Measurement of thermal diffusivity of thin films by the AC calorimetric method

Materials in the form of thin films are increasingly used in electronics and industry. In order to gain full control over fabricated thin-film ultimate properties, methods for their thermal characterisation are necessary. Development of a capability for their thermal characterisation is under way at the Institute of Nuclear Sciences "Vinca": Modified Ac calorimetry is the chosen method for thin-film characterisation. The sample is irradiated with modulated light. The propagation of temperature waves along the specimen is characterised by the rate of amplitude decay and phase lag increase. The frequency of modulation is varied between 0.25 and 1 Hz. Samples of commercial tantalum, nickel, and tungsten foils have been investigated. A PC-based data acquisition system recorded simultaneously the signals from the thermocouple (temperature waves) and from the optical detector (incident light intensity). Data processing and reduction included filtering and fitting. A sinusoidal signal had to be fitted to noisy temperature and light intensity signals, to determine the most probable values of amplitude and phase of recorded signals. Thermal diffusivity was determined on the basis of amplitude decay and phase lag increase, and therefore heat loss has been compensated for. Initial values of room temperature data for tantalum, nickel, and tungsten have been obtained, and also pointers to improvements in apparatus and software as well as extension of the temperature range of measurements.