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An Improved Write Operation to Reduce and Correct Disturb Error in Magnetic Random Access Memory

IP.com Disclosure Number: IPCOM000020220D
Original Publication Date: 2003-Nov-03
Included in the Prior Art Database: 2003-Nov-03
Document File: 3 page(s) / 78K

Publishing Venue

IBM

Abstract

Magnetic random access memory (MRAM) relies on the magnetic field generated by flowing current through wires to write information into memory cells. It is usually difficult to confine the field to the vicinity of any particular cell. The neighbor cells can be disturbed during the write operation if the write field is too large or the duration too long. This document describes a method to reduce this disturb sensitivity by tuning the write field or duration for each write operation, checking the most susceptible neighbors before and after the write operation to confirm their state, and writing the disturbed neighbors to their original state.

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  An Improved Write Operation to Reduce and Correct Disturb Error in Magnetic Random Access Memory

    Magnetic random access memory (MRAM) relies on the magnetic field generated by flowing current through wires to write information into memory cells. A small but noticeable amount of field (1-10%) is also present on the cells next to but not immediately underneath the wire. These neighbor cells can be disturbed during the write operation if the write field is too large or the duration too long. During the write operation, the neighbor cells are much closer to their switching threshold than in the standby mode. The energy barrier for the neighbor cells to switch spontaneously during the write operation is quite low. Given enough time (probably of the order of milliseconds), these neighbor cells will have significant chance of switching. The variation in the magnitude of the switching threshold of the bits makes this problem even more severe, especially in the case when a few 'soft' bits (low switching threshold) surround a 'hard' bit (high switching threshold).

    The present document describes a method to reduce the said disturb sensitivity by checking the most susceptible neighbor(s) before and after the write operation of the selected bit(s) to confirm their state, and restore the said disturbed neighbors to their original state. This method will reduce the disturb sensitivity of the worst case since all disturbed 'weak' cells will be restored, and the restoration process will not likely to disturb the 'strong' bits in the vicinity. This method will also correct the rare case of thermal induced switching because the chance of consecutive thermal disturbs is much lower than a single thermal activated switching.

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    The detailed flow chart of the write operation is shown in figure 1. The write operation starts by reading all the neighbor bits and/or any other bits susceptible to disturb and store this information in a temporary memory. The write operation then enters the write cycle, where repeated attempts to write the selected (target) bit are performed. Before each write attempt, the target bit is read and compared with the intended state of the target bit. If the target bit is in the correct state, the write cycle stops. If the target bit is in the wrong state, the write-current...