Method to form a device including an annealed lamina and having amorphous silicon on opposing faces
View Patent ↗A semiconductor assembly is described in which a support element is constructed on a surface of a semiconductor lamina. Following formation of the thin lamina, which may have a thickness about 50 microns or less, the support element is formed, for example by plating, or by application of a precursor and curing in situ, resulting in a support element which may be, for example, metal, ceramic, polymer, etc. This is in contrast to a rigid or semi-rigid pre-formed support element which is affixed to the lamina following its formation, or to a donor wafer from which the lamina is subsequently cleaved. Fabricating the support element in situ may avoid the use of adhesives to attach the lamina to a permanent support element; such adhesives may be unable to tolerate processing temperatures and conditions required to complete the device. In some embodiments, this process flow allows the lamina to be annealed at high temperature, then to have an amorphous silicon layer formed on each face of the lamina following that anneal. A device may be formed which comprises the lamina, such as a photovoltaic cell.
1. A method to fabricate a device, the method comprising:
providing a monocrystalline semiconductor lamina having a thickness about 50 microns or less, the lamina having a first surface and a second surface opposite the first;
annealing the semiconductor lamina to a temperature of 850 degrees C. or greater; and
fabricating a photovoltaic cell, wherein the completed cell has a first amorphous silicon layer in immediate contact with the first surface, and a second amorphous silicon layer in immediate contact with the second surface.
2. The method of claim 1 wherein the lamina has a thickness between about 4 microns and about 20 microns.
3. The method of claim 1 wherein the first amorphous silicon layer and the second amorphous silicon layer are both deposited after the annealing step.
4. The method of claim 1 wherein the first amorphous silicon layer comprises an emitter of the photovoltaic cell, or the second amorphous silicon layer comprises an emitter of the photovoltaic cell.
5. The method of claim 1 wherein at least a portion of the first amorphous silicon layer is heavily doped to a first conductivity type, and at least a portion of the second amorphous silicon layer is heavily doped to a second conductivity type opposite the first.
6. The method of claim 1 wherein the lamina comprises a base region of the photovoltaic cell.
7. The method of claim 1 wherein the step of providing a semiconductor lamina comprises:
implanting ions into a semiconductor donor body to define a cleave plane; and
cleaving the lamina from the donor body at the cleave plane.
8. The method of claim 1 wherein, during the annealing step, temperature reaches at least 900 degrees C.
9. The method of claim 1 wherein the annealing step lasts at least 60 seconds.
10. The method of claim 1 further comprising, following the annealing step, constructing a permanent support element on or over the first surface of the lamina, wherein, in the completed cell, the first amorphous silicon layer is disposed between the first surface of the lamina and the support element.
11. The method of claim 10 wherein the step of constructing a permanent support element comprises plating, and the permanent support element comprises metal.
12. The method of claim 10 wherein the step of constructing a permanent support element comprises:
applying a ceramic mixture to the first surface of the lamina; and
curing the ceramic mixture, wherein the permanent support element is ceramic.
13. The method of claim 10 wherein a reflective metal layer is disposed between the permanent support element and the first amorphous silicon layer.
14. The method of claim 13 wherein a transparent conductive oxide layer is disposed between the reflective metal layer and the first amorphous silicon layer.