A Phase Change in High-Density Data Storage

A new approach to storing bits of information in a rewritable medium substitutes electron beams for optical beams. Scientists at Hewlett Packard create individual bits in the form of tiny amorphous regions inside a thin indium-selenium layer. That layer, along with another layer beneath (gallium-selenium) and a silicon substrate, form the principal parts of a pn-junction diode. The read-write cycle goes like this: short, high-power bursts from an electron beam are used to write a "1" by melting a tiny portion of the InSe layer, turning it into a glassy blob.

Alternatively the blob can be erased by the use of a longer, low-power beam pulse, which recrystallizes the material. With the help of an even lower-power beam pulse the bit can be read out as either a 1 (the amorphous blob yields little or no detectable current in the pn-junction diode ) or a 0 (the crystalline material yields a high diode current). Electron-beam storage can potentially reach higher densities than optical storage due to the shorter wavelength of high-energy electrons. Ultimately, it may also enable faster data access through electrostatic deflection of the electron-beams.

The HP tests so far have used a laser beam rather than an electron beam to do the writing part (their electron beam isn't yet strong enough) but employ an e-beam (essentially a scanned electron microscope) to do the reading. The response of the diode storage medium is fast enough to allow reading rates of at least a million bits per second per electron-beam and more than 100 write/erase/rewrite cycles have been carried out successfully. The bit size right now is about 150 nm in lateral extent (for an area density of about 29 gigabits per square inch), but this will probably be made far smaller, maybe down to 10 nm. (Gibson et al., Applied Physics Letters, 31 January 2005; contact Gary Gibson, gary.gibson@hp.com, 650-857-2125 or Alison Chaiken, chaiken@hpl.hp.com, 650-23 6-2231