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>Electrical delay technique in the picosecond thermoreflectance method for thermophysical property measurements of thin films
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Electrical delay technique in the picosecond thermoreflectance method for thermophysical property measurements of thin films
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机译：Electrical delay technique in the picosecond thermoreflectance method for thermophysical property measurements of thin films
A new picosecond thermoreflectance measurement system using an electrical delay control has been developed in order to extend observation time of temperature response of thin films after pulse heating. The new system uses two picosecond titanium sapphire lasers, one for heating a specimen and the other for probing temperature change on the film surface. Temperature history of the film surface is observed by changing the delay time of the probe pulse from the pump pulse electrically instead of changing the difference of path length between the pump beam and the probe beam. It is not difficult to observe temperature history over longer time than the repetition period of the picosecond laser pulses using the electrical delay technique. The electrical delay technique can get rid of the fluctuation of focused position of the beam on the specimen surface which is hard to eliminate after traveling through the optical delay line of variable path length. Temperature history curves of sputtered tungsten thin films of 140, 200, and 300 nm thick on glass substrates have been observed much longer than the repetition period of picosecond laser pulses for the first time. An analytical model is proposed to explain temperature response after repetitive picosecond laser pulses considering heat diffusion across the thin film, heat diffusion parallel to the film face, and heat effusion into substrate. Based on this model, thermal diffusivity values of the tungsten thin films calculated from the observed temperature history curves are about 40 of that of bulk tungsten. This electrical delay technique can realize thermophysical property measurements of a wide variety of thin films such as thicker metal films, nonmetal thin films, low thermal diffusivity materials, and multilayered thin films, the heat diffusion time of which could not be covered by the optical delay technique. (c) 2005 American Institute of Physics.
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