Plasma-Enhanced Spatial Atomic Layer Deposition on 2D and 3D Surface Topologies: The Case of Amorphous and Crystalline TiO2

Abstract

Plasma-enhanced spatial ALD (PE-s-ALD) is an interesting technique for high-volume manufacturing of thin films at low-temperature. This technique is particularly appealing for conformal depositions on 3D surfaces for various applications, such as optical coatings, electrolyzers, and batteries. However, various crystallization and growth effects can influence the final film profile and properties, and understanding these effects and their interplay is key. This study investigates the complex growth mechanism of TiO₂ using PE-s-ALD. TiO₂ films are deposited on both planar and 3D substrates, while systematically varying the number of cycles, deposition temperature, and exposure times. Thickness, crystallinity, and composition are determined as a function of depth inside the structures. Conditions that result in the anatase phase on a planar surface only partially form this phase inside 3D structures, with the deepest part of the film being amorphous. This partial crystallization is ascribed to the film thickness inside the 3D structure gradually dropping below the critical thickness for crystallization. In turn, the partial crystallization is shown to have a significant effect on the resulting thickness profile, due to a difference in growth per cycle between the two phases. A framework of the interplay between effects is proposed, offering insights that enable better control of crystallinity and thickness throughout the entirety of coated surfaces of 3D structures by PE-s-ALD. Additionally, the recombination probability of oxygen radicals during this atmospheric-pressure PE-s-ALD process at 200 °C is determined to be 3 × 10^–5. This value is similar to low-pressure PE-ALD, indicating that differences in conformality between the two types of ALD are not the result of differences in recombination probability, but rather of differences in initial radical density and diffusion behavior.