We have developed a novel microfabrication technology applying Metal Assisted Chemical Etching. This technology makes it possible to chemically process the entire surface of Si wafer simultaneously with high productivity. When processing high aspect ratio trenches with this technology, it is important to suppress the generation of the fine-hole-shaped defects on the walls of trenches that reduce the strength of trenches. In this study, we estimated the model that the fine hole-shaped defects were caused by the Au atoms, and as a countermeasure, we added polymer additive that could inactivate Au diffusion region to the etching solution. As a result, polymer additive inhibited the dissolution of Si in Au diffusion region and suppressed the development of fine hole-shaped defects.