MRAM
- an overview or tutorial of MRAM, Magneto-resistive Random Access Memory.
MRAM is a new form of semiconductor memory that offers non-volatile
characteristics and consumes 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 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 technology could
become a major player in the electronics industry now that production processes
have been developed to enable it to be produced.
While MRAM, magneto-resistive 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 characteristics
MRAM technology is completely different to any other semiconductor technology
that is currently in use and it offers a number of advantages:
- MRAM 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 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.
MRAM operation
The operation of the new semiconductor memories 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.
Construction
One of the major problems with MRAM 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 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
mm. By using different masks, between 10 to 74
junctions of a size of approximately 80 x 80 mm 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.
Summary
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 technology. It opens up the
way for many more manufacturers to follow with their flavours of the new memory
technology
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