An Effective Approach for Restraining Galvanic Corrosion of Polycrystalline Silicon by Hydrofluoric-Acid-Based Solutions

This paper presents an effective method to restrain galvanic corrosion of polycrystalline silicon (polysilicon) that is electrically coupled with noble metals of microelectromechanical systems (MEMS) devices by hydrofluoric-acid (HF)-based solutions. A titanium (Ti) redox sacrificial layer is added on the noble-metal layer and then covered by photoresist. Benefiting from the lower electrochemical potential of Ti than that of polysilicon in HF-based solutions, Ti is preferentially corroded in HF-based solutions, and the polysilicon is well protected. The thickness of the Ti layer should be optimized for effectively suppressing galvanic corrosion of polysilicon; a 50-nm-thick Ti film is able to preserve the resistivities of polysilicon unchanged in concentrated HF (49% HF by weight percent) solution for 1 h. This approach is simple and compatible with MEMS batch-fabrication technology and provides a solution for the longstanding issue in microfabrication technology, i.e., galvanic attack to the polysilicon structural layer by HF-based solutions.