AVS 49th International Symposium
    Thin Films Wednesday Sessions
       Session TF-WeA

Paper TF-WeA6
Conformal Coating of Ultrahigh Aspect Ratio Anodic Alumina Membranes by Atomic Layer Deposition

Wednesday, November 6, 2002, 3:40 pm, Room C-101

Session: Atomic Layer Deposition - Applications of ALD
Presenter: J.W. Elam, University of Colorado
Authors: J.W. Elam, University of Colorado
D. Routkevitch, Nanomaterials Research Corporation
P.P. Mardilovich, Nanomaterials Research Corporation
S.M. George, University of Colorado
Correspondent: Click to Email
Anodic alumina (AA) membranes are unique nanostructures that are permeated by highly uniform, nanometer-scale pores arranged in a hexagonal close packed array. The application of AA membranes to gas sensors requires that the nanopores with ultrahigh aspect ratios of L/d~1000 be coated by uniform films of chemoresistive materials such as ZnO. In this study, AA membranes were coated with Al@sub 2@O@sub 3@ and ZnO atomic layer deposition (ALD) films in a viscous flow ALD reactor. The coated membranes were analyzed using cross-sectional scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). SEM analysis of individual nanopores revealed that the AA membranes with nanopore dimensions of d=65 nm and L=50 µm could be coated conformally by Al@sub 2@O@sub 3@ ALD using sufficient reactant exposure times. EPMA measurements with a spatial resolution of 3-5 µm were performed following ZnO ALD on the d=65 nm, L=50 µm AA membranes. The EPMA Zn concentration profiles showed the progr essive infiltration of the ZnO ALD into the nanopores with increasing exposure times. Monte Carlo simulations of these experiments assuming Knudsen diffusion accurately reproduced the experimental ZnO EPMA concentration profiles. Furthermore, this modelin g predicted that the diffusion-limited coating process would become reaction-limited given a sufficiently low ALD reaction probability. To test this idea, Fourier transform infrared absorption measurements were performed during the coating of the AA membranes by Al@sub 2@O@sub 3@ and SiO@sub 2@ ALD. In agreement with the predictions, diffusion-limited behavior with a t@super 1/2@ time dependence was observed for Al@sub 2@O@sub 3@ ALD during the AlOH* + Al(CH@sub 3@)@sub 3@ -> AlOAl(CH@sub 3@)@sub 2@* + CH@sub 4@ reaction. In contrast, reaction-limited behavior with a t@super 1@ time dependence was observed for SiO@sub 2@ ALD during the SiOH* + SiCl@sub 4@ -> SiOSiCl@sub 3@* + HCl reaction.