Ryutaro Suda

Development of ballistic hot electron emitter and its applications to parallel processing: active-matrix massive direct-write lithography in vacuum and thin-film deposition in solutions

Abstract. Making the best use of the characteristic features in nanocrystalline Si (nc-Si) ballistic hot electron source, an alternative lithographic technology is presented based on two approaches: physical excitation in vacuum and chemical reduction in solutions. The nc-Si cold cathode is composed of a thin metal film, an nc-Si layer, an n+-Si substrate, and an ohmic back contact. Under a biased condition, energetic electrons are uniformly and directionally emitted through the thin surface electrodes. In vacuum, this emitter is available for active-matrix drive massive parallel lithography. Arrayed 100×100 emitters (each emitting area: 10×10  μm2) are fabricated on a silicon substrate by a conventional planar process, and then every emitter is bonded with the integrated driver using through-silicon-via interconnect technology. Another application is the use of this emitter as an active electrode supplying highly reducing electrons into solutions. A very small amount of metal-salt solutions is dripped onto the nc-Si emitter surface, and the emitter is driven without using any counter electrodes. After the emitter operation, thin metal and elemental semiconductors (Si and Ge) films are uniformly deposited on the emitting surface. Spectroscopic surface and compositional analyses indicate that there are no significant contaminations in deposited thin films.

Deposition of thin Si and Ge films by ballistic hot electron reduction in a solution-dripping mode and its application to the growth of thin SiGe films

To enhance the usefulness of ballistic hot electron injection into solutions for depositing thin group-IV films, a dripping scheme is proposed. A very small amount of SiCl4 or GeCl4 solution was dripped onto the surface of a nanocrystalline Si (nc-Si) electron emitter, and then the emitter is driven without using any counter electrodes. It is shown that thin Si and Ge films are deposited onto the emitting surface. Spectroscopic surface and compositional analyses showed no extrinsic carbon contaminations in deposited thin films, in contrast to the results of a previous study using the dipping scheme. The availability of this technique for depositing thin SiGe films is also demonstrated using a mixture SiCl4+GeCl4 solution. Ballistic hot electrons injected into solutions with appropriate kinetic energies promote preferential reduction of target ions with no by-products leading to nuclei formation for the thin film growth. Specific advantageous features of this clean, room-temperature, and power-effective process is discussed in comparison with the conventional dry and wet processes.

Resistive switching effects in electromigrated Ni nanogaps

Recently, resistive switches have been constructed from nanogap electrodes. We have already reported a simple method for the fabrication of nanogaps with well-controlled tunnel resistance, which is called “activation”. Since the activation is considered to be the method of transferring atoms across the nanogap, it is expected that a resistive switching effect is caused in the nanogaps controlled by the activation method. In this study, we explore a resistive switching effect of Ni nanogaps using the activation. First, by applying the activation, the tunnel resistance of Ni nanogaps was sufficiently decreased with increasing the field emission current passing through the nanogaps. Then, during the subsequent voltage sweep, the characteristic electrical properties exhibited current increase and decrease, showing the typical set (higher conduction) and reset (lower conduction) processes, respectively. Actually, set/reset state can be controlled with applying pulse voltages, and the conduction states were successfully read out. In this method, the endurance was more than 100 cycles and the resistive ratio was obtained to be about 103. The results suggest that resistive switching properties are successfully observed using nanogaps controlled with activation method.

Improved quasiballistic electron emission from a nanocrystalline Si cold cathode with a monolayer-graphene surface electrode

The quasiballistic electron emission from a nanocrystalline porous silicon (nc-Si) diode is drastically enhanced by using a monolayer-graphene film as the surface electrode. Due to little scattering losses in monolayer-graphene, the electron emission efficiency at room temperature is increased up to 6.3% that is considerably higher than that in the case of conventional thin metal films. The peak energy of emitted electrons can be tuned by the applied voltage while keeping narrow energy dispersion. The energy distribution becomes more monochromatic at a low temperature of around 150 K. Monolayer-graphene acts as a highly transparent nanogrid for quasiballistic hot electrons.The quasiballistic electron emission from a nanocrystalline porous silicon (nc-Si) diode is drastically enhanced by using a monolayer-graphene film as the surface electrode. Due to little scattering losses in monolayer-graphene, the electron emission efficiency at room temperature is increased up to 6.3% that is considerably higher than that in the case of conventional thin metal films. The peak energy of emitted electrons can be tuned by the applied voltage while keeping narrow energy dispersion. The energy distribution becomes more monochromatic at a low temperature of around 150 K. Monolayer-graphene acts as a highly transparent nanogrid for quasiballistic hot electrons.

Simulation analysis of a miniaturized electron optics of the massively parallel electron-beam direct-write (MPEBDW) for multi-column system

In this study, a simulation analysis of a miniaturized electron optics for the Multi-Column Massively Parallel Electron Beam Writing system is demonstrated. Analytical evaluation of space charge effect with prototype Massively Parallel Electron Beam Writing (MPEBW) system showed 2.86 nm blur in radius occurs on each beam with a convergence half angle of 3 mrad. The angle of each beam was increased to 10 mrad to reduce the space charge effect, the coulomb blur amount can be kept to less than 1 nm in radius. However, there was limitation to increasing the angle due to a spherical aberration. Since the beam current density from the electron emitter array in the prototype MPEBW system was 100 μA/cm2 and the total beam current was 1μA with 100×100 array of 10μm square emitter, the influence of coulomb blur was small. By contrast, considerably increasing the number of beams and the beam current are planned in near future in MPEBW. The coulomb blur and other aberrations will not be controlled by merely adjusting the beam convergence angle. In order to increase total beam current, miniaturized electron optics have been designed for Multi-beam+Multi-column system. Reduction lens in the designed miniaturized electron optics with crossover free to reduce the influence of coulomb repulsion with narrow convergence half angle. Unlike conventional methods, the electron beams as principal rays do not intersect at one point, so even if the beam becomes extremely narrow, the coulomb repulsion effect does not increase at the crossover area. The reduction of the entire size of parallel beams in the designed electron optics was confirmed by simulation software. The simulation results showed that least confusion disk of 6.5 nm size was obtained at the beam convergence half angles of 3 mrad corresponding to the incident beam of ±0.1 mrad divergence angle. It showed that the miniaturized electron optics was suitable for 10 nm order EB writing. The crossover free electron optics of the miniaturized electron optics is possible due to dispersing the intersection points of the principal rays by a combination of a concentric electron optics and a tapered lens electrode of the reduction lens.

Development of ballistic hot electron emitter and its applications to parallel processing: active-matrix massive direct-write lithography in vacuum and thin films deposition in solutions

Making the best use of the characteristic features in nanocrystalline Si (nc-Si) ballistic hot electron source, the alternative lithographic technology is presented based on the two approaches: physical excitation in vacuum and chemical reduction in solutions. The nc-Si cold cathode is a kind of metal-insulator-semiconductor (MIS) diode, composed of a thin metal film, an nc-Si layer, an n+-Si substrate, and an ohmic back contact. Under a biased condition, energetic electrons are uniformly and directionally emitted through the thin surface electrodes. In vacuum, this emitter is available for active-matrix drive massive parallel lithography. Arrayed 100×100 emitters (each size: 10×10 μm2, pitch: 100 μm) are fabricated on silicon substrate by conventional planar process, and then every emitter is bonded with integrated complementary metal-oxide-semiconductor (CMOS) driver using through-silicon-via (TSV) interconnect technology. Electron multi-beams emitted from selected devices are focused by a micro-electro-mechanical system (MEMS) condenser lens array and introduced into an accelerating system with a demagnification factor of 100. The electron accelerating voltage is 5 kV. The designed size of each beam landing on the target is 10×10 nm2 in square. Here we discuss the fabrication process of the emitter array with TSV holes, implementation of integrated ctive-matrix driver circuit, the bonding of these components, the construction of electron optics, and the overall operation in the exposure system including the correction of possible aberrations. The experimental results of this mask-less parallel pattern transfer are shown in terms of simple 1:1 projection and parallel lithography under an active-matrix drive scheme. Another application is the use of this emitter as an active electrode supplying highly reducing electrons into solutions. A very small amount of metal-salt solutions is dripped onto the nc-Si emitter surface, and the emitter is driven without using any counter electrodes. After the emitter operation, thin metal (Cu, Ni, Co, and so on) and elemental semiconductors (Si and Ge) films are uniformly deposited on the emitting surface. Spectroscopic surface and compositional analyses indicate that there are no significant contaminations in deposited thin films. The implication is that ballistic hot electrons injected into solutions with appropriate kinetic energies induce preferential reduction of positive ions in solutions with no by-products followed by atom migration, nuclei formation, and the subsequent thin film growth. The availability of this technique for depositing thin SiGe films is also demonstrated by using a mixture solution. When patterned fine emission windows are formed on the emitter surface, metal and semiconductor wires array are directly deposited in parallel.

A Fully Customized Hardware System for Ultra-Fast Feedback-Controlled Electromigration Using FPGA

Electromigration (EM) method for the fabrication of nanogaps is specifically simple as compared with other methods because it is achieved by only passing a current through a metal nanowire. However, typical EM procedure induces an abrupt break that yields a nanogap with high tunnel resistance. Hence, various approaches have been reported to address this problem, and feedback-controlled electromigration (FCE) scheme has been successfully employed to make nanogaps safely and reliably. On the other hand, the formation of nanogaps by FCE method using a microprocessor-based controller with a general purpose operating system is considerably slow process. In this study, we designed a new system using field programmable gate array (FPGA). Furthermore, we applied this system to Au μm-wires. Consequently, the FCE experiments using FPGA-based control system were performed at 20 μsec of deterministic loop time. In addition, conductance was precisely controlled and adjusted from 10 mS to less than 77.5 μS for within 1 sec, which is 102-3 times shorter than that of conventional FCE procedure using microprocessor-based control system.

Deposition of Thin Si, Ge, and SiGe Films by Ballistic Hot Electron Reduction

Liquid-phase deposition scheme of thin group-IV films is presented under ballistic hot electron injection into solutions. Energetic electrons emitted from nanocrystalline Si diode reduce target ions at the interface followed by the deposition of thin amorphous Si, Ge, and SiGe films with no contaminations.

In-situ temperature measurements of Joule-heated graphene using near-infrared CCD imaging system

We report temperature distribution of graphene during Joule heating process using in-situ near-infrared (NIR) charge-coupled device (CCD) imaging system. Graphene layers were prepared using mechanical exfoliation of a pyrolytic graphite sheet (PGS), which is commercially available from an industrial materials company, and were then deposited on SiO2/Si substrates with approximately 780 nm thermally grown oxide. Thickness of the graphene layers was optically determined to be 20-80 nm using Fresnel theory. In order to investigate the heating process of the graphene, the temperature of the graphene under current flow was estimated using NIR microscopy with a CCD detector. A hand-made, in-situ experimental setup consists of an IR microscope, a NIR CCD, and an image enhancer. The CCD detector is mounted on the IR microscope with objective 20×. Heating experiments were carried out in obscurity. Joule heating process controlled with applied bias voltages was performed for the graphene in vacuum/ambient air, and the temperature distribution of the graphene during NIR emission was successfully studied by in-situ NIR CCD imaging system. The temperature of Joule-heated graphene was detected to be approximately 800 K. These results imply that NIR CCD imaging system is a useful tool for the investigation of temperature distribution of graphene.

Electrical properties of nanogap-based single-electron transistors fabricated by field-emission-induced electromigration

We report a simple and easy method for the control of electrical characteristics of planar-type nanogap-based single-electron transistors (SETs) using field-emission-induced electromigration, which is so-called “activation”. 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 the field emission current through a nanogap. When the activation with the preset current of 500 nA was applied to the nanogaps having initial gap separation of 48 nm, current-voltage characteristics of the activated nanogaps displayed the suppression of electrical current at low-bias voltages known as Coulomb blockade at room temperature. In addition, strong Coulomb staircases were clearly obtained, and the quasi-periodic current oscillations were also observed at room temperature. These results indicate that higher charging energy associated with a smaller Ni island structure within the multiple islands causes a bottleneck mechanism in conduction, improving the Coulomb staircase structures. Moreover, we successively performed the activation using the preset current Is of 500 nA to the nanogap with initial gap separation of 27 nm. Coulomb blockade voltage was clearly modulated by the gate voltage periodically at 16 K, resulting in the formation of single island in the SETs fabricated by the activation. These results imply that activation procedure allows us to simply and easily control electrical characteristics of planar-type nanogap-based SETs.