In this paper, we have directly buried microchannels in a fused silica substrate using femtosecond laser irradiation followed by chemical etching. By exploiting a synergic effect between gaseous hydrofluoric acid and fluorine (F₂), we have obtained microchannels having an outstanding aspect ratio of 86 with a relative etching speed of 17m min^?1. Using fluorine we successfully regenerated the etchant ‘in situ‘, thus providing a way to control the bore shape, in particular, the strong conically shaped microchannel can be adjusted by modifying the etching mixture composition. We have avoided water formation during the microchannel construction, thus overcoming one of the major limits that prevents any further increase in the length and the aspect ratio.