The microstructure of sputter‐deposited coatings

TLDR

Microstructure critically affects sputter‑deposited thin films used as microcircuit metallization layers and diffusion barriers, and low‑temperature deposition (T ≪ Tm) typically produces columnar, void‑filled growth superimposed on polycrystalline or amorphous structures, which is undesirable and results from atomic shadowing and limited adatom mobility exacerbated by surface roughness. This review investigates how deposition conditions shape sputter‑deposited film microstructure, focusing on the origins of columnar growth and its suppression via energetic particle bombardment. The authors synthesize recent studies on the interplay between deposition parameters, atomic shadowing, adatom mobility, and energetic particle bombardment to explain and mitigate voided growth in sputter‑deposited coatings.

Abstract

Microstructure is a critical consideration when polycrystalline or amorphous thin films are used for applications such as microcircuit metallization layers and diffusion barriers. The trend in device fabrication toward lower processing temperatures means that such coatings must often be deposited at substrate temperatures T that are low relative to the coating material melting point Tm. The structure of vapor deposited coatings grown under these conditions consists typically of a columnar growth structure, defined by voided open boundaries, which is superimposed on a microstructure which may be polycrystalline (defined by metallurgical grain boundaries) or amorphous. The voided growth structure is clearly undesirable for most applications. Its occurrence is a fundamental consequence of atomic shadowing acting in concert with the low adatom mobilities that characterize low T/Tm deposition, and its formation can be enhanced by the surface irregularities which are common to microcircuit fabrication. This paper reviews some of the recent developments in understanding the fundamental aspects of the relationship between the deposition conditions and the microstructure of sputter‐deposited thin films, with particular emphasis on the origin of the growth structure and its suppression through energetic particle bombardment.