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Thin non-porous silicon nitride membrane has been used for transmission electron microscopy (TEM) “windows” due to its transparency to high-energy electrons. However, to completely eliminate the background scattering, a porous membrane is desirable. Here the authors report the fabrication of porous silicon nitride membranes by CsCl self-assembly to form hemispherical islands, followed by “image reversal” via drying etching and liftoff steps. Through controlling the evaporated CsCl film thickness, the exposure time of the film in air, and the relative humidity of air, the authors were able to fabricate porous silicon nitride membranes with pore diameter and interpore spacing of order 100 nm. The pore diameter could be further shrunk by coating a conformal film onto the porous membrane. In addition, conductive porous TEM windows can be achieved by simply skipping the final metal removal step. Such a porous membrane would also find applications in separation or filtration of macromolecules or nanoparticles.