A crystalline Si based tandem solar cell composed of a nano-wall top cell and a conventional p/n junction bottom cell has been studied. To increase the bandgap of top cell by the quantum confinement effect, the width of nano-walls should decreased to 2 nm or narrower. In this work, we studied how to make nano-walls with a width of several nm by adopting top-down processes. As a result, an array of nano-walls with a width of about 2.0 nm was obtained. The fabrication process of nano-walls is described in detail. In addition, we measured the reflectance of the fabricated samples, and found that the reflectance in the range of 400 nm to 1000 nm was lower than 4%.