A method for forming a silicon trench, comprises the steps of: defining an etching area at a silicon substrate; forming metal catalysts at the surface of the etching area; immersing the silicon substrate in a first etching solution thereby forming anisotropic silicon nanostructures in the etching area; immersing the silicon substrate in a second etching solution thereby resulting in the silicon nanostructures being side-etched and detached from the silicon substrate, thus forming the silicon trench.