Paper TF+PS-MoA9
Effect of Precursor on Coating Uniformity in Mesoporous Metal Oxide Films during Steady and Hold-Step ALD Processes

Monday, November 10, 2014, 4:40 pm, Room 307

Mesoporous film substrates with surface areas greater than 100 m²/g see use in a variety of applications, most notably in photovoltaic and photoelectrochemical energy conversion. Pastes composed of 10-20 nm diameter metal oxide particles (ITO, FTO, ATO, TiO₂) are cast as a thick film and sintered to form conductive substrates. Atomic layer deposition (ALD) is uniquely suited to apply conformal coatings into these types of mesostructured films. To date, ALD coatings have been used in this fashion for dye sensitized solar cells, photoelectrochemical cells, and thermal photovoltaic devices. Beyond common “steady” or “continuous-flow” ALD processes, several research groups have explored the use of “gas hold steps”, where the reactor is isolated from the pump for some period of time during the precursor exposure, for example, to enhance precursor infusion into high surface area or porous substrates.

In this study we examine ALD processing under steady and hold-step sequences for applying TiO₂ coatings into mesoporous Sn-doped indium oxide (ITO). Typical mesoposous films are up to 10 µm thick, which represents the minimum distance (without tortuosity) that precursor vapors need to travel in order to reach the bottom of the mesostructure. We choose two Ti-containing precursors, titanium tetrachloride (TiCl₄) and titanium tetraisopropoxide (TTIP), to understand the influence of bulky functional groups on precursor diffusion. The TTIP diffusivity will be smaller than TiCl₄, but it is also sterically hindered by its larger molecular size. We characterize film uniformity in mesoporous substrates by dynamic time-of-flight secondary ion mass spectrometry (TOF-SIMS), in-situ quartz crystal microbalance (QCM), and Krypton gas adsorption experiments.

We show that the TiCl₄/H₂O process can readily infiltrate into nanoporous ITO films as thick as 15 µm using a typical ALD process sequence, without gas “hold” steps. On the other hand, SIMS analysis shows that TiO₂ films deposited using TTIP and H₂O under the same exposure condition reach a depth of only 6 µm before exhibiting a large decay in TiO^- secondary ion intensity. While the TiCl₄ shows much better coverage, the process suffers from potential contamination, for example, from Cl which is observed in SIMS analysis. Therefore the use of bulky precursors such as TTIP is critical for ALD infiltration into mesoporous substrates, especially under conditions where coating impurity content is an important concern.