A base layer, comprising an electrically conductive element, is formed. An upper layer, including a third, lower planarization stop layer, a second layer and a first, upper layer is formed on the base layer. A keyhole opening is formed through the upper layer to expose a surface of an electrically conductive...http://www.google.de/patents/US7473576?utm_source=gb-gplus-sharePatent US7473576 - Method for making a self-converged void and bottom electrode for memory cell
