Hugo E. Rothuizen

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.

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.

Ion-beam patterning of magnetic films using stencil masks

Previously, ion-beam irradiation has been shown to locally alter the magnetic properties of thin Co/Pt multilayer films. In this work, we have used ion-beam irradiation through a silicon stencil mask having 1-μm-diam holes to pattern a magnetic film. Regularly spaced micrometer-sized regions of magnetically altered material have been produced over areas of a square millimeter in Co/Pt multilayers. These magnetic structures have been observed by magnetic force microscopy. The patterning technique is demonstrated with mask–sample spacing as large as 0.5 mm. In addition, smaller regions of magnetic contrast, down to 100 nm, were created by using two masks with partially overlapping micrometer holes. Such patterned magnetic films are of interest for application in high-density magnetic recording.

Atomic force microscope cantilevers for combined thermomechanical data writing and reading

Heat conduction governs the ultimate writing and reading capabilities of a thermomechanical data storage device. This work investigates transient heat conduction in a resistively heated atomic force microscope cantilever through measurement and simulation of cantilever thermal and electrical behavior. The time required to heat a single cantilever to bit-writing temperature is near 1 μs and the thermal data reading sensitivity ΔR/R is near 1×10−4 per vertical nm. Finite-difference thermal and electrical simulation results compare well with electrical measurements during writing and reading, indicating design tradeoffs in power requirements, data writing speed, and data reading sensitivity. We present a design for a proposed cantilever that is predicted to be faster and more sensitive than the present cantilever.

Nanometer thin-film Ni-NiO-Ni diodes for detection and mixing of 30 THz radiation

Abstract We report on the realization and the experimental study of thin-film Ni–NiO–Ni diodes with integrated infrared antennas. These diodes are applied as detectors and mixers of 28-THz CO 2 -laser radiation with difference frequencies up to 176 GHz. They constitute a mechanically stable alternative to the point-contact MOM diodes used today in heterodyne detection of such high frequencies. Thus, they represent the extension of present millimeter-wave and microwave thin-film and antenna techniques to the infrared. Our thin-film Ni–NiO–Ni diodes are fabricated on SiO 2 /Si substrates with the help of electron-beam lithography at the IBM Research Laboratory (Ruschlikon, Switzerland). We have reduced the contact area to 110 nm×110 nm in order to achieve a fast response of the device. This contact area is in the order of those of point-contact diodes and represents the smallest ever reported for thin-film MOM diodes. The thin NiO layer with a thickness of about 35 A is deposited by sputtering. Our thin-film diodes are integrated with planar dipole, bow-tie and spiral antennas that couples the incident field to the contact. The second derivative I″ ( V ) of the nonlinear I ( V ) characteristics at the bias voltage applied to the diode is measured at a frequency of 10 kHz. It determines the detection and second-order mixing performed with the diode for frequencies from dc to at least 30 THz. The I″ ( V ) characteristics exhibit for low bias voltage V bias a linear dependence, which is followed by a saturation and a maximum for high V bias . The zero-bias resistance of the diode is in the order of 100 Ω. It is not strictly inversely proportional to the contact area of the diode. The first application of our thin-film diodes was the detection of cw CO 2 -laser radiation. The measured dc signal generated by the diode when illuminated with 10.6- μ m radiation includes a polarization-independent contribution, caused by thermal effects. This contribution is independent of the contact area and of the type of integrated antenna. The polarization-dependent contribution of the signal originates in the rectification of the antenna currents in the diode by nonlinear tunneling through the thin NiO layer. It follows a cosine-squared dependence on the angle of orientation of the linear polarization, as expected from antenna theory. For the linearly polarized dipole and bow-tie antennas, the maximum detection signals are therefore measured for the polarization parallel to the antenna axis. Bow-tie antennas with a half length of 2.3 μ m generate the highest detection signals. The full length of these antennas corresponds to 3/2 of the wavelength of the incident 10.6- μ m radiation in the supporting Si substrate. The relevance of the substrate wavelength confirms that our antennas are more sensitive to the radiation incident from the substrate side. The time of response of our thin-film diode is not limited by the speed of the electron-tunneling effect, but by the RC time constant of the diode circuitry. Thus, the overall best performances are attained by the diodes with the smallest contact areas and corresponding capacitances. The study of the polarization response of our integrated asymmetric spiral antennas revealed the contribution of an unbalanced mode propagating on the antenna arms beside the fundamental balanced mode. The imbalance is caused by the reactive impedance of the diode and by the asymmetry of the antenna arms in the feed region. In addition, the response of the diode is influenced by reflection of the antenna currents near the end of the spiral arms. The resulting polarization of our spiral antenna is therefore not the expected circular polarization, yet an elliptical polarization with an axial ratio in the order of 0.12. Furthermore, we have demonstrated the presence of two distinct additive thermal effects besides the fast antenna-induced contribution by the measurement of the response of our thin-film diodes to 35 ps optical-free-induction decay (OFID) CO 2 -laser pulses. The measured characteristic times of these two relatively slow relaxations are τ 1 ≈100 ns and τ 2 ≈15 ns. These exponential relaxations observed are explained by thermal diffusion in the SiO 2 and in the Ni layers of our structures. These time constants show that thermal effects influence mixing processes at low difference frequencies. For the first time, the operation of thin-film diodes as mixers of 28-THz radiation was demonstrated. Difference frequencies up to 176 GHz have been measured when the diode was irradiated by two CO 2 -laser beams and microwaves generated by a Gunn oscillator working at 58.8 GHz. These difference frequencies were generated in mixing processes from the second to the fifth order. These experiments were performed with thin-film Ni–NiO–Ni diodes with the minimum contact area of 0.012 μ m 2 and integrated resonant bow-tie antennas. The transmission of the high-frequency signals to the spectrum analyzer was accomplished using integrated rhodium waveguides and flip-chip connections. The diode and the antenna were irradiated through the substrate, taking advantage of the better sensitivity of the antenna to radiation incident from the substrate side. The dependence on the linear polarization of the mixing signal matches almost perfectly the ideal cosine-squared dependence predicted by antenna theory for bow-tie antennas. A ratio of the mixing signals for the polarization parallel to the axis vs. the cross-polarization of over 50 was attained. The signal-to-noise ratios of our mixing signals demonstrate the potential of our type of diodes to respond to even higher carrier and difference frequencies. Also higher-order mixing can be achieved with our thin-film diodes.

Design of atomic force microscope cantilevers for combined thermomechanical writing and thermal reading in array operation

In thermomechanical data writing, a resistively-heated atomic force microscope (AFM) cantilever tip forms indentations in a thin polymer film. The same cantilever operates as a thermal proximity sensor to detect the presence of previously written data bits. This paper uses recent progress in thermal analysis of the writing and reading modes to develop new cantilever designs for increased speed, sensitivity, and reduced power consumption in both writing and reading operation. Measurements of cantilever electrical resistance during heating reveals physical limits of cantilever writing and reading, and verifies a finite-difference thermal and electrical simulation of cantilever operation. This work proposes two new cantilever designs that correspond to fabrication technology benchmarks. Simulations predict that the proposed cantilevers have a higher data rate and are more sensitive than the present cantilever. The various cantilever designs offer single-bit writing times of 0.2 /spl mu/s-25 /spl mu/s for driving voltages of 2-25 V. The thermal reading /spl Delta/R/R sensitivity is as high as 4/spl times/10/sup -4/ per vertical nm in near steady-state operation.

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.

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.