Thermal diffusivity measurement of a thin metal film with a picosecond thermoreflectance technique

A picosecond thermoreflectance measurement system in the National Research Laboratory of Metrology has been improved by introducing a 'rear heating/front probing'configuration. Both rear heating/front probing and conventional front heating/front probing thermoreflectance signals can be observed for thin films deposited on transparent substrates. The analytical solution of the temperature response is presented. It considers the penetration depth of a heating beam and a probe beam, and is derived based on the response function method. These analyses were applied to an aluminium thin film, nominally 100 nm thick, deposited on a Pyrex 7740 glass substrate. The thermal diffusivity calculated from the front heating/front probing thermoreflectance signal is 30% smaller than that of the bulk aluminium. The thermal diffusivity calculated from the rear heating/front probing thermoreflectance signal is 6.2% smaller than that of the bulk aluminium.