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

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.

Development of DLC coating architectures for demanding functional surface applications through nano- and micro-mechanical testing

“…It is also important that the test instrumentation has sufficiently high lateral rigidity (as the NanoTest Vantage) so that when using a sensitive friction transducer the scratch track is not significantly affected by surface roughness. The method has previously been validated for DLC films deposited on Silicon with good agreement found between scratch track widths determined from the analytical method and assessed by microscopic examination [16].…”

“…true depth data. The mean pressure during the nano-and micro-scratch tests was estimated by the application of a Hertzian treatment previously applied to the nanoscratch testing of carbon films with spherical probes [15,16]. The method enables the yield stresses and the pressure required for the failure of the film to be estimated from contact mechanics, assuming the geometry of indentation, provided spherical indenters are used.…”

Advanced Nanomechanical Test Techniques

“…Recently the capability of the progressive load multi-pass scratch technique has been extended by the development of a novel analytical treatment to obtain accurate scratch depth data after correction of the raw data for the contribution from instrument compliance, sample topography and sample slope [54]. The method described in reference 54 enables the yield stresses and the pressure required for the failure of the film to be estimated from contact mechanics, assuming the geometry of indentation, provided spherical indenters are used.…”

“…In practice it appears that these conditions can often be met in the nano-scratch test [54,55], although the approximation becomes less valid as the geometry moves away from Hertzian. The methodology has been validated for thin films on Silicon wafers with good agreement between (i) scratch hardness independently determined from optical measurements of scratch widths (ii) contact pressures for film and substrate yield events.…”

“…Beake and co-workers have reported nanomechanical and nanotribological (nano-scratch and nano-wear) characterisation of a wide range of thin films on Si, with thicknesses from 5 nm (ta-C) to 1500 nm (TiN) performing nanoindentation with Berkovich indenters combined with nano-scratch testing with spherical indenters [49,[54][55][56][57]. A key motivation for studying several of these was to understand the interplay between film thickness and interfacial toughness for MEMS and protective thin film applications.…”