On the characterization of thin film-only mechanical property based on the indentation image analysis

초록

Conventional nanoindentation test generally controls its peak penetration depth shallower than 10 % of thin film thickness in order to measure the thin film-only mechanical property without considering detailed information on the critical indentation depth for a given thin film-substrate system. This uncertain testing condition makes hardness measurement more difficult. Thus, we proposed a new method for determining the critical indentation depth for general thin film-substrate systems; an impression volume analyzed from the remnant indent image was used as an important parameter in this study. A series of nanoindents were made on soft Au thin film by applying various indentation loads and then observed with an atomic force microscope. The remnant indentation volumes were quantitatively calculated from the three-dimensional indentation morphologies. In shallow indentation regime, a trend of the indentation volume depended on three-dimensional morphology of the Berkovich indenter. When the indenter penetration depth surpasses the targeted critical indentation depth, the remnant indentation volume deviated from its original trend due to severe elastic recovery generated by the hard Si substrate. The critical indentation depth of Au/Si system was measured by superposing two indentation volume trends from shallow and deep indentation regimes. Within the critical indentation depth, the hardness of the Au film was nearly constant 0.95±0.02 GPa and the validity of the proposed method was confirmed empirically.