Paper TF-FrM3
A Fundamental Study of Thermal Conductivity in ALD-deposited Amorphous Oxide Thin Films of Varying Density

Friday, November 11, 2016, 9:00 am, Room 105A

Non-crystalline materials are believed to follow the minimum thermal conductivity model first proposed by Einstein in 1911. This model predicts that the thermal conductivity (Λ) of an amorphous solid is proportional to the atomic density (n) via a Λ proportional to n^2/3 relationship. This theory implies that the thermal conductivity of amorphous oxide materials can be controlled via their density. While processing conditions in the microelectronics industry often focus on optimizing the dielectric and electrical resistivity properties of amorphous oxide materials, less attention is given to these layers’ thermal properties. However, in high-power applications, the thermal conductivity of these materials begins to have importance. In this presentation, we will report on our new fundamental understanding of two industrially important amorphous metal oxide thin films: Al₂O₃ and TiO₂. ALD deposition of these materials—besides having industrial relevance—also enables direct control over atomic density of these amorphous materials via deposition temperature. In this study, amorphous thin films of Al₂O₃ and TiO₂ of varying density were deposited with ALD over a range of temperatures from 25 °C to 250 °C. The atomic density of these films is assessed with multiple techniques including ellipsometry, x-ray reflectivity, and gravimetric measurements. Time-domain thermoreflectance (TDTR) is used to measure thermal conductivity. TDTR is an ultrafast optical pump-probe measurement that is particularly well-suited for evaluating the thermal conductivity of thin films and other nanostructures. In this study, the density of Al₂O₃ films was increased by 15%, leading to an increase in thermal conductivity from 1.2 W/m-K to 1.7 W/m-K, a 42% change. TiO₂ films saw an increase from 1.4 W/m-K to 1.9 W/m-K (36%) with a 12% increase in density. Thermal conductivities as a function of film densities were fit with the Einstein minimum effective limit model modified with a differential effective-medium approximation, affirming the applicability of the amorphous limit to metal oxide systems. For the case of TiO₂, a discrete jump in thermal conductivity to 2.5 W/m-K was detected at the onset of film crystallization (125°C). This result suggests that TDTR can be more sensitive than XRD in detecting the onset of crystallization in amorphous thin films.