Daniel Wheeler

Bio: Daniel Wheeler is an academic researcher from National Institute of Standards and Technology. The author has contributed to research in topics: Flip chip & Multiphysics. The author has an hindex of 29, co-authored 69 publications receiving 3306 citations. Previous affiliations of Daniel Wheeler include University of Greenwich.

FiPy: Partial Differential Equations with Python

Abstract: Many existing partial differential equation solver packages focus on the important, but arcane, task of numerically solving the linearized set of algebraic equations that result from discretizing a set of PDEs. Many researchers, however, need something higher level than that.

Superconformal film growth: mechanism and quantification

Abstract: Superconformal electrodeposition of copper is explained by the recently developed curvature-enhanced-accelerator coverage (CEAC) model, which is based on the assumptions that 1) the local growth velocity is proportional to the surface coverage of the accelerator, or catalyst, and 2) the catalyst remains segregated at the metal/electrolyte interface during copper deposition. For growth on nonplanar geometries, this leads to enrichment of the catalyst on advancing concave surfaces and dilution on advancing convex sections, thereby giving rise to bottom-up superfilling of submicrometer trenches and vias. In this paper the robustness of the CEAC model is demonstrated by characterizing the kinetics of catalyst accumulation and consumption in a series of electroanalytical experiments on planar electrodes. The model is then used to successfully predict interface shape evolution during feature filling in a variety of experiments, without using adjustable parameters.

Superconformal Electrodeposition of Copper

Abstract: A model of superconformal electrodeposition is presented based on a local growth velocity that is proportional to coverage of a catalytic species at the metal/electrolyte interface. The catalyst accumulates at the interface through reaction with the electrolyte. More importantly, if the concentration of the catalyst precursor in the electrolyte is dilute, then surface coverage within small features can change far more rapidly due to changing interface area. In such a case, the catalyst effectively floats on the interface during deposition, with changes in coverage coupled to alterations in arc-length of the moving surface. The local coverage therefore increases during conformal growth on a concave surface, resulting in a corresponding increase in the local deposition rate. The opposite is true for a convex surface. The model is supported by experiments and simulations of superconformal copper deposition in 350-100 nm wide features. The model also has significant implications for understanding the influence of adsorbates on the evolution of surface roughness during electrodeposition. © 2001 The Electrochemical Society. @DOI: 10.1149/1.1354496# All rights reserved.

Electrodeposition of Copper in the SPS-PEG-Cl Additive System I. Kinetic Measurements: Influence of SPS

Abstract: 1to form a passivating film that inhibits the metal deposition rate by two orders of magnitude. Subsequent adsorption of short chain disulfide or thiol molecules with a sulfonate-end group~s! leads to the disruption and/or displacement of the passivating surface complex and acceleration of the metal deposition rate. The effect of submonolayer quantities of catalytic SPS is sustained even after extensive metal deposition, indicating that the catalyst largely remains segregated on the growth surface. Multicycle voltammetry reveals a significant potential dependence for SPS adsorption as well as its subsequent deactivation. Catalyst deactivation, or consumption, was examined by monitoring the quenching of the metal deposition rate occurring on SPS-derivatized electrodes in a SPS-free electrolyte. Catalyst consumption is a higher order process in terms of its coverage dependence and a maximum deactivation rate is observed near an overpotential of 20.1 V. Derivatization experiments are shown to be particularly effective in revealing the influence of molecular functionality in additive electroplating. Specifically, the charged sulfonate end group is shown to be central to effective catalysis. In the last three years, a curvature-enhanced accelerator coverage ~CEAC! mechanism has been shown to quantitatively describe superconformal film growth which is responsible for ‘‘bottom-up superfilling’’ of submicrometer features in damascene processing. 1-3 The mechanism has also been shown to apply to silver electrodeposition 4 as well as copper chemical vapor deposition. 5 A key characteristic of superfilling electrolytes, disclosed to date, is the competition between inhibitors and accelerators for electrode surface sites. According to the CEAC model, a thiol or disulfide accelerator, or catalyst, displaces an inhibiting halide-cuprouspolyether species from the interface and remains segregated at the surface during metal deposition. 1-3,6,7 A key consequence of these two stipulations is the possibility that local area change associated with metal deposition on a nonplanar surface may give rise to changes in the local catalyst coverage, ~e.g., increases on concave sections and decreases on convex segments! and thereby superconformal film growth. This process is particularly important for surface profiles with dimensions in the submicrometer regime and naturally provides an explanation for the beneficial effects induced by certain additives known as ‘‘brighteners.’’ 1,6 In this first of a series of papers, a more complete assessment of the electrochemical response of planar electrodes in copper superfilling electrolytes is presented. A typical electrolyte contains a dilute, i.e., micromolar, concentration of accelerator in the presence of an inhibitor concentration that is usually an order of magnitude greater. This configuration gives rise to hysteretic voltammetric curves, rising chronoamperometric transients, and decreasing chronopotentiometric traces, all of which reflect the competitive adsorption dynamics occurring between the two species. An underdeveloped aspect of this system is a quantitative description of the mass balance of the additives during plating. Of specific interest is the partitioning of the catalyst between segregation to the free surface vs. deactivation by either incorporation into the growing deposit or desorption into the electrolyte. Examination of the metal deposition kinetics on catalyst-derivatized electrodes in a catalyst-free electrolyte is shown to be particularly helpful in quantifying the deactivation process. These experiments also provide an avenue for exploring the impact of various additive functional groups on the metal deposition kinetics. Experimental

Superconformal Electrodeposition in Submicron Features

Abstract: Superconformal electrodeposition is explained based on a local growth velocity that increases with coverage of a catalytic species adsorbed on the copper-electrolyte interface. For dilute concentration of the catalyst precursor in the electrolyte, local coverage in fine features changes more due to interface area change than by accumulation from the electrolyte, yielding superconformal growth. The model is supported by experiments and simulations of copper deposition in 350-100 nm wide features, helping to explain the influence of adsorbates on roughness evolution.

Machine learning in materials informatics: recent applications and prospects

Abstract: Propelled partly by the Materials Genome Initiative, and partly by the algorithmic developments and the resounding successes of data-driven efforts in other domains, informatics strategies are beginning to take shape within materials science. These approaches lead to surrogate machine learning models that enable rapid predictions based purely on past data rather than by direct experimentation or by computations/simulations in which fundamental equations are explicitly solved. Data-centric informatics methods are becoming useful to determine material properties that are hard to measure or compute using traditional methods—due to the cost, time or effort involved—but for which reliable data either already exists or can be generated for at least a subset of the critical cases. Predictions are typically interpolative, involving fingerprinting a material numerically first, and then following a mapping (established via a learning algorithm) between the fingerprint and the property of interest. Fingerprints, also referred to as “descriptors”, may be of many types and scales, as dictated by the application domain and needs. Predictions may also be extrapolative—extending into new materials spaces—provided prediction uncertainties are properly taken into account. This article attempts to provide an overview of some of the recent successful data-driven “materials informatics” strategies undertaken in the last decade, with particular emphasis on the fingerprint or descriptor choices. The review also identifies some challenges the community is facing and those that should be overcome in the near future.

Damascene copper electroplating for chip interconnections

Abstract: Damascene copper electroplating for on-chip interconnections, a process that we conceived and developed in the early 1990s, makes it possible to fill submicron trenches and vias with copper without creating a void or a seam and has thus proven superior to other technologies of copper deposition. We discuss here the relationship of additives in the plating bath to superfilling, the phenomenon that results in superconformal coverage, and we present a numerical model which accounts for the experimentally observed profile evolution of the plated metal.

Recent advances on electromigration in very-large-scale-integration of interconnects

Abstract: Today, the price of building a factory to produce submicron size electronic devices on 300 mm Si wafers is over billions of dollars. In processing a 300 mm Si wafer, over half of the production cost comes from fabricating the very-large-scale-integration of the interconnect metallization. The most serious and persistent reliability problem in interconnect metallization is electromigration. In the past 40 years, the microelectronic industry has used Al as the on-chip conductor. Due to miniaturization, however, a better conductor is needed in terms of resistance–capacitance delay, electromigration resistance, and cost of production. The industry has turned to Cu as the on-chip conductor, so the question of electromigration in Cu metallization must be examined. On the basis of what we have learned from the use of Al in devices, we review here what is current with respect to electromigration in Cu. In addition, the system of interconnects on an advanced device includes flip chip solder joints, which now tend ...

Advances in lead-free electronics soldering

Abstract: Lead-free soldering has emerged as one of the key technologies for assembling in environmental-conscious electronics. Among several candidate alloys, the Sn–Ag–Cu alloy family is believed to be the first choice with the combination of other alloys such as Sn–Zn–Bi, Sn–Cu and Sn–Bi–Ag. Phase diagrams of lead-free alloy systems have been intensively examined by using careful thermal and microstructural analysis combined with the thermodynamic calculation such as the CLAPHAD method. The Cu6Sn5/Cu3Sn layers are formed at most lead-free solder alloy/Cu interfaces, while Cu–Zn compound layers are formed in the Sn–Zn/Cu system. Growth kinetics of intermetallic layers both in solid-state and in soldering are also discussed. Creep and fatigue phenomena are also reviewed. In many aspects of lead-free soldering, much more work is required to establish a sound scientific basis to promote their applications.