L. R. Tocci

Magnetic bubble domain devices

A review is given of recent advances which have been made in the area of magnetic bubble domain devices with emphasis being placed on new device components and the problem of device optimization. The subject of bubble domain propagation is introduced by considering the results obtained on the basis of a simplified model which is applied to individual bars and bar pairs to illustrate the basic principles involved in field access drive circuits. Experimental data is presented which qualitatively confirms various aspects of the model. New device component designs are then introduced, such as the multiple-bar keyhole generator and the chevron stretcher detector, in which bubble stretching occurs orthogonal to bubble motion. Preliminary data on these as well as on various other circuit components are presented and discussed in the light of future device applications.

Device component margin evaluation using generalized field interruption technique

A generalized field interruption technique is evaluated for bubble memory chip margin analysis. Using a short bubble stream and measuring the averaged propagation steps for the first error in a given data pattern, field interruption offers a statistically significant measurement of the failure probability for a small segment of a complete bubble memory device. This technique also allows an individual component margin measurement independent of the rest of the device. A practical example is given using a 100K bit serial loop memory chip which is composed of various components and which also contains a weak processing defect. The component margin measurements show that at low driving fields the chip operation is limited by the passive replicator component, while at high driving fields operation is limited by the permalloy defect.

Experimental techniques for studying the reliability of bubble memory devices

The reliability of a large capacity bubble memory device was studied by measuring the error accumulation in the stored data resulting from the random annihilation of bubbles during the operation of the device. The bubble‐population dependence of the error‐rate plot indicates a strong dependence of the device reliability on the bubble pattern. To compare the long‐term margin degradation the initial error rate at a fixed bubble population, the inverse of the mean step to failure, is used as the comparison parameter rather than a 50% failure criterion. In this manner, the measurement time required to provide good statistics is greatly reduced when using large capacity chips. In addition, chips of this size are practical ones which can be used in real memory systems. The analysis of the chip reliability is complicated by the many different types of components in the chip and the higher probability of material or processing defects. A generalized field interruption technique is also used to characterize localized areas or components within the large chip. Using this technique in conjunction with the technique of measuring the bubble decay rate at different sites of a repetitive bubble pattern, the direct bubble‐bubble interaction in the circuit can be related to the Permalloy geometry. For example, the bent‐H‐type 180° corner with a diagonal bar was found to cause a significant increase in the error rate in the third bit site of the word pattern 01110111 over that of the pattern 01110000. When the device is operated in start–stop mode, the error rate is further complicated by the static coupling between the bubble and the Permalloy. The effect of bias‐field tilting on the device operation reliability can also be studied through a margin degradation‐type measurement for the start–stop mode.

A magnetic bubble domain flight recorder

An all electronic bubble memory system is aimed toward direct replacement of present flight tape recorders which have a high rate of failure due to the tape transport mechanism. A small 60k bit bubble recorder has been built to demonstrate the feasibility of this approach. It consists of six 10k bit chips mounted in three separate packages operating as a FIFO at 150 KHz bubble data rate. In addition to a FIFO the bubble recorder can be organized as a block oriented memory and also can be operated asynchronously to very low frequencies adding dimensions of flexibility and reliability not found in the present tape recorder system. Recorder nonvolatility is achieved by power shutdown protection circuits and a magnetically closed structure which also prevents magnetic contamination of surrounding equipment. A 10-8detection error rate is achieved with a 300μV detector signal at 150 kHz using power strobing and d.c. restore. Projection on a 50M bit prototype recorder is made. A 105bit chip capacity is proposed based on the electronic cost and MTBF considerations. This prototype operating at 150 kHz is capable of meeting or surpassing the capacity, data rate and reliability specifications of most present flight tape recorders.

The effect of DC in plane field on the operation of field access bubble memory devices

The effect of a small dc in-plane field on the start-stop operation of field access bubble devices has been studied. Experimental results show that the bias margin in this mode is very sensitive to the magnitude of the field and its orientation relative to the start-stop direction of the drive field. In a T-I circuit a complete margin loss was observed for an in-plane field of 3 Oe oriented antiparallel to the start-stop direction. For parallel orientations of the in-plane field the start-stop margin improved and approached that of the continuous propagation margin at an in-plane field of approximately 6 Oe. Dependence of the start-stop margin on the orientation of the start-stop direction relative to the pattern was also observed. Measurements of the bubble collapse field at various points in the pattern show a very strong dependence on the in-plane field and the permalloy geometry. The collapse-field results and magnetostatic energy considerations which take into account local field variations and bubble-bubble interactions provide a basis for understanding the experimentally observed start-stop margins. These results show that a small tilt (2 to 3°) should be introduced in the bias field to overcome normal alignment tolerances and ensure that a favorable in-plane field is always present. This assures reliable start-stop operation.

High Speed Characteristics of a Chevron Stretcher Detector

The characteristics of a chevron stretcher detector consisting of a thin permalloy bar (200–900A thick) placed transverse to a multiple chevron track are presented for frequencies up to 100 KHz. Data has been taken at quasi‐static and high frequencies on the output as a function of bias field, detector current, in‐plane field and the length of bubble stretching. The characteristics of the bubble signal were also observed as a function of the rotating field and the length of bubble stretching. The characteristics of the bubble signal were also observed as a function of the rotating field direction for detector elements at two positions on the chevron propagation track. The data indicates that the position is very important in determining the structure of the bubble signal. The output drops off with increasing bias and drive field and can be found to vary with frequency depending upon the value of the in‐plane drive field. The output is also linear with the length of bubble stretching.

Data unreliability in bubble memory devices caused by spontaneous annihilation

The unreliability of the bubble devices caused by spontaneous bubble annihilation in a large capacity chip is different from that measured in a small test loop because of the direct interaction between neighboring bubbles. This direct interaction can be separated from the indirect interaction by measuring the individual bit decay rates for different word patterns. Measurement on a 16 micron 20 K bit chip shows that the bubble annihilation rate caused by direct interaction can be several orders higher than that caused by indirect bubble interaction through the remanence effect in the permalloy overlay. Because of this strong pattern dependence, the inverse of the initial slope of the error accumulation (mean step to failure, MSTF) should be used as the comparison parameter to evaluate the long term reliability effects in large capacity bubble memory devices.

Compact replicate/transfer switch for field access bubble devices

A compact transfer/replicate switch has been developed which is also capable of bubble annihilaation. The switch utilizes a new 180 degree multi‐chevron corner adapted to a conventional T‐I propagation pattern (Figure 1) and permits two bit spacing of the minor loops on the access track resulting in minimum access time for any given data block. A two level version of the switch has been employed in a 64K bit block access chip design.1 The manipulation of the domains is accomplished in a fashion similar to an all chevron, one level switch previously introduced by other workers.2 The propagation margin of the minor loop at −25°, 25°, and 70°C is 16, 17, and 13 Oe respectively, for a drive field of 45 Oe at 100 kHz. The annihilation function overlaps the propagation margin completely over this temperature range. At −25°, 25°, and 70°C replicate‐in has 12, 13.5, and 10 Oe; and transfer‐out has 8, 14, and 9 Oe. All the switching functions have at least 40 degree margins.

NICMOS flight hardware 256 x 256 MCT detector array configuration

Features of the flight hardware version of the NICMOS 256 X 256 mercury-cadmium- telluride (MCT) detector array for the Hubble Space Telescope (HST) are presented and described. Detector flowdown requirements for flight production are reviewed and discussed. Detector cross section and array architecture features are analyzed in relation to quantum efficiency and crosstalk behavior. Features of the charge integration scheme employed are analyzed in assessing dynamic range.

2µm - bubble device optimization

We report on the improvement in 8-9μm period devices as compared to simple scaling of the present state-of-the-art 16μm period devices. Optimizations of garnet, and processing parameters and device component design were made to reduce the chip drive (field, voltage). We found that the using larger bubble size than the scaled down value (2.2μm instead of 1.7μm) is most effective in reducing the drive field and improving gap tolerance. Conductor and permalloy thickness were kept at 3500A (scaled down value is ≈2000A). The spacer thickness, however, was close to the scaled down value of 4000 to 4500A to improve the stretcher upper bias margin. Although non-planar devices sometimes showed very good margins at low drive fields (10% at 50 Oe drive), it is felt that planar processing is necessary to maintain reproducibility. Device component design based on extra large permalloy element such as double period major loops and 3π-corner in the replicator switch were found to have good overall bias margins of 26 Oe at 45 to 55 Oe drives. Operation at 150 kHz with 55 Oe drive showed as bias margin of 10% or more in the temperature range of -25 to 80°C. At 250 kHz an increase of 5 Oe in the minimum drive field was observed.