A method and apparatus is provided for locating with improved vertical positioning accuracy a discrete element on a semiconductor optoelectronic integrated circuit. The method employs an etch stop layer located beneath a series of semiconductor layers. The semiconductor layers may include waveguides...http://www.google.se/patents/US5793913?utm_source=gb-gplus-sharePatent US5793913 - Method for the hybrid integration of discrete elements on a semiconductor substrate 
