Toshitsugu Sakamoto

Nanometer-scale switches using copper sulfide

We describe a nanometer-scale switch that uses a copper sulfide film and demonstrate its performance. The switch consists of a copper sulfide film, which is a chalcogenide semiconductor, sandwiched between copper and metal electrodes. Applying a positive or negative voltage to the metal electrode can repeatedly switch its conductance in under 100 μs. Each state can persist without a power supply for months, demonstrating the feasibility of nonvolatile memory with its nanometer scale. While biasing voltages, copper ions can migrate in copper sulfide film and can play an important role in switching.

A nonvolatile programmable solid electrolyte nanometer switch

A reconfigurable LSI employing a nonvolatile nanometer-scale switch, NanoBridge, is proposed, and its basic operations are demonstrated. The switch, composed of solid electrolyte copper sulfide, has a <30-nm contact diameter and <100-/spl Omega/ on-resistance. Because of its small size, it can be used to create extremely dense field-programmable logic arrays. A 4 /spl times/ 4 crossbar switch and a 2-input look-up-table circuit are fabricated with 0.18-/spl mu/m CMOS technology, and operational tests with them have confirmed the switchs potential for use in programmable logic arrays. A 1-kb nonvolatile memory is also presented, and its potential for use as a low-voltage memory device is demonstrated.

Electronic transport in Ta2O5 resistive switch

The authors examined the electronic transport of a solid electrolyte resistive switch. Using element analysis and the temperature dependence of its electronic transport, they deduced that the conductive path is composed of Cu metal precipitated in the solid electrolyte film by an electrochemical reaction. Furthermore, they observed Coulomb blockade phenomena at 4K when the switch was in the off state. Their observations and experimental results suggest that the metallic conductive path consists of metallic islands separated by tunneling barriers and that switching between the on and off states originates from modulation in the tunneling barriers.

Diffusivity of Cu Ions in Solid Electrolyte and Its Effect on the Performance of Nanometer-Scale Switch

A novel solid-electrolyte nonvolatile switch that we previously developed for programmable large-scale-integration circuits turns on or off when a conducting Cu bridge is formed or dissolved in the solid electrolyte. Cu⁺ ion migration and an electrochemical reaction are involved in the switching process. For logic applications, we need to adjust its turn-on voltage (<i>V</i> _ON), which was too small to maintain the conductance state during logic operations. In this paper, we clarified that <i>V</i> _ON is mainly affected by the rate of Cu⁺ ion migration in the solid electrolyte. Considering the relationship between the migration rate and <i>V</i> _ON, we replaced the former electrolyte, Cu_2-alphaS, with Ta₂O₅, which enabled us to appropriately adjust <i>V</i> _ON with a smaller Cu⁺ ion diffusion coefficient.

Observation and characteristics of mechanical vibration in three-dimensional nanostructures and pillars grown by focused ion beam chemical vapor deposition

The Young’s modulus of diamond-like carbon (DLC) pillars was measured by means of mechanical vibration using scanning electron microscopy. The DLC pillars were grown using Ga+ focused ion beam-induced chemical vapor deposition with a precursor of phenanthrene vapor. The Young’s modulus of the DLC pillars was around 100 GPa at vapor pressure of 5×10−5 Pa and it had a quality (Q) value of resonance exceeding 1200. There seemed to be a balance between the DLC growth rate and surface bombardment by the ions, and this played an important role in the stiffness of the pillars. Some of the DLC pillars showed a very large Young’s modulus over 600 GPa at low gas pressure conditions.

DNA size separation using artificially nanostructured matrix

We have demonstrated two types of size separation of biomolecules using a nanostructured matrix artificially fabricated using electron-beam lithography: sieve-type separation using a regular pillar array structure and size exclusion chromatography (SEC) type separation using a structure with narrow and wide gaps. With these devices, samples of double-stranded DNA molecules (2, 5, and 10 k base pairs) were clearly separated into bands; smaller molecules eluted earlier in the sieve type while they eluted later in the SEC type. The nanostructured matrix enables various types of molecular separation by changing the design of the nanostructure. Moreover, it should be easy to integrate the matrix with other biomolecular fluidic devices because it does not require a filling medium.

Observation of source-to-drain direct tunneling current in 8 nm gate electrically variable shallow junction metal–oxide–semiconductor field-effect transistors

We investigated quantum mechanical effects in electrically variable shallow junction metal–oxide–semiconductor field-effect transistors with an 8 nm long gate. We clearly observed the direct tunneling current from the source to the drain below 77 K, in good agreement with the calculation. We also showed that the direct tunneling current will exceed the thermal current and will become detrimental to low-voltage operation of MOSLSIs in the 5 nm gate generation.

A chip-stacked memory for on-chip SRAM-rich SoCs and processors

Advanced SoC chips used in multimedia devices such as mobile phones have a number of dedicated functional IP cores, including 3D graphics and video codec, and require local memories with high bit density. Each IP core is connected to closely positioned local memories for fast access and wide bandwidth. The simultaneous operation of all of IP cores on a chip is an extremely rare situation and we anticipate that future integration of more IP cores onto a chip will increase the average number of sleeping IP cores at any given time. Therefore, current chip architectures that allocate local memories to individual IP cores will become increasingly inefficient in thier use of memory resources. In contrast to this, is the use of an off-chip external memory shared by a number of IP cores.

Transistor characteristics of 14-nm-gate-length EJ-MOSFETs

We have fabricated electrically variable shallow junction metal-oxide-silicon field-effect transistors (EJ-MOSFETs) to investigate transport characteristics of ultrafine gate MOSFETs. By using EB direct writing on an ultrahigh-resolution negative resist (calixarene), we could achieved a gate length of only 14 nm. Despite such an ultrafine gate, the device exhibited transistor operation at room temperature. From studying the devices with the gate lengths from 14 nm to 98 nm, we found that when the gate length was below 30 nm the subthreshold leakage current increased. The low-temperature measurements showed that the leakage current was caused by the classical thermal process and that quantum effects do not play an important role in subthreshold characteristics at room temperature.

Calixarene Electron Beam Resist for Nano-Lithography

New electron beam (EB) resists made of calixarene resists are introduced. Typical sensitivities of calixarene resists range from 700 µ C/cm2 to 7 mC/cm2. High-density dot arrays with 15 nm diameter constructed using calixarene resist were easily delineated using a point EB lithography system. Our results suggest that the resolution limit of calixarene resists is dominated by a development process such as adhesion to a substrate rather than by the EB profile. Calixarene resists are resistant to etching by halide plasma. We also demonstrated nanoscale devices processed by using calixarene resists. Calixarene resists are promising materials for nanofabrication.