DE102011084346A1 - Process for treating silicon wafers, treatment liquid and silicon wafers - Google Patents

Abstract

The invention relates to a method for the treatment of silicon wafers, in particular as substrates for the production of solar cells, wherein the surface of the silicon wafer with a treatment liquid containing water, at least one alkaline etching component and at least one etching additive from the class of cellulose ethers wets, thereby texturizing becomes. The invention also relates to a corresponding treatment liquid and silicon wafers produced by such a process or treated with such a treatment liquid.

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 anti-reflection of the solar cell surfaces in order to reflect as little as possible of the incident light again unused.

This is generally accomplished by roughening the wafer and applying one or more anti-reflective layers.

Roughening 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.

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 plane, 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 use of the solar cell, not only is the low weighted reflection - in which the reflection data is linked to the intensity distribution of the wavelengths of the solar spectrum - important, but also the macroscopic homogeneity over the entire wafer, ie a uniform optical appearance.

In order to achieve low manufacturing costs, a high throughput of the treatment process is important. Condition for this is a short 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 continuous-flow systems (inline systems), 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 high costs. Furthermore, provision must be made in the system technology for 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, in WO 2008/145231 describes an aqueous treatment solution to which water-soluble hydroxycarbonyls, hydroxyaromatics, aliphatic or alicyclic hydroxy compounds are added. WO 2009/071333 describes the addition of carboxylic acids, polyalcohols and hydroxyaromatics.

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 is proposed as Ätzermerator et al. Cellulose acetate, a cellulose ester. 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 etch pastes are provided, which are intended for structuring. Such pastes have the disadvantage that their structuring depths are limited to only a few 100 nm.

A deeper Strukturierungsätzung in the range of several microns, as it is necessary for the texturing, is not possible with them.

Thus, these pastes serve to locally pattern thin films such as the solar cell emitter, e.g. in the selective emitter process or for edge isolation. They thus offer alternatives to other 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 an object of the invention to provide an aforementioned method, 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 object is achieved by a method having the features 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 described only in the context of the process or the treatment liquid, but 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.

The aqueous treatment solution according to the invention or the aqueous treatment solution used in the process according to the invention has, as the alkaline etching component, in particular potassium hydroxide and / or sodium hydroxide and / or one or more amines, eg. As ammonia, triethanolamine, ethylenediamine, ethanolamine, tetramethylammonium, on.

The proportion of the alkaline etching component in the treatment solution is preferably 1% by weight to 10% by weight, particularly preferably 2% by weight to 5% by weight.

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 an etching additive are water-soluble.

The specific 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, it may be possible to resort to its water-soluble salts, for example Na salts. 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 is added to the treatment liquid.

Advantageously, the method is at a ggü. Room temperature increased temperature of the treatment liquid performed. For this purpose, the treatment liquid is 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 accelerates the etching process.

Advantageously, the method is carried out such that the duration of the treatment of the wafer surface with the treatment liquid is between 5 minutes and 30 minutes, particularly advantageously 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 achieved by special nozzles or so-called baffles e.g. with slits in the vicinity of the surface of the silicon wafers, also in a full bath, or generally by moving 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 immersion method with such a treatment liquid with clocking operation great advantages, namely a better replacement of the treatment liquid on the wafer surface by the horizontal position and a faster treatment by the possibility of the continuous process. Therefore, the implementation of the method according to the invention is preferred as a continuous process.

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

With such patterning etching, 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 re-dosing nor disposal of the isopropanol are necessary.

Due to the low concentration of cellulose ethers as an 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 × 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 100 ppm Hydroxyethylcellulose (average molecular weight of 1,500,000), etched in demineralized water for 20 min at 88 ° C.

Result:

This gives a macroscopically homogeneous texture whose pyramidal structures have average base lengths of 3 μm-6 μm (s. ).

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

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 × 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 hydroxyethyl cellulose (average molecular weight of 1,500,000), etched in demineralised water at 95 ° C for 12 min.

Result:

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

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2008/145231 [0020]
• WO 2009/071333 [0020]
• EP 1890338 A1 [0022]
• JP 2005-19605 A [0023]
• WO 2004/032218 [0025]
• WO 2004/023567 [0025]

Claims (9)

Process for the treatment of silicon wafers, in particular as sub strate 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 is water, at least one alkaline etching component and at least one etching additive from the class of cellulose ethers contains. Process according to Claim 1, characterized in that the alkaline etching component is sodium hydroxide and / or potassium hydroxide. Method according to claim 1 or 2, characterized in that the etching additive is methylhydroxycellulose and / or hydroxyethylcellulose. Method according to one of the preceding claims, characterized in that the proportion of Ätzadditivs in the treatment liquid is between 10 ppm and 1%, preferably between 100 ppm and 300 ppm. Method according to one of the preceding claims, characterized in that the treatment liquid in carrying out the method has a temperature between 80 ° C and 100 ° C, preferably between 85 ° C and 98 ° C. Method according to one of the preceding claims, characterized in that the duration of 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 in that the treatment liquid contains water, at least one alkaline etching component and at least one etching additive from the class of cellulose ethers. Treatment fluid according to claim 7, characterized in that the proportion of the additive in the treatment liquid is between 10 ppm and 1%, preferably between 100 ppm and 300 ppm. Silicon wafer, 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 with a treatment liquid according to one of claims 7 or 8.

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