Method of making diffused doped areas for semiconductor componen

Details Method of making diffused doped areas for semiconductor componen Method of making diffused doped areas for semiconductor componen

1993-01-14

1995-10-24

Chaudhuri, Olik

Fishing, trapping, and vermin destroying

437164, 148DIG7, H01L 21385, H01L 21225

Patent

active

054610028

DESCRIPTION:

BRIEF SUMMARY
The invention concerns a method of making doped areas on semiconductor components, preferably solar cells. The invention additionally concerns a solar cell with incorporated bypass diode, made according to the invention.
A solar cell basically consists of a thin silicon wafer having a single large area of typically a PN junction which covers one side of the wafer facing the source of light. The photo-excited charge carriers flow to front side metal contact and back side metal contact, respectively, the former having a geometry such that substantially maximum collection of the charge carriers is accomplished, while the area of the metal contact covering the surface of the cell and thereby blocking the light is minimized. The described solar cell is monofacial, i.e. the solar cell is only photo-active with respect to light impinging on the front side of the cells. By additionally providing a thin junction layer on the back side of the cell, this too may be made photo-active to form a bifacial solar cell. The solar cells may either have an intrinsic, P or N type substrate, while the doped layer or layers may either be N.sup.+ or P.sup.+.
It is known in the art to use an oxide containing dopant. The U.S. Pat. No. 4,101,351 describes how an oxide is grown on a crystal, following which parts of the crystal are exposed again to oxide to define the active areas. This exposure typically takes place by adding to this oxide coated crystal a mask layer resistant to hydrofluoric acid, whereby the oxide layer will be etched away with hydrofluoric acid in the areas where the crystal is not coated with a photoresist layer. The crystal areas thus exposed can then be doped after removal of the photoresist layer by diffusion of a suitable substance into the crystal at a high temperature, typically in the region about 1000 .degree. C. The dopant source material may be added by depositing on the exposed area of the crystal a layer of e.g. silica containing dopant source material. No undesirable auto-doping will take place since the rest of the crystal is protected with the oxide layer. The oxidation, photo-resist masking, etching and doping process can be repeated several times so that the crystal will have the desired number of doped areas. The stated method has several shortcomings, including e.g. many complicated process steps, such as growing of the oxide on the surface of the crystal, which requires a temperature treatment of about 1000.degree. C., as well as requirements concerning pure surroundings in either pure oxygen or water vapour. Exposure of parts of the crystal surface normally requires the use of hydrofluoric acid which is an extremely difficult chemical to work with because of its toxicity.
The objective of the invention is to provide a method of making doped areas on semiconductor components, preferably solar cells.
This objective is achieved by applying to a portion of the surface of a semiconductor substrate an oxide forming mask layer containing dopant, said semiconductor substrate with applied mask layer being heated to a temperature sufficient for diffusion of part of the dopant from the mask layer to the semiconductor substrate, where also undesirable auto-doping of the naked surface of a semiconductor substrate takes place during the doping process. The auto-doped areas of the semiconductor substrate are etched away e.g. by alkaline etching, including choline, potassium or sodium hydroxide or plasma etching, while the mask layer constitutes a protection barrier for the doped areas below the mask layer. This makes the use of photoresist masking superfluous, since the employed dopant source layers of silica satisfy several functions and thus also serve as a protection barrier for a doped area in subsequent doping processes. Further, the number of high temperature steps is reduced so that it substantially corresponds to the number of doping processes and may even be smaller than this number if a dopant source layer also serving as a barrier to a gas phase source is used. This is a significant advantage si

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