Feedback-controlled Electromigration Research Articles

Electromigrated Gold Nanogap Tunnel Junction Arrays: Fabrication and Electrical Behavior in Liquid and Gaseous Media.

Tunnel junctions have been suggested as high-throughput electronic single molecule sensors in liquids with several seminal experiments conducted using break junctions with reconfigurable gaps. For practical single molecule sensing applications, arrays of on-chip integrated fixed-gap tunnel junctions that can be built into compact systems are preferable. Fabricating nanogaps by electromigration is one of the most promising approaches to realize on-chip integrated tunnel junction sensors. However, the electrical behavior of fixed-gap tunnel junctions immersed in liquid media has not been systematically studied to date, and the formation of electromigrated nanogap tunnel junctions in liquid media has not yet been demonstrated. In this work, we perform a comparative study of the formation and electrical behavior of arrays of gold nanogap tunnel junctions made by feedback-controlled electromigration immersed in various liquid and gaseous media (deionized water, mesitylene, ethanol, nitrogen, and air). We demonstrate that tunnel junctions can be obtained from microfabricated gold nanoconstrictions inside liquid media. Electromigration of junctions in air produces the highest yield (61-67%), electromigration in deionized water and mesitylene results in a lower yield than in air (44-48%), whereas electromigration in ethanol fails to produce viable tunnel junctions due to interfering electrochemical processes. We map out the stability of the conductance characteristics of the resulting tunnel junctions and identify medium-specific operational conditions that have an impact on the yield of forming stable junctions. Furthermore, we highlight the unique challenges associated with working with arrays of large numbers of tunnel junctions in batches. Our findings will inform future efforts to build single molecule sensors using on-chip integrated tunnel junctions.

Searching for optimal experimental parameters with D-Wave quantum annealer for fabrication of Au atomic junctions

Feedback-controlled electromigration (FCE) is employed to control metal nanowires with quantized conductance and create atomic junctions. The FCE scheme is commonly implemented using many experimental parameters, and determining the optimal FCE parameters can be regarded as a combinatorial optimization problem. The D-Wave quantum annealer has attracted attention for its hardware implementation of quantum annealing (QA), which is a heuristic algorithm for solving combinatorial optimization problems. This study developed a QA approach for optimizing the FCE parameters and executed it on a quantum annealer. The results and performance were compared with those of a classical solver.

3 nm Channel MoS2 Transistors by Electromigration of Metal Interconnection

Further scaling down the feature size of transistors is central for the development of next-generation electronic devices. However, fabrication of transistors with channel lengths down to 5 nm has been challenging due to lithography limit and short channel effects (SCEs). Here, we demonstrate an MoS2-based device with the shortest 3 nm channel length among global back-gated transistors by feedback-controlled electromigration of metal interconnection. The Si/SiO2-back-gated model device shows on/off ratios of up to 2 × 105 and exhibits a field-effect mobility of up to 33.5 cm2 V–1 s–1, which is, to the best of our knowledge, the highest value in the as-yet-reported same-type transistors with a sub-10 nm channel length. This good immunity of the device to SCEs is also corroborated by the COMSOL Multiphysics simulation. After replacing the thicker physical gate SiO2 dielectric and Si electrode with the 2D hexagonal boron nitride (h-BN) and graphene monolayer, respectively, for better gate control, the field-effect mobility is pushed to 51.2 cm2 V–1 s–1 and displays excellent switching characteristics with near-ideal subthreshold swing of 67 mV dec–1 and drain-induced barrier lowering as low as 0.378 V V–1. This work can promote further transistor downscaling and extend Moore’s law.

Nonlinear flexural response of a suspended Au nanobeam structure undergoing an electromigration-lead breakdown

The nonlinear resonance response of electromechanical structures, such as Duffing resonators, can discern both geometrical and internal anomalies, such as the “softening” response attributed to deviations from an ideal parallel plane for the former and the “hardening” response attributed to internal strains for the latter. Herein, we study the evolution of the nonlinear resonance response of a suspended Au nanobeam structure undergoing a mechanical breakdown due to an electromigration-lead process. Nanogaps are formed by utilizing a feedback-controlled electromigration technique while simultaneously electrostatically driving the free-standing beam. The morphological evolution of the metallic nanobeam structures is further ascertained between feedback iterations by a scanning electron microscopy. We detect a rich nonlinear response when changing from softening to hardening, and vice versa, before the ultimate mechanical breakdown.

Machine learning-based approach for automatically tuned feedback-controlled electromigration

Feedback-controlled electromigration (FCE) has been employed to control atomic junctions with quantized conductance. An FCE scheme is controlled by many parameters, such as the threshold differential conductance GTH, feedback voltage VFB, and voltage step VSTEP. It is considered possible to achieve a precise and stable control of the quantized conductance by automatically optimizing the FCE parameters. This motivated us to develop an approach based on machine learning (ML) to tune the feedback parameters of FCE. The ML system is composed of three kinds of engines, namely, learning, evaluation, and inference. The learning engine performs the FCE procedure with random parameters, collects various experimental data, and updates the database. Subsequently, four variables and a cost function are defined to evaluate the controllability of the quantized conductance. The evaluation engine scores the experimental data by using the defined cost function. Then, the control quality is evaluated in real time during the FCE procedure. The inference engine selects the new FCE parameter according to the evaluated data. These engines determine the optimal parameters without human intervention and according to the situation. Finally, we actually applied this system to the FCE procedure. The parameter is selected from sample data in the database according to the variation in controllability. As a result, the controllability gradually improves during the FCE procedure that uses the ML system. The results indicate that the proposed ML system can evaluate the controllability of the FCE procedure and change the VFB parameter in real time according to the situation.

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.

Critical temperature in feedback-controlled electromigration of gold nanostructures

This paper presents several experiments demonstrating the need for a more nuanced picture of electromigration (EM) than that of a fixed critical junction temperature at which EM onset occurs. Our data suggests that even for a fixed cross-sectional geometry the critical junction temperature for EM, Tc, varies with environmental temperature, thermal resistance of adjacent regions, and even the direction of the current flow in asymmetric structures. We have performed feedback-controlled EM on nanowires at environmental temperatures between 75 and 260 K and fit the EM onset points with a constant junction power model. We find that average fit critical power is monotonically increasing with decreasing temperature, but is decidedly nonlinear at lower temperatures. We extract and compare the corresponding Tc values using several different thermal models which utilize measured values of nanowire thermal conductivity for our devices: these models all agree on a moderately increasing Tc with decreasing environmental temperature. This is tentatively explained by enhanced current-driven annealing on the voltage ramp prior to EM onset which decreases structural scattering, thereby increasing the critical temperature at which wind-force-driven hopping events will achieve a critical atomic flux. We also obtain fit critical power for a series of bowtie structures of identical constriction but varying adjacent thermal resistance (Rth), and estimate that Tc in the constriction varies with Rth for higher resistance structures. Critical power measurements on a second series of asymmetric bowties further suggests that Tc also depends on the alignment of the electron flow with the temperature gradient at the constriction.

Evolution of local temperature in Au nanowires during feedback-controlled electromigration observed by atomic force microscopy

Feedback-controlled electromigration (FCE) has been developed to enable more reproducible fabrication of nanoscale gaps between two metallic electrodes. However, there remains considerable uncertainty about some aspects of the FCE process. In this study, electromigration (EM)-induced mass transport in Au nanowires during the application of a voltage feedback technique was directly observed by in situ atomic force microscopy (AFM). The measured results unambiguously revealed a decrease in the cross-sectional area of the nanoconstriction early in the FCE process. In addition, the local temperature in the biased nanoconstriction was estimated using the diffusive heat transport relation. During FCE, the onset of EM occurred at local temperatures ranging from 420 K to 557 K in a room-temperature environment when the current density was held constant at 108 A/cm2. We found that the local temperature at the onset of EM increased in our results when the Joule heating power in the nanoconstriction was not constant. In these cases, the experimental procedure enables the local temperature to rise in a controlled manner. Our results imply that the controlled EM proceeded without causing the nanowires to melt due to Joule heating. The in situ AFM results also enabled the changes in the electrical properties to be correlated directly with the physical modifications occurring in the Au nanowires during FCE.

In situ atomic force microscopy imaging of structural changes in metal nanowires during feedback-controlled electromigration

The authors present the real-time atomic force microscopy (AFM) imaging of structural changes in gold (Au) nanowires during the feedback-controlled electromigration (FCE) process. The resistance increases during the FCE process and is associated with drastic changes in the nanowire morphology, suggesting successful control of electromigration (EM) through the FCE scheme. Moreover, the AFM images taken after performing FCE indicate a redeposition of matter along the nanowire in the direction of the anode side. The grains show faceting structures at the anode side. Furthermore, to obtain quantitative information on the height of structures, cross-sections of the nanowire obtained from the AFM images during FCE were investigated. The height evolution of the narrowest part of the wire perpendicular to the electron flow was obtained, showing that void nucleation and void growth along the grain boundaries, which are located on the border of the nanowire, start in the vicinity of the nanowire constriction at the cathode side. Furthermore, a maximum relative mass transport value of 19% and a mass transport rate of 106 atoms/s were found. These results imply that the in situ AFM technique provides insight into the behavior of EM-induced voids in metal nanowires during FCE.

Ultrafast feedback-controlled electromigration using a field-programmable gate array

Feedback-controlled electromigration (FCE) has been a useful technique for creating nanoscale gaps between metal electrodes. In general, it is considered that the fabrication of nanogaps by the conventional FCE method using a microprocessor-based control system is a very time consuming and slow process. Here, the authors propose an ultrafast FCE method using a field-programmable gate array (FPGA) to immediately and precisely control the channel resistance of metal nanowires at room temperature. Using the ultrafast FCE method based on an FPGA, the channel resistance of Au micrometer wires was precisely controlled from 102 to 104 Ω within 1 s at room temperature, and without catastrophic breaks of the Au micrometer wires. Furthermore, the total process time of the FPGA-based ultrafast FCE procedure was considerably shortened without degradation of the controllability of the channel resistance of the Au micrometer wires, and is 103 to 104 times shorter than that of conventional FCE. These results imply that ultrafast FCE using an FPGA can precisely and stably control the channel resistance of Au micrometer wires with a shortened process time.

In situ electrical characterization of palladium-based single electron transistors made by electromigration technique

We report the fabrication of single electron transistors (SETs) by feedback-controlled electromigration of palladium and palladium-nickel alloy nanowires. We have optimized a gradual electromigration process for obtaining devices consisting of three terminals (source, drain and gate electrodes), which are capacitively coupled to a metallic cluster of nanometric dimensions. This metal nanocluster forms into the inter-electrode channel during the electromigration process and constitutes the active element of each device, acting as a quantum dot that rules the electron flow between source and drain electrodes. The charge transport of the as-fabricated devices shows Coulomb blockade characteristics and the source to drain conductance can be modulated by electrostatic gating. We have thus achieved the fabrication and in situ measurement of palladium-based SETs inside a liquid helium cryostat chamber.

Tuning the local temperature during feedback controlled electromigration in gold nanowires

Feedback controlled electromigration (FCE) in metallic nanowires has been widely used for various purposes. However, the control of the local temperature during FCE remains a challenge. Here, we report that the environment temperature can be used as a knob to tune the local temperature during FCE in gold nanowires. FCE was performed in gold nanowires at various environment temperatures ranging from 4.2 K to 300 K. We find that the dissipated power normalized by the cross section area of the nano constriction is linearly proportional to the environment temperature. Interestingly, the estimated local maximum temperature parabolically depends on the environment temperature. A minimum in the local temperature can be reached if an appropriate environment temperature is chosen. Our findings are well supported by the finite element simulation. Moreover, the data indicates the coupling between FCE triggering current density and local temperature.

Two-dimensional simulations of temperature and current-density distribution in electromigrated structures

We report on the application of a feedback-controlled electromigration technique for the formation of nanometre-sized gaps in mesoscopic gold wires and rings. The effect of current density and temperature, linked via Joule heating, on the resulting gap size is investigated. Our experiments include in situ measurements of the evolution of the electrical resistance and of the structure of the device during electromigration. Experimentally, a good thermal coupling to the substrate turned out to be crucial to reach electrode spacings below 10 nm and to avoid overall melting of the nanostructures. This finding is supported by numerical calculations of the current-density and temperature profiles for structure layouts subjected to electromigration. The numerical method can be used for optimizing the layout so as to predetermine the location where electromigration leads to the formation of a gap.

Feedback-controlled electromigration for the fabrication of point contacts

Lithographically fabricated point contacts serve as important examples of mesoscopic conductors, as electrodes for molecular electronics, and as ultra-sensitive transducers for mechanical motion. We have developed a reproducible technique for fabricating metallic point contacts through electromigration. We employ fast analog feedback in a four-wire configuration in combination with slower computer controlled feedback to avoid catastrophic instability. This hybrid system allows electromigration to proceed while dissipating approximately constant power in the wire. We are able to control the final resistance of the point contact precisely below 5 kΩ and to within a factor of three when the target resistance approaches 12 kΩ where only a single conducting channel remains.

High-Throughput Fabrication of Photoconductors with High Detectivity, Photosensitivity, and Bandwidth

Nanocrystalline cadmium selenide (nc-CdSe) was electrodeposited within a sub-50 nm gold nanogap, prepared by feedback-controlled electromigration, to form a photoconductive metal-semiconductor-metal nanojunction. Both gap formation and electrodeposition were rapid and automated. The electrodeposited nc-CdSe was stoichiometric, single cubic phase with a mean grain diameter of ∼7 nm. Optical absorption, photoluminescence, and the spectral photoconductivity response of the nc-CdSe were all dominated by band-edge transitions. The photoconductivity of these nc-CdSe-filled gold nanogaps was characterized by a detectivity of 6.9 × 10(10) Jones and a photosensitivity of 500. These devices also demonstrated a maximum photoconductive gain of ∼45 and response and recovery times below 2 μs, corresponding to a 3 dB bandwidth of at least 175 kHz.

Ultra-short suspended single-wall carbon nanotube transistors

We describe a method to fabricate clean suspended single-wall carbon nanotube (SWCNT) transistors hosting a single quantum dot ranging in length from a few 10 s of nm down to ≈3 nm. We first align narrow gold bow-tie junctions on top of individual SWCNTs and suspend the devices. We then use a feedback-controlled electromigration to break the gold junctions and expose nm-sized sections of SWCNTs. We measure electron transport in these devices at low temperature and show that they form clean and tunable single-electron transistors. These ultra-short suspended transistors offer the prospect of studying THz oscillators with strong electron-vibron coupling.

Magnetoresistance signature of resonant states in electromigrated Ni nanocontacts

Fundamental insight is reported into magnetoresistance properties of ballistic-type atomic size Ni nanojunctions obtained at low temperatures. Feedback-controlled electromigration was used to reveal the ballistic nature of the transport and stabilize samples of conductance values in the range of G0 (G0=2e2/h). Bias voltage dependent measurements identify a clear magnetoresistance fingerprint of resonant tunneling, revealing that localized states in the nanojunctions can be responsible for nonlinear behavior in the IV curves and the related magnetoresistance properties.

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.

High‐On/Off‐Ratio Graphene Nanoconstriction Field‐Effect Transistor

A method is reported to pattern monolayer graphene nanoconstriction field-effect transistors (NCFETs) with critical dimensions below 10 nm. NCFET fabrication is enabled by the use of feedback-controlled electromigration (FCE) to form a constriction in a gold etch mask that is first patterned using conventional lithographic techniques. The use of FCE allows the etch mask to be patterned on size scales below the limit of conventional nanolithography. The opening of a confinement-induced energy gap is observed as the NCFET width is reduced, as evidenced by a sharp increase in the NCFET on/off ratio. The on/off ratios obtained with this procedure can be larger than 1000 at room temperature for the narrowest devices; this is the first report of such large room-temperature on/off ratios for patterned graphene FETs.

Magnetoresistance Properties of Planar-Type Tunnel Junctions With Ferromagnetic Nanogap System Fabricated by Electromigration Method

We report electromigration techniques for the fabrication of planar-type tunnel junctions with ferromagnetic nanogap system. In these techniques, by monitoring the current passing through the devices, we are easily able to obtain the planar-type Ni-Vacuum-Ni tunnel junctions. In this paper, magnetoresistance (MR) properties of the planar-type Ni-based tunnel junctions formed by stepwise feedback-controlled electromigration (SFCE) and field-emission-induced electromigration (activation) are studied. We performed the SFCE method for Ni nanoconstrictions connecting asymmetrical butterfly-shape electrodes. Furthermore, the activation technique was applied to Ni nanogaps with separations of 15-45 nm. MR ratio of the devices formed by the SFCE exhibited approximately 4% at 16 K . On the other hand, the devices fabricated by the activation showed MR ratio of above 300% at 16 K. These results suggest that it is possible to fabricate planar-type ferromagnetic tunnel junctions with vacuum barriers by electromigration techniques.