Abstract: A method is described for obtaining high temperature resistant assemblies comprising isolated silicon islands bonded to a substrate in view of the manufacture of bipolar or MOS circuit element built-in devices. The flat surface of an initial wafer comprising a silicon layer wherein unisolated silicon islands are nested within a silicon oxide layer otherwise having a planar interface with the silicon layer is totally oxidized and coated with a thicker insulating layer at least part of which is comprised of silicon oxide up to an exposed surface, and a second substrate at least the surface of which is comprised of silicon oxide is bonded to the said exposed oxide surface by fusion of an intermediate phosphosilicate glass layer between said two oxide surfaces. Thereafter, the silicon material of the initial wafer is removed up to the said interface.