Silicon membranes of 100 nm thick (and below) over several mm(2) have been fabricated for high efficiency extreme ultraviolet (EUV) transmission grating. The perfect planarity (< 1 nm) and the high transparency (>80%) at 13.5 nm wavelength have been measured. In comparison to SiNx membranes (40% transmission), the silicon membranes allow high resolution EUV interference lithography using the second order diffraction of the molybdenum grating. The fabrication of such phase gratings has been performed and the diffraction efficiency measurement is 27% for the silicon membrane and the Mo grating.