A MULTISCALE MODEL FOR AN ATOMIC LAYER DEPOSITION PROCESS

Abstract

Atomic layer deposition (ALD) is a deposition technique suitable for the con- trolled growth of thin films. During ALD, precursor gasses are supplied to the reactor in an alternating sequence producing individual atomic layers through self- limiting reactions. Thin films are grown conformally with atomic layer control over surfaces with topographical features. A very promising material system for ALD growth is aluminum oxide. Alu- minum oxide is highly desirable for both its physical and electronic characteristics. Aluminum oxide has a very high band gap (~ 9 ev) and a high dielectric constant (k ~ 9). The choice of precursors for aluminum oxide atomic layer deposition vary from aluminum halide, alkyl, and alkoxides for aluminum-containing molecules; for oxygen-containing molecules choices include oxygen, water, hydrogen peroxide and ozone. For this work a multiscale simulation is presented where aluminum oxide is deposited inside anodic aluminum oxide (AAO) pores for the purposes of tuning the pore diameter. Controlling the pore diameter is an import step in the conversion of AAO into nanostructered catalytic membranes (NCM). Shrinking the pore size to a desired radius allows for the control of the residence time for molecules entering the pore and a method for molecular filtration. Furthermore pore diameter control would allow for the optimization of precursor doses making this a green process. Inherently, the ALD of AAO is characterized by a slow and a faster time scale where film growth is on the order of minutes and hours and surface reactions are near instantaneous. Likewise there are two length scales: film thickness and composition on the order of nanometers and pore length on the order of microns. The surface growth is modeled in terms of a lattice Monte Carlo simulation while the diffusion of the precursor gas along the length of the pore is modeled as a Knudsen diffusion based transport model.