a method for forming microscopic 3d structures . in the method according to the invention a substrate is placed in a scanning electron microscope . the sem is equipped with a gas injection system for directing a jet of precursor fluid to the substrate . the substrate is cooled below the freezing point of the precursor gas so that a frozen layer of the precursor gas can be applied to the substrate . by now repeatedly applying a frozen layer of the precursor to the substrate and irradiate the frozen layer with an electron beam , a stack of frozen layers is built , each layer showing an irradiated part in which the precursor is converted to another material . after applying the last layer the temperature is raised so that the unprocessed precursor can evaporate . as a result 3d structures with overhanging features can be built .