Near-surface mechanical properties and surface morphology of hydrogenated amorphous carbon thin films

The study of the near-surface nanomechanical properties of thin films is a very ambitious task and can be accomplished by advanced surface sensitive techniques. Depth-sensing nanoindentation (NI) is a widely used technique for the nanomechanical characterization of thin films, but it has the inherent limitation of the substrate influence to the measured hardness (H) and elastic modulus (E). Thus, sophisticated modeling is required to determine H and E at the surface. On the other hand, surface acoustic methods seem more promising for such a study. Among them, atomic force acoustic microscopy (AFAM) is a scanning probe microscopy technique based on the resonant vibration of the AFM cantilever. In this work, we study the near-surface nanomechanical properties and the surface morphology of soft hydrogenated amorphous carbon (a-C:H) thin films. We use NI, employing the continuous stiffness measurements technique, and AFAM for the imaging of the variations of the surface mechanical properties and the accurate determination of the elastic modulus. We analyze NI data using empirical models in order to estimate near-surface E and H and the results are compared to those obtained by AFAM. From depth-sensing NI, it was found that the a-C:H thin films present “pop-in” events, which are eliminated by changing the deposition conditions e.g. increasing the negative bias voltage to the substrate (Vb). Near-surface H and E of the a-C:H thin films measured with nanoindentation were found to initially increase with increasing |Vb|. Further increase of |Vb| has no effect on the E and H values. Finally, by comparing the results from AFAM and NI, we conclude that the obtained values for E by NI are lower for all the a-C:H thin films.