By Will Soutter
Topics Covered
Introduction
Current
Memory Technologies
Universal Memory
Carbon Nanotube RAM (CNT RAM)
Phase Change RAM (PCRAM)
Magnetoresistive RAM (MRAM)
Quantum Dot RAM (QD RAM)
Conclusions
References
Introduction
Memory is crucial to all computing devices, both for long-term
storage of data, and for short-term storage whilst information is being
processed.
Currently, different technologies are used for different types of
memory, as the properties of each type of memory are quite restrictive.
Current Memory
Technologies
SRAM (Static Random Access Memory) is mainly used in high
performance embedded computing, and in cache memory for processors and
hard drives, where its high speed and low energy consumption is
helpful. It is very costly, however, and has a very low density
compared to other forms of memory.
DRAM (Dynamic Random Access Memory) is also quite fast, and much
more dense and cheaper than SRAM, making it the current choice for the
main memory banks in computers, shuttling information between the
storage drives and the processor.
Flash memory is used where permanent storage is required - DRAM
requires power to maintain the arrangement of 1s and 0s on the chip,
but a Flash drive is non-volatile, and so will store data indefinitely
with or without power. It is relatively cheap and high-density, but is
not fast enough for RAM applications. The properties that keep the data
stored on a flash chip stable for up to 10 years also means that a
large amount of energy is required to write to the chip, slowing down
the process. Writing data also damages the flash chip, limiting its
useful lifetime.
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Figure 1. RAM, or Random
Access Memory,
comes in lots of different forms. DRAM, pictured, is too slow for high
performance applications, and cannot store data without a constant
power source. It is used in computers to shuttle data between the hard
drive and the processor cache.
Universal Memory
Semiconductor manufacturers are now competing to produce "universal
memory" technologies, which combine the benefits of each of these
technologies. The primary aim is memory with the access speed of SRAM,
but with the non-volatility of Flash. There are several potential
candidates, explored in more detail below, which are likely to become
commercially competitive with current technologies within the next five
to ten years.
Another driver for developing these new technologies is to keep up
with the exponential progression of Moore's Law. The feature size in
silicon-based integrated circuits has halved roughly every two years
since the 1960's, but physical limits to this progression are within
sight. Many of the universal memory technologies which are being
explored have the capability to be scaled down beyond the limits of
silicon CMOS circuits.
Carbon
Nanotube RAM (CNT RAM)
Carbon nanotubes (CNTs) have great potential as the basis for memory
chips. their small size and unique dimensionality allows for
interactions between their electrical and mechanical properties which
can be used to design fast, dense, and non-volatile data storage
devices.
Whilst many CNT-based memory designs have been proposed, the
difficulty in producing the nanotubes in sufficient purity and quality,
and with integrating the nanomaterials with current semiconductor
fabrication techniques, has prevented their widespread adoption.
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Figure 2. Carbon Nanotubes
have unique electronic properties that could be used to make highly
efficient, fast, non-volative memory chips. However, there are many
challenges in bringing the technology to market - mainly manufacturing
the nanotubes in high enough purities.
Phase Change RAM (PCRAM)
In 2011, IBM demonstrated a breakthrough in Phase Change RAM
(PCRAM), which has been in development as a potential universal memory
technology for some time. There will no doubt still be difficulties in
translating the technology to a large-scale fabrication process, but
the properties are very promising. In June 2012, IBM announced a deal
with SK Hynix to take the commercialisation of this technology further.
Phase Change memory is based on a special material which has two
possible phases - crystalline and amorphous - and can be switched
between the two phases using a short electrical pulse. The write speed
is around 100 times faster than currently available flash memory,
although extra operations are required to check for write errors and
correct for drift.
Magnetoresistive RAM (MRAM)
Magnetoresistive technology is a mature technology, which is behind
modern high-density hard drives. There has been a recent research drive
to adapt this technology to higher speed, non-volatile solid state
memory. The main challenge to this is to create a large, high-density
array of magnetic tunnel junctions, which are used to write to the
storage layer. Hard drives contain just one of these, whereas an MRAM
chip would need one for each bit of stored information.
Because it based on a well-known technology, MRAM is hotly tipped as
a
candidate for the first commercial universal memory. Companies such as
Samsung, Toshiba, IBM, Hitachi and Motorola are all involved in the
development of MRAM.
Quantum Dot RAM (QD RAM)
Quantum Dot RAM uses 3nm-wide spots of semiconduictor material,
called
quantum dots, embedded in a layer of insulating material and covered
with a metal film. This structure forms an array of transistors, which
are used to store data by changing the state of each quantum dot using
a millisecond laser pulse.
This technology is highly promising, as it can achive read/write
speeds up to hundres of times faster than existing types of memory, and
is also reasonably easy to integrate into existing manufacturing
processes, as the chips can still be constructed from silicon. There
will be challenges in scaling up the process, but not to the same
extent as with implementing a totally new material like carbon
nanotubes.
.jpg)
Figure 3. Quantum dots are
tiny crystals containing just a few hundred atoms. Memory based on
quantum dots could achive much higher storage densities than existing
technologies, and would have a vastly longer lifetime.
Conclusions
The main challenges to all of these technologies is to get them to a
stage where they can be manufactured affordably, preferably using
minimally adapted exisiting equipment. They must also compete with
flash memory on price, speed and data density. In the five to ten years
it could take to get these new technologies to market, flash will also
have advanced considerably, so the real requirements for any new
replacement technology are very high.
References