Method for preparing a donor surface for reuse
View Patent ↗A donor wafer, for example of silicon, has an irregular surface following cleaving of a lamina from the surface, for example by exfoliation following implant of hydrogen and/or helium ions to define a cleave plane. Pinholes in the lamina leave column asperities at the exfoliated surface of the donor wafer, and the beveled edge may leave an edge asperity which fails to exfoliate. To prepare the surface of the donor wafer for reuse, mechanical grinding removes the column and edge asperities, and minimal additional thickness. Following cleaning, growth and removal of an oxide layer at the surface rounds remaining peaks. The smoothed surface is well adapted to bonding to a receiver element and exfoliation of a new lamina. A variety of devices may be fabricated from the lamina, for example a photovoltaic cell.
1. A method for treating a surface of a silicon donor body, the method comprising the steps of:
introducing a metal layer to at least a portion of a donor wafer;
thermally cleaving a first lamina from the donor wafer to create an exfoliated surface of the donor wafer, wherein the first lamina has a first surface and second surface opposite the first surface, and wherein a thickness of the lamina between the first and the second surface is between two and 10 microns and wherein the exfoliated surface of the donor body has silicon asperities having relief of at least the thickness of the lamina;
mechanically grinding the surface to remove or reduce the asperities;
before the grinding step, etching to remove at least a portion of the metal layer;
after the grinding step, growing an oxide at the surface;
etching to remove the oxide; and
after the etching to remove the oxide step, implanting ions through the surface to define a cleave plane within the donor body,
wherein between the step of thermally cleaving a first lamina and the step of implanting ions, between two and ten microns of thickness of silicon is removed from the donor body, including the relief of the asperities, and
wherein, between the step of thermally cleaving a first lamina and the step of implanting ions, no chemical-mechanical polishing step is performed at the surface.
2. The method of claim 1 wherein, between the step of thermally cleaving a first lamina and the step of implanting ions, no more than about eight microns of thickness is removed from the donor body, including the relief of the asperities.
3. The method of claim 1 wherein the donor body is a silicon wafer.
4. The method of claim 3 wherein the donor body is a monocrystalline silicon wafer.
5. The method of claim 1 further comprising the step of cleaving a lamina from the donor body at the cleave plane.
6. The method of claim 1 wherein, immediately following the step of etching to remove the oxide, the surface has a peak-to-valley roughness between about 50 angstroms and about 250 angstroms.
7. The method of claim 1 wherein the step of etching to remove the oxide comprises etching with an etchant comprising hydrofluoric acid.
8. A method for treating a surface of a donor wafer, the method comprising the steps of:
introducing a metal layer to at least a portion of the donor wafer;
thermally cleaving a first lamina from the donor wafer, creating an exfoliated surface of the donor wafer wherein the first lamina has a first surface and second surface opposite the first surface, and the lamina has a thickness between the first and the second surface;
mechanically grinding the exfoliated surface to remove or reduce asperities, wherein the step of mechanically grinding the exfoliated surface does not include use of a chemical etchant;
wherein between the steps of thermally cleaving a first lamina and grinding the exfoliated surface, etching to remove at least a portion of the metal layer;
growing oxide on the exfoliated surface;
etching to remove the oxide, wherein after the steps of grinding, growing and etching to remove the oxide, a thickness of silicon is removed from the donor wafer, the thickness of silicon being at least the thickness between the first and second surface of the first lamina and up to 10 microns;
implanting ions through the exfoliated surface to define a cleave plane within the donor wafer, wherein between the step of thermally cleaving a first lamina and the step of implanting ions, no chemical-mechanical polishing step is performed at the surface; and
thermally cleaving a second lamina from the donor wafer at the cleave plane, wherein the second lamina is suitable for use in a photovoltaic cell.
9. The method of claim 8 further comprising fabricating a photovoltaic cell, wherein the photovoltaic cell comprises the second lamina.
10. The method of claim 8 wherein the first lamina has a thickness of at least three microns.
11. The method of claim 8 wherein the donor wafer is a silicon wafer.
12. The method of claim 11 wherein the donor wafer is a monocrystalline silicon wafer.
13. The method of claim 8 wherein, following the step of etching to remove the oxide, the exfoliated surface has a peak-to-valley roughness between about 50 angstroms and about 250 angstroms.
14. The method of claim 8 wherein the ions include hydrogen ions.