A process based on deep reactive ion etching (DRIE) has been developed and optimized for the fabrication of millimeter deep silicon cavities with smooth sidewalls. The process combines two approaches which involve an optimized etching process based on the classical Bosch process (Alcatel A601E equipment) followed by the use of an aqueous etchant solution of potassium hydroxide (KOH) to smooth the surface and remove the fluorocarbon contaminants remaining after the DRIE process. As DRIE highly depends on the opening size of the patterned etch mask, different opening sizes have been tested to completely etch through a 1.4 mm thick silicon wafer. Additionally, the effect of different etch-stop materials onto the sidewalls quality has also been characterized. Sidewall quality of etched-through cavities was characterized by scanning electron microscopy (SEM) and contact surface profilometry. This single-step DRIE etching followed by short exposure to KOH solution permits to smooth sidewalls and achieve a surface roughness as low as 50 nm, which is the roughness typically obtained with the Bosch process on standard depths.