Multilayer interconnections of integrated circuits are composed of Cu, an SiO₂-based dielectric, with liner/barrier layers between them. With continuous device scaling, line resistivity increases rapidly because of the presence of the liner/barrier. As a possible solution, we developed a self-forming barrier layer using a Cu-Mn alloy. This paper explains the underlying physical concepts to find a proper alloying element for the self-forming barrier, its chemical composition, and the formation mechanisms. Furthermore, the properties and reliability of dual-damascene lines at a 90 nm technology node are explained in comparison with the lines made with conventional materials.

Atom probe tomography (APT) is a method for obtaining three-dimensional (3D) distributions of elements in materials with nearly atomic-scale resolution. While APT has attracted considerable attention as a promising 3D analysis for various materials and their device structures owing to upgrading the APT system in terms of hardware and software, successful data acquisition relies substantially on a desired preparation of needle-shaped specimens. In this article, technical advances for near-surface analysis on textured (non-flat) samples using a site-specific lift-out method processed with a focused ion beam are provided. As an application of research with APT, the author reviews a trial of the quantification of hydrogen in hydrogenated amorphous Si (a-Si:H) in high-performance Si solar cells and 3D mapping of nanocrystallites embedded in a-Si:H via hydrogen depletion.

We introduce our recent research on two-dimensional materials based on our new concept of “2.5-dimensional materials science”. In particular, our work on the controlled CVD growth of bilayer graphene and the successive intercalation of metal chloride molecules, multilayer hBN growth and application to the insulating substrate of other two-dimensional materials, and molecular doping in monolayer WSe₂ for p- and n-type semiconductor device applications, are reviewed. The concept of 2.5-dimensional materials will open a new research field enriched with fascinating properties and promising applications.

In recent years, room-temperature or low-temperature bonding technologies have attracted considerable attention and have become increasingly important for realizing high-performance multifunctional semiconductor devices featuring small sizes, low power consumption, high thermal dissipation, and high output power, among other properties. Sensors and microelectromechanical systems (MEMS) are typically made of Si and various non-Si materials, including glass, polymers, and piezoelectric materials, and require wafer-level hermetic packaging with small cavity volumes. Therefore, room-temperature or low-temperature bonding technologies with advanced features, such as low thermal damage and low residual stress due to the mismatch of the thermal expansion coefficients of dissimilar materials, are rapidly becoming popular for the fabrication of sensors and MEMS. This study focuses on surface activated bonding (SAB) using Au thin films for room-temperature bonding and introduces some recent research topics on hermetic and vacuum sealing of sensors and MEMS. It is expected that such technology can be used to fabricate gas cells for miniaturized atomic clocks.

Fundamental knowledge of point defects is essential for developing epitaxial growth, device processes and device design and characterization. However, the knowledge with regard to GaN is quite limited compared to Si and GaAs. The authors have carried out an extensive investigation of point defects in GaN by using deep-level transient spectroscopy (DLTS). In this paper, the origin of the E3 trap, the development of quantitative measurement techniques for carbon-related hole traps in GaN by using sub-bandgap photoexcitation and studies of nitrogen-displacement-related point defects (nitrogen vacancy and interstitial) intentionally formed by electron beam irradiation are reviewed.

The world is moving towards a zero-CO₂ emission society. The key to this is to make renewable energy sources the main source of power. In parallel with the development of energy storage devices and hydrogen transport and storage technologies, it is essential to develop and expand systems for the efficient transport of electricity from these large-scale renewable energy sources. Since renewable energy sources are compatible with DC power transmission, a “next generation power grid” has been proposed, in which the existing AC grid is connected to a DC grid via voltage-sourced converters at multiple points. This article introduces the features of the AC grid, the historical background to its formation, and the status of the DC grid, which is currently under development.