Very thin and flat silicon membrane is the most important part for fabricating high sensitivity infrared detectors. The investigation of anisotropic etching of silicon with organic (EPW) and inorganic (KOH, KOH+IPA) solutions is presented to fabricate this very thin Si membrane. These two etching technologies are compared in point of the etching rate, the quality of etched surface, etch-stop character, the morphology of etching edges and etching procedures. Very thin silicon membranes of less 1μm thick are fabricated with these technologies respectively. The mask materials resistant in etching solutions are presented. These works provide the useful technology foundation for the fabrication of the high sensitivity infrared detector.