Dimensional scaling of materials in integrated circuits have resulted in major challenges in power dissipation and thermal management. While the characteristic length scales over which these materials are utilized has continued to scale, thermal metrologies to characterize the properties of these materials at the scales on which they are integrated have not kept pace. The continued scaling of devices and their constituent materials places an ever increasing importance on understanding the role of interfacial thermal transport and the physics driven reductions of thermal conductivity that take place in geometrically confined materials, especially those where phonons are the dominant heat carriers. Many common assumptions that hold in electron dominated systems break down in phonon dominated systems and can lead to dramatic underestimates of the overall thermal resistance at the device scale.
While theoretical understanding of the physics that dominate thermal transport in phonon dominated systems is important, it is ultimately important to measure the fundamental behavior of materials as they are deposited, integrated, and treated to understand their intrinsic thermal properties. Thermoreflectance based techniques, which rely on using laser to source a thermal perturbation and measure small changes in the reflectivity of a material due to this perturbation, have been used successfully over the past couple decades to characterize materials on these length scales. Traditionally time-domain and frequency-domain thermoreflectance techniques have been used, however they suffer from relatively shallow probing depths, often on the order of tens to hundreds of nanometers, depending on the material in question. Additionally, these techniques measure thermal effusivity, a quantity that is related to both the thermal conductivity and the volumetric heat capacity, requiring knowledge of either in order to calculate the other. Steady-state thermoreflectance has emerged as a new technique that offers increased depth sensitivity, on the order of the radius of the laser spots used in the measurement, and also provides a direct measurement of thermal conductivity.
Here we present the fundamentals of Steady-State Thermoreflectance measurements and several applications that demonstrate its usefulness in understanding the thermal behavior of material systems at device relevant length-scales. We will demonstrate material systems and measurements where the thickness of the material causes a reduction in thermal conductivity from expected bulk values, use of SSTR to measure the in-plane thermal conductivity of thin conductive films, ultra-thin film measurements on the order of single digit nanometers and how to extract thermal properties, and the use of SSTR to measure the temperature dependent properties of materials up through relevant device operating temperatures.
