WO2013053472A1

WO2013053472A1 - Method for the treatment of silicon wafers, treatment liquid and silicon wafers

Info

Publication number: WO2013053472A1
Application number: PCT/EP2012/004251
Authority: WO
Inventors: Knut Vaas; Berthold Schum; Agata Lachowicz
Original assignee: Schott Solar Ag
Priority date: 2011-10-12
Filing date: 2012-10-11
Publication date: 2013-04-18
Also published as: DE102011084346A1; TW201322326A

Abstract

The invention relates to a method for the treatment of silicon wafers, in particular as substrates for producing solar cells, wherein the surface of the silicon wafers is wetted with a treatment liquid, which contains water, at least one alkaline etching component and at least one etching additive from the substance class of cellulose ethers, and is textured in the process. The invention also relates to a corresponding treatment liquid, and to silicon wafers which have been produced by such a method or have been treated using such a treatment liquid.

Description

Process for treating silicon wafers, treatment liquid and silicon wafers

description

The invention relates to a method for the treatment of silicon wafers, as they are used in particular for the production of solar cells, wherein a treatment liquid is applied to the surface of the silicon wafer. Furthermore, the invention relates to a corresponding treatment liquid and silicon wafers which have been produced by such a method or have been treated with such a treatment liquid.

In order to obtain the most efficient solar cell, it is necessary to use as much of the incident light as possible for conversion into electric current. An essential factor for this is the antireflection coating

Solar cell surfaces to reflect as little as possible of the incident light again unused.

This is generally achieved by roughening the wafer and applying one or more antireflection coatings.

A roughening of the surface causes the light rays to be better coupled into the solar cell than on a smooth surface. This process is called surface texturing.

For reasons of cost, mainly wet-chemical etching processes are used today for texturing crystalline solar cells.

For multicrystalline silicon, the so-called acidic isotexture, a mixture of hydrofluoric acid and nitric acid, has become established. This texture etches trough-shaped structures into a silicon surface.

CONFIRMATION COPY Monocrystalline wafers are usually patterned in an alkaline etching mixture. In this case, the different etch rates are exploited on the different silicon crystal directions. The densest level, the

(111) - Crystal plane is etched the slowest, a (100) plane much faster. If one now etches a wafer surface with the crystal orientation <100>, then pyramidal structures in the <111> direction are formed, which allow a very good antireflection of the surface.

Common solutions are NaOH or KOH, but other basic compounds can be used as well.

The denser the pyramidal structures on the surface, the lower the reflection of the incident light and the better the light coupling into the silicon wafer and thus the energy yield of a solar cell produced therewith.

For a use of the solar cell is not only the low weighted

Reflection - with it, the reflection data are linked to the intensity distribution of the wavelengths of the solar spectrum - but also the

macroscopic homogeneity over the entire wafer, ie one

uniform optical appearance, important.

In order to achieve low manufacturing costs, a high throughput of the treatment process is important. Condition for this is a small one

Treatment time.

A variety of alkaline texture solutions for monocrystalline wafers are known. Most commonly, a mixture of water, KOH (or NaOH) and isopropanol is used. Typical exposure times are 30 minutes at temperatures around 80 ° C. Isopropanol plays an essential role as Ätzmoderator. Its function is to slow down the etch reaction and provide more seeds for pyramid etching. Furthermore, isopropanol decreases the surface tension of the solution, which improves the wetting of the etching solution to the wafer. As a result, the pyramid density increases on the surface, lower reflections can be achieved, and the homogeneity over the wafer improves. However, with the use of isopropanol, there are some fundamental disadvantages due to its properties.

The maximum process temperature is limited by the boiling point of isopropanol, which is at 82 ° C. However, higher process temperatures would be desirable since the reaction rate at temperatures around 80 ° C is only moderately fast for an industrial application. Especially for the

Continuous-flow system technology (inline systems) means that the reaction times of about 30 minutes required for the use of isopropanol are far too high.

An increase in the etching rate by increasing the lye concentration and / or the isopropanol is not helpful because with higher hydroxide concentrations, the etching rate increases too much and the texture quality suffers.

Nevertheless, in order to obtain the lowest possible process times, work is usually carried out close to the boiling point of the isopropanol, which leads to a high rate of evaporation thereof. This requires high replenishment, which, together with the disposal problem of organic solvents and their vapors causes high costs. Furthermore, in the

Plant engineering precautions to ensure the fire and explosion protection.

Thus, a replacement of isopropanol is in the focus of the search for less expensive and therefore less expensive textur mixtures that provide the same good or better texture results. For example, WO 2008/145231 describes an aqueous treatment solution to which water-soluble hydroxycarbonyls, hydroxyaromatics, aliphatic or alicyclic hydroxy compounds have been added.

WO 2009/071333 describes the addition of carboxylic acids, polyalcohols and

Hydroxyaromatic.

A disadvantage of the large classes of substances mentioned here is the toxicity and / or environmental compatibility of many of their specific compounds and the associated disposal problem.

Furthermore, surfactants are described as additives (see, for example, EP 1 890 338 A1). But they often have the disadvantage of strong foaming. In addition, these surfactants like surface-active substances like to remain on the surface, even after intensive rinsing. This can be critical for subsequent processes.

In JP 2005-19605 A, as etch moderator i.a. Cellulose acetate, a

Cellulose ester, proposed. However, cellulose esters are not water-soluble, so the process requires organic solvents that cause the same problems as isopropanol.

It was also the way to use no treatment liquids, solutions, but pastes with additives.

In WO 2004/032218 or WO 2004/023567 etching pastes are described, which are provided for structuring. Such pastes have the disadvantage that their structuring depths are limited to only a few 100 nm. A deeper structuring etch in the range of several m, as it is necessary for the texturing, is not possible with them.

Thus, these pastes are used for local structuring of thin layers such as the solar cell emitter, such as the selective emitter process or for an edge insulation. They offer alternatives to others

Masking methods such as laser processing or photolithography.

These pastes contain thickeners for viscosity adjustment, including cellulose and cellulose ethers. They contain only small Ätzstoffmengen and are applied only with small paste thicknesses. Therefore, a deep etching is not possible. A higher paste order does not solve this problem either. The slow diffusion of the substances prevents mass transfer over greater distances in periods relevant to production technology.

By comparison with e.g. aqueous solutions slow diffusion of the substances in the paste depletes the paste at the interface with the silicon relatively quickly to the etchant. As a result, the etching reaction is slowed to a complete Abebben. The removal of the etching products from the interface is also difficult.

The invention is therefore the object of an aforementioned

Method to provide a corresponding treatment liquid and a silicon wafer treated therewith, with which the problems of the prior art can be avoided. It should be dispensed with organic solvents and a homogeneous texturing with low reflection can be achieved.

This problem is solved by a method with the characteristics of

Claim 1, a treatment liquid having the features of claim 7 and a silicon wafer having the features of claim 9. Advantageous and preferred embodiments of the invention are the subject of the other claims. Some of the features listed below are only in connection with the process or the treatment liquid

however, they should apply to both as well as to the silicon wafer independently. The wording of the claims is incorporated herein by express reference. The method for treating silicon wafers preferably serves for the treatment of monocrystalline silicon wafers.

Preferably, the treatment liquid consists of the components mentioned as contained therein.

The aqueous treatment solution according to the invention or in the

aqueous treatment solution used according to the invention has as alkaline etching component in particular potassium hydroxide and / or

Sodium hydroxide and / or one or more amines, for. Ammonia,

Triethanolamine, ethylenediamine, ethanolamine, tetramethylammonium hydroxide. In addition to such salts acting as an alkaline etching component, the treatment solution preferably contains no metal salts with inorganic anions.

The proportion of the alkaline etching component in the

Treatment solution 1 wt .-% to 10 wt .-%, particularly preferably 2 wt .-% to 5 wt .-%.

The treatment process is carried out on a water basis. On organic solvents can be omitted.

The cellulose ethers used according to the invention as Ätzadditiv are

water soluble.

The concrete properties of the cellulose ethers depend on the nature of the substituents, the number of substituted hydroxy groups and the

Molecular size of the cellulose chain. Their water solubility depends on the type and number of ether groups introduced. While hydroxyethylcellulose with low substitution is still completely water-soluble, completely ethoxylated ethylcellulose can no longer be dissolved in water. If the Cellulose ether itself is insoluble in water, if necessary, its water-soluble salts, such as Na salts, are used. Therefore, in this patent application, the salts of cellulose ethers are included in the term cellulose ethers.

The most uniform possible substance property is achieved with the smallest possible molecular weight distribution of the polymeric cellulose chain.

The cellulose ethers used are non-toxic.

Preference is given to using methylhydroxycellulose and / or hydroxyethylcellulose and / or hydroxypropylcellulose and / or carboxymethylhydroxyethylcellulose and / or the sodium salt of carboxymethylcellulose.

Preference is given to using methylhydroxycellulose and / or hydroxyethylcellulose.

To carry out the process, the amount of cellulose ether in the treatment liquid may be between 10 ppm and 1%. Advantageously, between 100 ppm and 300 ppm of cellulose ether to the

Treatment liquid added.

Advantageously, the method is at a ggü. Increased room temperature

Temperature of the treatment liquid performed. This is the

Treatment liquid heated. Advantageously, it can be heated to a temperature between 80 ° C and 100 ° C, particularly advantageous to a temperature between 85 ° C and 98 ° C. This improves and speeds up the etching process.

Advantageously, the method is performed such that the duration of the

Treatment of the wafer surface with the treatment liquid between 5 minutes and 30 minutes, especially between 10 minutes and 20 minutes.

In this time, a sufficient, in number and depth of the pyramidal structures at least the state of the art texturing occurs.

At least for silicon wafers for solar cells, which are designed for unilateral incidence of light, it is sufficient to texture a page. This side is then completely wetted with treatment liquid, for example sprayed or dipped, advantageously with the side to be treated upwards. It is also possible to texture both sides of a silicon wafer, either in the aforementioned manner or by completely immersing the entire silicon wafer in a bath with the treatment liquid.

In order to both accelerate the etching process and to achieve a better etching result, it may be provided to relatively quickly renew or exchange or circulate the treatment liquid on the surface of the silicon wafer. For this purpose, a targeted flow against treatment liquid can take place, for example as a turbulent flow or by so-called swelling. This can be done by special nozzles or so-called

Surge devices e.g. with slots near the surface of the

Silicon wafers are made, even in a full bath, or generally by agitating the treatment liquid, for example by stirring.

Furthermore, it can be provided in the method according to the invention that the silicon wafers are aligned horizontally in the treatment with the

Treatment liquid, preferably in a continuous process. Such a horizontal method, in particular as a continuous process, offers over the known vertical dipping method with such

Treatment liquid with clocking operation great advantages, namely a better exchange of the treatment liquid on the wafer surface by the horizontal position and a faster treatment by the possibility of Continuous process. Therefore, the implementation of the method according to the invention is preferred as a continuous process.

With the described method for the treatment of wafers with the described treatment liquid structuring etchings of a few pm to 20 pm can be made, as it is necessary for an effective texturing.

With such a structuring etch, a weighted reflection in the range 400 nm-1000 nm of 11.0% to at most 12.5% can be achieved.

The wafer has a macroscopic homogeneity over the entire textured area, thus having a uniform optical appearance.

Due to the higher process temperatures, shorter process times and higher throughputs are possible, which is particularly advantageous in industrial continuous flow processes. By using a purely aqueous solution, ie the absence of isopropanol and any other organic solvents, the process is enormously simplified, since neither subsequent dosing nor disposal of isopropanol are necessary.

Due to the low concentration of the cellulose ethers as etching additive, which is sufficient to produce the same or better texturing results than with isopropanol or other etching additives, the treatment solution is very

inexpensive.

For better understanding and to clarify the invention, two exemplary embodiments are described below. Embodiment 1

In Embodiment 1, a 156 mm x 156 mm (100) Cz "as cut" p-type, monocrystalline wafer in an aqueous texture solution consisting of 4 wt% KOH (ultra-pure or electronic-grade) and 00 ppm

Hydroxyethylcellulose (average molecular weight of 1,500,000), in

demineralized water for 20 min at 88 ° C etched.

Result:

This gives a macroscopically homogeneous texture whose pyramidal structures have on average base lengths of 3 pm-6 pm (see Fig. 1).

The reflection minimum of the texture is 960 nm and is 9.6% (s.

Fig.2).

The weighted reflection from 400 nm to 1000 nm is 11.5%.

(Measured with Cary5000 UV-VIS spectrometer from Varian and calibrated

White-Standard)

Embodiment 2:

In Embodiment 2, a 156 mm x 156 mm (100) Cz "as cut" p-type monocrystalline wafer in an aqueous texture solution consisting of 6 wt% KOH (ultra-pure or electronic-grade) and 150 ppm

Hydroxyethylcellulose (average molecular weight of 1,500,000), in

demineralized water at 95 ° C etched for 12 min.

Result:

This gives a macroscopically homogeneous texture whose pyramidal structures have on average base lengths of 2 pm-5 pm.

The reflection minimum of the texture is 965 nm and is 9.8%.

The weighted reflection from 400 nm to 1000 nm is 11, 9%.

Claims

claims

1. A method for treating silicon wafers, in particular as

Substrates for the production of solar cells, wherein the surface of the silicon wafer is wetted with treatment liquid and thereby textured, characterized in that the treatment liquid contains water, at least one alkaline Ätzkomponente and at least one Ätzadditiv from the class of cellulose ethers.

2. The method according to claim 1,

characterized,

that the alkaline etching component is sodium hydroxide and / or potassium hydroxide.

3. The method according to claim 1 or 2,

characterized,

in that the etching additive is methylhydroxycellulose and / or hydroxyethylcellulose.

4. The method according to any one of the preceding claims,

characterized,

the proportion of the etching additive in the treatment liquid is between 10 ppm and 1%, preferably between 100 ppm and 300 ppm.

5. The method according to any one of the preceding claims,

characterized,

that the treatment liquid has a temperature between 80 ° C and 100 ° C, preferably between 85 ° C and 98 ° C when carrying out the method. Method according to one of the preceding claims,

characterized,

the duration of the treatment with the treatment liquid is between 5 minutes and 30 minutes, preferably between 10 minutes and 20 minutes.

Treatment liquid for the treatment of silicon wafers, in particular as silicon wafer for the production of solar cells, for wetting and texturing the surface of the silicon wafer with the

Treatment liquid,

characterized,

the treatment liquid contains water, at least one alkaline etching component and at least one etching additive from the substance class of the cellulose ethers.

Treatment liquid according to claim 7,

characterized,

the proportion of the additive in the treatment liquid is between 10 ppm and 1%, preferably between 100 ppm and 300 ppm.

Silicon wafers, in particular for the production of solar cells,

characterized,

in that it has been treated by a method according to one of claims 1 to 6 or by a treatment liquid according to one of claims 7 or 8.