24-11-2012, 04:13 PM
Atomic Scale Memory at A Silicon Surface
Atomic-Scale-Memory-At-A-Silicon-Surface.doc (Size: 1.74 MB / Downloads: 140)
Abstract
The limits of pushing storage density to the atomic scale are explored with a memory that stores a bit by the presence or absence of one silicon atom. These atoms are positioned at lattice sites along self-assembled tracks with a pitch of five atom rows. The memory can be initialized and reformatted by controlled deposition of silicon. The writing process involves the transfer of Si atoms to the tip of a scanning tunneling microscope. The constraints on speed and reliability are compared with data storage in magnetic hard disks and DNA.
Introduction
In 1959 physics icon Richard Feynman estimated that “all of the information that man has carefully accumulated in all the books in the world, can be written in a cube of material one two-hundredth of an inch wide”. Thereby, he uses a cube of 5×5×5 = 125 atoms to store one bit, which is comparable to the 32 atoms that store one bit in DNA. Such a simple, back-of-the-envelope calculation gave a first glimpse into how much room there is for improving the density of stored data when going down to the atomic level.
In the meantime, there has been great progress towards miniaturizing electronic devices all the way down to single molecules or nanotubes as active elements. Memory structures have been devised that consist of crossed arrays of nanowires linked by switchable organic molecules or crossed arrays of carbon nanotubes with electro statically switchable intersections.
Conventional Storage Media
We are going to discuss about atomic scale memory at a silicon surface .But some knowledge about the conventional storage media will help us to understand the atomic scale memory deeply.
The highest commercial storage density is achieved with magnetic hard disks, whose aerial density has increased by seven orders of magnitude since their invention in Feynman's days. Currently, the storage density is approaching 100 Gigabits per square inch in commercial hard disks. Typical storage media consist of a combination of several metals, which segregate into magnetic particles embedded into a non-magnetic matrix that keeps them magnetically independent. A strip of particles with parallel magnetic orientation makes up a bit, as color coded red and turquoise in the figure below. (The dimensions keep getting smaller.) When such a bit is imaged by a magnetic force microscope the collection of these particles shows up as white or dark line, depending on the magnetic orientation.
Silicon Memory structure
The new memory was made without the use of lithography as required to make conventional memory chips. To make conventional memory chips, light is used to etch patterns on a chemically treated silicon surface. To use lithography to make chips that are denser than the best available chips is prohibitively expensive and difficult.
The self-assembled memory structure shown in figures 1 and 2 is obtained by depositing 0.4 monolayer of gold onto a Si(111) surface at 700 ◦C with a post-anneal at 850 ◦C, thereby forming the well-known Si(111)5 × 2–Au structure. All images are taken by STM with a tunneling current of 0.2 nA and a sample bias of −2 V. At this bias the extra silicon atoms are enhanced compared to the underlying 5 × 2 lattice. A stepped Si(111) substrate tilted by 1◦___ towards the azimuth is used to obtain one of the three possible domain orientations exclusively. The surface arranges itself into tracks that are exactly five atom rows wide (figure 1). They are oriented parallel to the steps. Protrusions reside on top of the tracks on a 5 × 4 lattice. Only half of the possible sites are occupied in thermal equilibrium (figure 4(a)).When varying the Au coverage the occupancy remains close to 50%. Excess Au is taken up by patches of the Au-rich Si(111)√3 × √3–Au phase, and Au deficiency leads to patches of clean Si(111)7 × 7. In order to find out whether the protrusions are Si or Au, we evaporate additional Si and Au at low temperature (300 ◦C). Silicon fills the vacant sites (figures 4(b) and (d)), but gold does not.
S T M
We have a technology that lets us happen some materials like electrical wires so that their end terminates in a single atom. We are also able to move this upper wire in atomic scale increments across the bottom surface. Closer the atomically sharp tip is to atoms on the bottom surface, the more electricity will flow between the two, when we make them part of electrical circuit. By measuring this electricity as the tip and surface are moved relative to one another, we can see how atoms are arranged on the bottom surface. The instrument that is used for this measurement is called scanning tunneling microscope.
OUTLOOK
An interesting yardstick is the storage and transcription of data in biological syatems,5*4=20 surface atoms store one bit on silicon compared to 32 atoms used by DNA (64 atoms for an AT base pair plus backbone,68 atoms for CG, with each base pair coding the four combinations AT,TA,CG,GC,ie 2 bits ).The transcription rate from DNA to RNA is ≈60 nucleotides s−1 for E-coli at 37 ◦C and 10 times faster for DNA replication .The STM acquisition rate on silicon is comparable (120 bits s−1 in figure 2©.It could be as high as 107 bit s−1 at the statistical noise limit .Parallel readout can be used in both cases, with ≈ 101 subsections of DNA being replicated simultaneously and an array of 103 tips scanning in parallel. Cells use a similar parallelism of 103–104 for protein synthesis, where speed is more important. The error rate achieved in DNA replication is as low as 10−7–10−11 with error correction by DNA polymerase.
Advantages and disadvantages
An intriguing aspect of atomic scale memory is that memory density is comparable to the way nature stores data in DNA molecules. The Wisconsin atomic-scale silicon memory uses 20 atoms to store one bit of information, including the space around the single atom bits. DNA uses 32 atoms to store information in one half of the chemical base pair that is the fundamental unit that makes up genetic information. Compared to conventional storage media, both DNA and the silicon surface excel by their storage density.
Obviously there are some drawbacks. The memory was constructed and manipulated in a vacuum, and that a scanning tunneling microscope is needed to write memory which makes the writing process very time consuming.
Conclusion
The push towards the atomic density limit requires a sacrifice in speed, as demonstrated in figure 5. Practical data storage might evolve in a similar direction, with the gain in speed slows down as the density increases. Somewhere on the way to the atomic scale ought to be an optimum combination of density and speed.
If the reading and writing speed is improved and the memory is made cost effective, this will revolutionize the field of secondary storage devices. Researchers are working on manufacturing STM with multiple tips or heads that can perform parallel read-write processes.
This type of memory may eventually become useful for storing vast amounts of data, but because the stability of each bit of information depends on one or a few atoms, it likely to be used for applications where a small number of errors can be tolerated.