To completely isolate an island of silicon, a trench is cut into an epitaxial layer to provide access to a differently doped buried layer. While suspending the portion of the epitaxial layer surrounded by the trench by means of an oxide bridge, the underlying region of the buried layer is etched away...http://www.google.de/patents/US4888300?utm_source=gb-gplus-sharePatent US4888300 - Submerged wall isolation of silicon islands
