Galvanic Corrosion during Processing of Polysilicon Microelectromechanical Systems

Immersion in aqueous hydrofluoric acid (HF), a standard processing step in fabricating microelectromechanical systems devices, causes rougher and weaker polycrystalline silicon (polysilicon) films in the presence of Au metallization and leads to relatively thick surface oxides. The origin of these effects is the galvanic corrosion that takes place due to the difference in electrochemical potential between the noble Au metallization and the polysilicon in HF solution, resulting in the accelerated formation of a porous surface layer due to corrosion of the silicon. The electrochemical behavior and the evolving microstructural morphology are correlated. In HF solution, Si is dissolved and porous silicon (PS) is formed. The PS first develops along the polysilicon grain boundaries and then extends into the grain interiors. After rinsing in water and exposure to air, the pore walls form the usual oxide, leading to a composite with a gradient in composition—from a high fraction of near the surface to 100% Si deeper into the polysilicon. Inasmuch as the galvanic corrosion requires positively charged holes, these effects can be minimized by using n-type dopants in the polysilicon and performing the HF immersion in the absence of illumination.