The demands for mass storage are increasing with progress in information technology. A considerable increase in data recording density will lead to even higher storage capacity and the realization of a small storage device.
There are a number of different areas of data storage research carried out very actively at the moment. These are: magnetic hard disk, magnetic tape and advanced optical data storage.
The traditional storage technique currently pushing the areal storage densities the furthest is magnetic data storage (hard disks) where densities of around 430Gb/in2 have been achieved to date. While there is undoubtedly some mileage left in hard disk densities, the rate of improvement is slowing as the technology approaches the limitations due to super-paramagnetic limit. Techniques such as perpendicular recording HAMR (heat assisted magnetic recording) are being developed to extend the technology beyond what will be possible with current longitudinal recording. And it is predicted that it might be possible to reach densities in the region of 1Tb/sq.in using such ideas, perhaps by 2010 according to industry roadmaps. However, magnetic hard disks are far from ideal for archival storage (the data storage area which we are targeting) due to the relatively short data lifetime and the relatively high-energy costs of hard disk-based archival storage systems.
Tape storage densities are currently improving significantly, though they are currently only around 0.7Gb/in2 they are projected to be around 5Gb/in2 in several years time. The lifetime of magnetic tape is however also questionable, which is one of the reasons it is generally restricted to the backup data storage market.
In the optical storage sector, the current physical barrier to progress using conventional optical readout is the resolution limit caused by the classical optical diffraction limit. Several ways of circumventing the diffraction limit are being researched, some of which might result in optical storage systems suited to archival storage:
Phase-change blue laser based optical discs currently hold about 30GB/side (20 Gb/in2) and provide data rates of 12MBytes/s, and prototype discs have shown that 4-layer discs are possible - but currently only WO form – giving a capacity of up to 120GB/side (80Gb/in2) at data rates around 20-30MBytes/s. The data is of course removable (using an automated picker for example) to increase the system capacity, but the overall size (volume) of systems like this are a drawback (room-sized for very large archives, filing cabinet size for multi-TByte systems).
Near-field systems being developed by Philips look promising and give data densities around 100Gb/in2 though the systems are not yet proven to be capable against dust, dirt and fingerprints. Also, the relatively small gains they provide compared to “conventional” multilayer media may not give them sufficient competitive advantage.
Holographic data storage is also quite well developed, though the main issue is the media, with the only viable media available currently being “Tapestry” produced by the start-up company InPhase. In addition to a number of drive and optics complexity issues (e.g. temperature stability, drive interchangeability, size), holographic media can only write a “page” of data at once, so is not suitable for many applications where data comes in smaller packages (or else storage becomes inefficient).
Another drawback, and one for which there seems to be no solutions forthcoming, is that the media is very sensitive to light requiring the disc to be “fixed” after the whole disc is written- not an ideal situation. Making rewritable holographic media also looks to be very difficult. Although holographic has the potential for very high data densities, (equivalent to around 500Gb/in2 but stored in the volume of the disc) the significant media and hardware challenges may limit this in a practical system and current prototype demonstrations are at around 50Gb/in2.
Against this background of possible technologies that might be used for multi-Terabit/sq.in, multi-TeraByte archival data storage, with their various different limitations, there is the new and exciting possibility that we are proposing, that based on scanning probe storage.
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