Wafer-Level Simultaneous\nFilm Thickness and Thermal\nConductivity Characterizations for Bonded Si-on-Si

Wafer bonding and thinning processes in three-dimensional\nSi–Si\nbonding technology may cause the nonuniform thickness of the Si thin\nfilm. This in turn will lead to nonuniform thermal conductivity (TC)\nas phonon boundary scattering dominates TC at the submicron scale.\nThe simultaneous scanning thickness and TC of the Si thin film are\nchallenging. In this paper, we experimentally investigate the transient\nthermoreflectance (TTR) technique’s capability and effectiveness\nof simultaneously measuring the TC and thickness of the Si thin film\nfor bonded Si–Si samples. Uniform samples, combined with scanning\nelectron microscopy and phonon-based thermal conductivity models,\nwere used to verify the technique’s capability. A small deviation\n(∼10%) was found between TTR and SEM measurements for a film\nthickness less than 1 μm. From the Monte Carlo error analysis,\nwe obtained a fitting error of ∼3% for the film thickness and\n∼10–25% for TC. The fitting uncertainty was then elaborated\nthrough sensitivity analysis. We also developed a wafer-level high-throughput\nthickness scanning system, achieved by a high-precision automatic\nscanning stage combined with the deep learning-based fast-fitting\nalgorithm. We performed the scanning measurements on the sample with\na nonuniform thickness, observing local inhomogeneity, which is difficult\nto detect with the general single-point method. The scanning area\nis 16 mm × 6 mm and the spatial resolution is ∼60 μm.\nThe TTR technique enables a platform to characterize the structure\nand thermal properties of advanced Si–Si bonded materials.