Flash memory has definitely taken the world of embedded applications by storm, but new technologies are on the horizon. One is Motorola's MRAM (magnetoresistive random access memory). This technology has the nonvolatility characteristic of flash memory and the advantage of DRAM density, but without the need for refresh. With speed that's comparable to SRAM, MRAM is relatively immune to soft bit errors.
Motorola's current design employs a magnetic film surrounding the conductor (see the figure). This doubles the field for a given current. The design is based on a single transistor cell with a magnetic-tunnel-junction (MTJ) structure. The structure consists of two electrodes of magnetic material surrounding a layer of insulating material. Current tunnels perpendicularly from one magnetic layer to the other through the insulator. At the base of one electrode is a fixed ferromagnetic layer that creates a strong pinning field to hold the magnetic polarization of the layer in one specific direction. The other ferromagnetic layer is free to rotate and hold polarization in either of the two directions.
A 1-Mbit MRAM in a 64k by 16 design has been built using a 0.6-µm, five-metal, double-poly process. It has an access time and a cycle time of 50 ns. In addition, its current-cell lifetime is several billion cycles, much greater than that of flash memory technology. Future enhancements may eliminate this problem.
MRAM is not in production yet. But once in production, it may prove to be the preferred memory technology.
