Magnetoresistive random access memory (MRAM) is a revolutionary memory technology that can replace many of today’s semiconductor memory technologies. MRAM combines the speed of eSRAM and the non–volatility of Flash onto a single chip. MRAM uses magnetic moments, rather than an electric charge, to determine the on–off state of the memory bit cell. It allows a single memory solution to replace multiple memory options within one chip—helping to enable faster, more cost–effective solutions for next-generation memory–intensive products. MRAM is a nonvolatile memory technology that protects data in the event of power loss and does not require periodic refreshing. The MR2A16A is the ideal memory solution for applications that must permanently store and retrieve critical data quickly.

Micrograph of the MR2A16A MRAM device.
Introduction
Magnetoresistive Random Access Memory (MRAM) combines a magnetic device with standard silicon-based microelectronics to obtain the combined attributes of non-volatility, high-speed operation and unlimited read and write endurance not found in any other existing memory technology. In this paper we provide an overview of Freescale’s MRAM technology and describe the MR2A16A, a 4 Mbit MRAM device. As shown in Figure 1, the memory is based on a 1-transistor, 1-magnetic tunnel junction (1T1MTJ) memory cell that employs a novel bit structure and approach for operation. The MR2A16A is fabricated with a 0.18µm CMOS process using five levels of metal, including program current lines clad with highly permeable material for magnetic flux concentration. We describe how the cell architecture, bit structure, and the toggle switching mode are combined to provide significantly improved operational performance and manufacturability as compared to MRAM based on conventional switching.

Schematic of a 1-transistor, 1-MTJ memory cell showing the write lines above and below the bit and the read current path.

MRAM Description
MRAM is based on magnetic memory elements integrated with CMOS. Each memory element uses a magnetic tunnel junction (MTJ) device for data storage. The MTJ is composed of a fixed magnetic layer, a thin dielectric tunnel barrier, and a free magnetic layer. When a bias is applied to the MTJ, electrons that are spin polarized by the magnetic layers traverse the dielectric barrier through a process known as tunneling. The MTJ device has a low resistance when the magnetic moment of the free layer is parallel to the fixed layer and a high resistance when the free layer moment is oriented anti-parallel to the fixed layer moment. This change in resistance with the magnetic state of the device is an effect known as magnetoresistance, hence the name “Magnetoresistive” RAM.
Unlike most other semiconductor memory technologies, the data is stored as a magnetic state, rather than charge, and sensed by measuring the resistance without disturbing the magnetic state. Using a magnetic state for storage has two main benefits: 1) the magnetic polarization does not leak away with time like charge does, so the information is stored even when the power is turned off; and 2) switching the magnetic polarization between the two states does not involve actual movement of electrons or atoms and thus has no known wear-out mechanism. The magnetoresistive device used in MRAM is very similar to the device used for the reader in hard disk drives.

A memory array consisting of many MRAM cells with digit and bit lines for cross-point writing and isolation transistors controlled by word lines.

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