Effect of Synthesis Condition on Thermal Diffusivity of Molybdenum Thin Films Observed by a Picosecond Light Pulse Thermoreflectance Method

Abstract

The relation between out-of-plane thermal diffusivity of Mo thin film and its synthesized condition is studied. Two types of molybdenum thin film with nominal 100 nm thickness are prepared, one is dc-sputtered at 0.4 Pa in Ar pressure on a Pyrex 7740 glass substrate (film A), and the other is rf-sputtered at 11 Pa in Ar pressure on the Pyrex 7740 glass substrate (film B). Each thermal diffusivity was measured by rear-heating front-detection type (RF-type) picosecond light pulse thermoreflectance method. The thermal diffusivity of film A was 3.9 ×10-5 m2·s-1 and that of film B was 4.4 ×10-6 m2·s-1. This demonstrates that the synthesis condition is strongly related to the out-of-plane thermal diffusivity and that the thermal diffusivity can become less than one-tenth of that for bulk Mo. To elucidate the heat transfer mechanism, transmission electron microscopy (TEM), X-ray diffraction (XRD), and in-plane electrical conductivity measurements were performed. Mean free paths calculated from in-plane resistivities were smaller than the grain sizes estimated from XRD peaks and TEM image. Those results suggested that not only grain boundary but also other impurities or defects might play a role as scattering objects.