-implanted circuits were operated over -55° to +110°C. The circuits used bidirectional transfer and nondestructive detection and were made on bismuth-containing magnetic garnet films. Passivated circuits that incorporated an improved transfer conductor design were operated on films with higher Gilbert damping parameters. Potential advantages to using films wih lower bubble aspect ratios for ion-implanted circuits are discussed. However nondestructive readout (NDRO) detection was not realized with these "flat" bubbles.