Abstract In appropriate mixtures of bromine and fluorine BrF3 is generated, which can then be used for the structuring of silicon under room conditions without plasma support. Direct reaction of the intermediary BrF3 with silicon results in the formation of SiF4 and bromine. By further addition of fluorine, the etching reaction can be started again. Bromine then acts as a catalyst. In spite of the high etching rate, the roughness of the etched surfaces remains small. By adding xenon to the etching gases, the roughness can be reduced to a minimum. Thermally produced SiO2 can be applied as the etching mask. Complete isotropy of the etching process allows underetching of closely adjacent LIGA structures. Moreover, optical application is possible due to the good quality of the etched surfaces. When underetching small circular holes in the SiO2 mask, spherical depressions are generated. After the SiO2 mask has been removed, these structures can be moulded in plastic and used as microlenses.