Thermal annealing effect of tetrahedral amorphous carbon films deposited by filtered vacuum arc

1. Thermal annealing effect of tetrahedral amorphous carbon films deposited by filtered vacuum arc Youngkwang Lee *†,Tae-Young Kim*†, Kyu Hwan Oh†, Kwang-Ryeol Lee* *Future Technology Research Division, Korea Institute of Science and Technology †School of Materials Science & Engineering, Seoul National University Filtered Vacuum Arc Carbon target H.F. bias voltage : 600V Ar environment (10-4 Torr) Film thickness : ~5nm I. Abstract IV.Result -2. Raman Spectrum Analysis Thermal annealing of tetrahedral amorphous carbon (ta-C) film has been widely used to reduce its high level of compressive stress. In the present work, we investigated the changes in atomic bond structure and the mechanical properties caused by the thermal annealing. The ta-C films were prepared by a filtered vacuum arc process. In order to obtain ta-C films of various atomic bond structures, negative bias voltage of the substrate were varied from 0 to 650V. The specimens were annealed in vacuum at 600℃ using rapid thermal annealing (RTA) system. The residual stress of the annealed ta-C film changed in contrasting manner depending on the initial atomic bond structure or negative bias voltage applied to the substrate during deposition. At the lower bias voltage ranging from 0 to 150V, residual compressive stress of the film significantly decreased by the annealing. In contrast, the residual compressive stress increased by the thermal annealing when the applied negative bias voltage was larger than 150V. However, no significant change in the hardness was observed after the thermal annealing in this range of the negative bias voltage. This contrasting behavior will be discussed in terms of the changes in atomic bond structure analyzed by Raman spectroscopy. As-deposited Annealed G-Peak D-Peak G-Peak Different Behavior with Bias Voltage 0~50V : No significant change >200V : D-peak increases  significant increase in 6 fold ring Intensity (a.u.) Intensity (a.u.) G-peak Position of Annealed ta-C Show Higher Shift in All Range of Bias Voltage Increase of sp2 Contents II. Introduction and Motivation Our Goal Experimental Procedure Characteristic of ta-C * Corrected data considering the stress effect • High Hardness • Smooth Surface • Chemical Inertness Change of Bond Structure Change of Mechanical Property (Shin et al. APL 78, No. 5 (2001) p.631) Thermal Annealing of ta-C V.Discussion Summary of Changes by Thermal Annealing • High Compressive Residual Stress Build Model of Relationship between Change of Bond Structure and Mechanical Property by Thermal Annealing • Observe Change of • Residual Stress • Hardness ta-C deposition Moon et al.Acta Materialia 50 (2002) p.1219 III. Experimental Procedure Model of Annealing Effect on Residual Stress Thermal Annealing ta-C Deposition Buffer layer deposition Bias Voltage : 0V ~ 50V Bias Voltage : 200V ~ 500V Rapid Thermal Annealing Vacuum : below 8 mTorr Temperature : 600 ºC Time : 7 min • Six fold rings increase with increasing sp2 sites by thermal annealing • Six fold rings can be highly distorted in the amorphous carbon matrix as the size of the ring increases. • sp2 contents increases by thermal annealing without generating six fold ring. Filtered Vacuum Arc Carbon target H.F. bias voltage : 0 V~650 V Vacuum (10-5 Torr) Film thickenss : 90~120 nm • This structural change will reduce the 3-dimensional constraint in atomic bond configuration. • Compressive residual stress increases as the distortion of the large six fold ring becomes significant. • Compressive residual stress is relieved as the distortion of atomic bond is relaxed. IV. Result -1. Change of Mechanical Properties by Thermal Annealing Hardness Residual Stress VI. Conclusion • Thermal annealing affects the structure and the residual stress of ta-C film in a different manner depending on the initial structure of the film. • For ta-C of higher sp3 fraction, the residual compressive stress decreased as the distortion of atomic bond is relaxed with increasing sp2 hybrid bond. • For ta-C of smaller sp3 fraction, the residual compressive stress increased as the distortion of the large six fold ring becomes significant. Different Behavior with Bias Voltage 0V~50V : Residual Stress Decreased 200V~500V : Residual Stress Increased No Significant Change by Thermal Annealing