A target layer with holes is formed on a bottom conducting layer. Nanotubes are formed in the holes from the bottom conducting layer. A flat insulating layer is then deposited on the target layer, the nanotubes passing through the insulating layer. Air gaps are then formed by selective decomposition of the target layer. The decomposition agent and/or decomposition by-products use the walls and the central holes of the nanotubes to pass between the target layer and the outside. After decomposition, the top conducting layer is formed on the insulating layer. The nanotubes then electrically connect the conducting layers.