Antonia Antoniou

Controlled nanoporous Pt morphologies by varying deposition parameters

Typically, dealloying of an alloy can result in an open cell nanoporous structure of the least electrochemically active element. Here, we show that a wider range of nanoporous structures is possible by controlling the composition and deposition parameters of the as-synthesized alloy as a way to provide sites for preferential etching. We demonstrate this by synthesizing nanoporous platinum (np-Pt) through electrochemical dealloying in aqueous HF from co-sputtered Pt{sub x}Si{sub 1-x} amorphous films. For increased Pt fraction of the amorphous alloy, silicon dissolution is favored along pre-existing features of the amorphous film (e.g. column boundaries or surface asperities). The resulting np-Pt depends on the manner in which silicon is preferentially removed. In addition to the expected isotropic open cell structure, columnar and Voronoi (radial) np-Pt are observed. A processing-structure map is developed to correlate np-Pt morphology to the initial composition and thickness of the amorphous Pt{sub x}Si{sub 1-x} film and the negative substrate bias used in magnetron sputtering.

Electrochemically dealloyed platinum with hierarchical pore structure as highly active catalytic coating

Micro structured reactors are attractive candidates for further process intensification in heterogeneous catalysis. However, they require catalytic coatings with significantly improved space-time yields compared to traditional supported catalysts. We report the facile synthesis of homogeneous nanocrystalline Pt coatings with hierarchical pore structure by electrochemical dealloying of amorphous sputter-deposited platinum silicide layers. Thickness, porosity and surface composition of the catalysts can be controlled by the dealloying procedure. XPS analysis indicates that the catalyst surface is primarily composed of metallic Pt. Catalytic tests in gas-phase hydrogenation of butadiene reveal the typical activity, selectivity and activation energy of nanocrystalline platinum. However, space time yields are about 13 to 200 times higher than values reported for Pt-based catalysts in literature. The highly open metallic pore structure prevents heat and mass transport limitations allowing for very fast reactions and reasonable stability at elevated temperatures.

Synthesis and mechanical behavior of nanoporous nanotwinned copper

We synthesize nanoporous copper (NP Cu) through electrochemical dealloying of amorphous Cu0.41Si0.59 under compressive residual stress. Transmission Electron Microscopy reveals that struts are nanocrystalline with grain size equal to the strut thickness. Moreover, a significant population of twins with spacing ∼7 nm is present within each imaged grain. The hardness of this nanocrystalline, nanotwinned NP Cu is approximately one order of magnitude greater than reports on NP Cu in the literature. The yield strength of individual struts inferred through dimensional analysis is approximately an order of magnitude greater than bulk copper and compares well with other nanostructured copper systems.

Fluid and Resistive Tethered Lipid Membranes on Nanoporous Substrates.

Cell membranes perform important biological roles including compartmentalization, signaling, and transport of nutrients. Supported lipid membranes mimic the behavior of cell membranes and are an important model tool for studying membrane properties in a controlled laboratory environment. Lipid membranes may be supported on solid substrates; however, protein and lipid interactions with the substrate typically result in their denaturation. In this report, we demonstrate the formation of intact lipid membranes tethered on nanoporous metal thin films obtained via a dealloying process. Uniform lipid membranes were formed when the surface defect density of the nanoporous metal film was significantly reduced through a two-step dealloying process reported here. We show that the tethered lipid membranes on nanoporous metal substrates maintain both fluidity and electrical resistivity, which are key attributes to naturally occurring lipid membranes. The lipid assemblies supported on nanoporous metals provide a new platform for investigating lipid membrane properties, and potentially membrane proteins, for numerous applications including next generation biosensor platforms, targeted drug-delivery, and energy harvesting devices.

Mechanical behavior of mesoporous titania thin films

Nanoindentation is used to assess mechanical properties of ordered mesoporous titania thin films with pore sizes in the range of 8–16 nm. Estimates of strut properties are obtained under the standard scaling assumptions widely used in porous media. The inferred hardness and fracture toughness of individual struts are found to correspond to anatase titania, indicating the absence of obvious size effects in the nanostructured ceramic. This is in marked contrast to nanoporous metals, where size effects often play a crucial role in determining material properties at similar length scales.

Microcantilever bend testing and finite element simulations of HIP-ed interface-free bulk Al and Al–Al HIP bonded interfaces

Abstract We report on the strength of Al–Al interfaces and the effects of chemical segregation and interfacial void formation on bond strength using microcantilever bend testing. Interfaces are synthesised via hot isostatic pressing. Microcantilevers of several nominal dimensions were fabricated via focused ion beam and deformed in a nanoindenter. We find increased cantilever strength as a function of decreasing sample size, with a linear dependence of the yield strength on the inverse square root of the length scale characteristic to the cantilever cross-section. The presence of pores and chemical segregation decreases the yield strength of the material by 17% and the accommodated strain energy by 10–15% for strain values in the 6–12% range.

Low-temperature, organics-free sintering of nanoporous copper for reliable, high-temperature and high-power die-attach interconnections

A novel die-attach joining technique based on low-temperature film sintering of nanoporous Cu is demonstrated. Nanoporous Cu films are proposed as a low-cost replacement of nano-sintering pastes with the following benefits: (i) synthesis by electrochemical dealloying, compatible with standard lithography processes; (ii) no organic content to minimize risks of voiding and corrosion; and (iii) controllable physical properties post sintering through tailorable initial nanostructure and morphology. As a first proof-of-concept, thin films of nanoporous Cu with 25–50nm feature size and ∼60% relative density were synthesized by dealloying of Cu-Si films. The nanoporous Cu films were then sintered on bulk Cu metallizations at temperatures of 200–250°C for 5–15min with an applied pressure of 6–9MPa, in reducing atmosphere. A maximum shear strength of 4.2kgf was achieved and analysis of the fracture profiles showed failure through the sintered joints, confirming strong metallurgical bonding to bulk Cu. Cross-sections of joints formed at 200°C and 250°C −15min observed by SEM showed relative density as high as 85%, achieved for the first time with sintered copper.

Enabling Chip-to-Substrate All-Cu Interconnections: Design of Engineered Bonding Interfaces for Improved Manufacturability and Low-Temperature Bonding

This paper presents the design and implementation of engineered nanoscale bonding interfaces as an effective strategy to improve manufacturability of Cu-Cu bonding to the level where it can, for the first time, be applied to chip-to-substrate (C2S) assembly. All-Cu interconnections are highly sought after to meet the escalating electrical, thermal, and reliability requirements of a wide range of emerging digital and analog systems. Such applications require low-cost processes with bonding temperatures and pressures ideally below 200°C and 20MPa, respectively, far from existing solutions established in wafer-level packaging. GT-PRC and its industry partners address this technology gap through innovative designs of bonding interfaces, introducing: 1) novel ultra-thin surface finish metallurgies applied on Cu bumps and pads to prevent oxidation and achieve low-temperature assembly, 2) low-cost fly-cut planarization technique to lower bonding pressures, and 3) low-modulus nanocopper foam caps to provide tolerance to non-coplanarities, and further reduce bonding temperatures and pressures.