Megan Frary

Simulation of Plasticity in Nanocrystalline Silicon

M. J. Demkowicz — Thu, 23 Apr 2009 18:14:58 PDT

Molecular dynamics investigation of plasticity in a model nanocrystalline silicon system demonstrates that inelastic deformation localizes in intergranular regions. The carriers of plasticity in these regions are atomic environments that can be described as high-density liquid-like amorphous silicon. During fully developed flow, plasticity is confined to system-spanning intergranular zones of easy flow. As an active flow zone rotates out of the plane of maximum resolved shear stress during deformation to large strain, new zones of easy flow are formed. Compatibility of the microstructure is accommodated by processes such as grain rotation and formation of new grains. Nano-scale voids or cracks may form if there emerge stress concentrations that cannot be relaxed by a mechanism that simultaneously preserves microstructural compatibility.

Determination of Three-Dimensional Grain Boundary Connectivity from Two-Dimensional Microstructures

Megan Frary — Thu, 23 Apr 2009 18:14:58 PDT

The connectivity of so-called "special" and "general" grain boundaries at a quadruple node is known to be nonrandom as a result of crystallographic constraints. Although a quadruple node is a three-dimensional feature, there exist two-dimensional features which are topologically identical. Therefore, the distribution of these two-dimensional features may be used to determine the three-dimensional connectivity. Computer simulations of a three-dimensional microstructure which is virtually serial sectioned are used to validate the proposed approach.

Correlations Beyond the Nearest-Neighbor Level in Grain Boundary Networks

C. A. Schuh — Thu, 23 Apr 2009 18:14:57 PDT

Correlations among 'special' and 'general' grain boundaries are studied on two-dimensional networks, by examining the configurational entropy of boundary structures as well as percolation thresholds. Consideration of crystallographic constraints at various length scales reveals that higher-order constraints play a role in boundary connectivity and network structure. Implications for grain boundary engineering are discussed and directions for future work highlighted.

Correlation-Space Description of the Percolation Transition in Composite Microstructures

Megan E. Frary — Tue, 14 Apr 2009 09:55:15 PDT

We explore the percolation threshold shift as short-range correlations are introduced and systematically varied in binary composites. Two complementary representations of the correlations are developed in terms of the distribution of phase bonds or, alternatively, using a set of appropriate short-range order parameters. In either case, systematic exploration of the correlation space reveals a boundary that separates percolating from nonpercolating structures and permits empirical equations that identify the location of the threshold for systems of arbitrary short-range correlation states. Two- and three-dimensional site lattices with two-body correlations, as well as a two-dimensional hexagonal bond network with three-body correlations, are explored. The approach presented here should be generalizable to more complex correlation states, including higher-order and longer-range correlations.

Characterization of Developing Bovine Cartilage Using Immuno-SEM

Michelle Gerritsen — Mon, 13 Apr 2009 13:48:38 PDT

Collagen is an important material in tissues of living organisms. Found almost everywhere in the human body, it is important in connective tissues, bone growth, and cartilage. In this work, collagen XI and two of its isoforms, V1b and V2, present in developing cartilage, are investigated using the technique of immuno-SEM. The efficacy of this technique to examine fundamental issues related to ossification is presented. Prior work using alternative techniques has shown that both isoforms were observed in the longitudinal septa, and in a very restricted pericellular pattern in the resting zone. Prior to primary ossification, V1b was detected only in the diaphysis, primarily adjacent to the perichondrium, and not in the epiphysis. The V2-containing isoforms were most strongly expressed in areas of newly forming cartilage, and disappeared as chondrocyte maturation proceeded. The technique of immuno-SEM will yield a better understanding of protein composition and organization within the osteochondral junction.

Influence of Copper Microstructure on Aggressive Chemical Mechanical Planarization Processes

Patrick Andersen — Mon, 13 Apr 2009 13:18:30 PDT

Novel die-stacking schema using through-wafer vias may require thick electroplated copper and aggressive first-step CMP. However, the effect of microstructural parameters, including surface orientation and grain size, on the CMP behavior of thick electroplated copper is not well understood. Here we explore the relationship between the surface orientation of copper grains and local CMP removal parameters using electron backscatter diffraction (EBSD) and atomic force microscopy (AFM) correlation techniques. EBSD is used to map crystal orientation across a large area of the sample, and AFM is subsequently used to determine relative surface heights between grains of known orientation. In the present work, aggressive CMP is performed on thick electroplated copper (30 micrometers) on silicon. In addition, solid copper disks that are annealed under varying conditions to produce samples with differing grain sizes are also studied. At the bulk level, variations in grain size and overall crystallographic texture are found to impact the removal rate and surface roughness. Furthermore, the nature of the grain boundaries (e.g. coincidence site lattice (CSL) vs. non-CSL boundaries) is shown to impact the depth of grooving at the grain boundaries. A relationship between total grain boundary length and CMP removal rate is proposed.

Microstructural Modification of Thick Copper Films to Optimize Chemical-Mechanical Planarization of Through-Wafer Interconnects

Patrick Andersen — Mon, 13 Apr 2009 13:03:52 PDT

Large through-wafer interconnects may be required for high current applications or novel devices like backside connected solar cells. The electroplated copper film required to fill large vias is typically much thicker than conventional films and has a much different microstructure. Previous work has shown films with smaller, more equiaxed grains tend to have lower removal rates and different topography after chemical mechanical planarization (CMP). Here we alter the plating parameters (bath chemistry and current density) and thermal treatments in order to create different microstructures and measure their effects on CMP outputs. Electron backscatter diffraction is used to quantify microstructural parameters such as grain size and crystallographic texture. We find that annealing thick copper films after complete room-temperature relaxation has a significant impact on CMP outputs such as removal rate and surface roughness. The results of this work could be applied to optimize CMP of thick copper films and through-wafer interconnects.

Microstructural Stability in Grain Boundary Engineered Materials

Scott Schlegel — Mon, 13 Apr 2009 13:00:17 PDT

Grain boundary engineering (GBE) is a thermomechanical process in which sequential straining and annealing cycles are used to increase the fraction of special, low-energy grain boundaries (with Σ ≤ 29 according to the coincidence site lattice model). In the present work, two cubic face-centered materials, pure copper and Inconel 617, were processed by GBE. In addition, a conventionally processed sample was processed with a single strain step, equal to the total strain in the GBE sample. The thermomecanically-processed samples were subjected to elevated temperatures for varying times. The GBE samples exhibited a resistance to change in the fraction of special boundaries and grain size (as determined by Orientation Imaging Microscopy (OIM)), while the conventionally-processed samples experienced abnormal grain growth. Monte Carlo grain growth simulations on the OIM-determined microstructures confirm the increased microstructural stability of the GBE samples. Therefore, GBE processing can produce more stable and predictable microstructures than can conventional processing.

Effects of Surface Orientation on High-Temperature Oxidation Behavior

Louis Bonfrisco — Mon, 13 Apr 2009 12:56:39 PDT

Surface orientation plays an important role in oxidation behavior; studies of single crystals suggest that {111} surfaces are most resistant to oxidation. However, most materials are polycrystalline and contain numerous orientations which contribute to the oxidation process. Here we determine the effect of orientation on oxidation behavior of metals over all surface orientations. The microstructure is characterized with electron backscatter diffraction (EBSD). After high temperature oxidation, the oxide topography is characterized using optical profilometry (OP). By correlating results from EBSD and OP, the oxide height can be determined for each orientation. The data suggests that as the surface normal deviates from the <111> direction, the oxide thickness increases. The oxidation rate may depend on not only the surface orientation, but the character of the grain boundaries. This technique allows a thorough understanding of the role of surface orientation on oxidation and may provide insight to the production of oxidation-resistant surfaces.

Thermal Stability of Cu-Sn Metal-Metal Interconnects

Jemima Fernandez — Mon, 13 Apr 2009 12:52:31 PDT

Megan Frary1; Amy Moll1; 1Boise State University Cu-Sn is currently being investigated as an alternative to Pb-Sn solders. It is especially interesting for small scale solder bumps and fine pitches. With the appropriate bonding conditions, the preferred phase of Cu-Sn (Cu3Sn) can be formed at the interface of two bond pads. This phase should be thermodynamically stable (for up to 350�C) and withstand multiple reflow cycles encountered during the assembly process of a multilayer interconnect stack. This paper investigates the thermal stability and reliability of Cu-Sn bonded die with different Sn thicknesses and bonding pressures. The samples are isothermally aged at 125�C � 10�C and also subjected to thermal cycling from 125�C to -55�C. The samples are analyzed before and after experiments to track any changes in inter-metallic growth, grain-structure, die cracking, package cracking, and bond lifting with analytical tools including Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Electron Back Scattered Diffraction (EBSD).