SUBSTRATES FOR SEMICONDUCTOR DEVICES

A method of manufacturing a composite substrate for a semiconductor device, the method comprising: selecting a substrate wafer comprising: a first layer of single crystal material suitable for epitaxial growth of a compound semiconductor thereon and having a thickness of 100 mum or less;a second layer having a thickness of no less than 0.5 mum and formed of a material having a lower thermal expansion coefficient than the first layer of single crystal material and/or is formed of a material which has a higher fracture strength than that of the first layer of single crystal material; and a third layer forming a handling wafer on which the first and second layers are disposed, wherein the substrate wafer has an aspect ratio, defined by a ratio of thickness to width, of no less than 0.25/100; growing a first polycrystalline CVD diamond layer on the first layer of single crystal material using a chemical vapour deposition technique to form a composite comprising the substrate wafer bonded to the polycrystalline diamond layer via the first layer of single crystal material, wherein during growth of the first polycrystalline CVD diamond layer a temperature difference at a growth surface between an edge and a centre point thereof is maintained to be no more than 80°C; and removing the second and third layers of the substrate wafer to form a composite substrate comprising the polycrystalline diamond layer directly bonded to the first layer of single crystal material.