Circuit and method of writing a toggle memory

Details Circuit and method of writing a toggle memory Circuit and method of writing a toggle memory

2002-06-28

2004-02-17

Le, Thong (Department: 2818)

Static information storage and retrieval

Systems using particular element

Magnetoresistive

C365S189040, C365S230010

Reexamination Certificate

active

06693824

ABSTRACT:

DELATED APPLICATIONS
This application is related to:
U.S. patent application Ser. No. 09/978859, entitled “A Method of Writing to a Scalable Magnetoresistance Random Access Memory Element,” filed Oct. 16, 2001, and assigned to the assignee hereof; and
U.S. patent application Ser. No. 10/185,868, entitled “MRAM Architecture with Electrically Isolated Read and Write Circuitry,” filed simultaneously herewith, and assigned to the assignee hereof.
FIELD OF THE INVENTION
This invention relates to writing memories, and more particularly to writing to memories that toggle.
BACKGROUND OF THE INVENTION
Non-volatile memory devices are an extremely important component in electronic systems. FLASH is the major non-volatile memory device in use today. Typical non-volatile memory devices use charges trapped in a floating oxide layer to store information. Disadvantages of FLASH memory include high voltage requirements and slow program and erase times. Also, FLASH memory has a poor write endurance of 10
4
-10
6
cycles before memory failure. In addition, to maintain reasonable data retention, the scaling of the gate oxide is restricted by the tunneling barrier seen by the electrons. Hence, FLASH memory is limited in the dimensions to which it can be scaled.
To overcome these shortcomings, magnetic memory devices are being evaluated. One such device is magnetoresistive RAM (hereinafter referred to as “MRAM”). To be commercially practical, however, MRAM must have comparable memory density to current memory technologies, be scalable for future generations, operate at low voltages, have low power consumption, and have competitive read/write speeds.
For an MRAM device, the stability of the nonvolatile memory state, the repeatability of the read/write cycles, and the memory element-to-element switching field uniformity are three of the most important aspects of its design characteristics. A memory state in MRAM is not maintained by power, but rather by the direction of the magnetic moment vector. Storing data is accomplished by applying magnetic fields and causing a magnetic material in a MRAM device to be magnetized into either of two possible memory states. Recalling data is accomplished by sensing the resistive differences in the MRAM device between the two states. The magnetic fields for writing are created by passing currents through strip lines external to the magnetic structure or through the magnetic structures themselves.
As the lateral dimension of an MRAM device decreases, three problems occur. First, the switching field increases for a given shape and film thickness, requiring a larger magnetic field to switch. Second, the total switching volume is reduced so that the energy barrier for reversal decreases. The energy barrier refers to the amount of energy needed to switch the magnetic moment vector from one state to the other. The energy barrier determines the data retention and error rate of the MRAM device and unintended reversals can occur due to thermofluctuations (superparamagnetism) if the barrier is too small. A major problem with having a small energy barrier is that it becomes extremely difficult to selectively switch one MRAM device in an array. Selectablility allows switching without inadvertently switching other MRAM devices. Finally, because the switching field is produced by shape, the switching field becomes more sensitive to shape variations as the MRAM device decreases in size. With photolithography scaling becoming more difficult at smaller dimensions, MRAM devices will have difficulty maintaining tight switching distributions.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.


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PCT/US03/13179 PCT International Search Report.

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