Metal catalyst technique for texturing silicon solar cells

Details Metal catalyst technique for texturing silicon solar cells Metal catalyst technique for texturing silicon solar cells

2000-08-09

2001-12-11

Powell, William A. (Department: 1765)

Semiconductor device manufacturing: process

Chemical etching

Vapor phase etching

C216S002000, C216S011000, C216S067000, C216S079000, C438S719000, C438S735000

Reexamination Certificate

active

06329296

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
This invention pertains generally to the field of multicrystalline silicon solar cells and manufacturing techniques directed to yield improvements in optical performance of silicon solar cells. More specifically, the invention pertains to use of reactive ion etching (RIE) in the presence of metal catalysts to texture the surface of silicon solar cell material. The techniques described can controllably generate randomly distributed surface features optimally having dimensions comparable, or smaller than solar spectrum wavelengths.
2. Description of the Related Art
Multicrystalline silicon (mc-Si) is a frequently-used semiconductor substrate in the manufacture of silicon solar cells. Single crystal silicon (c-Si) is also used in manufacture of silicon solar cells, but it has certain disadvantages including the fact that it is more expensive than mc-Si. U.S. Pat. No. 6,091,021 describes some aspects of mc-Si solar cell fabrication developed by the present inventors, including use of micromasks (for example, a thin polycrystalline coating such as SiO
2
) to aid in etched texturing of mc-Si surfaces. In that patent, techniques are disclosed which demonstrate that minute, randomly distributed micro-craters and pyramidal structures can be formed in the surface of the emitter of a solar cell (or any silicon surface, for that matter). This is accomplished, according to that patent, by performing a deposition of SiO
2
, for example, with standard chemical vapor deposition methodologies in common use in the photovoltaics industry. As taught by the '021 patent, the “micro-mask” created by such deposition allows etchant (e.g., SF
6
/O
2
) to pass through micro-gaps to selectively etch exposed Si in the gap locations before reaching Si underneath the microcrystals. As a result, a random texturing of the Si surface is achieved. These randomly textured surfaces significantly reduce surface reflection over the usable spectral range, thus, increasing the light absorption in the solar cells. This, in turn, increases the likelihood that additional electron-hole pairs will be generated within the silicon matrix as a result of the photovoltaic effect.
Additional relevant background information can be found in U.S. Pat. No. 6,091,021, which is hereby incorporated by reference, in its entirety. Although the '021 patent represents one approach to silicon (especially, mc-Si) texturing, the need remains for more flexibility in the available texturing technologies compatible with solar cell manufacturing techniques. The present invention offers alternate processes aimed at engineering randomly textured surfaces compatible with existing etching methods and standard solar cell manufacturing processes. The present invention, according to at least one embodiment, also can be adapted to allow optimal random texturing of silicon solar cell material processed at a high throughput rate.
BRIEF SUMMARY OF THE INVENTION
The technology described in this disclosure is compatible with various manufacturing processes and applications for which surface-textured silicon is needed or advantageous. Various objects and advantages can be attained as a result of use of the present invention in which metal “impurities” (particles) are intentionally introduced into the environment inside a reactive ion etching (RIE) chamber while the random ion etching of the silicon surface takes place. As more fully explained in the detailed description, tailored texturization can be engineered according to one embodiment by a strategic placement of metal-coated silicon samples at various locations relative to the surface of the silicon wafer inside the RIE chamber. Some of the suitable metal species include, for example, Cr, Au, Cu, Al, Pd, and Ti.
According to another embodiment, desired texturing results and high throughput can be achieved by conditioning the RIE chamber with a suitable choice of metal catalysts. This can include, for example, forming a thin layer of metal, or metals on the inner surfaces of the chamber prior to RIE texturing of silicon in the chamber. (Suitable metal species include those just mentioned in conjunction with the embodiment utilizing metal-coated Si samples.)
Metal thus introduced into the RIE chamber has the effect of introducing “micro-masks” (or microscopic particles of metal) into the RIE environment. Metal present in the reaction chamber “catalyzes” the formation of random nanoscale features on the surface of silicon etched in the chamber, thus resulting in an optically-favorable texturing beneficial for solar cell applications. Silicon surfaces textured in this fashion include, for example, columnar and pyramidal features with dimensions that can be much smaller than the wavelength of incident light. This sub-wavelength fabrication technology enables creation of a layer of a tailored index of refraction, spanning the range defined by the refractive indices of air and silicon. Therefore, the invention enables fabrication of an optimum anti-reflective layer on, for example, a mc-Si solar cell, to optimize photon absorption over a broad spectral range and enhance solar cell efficiency. Moreover, the processes of the invention induce only slight surface damage to the Si, if any, which does not significantly diminish current extraction from the solar cell. Such damage, if it occurs, can be easily removed without compromising the optically favorable texture features.
It is noted that the terms “catalyze”, “catalysis”, “catalyst” and other similar words and phrases used throughout this disclosure refer and relate to the enabling or facilitating effect that certain metals have on random texturing of Si, when they are added to the RIE environment. It is not certain, at this time, whether a classic chemical catalysis is involved, wherein the plasma process is changed due to alteration of reaction pathways. Indeed, the precise chemical and/or physical mechanism(s) responsible for the formation of desirable surface texture features evidently mediated by the presence of metal in the chamber are not presently known, although studies aimed at a complete understanding of underlying physical mechanisms are underway. Rather, for purposes of this application, “catalysis” and other related language is intended to convey the fact that, although surface texturing of silicon can be accomplished using techniques other than those described herein, speed, control and quality of results are all significantly enhanced by introducing the metals in the fashion herein disclosed.
Accordingly, an advantage of the invention is that it provides a method forming a textured surface on a substrate by generating randomly distributed surface features thereon.
Another advantage of the invention is that it includes exposing a surface of such substrate to a reactive ion etching plasma comprising at least one etchant and particles of metal introduced into the plasma.
Another advantage of the invention is that the techniques of the invention result in nanoscale surface texture features having dimensions smaller than the wavelength of light in the solar spectrum (meaning, specifically, light that is capable of causing the photovoltaic effect in silicon solar cells).
Yet another advantage of the present invention is that metal catalysts can be introduced in to a reactive ion etching chamber in various ways including by placing discrete metal sources in the chamber in the presence of a substrate to be textured, and pre-conditioning the interior surfaces of a reactive ion etching chamber using, for example, a thin coating of metal.
Upon further study of the specification and appended claims, further objects and advantages of the invention will become apparent to those skilled in the art.


REFERENCES:
J. I. Gittleman, E. K. Sichel, H. W. Lehmann and R. Widmer, Textured silicon: A selective absorber for solar themal conversion, 1979 American Institute of Physics, Appl. Phys. Lett. 35(10), Nov. 15, 1979, accepted for publication Sep. 4, 1979, pp. 742-744.
Saleem H. Zaidi, James M. Gee, Dougla

Affiliated with

Also associated with

Rating

Metal catalyst technique for texturing silicon solar cells does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Metal catalyst technique for texturing silicon solar cells, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Metal catalyst technique for texturing silicon solar cells will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2597128