Nanotechnological data storage

Illustration 18: Millipede: ultra dense data storage on basis of scanning probe technology (Source: IBM)

Also in the area of data storage, nanotechnology offers potentials for the production of miniaturized mass memories with extremely high storage density as well as for the development of new non volatile working memories for computer systems, which will compete in the future with conventional memory chips like DRAM.

Nanotechnology could lead to improved mass data storage systems in the future based on thermomechanical, optical or holografic principles, which at present are still under basic research. The IBM research department in Rueschlikon works on the development of the so-called Millipede memory, which is a micromechanical device with an array of nanosca-le read/write/erase tips based on scanning probe technology. The active storage medium is a thin polymer film on the surface of the chip that represents bits in the form of 10-nanometer-diameter holes. For writing the tips are heated to 400 °C causing intendations in the polymer film. To read out, the same tips are heated to just 300°C to prevent damaging the polymer. When the tip drops into a hole marking a bit, it is abruptly cooled by the better heat transport, and a measurable change in its resistance can be detected that is enough to distinguish "1" from "0." Advantages of the millipede data storage are nonvolatility, low power and large capacity storage up to 1 Tbit per square inch, thus making the millipede interesting for mobile applications and perhaps also for space applications. If existing technological problems could be solved, the millipede will become competitive especially for mobile applications as a replacement for flash memories in some years.50

50 EE Times news from June, 24 2002: „IBM stores terabits of memory on a single chip" (www.eetimes.com)

Optical data memories with a 3-d array of optically adressable quantum dots offer likewise the potential for a substantially increased data storage density. Data memories can in principle also be realized by making use of biological molecules. In particular bacteriorhodopsin (bR) has been examined intensively for applications in data memories. This protein complex can be switched into different configurations by laser light, which can be used for data reading and writing in a three-dimensional medium. Such three-dimensional optical memories have been investigated for several years now, but are still in their infancy (Birge et al. 1999). Problematic among other things, are the high demands regarding the laser arrangement and control as well as the production of the storage medium. At present efforts are made on genetic mutations of bR in order to stabilize individual configurations of the protein for increasing the data stability. The development target is a mass storage, which however is hardly regarded as a serious competitor for established storage media in the near future.51

Another approach for a biological memory is developed by the company NanoGen. This memory consists of micro arrays, on which modified DNA molecules with different fluorescence markers for different colors are attached. These can be utilized to read out the data, which are stored as specific configurations of the DNA strands. Since the data writing procedure is extremely slow, first applications of such storage systems might be the large archiving of large data sets.

In the range of main memories, different nanotechnologically influenced storage types are in development, which will step into competition to mram as nonvolatile DRAM chips in the near future. The research activities of the main chip radiation hard data manufacturers here essentially focus on both competitive ferroelectrical memory for space appli-(FRAM) and magnetoelectronic (MRAM) storage technologies. The main advantages of both storage types lie in the non-volatile information storage, i.e. the data remains without external current supply. That means data cannot be lost upon a sudden power failure and the booting procedure of PC's would become unnecessary. Beyond that, the necessity for the data refresh is cancelled clearly, which reduces time lags and the energy dissipation as compared with DRAM. MRAM exhibit here, in comparison to other non volatile storage types such as EEPROM, Flash or FRAM some advantages, which are particularly interesting for aerospace and military applications: 52

• Low energy consumption

• Inherent radiation resistance

• Suitability for high temperatures

The temperature stability of MRAM is clearly better than that of FRAM, whereby data durability already decreases significantly at temperatures of cations

52 Honeywell Solid State Electronics Center (http://www.ssec.honeywell.com)

70 to 85 °C. Magnetoresistive MRAM memory at present still possess no market readiness, although the company Honeywell already manufactured MRAM chips for special space applications some years ago. These MRAM chips were based on the AMR resistance effect, while modern concepts utilize the GMR or TMR effect. Meanwhile Honeywell has developed GMR based prototype MRAM chips for military applications, while the readiness for the civilian terrestrial market is expected for 2004 (see chapter 5.3.4).

FRAM are based on the ferroelectricity of certain crystals, in which lattice mobile atoms with stable configurations can be found, which can be switched by electrical fields in nanoseconds. The ferroelectrical memory cells retain the written data for more than 10 years. A disadvantage of FRAM is that the life span is limited due to material fatigue on approx. 10 billion writing cycles. FRAM were manufactured as 8Mbit-chips and were already used for example in SmartCards.53 In particular Japanese and Korean companies (Toshiba, Fujitsu, Samsung) but also German companies (Infineon) accomplish intensified efforts in order to develop ferroelectrical memories.

As further nanotechnological storage concepts with potential for space application SOI memory (silicon on insulator) and phase change memories (PC RAM) should be mentioned. SOI memory chips can be used for SOI memory chips for space applications with moderate storage requirements. Honeywell intro-

space applications with duced radiation-hard 4 Mbit SOI SRAM for space applications, which moderate storage de- exhibit access times of 25 ns and are suitable for temperature ranges from mand -55 to 125 °C.54 SOI chips utilize a thin SiO2 isolation layer (about 25 nm thin) deposited on Si wafers, on which the transistor is built. SOI memory exhibits a higher speed and a smaller energy consumption in comparison to CMOS technology (Isaac 2000). The Naval Research Laboratory holds a patent on microelectronic components based on SOI technology for space applications.

In phase change memories (PC RAM), which for example are subject to development work done by the companies Intel and Ovonyx, data are stored by electronically excited phase transitions of semiconductor alloys in an amorphous (high electrical resistance) respective crystalline state (low electrical resistance). Possible advantages of this technology are the simplified production process and a high integrateability into circuits.

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