ChBE/ISR Ph.D. Proposal Defense: Curtisha D. Travis

Tuesday, January 24, 2012
3:00 p.m.
2168 A.V. Williams Bldg., ISR Conference Room
Curtisha D. Travis
cdtravis@umd.edu

Modeling ALD Surface Reaction Dynamics Using Absolute Reaction Rate Theory

Presented by Curtisha D. Travis

Advisor: Professor Raymond A. Adomaitis

Committee: Professors Panagiotis Dimitrakopoulos, Sheryl Ehrman, Gregory Jackson, and Jeffrey Klauda

Atomic layer deposition (ALD) is a thin-film manufacturing process in which a surface is exposed to alternating precursor gases which react in a self-limiting fashion, producing conformal films on various substrate geometries with molecular level precision in film thickness. Often the surface reaction mechanisms in ALD processes are not well understood beyond the initial deposition cycle and optimal operating conditions are found through experiment, which can be both timely and costly. Ab initio computational chemistry (e.g. density functional theory) and Monte Carlo methods have been employed to gain insight into reaction mechanisms and energetics, but such studies have fallen short of predicting reaction kinetics for the ALD processes.

In this study, known reaction mechanisms and thermodynamics of the alumina and boron nitride ALD processes are coupled to an ALD surface reaction model that describes precursor chemisorption, desorption, and surface reactions that form the ALD film and gas-phase by-products. This model is developed from first principles investigation of precursor species, Lewis acid-base adduct, and transition state structures which, using absolute rate theory and equilibrium statistical thermodynamics, allows for reaction rate expressions and an array of nonlinear differential equations describing surface coverage dynamics. Numerical limit-cycle solutions are computed using a collocation method whereby the differential equations are fully discretized and the resulting set of nonlinear algebraic expressions are solved using a Newton-Rhapson method.

Predictions from this model (e.g. growth per cycle) are compared to previously published ALD studies to validate our theory and to gain further insight into the surface mechanisms at work in ALD processes. Results for the alumina and boron nitride systems show close agreement with published growth per cycle observations. Overall, our goal is to establish a baseline from which more refined models can be developed, to explore more realistic surface reaction mechanisms and kinetic expressions. Ultimately, physically based models of the type we develop (coupling surface reactions with transport) can be exploited to determine optimal precursor exposure levels and pulse times for ALD-based nanomanufacturing operations.

Audience: Graduate  Faculty  Post-Docs 

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