Ultrahigh density probe-based nonvolatile memory technology using ferroelectric recording media [electronic resource] : fundamental issues and novel solutions
Keywords: Electrical Engineering

The probe-based seek-and-scan data storage system is an ideal candidate for future ultrahigh-density (> 1 Tbit/inch2) nonvolatile memory devices. In such a system, an array of atomic force microscope (AFM) probes is used to write and read data on a nonvolatile medium. While various writing mechanisms have been proposed for probe-based storage, a great deal of attention has recently been devoted to the pulse writing on ferroelectric films due to the non-destructive nature of the write-erase mechanism. When a short electrical pulse is applied through a conductive probe on a ferroelectric film, the highly concentrated electric field can invert the polarization of a local film volume, resulting in a nonvolatile ferroelectric domain that is the basis of data recording. This mechanism allows for longer medium longevity, i.e., larger number of write-erase cycles that is comparable to hard disk drives, faster write and read times, smaller bit size and higher storage densities. However, no commercial product has reached the market yet. This is due to three fundamental issues that have remained a bottleneck for the development of this technology. These are ultrahigh density writing over large areas, stability of single-digit nanometer inverted domains and probe-tip mechanical wear. In this dissertation, we demonstrate a scheme which allows for the writing of a stable 3.6 Tbit/inch2 storage density over a 1 x 1 [Mu]m2 area, which is the highest density ever written on ferroelectric films over such a large area. Such a high density is enabled by the growth of atomically smooth single-crystal Pb(Zr1-xTix)O3 (PZT) ferroelectric films. We also demonstrate the application of a novel conductive AFM procedure, which allows for the mapping of dead spots in these films, thereby enabling the optimization of their growth conditions. Using novel dielectric-sheathed single-walled carbon nanotube (SWNT) probes, we then demonstrate that single-digit nanometer domains remain stable only if they are fully inverted through the entire PZT film thickness, which is dependent on a critical ratio of electrode size to the film thickness. This understanding enables the formation of stable domains as small as 4 nm in diameter, corresponding to 10 unit cells in size. Such domain size corresponds to potential 40 Tbit/inch2 data storage densities. Furthermore, we show that the built-in bias, which is mainly due to near-surface lead (Pb) vacancies in the PZT films, can be tuned and suppressed by repetitive hydrogen and oxygen plasma treatments, allowing for nanometer-size domain stability in both up and down polarizations. Such treatments compensate for charges induced by the Pb vacancies. Finally, we show that a platinum-iridium probe-tip retains its write-read resolution on such surfaces over 5 km of sliding at a 5 mm/s velocity. This tip-wear endurance is enabled by introducing a thin water layer at the tip-media interface -- thin enough to form a liquid crystal. By modulating the force at the tip-surface contact, this water crystal can act as a viscoelastic material, which reduces the stress level on atomic bonds taking part in the wear process. Furthermore, we show that the wear endurance distance can be increased by as much as two folds through the use of ultra-hard HfB2 metal coatings.