In this paper, a new fabrication technique to obtain silicon crystalline flat planes is described. The selected Si crystalline planes, that are not always the planes having the lowest etching rate, are pre-obtained by deep reactive ion etching (RIE). The flat crystalline planes are generated by Si anisotropic wet etching with ethylenediamine pyrocatechol and water after the deep-RIE. We demonstrate the proposed technique by fabricating a solid polymer dye microcavity laser, which is molded with the etched Si cavity. The etched condition for obtaining the very smooth sidewall of the cavity of the Si mold has been investigated. The lasing spectra confirm the flatness of the etched surface that is measured by an interferometer and an atomic force microscope.