Thermal Property Imaging of Aluminum Nitride Composites

Frequency domain thermoreflectance (FDTR) imaging is used to create quantitative thermal conductivity maps of porous Aluminum Nitride (AlN) particles embedded in epoxy. The AlN-epoxy composite is polished and coated with a metal layer. A piezo stage is used to move the sample for imaging with our FDTR system. For each pixel, a periodically modulated continuous-wave laser (the red pump beam) is focused to a Gaussian spot, less than 2 um in diameter, to locally heat the sample, while a second beam (the green probe beam) monitors the surface temperature through a proportional change in the metals' reflectivity. The pump beam is modulated simultaneously at six frequencies and the thermal properties of the AlN composite are extracted by minimizing the error between the measured probe phase lag at each frequency and an analytical solution to the heat diffusion equation in a multilayer stack of materials. A schematic of the AlN sample in our measurement system and an optical image of the polished surface of the AlN-epoxy composite before coating with metal is shown in a. Two scanning electron microscope images of the porous AlN particles prior to embedding in epoxy are shown in b. One of the six simultaneously collected phase images of the probe laser is shown in c. The dark blue regions in the phase image are pits on the sample surface. We fit the six phase images to our thermal model and obtain thermal conductivity maps. The conductivity maps of four particles are shown in d. A log color bar is used to highlight the contrast of thermal conductivity in a single particle. The thermal conductivity of the AlN particles ranges from 80W/mK in the dense regions to 5W/mK in the porous regions.