AFM-type storage
Areal density of magnetic disk is still following the so-called "Moore's Law" (originally proposed for processor performance by Gordon Moore, cofounder of Intel): it is doubling every eighteen month (60% compound annual growth rate). However, expected problems with magnetic recording principles at the densities on the order of 100 GB/inch2 force researches to look for alternative storage solutions.
One of the possible storage technologies of the future is based on the principles of atomic force microscopy (AFM) and can be used to achieve and exceed 300 GB/inch2.
This chapter is relatively short since this technology is still in its development and testing stage, and there is not too much we can say about it. Also, the pace of traditional recording methods is so high that if AFM-based storage will not reach its potential customers in the next few years, it may be overtaken by its more traditional competitors and become obsolete...
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Atomic Force Microscopy
(AFM)
AFM could be called simply a high-resolution surface profiler if not one important difference - AFM cantilever is so thin and sensitive that it can sense the minute surface forces, such as Van-der-Waals forces, magnetic forces, electrostatic forces, etc. It allows to use AFM to investigate not only surface topography, but probe surface physical, chemical, and magnetic properties.
The next figure shows a schematic of the typical AFM tool (one of a few designs used). The main components of this tool are thin cantilever with extremely sharp (10 A to 100 A in radius) probing tip, a 3D piezo-electric scanner, and optical system to measure deflection of the cantilever. When the tip is brought into the contact with the surface or in its proximity, or is tapping the surface, it being affected by a combination of the surface forces (attractive and repulsive). Those forces cause cantilever bending and torsion, which is continuously measures via. the deflection of the reflected laser beam. 3D scanner moves the sample or, in alternative designs, the cantilever, in 3 dimensions thus scanning predetermined area of the surface. A vertical resolution of this tool is extremely high reaching 0.1 A (1 A is on the order of atomic radius).
AFM can image the very fine details of the surface topography. Another good thing about AFM measurements is that the contact force between the probing tip and the surface is very small - on the order of a few nN (10-10 kg) which minimizes damage to the most delicate surfaces.
Below you can find some examples of AFM use for nano-scale imaging of different objects.
Low pass-filtered image of mica surface atoms taken with a BioScope AFM system. Atomic-level stability of the BioScope is demonstrated using the standard BioScope scanner which has an X-Y range of 90um. The BioScope can be operated at all magnifications and be calibrated at all scan sizes. The user does not need to switch scanners to achieve good stability and resolution. 5nm scan. Freshly cleaved mica was attached to a standard microscope slide by double-sided adhesive and mounted onto a standard BioScope sample stage atop a Zeiss Axiovert 135 inverted optical microscope. The mica was imaged in contact mode using standard 450um long, single arm, etched silicon probes. Shown is a deflection image taken at a scan rate of 24.4Hz with integral gain set to 0.665 and proportional gain set to 0.176.
Image/photo taken with NanoScope SPM, courtesy Digital Instruments, Veeco Metrology Group, Santa Barbara ,CA.
Protein/DNA complex imaged in air. The protein is a restriction endonuclease (Eco RI) bound to plasmid DNA. The ability of the AFM to image DNA/protein complexes rapidly and without fixation makes it an extremely useful tool for numerous investigations. 1.2um scan.
Image/photo taken with NanoScope SPM, courtesy Digital Instruments, Veeco Metrology Group, Santa Barbara ,CA.
Magnetic domains in low-coercivity, amorphous CoZrNb film used in emerging, high-Ms thin-film heads. 50um scan. Visible are Landau-Lifshitz and cross-tie domains, and the effects of edge roughness. Such images surpass the resolution of optical Kerr-effect. Captured with LiftMode (lift height 75nm) and a low-moment tip to prevent domain perturbation.
Image/photo taken with NanoScope SPM, courtesy Digital Instruments, Veeco Metrology Group, Santa Barbara ,CA.
Nowadays, AFM remains one of the most popular and indispensable research tools in physics, chemistry, material science, etc.
It is not surprising that the idea of AFM attracted attention of the storage system designers: high resolution and accuracy of the tool, and its ability to create small indentation in the surfaces and image these indentations with the same tip seemed to be more than sufficient for the task. AFM systems work in contact or semi-contact modes, when the deflection of the AFM cantilever is measured optically (most common technique), but could be also measured by using an integrated sensor, which changes electric resistance when deformed. The later technique is preferable for data storage system due to space and cost constrains.
Today's AFM storage prototypes (see IBM Research web page) use the following technique for data recording:
- The tip is being positioned over the desired area of the recording media
- A current is passed trough the tip thus heating it and the media below the tip
- An indentation is made in the softened media. This indentation is an equivalent of 1 bit of data
- A tip is moving to another location...
The reading sequence includes positioning of the tip and imaging the surface topography in the search of indentation.
Practically speaking, this process seems to be too slow to be used in real time, but it could be used to create a master disk - an equivalent of the master disk used do print CD. Once the disk is created it could be used to replicate thousands of disks of enormously high areal recording density (300 GB/sq. inch and higher). Therefore, even if this type of data storage is still far from its final readiness, there is possibly a large potential to it.
The AFM-ROM system could be designed using a small rotating disk, which should provide high data transfer rates (more then 1 MB/sec). The AFM drive will store data as bits or bumps in a polymer disk and detect this bits as the cantilever deflections. The lever could be made from doped silicon to turn it into a piezoresistive material (changes electrical resistance when deformed). The tip will access the track of bits in a similar way to a typical hard drive - using a kind of high-precision voice-coil actuator (VCA). A very small contact load of less then 100 nN is needed to reduce wear of the dip and media and will be set and controlled again with the help of resistance measurements. The bits could be smaller than 50 nm in size and 50 nm in height.
The signal encoding used in AFM storage system can be similar to that used in compact disks, where every transition from pit to land and back is interpreted as 1. No transition means 0, and the length of each land represents the number of 0s in the data stream.
Coming soon...
| Address | Comments |
|---|---|
| IBM Zurich | Place where AFM was born |
| Digital Instruments | Place, where the most popular in the US AFMs are being born today. Find great images there. |
| Blaine's SPM Page | SPM links, Bio AFM Links, STM/AFM Links, etc. |