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Photoelectrochemical etching of silicon in hydrofluonc acid solutions is proposed as a method for realising three-dimensional sensor and actuator elements. It is shown that high selectivity in etch rates between illuminated and dark areas can be obtained by a proper electrochemical bias. This can be implemented with an inert metal electrode, e.g. platinum. The technique could be useful for fabrication of beam and membrane structures for holographic gratings and for maskless small-volume production.