Near-surface mechanical properties and surface morphology of hydrogenated amorphous carbon thin films S.N. Kassavetis, S. Logothetidis ⁎ , G.M. Matenoglou Aristotle University of Thessaloniki, Department of Physics, Laboratory for Thin Films - Nanosystems and Nanometrology, GR-54124, Thessaloniki, Greece Abstract 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 (V b ). Near- surface H and E of the a-C:H thin films measured with nanoindentation were found to initially increase with increasing |V b |. Further increase of |V b | 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. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogenated amorphous carbon; Surface morphology; Nanoindentation; Atomic force acoustic microscopy 1. Introduction In recent years, several techniques have been developed to study the nanomechanical properties of thin films. Among them, nanoindentation (NI) is a widely used technique for the measure- ment of the elastic modulus (E) and the hardness (H), either on a single loading–unloading cycle (CI) [1] or during dynamic load- ing (continuous stiffness measurements, CSM) [2]. In the case of thin and ultra thin films, a limitation to the measurement of the nanomechanical properties with this technique arises from the small thickness of the films and as a consequence the measured values of H and E are affected by the mechanical response of the substrate. The main advantage of CSM technique, compared to CI, is that it provides the H and E values continuously versus the penetration (contact) depth of the indenter. Thus, the appropriate models, which take into account the substrate effect, can be applied, in order to estimate the real hardness (H f ) and elastic modulus (E f ) values of the surface of the thin films. In addition, surface acoustic methods seem more promising for the measurement of the nanomechanical properties of the surface. Several scanning techniques were used for the detection of the surface mechanical response to the acoustic waves. Among them near-field techniques, like atomic force microscopy (AFM), provide better lateral resolution compared to optical scanning techniques, which are restricted by Abbe's principle. In this study we employ atomic force acoustic microscopy (AFAM) [3], a non-destructive technique for the imaging of the variations of the nanomechanical properties of the surface and the quanti- tative determination of the near-surface elastic modulus. AFAM is based on the vibration at the resonant frequency of an AFM cantilever, which is in continuous contact with the surface of the film, when an external ultrasonic signal is applied to the sample. Hydrogenated amorphous carbon (a-C:H) thin films are candidate materials for many cutting edge technological appli- cations, such as protective coatings for optical systems, barrier Surface & Coatings Technology xx (2005) xxx – xxx + MODEL SCT-11907; No of Pages 5 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +30 2310 998174; fax: +30 2310 998390. E-mail address: logot@auth.gr (S. Logothetidis). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.11.010 ARTICLE IN PRESS