Improved TMAH Si-etching solution without attacking exposed aluminum

In MEMS fabrication technology, it is often desirable to etch silicon substrate or polysilicon sacrificial layer after metallization is completed. Most of the etching solution would attack the aluminum. In this paper, an improved tetramethyl ammonium hydroxide (TMAH) etching method is reported. The advantages of the process are to achieve high silicon etching rate (0.9-1.0 μm/min at 85°C) and smooth silicon surface (less than 0.2 μm root-mean-square). Another important advantage is that no significant aluminum etching is observed during this process. The etchant used in the study consists of 5 wt.% TMAH solution, 1.4 wt.% (or above) dissolved silicon, and 0.4-0.7 wt.% (NH4)2S2O8ammonium peroxodisulfate. The aluminum etching rate is significantly reduced by the addition of the ammonium peroxodisulfate, which also significantly improved the silicon surface roughness. When dissolving a suitable amount of silicon in TMAH etchant, along with a suitable amount of ammonium peroxodisulfate, the aluminum etching rate is reduced to zero. After TMAH etching the aluminum surfaces are protected by the coating of by-products, which prevents the underlying aluminum film from being etched by TMAH solution. The etching process demonstrated in this work is readily applicable to MEMS (micro-electromechanical system) device fabrication, such as polysilicon sacrificial layer removal, after the metallization is completed.