On the Characterization of Thin Film-Only Mechanical Property Based on the Indentation Image Analysis
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0976-EE06-10
On the Characterization of Thin Film-Only Mechanical Property Based on the Indentation Image Analysis Yun-Hee Lee1, Yong-Il Kim1, Hoon-Sik Jang1, Seung-Hoon Nahm1, Ju-Young Kim2, Dongil Kwon2, and Jae-il Jang3 1 Division of Metrology for Quality Life, Korea Research Institute of Standards and Science, Daejeon, 305-340, Korea, Republic of 2 School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Korea, Republic of 3 Department of Materials Science and Engineering, Hanyang University, Seoul, 133-791, Korea, Republic of
ABSTRACT Conventional nanoindentation testing generally uses a peak penetration depth of less than 10 % of thin-film thickness in order to measure film-only mechanical properties, without considering the critical depth for a given thin film-substrate system. The uncertainties in this testing condition make hardness measurement more difficult. We propose a new way to determine the critical relative depth for general thin-film/substrate systems; an impression volume analyzed from the remnant indent image is used here as a new parameter. Nanoindents made on soft Cu and Au thin films with various indentation loads were observed by atomic force microscope. The impression volume calculated from 3D remnant image was normalized by the indenter penetration volume. This indent volume ratio varied only slightly in the shallow regime but decreased significantly when the indenter penetration depth exceeded the targeted critical relative depth. Thus, we determined the critical relative depth by empirically fitting the trend of the indent volume ratio and determining the inflection point. The critical relative depths for Cu and Au films were determined as 0.170 and 0.173, respectively, values smaller than 0.249 and 0.183 determined from the hardness variation of the two thin films. Hence the proposed indent volume ratio is highly sensitive to the substrate constraint, and stricter control of the penetration depth is needed to measure film-only mechanical properties. INTRODUCTION Nanoindentation testing is a powerful technique for evaluating the mechanical properties of thin films because materials must be deformed on only a very small scale [1]. This technique thus has not only the merits of conventional hardness measurement such as testing simplicity, nondestructiveness, and inexpensiveness but also the merits of instrumented techniques such as simultaneous data gathering and property analysis without additional specimen observation. However, when a thin film-substrate system is indented, the indentation load versus depth curve is influenced by not only the film but also the substrate. A general rule of thumb in film-substrate hardness measurement suggests that nanoindentations should be made at penetration depths less than 10 % of the film thickness in order to obtain substrate-independent film properties. However, this critical depth criterion is too strict to apply to ultra-thin-films (less than 100 nm);
in addition, a previous study [2] has reported that the critic
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