The evolution of copper-based interconnects requires the realization of an ultrathin diffusion barrier layer between the Cu interconnect and insulating layers. The present work reports the use of atomically thin layer graphene as a diffusion barrier to Cu metallization. The diffusion barrier performance is investigated by varying the grain size and thickness of the graphene layer; single-layer graphene of average grain size 2 ± 1 μm (denoted small-grain SLG), single-layer graphene of average grain size 10 ± 2 μm (denoted large-grain SLG), and multi-layer graphene (MLG) of thickness 5-10 nm. The thermal stability of these barriers is investigated after annealing Cu/small-grain SLG/Si, Cu/large-grain SLG/Si, and Cu/MLG/Si stacks at different temperatures ranging from 500 to 900 °C. X-ray diffraction, transmission electron microscopy, and time-of-flight secondary ion mass spectroscopy analyses confirm that the small-grain SLG barrier is stable after annealing up to 700 °C and that the large-grain SLG and MLG barriers are stable after annealing at 900 °C for 30 min under a mixed Ar and H2 gas atmosphere. The time-dependent dielectric breakdown (TDDB) test is used to evaluate graphene as a Cu diffusion barrier under real device operating conditions, revealing that both large-grain SLG and MLG have excellent barrier performance, while small-grain SLG fails quickly. Notably, the large-grain SLG acts as a better diffusion barrier than the thicker MLG in the TDDB test, indicating that the grain boundary density of a graphene diffusion barrier is more important than its thickness. The near-zero-thickness SLG serves as a promising Cu diffusion barrier for advanced metallization.

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This study empirically examines the relationship between the role of R&D project leaders and their team performance using data from 87 project teams in 6 R&D organizations in Korea. The results reveal that: (1) R&D project leaders played five different roles in performing their jobs – strategic planner, team builder, gatekeeper, technical expert, and champion; (2) All but the champion role of a leader is positively related with project team performance; (3) However, this relationship between the role of leader and project team performance varies according to the characteristics of R&D project teams and their tasks. Specifically, it becomes important for a leader to focus less on the team building role as the team gets older. The team building role of a leader, however, is more important for higher performance of relatively certain R&D projects, while for uncertain R&D projects, the strategic planning role appears to be more crucial. Based on these results, this study discusses several managerial and theoretical issues related to the role of a leader in R&D project teams.

Abstract-In this paper, we investigate a radio frequency (RF) lens-embedded massive multiple-input multiple-output (MIMO) system and evaluate the system performance of limited feedback by utilizing a technique for generating a suitable codebook for the system. We fabricate an RF lens that operates on a 77 GHz (mmWave) band. Experimental results show a proper value of amplitude gain and an appropriate focusing property. In addition, using a simple numerical technique-beam propagation method (BPM)-we estimate the power profile of the RF lens and verify its accordance with experimental results. We also design a codebook-multi-variance codebook quantization (MVCQ)-for limited feedback by considering the characteristics of the RF lens antenna for massive MIMO systems. Numerical results confirm that the proposed system shows significant performance enhancement over a conventional massive MIMO system without an RF lens.Index Terms-Massive MIMO, RF lens antenna, beam propagation method, limited channel feedback, multi-variance codebook quantization method.

Ultra-thin two-dimensional semiconducting crystals in their monolayer and few-layer forms show promising aspects in nanoelectronic applications. However, the ultra-thin nature of two-dimensional crystals inevitably results in high contact resistance from limited channel/ contact volume as well as device-to-device variability, which seriously limit reliable applications using two-dimensional semiconductors. Here, we incorporate rather thick twodimensional layered semiconducting crystals for reliable vertical diodes showing excellent Ohmic and Schottky contacts. Using the vertical transport of WSe 2 , we demonstrate devices which are functional at various frequency ranges from megahertz AM demodulation of audio signals, to gigahertz rectification for fifth-generation wireless electronics, to ultraviolet-visible photodetection. The WSe 2 exhibits an excellent Ohmic contact to bottom platinum electrode with record-low contact resistance (~50 Ω) and an exemplary Schottky junction to top transparent conducting oxide electrode. Our semitransparent vertical WSe 2 Schottky diodes could be a key component of future high frequency electronics in the era of fifth-generation wireless communication.

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The fabrication of well-ordered metal nanoparticle structures onto a desired substrate can be effectively applied to several applications. In this work, well-ordered Ag nanoparticle line arrays were printed on the desired substrate without the use of glue materials. The success of the method relies on the assembly of Ag nanoparticles on the anisotropic buckling templates and a special transfer process where a small amount of water rather than glue materials is employed. The anisotropic buckling templates can be made to have various wavelengths by changing the degree of prestrain in the fabrication step. Ag nanoparticles assembled in the trough of the templates via dip coating were successfully transferred to a flat substrate which has hydrophilic surface due to capillary forces of water. The widths of the fabricated Ag nanoparticle line arrays were modulated according to the wavelengths of the templates. As a potential application, the Ag nanoparticle line arrays were used as SERS substrates for various probing molecules, and an excellent surface-enhanced Raman spectroscopy (SERS) performance was achieved with a detection limit of 10(-12) M for Rhodamine 6G.

Abstract-In this article, we present a real-time threedimensional (3D) hybrid beamforming for fifth generation (5G) wireless networks. One of the key concepts in 5G cellular systems is the small cell network, which settles the high mobile traffic demand and provides uniform user-experienced data rates. The overall capacity of the small cell network can be enhanced with the enabling technology of 3D hybrid beamforming. This study validates the feasibility of the 3D hybrid beamforming, mostly for link-level performances, through the implementation of a realtime testbed using a software-defined radio (SDR) platform and fabricated antenna array. Based on the measured data, we also investigate system-level performances to verify the gain of the proposed smart small cell system over long term evolution (LTE) systems by performing system-level simulations based on a 3D ray-tracing tool.Index Terms-Small cell, three-dimensional (3D) hybrid beamforming, fifth generation (5G) communications.