Substrates for semiconductor devices

A composite substrate comprising a silicon wafer 10 and a silicon oxide layer 12 is provided with a bonding layer 14 on which a polycrystalline diamond layer 16 is grown. The composite wafer is removed using preferential etching which does not etch the bonding layer or the diamond layer. The bonding layer may be a different material to silicon or silicon dioxide or it may be silicon having a different crystal orientation to the underlying silicon portion of the composite wafer. An edge etch stop may be provided around edge regions of the bonding layer attached to the polycrystalline diamond layer (see figure 3). In an alternative method a doped, electrically conductive region is provided in a portion of the substrate to be removed and an electrically resistive layer is bonded to the polycrystalline diamond layer and the doped, electrically conductive portion of the substrate is removed by electrochemical etching or electrochemically assisted etching.