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.