Fabrication of atomic junctions with experimental parameters optimized using ground-state searches of Ising spin computing.

Feedback-controlled electromigration (FCE) is employed to control metal nanowires with quantized conductance and create nanogaps and atomic junctions. In the FCE method, the experimental parameters are commonly selected based on experience. However, optimization of the parameters by way of tuning is intractable because of the impossibility of attempting all different combinations systematically. Therefore, we propose the use of the Ising spin model to optimize the FCE parameters, because this approach can search for a global optimum in a multidimensional solution space within a short calculation time. The FCE parameters were determined by using the energy convergence properties of the Ising spin model. We tested these parameters in actual FCE experiments, and we demonstrated that the Ising spin model could improve the controllability of the quantized conductance in atomic junctions. This result implies that the proposed method is an effective tool for the optimization of the FCE process in which an intelligent machine can conduct the research instead of humans.

Gesture Prediction Using Wearable Sensing Systems with Neural Networks for Temporal Data Analysis.

A human gesture prediction system can be used to estimate human gestures in advance of the actual action to reduce delays in interactive systems. Hand gestures are particularly necessary for human⁻computer interaction. Therefore, the gesture prediction system must be able to capture hand movements that are both complex and quick. We have already reported a method that allows strain sensors and wearable devices to be fabricated in a simple and easy manner using pyrolytic graphite sheets (PGSs). The wearable electronics could detect various types of human gestures with high sensitivity, high durability, and fast response. In this study, we demonstrated hand gesture prediction by artificial neural networks (ANNs) using gesture data obtained from data gloves based on PGSs. Our experiments entailed measuring the hand gestures of subjects for learning purposes and we used these data to create four-layered ANNs, which enabled the proposed system to successfully predict hand gestures in real time. A comparison of the proposed method with other algorithms using temporal data analysis suggested that the hand gesture prediction system using ANNs would be able to forecast various types of hand gestures using resistance data obtained from wearable devices based on PGSs.

Quantifying Joule Heating and Mass Transport in Metal Nanowires During Controlled Electromigration.

The nanoscale heat dissipation (Joule heating) and mass transport during electromigration (EM) have attracted considerable attention in recent years. Here, the EM-driven movement of voids in gold (Au) nanowires of different shapes (width range: 50⁻300 nm) was directly observed by performing atomic force microscopy. Using the data, we determined the average mass transport rate to be 105 to 106 atoms/s. We investigated the heat dissipation in L-shaped, straight-shaped, and bowtie-shaped nanowires. The maximum Joule heating power of the straight-shaped nanowires was three times that of the bowtie-shaped nanowires, indicating that EM in the latter can be triggered by lower power. Based on the power dissipated by the nanowires, the local temperature during EM was estimated. Both the local temperature and junction voltage of the bowtie-shaped nanowires increased with the decrease in the Joule heating power and current, while the current density remained in the order of 108 A/cm². The straight-shaped nanowires exhibited the same tendency. The local temperature at each feedback point could be simply estimated using the diffusive heat transport relationship. These results suggest that the EM-driven mass transport can be controlled at temperatures much lower than the melting point of Au.

Formation scheme of quantum point contacts based on nanogaps using field-emission-induced electromigration.

We propose a new fabrication scheme of quantum point contacts (QPCs) composed of nanogaps at room temperature. This scheme is based on electromigration induced by a field emission current, which is so-called "activation." By applying the activation to ferromagnetic Ni nanogaps with sub-10 nm separation, QPCs can be easily obtained at room temperature. The conductance changed in quantized steps of 0.5G0 (G0 = 2e2/h) at the final stage of activation with a preset current Is of 0.5 mA. Then, the conductance during the activation was varied from 2G0 to 9.5G0 by increasing the preset current Is from 0.5 mA to 1.5 mA. Furthermore, after performing the activation with the preset current Is of 1.5 mA, the QPC device formed by the activation exhibited magnetoresistance (MR) ratio of approximately 1.5%. These results indicate that few-atom Ni contacts are achieved using Ni nanogaps controlled by the activation with precisely tuned preset current.

Control parameters for fabrication of single-electron transistors using field-emission-induced electromigration.

We report a simple method for the control of electrical characteristics of planar-type metal-based single-electron transistors (SETs) using field-emission-induced electromigration. The advantages of this method are as follows: (1) the fabrication of SETs is achieved by only passing a field emission current through a nanogap and (2) the charging energy of SETs can be controlled by adjusting the magnitude of the applied current during the procedure. In order to better control the electrical properties of the SETs, we investigate the relation between control parameters of the method and electrical characteristics of the SETs. When the field-emission-induced electromigration with the preset current of 500 nA was applied to the nanogaps, current-voltage characteristics of the nanogaps displayed the suppression of electrical current at low-bias voltages known as Coulomb blockade at 16 K. In addition, Coulomb blockade voltage was clearly modulated by the gate voltage periodically at 16 K, resulting in the formation of single island in the SETs by the field-emission-induced electromigration. Furthermore, as the preset current was increased, the charging energy of the SETs was decreased with decreasing the initial gap separation of the nanogaps. These results imply that the electrical characteristics of the SETs are controllable by the preset current of the method and the initial gap separation of the nanogaps. Field-emission-induced electromigration procedure allows us to simply control electrical characteristics of planar-type metal-based SETs.

Integration of single-electron transistors using field-emission-induced electromigration.

A novel technique for the integration of planar-type single-electron transistors (SETs) composed of nanogaps is presented. This technique is based on the electromigration procedure, which is caused by a field emission current. The technique is called "activation." By applying the activation to the nanogaps, SETs can be easily obtained. Furthermore, the charging energy of the SETs can be controlled by adjusting the magnitude of the applied current during the activation process. The integration of two SETs was achieved by passing a field emission current through two series-connected initial nanogaps. The current-voltage (I(D)-V(D)) curves of the simultaneously activated devices exhibited clear electrical-current suppression at a low-bias voltage at 16 K, which is known as the Coulomb blockade. The Coulomb blockade voltage of each device was also obviously modulated by the gate voltage. In addition, the two SETs, which were integrated by the activation procedure, exhibited similar electrical properties, and their charging energy decreased uniformly with increasing the preset current during the activation. These results indicate that the activation procedure allows the simple and easy integration of planar-type SETs.

Tuning of tunnel resistance of nanogaps by field-emission-induced electromigration using current source mode.

A newly investigated technique for the tuning of the tunnel resistance of nanogaps using electromigration method induced by a field emission current is presented to reduce the power consumption during the process. The method is called "activation" and is demonstrated with a current source. Planar-type initial nanogaps of Ni separated by 20-80 nm were defined on SiO2/Si substrates via electron-beam lithography and the lift-off process. Then, a bias current was applied to the initial nanogaps at room temperature, using a current source. The applied current was slowly ramped up until it reached the preset value. As a result, the process time of the current-source-based activation was 16 times shorter than activation using a voltage source. Furthermore, the tunnel resistance of the nanogaps was reduced from 100 T ohms to 70 M ohms by increasing preset current I(s) from 1 nA to 3.5 microA. Regarding the average power required for current-source-based activation, it can be successfully suppressed compared with that of voltage-source-based activation. These results imply that the current source directly and precisely tunes the field emission current passing through the nanogaps, and effectively causes the migration of atoms across the nanogaps, resulting in the successful control of the tunnel resistance of the nanogaps.

Scanning probe microscope lithography at the micro- and nano-scales.

Scanning probe microscopy (SPM)-based lithography at the micro- and nano-scales is presented. Our method in SPM local oxidation involves two SPM tips, one having a robust blunt tip, a "micrometer tip," and the other having a sharp tip, a "nanometer tip." In tapping-mode SPM local oxidation experiments, Si oxide wires with sub-10 nm resolution were produced by precisely tuning the dynamic properties of the nanometer tip such as drive amplitude and quality factor. On the other hand, in order to perform large-scale oxidation, SPM tip with a contact area of microm2, which is about 10(4) times larger than that of the conventional nanometer tip, was prepared. We propose and demonstrate a method of performing micrometer-scale SPM local oxidation using the micrometer tip under contact-mode operation. The width of the Si oxide produced was clearly determined by the contact length of the tip. Furthermore, we explore the possibility of performing the sub-20 nm lithography of Si surfaces using SPM scratching with a diamond-coated tip. The influence of various scan parameters on the groove size was investigated. The groove size could be precisely controlled by the applied force, scan direction, and the number of scan cycles. There is no effect of the scan speed on the groove size. It is concluded that high-speed nanolithography can be achieved without the degradation of patterns by SPM scratching. SPM-based lithography has the advantage of being able to fabricate a desired structure at an arbitrary position on a surface and plays an important role for bridging the gap between micro- and nano-scales.

10 micrometer-scale SPM local oxidation lithography.

10 micrometer-scale scanning probe microscopy (SPM) local oxidation lithography was performed on Si. In order to realize large-scale oxidation, an SPM tip with a contact length of 15 microm was prepared by focused-ion-beam (FIB) etching. The oxidation was carried out in contact mode operation with the contact force ranging from 0.1 to 2.1 microN. The applied bias voltage was 50 V, and scanning speed was varied from 10 to 200 microm/s. The scan length was 15 microm for one cycle. The influence of contact force on the large-scale oxidation was investigated. At high contact force, the Si oxide with good size uniformity was obtained even with high scanning speed. The SPM tip with larger contact length may increase the spatial dimensions of the water meniscus between the SPM tip and sample surface, resulting in the larger dimensions of the fabricated oxide. Furthermore, the throughput of large-scale oxidation reached about 10(3) microm2/s by controlling the scanning speed and contact force of the SPM tip. It is suggested that SPM local oxidation can be upscaled by using a SPM tip with large contact length.

Fabrication of single-electron transistors using field-emission-induced electromigration.

We report a novel technique for the fabrication of planar-type Ni-based single-electron transistors (SETs) using electromigration method induced by field emission current. The method is so-called "activation" and is demonstrated using arrow-shaped Ni nanogap electrodes with initial gap separations of 21-68 nm. Using the activation method, we are easily able to obtain the SETs by Fowler-Nordheim (F-N) field emission current passing through the nanogap electrodes. The F-N field emission current plays an important role in triggering the migration of Ni atoms. The nanogap is narrowed because of the transfer of Ni atoms from source to drain electrode. In the activation procedure, we defined the magnitude of a preset current Is and monitored the current I between the nanogap electrodes by applying voltage V. When the current I reached a preset current Is, we stopped the voltage V. As a result, the tunnel resistance of the nanogaps was decreased from the order of 100 T(omega) to 100 k(omega) with increasing the preset current Is from 1 nA to 150 microA. Especially, the devices formed by the activation with the preset current from 100 nA to 1.5 microA exhibited Coulomb blockade phenomena at room temperature. Coulomb blockade voltage of the devices was clearly modulated by the gate voltage quasi-periodically, resulting in the formation of multiple tunnel junctions of the SETs at room temperature. By increasing the preset current from 100 nA to 1.5 microA in the activation scheme, the charging energy of the SETs at room temperature was decreased, ranging from 1030 meV to 320 meV. Therefore, it is found that the charging energy and the number of islands of the SETs are controllable by the preset current during the activation. These results clearly imply that the activation procedure allows us to easily and simply fabricate planar-type Ni-based SETs operating at room temperature.

Influence of feedback parameters on resistance control of metal nanowires by stepwise feedback-controlled electromigration.

We propose a stepwise feedback-controlled electromigration (SFCE) approach to control the channel resistance of metal nanowires at room temperature. SFCE procedure finely divides a conventional feedback-controlled electromigration (FCE) scheme into several FCE cycles. This approach effectively removes thermal instability caused by large current passing through a metal nanowire, because process time of each FCE cycle can be successfully reduced. Using the SFCE approach, a wide-range control of the channel resistance of Ni nanowires was achieved ranging from the order of 10(2) omega to 10(5) omega at room temperature, without catastrophic breaks of the nanowires. Furthermore, total process time of the SFCE procedure was considerably shortened without degradation of the controllability of the resistance of the nanowires. The channel resistance of a Ni nanowire was precisely controlled from 0.2 to 600 k(omega) for 20 min at room temperature, which is 3000 times larger than the initial resistance of the channel. These results clearly indicate that a wide-range control of the channel resistance of metal nanowires can be achieved with a shortened process time using SFCE scheme.

Nanomachining of permalloy for fabricating nanoscale ferromagnetic structures using atomic force microscopy.

It is well-known that the tip of an Atomic Force Microscope (AFM) can act as a cutting tool for machining various types of materials. In this article, AFM machining experiments have been conducted to investigate the machining characteristics of a nickel-iron thin film material. The influences of the machining parameters on the resulting machined geometries and surfaces are specifically investigated. The machining parameters considered include the normal applied force, number of machining cycles, machining speed, and machining direction. To demonstrate its versatility, the machining technique developed has been applied for fabricating a NiFe based nanostructure required by many ferromagnetic devices. All results indicate that the machined groove size can be well correlated with and precisely controlled by the applied force and the machining cyclic number. The AFM machining technique is indeed simple and predictable for machining nanostructures with specified dimension and controllable precision.