Role of Ambient Composition on the Formation and Shape of Watermarks on a Bare Silicon Substrate

Wetting and drying in wet cleaning process received a lot of attention to meet the requirements of advanced technology nodes. Drying performance is evaluated by the number of watermarks produced during the rinsing and drying steps. Watermarks appear by evaporation of a very small sized water droplet remaining at the end of the drying step. In this article the influence of relative humidity and oxygen concentration of ambient on formation of different watermark shapes is studied by evaporation of ultra-pure water droplet on HF-last silicon surfaces. A qualitative model is presented taking into account: evaporation and gas-ambient (O2) exchange, chemical reactions forming the residue species starting from dissolved O2 and Si, transport of species by diffusion and evaporation driven convective radial outward flow. The watermarks are subdivided in two components: an outer ring shaped component and a more or less flat residue bed inside the ring. The experimental results showed a transition between ring shape to somehow uniform circular shape of watermark (bed part) by increasing the relative humidity of ambient. The low humidity resulted in the shorter drying time and in the smallest drying residue, consisting mainly of a ring-shaped component, which is in agreement with the model.