Experimental techniques for studying the reliability of bubble memory devices

Abstract

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.