Thermoreflectance Technique for Thermal Properties Measurement of Micro/Nanoscale Cantilever Beam

Thermal property analysis of thin film materials on a semi-infinite substrate is a very important area of research in last few decades due to the deviation of their properties from its bulk values and measurement technique plays very important role to be free from the effect of interface conductance, surface impurities, convective and radiative losses and fabrication tolerances. Non-contact optical measurement technique drew attention to be applicable to measure thermal properties using pump-probe thermoreflectance where short pulse Laser beam applied to the film to avoid heat loss with the inclusion of the interfacial conductance and extract thermal diffusivity which is coupled to the thermal conductivity and the heat capacity of the film. This work extends the technique using Continuous Wave laser to freestanding cantilever beams for thermal properties measurement more accurately and without having any affect from interfacial conductance where heat flow is dominantly driven by 1-D heat conduction due to the device structure and experimental conditions. The pump beam applies a constant flux to the free end of the cantilever thereby inducing a thermal gradient along its length which is measured by the probe beam. Measurement of the thermal gradient allows for determination of the thermal conductivity of the material. First of all, the cantilever beam structure was chosen due to its 1D structure and free from the effect of the interface conductance and convective and radiative heat losses are minimized by performing the experiment at high vacuum and removing the substrate underneath the beam. Specific heat capacity of the cantilever beam also can be extracted by applying harmonic signal of the pump flux at the free end of the beam, furthermore the specific heat capacity verified using transient analysis of the heat flux using probe laser thermoreflectance. We demonstrate the technique by measuring the thermal conductivity of a 1.29 mum thick piece of Si. The sample demonstrated ∼30% reduction in thermal conductivity when compared to the bulk value and the specific heat capacity ∼ 760JKg--1 K--1, does not have much change from its bulk.