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　lμm　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.