Magnetron sputtering of transparent conductive zinc oxide: relation between the sputtering parameters and the electronic properties

TLDR

Magnetron sputtering of transparent conductive oxides such as zinc oxide, indium tin oxide, and tin oxide enables low‑temperature deposition of films with good optical and electronic properties and scalable to large areas. This paper reviews the principles underlying magnetron sputtering. During growth, the film is bombarded by sputtered atoms, argon ions, and reflected neutrals, and the resulting film properties depend on the species energies and ion‑to‑neutral ratio, with low ion energies (<50 eV) and a high ratio being advantageous. High‑energy bombardment (≈100 eV) at low sputtering pressures damages the film, causing stress, small crystallites, and poor electrical performance, while ion‑assisted growth with low ion energies and a high ion‑to‑neutral ratio yields polycrystalline films with a limiting resistivity of 2 × 10⁻⁴ cm, suggesting new research directions.

Abstract

Magnetron sputtering of transparent conductive oxides (zinc oxide, indium tin oxide, tin oxide) is a promising technique which allows the deposition of films at low temperatures with good optical and electronic properties. A special advantage is the scalability to large areas. The principles underlying magnetron sputtering are reviewed in this paper. The growth process during magnetron sputtering is characterized by the bombardment of the growing film with species from the sputtering target and from the plasma. In addition to sputtered atoms with energies in the eV range, ions from the plasma (mostly argon) and neutral atoms (also argon) reflected at the target hit the growing film. Depending on the energy of these species and on the ion-to-neutral ratio the properties of the films vary. High energies ( 100 eV), which occur mainly at low sputtering pressures lead to damage of the growing film, connected with mechanical stress, small crystallites and bad electrical parameters. Ion assisted growth with low ion energies (below about 50 eV) is advantageous as is a high ion-to-neutral ratio. A compilation of resistivities of magnetron sputtered zinc oxide films yields a limiting resistivity of 2 × 10-4 cm for polycrystalline films. Based on the correlation between plasma parameters and film properties new research fields are anticipated.