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Pulsed photothermal measurement of tetrahedral amorphous carbon (ta-C) film

Pulsed photothermal reflection technique is developed to measure the thermal conductivity of tetrahedral amorphous carbon (ta-C) films. This technique is basically a pump and probe type. The pump source is Nd YAG laser, pulse width 8 ns, is operated at 532 nm with 1 mJ. The sample is deposited on silicon wafer and over-coated with a gold layer for heat absorption. Upon the laser pulse strikes on the sample surface, the surface temperature rises sharply and then relaxes with time. A continuous HeNe laser (1 mW) probes the surface temperature, measuring the reflectivity changes on the sample's surface. Since the reflectivity and temperature has an inverse linear relationship, the relaxed temperature profile can be obtained by inverting the captured reflectivity profile. Using either 2-layer or 3-layer heat conduction model, thermal properties of the ta-C film are then varied to fit the obtained profile. The fitted thermal conductivity value of the ta-C film is approximately 2 W/mK.