Motonobu Sato

Intercalated multi-layer graphene grown by CVD for LSI interconnects

We have fabricated multi-layer graphene (MLG) wiring and demonstrated a resistivity of the same order as Cu and reliability better than Cu. The MLG was synthesized epitaxially by chemical vapor deposition (CVD) on an epitaxial Co film, resulting in quality and electrical properties as good as those of a graphite crystal. The MLG was further intercalated with FeCl3 to achieve a resistivity as low as 9.1 μΩ cm. Our results show that intercalated MLG is really promising for future LSI interconnects.

Intercalated multilayer graphene wires and metal/multilayer graphene hybrid wires obtained by annealing sputtered amorphous carbon

We have fabricated an intercalated multilayer graphene (i-MLG) wire and a cobalt metal/multilayer graphene (Co/MLG) hybrid wire with decreased resistivity relative to a multilayer graphene (MLG) wire by annealing sputtered amorphous carbon. The resistances of the i-MLG and Co/MLG wires decrease by 1 and 2 orders of magnitude relative to that of the MLG wire, respectively. The breakdown tests of the i-MLG and MLG wires indicate that the current tolerance per graphene layer remains almost the same, independent of whether the wires are intercalated. On the other hand, the Co/MLG hybrid wire exhibits not only decreased resistivity but also increased current tolerance. Moreover, with the optimization of the film thickness and annealing temperature, the obtained resistivities of Co/MLG, MLG, and i-MLG are 10, 80, and 6 µΩ cm, respectively.

High-Current Reliability and Growth Conditions of Multilayer Graphene Wire Obtained by Annealing Sputtered Amorphous Carbon

We fabricated multilayer graphene directly on SiO2 by annealing of sputtered amorphous carbon under a catalyst layer without complicated transfer processes, and investigated the effects of the catalysts and the annealing ambient gases on obtaining large-grain, multilayer graphene. As a result, it was found that annealing conditions with a Co catalyst layer in a nitrogen gas atmosphere are important for increasing the ratio of oriented graphene sheets, corresponding to a lower resistivity of the film. Furthermore, it was confirmed that the multilayer graphene wire obtained by optimizing the growth conditions can sustain a high current density of 107 A/cm2, that is, the lifetime of the multilayer graphene wire is over two orders of magnitude longer than that of a Cu wire with the same current density; this current density is over one order of magnitude higher than the current density that can be carried by a Cu wire for the same lifetime.

Spectroscopic Analysis of Graphitization and Grain Orientation of Carbon Films Grown by Photoemission-Assisted Plasma-Enhanced Chemical Vapor Deposition

The degrees of graphitization of carbon films grown by photoemission-assisted plasma-enhanced chemical vapor deposition were evaluated by hard-X-ray photoemission spectroscopy (HAXPES). The films were grown with a CH4/He or CH4/Ar mixture at growth temperatures from 400 to 1000 °C. Low-temperature growth was mainly focused on. The result of HAXPES showed that the films dominantly have sp2 bonding states. The film grown at 400 °C with CH4/Ar had an sp2 content of 84%, which was comparable to those of the films grown at temperatures ≥800 °C. The orientation of the graphitic grains was also examined by soft-X-ray absorption spectroscopy. The degrees of orientation of the films were up to 70%, compared to that of highly oriented pyrolytic graphite. The film grown at 400 °C with CH4/Ar had the degree of graphitization of ~40%, which was comparable to that for the film grown at 800 °C with CH4/He.

Improved thermal conductivity by vertical graphene contact formation for thermal TSV

This paper reports the tailoring thermal conductivity of novel dense vertical and horizontal graphene (DVHG) structures, which we previously discovered. By removing horizontal graphene layers, resulting in forming vertical graphene contacts to the electrode, we not only improved the thermal conductivity by a factor of 10 but also improved the electrical conductivity by a factor of 100. The pyrolytic graphite, grown at a higher temperature than the DVHG, showed a high thermal conductivity of 1426 W/mK by forming vertical graphene contacts. Although the DVHG showed poor thermal properties at this point, we found that the vertical graphene contact formation can be an important technology to realize high thermal conductivity for carbon-based thermal through-silicon-vias (TSV).

Novel implantation process of carbon nanotubes for plugs and vias, and their integration with transferred multilayer graphene wires

We have developed a novel process, the implantation of carbon nanotubes (CNTs) into holes like plugs, vias, and through-silicon vias (TSVs) for the first time. The developed low-temperature process is suitable for back-end-of-line (BEOL) processes of LSI. With this approach, we can use high-quality CNTs grown at high temperature on a different substrate and densify CNTs. Consequently, the implanted CNT plugs had a resistance one order of magnitude lower than the directly grown CNT plugs. In addition, we successfully integrated the implanted CNT plug with transferred multilayer graphene (MLG) wires for all carbon interconnects.

Picoampere Resistive Switching Characteristics Realized with Vertically Contacted Carbon Nanotube Atomic Force Microscope Probe

The resistive switching characteristics of a TiO2/Ti structure have been investigated using a conductive atomic force microscopy (AFM) system with 5-nm-diameter carbon nanotube (CNT) probes. The resistive switching showed bipolar resistive random access memory (ReRAM) behaviors with extremely low switching currents in the order of Picoamperes when voltages were applied. From transmission electron microscopy (TEM) observation, we confirmed that filament-like nanocrystals, having a diameter of about 10 nm, existed in TiO2 films at resistive switching areas after not only set operation but also reset operation. Moreover, photoemission electron microscopy (PEEM) analysis showed that the anatase-type TiO2 structure did not change after set and reset operations. From these results, we suggested that the Picoampere resistive switching occurred at the interface between the TiO2 dielectric and conductive nanocrystal without any structural changes in the TiO2 film and nanocrystal. The resistive switching mechanism we suggested is highly promising to realize extremely low-power-consumption ReRAMs with vertically contacted CNT electrodes.

CNT/graphene technologies for future carbon-based interconnects

With the aim of achieving low-power consumption, high-performance LSIs, our work focuses on the development of low-resistance carbon nanotube (CNT)/graphene interconnect technologies. For vertical interconnects, we report on a previously unseen dense vertical and horizontal graphene (DVHG) structure, which is expected to lead to a low electrical resistance. For horizontal interconnects, we have succeeded in forming multi-layer graphene (MLG) directly on SiO2 by annealing sputtered amorphous carbon. Furthermore, achieving low-resistance contacts between the vertical CNTs and horizontal graphene represents a critical issue for 3D interconnects. We grew vertically aligned CNTs on MLG, and analyzed the contact structure using cross-sectional TEM-EELS measurements. Migration of Ti and Co into the graphene layers was clearly observed. This may contribute to the reduction of contact resistance between CNTs and graphene.

Multi-layer graphene wire grown by annealing sputtered amorphous carbon

We have fabricated multi-layer graphene directly on SiO2 by annealing sputtered amorphous carbon with a Co catalyst without use of complicated transfer processes. Structural analysis revealed that the graphene sheets formed an epitaxial structure, aligned to the Co (111) surface. An MLG wire can sustain a high current density of 107 A/cm2 which is over one order of magnitude higher than that of a Cu wire.

Mechanism of resistivity decrease in networked-nanographite wires for multi-layer graphene interconnects

We investigated the mechanism of resistivity decrease in networked-nanographite wires. The wires had no failure during over 200 hours under a current density of 5E5A/cm2 at about 400°C. However, their resistivity decreased gradually owing to thermal stress. Raman and x-ray photoelectron spectroscopy revealed that the decrease in resistivity can be attributed to an increase in sp2 bonding corresponding to the formation of a graphene sheet. It is important to clarify the mechanism for the decrease in resistivity, not only to improve thermal stability but also to obtain lower resistivity in carbon interconnects.