Modeling of Cu Surface Precipitation and Out-Diffusion from Silicon Wafers
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0994-F09-01
Modeling of Cu Surface Precipitation and Out-Diffusion from Silicon Wafers Hsiu-Wu Guo, and Scott T. Dunham Electrical Engineering, University of Washington, Seattle, WA, 98195-2500
ABSTRACT It has been observed by Ohkubo et al. [Jpn. J. Appl. Phys. 44, 3793(2005)] that Cu outdiffusion is much slower than expected based on Cu diffusivity and that the presence of both organics and Cu on the surface increases Cu out-diffusion rate and reduces surface potential. In this work, physically based models are developed to provide the foundation for simulating Cu out-diffusion to the wafer surface. These models include built-in electric field due to surface states as well as nucleation and growth of Cu silicide precipitates on wafer surface. The resulting models are verified by comparison to the experimental measurements. INTRODUCTION Copper is often used as the interconnect material in todayís CMOS fabrication processes due to its low resistivity, but it is especially problematic among the transition metals in silicon devices due to its diffusivity in silicon. Cu atoms have the tendency to precipitate at the siliconsilicon dioxide interface and thus degrade the gate-oxide integrity, and Cu precipitates are electrically active with deep energy levels, which act as the generation/recombination centers for free carriers increasing leakage currents. Therefore, with the downscaling of the IC dimensions, there is a strong demand for suppressing the Cu contamination level to as low as possible. It is reported that Cu out-diffusion depends on the wafer surface condition, which can be changed by using different wet treatments [1, 2, 3] or depositing a metal thin film [4]. Experiments from McCarthy et al. [2] and Shabani et al. [3] found Cu out-diffusion behavior to have a strong dependence on the surface treatment, while Ohkubo et al. [1] investigated the correlation between Cu out-diffusion and the change of surface conditions quantitatively. In this work, physically based models are developed, including the change of surface potential due to the amount of Cu precipitates and absorbed organic molecules and kinetics for surface precipitation process, to provide the understanding of Cu out-diffusion. EXPERIMENT Fig. 1 shows the experiment procedures and results reported by Ohkubo et al. [1]. Samples were boron-doped (10 Ωcm), single side polished, 200mm Czochralski silicon wafers. Cu contamination was driven into wafers at 900oC for 1 hour after immersing in deionized water containing Cu. Out-diffused Cu (C1st) was measured by total reflection X-ray fluorescence (TXRF) after the first bake in either a quartz box or a plastic box at 60oC for varying times up to 48h. Surface cleaning was performed with solution to remove the surface Cu. TXRF measurements (C2nd) were repeated again after second bake at 80oC for 24 hours with the wafers loaded in a plastic box. Surface precipitation ratio (Cs/C0) as shown in Fig. 1b was defined as the ratio of surface precipitated Cu after first step (C1st) to the initial total Cu in silicon
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