Effect of Pre-Wet Cleaning Conditions on the Pad Oxide Thickness

The thickness of the furnace grown pad oxide is limited by the balance between the bird’s beak effect and the stress induced crack of the silicon nitride. Absence of pad oxide can completely eliminate the bird’s beak effect; however it also introduces stress generated cracks at the Si and silicon nitride interface which ultimately degrades the transistors. Additionally, the silicon wafers are also cleaned by wet processes to remove any incoming contamination, surface particles and other defects before the growth of this pad oxide. Since this wet cleaning process directly affects the pad oxide thickness, it is important to understand how the pre-clean chemistries and process conditions control the oxide growth.

In this study, effects of temperature, process time and concentration of SC1 and SC2 cleaning chemistries are investigated. Furthermore, effect of removing native SiO₂ by initial dHF treatment is also evaluated as one of the variables. A set of experiments were designed to evaluate the effects and interactions between these variables within wide windows. A difference of >2A in pad oxide thickness and >1nm in mean roughness of the silicon surface after the pre-wet clean were observed. For example, SC1 process at higher temperature always showed thicker oxide growth compared to room temperature SC1 when the SC1 process time was identical. This article provides a detailed understanding of how the growth of pad oxide can be affected by the pre cleaning process parameters.