Yusuke Ominami

Structural and Electrical Characterization of Carbon Nanofibers for Interconnect Via Applications

We present temperature-dependent electrical characteristics of vertically aligned carbon nanofiber (CNF) arrays for on-chip interconnect applications. The study consists of three parts. First, the electron transport mechanisms in these structures are investigated using I-V measurements over a broad temperature range (4.4 K to 350 K). The measured resistivity in CNF arrays is modeled based on known graphite two-dimensional hopping electron conduction mechanism. The model is used because of the disordered graphite structure observed during high-resolution scanning transmission electron microscopy (STEM) of the CNF and CNF-metal interface. Second, electrical reliability measurements are performed at different temperatures to demonstrate the robust nature of CNFs for interconnect applications. Finally, some guidance in catalyst material selection is presented to improve the nanostructure of CNFs, making the morphology similar to multiwall nanotubes.

Current-induced breakdown of carbon nanofibers

We present a study of high-field transport in carbon nanofibers (CNFs) and breakdown phenomena due to current stress. In situ measurements with scanning transmission electron microscopy reveal that the failure mode of CNFs is strongly related to the morphology of graphite layers comprising CNFs. Comparison with carbon nanotube (CNT) breakdown is made, demonstrating that the current capacity of CNFs is described by a similar model as that of CNTs with a modification of the current capacity of each graphitic layer. The maximum current density is correlated with resistivity, leading to the conclusion that lower resistivity results in higher current capacity in CNFs.

Structural characteristics of carbon nanofibers for on-chip interconnect applications

In this letter, we compare the structures of plasma-enhanced chemical vapor deposition of Ni-catalyzed and Pd-catalyzed carbon nanofibers (CNFs) synthesized for on-chip interconnect applications with scanning transmission electron microscopy (STEM). The Ni-catalyzed CNF has a conventional fiberlike structure and many graphitic layers that are almost parallel to the substrate at the CNF base. In contrast, the Pd-catalyzed CNF has a multiwall nanotubelike structure on the sidewall spanning the entire CNF. The microstructure observed in the Pd-catalyzed fibers at the CNF-metal interface has the potential to lower contact resistance significantly, as our electrical measurements using current-sensing atomic force microscopy indicate. A structural model is presented based on STEM image analysis.

Interface characteristics of vertically aligned carbon nanofibers for interconnect applications

The authors characterize the detailed interface structure of Ni-catalyzed vertically aligned carbon nanofibers (CNFs) prepared by plasma-enhanced chemical vapor deposition for interconnect applications. Stacked graphitic layers and cup-shape structures of CNFs around the interface region have been observed using high-resolution scanning transmission electron microscopy. The interaction between the Ni catalyst and Ti layer dramatically affects the CNF structure during initial growth. The effect of interface nanostructures on contact resistance is also discussed.

Bright contrast imaging of carbon nanofiber-substrate interface

We present the contrast mechanisms of scanning electron microscopy (SEM) for visualizing the interface between carbon nanofibers (CNFs) and the underlying substrate. SEM imaging with electron beam energies higher than a certain threshold provides different image contrasts depending on whether CNFs are in contact with the substrate or suspended above the substrate. CNFs with diameters ranging from 25to250nm are examined with various electron beam energies. It is found that the threshold energy corresponds to the energy required to penetrate the CNF and its dependence on CNF diameter can be understood using the theory of electron range. This knowledge will be quite useful for interface imaging of all nanostructure devices.

Secondary electron imaging of embedded defects in carbon nanofiber via interconnects

Carbon nanofiber (CNF) via interconnect test structures are fabricated with the bottom-up process proposed by Li et al. [Appl. Phys. Lett. 82, 2491 (2003)] for next-generation integrated circuit technology. Critical defects in the interconnect structure are examined using scanning electron microscopy. It is shown that secondary electron signal with optimized incident beam energy is useful for detecting embedded defects, including unexposed CNF plugs and voids in the dielectric layer. The defect imaging mechanisms are elucidated based on beam-induced charging of the specimen surface.

Electrical characterization of carbon nanofibers for on-chip interconnect applications

Recent process and reliability issues in state-of-the-art copper on-chip interconnects systems have spurred an interest in finding new materials to augment, or in some cases, replace copper as an interconnect material for future technology nodes. Carbon nanofiber (CNF) is one such candidate material for on-chip interconnect integration. Three main aspects of our research are discussed, controllable CNF synthesis, CNF-metal interface characterization, and development of new test structures and methods. We present fundamental electrical measurements of vertically-aligned CNF arrays for use as vias and on-chip interconnect wiring. CNF arrays are synthesized using plasma enhanced chemical vapor deposition (PECVD). The synthesis process is discussed as it relates to device architecture and electrical characterization.

Bright-field transmission imaging of carbon nanofibers on bulk substrate using conventional scanning electron microscopy

The authors present scanning transmission electron microscopy (STEM) of carbon nanofibers (CNFs) on a bulk substrate using conventional scanning electron microscopy (SEM) without specimen thinning. By utilizing the electron beam tilted >85° from the substrate normal, bright-field STEM contrast is obtained for the CNFs on substrate with conventional SEM. Analysis of the observed contrast using Monte Carlo simulation shows that the weakly scattered electrons transmitted from the CNF are selectively enhanced by the largely tilted substrate and result in the observed STEM contrast. This mechanism provides a useful STEM imaging technique to investigate the internal structure of materials on bulk substrates without destructive specimen thinning.

Growth of carbon nanofibers on nanoscale catalyst strips fabricated with a focused ion beam

We studied the growth mode of vertically aligned carbon nanofibers (CNFs) on Ni catalyst strips fabricated using a focused ion beam (FIB). We found that the CNF growth on Ni catalysts was strongly affected by the geometry of the microfabricated Ni catalyst strips. Selective growth of vertically aligned CNFs requires ion milling from the outside edge of the sample so that the milled materials are effectively evacuated. The CNF diameter and density on the strip depends on its width. Possible mechanisms to control CNF growth using microfabricated catalysts are analyzed with a liquid model using surface free energies.

Image Formation Mechanisms in Scanning Electron Microscopy of Carbon Nanofibers on Substrate

Nanostructures fabricated on thick substrates (typically a silicon wafer) are building blocks for high-performance electronic devices. While detailed structural analysis using high-resolution electron microscopy usually requires a thinning process to obtain an electron transparent specimen, a non-destructive approach is also required to analyze the structure. A recent study showed that heat dissipation into the substrate from a nanotube device governs its electronic transport characteristics [1]. In such a system, structural analysis that can be performed without any sample modification is favorable since imaging can be performed before, after, and even during the electrical operation of devices to investigate structural change. In this paper, we present two kinds of imaging techniques using conventional scanning electron microscopy (SEM) developed for the characterization of carbon nanofiber (CNF) devices fabricated on a thick Si substrate without any sample modification.