Deep isolation trenches having sides and a bottom are formed in a semiconductor substrate. The sides and the bottom are coated with an electrically insulating material that delimits an empty cavity, and forms a plug to close the cavity. The sides of the trench are configured with a neck that determines the depth of the plug, and a first portion that tapers outwards from the neck as the distance from the bottom increases. Deep isolation trenches may be applied, in particular, to bipole and BiCMOS circuits.