Nanomechanical and nanotribological testing of ultra-thin carbon-based and MoST films for increased MEMS durability

TLDR

Reliability of MEM devices can be limited by stiction forces that develop in use. It is desirable to alter the mechanical and interfacial behaviour of silicon surfaces by applying very thin, low surface energy and low stress coatings. We characterize 5–150 nm thin films deposited on silicon by filtered cathodic vacuum arc and closed field unbalanced magnetron sputtering, and propose a method of analysing nano‑scratch data with spherical indenters. The onset of non‑elastic deformation in nano‑scratch tests is mainly due to substrate yield, the critical load for film failure depends strongly on indenter radius, hardness‑to‑modulus ratio, and film thickness, and <100 nm amorphous carbon films on Si show no stress‑related delamination, suggesting a promising strategy for MEMS reliability.

Abstract

Reliability of MEM (microelectromechanical) devices can be limited by stiction forces that develop in use. It is desirable to alter the mechanical and interfacial behaviour of the silicon surfaces by the application of very thin, low surface energy and low stress coatings. In this publication we report the nanomechanical and nanotribological characterization of a range of 5–150 nm thin films deposited on silicon by filtered cathodic vacuum arc (FCVA) and closed field unbalanced magnetron sputtering. A method of analysing nano-scratch data with spherical indenters is proposed. The method suggests the onset of non-elastic deformation in the nano-scratch test is due to substrate yield rather than film deformation on all but the softest films studied in this publication. The critical load for total film failure is a marked function of indenter radius, the ratio of hardness to modulus and the film thickness. The FCVA films were tested with probes of different radii (1.1, 3.1 and 9.0 µm) and the critical load for film failure was found to vary strongly with probe radius. The deposition of <100 nm amorphous carbon films on Si could be a promising strategy for improving the reliability of Si-based MEMS devices as none of the very thin films tested underwent stress-related delamination failures that occur behind the indenter during the nano-scratch testing of thicker amorphous carbon films.