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Memory types: DRAM EEPROM Flash FRAM MRAM Phase change memory SDRAM SRAM
Magneto-resistive RAM, Magnetic RAM or just MRAM is a form of non-volatile random access memory technology that uses magnetic charges to store data instead of electric charges.
MRAM memory technology also has the advantage that it is a low power technology as it does not require power to maintain the data as in the case of many other memory technologies.
While MRAM memory technology has been known for over ten years, it is only recently that the technology has been able to be manufactured in large volumes. This has now brought MRAM technology to a point where it is commercially viable.
What is MRAM: basics
MRAM technology is completely different to any other semiconductor technology that is currently in use and it offers a number of advantages:
- MRAM memory technology retains its data when the power is removed
- It offers a higher read write speed when compared to other technologies including Flash and EEPROM
- Consumes a comparatively low level of power
- MRAM data does not degrade over time
The new MRAM memory development is of huge significance. Several manufacturers have been researching the technology, but Freescale was the first company to have developed the technology sufficiently to enable it to be manufactured on a large scale. With this in mind, they already have already started to build up stocks of the 4 megabit memories that form their first offering, with larger memories to follow.
MRAM structure & fabrication
One of the major problems with MRAM memory technology has been developing a suitable MRAM structure that will allow the memories to be manufactured satisfactorily. A wide range of structures and materials have been investigated to obtain the optimum structure.
Some early MRAM memory technology development structures employed fabricated junctions using computer-controlled placement of up to 8 different metal shadow masks. The masks were successively placed on any one of up to twenty 1 inch diameter wafers with a placement accuracy of approximately ± 40 µm. By using different masks, between 10 to 74 junctions of a size of approximately 80 x 80 µm could be fashioned on each wafer.
The tunnel barrier was formed by in-situ plasma oxidation of a thin Al layer deposited at ambient temperature. Using this technique, large levels of variation in resistance due to magneto-resistive effects were seen. Investigations into the dependence of MR on the ferromagnetic metals comprising the electrodes were made.
It was anticipated that the magnitude of the MR would largely be dependent on the interface between the tunnel barrier and the magnetic electrodes. However it was found that thick layers of certain non-ferromagnetic metals could be inserted between the tunnel barrier and the magnetic electrode without quenching the MR effect. However it was found that the MR was quenched by incomplete oxidation of the Al layer.
The operation of the new semiconductor memory is based around a structure known as a magnetic tunnel junction (MJT). These devices consist of sandwiches of two ferromagnetic layers separated by thin insulating layers. A current can flow across the sandwich and arises from a tunnelling action and its magnitude is dependent upon the magnetic moments of the magnetic layers. The layers of the memory cell can either be the same when they are said to be parallel, or in opposite directions when they are said to be antiparallel. It is found that the current is higher when the magnetic fields are aligned to one another. In this way it is possible to detect the state of the fields.
Magnetic tunnel junctions (MTJ) of the MRAM comprise sandwiches of two ferromagnetic (FM) layers separated by a thin insulating layer which acts as a tunnel barrier. In these structures the sense current usually flows parallel to the layers of the structure, the current is passed perpendicular to the layers of the MTJ sandwich. The resistance of the MTJ sandwich depends on the direction of magnetism of the two ferromagnetic layers. Typically, the resistance of the MTJ is lowest when these moments are aligned parallel to one another, and is highest when antiparallel.
To set the state of the memory cell a write current is passed through the structure. This is sufficiently high to alter the direction of magnetism of the thin layer, but not the thicker one. A smaller non-destructive sense current is then used to detect the data stored in the memory cell.
MRAM memory is becoming available from a number of companies. Its development shows that memory technology is moving forwards to keep pace with the ever more demanding requirements of computer and processor based systems for more memory. Although relatively new to the market MRAM, magnetoresistive RAM, when looking at what is MRAM, it can be seen to have some significant advantages to offer.
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