Wafer Surface Charging Model for Single-Wafer Wet-Spin Processes

Wet chemical processes in integrated circuit (IC) manufacturing are used in many applications, e.g., post-etch residue removal and pre-deposition surface treatment. While advanced single-wafer wet spin tools are part of the critical tool-set for advanced IC fabrication, non-optimized tool hardware and/or process may induce different types of wafer surface charging issues. In this paper, a physical model to fundamentally explain surface charging induced by a single-wafer wet spin tool is described. The model is based on the advection of surface charges from wafer-center to wafer-edge resulting from the shear flow of the liquid. The charge distribution in the diffuse layer adjacent the wafer surface is calculated by solving the coupled Poisson's and current continuity equations. As often practiced in the industry in characterizing this type of wafer surface charging, a thermally grown silicon dioxide surface is used as the model surface and de-ionized water as the liquid medium. Good agreement is obtained between experimental and calculated surface charging potentials for radial positions extending from wafer-center to approximately 130 mm on standard 300 mm diameter wafers. The observed charging potential trends with respect to radial position, wet process time, and wafer spin speed are well explained by the current model.