Sputter deposition of porous thin films from metal/NaCl powder targets

Abstract

A method to deposit porous thin films is elucidated. For this purpose, NaCl powder was mixed with a metal powder, cold pressed, and used as a target material in order to deposit a metal/NaCl thin film by DC magnetron sputtering. The thin film was immersed in water after deposition to remove the salt and to obtain a porous film. The low thermal conductivity of the target results in target heating and salt sublimation. In this way, the salt content in the layer and hence the film porosity are controlled by the discharge power. This procedure was carried out for Cu and Ti. The study focuses on the deposition of porous Cu thin films. From scanning transmission electron microscopy images, two film structures were observed. Films with a density higher than ≈40% of the bulk density exhibit a homogeneous spongelike microstructure with pores around 20 nm. At lower density, a noncontinuous, fractured layer is formed. The blocks between the observed cracks manifest itself in the form of columnar pores. The lowest measured density was ≈23% of the bulk density. This approach combines the flexibility of powder targets and the scalability of magnetron sputtering and avoids the usage of aggressive chemicals.