MRAM Memory Tutorial
- an overview or tutorial about the basics of MRAM memory technology and how this new form of semiconductor memory can be used in circuit design to offer non-volatile memory while consuming low levels of power
Magneto-resistive RAM, or Magnetic RAM is a form of non-volatile RAM memory technology that uses magnetic charges to store data instead of electric charges. Unlike technologies including DRAM, which require a constant flow of electricity to maintain the integrity of the data, MRAM memory technology retains data even when the power is removed. An additional advantage is that MRAM only requires low power for active operation. As a result this memory echnology is becoming a major player in the electronics industry now that production processes have been developed to enable it to be produced.
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.
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
- MRAM data does not degrade over time
- Consumes a comparatively low level of power
The new MRAM memory development is of huge significance. Several manufacturers have been researching the technology, but Freescale is 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.
The new MRAM memory technology is an exciting new development. Analysts have said that it is the most significant memory development for a decade. Additionally with one manufacturer entering the market, others are likely to follow. Whether it spells the end of memories like Flash remains for be seen, but it will certainly be a major contender for the enormous non-volatile memory market, challenging the dominating position held by Flash. Only time will tell how the market will change, but the new memories will certainly become a major contender, especially when the process matures and larger memories become available.
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.
One of the major problems with MRAM memory technology has been developing a suitable construction 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.
Now the first commercially available MRAM memories have been launched onto the market by Freescale. This represents a major step forward not only in general semiconductor memory technology, but also in MRAM memory technology. It opens up the way for many more manufacturers to follow with their flavours of the new semiconductor memory technology
By Ian Poole
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