Probe storage R&D institutions

Background of the Scanning probe storage technologies



The field of scanning probe microscopy (SPM) has developed tremendously since the invention of the Scanning Tunnelling Microscope back in the 1980s. It is now practicable, for example, to use SPM technology to modify the surfaces of materials on the nanoscale, rather than just for microscopic imaging. Such surface modification might comprise the writing and reading of data, so providing a storage system with, ultimately, atomic resolution. However this approach, and similar atomic-level storage demonstrations by others, was exceedingly slow and, from a system perspective, offered an impracticably low data rate.

The probe storage field was of course given huge impetus by the impressive work at IBM (Zurich) into the 'Millipede' system. The Millipede is an alternative storage technology aiming at ultrahigh storage densities of 1 Tb/in2 and beyond by using local-probe techniques to write, read, and erase data in very thin polymer films. It uses thermo-mechanical recording and readback processes, and combines ultrahigh density, small form factor, and data rates suitable for mobile applications. The Millipede project has been pursued at the IBM Zurich Research Laboratory over the past seven years. A first small-scale prototype storage system with servo navigation and parallel read/write/erase capability using scanning-probe thermo-mechanical recording technology was completed in November of 2004. This is the first time a scanning-probe recording technology has reached this level of technical maturity, demonstrating the joint operation of all basic building blocks of a storage device and, as a consequence, the potential of the probe technology for commercialisation. The small-scale storage prototype comprised a MEMS assembly (in form-factor), readback electronics (in non-form-factor) supporting parallel operation of up to eight levers, a navigation/servo system based on thermal positioning sensors, a Reed-Solomon encoder and decoder, a microcontroller and a compact flash interface to the host. The key building blocks of the small-scale prototype were:

  • The (16.5 x 17.5 x 1.2 mm) form-factor MEMS assembly comprising a 2D probe-array (based on the lever-transfer technique onto a dummy CMOS wafer), high-accuracy thermal-position sensors for providing x/y-position information to the servo controller during the closed-loop operation of the micro-scanner, and a silicon-based microscanner with 120 mm x/y-displacement capabilities.

  • The electronics read channel and system controller functions including: (i) a data detection scheme suitable for extracting the useful signal information in the presence of media noise and signal distortion, (ii) a robust protocol for identifying the track centre lines with high accuracy and for precise track-following during write and read operations, and (iii) a servomechanism that provides track-following and random medium-access capability achieving a positioning accuracy of around 1.5 nm.

  • The system controller algorithms and software for transferring user data between the host and the cantilever probes.

Using this prototype IBM demonstrated parallel operation of eight levers and, for the first time, reliable sector storage and retrieval at 517 Gb/in2. The potential for much higher density storage using the thermo-mechanical read/write process with polymer media was also demonstrated in stringent areal density tests. Employing single cantilevers and thermo-mechanical recording a record 1.217 Tb/in2 areal density (bit pitch 13.3nm, track pitch 26.6 nm) was demonstrated with a raw BER error rate of less than 1 in 104, thus meeting the criteria accepted by the magnetic recording industry for demonstrations of data recording capabilities. This density is 3 times higher than the current record highest density demonstration of 430 Gb/in2 achieved using magnetic recording in perpendicular mode. However, the thermomechanical probes up to now have only been fabricated in sample volumes using IBM’s laboratory equipment. The manufacturability in volumes in a MEMS foundry has not yet been demonstrated.

A variation on the Thermo-mechanical probe storage approach of IBM was also investigated by Samsung/LG laboratories and by researchers at Shanghai Institute of Microsystems, who proposed a piezoelectric readout method that offered lower readout power consumption than the Millipede system. However, the writing method was identical to IBMs approach. Hewlett-Packard have also recently announced a probe storage research programme based on a thermo-mechanical writing process similar to that proposed by IBM. Readout is via a second order electrical effect. This HP interest follows on from their recent work on electron-beam based system (originally called Atomic Resolution Storage, ARS, although this acronymn is being carried forward to their new thermo-mechanical approach) where e-beam heating was used to induce phase changes in InSe/GaSe material to write bits – with readout being a form of EBIC (electron beam induced current). Unfortunately, this e-beam solution requires a relatively high voltage and vacuum packaging and has been discontinued.

Probe storage using magnetic storage media has also been investigated by various research groups, for example at Seagate Research Pittsburgh, Carnegie Mellon University. As for hard disk recording, the density of magnetic-based probe storage is limited by the superparamagnetic effect. Writing can be achieved by applying a magnetic field, possibly assisted by heating of some kind. Read-out can be performed by force-mode, as used in a Magnetic Force Microscope. This however requires a compliant cantilever and a sensitive force sensor, complicating the array design. Another option for read-out of magnetic bits is to use the magneto-resistance effect, as in a hard disk. This solution is power-hungry and complex (in its adaptation to a form suitable for probe storage).

The electric counterpart of magnetic recording, Ferroelectric Storage, has been investigated for decades by Samsung, Seagate Reseach Pittsburgh, Tohoku University in Japan, Carnegie Mellon University, Pineer Corporation, Canon etc. This method is now being considered for probe storage [11]. Since contact writing can be used, very high densities over 1 Tbit/in2 can be obtained. The read-out mechanism is however rather complicated and not convenient for probe array integration, since it involves high frequency detection of minute changes in the storage medium’s capacitance caused by the effect on the non-linear part of the permittivity tensor on reversal of the ferroelectric polarization. The writing method could in principle also be performed in non-contact mode, but at the cost of a reduction in data density.

Another category of probe storage might be termed 'electrical probe storage'. From a generic point of view, ‘Electrical Probe Storage’ might be viewed as using an electrical potential applied to a probe that is in contact (or quasi contact) with a medium whose properties are altered in some way by the resulting flow of electrical current through the medium toward a counter electrode. The change in medium properties should be electrically detectable, e.g. by a change in electrical resistance. Several groups worldwide such as Exeter University, CEA-Grenoble, Tohoku University, Hokkaido University, Pioneer Corporation, Korea Advance Institute of Science & Technology etc. are pursuing such and electrically-based approach. Indeed, as part of an EU FP5 funded project (InProM, IST-2001-33065) researchers at CEA (in collaboration with the Universities of Exeter and Twente) have developed a new type of scanning probe storage, that relies on an electro-thermal recording process in a phase-change material to provide an ultra-low power (<<1W), ultra-high density (1Tbit/in2 and beyond), ultra-compact storage system. Electrical probe recording as a generic approach has several attractions, in particular:

  • The power consumption for the writing process is low with respect to other technologies (~ 0.1 nJ per written bit). This is because only the dot memory volume, as opposed to the entire tip volume, is heated.

  • The spatial resolution obtained with electrical probe memory is expected to be very good. It can, for example, be enhanced by a self-focusing effect of the current lines linked to the non-linear thermal and electrical responses of certain types of media. Also, since the electrical current only passes through the Hertzian contact area between tip and media, the tip/media contact area could be very small (for hard materials) even if the tips themselves are not necessarily sharp (and a ‘smoother’ tip should alleviate tribology and wear issues).

  • Electrical probe recording may also offer the potential for true molecular-scale storage - demonstrations of molecular-scale conductance transitions using ST-ITALY tips have been made at Oak Ridge National Labs in the USA (although interpretation of the physical mechanisms involved remains a point of debate).

In a recent and interesting development, a US start-up company, Nanochip has adopted electrical probe recording and phase-change media as the platform for and attempt to commercialise probe storage as a Flash NAND competitor by 2006/7.

The immediate plans of commercial ventures into probe recording, such as by Nanochip and HP, appear to be targeting memory markets traditionally addressed by Flash NAND and perhaps the micro-hard drives. In the longer term however we envisage two main routes for probe storage research:

1) Towards very high density in a small volume (chip component type)
2) Towards huge capacity for mass information archiving in larger formats

The primary aim of ProTeM is to investigate the micro-nano technology, systems, tips, media and recording techniques to realise the second of these two routes, primarily in the context of the application area of digital archiving (for administrations, financial institutions, health systems, business archives, personal archives….) in which the EU can compete globally in terms of both intellectual property and EU-based manufacturing. This requires a host of new ideas, techniques and expertise to be developed. The first route, towards the provision of probe storage solutions for mobile applications, is not a direct aim of ProTeM. However, it is to be expected that scientific and technological research brought about by ProTeM- for example in new R/W probes, techniques and media - will have a significant indirect impact on the mobile storage sector.






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