BACKGROUND OF THE INVENTION
The present invention relates to a method for the wet chemical pyramidal texture etching of silicon surfaces.
A textured surface reduces the reflection of incident light across wide bands, thereby increasing the absorbed light intensity. When manufacturing crystalline silicon solar cells, a systematic structuring of the silicon surface leads to an increase in solar cell performance. Such a wet chemical structural etching is not, however, restricted just to the processing and production of solar cells, but is also suitable for optical and electrochemical detectors/sensors, biodetectors/biosensors, catalysts, electrodes and the like.
Known techniques for the wet chemical pyramidal structural etching of (100)-oriented silicon surfaces use alkaline media, mainly with solutions of alkali hydroxides, alkali carbonates, ammonia or choline. Solutions of hydrazine or ethylene diamine/pyrocatechol can also be used, but they are disadvantageous on account of their toxicity. The most common recipes comprise water, sodium or potassium hydroxide and alcohol. The alcohol component used is either ethylene glycol or isopropanol. Nevertheless, the known methods for the wet chemical structural etching of silicon merely relate, purely and simply, to the generation of a pyramidal (tetragonal) texture.
In terms of fine line metalization and the application of doping resists or photoresist, however, a texture that exhibits small pyramids is desirable. A method based on an etching solution that contains ethylene glycol is described in EP 0 477 424 A1. The etching solution recipe used here contains water, potassium hydroxide, ethylene glycol and silicon. Oxygen is also added as a further component. After the silicon surface has undergone wet chemical structural etching, reproducibly uniform pyramids are obtained by aerating the etching solution with oxygen. The pyramid height can be varied by the aerating duration of the etching solution. Introducing oxygen over a longer period, i.e. greater oxidation of the glycol, results in smaller pyramids. In this way, pyramid sizes of ≦2 μm can be produced even in terms of manufacturing engineering. But the disadvantage is that the etching solution based on ethylene glycol cannot be used ad hoc because it requires a preceding dissolution of silicon. Studies have shown that a subsequent rest phase of several hours proves to be beneficial for formation of smooth (111) faces. A simple addition of silicate solution does not lead to success.
Methods which use an etching solution based on isopropanol are known for example from U.S. Pat. No. 3,998,659, from “Uniform Pyramid Formation on Alkaline-etched Polished Monocrystalline (100) Silicon Wafers”, Bressers et al., Progress in Photovoltaics, Vol. 4, 435-438 (1996), and from “Experimental Optimization of an Anisotropic Etching Process for Random Texturization of Silicon Solar Cells”, King et al., IEEE 1991, 303-308. In contrast to the etching solution based on ethylene glycol, an IPA-based etching solution can be used immediately for the purpose of texture etching. This etching solution can be used with and without silicate. This type of solution nevertheless suffers from the drawback of a high rate of evaporation, on account of the isopropanol's boiling point of just 82° C. This means that problems arise with regard to uniform etching and reproducibility of the pyramidal surface. In contrast to the etching solution based on ethylene glycol, the IPA-based etching solution tends toward large pyramids. According to Bressers et al., small pyramids can be achieved by using nitrogen to displace the dissolved oxygen during the etching process. But there are reports of laboratory experiments in which a 10 vol. % isopropanol etching solution, a nitrogen flow of 25 l/min and a process temperature of 80° C. are used. An enormous consumption of isopropanol must be expected for production application in tanks. This entails high costs since the durability of the etching solution is low on account of such a high volatilization of IPA. This simultaneously leads to high waste-disposal costs for the spent chemicals.
As regards the industrial production of solar cells, it is important to guarantee a constantly good electrical quality of the solar cells produced in large quantities. Constant good quality in terms of the shape of the pyramid structure and a uniform structuring of the total silicon surface must be ensured. The ability to vary the pyramid size within a certain range is also advantageous.
Neither the etching solution based on ethylene glycol nor the isopropanol-based solution are able, however, to meet these requirements simultaneously to a satisfactory extent. As summarized above, the aqueous alkaline etching solution to which ethylene glycol is added is indeed able to produce pyramids with a height of ≦2 μm, but it cannot be used immediately. Yet precisely this is a drawback as regards industrial use. Although the etching solution that contains isopropanol can, on the other hand, be used immediately for texture etching, it does tend toward large pyramids and proves to be disadvantageous as a result of the isopropanol's high rate of evaporation, since this impairs the reproducibility of a uniform pyramidal texture.
The object of the present invention is therefore to provide a method for the wet chemical pyramidal texture etching of silicon surfaces which is able—analogous to the etching solution based on ethylene glycol—to generate reproducibly small (≦2 μm) pyramid sizes, with different sizes of pyramids being adjustable within a certain range or scatter, while simultaneously ensuring complete texturization of the silicon surface and at the same time reducing costs.
SUMMARY OF THE INVENTION
In an embodiment, the present invention provides a new and improved etching solution used for the pyramidal structuring of the silicon surface which comprises both isopropanol and an aqueous alkaline ethylene glycol solution. The aqueous alkaline ethylene glycol solution is advantageously reacted with oxygen, as is known for example from EP 0 477 424 A1. The greater the amount of oxygen reacted, the smaller the amount of ethylene glycol solution used in the etching solution. The object is surprisingly solved just by the simultaneous use of isopropanol and ethylene glycol in the etching solution, which also entails the following advantages. At the start of etching, the use of water which is already preheated and is hence low in oxygen—as also described by Bressers et al.—has a supportive effect in terms of small pyramids, but is not absolutely necessary because it can be compensated by a corresponding correction in the ratio of glycol. Preheated isopropanol—also described in Bressers et al.—is problematic for reasons of production technology.
The etching solution according to the invention is capable of texture immediately after production, i.e. an etching process can be performed directly after the solution's production.
The etching result is also relatively insensitive to the exact composition of the etching solution, i.e. precise optimization of the ratio of the individual dissolved components is not necessary, which is beneficial particularly in terms of industrial application.
Another advantage is the good reproducibility of the etching result. At the same time, the pyramid size is adjustable, while simultaneously ensuring complete texture of the silicon surface.
A further advantage is obtained in that the present invention's etching solution does not suffer as much from the disadvantage of a conventional etching solution based just on isopropanol, i.e. a considerable rate of evaporation. This means that the etching solution according to the invention can be used for etching over a much longer period than a conventional solution that is based on isopropanol alone. The etching solution therefore has to be replaced less frequently. This lowers costs since downtime is decreased, thus reducing the amount of waste chemicals to be disposed of.