Scanning probe-type data storage beyond hard disk drive and flash memory
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Brief overview of probe data storage A “probe data storage device” can be defined as a storage device where a mobile “probe” serves as the reader/writer of a storage device. Edison’s phonography, CDs, DVDs, and hard disk drives (HDDs) are good examples of “probe data storage devices”—they all incorporate a stylus, optical pickup or head as mobile probes, and their media vary from polymers to magnetic coatings.1,2 HDDs have a separate reader and writer,3 where the writer typically is an inductive coil, while the reader has led the exponential increase in the density of stored information, starting from the inductive coil through magnetoresistance (MR) heads to giant magnetoresistance, and more recently, to tunneling magnetoresistance heads. Media today have evolved from longitudinal magnetic recording media to perpendicular magnetic recording media due to the superparamagnetic limit of the media, at which point the magnetic moment fluctuates randomly by thermal energy.4 In the early 2000s, the need for smaller form factor was driving the storage device industry toward mobile high-density devices. However, these devices were facing big challenges for further miniaturization while maintaining high information density. This led to a paradigm shift from traditional mechanical assembly to silicon processing, similar to microelectromechanical systems (MEMS) design for further miniaturization of probe data-storage devices. The first step was provided by IBM researchers, when they invented the scanning probe microscope in 1981.5,6
IBM further developed the idea of using the probe for simple measurement and imaging to probe storage devices; they realized that the same probe technology could be applied to storage devices if combined with MEMS and HDD technology.7 The working principle was similar to the old phonograph in the sense that data written in the form of nano- or atomicscale grooves indented by a heated tip. The doped silicon probe acted as a heater and indenter when it heats itself and hits the media with a mechanical force to form an indentation mark (Figure 1).8 The silicon heater also acts as a reader when it scans over the indentation by using the temperaturedependent resistance of the doped silicon probe. The heatdissipation rate changes depending on the air gap between the cantilever and the media surface, which, in turn, determines the temperature and accordingly the resistance of the probe. Thus, it was named thermomechanical probe storage.8
Thermomechanical data storage (Millipede) IBM developed the “Millipede,” a thermomechanical probe storage device (Figure 2),9 which is composed of thousands of atomic force microscope (AFM) tips that run in parallel to write and read data in the form of tiny depressions in a polymer media—data densities were shown to exceed 1 Tbit/in2 (1.55 Pbit/m2) in 2004. IBM’s most recent array design consists of an array of 64 × 64 cantilevers on a 100-µm pitch, where the 6.4 × 6.4 mm2 array is fabricated on a 10 × 10 mm2
Yasuo Cho, Research Institute of Electrical Communication,
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