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The fabrication of free-standing ultra-thin porous silicon membrane structures using xenon difluoride (XeF2)-based isotropic dry etching of thin silicon wafers is reported. Using this technique, the authors demonstrate 1, 5 and 10 m-thick porous silicon membranes that are stable and self-supporting and of relatively large surface area. By strategically choosing the etching parameters and conditions, membrane thickness and pore size can be tuned to produce porous silicon membranes with attractive features that could allow structural optimisation for different applications including biological sample filtering, sensing and drug delivery. The pore size, porosity and thickness of the various developed ultra-thin free-standing porous silicon membranes were characterised with scanning electron microscopy and optical transmittance measurements.