Atomic layer deposited lithium aluminum oxide: (In)dependency of film properties from pulsing sequence

Abstract

Atomic layer deposition (ALD) holds markedly high potential of becoming the enabling method for achieving the three-dimensional all-solid-state thin-film lithium ion battery (LiB). One of the most crucial components in such a battery is the electrolyte that needs to hold both low electronic conductivity and at least fair lithium ion conductivity being at the same time pinhole free. To obtain these desired properties in an electrolyte film, one necessarily has to have a good control over the elemental composition of the deposited material. The present study reports on the properties of ALD lithium aluminum oxide (LixAlyOz) thin films. In addition to LiB electrolyte applications, LixAlyOz is also a candidate low dielectric constant (low-k) etch stop and diffusion barrier material in nanoelectronics applications. The LixAlyOz films were deposited employing trimethylaluminum-O3 and lithium tert-butoxide-H2O for Al2O3 and Li2O/LiOH, respectively. The composition was aimed to be controlled by varying the pulsing ratio of those two binary oxide ALD cycles. The films were characterized by several methods for composition, crystallinity and phase, electrical properties, hardness, porosity, and chemical environment. Regardless of the applied pulsing ratio of Al2O3 and Li2O/LiOH, all the studied ALD LixAlyOz films of 200 and 400 nm in thickness were polycrystalline in the orthorhombic β-LiAlO2 phase and also very similar to each other with respect to composition and other studied properties. The results are discussed in the context of both fundamental ALD chemistry and applicability of the films as thin-film LiB electrolytes and low-k etch stop and diffusion barriers.