Deposition and removal of sodium contamination on silicon wafers

In this paper investigations are performed in order to understand the sourc e of sodium contamination in clean-room environments and to find different cleaning processes able to limit or suppress sodium deposition. In a clean- room environment, the human being has been identified as one of the major s ources of sodium. The airborne sodium contamination is essentially transmit ted in particle form. In order to limit sodium deposition from the air, the wafers awaiting subsequent processing need to be stored in a protective bo x or placed far from the human environment and should not be left for much more than 1 week in a class 1 clean room. Also, wet chemistries could cause sodium contamination on wafers particularly during the deionized water rin se. In order to limit the possible contamination, the sodium deposition mec hanisms have been studied: they show the typical characteristics of Langmui r adsorption. Temperature and ionic concentration are both parameters which influence the deposition. In water, sodium deposition can be avoided by in troducing acid or alkaline solutions or increasing the temperature: it can be drastically reduced by adding traces of HCl (0.01%). Finally, other clea ning chemistries such as SCI (NH4OH-H2O2-H2O) in 0.25:1:5 proportion, SC2 ( HCl-H2O2-H2O) in 1:1:5 proportion, 0.1% HF and SPM (H2SO4-H2O2) in 3:1 prop ortion reduce the contamination as well.