METHOD FOR POLISHING HETEROSTRUCTURES

1 .- 17 . (canceled)

18 . A method for polishing a heterostructure comprising at least one relaxed superficial heteroepitaxial layer on a substrate made from a material that is different from that of the heteroepitaxial layer, which method comprises:

performing a first chemical mechanical polishing on the surface of the heteroepitaxial layer for a first period of time with a polishing cloth having a first compressibility ratio and with a polishing solution having a first silica particle concentration,

subsequently performing a second chemical mechanical polishing step on the surface of the heteroepitaxial layer for a second period of time with a polishing cloth having a second compressibility ratio, higher than the first compressibility ratio, and with a polishing solution having a second silica particle concentration lower than the first concentration, to reduce surface roughness compared to performing a single chemical mechanical polishing step using a single polishing cloth and a single polishing solution for a time period that is the same as that of the combined first and second periods of time of the first and second chemical mechanical polishing steps.

19 . The method of claim 18 , wherein the first polishing step is performed with a polishing solution that contains silica particles having diameters within a first range of values and the second polishing step is performed with a polishing solution that contains silica particles having diameters within a second range of values, wherein the second range of values is at least partly lower than the first range of values.

20 . The method of claim 18 , wherein the first polishing step is performed with a polishing cloth having a first compressibility ratio that is between 2% and 4%.

21 . The method of claim 18 , wherein the second polishing step is performed with a polishing cloth having a second compressibility ratio that is between 5% and 9%.

22 . The method of claim 18 , wherein the first polishing step is performed with a polishing solution having a first silica particle concentration that is between 28% and 30%.

23 . The method of claim 18 , wherein the second polishing step is performed with a polishing solution having a second silica particle concentration that is between 8% and 11%.

24 . The method of claim 19 , wherein the first polishing step is performed with the silica particles of the polishing solution having a diameter that is between 70 nm and 100 nm.

25 . The method of claim 19 , wherein the second polishing step is performed with the silica particles of the polishing solution having a diameter that is 60 nm and 80 nm.

26 . The method of claim 18 , wherein the heteroepitaxial layer is a silicon-germanium layer, and the substrate includes a silicon support.

27 . The method of claim 26 , wherein after the second chemical mechanical polishing step is performed, the surface roughness of the silicon-germanium heteroepitaxial layer is reduced to less than 0.1 nm RMS for a roughness measurement made with an atomic force microscope on 2*2 μm 2 and 10*10 μm 2 scan areas.

28 . The method of claim 26 , wherein after the second chemical mechanical polishing step is performed, the silicon-germanium heteroepitaxial layer presents a surface macroroughness corresponding to a surface haze level of less than 0.5 ppm.

29 . The method of claim 18 , wherein the first and second chemical mechanical polishing steps are performed in a polishing tool comprising a polishing head in which the heterostructure is arranged and a plate covered by a polishing cloth in contact with the surface of the heteroepitaxial layer to be polished, with the polishing solution being dispensed from the polishing head.

30 . A fabrication method of a sSOI structure which comprises:

polishing a silicon-germanium heteroepitaxial layer belonging to a donor substrate according to the method of claim 18 ;

forming a strained silicon layer on the polished silicon-germanium heteroepitaxial layer;

implanting at least one atomic species in the donor substrate designed to form a weakened layer;

bonding a surface of the donor substrate with a surface of a receiver substrate; and

detaching a layer of the donor substrate in contact with the receiver substrate by cleavage at the weakened layer to form the sSOI structure.

31 . The method of claim 30 , wherein the receiver substrate comprises a thermal oxide layer on its bonding surface.

32 . A heterostructure comprising at least one relaxed silicon-germanium superficial heteroepitaxial layer on a silicon substrate; wherein the heteroepitaxial layer has a surface that is polished to present a surface microroughness of less than 0.1 nm RMS for a roughness measurement made with an atomic force microscope on 2*2 μm 2 and 10*10 μm 2 scan areas.

33 . The heterostructure of claim 32 , wherein the polished heteroepitaxial layer surface also presents a surface macroroughness corresponding to a surface haze level of less than 0.5 ppm.

34 . A donor substrate designed to be used as a crystalline growth seed for formation by epitaxy of at least one strained silicon layer thereon, which comprises a heterostructure according to claim 32 .

35 . A donor substrate designed to be used as a crystalline growth seed for formation by epitaxy of at least one strained silicon layer thereon, which comprises a heterostructure according to claim 33 .