The present invention relates to a method for doping one side of a semiconductor body, especially for doping one side of silicon wafers.
In the production of semiconductor structural elements, a targeted doping of the substrate or of a semiconductor layer is necessary in order to provide for the desired conductive properties. For some applications it is advantageous to perform doping on one side, i.e. to introduce the doping substance by diffusion into the body that is to be doped from one side only while protecting the other side from unintended doping.
A method of this sort is required for example in the production of solar cells having what is called a back-surface field. In this case, a highly doped layer is deposited on the back side of the solar cell in order to generate a field to counter the diffusion of minor charge carriers (back-surface field).
A known method for doping of one side, as described for example in PCT application WO96/28851, provides that first a protective layer (e.g. a protective lacquer) be deposited on the side of semiconductor body that is not to be doped, or that a thermal oxide is grown on both sides that subsequently is removed by etching one side. Then, the unprotected side of the semiconductor body may be doped for example out of the gaseous phase inside a diffusion oven. A doping layer can also be applied to the side to be doped, which is not covered by a protective cover. The doping agent contained in the doping lacquer then diffuses within one temperature step. After the diffusion process, the protective layer is removed by etching.
One possibility to enable doping on one side of a semiconductor body is to apply to both sides a protective layer, for example by thermally oxidizing the semiconductor body and then separating from one side the protective film disposed on the side of the semiconductor body that is to be doped. This side can then also be doped out of the gaseous phase within a diffusion oven, as described above, or a doping layer (e.g. doping lacquer or doping paste) may be applied to the side of the semiconductor body that is to be doped and may be diffused thereinto by a subsequent temperature treatment. After the diffusion step, again the protective layer is removed by etching.
In order to achieve regular, uniform properties and a reliable functioning of semiconductor structural elements produced in this manner, uniform homogenous doping is necessary. One tear formation or a bursting off of the doping lacquer that has been applied and dried or any damage to the doping agent will negatively affect the functioning and reliability of the structural element.
The present invention therefore pursues the object of making available a method for doping of one side of a semiconductor body, especially of a silicon wafer, enabling a reliable homogenous doping of the desired side while simultaneously reliably protecting the side that is not to be doped.
The object is solved according to the present invention by the method described in claim 1. Advantageous embodiments of the invention are derived from the subclaims.
A central thought here is that the doping agent contained within the doping layer passes through a conventional oxide layer during the diffusion process and diffuses into the side to be doped, where the oxide layer simultaneously serves as a protective coat on the side that is not to be doped and prevents penetration of doping agent on this side.
By applying doping lacquer onto an oxide layer that has been deposited beforehand, a better lateral distribution of the doping agent is achieved and a xe2x80x9cdoping throughxe2x80x9d the oxide layer positively supports a homogenous doping.
In addition, the inventive method also provides the advantage that in the case of a non-homogenous spreading of doping agent upon treatment or coating, especially on rough surfaces (e.g. with textured solar cells), homogenous doping is also guaranteed.
In the following, the invention is described using individual method steps and with reference to the attached diagrams.