Harry K. Moffat

Design of gas inlets for the growth of gallium nitride by metalorganic vapor phase epitaxy

Abstract The problem of gas inlet design for metalorganic vapor phase epitaxy (MOVPE) of group III nitrides from metal alkyls and ammonia is addressed. The focus is on GaN growth from trimethyl-gallium and ammonia. In traditional reactors with well-mixed inlet streams, parasitic gas-phase reactions between the two precursors may lead to the formation of stable adducts that can condense on cool inlet walls, thus reducing the film growth efficiency. Such reactions may also lead to the formation of particulates through gas-phase condensation reactions (e.g. during AlN growth). A fundamentally-based model was developed to describe the MOVPE of GaN and was used to study the effect of inlet design and reactor operating conditions on film thickness uniformity in vertical stagnation-flow and rotating-disk reactors. The model includes a description of gas-phase kinetics and a simple gas-surface reaction mechanism. The kinetic model was coupled to a two-dimensional transport model describing flow, heat and mass transfer in a vertical MOVPE reactor. Predictions of growth rate compare well to experimental observations from a vertical rotating-disk reactor, without any adjustable parameters. The model was also used to study the distribution of gaseous species in the reactor and their role in film growth. Finite element simulations using a massively parallel computer code (MPSalsa) indicate that the species responsible for film growth are Ga-alkyls and not their adducts with ammonia. Sensitivity analysis was also performed to assess the relative importance of each reaction in determining the growth rate. The model was subsequently employed in the design of axisymmetric, multi-aperture gas inlets feeding precursors into the reactor in an alternating (not well-mixed) fashion. Simulations were performed to study the effect of key design parameters, such as inlet velocities, susceptor rotating speed, inlet to susceptor distance as well as the number and distribution of inlets, on GaN film growth rate and uniformity in industrial scale reactors. Optimal cases are presented that lead to uniform films over large-area substrates. An alternating precursor feed scheme based on concentric rings was found to lead to more uniform films as the number of inlets increased. The other important reactor parameters were inlet velocity, relative size of inlet “rings” and susceptor distance from the inlet.

Chemical kinetics in chemical vapor deposition: growth of silicon dioxide from tetraethoxysilane (TEOS)

Abstract Chemical reactions in the gas-phase and on surfaces are important in the chemical vapor deposition (CVD) of materials for microelectronic applications. General approaches for modeling the homogeneous and heterogeneous kinetics in CVD are discussed. A software framework for implementing the theory utilizing the CHEMKIN suite of codes is presented. Specific examples are drawn from the CVD of SiO 2 using tetraethoxysilane (TEOS). Experimental molecular beam reactive-sticking coefficient studies were employed to extract surface-reaction rate constants. Numerical simulations were used to analyze the molecular-beam experiments and low-pressure tube furnace data, illustrating the general modeling approach.

Fundamental models of the metalorganic vapor-phase epitaxy of gallium nitride and their use in reactor design

A fundamental reaction-transport model describing the metalorganic vapor-phase epitaxy (MOVPE) of GaN from trimethyl-gallium (TMG) and ammonia has been developed. This model has been tested against experimental data from research-scale and industrial-scale reactors, A simplified version of the model that includes only transport phenomena and a unity sticking coefficient of the limiting film precursor (TMG) to the surface of the growing film was found to accurately capture observed film deposition variations in an early variant of the Thomas Swan close-coupled-showerhead 3 × 2 reactor. Modifications of the Thomas Swan reactor, in line with the findings suggested by this work, enabled state-of-the-art thickness uniformity to be achieved. The model has been used to develop performance diagrams for conceptual multi-aperture MOVPE reactors and for the Thomas Swan system. These performance diagrams identify regions of the parameter space of the reactor which correspond to minimal variations in film growth rate across large-area substrates. Published by Elsevier Science B.V.

Efficient parallel computation of unstructured finite element reacting flow solutions

Abstract A parallel unstructured finite element (FE) reacting flow solver designed for message passing MIMD computers is described. This implementation employs automated partitioning algorithms for load balancing unstructured grids, a distributed sparse matrix representation of the global FE equations, and parallel Krylov subspace iterative solvers. In this paper, a number of issues related to the efficient implementation of parallel unstructured mesh applications are presented. These issues include the differences between structured and unstructured mesh parallel applications, major communication kernels for unstructured Krylov iterative solvers, automatic mesh partitioning algorithms, and the influence of mesh partitioning metrics and single-node CPU performance on parallel performance. Results are presented for example FE heat transfer, fluid flow and full reacting flow applications on a 1024 processor nCUBE 2 hypercube and a 1904 processor Intel Paragon. Results indicate that very high computational rates and high scaled efficiencies can be achieved for large problems despite the use of sparse matrix data structures and the required unstructured data communication.

Organometallic vapor phase epitaxy (OMVPE)

Abstract Organometallic vapor phase epitaxy (OMVPE) has emerged in this past decade as a flexible and powerful epitaxial materials synthesis technology for a wide range of compound–semiconductor materials and devices. Despite its capabilities and rapidly growing importance, OMVPE is far from being well understood: it is exceedingly complex, involving the chemically reacting flow of mixtures of organometallic, hydride and carrier-gas precursors. Recently, however, OMVPE technologies based on high-speed rotating disk reactors (RDRs) have become increasingly common. As fluid flow in these reactors is typically cylindrically symmetric and laminar, its effect on the overall epitaxial growth process is beginning to be unraveled through quantitative computer models. In addition, over the past several years, a combination of well-controlled surface science and RDR-based growth-rate measurements has led to a richer understanding of some of the critical gas and surface chemistry mechanisms underlying OMVPE. As a consequence, it is becoming increasingly possible to develop a quantitative and physically based understanding of OMVPE in particular chemical systems. In this article, we review this understanding for the important specific case of AlGaAs OMVPE in an RDR under conditions used for growing typical device heterostructures. Our goal is to use typical growth conditions as a starting point for a discussion of fundamental physical and chemical phenomena, beginning with the fluid flow through an RDR and ending with the chemical reactions on the surface. By focusing on one particularly important yet relatively simple specific case, this review differs from more comprehensive previous reviews. Viewed as a case study, though, it complements these previous reviews by illustrating the wide diversity of research that is related to OMVPE. It can also serve as a good starting point for the development and transfer of insights into other more complex cases, such as: OMVPE of materials families containing Sb, P or N species, of other devices types, and in other more complex reactor geometries.

Analysis of gallium arsenide deposition in a horizontal chemical vapor deposition reactor using massively parallel computations

Abstract A numerical analysis of the deposition of gallium arsenide from trimethylgallium (TMG) and arsine in a horizontal CVD reactor with tilted susceptor and a 3″ diameter rotating substrate is performed. The three-dimensional model includes complete coupling between fluid mechanics, heat transfer, and species transport, and is solved using an unstructured finite element discretization on a massively parallel computer. A reaction mechanism consisting of three surface and two bulk species, four surface reactions, and four gas phase species was used to model the deposition. The effects of three operating parameters (the disk rotation rate, inlet TMG fraction, and inlet velocity) and two design parameters (the tilt angle of the reactor base and the reactor width) on the growth rate and uniformity are presented. The nonlinear dependence of the growth rate uniformity on the key operating parameters is discussed in detail. Efficient and robust algorithms for massively parallel reacting flow simulations, as incorporated into our analysis code MPSalsa, make detailed analysis of this complicated system feasible.

Mechanistic feature-scale profile simulation of SiO2 low-pressure chemical vapor deposition by tetraethoxysilane pyrolysis

Simulation of chemical vapor deposition in submicron features typical of semiconductor devices has been facilitated by extending the EVOLVE [T. S. Cale, T. H. Gandy, and G. B. Raupp, J. Vac. Sci. Technol. A 9, 524 (1991)] thin film etch and deposition simulation code to use thermal reaction mechanisms expressed in the Chemkin format. This allows consistent coupling between EVOLVE and reactor simulation codes that use Chemkin. In an application of a reactor-scale simulation code providing surface fluxes to a feature-scale simulation code, a proposed reaction mechanism for tetraethoxysilane [Si(OC2H5)4] pyrolysis to deposit SiO2, which had been applied successfully to reactor-scale simulation, does not correctly predict the low step coverage over trenches observed under short reactor residence time conditions. One apparent discrepancy between the mechanism and profile-evolution observations is a reduced degree of sensitivity of the deposition rate to the presence of reaction products, i.e., the by-product i...

Aria 1.5 : user manual.

Aria is a Galerkin finite element based program for solving coupled-physics problems described by systems of PDEs and is capable of solving nonlinear, implicit, transient and direct-to-steady state problems in two and three dimensions on parallel architectures. The suite of physics currently supported by Aria includes the incompressible Navier-Stokes equations, energy transport equation, species transport equations, nonlinear elastic solid mechanics, and electrostatics as well as generalized scalar, vector and tensor transport equations. Additionally, Aria includes support for arbitrary Lagrangian-Eulerian (ALE) and level set based free and moving boundary tracking. Coupled physics problems are solved in several ways including fully-coupled Newtons method with analytic or numerical sensitivities, fully-coupled Newton-Krylov methods, fully-coupled Picards method, and a loosely-coupled nonlinear iteration about subsets of the system that are solved using combinations of the aforementioned methods. Error estimation, uniform and dynamic h-adaptivity and dynamic load balancing are some of Arias more advanced capabilities. Aria is based on the Sierra Framework.

Pore corrosion model for gold-plated copper contacts

The goal of this study is to model the electrical response of gold plated copper electrical contacts exposed to a mixed flowing gas stream consisting of air containing 10ppb H2S at 30degC and a relative humidity of 70%. This environment accelerates the attack normally observed in a light industrial environment (essentially a simplified version of the Battelle class 2 environment). Corrosion rates were quantified by measuring the corrosion site density, size distribution, and the macroscopic electrical resistance of the aged surface as a function of exposure time. A pore corrosion numerical model was used to predict both the growth of copper sulfide corrosion product which blooms through defects in the gold layer and the resulting electrical contact resistance of the aged surface. Assumptions about the distribution of defects in the noble metal plating and the mechanism for how corrosion blooms affect electrical contact resistance were needed to close the numerical model. Comparisons are made to the experimentally observed number density of corrosion sites, the size distribution of corrosion product blooms, and the cumulative probability distribution of the electrical contact resistance. Experimentally, the bloom site density increases as a function of time, whereas the bloom size distribution remains relatively independent of time. These two effects are included in the numerical model by adding a corrosion initiation probability proportional to the surface area along with a probability for bloom-growth extinction proportional to the corrosion product bloom volume. The cumulative probability distribution of electrical resistance becomes skewed as exposure time increases. While the electrical contact resistance increases as a function of time for a fraction of the bloom population, the median value remains relatively unchanged. In order to model this behavior, the resistance calculated for large blooms has been weighted more heavily.

Final Report on LDRD Project: Coupling Strategies for Multi-Physics Applications

Many current and future modeling applications at Sandia including ASC milestones will critically depend on the simultaneous solution of vastly different physical phenomena. Issues due to code coupling are often not addressed, understood, or even recognized. The objectives of the LDRD has been both in theory and in code development. We will show that we have provided a fundamental analysis of coupling, i.e., when strong coupling vs. a successive substitution strategy is needed. We have enabled the implementation of tighter coupling strategies through additions to the NOX and Sierra code suites to make coupling strategies available now. We have leveraged existing functionality to do this. Specifically, we have built into NOX the capability to handle fully coupled simulations from multiple codes, and we have also built into NOX the capability to handle Jacobi Free Newton Krylov simulations that link multiple applications. We show how this capability may be accessed from within the Sierra Framework as well as from outside of Sierra. The critical impact from this LDRD is that we have shown how and have delivered strategies for enabling strong Newton-based coupling while respecting the modularity of existing codes. This will facilitate the use of these codes in a coupled manner to solve multi-physic applications.