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The invention relates to a of manufacturing a silicon dioxide layer of low roughness, that includes depositing a layer of silicon dioxide over a substrate by a low pressure chemical vapor deposition (LPCVD) process, the deposition process employing simultaneously a flow of tetraethylorthosilicate (T

The invention relates to a of manufacturing a silicon dioxide layer of low roughness, that includes depositing a layer of silicon dioxide over a substrate by a low pressure chemical vapor deposition (LPCVD) process, the deposition process employing simultaneously a flow of tetraethylorthosilicate (TEOS) as the source material for the film deposition and a flow of a diluant gas that it not reactive with TEOS, so that the diluant gas/TEOS flow ratio is between 0.5 and 100; and annealing the silicon dioxide layer at a temperature between 600° C. and 1200° C., for a duration between 10 minutes and 6 hours.

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What is claimed is: 1. A method of manufacturing a composite substrate having a buried silicon dioxide layer which comprises: depositing a silicon dioxide layer on a source substrate by: depositing a layer of silicon dioxide upon the substrate using a low pressure chemical vapour deposition (LPCVD)

What is claimed is: 1. A method of manufacturing a composite substrate having a buried silicon dioxide layer which comprises: depositing a silicon dioxide layer on a source substrate by: depositing a layer of silicon dioxide upon the substrate using a low pressure chemical vapour deposition (LPCVD) process, wherein the layer is deposited employing simultaneously a flow of tetraethylorthosilicate (TEOS) as the source material for the film deposition and a flow of a diluent gas that is not reactive with TEOS, with the diluent gas and TEOS being present in the flow at a diluent gas/TEOS ratio of between about 0.5 and 100; and annealing the silicon dioxide layer at a temperature between about 600° C. and 1200° C., for a duration between about 10 minutes and 6 hours to reduce surface roughness compared to silicon dioxide layers provided from TEOS alone or at a different diluent gas/TEOS ratio; implanting atomic species within the source substrate to define therein a zone of weakness which demarcates a thin top layer from a remainder of the substrate, with the thin top layer being in contact with the silicon dioxide layer; directly bonding a support substrate to the silicon dioxide layer by molecular adhesion, detaching the remainder of the source substrate along the zone of weakness to obtain the composite substrate. 2. The method according to claim 1 wherein source substrate comprises silicon, germanium, silicon germanium or strained silicon. 3. The method of claim 1, wherein the diluent gas is nitrogen, argon, helium or a mixture thereof. 4. The method of claim 1, wherein the annealing is performed at a temperature between about 700° C. and 900° C. for a duration between 1 and 4 hours. 5. The method of claim 1, wherein the annealing is carried out in an inert atmosphere comprising nitrogen, argon, helium or a mixture thereof. 6. The method of claim 1, wherein the dilute gas is nitrogen and the nitrogen/TEOS ratio during the deposition process is between about 2 and 10. 7. The method of claim 6 wherein the nitrogen/TEOS ratio during the deposition process is between about 3 and 4. 8. The method of claim 1, wherein the silicon dioxide layer is deposited at a temperature of between about 625° C. and 725° C. 9. The method of claim 8, wherein the deposition temperature is between about 650° C. and 700° C. 10. The method of claim 9 wherein the diluent is nitrogen, the deposition temperature is about 675° C. and the nitrogen/TEOS ratio is about 3.6. 11. The method of claim 1, wherein the silicon dioxide layer is deposited at a deposition pressure of between about 10 and 1000 Pa. 12. The method of claim 11 wherein the deposition pressure is between about 50 and 150 Pa. 13. The method of claim 12 wherein the deposition pressure is about 100 Pa. 14. The method of claim 1, wherein the TEOS flow is between about 10 and 200 sccm, the diluent is nitrogen and the nitrogen flow is between about 10 and 2000 sccm. 15. The method of claim 14 wherein the TEOS flow is between about 50 and 90 sccm and the nitrogen flow is between about 100 and 500 sccm. 16. The method of claim 15 wherein the TEOS flow is about 70 sccm and the nitrogen flow is about 250 sccm. 17. The method of claim 14, which further comprises providing an oxygen flow with the nitrogen and TEOS flows, with the oxygen flow being between about 5 and 100 sccm. 18. The method of claim 17 wherein the oxygen flow is between about 10 and 40 sccm. 19. The method of claim 18 wherein the oxygen flow is about 25 sccm. 20. The method of claim 1, wherein the silicon dioxide layer is deposited at a thickness of between about 10 and 400 nm.