Morphological Characterization of ALD and Doping Effects on Mesoporous SnO2 Aerogels by XPS and Quantitative SEM Image Analysis

Atomic layer deposition (ALD) is unsurpassed in its ability to
create thin conformal coatings over very rough and/or porous materials. Yet
although the coating thickness on flat surfaces can be measured by
ellipsometry, characterization of these coatings on rough surfaces is difficult.
Here, two techniques are demonstrated to provide such characterization of
ALD-coated TiO2 over mesoporous SnO2 aerogel films on glass substrates,
and insights are gained as to the ALD process. First, X-ray photoelectron
spectroscopy (XPS) is used to determine the coating thickness over the
aerogel, and the results (0.04 nm/cycle) agree well with ellipsometry on flat
surfaces up to a coating thickness limit of about 6 nm. Second, quantitative
analysis of SEM images of the aerogel cross section is used to determine porosity and roughness, from which coating thickness can be inferred. The analysis reveals increasing porosity from the aerogel/air interface to the aerogel/substrate interface, indicating a thicker ALD coating near the air side, which is consistent with tortuous diffusion through the pores limiting access of ALD precursors to deeper parts of the film. SEM-derived porosity is generally useful in a thin film because bulk methods like nitrogen physisorption or mercury porosimetry are impractical for use with thin-film samples. Therefore, in this study SEM was also used to characterize quantitatively the morphological changes in SnO2 aerogel thin films due to doping with Sb. This study can be used as a methodology to understand morphological changes in different types of porous and/or rough materials.