H. Pozidis

Millipede: a MEMS-based scanning-probe data-storage system

Ultrahigh storage densities of up to 1 Tbit/in./sup 2/ or more can be achieved by local-probe techniques to write, read back, and erase data in very thin polymer films. The thermomechanical scanning-probe-based data-storage concept called Millipede combines ultrahigh density, small form factor, and high data rate. After illustrating the principles of operation of the Millipede, we introduce system aspects related to the read-back process, multiplexing, and position-error-signal generation for tracking.

Demonstration of thermomechanical recording at 641 Gbit/in/sup 2/

Ultrahigh storage areal densities can be achieved by using thermomechanical local-probe techniques to write, read back, and erase data in the form of nanometer-scale indentations in thin polymer films. This paper presents single-probe experimental results in which large data sets were recorded at 641 Gbit/in/sup 2/ and read back with raw bit-error rates better than 10/sup -4/. (d,k) modulation coding is used to mitigate the effect of partial erasing, occurring when subsequent indentations are spaced too closely together, and to increase the effective areal density. The physical indentation profile, the sensitivity of the probe in readback mode, and noise sources that affect data detection are also discussed. Quantitative measurements of the partial erasing effect in both the on-track and cross-track directions are reported.

A servomechanism for a micro-electro-mechanical-system-based scanning-probe data storage device

Micro-electro-mechanical-system (MEMS)-based scanning-probe data storage devices are emerging as potential ultra-high-density, low-access-time, and low-power alternatives to conventional data storage. One implementation of probe-based storage uses thermomechanical means to store and retrieve information in thin polymer films. One of the challenges in building such devices is the extreme accuracy and the short latency required in the navigation of the probes over the polymer medium. This paper focuses on the design and characterization of a servomechanism to achieve such accurate positioning in a probe-based storage prototype. In our device, the polymer medium is positioned on a MEMS scanner with x/y-motion capabilities of about 100 µm. The device also includes thermal position sensors that provide x/y-position information to the servo controller. Based on a discrete state-space model of the scanner dynamics, a controller is designed using the linear quadratic Gaussian approach with state estimation. The random seek performance of this approach is evaluated and compared with that of the conventional proportional, integrator, and derivative (PID) approach. The results demonstrate the superiority of the state-space approach, which achieves seek times of about 4 ms in a ± 50 µm range. Finally, the experimental results show that closed-loop track following using the thermal position-sensor signals is feasible and yields a position-error standard deviation of approximately 2 nm.

Nanopositioning for probe-based data storage [Applications of Control]

Probe-based data-storage devices are being considered as an ultra-high-density, small- form-factor alternative to conventional data storage. The probe-based data-storage concept is derived from scanning-probe microscopy, where nanometer-sharp tips are used to interrogate and manipulate matter down to the atomic scale. One implementation of this concept is based on a thermomechanical principle for storing and retrieving data encoded as nanometer-scale indentations in thin polymer films. Ultra-high densities of more than 1 Tb/in2 have been achieved with this scheme. A small-scaleprototype system comprising all of the elements of a probe- based data storage device has been developed. One of the key challenges is the positioning of the storage medium relative to the read/write probes with nanometer-scale accuracy. A microscanner with X/Y motion capability is used to position the storage medium. Position information along both scan directions is provided by a pair of thermal position sensors. In addition, medium-derived position information provides a measure of the cross-track deviations in the Y-scan direction. Control architectures for both scan directions are presented. The X control architecture relies on thermal position sensors alone, whereas the Y control architecture relies on both the thermal position sensors and the medium-derived PES. Nanometer-scale positioning accuracies are achieved over a bandwidth of a few hundred hertz. Read/write demonstrations with sufficiently low error rates demonstrate the efficacy of the nanopositioning schemes employed.

Drift-resilient cell-state metric for multilevel phase-change memory

A new cell-state metric is proposed for multilevel phase-change memory (PCM) that is more representative of the fundamental programmed entity, i.e., the amorphous/crystalline phase configuration in the PCM cell. This metric exhibits improved performance in terms of drift and better sensing resolution of cell states with a large amorphous-phase fraction when compared to the conventional low-field resistance metric. Experimental results using PCM test devices of mushroom type demonstrate the efficacy of the new metric.

A Framework for Reliability Assessment in Multilevel Phase-Change Memory

Multilevel-cell (MLC) storage is the preferred way for achieving increased capacity and thus lower cost-per-bit in memory technologies. In phase-change memory (PCM), MLC storage is hampered by noise and resistance drift. In this paper the issue of reliability in MLC PCM devices is addressed at the array level. The purpose of this study is to identify the dominant reliability issues in PCM arrays and to provide a practical methodology to assess the reliability and predict the retention of multilevel states. Experimental data are used to derive and fit simple empirical models which can be used to assess the device reliability over the course of time.

Nanopositioning for probe storage

Scanning-probe data-storage devices are currently being explored as alternatives to conventional data storage. Ultra-high density, small form factor, and low cost are thought to be the primary advantages of probe storage. The ultra-high a real density makes nanopositioning a significant challenge in probe storage. In this paper, we discuss the control of a MEMS scanner used in a probe storage device which uses thermo-mechanical means to store and retrieve information on thin polymer films. The MEMS scanner has X-Y motion capabilities with a travel range of approx 120 /spl mu/m. Thermal position sensors are used to provide positioning information. This paper describes the dynamics of the micro-scanner, the primary control challenges, and the way they are addressed.

Reliable MLC data storage and retention in phase-change memory after endurance cycling

For phase-change memory to be considered a true universal memory it would have to combine MLC storage, for low cost per bit, with adequately high endurance and at least moderate data retention. However, this appears to be particularly difficult to achieve, because of phenomena such as material segregation, which comes as an effect of cycling, and resistance drift, which is inherent in the amorphous phase and affects the stability of stored data. We present a combination of a memory cell with stable programming behavior over cycling, electrical sensing techniques and signal processing technologies, to demonstrate the viability of reliable, non-volatile, MLC storage in phase-change memory cells after extended endurance cycling.

Estimation of amorphous fraction in multilevel phase change memory cells

The effective thickness of the amorphous chalcogenide part within the active element of a phase change memory (PCM) cell is estimated through electrical measurements. Current-voltage characteristics obtained at various intermediate cell states are fitted with the trap-limited subthreshold transport model and the amorphous part thickness is then extracted. Several cell electrical measures, such as the resistance and the threshold voltage, are shown to closely relate to the estimated parameter. The results serve to further validate the trap-limited conduction model, as well as the series phase distribution hypothesis in the active layer of a PCM cell.

Scanning Probes Entering Data Storage: From Promise to Reality

Micro-electro-mechanical-system (MEMS)-based scanning-probe data storage devices are emerging as ultra-high-density, low-access-time, and low-power alternatives to conventional data storage. The probe-storage technique explored at IBM utilizes AFM probes and thermomechanical means to store and retrieve information in thin polymer films. High data rates are achieved by parallel operation of large 2D arrays with thousands of micro/nanomechanical cantilevers/tips that can be batch-fabricated by silicon surface micromachining techniques. The very high precision required to navigate the probe-tips over the storage medium is achieved by MEMS-based x/y actuators that position the large arrays of probe tips for parallel write/read/erase operations. For thermomechanical scanning-probe storage the polymer medium plays a crucial role. Based on a systematic study of different polymers it has been identified that the glass-transition temperature is the most important property that needs to be controlled for indentation writing and erasing at very narrow spacing. A prototype system demonstrating all the basic functions of a storage device based on scanning probes has been built and its main building blocks will be described in this paper. The inherent parallelism, the ultrahigh areal densities and the small form factor that probe storage techniques offer may open up new perspectives and opportunities for application in areas beyond those envisaged today.