A micro-device with a cavity, the micro-device including a substrate. A method of forming the micro-device includes the steps of: A) providing the substrate having a surface and comprising a sacrificial oxide region at the surface; B) covering the sacrificial oxide region with a porous layer being permeable to a vapor HF etchant; and C) selectively etching the sacrificial oxide region through the porous layer using the vapor HF etchant to obtain the cavity. This method may be used in the manufacture of various micro-devices with a cavity , i.e. MEMS devices, and in particular in the encapsulation part thereof, and semiconductor devices, and in particular the BEOL-part thereof.