Phase field modeling of electrochemistry I: Equilibrium

A diffuse interface (phase field) model for an electrochemical system is developed. We describe the minimal set of components needed to model an electrochemical interface and present a variational derivation of the governing equations. With a simple set of assumptions: mass and volume constraints, Poisson's equation, ideal solution thermodynamics in the bulk, and a simple description of the competing energies in the interface, the model captures the charge separation associated with the equilibrium double layer at the electrochemical interface. The decay of the electrostatic potential in the electrolyte agrees with the classical Gouy-Chapman and Debye-H\"uckel theories. We calculate the surface energy, surface charge, and differential capacitance as functions of potential and find qualitative agreement between the model and existing theories and experiments. In particular, the differential capacitance curves exhibit complex shapes with multiple extrema, as exhibited in many electrochemical systems.Comment: v3: To be published in Phys. Rev. E v2: Added link to cond-mat/0308179 in References 13 pages, 6 figures in 15 files, REVTeX 4, SIUnits.sty. Precedes cond-mat/030817

The Effect of CoPt Crystallinity and Grain Texturing on Properties of Exchange-Coupled Fe/CoPt Systems

The effect of the crystallinity and the grain texturing of CoPt hard layers on exchange coupled Fe/CoPt soft/hard magnetic systems was studied using gradient thickness multilayer thin films. We have studied the hard layer structures by transmission electron microscopy and x-ray diffraction, and characterized the exchange coupling interaction through magnetization loops obtained by the magneto-optical Kerr effect measurement. We found that exchange coupling strongly depends on the crystalline characteristics of the CoPt hard layer. There is correlation between the mixture of different grain orientations of the CoPt hard layer and coupling efficiency. In particular, interlayer coupling is enhanced when there is only one orientation, namely, the L10 CoPt structure with its c-axis inclined at 45° with respect to the substrate plane. An increased degree of mixture of the latter with the in-plane-c-axis L10 CoPt structure is detrimental to obtaining one-phase-like magnetization loops. The present work points to the importance of controlling the crystalline properties of the hard layer in order to enhance the maximum energy product (BH)max in hard/soft nanocomposite magnets

Mechanical/Structural Properties of the Key Thin Film Materials Ag, Cu, & Ni for Electronics Applications

Thin metallic films and coatings of Ag, Cu, and Ni for interconnect and contact in next generation nanoelectronic are expected to have high conductivity, high temperature stability, good ohmic contacts to both p- and n- type semiconductors, and also superb mechanical properties. These metallic thin film are used in a variety of electronic applications due to their low cost of production, non-toxicity and excellent electrical conductivity. However, their applications in nanoelectronics are still limited due to diffusion into the commonly used silicon substrates and the tendency of developing oxides under atmospheric conditions. Thin oxide layers form immediately on metallic films surfaces’ upon contact with air even at room temperature. Thin metallic oxide films `tend to exhibit brittle behavior with a sharp jump in the mechanical properties such as hardness and modulus and results in high resistivity. Although the formation of oxides is still perceived as the primary impediment in using thin metallic films in nanoelectronics, thin metallic films exhibit promising applications in large-area electronics such as memories, MEMS, and microprocerssors. Metallic films of Ag, Cu, and Ni each of 150, 300, 600 and 1000 nm thick were deposited on Si using E-beam evaporation and deposition technique. A shutter was used to successively cover increments of 1² of the wafer at a time, starting with the wafer fully uncovered. A thin titanium layer of 10 nm is first deposited over the entire Si wafers followed by 150, 300, 600, and 1000 nm of the 2² target. Vacuum conditions at the start were ~1x10-7 torr and reached as high as 2x10-6 torr during the copper deposition due to outgassing. Similar vacuum conditions were maintained for the other films. The Ag, Cu, and Ni films thickness were verified using field emission scanning electron microscopy. A sample cross sectional FE-SEM of the 150 nm thick Ag film is shown in Figure1. Nanocrstalline grain structure formation is observed to dominate the crystal film growth. The nanomechanical properties were measured using nanoindentation to determine the modulus and hardness of the Ag, Cu, and Ni films. Indentations of 1/3 and 2/3 of the film thickness in addition to 2 µm deep indents were performed on each film to study the film properties irrespective of the Si substrate influence as well as the substrate influence on the mechanical properties. Figures 2 and 3 depict hardness versus normalized depth of indentation to film thickness for Ag and Cu. The hardness increases as the film thickness decreases and remains nearly flat (no size effects) for the same film with depth of indentation except for the very thin films as they suffer what is known as tapping effect [1]. References: [1] David E. Stegal, Md. Abdullah Mamun, Bryan Crawford, and Abdelmageed Elmustafa, J. Mater. Res., Vol. 27, No. 12, Jun 28, 2012. Figure 1 <jats:p /

Mechanical/Structural Properties of the Key Thin Film Materials Ag, Cu, &amp; Ni for Electronics Applications

Metallic films of Ag, Cu, and Ni each of 150, 300, 600 and 1000 nm thick were deposited on Si using E-beam evaporation deposition technique. A shutter was used to successively cover increments of 1² of the wafer at a time, starting with the wafer fully uncovered. A thin titanium layer of 10 nm is first deposited over the entire Si wafers followed by 150, 300, 600, and 1000 nm of the 2² target material. Vacuum conditions at the start were ~1x10-7 torr and reached as high as 2x10-6 torr during the copper deposition due to outgassing. The structural and surface properties were explored using field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). The nanomechanical properties were measured using nanoindentation to determine the modulus and hardness of the Ag, Cu, and Ni films. Nanocrystalline grain structure formation is observed to dominate the crystal film growth. The hardness increases as the film thickness decreases and remains nearly flat for the same film with depth except for the very thin films as they suffer what is known as tapping effect.</jats:p

Robust Bottom-Up Gold Filling of Deep Trenches and Gratings

This work extends an extreme variant of superconformal Au electrodeposition to deeper device architectures while exploring factors that constrain its function and the robustness of void-free processing. The unconventional bottom-up process is used to fill diffraction gratings with trenches 94 μm deep and 305 μm deep, with aspect ratios (height/width) of just below 20 and 15, respectively, in near-neutral 0.16 mol∙l−1 Na3Au(SO3)2 + 0.64 mol∙l−1 Na2SO3 electrolyte containing 50 μmol∙l−1 Bi3+. Although the aspect ratios are modest compared to previously demonstrated void-free filling beyond AR = 60, the deepest trenches filled exceed those in previous work by 100 μm—a nearly 50% increase in depth. Processes that substantially accelerate the start of bottom-up deposition demonstrate a linkage between transport and void-free filling. Final profiles are highly uniform across 65 mm square gratings because of self-passivation inherent in the process. Electron microscopy and electron backscatter diffraction confirm the fully dense Au and void-free filling suggested by the electrochemical measurements. X-ray transmission “fringe visibility” averages more than 80% at 50 kV X-ray tube voltage across the deeper gratings and 70% at 40 kV across the shallower gratings, also consistent with uniformly dense, void-free fill across the gratings.</jats:p

Biaxial Zero Creep Measurements of Interface Energies in Ni/Ag Multilayers

ABSTRACTBiaxial zero creep experiments were performed on Ni/Ag multilayer films on sapphire substrates. The equilibrium curvature was measured using a scanning laser and position sensitive photodetector. The experiments were designed to measure the free energy of Ni/Ag interfaces and to investigate their effect on the structural stability of multilayered materials. For the Ni/Ag multilayers studied, significant plastic straining occurs at temperatures above 400°C, enabling the growth stresses and thermal stresses in the multilayers to decay to zero. After a long time at elevated temperatures, the equilibrium curvature is reached for the film/substrate couple. This curvature is determined by the number and the energy of the Ni/Ag interfaces. Using this equilibrium technique, a free energy of 0.44 ± 0.03 N/m was measured for Ni/Ag interfaces at an equilibrium temperature of 550°C.</jats:p

Investigation of Hydrogen Storage Using Combinatorial Thin Films and IR Imaging

AbstractThree 100 nm-thick Mgx(TM)1-x (TM = Ni and Ti) composition spread thin films having compositional variation 0.4&lt;x&lt;0.95 and capped with a 5 nm-thick Pd layer were deposited in combinatorial electron-beam (e-beam) deposition chamber. Crystallinity of the films was characterized by scanning x-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM). Hydrogen absorption and desorption of the films were monitored with an infrared (IR) camera that could image a full area of the films. The observed changes in IR intensity due to hydrogen absorption/desorption demonstrated sensitivity of the method to the differences in compostion, microstructure and type of TM.</jats:p

Energy-Filtered High-Resolution Electron Microscopy for Quantitative Solid State Structure Determination

Energy-filtered (or selected) electron imaging is one of the future directions of highresolution electron microscopy (HREM). In this paper, the characteristics and applications of energy-selected electron imaging at high-resolution for structure determinations are illustrated. It is shown that image contrast can be dramatically improved with the use of an energy filter. High-resolution chemical-sensitive imaging using ionization-loss electrons is demonstrated in studies of Ni/Ti and Al/Ti multilayer thin films. It is also shown that the spatial resolution of energy-selected ionization edge electron images is dominated by the signal-to-noise ratio. Experimental parameters which may be selected to improve the signal-to-noise ratio are discussed