Ute Drechsler

The Millipede: more than one thousand tips for future AFM data storage

We report on a new atomic force microscope (AFM)-based data storage concept called the “Millipede” that has a potentially ultrahigh density, terabit capacity, small form factor, and high data rate. Its potential for ultrahigh storage density has been demonstrated by a new thermomechanical local-probe technique to store and read back data in very thin polymer films. With this new technique, 30–40-nm-sized bit indentations of similar pitch size have been made by a single cantilever/tip in a thin (50-nm) polymethylmethacrylate (PMMA) layer, resulting in a data storage density of 400–500 Gb/in. 2 High data rates are achieved by parallel operation of large two-dimensional (2D) AFM arrays that have been batch-fabricated by silicon surface-micromachining techniques. The very large scale integration (VLSI) of micro/nanomechanical devices (cantilevers/tips) on a single chip leads to the largest and densest 2D array of 32 × 32 (1024) AFM cantilevers with integrated write/read storage functionality ever built. Time-multiplexed electronics control the write/read storage cycles for parallel operation of the Millipede array chip. Initial areal densities of 100–200 Gb/in. 2 have been achieved with the 32 × 32 array chip, which has potential for further improvements. In addition to data storage in polymers or other media, and not excluding magnetics, we envision areas in nanoscale science and technology such as lithography, high-speed/large-scale imaging, molecular and atomic manipulation, and many others in which Millipede may open up new perspectives and opportunities.

A cantilever array-based artificial nose

We present quantitative and qualitative detection of analyte vapors using a microfabricated silicon cantilever array. To observe transduction of physical and chemical processes into nanomechanical motion of the cantilever, swelling of a polymer layer on the cantilever is monitored during exposure to the analyte. This motion is tracked by a beam-deflection technique using a time multiplexing scheme. The response pattern of eight cantilevers is analyzed via principal component analysis (PCA) and artificial neural network (ANN) techniques, which facilitates the application of the device as an artificial chemical nose. Analytes tested comprise chemical solvents, a homologous series of primary alcohols, and natural flavors. First differential measurements of surface stress change due to protein adsorption on a cantilever array are shown using a liquid cell.

Ultrahigh-density atomic force microscopy data storage with erase capability

We report a simple atomic force microscopy-based concept for a hard disk-like data storage technology. Thermomechanical writing by heating a Si cantilever in contact with a spinning polycarbonate disk has already been reported. Here the medium has been replaced with a thin polymer layer on a Si substrate, resulting in significant improvements in storage density. With this new medium, we achieve bit sizes of 10–50 nm, leading to data densities of 500 Gbit/in.2. We also demonstrate a novel high-speed and high-resolution thermal readback method, which uses the same Si cantilevers that are used in the writing process, and the capability to erase and rewrite data features repeatedly.

Ultralow nanoscale wear through atom-by-atom attrition in silicon-containing diamond-like carbon

Understanding friction and wear at the nanoscale is important for many applications that involve nanoscale components sliding on a surface, such as nanolithography, nanometrology and nanomanufacturing. Defects, cracks and other phenomena that influence material strength and wear at macroscopic scales are less important at the nanoscale, which is why nanowires can, for example, show higher strengths than bulk samples. The contact area between the materials must also be described differently at the nanoscale. Diamond-like carbon is routinely used as a surface coating in applications that require low friction and wear because it is resistant to wear at the macroscale, but there has been considerable debate about the wear mechanisms of diamond-like carbon at the nanoscale because it is difficult to fabricate diamond-like carbon structures with nanoscale fidelity. Here, we demonstrate the batch fabrication of ultrasharp diamond-like carbon tips that contain significant amounts of silicon on silicon microcantilevers for use in atomic force microscopy. This material is known to possess low friction in humid conditions, and we find that, at the nanoscale, it is three orders of magnitude more wear-resistant than silicon under ambient conditions. A wear rate of one atom per micrometre of sliding on SiO(2) is demonstrated. We find that the classical wear law of Archard does not hold at the nanoscale; instead, atom-by-atom attrition dominates the wear mechanisms at these length scales. We estimate that the effective energy barrier for the removal of a single atom is approximately 1 eV, with an effective activation volume of approximately 1 x 10(-28) m.

VLSI-NEMS chip for parallel AFM data storage

Abstract We report the microfabrication of a 32×32 (1024) 2D cantilever array chip and its electrical testing. It has been designed for ultrahigh-density, high-speed data storage applications using thermomechanical writing and readout in thin polymer film storage media. The fabricated chip is the first very large scale integration (VLSI)-NEMS (NanoEMS) for nanotechnological applications. For electrical and thermal stability, the levers are made of silicon, and the heater/sensor element is defined as a lower, doped platform with the tip on top. Freestanding cantilevers are obtained with surface-micromachining techniques, which yield better mechanical stability and heatsinking of the chip than bulk-micromachining releasing techniques do. Two-wiring levels interconnect the cantilevers for a time-multiplexed row/column addressing scheme. By integrating a Schottky diode in series with each cantilever, a considerable reduction of crosstalk between cantilevers has been achieved.

Probe-based ultrahigh-density storage technology

Ultrahigh storage densities can be achieved by using a thermomechanical scanning-probe-based data-storage approach to write, read back, and erase data in very thin polymer films. High data rates are achieved by parallel operation of large two-dimensional arrays of cantilevers that can be batch fabricated by silicon-surface micromachining techniques. The very high precision required to navigate the storage medium relative to the array of probes is achieved by microelectromechanical system (MEMS)- based x and y actuators. The ultrahigh storage densities offered by probe-storage devices pose a significant challenge in terms of both control design for nanoscale positioning and read-channel design for reliable signal detection. Moreover, the high parallelism necessitates new dataflow architectures to ensure high performance and reliability of the system. In this paper, we present a small-scale prototype system of a storage device that we built based on scanning-probe technology. Experimental results of multiple sectors, recorded using multiple levers at 840 Gb/in2 and read back without errors, demonstrate the functionality of the prototype system. This is the first time a scanning-probe recording technology has reached this level of technical maturity, demonstrating the joint operation of all building blocks of a storage device.

Selective Transfer Technology for Microdevice Distribution

We have developed a generic cost-efficient CMOS-compatible heterogeneous device integration method at wafer-scale level. This method enables the distribution of devices from one to numerous wafers using selective transfer technology. We have applied this method for the distribution of atomic force microscopy (AFM) cantilevers and successfully demonstrated the population of multiple wafers from one source wafer. The distribution function has been designed such as to populate 42 wafers with only one source wafer. This CMOS back-end-of-the-line compatible method is particularly suitable for microelectromechanical systems and integrated circuits. Electrical interconnects are compatible with this technology. We present the concept, the selective transfer method, including a laser ablation technique used for the transfer, as well as the process and results of the application for AFM cantilever distribution.

5×5 2D AFM cantilever arrays a first step towards a Terabit storage device

Abstract In this paper we report on the microfabrication of a 5×5 2D cantilever array and its successful application to parallel imaging. The 5×5 array with integrated force sensing and tip heating has been fabricated using a recently developed, all dry, silicon backside etching process. The levers on the array have integrated piezoresistive sensing, and are placed on a constriction in the lever to improve sensitivity. The array is scanned in x and y directions using voice coil actuators. Three additional voice coil z actuators are used in a triangular arrangement to approach the sample with the array chip. The system is thus leveled in the same way as an air table. We report details of the array fabrication, the x – y scanning and approach system as well as images taken with the system. The results are encouraging for the development of large-scale VLSI-Nano EMS, allowing the fabrication and operation of large AFM cantilever arrays to achieve high-data-rate Terabit storage systems.

Soft, entirely photoplastic probes for scanning force microscopy

A new probe made entirely of plastic material has been developed for scanning probe microscopy. Using a polymer for the cantilever facilitates the realization of mechanical properties that are difficult to achieve with classical silicon technology. The new cantilever and tip presented here are made of an epoxy-based photoplastic. The fabrication process is a simple batch process in which the integrated tip and the lever are defined in one photolithography step. The simplicity of the fabrication step, the use of a polymer as material, and the ability to reuse the silicon mold lead to a soft low-cost probe for scanning force microscopy. Imaging soft condensed matter with photoplastic levers, which uses laser beam deflection sensing, exhibits a resolution that compares well with that of commercially available silicon cantilevers.

A Vibration Resistant Nanopositioner for Mobile Parallel-Probe Storage Applications

We describe a planar microelectromechanical systems (MEMS)-based x/y nanopositioner designed for parallel-probe storage applications. The nanopositioner is actuated electromagnetically and has x/y motion capabilities of plusmn60 mum. The mechanical components are fabricated from a single-crystal silicon wafer using a deep-trench-etching process. To render the system robust against vibration, we utilize a mass-balancing concept that makes the system stiff against linear shock, but still compliant for actuation, and therefore results in low power consumption. We present details of the finite-element model used to design the device as well as experimental results for the frequency response, actuation, and vibration-rejection properties of the nanopositioner