The passivation layer formation in the cryo-etching plasma process

The growth and destruction of the SiOxFy passivation layer is investigated in the so-called cryogenic process used for silicon etching. We show that etching products (SiF4) can play an important role in its formation. It can explain why overpassivating regime characterized by the appearance of black silicon is preferentially obtained in large structures. Test experiments clearly show that both SiFx and O radicals are necessary to create the passivation layer. By separating SiF4 plasma and O-2 plasma, we could conclude that the reaction mainly occurred at the sidewalls of the structure. If we increase the power of the oxygen plasma, the passivation layer can be reinforced. Finally, we grew the passivation layer on a flat surface by using a system of electrostatic grids to get rid of ions and electrons in order to enhance the deposition on the surface. Ellipsometric analysis is reported during the growth of the passivation layer and its destruction, which occurs when the wafer is warmed back up to ambient temperature. (c) 2007 Elsevier B.V. All rights reserved.