Cross-linked PMMA on Porous Silicon: An Effective Nanomask for Selective Silicon Etching

Abstract

Combining a porous silicon (PS) buffer layer with cross-linked poly(methyl methacrylate) (PMMA), we have obtained masks that show high resistance to the electrochemical process usually employed to produce PS. PMMA is normally dissolved in an HF/ethyl alcohol mixture, but it becomes resistant to such solution after cross-linking of the polymer. This can be achieved by high-dose electron irradiation, obtaining a mask for the subsequent electrochemical etching. Due to the strong electric field across the masking layer during the electrochemical process, the time duration of such a mask is limited. We demonstrate that the presence of a highly porous silicon thin film lying under the resist is sufficient for an evident improvement of the masking power during the electrochemical process, allowing for the selective formation of thick PS regions. Furthermore, a final PS removal in alkaline solution leads to the formation of silicon micro- and nanostructures in relief, such as microtips and nanomolds.