Chee Lip Gan

Synthesis of hexagonal close-packed gold nanostructures

Solid gold is usually most stable as a face-centred cubic (fcc) structure. To date, no one has synthesized a colloidal form of Au that is exclusively hexagonal close-packed (hcp) and stable under ambient conditions. Here we report the first in situ synthesis of dispersible hcp Au square sheets on graphene oxide sheets, which exhibit an edge length of 200-500 nm and a thickness of ~ 2.4 nm (~ 16 Au atomic layers). Interestingly, the Au square sheet transforms from hcp to a fcc structure on exposure to an electron beam during transmission electron microscopy analysis. In addition, as the square sheet grows thicker (from ~ 2.4 to 6 nm), fcc segments begin to appear. A detailed experimental analysis of these structures shows that for structures with ultrasmall dimensions (for example, <~ 6 nm thickness for the square sheets), the previously unobserved pure hcp structure becomes stable and isolable.

Graphene Oxide‐Templated Synthesis of Ultrathin or Tadpole‐Shaped Au Nanowires with Alternating hcp and fcc Domains

Ultrathin Au nanowires (AuNWs) and tadpole-shaped nanowires are synthesized on graphene oxide (GO) sheet templates. For the first time, 1.6 nm-diameter AuNWs are shown to contain hexagonal close-packed (hcp) crystal domains, and the tadpole-shaped nanowires exhibit alternating sets of hcp and face-centered cubic (fcc) structures, associated with variation in wire thickness.

Effect of Copper TSV Annealing on Via Protrusion for TSV Wafer Fabrication

Three-dimensional (3D) integrated circuit (IC) technologies are receiving increasing attention due to their capability to enhance microchip function and performance. While current efforts are focused on the 3D process development, adequate reliability of copper (Cu) through-silicon vias (TSVs) is essential for commercial high-volume manufacturing. Annealing a silicon device with copper TSVs causes high stresses in the copper and may cause a “pumping” phenomenon in which copper is forced out of the blind TSV to form a protrusion. This is a potential threat to the back-end interconnect structure, particularly for low-κ materials, since it can lead to cracking or delamination. In this work, we studied the phenomenon of Cu protrusion and microstructural changes during thermal annealing of a TSV wafer. The extruded Cu-TSV was observed using scanning electron microscopy (SEM), 3D profilometry, and atomic force microscopy (AFM). The electron backscatter diffraction (EBSD) technique was employed to study the grain orientation of Cu-TSV and evolution of the grain size as a function of annealing temperature. The elastic modulus and yield stress of copper were characterized using nanoindentation. A model for Cu protrusion is proposed to provide insight into the failure mechanism. The results help to solve a key TSV-related manufacturing yield and reliability challenge by enabling high-throughput TSV fabrication for 3D IC integration.

Surface modification-induced phase transformation of hexagonal close-packed gold square sheets

Conventionally, the phase transformation of inorganic nanocrystals is realized under extreme conditions (for example, high temperature or high pressure). Here we report the complete phase transformation of Au square sheets (AuSSs) from hexagonal close-packed (hcp) to face-centered cubic (fcc) structures at ambient conditions via surface ligand exchange, resulting in the formation of (100)f-oriented fcc AuSSs. Importantly, the phase transformation can also be realized through the coating of a thin metal film (for example, Ag) on hcp AuSSs. Depending on the surfactants used during the metal coating process, two transformation pathways are observed, leading to the formation of (100)f-oriented fcc Au@Ag core-shell square sheets and (110)h/(101)f-oriented hcp/fcc mixed Au@Ag nanosheets. Furthermore, monochromated electron energy loss spectroscopy reveals the strong surface plasmon resonance absorption of fcc AuSS and Au@Ag square sheet in the infrared region. Our findings may offer a new route for the crystal-phase and shape-controlled synthesis of inorganic nanocrystals.

Effect of current direction on the lifetime of different levels of Cu dual-damascene metallization

Electromigration in the lower metal (M1) and the upper metal (M2) of Cu dual-damascene interconnections has been studied. The failure times of M2 test structures are significantly longer than those of identical M1 structures. It is proposed that this asymmetry is the result of a difference in the location of void formation and growth, which is believed to be related to the ease of electromigration-induced void nucleation and growth at the Cu/Si3N4 interface. Asymmetric via reliability is therefore an intrinsic characteristic of current Cu interconnect technology.

Photochemically Controlled Synthesis of Anisotropic Au Nanostructures: Platelet-like Au Nanorods and Six-Star Au Nanoparticles

We report the shape-controlled synthesis of anisotropic Au nanostructures through TiO(2)-assisted photochemical reduction of HAuCl(4). By using this method, we have successfully synthesized the platelet-like Au nanorods and six-star Au nanoparticles. Importantly, the platelet Au nanorod exhibits the unique asymmetric five-twinned structure. The colloidal TiO(2) sols were used as both the photocatalyst to initiate the reaction and the stabilizing agent for the produced Au nanostructures. Significantly, in this photochemical process, the tunable irradiation intensity allows us to kinetically control the crystal evolution at various growth stages, leading to the shape difference of ultimate gold nanostructures. Our synthetic method shows a great potential as an alternative or supplement to the other wet chemical approaches for the shape-control of metallic nanostructures.

Stabilization of 4H hexagonal phase in gold nanoribbons

Gold, silver, platinum and palladium typically crystallize with the face-centred cubic structure. Here we report the high-yield solution synthesis of gold nanoribbons in the 4H hexagonal polytype, a previously unreported metastable phase of gold. These gold nanoribbons undergo a phase transition from the original 4H hexagonal to face-centred cubic structure on ligand exchange under ambient conditions. Using monochromated electron energy-loss spectroscopy, the strong infrared plasmon absorption of single 4H gold nanoribbons is observed. Furthermore, the 4H hexagonal phases of silver, palladium and platinum can be readily stabilized through direct epitaxial growth of these metals on the 4H gold nanoribbon surface. Our findings may open up new strategies for the crystal phase-controlled synthesis of advanced noble metal nanomaterials.

Synthesis of Ultrathin Face-Centered-Cubic Au@Pt and Au@Pd Core–Shell Nanoplates from Hexagonal-Close-Packed Au Square Sheets†

The synthesis of ultrathin face-centered-cubic (fcc) Au@Pt rhombic nanoplates is reported through the epitaxial growth of Pt on hexagonal-close-packed (hcp) Au square sheets (AuSSs). The Pt-layer growth results in a hcp-to-fcc phase transformation of the AuSSs under ambient conditions. Interestingly, the obtained fcc Au@Pt rhombic nanoplates demonstrate a unique (101)f orientation with the same atomic arrangement extending from the Au core to the Pt shell. Importantly, this method can be extended to the epitaxial growth of Pd on hcp AuSSs, resulting in the unprecedented formation of fcc Au@Pd rhombic nanoplates with (101)f orientation. Additionally, a small amount of fcc (100)f -oriented Au@Pt and Au@Pd square nanoplates are obtained with the Au@Pt and Au@Pd rhombic nanoplates, respectively. We believe that these findings will shed new light on the synthesis of novel noble bimetallic nanostructures.

Forest of gold nanowires: a new type of nanocrystal growth.

We report a nanowire growth that is highly unconventional: (1) nanowires can grow from substrate-bound seeds but cannot from colloidal seeds under otherwise the same conditions; (2) the nanowires grow from only one side of the seeds, with their diameter independent of the size of the seeds; and (3) vertically aligned ultrathin nanowires are obtained on substrates, using aqueous solution and ambient conditions. With carefully designed experiments, we propose and test a new mechanism that can explain these unusual phenonmena. It turns out that the strong binding of ligands in this system forces selective deposition of Au at the ligand-deficient interface between Au seeds and oxide substrates. This means of promoting anisotropic growth of nanocrystals into nanowires is previously unknown in the literature. We are able to pinpoint the site of active growth and explain the control of nanowire width. The sustained growth at the active site and the inhibited growth at its parameter push the nanocrystals upward into wires; their diameter is dependent on the dynamic competition of the two processes. The site-specific growth from substrate-anchored seeds provides a rare means to create substrate-nanowire hierarchical structures in aqueous solution under ambient conditions. Rendering a surface conductive, particularly one with complex surface morphology, is now made easy.

Experimental characterization and modeling of the reliability of three-terminal dual-damascene Cu interconnect trees

Electromigration tests on Cu dual-damascene interconnect tree structures consisting of straight via-to-via (or contact-to-contact) lines with an extra via in the middle of the line have been carried out. Like Al-based interconnects, the reliability of a segment in a Cu-based interconnect tree strongly depends on the stress conditions of connected segments. The analytic model based on a nodal analysis developed for Al trees gives a conservative estimate of the lifetime of Cu-based interconnect trees. However, there are important differences in the results obtained under similar test conditions for Al-based and Cu-based interconnect trees. These differences are thought to be associated with variations in the architectural schemes of the two metallization systems. The absence of a conducting electromigration-resistant overlayer in Cu technology and the low critical stress for void nucleation at the Cu/interlevel diffusion barrier interface (e.g., the Cu/Si3N4 interface) leads to different failure modes betwe...