Method of etching a cavity in a stack of layers

A method, comprising:

• a) forming a stack of layers which includes a first layer made of a first material, a second layer made of a second material, the first layer forming a lower layer of the stack, and an upper surface of the first layer being in contact with a lower surface of the second layer, a third layer made of the first material above the second layer, and a fourth layer made of the second material above the third layer, wherein the third layer includes a layer opening such that a bottom of the fourth layer extends through the layer opening and is in contact with a top of the second layer;
• b) forming, on an upper surface of the stack of layers, a first etch mask having a first opening;
• c) etching the stack of layers opposite the first opening to a depth located at least at the second layer and passing through the layer opening;
• d) forming, on the upper surface of the stack, a second mask having a second opening, wherein dimensions of the second opening are, in top view, smaller than dimensions of the first opening, the second opening being located, in top view, opposite an area etched at step c);
• and
• e) etching the second layer opposite the second opening by an etch method capable of etching the second material selectively over the first material, to reach the upper surface of the first layer.