A method of making a silicon on insulator (SOI) wafer having an etch stop layer

A method comprising: forming an etch stop layer (130; 330) in an SOI wafer (100; 300), wherein the etch stop layer (130; 330) of an insulating layer (120; 320) of the SOI wafer (100; An etch stop layer (130; 330) comprising at least two materials selected from a group consisting of silicon nitride, nitrogen doped silica and silicon oxynitride, wherein forming the etch stop layer (130; 330) comprises: implanting (210) nitrogen in the SOI wafer ( 100; 300), wherein a maximum concentration of implanted nitrogen is at an interface between an upper silicon layer (140; 340) of the SOI wafer (100; 300) and the insulating layer (120; 320); and annealing (220) the SOI wafer (100; 300) to form the etch stop layer (130; 330), the method further comprising forming integrated circuits for a number of dies (102) in the top silicon layer (140; 340) wherein the etch stop layer (130; 330) acts as an etch stop layer in forming the integrated circuits.