Modulation of the time-domain reflectance signal induced by thermal transport and acoustic waves in multilayered structure

Structure of chip on chip and three-dimension chip of integration circuits provides promising methods to satisfy the miniaturization of integration. Such design may affect the performance of circuits considering the heat accumulation and thermal strain in devices resulted from poor thermal conductivity in small size near nanoscale. Hence, it is crucial to characterize the thermal transport and strain of multilayer structures in micro/nanoscale. In this paper, the ultrashort pulse laser pump-probe measurement is used to study the thermal transport and induced thermal strain simultaneously with sample of 94 nm film deposited on glass substrate. Superposition theory is developed to characterize the reflectance signal by stacking the thermoreflectance variation and interference of the probe beams reflected by the interface and the coherent acoustic-phonon waves. Rigorous theoretical derivation is performed to modulate the phase and amplitude of the reflectance signal. The theoretical prediction matches well with the experimental data, and the extracted thermal conductivity of the Pt film exhibits significant size effects.