TW201322326A - Method for processing silicon wafers, processing liquid and germanium wafer - Google Patents

Abstract

The present invention relates to a method for processing a germanium wafer, which is specifically used as a substrate in solar cell manufacturing, wherein a surface of the germanium wafer is wetted with a processing liquid and the surface of the germanium wafer is woven. The treatment liquid comprises water, at least one alkaline etching component, and at least one etching additive selected from the group consisting of cellulose ethers. The invention further relates to a corresponding treatment liquid and a tantalum wafer produced by or treated with the treatment liquid.

Description

Method for processing silicon wafers, processing liquid and germanium wafer

The present invention relates to a method of processing a tantalum wafer for use in the manufacture of solar cells, wherein a treatment liquid is applied to the surface of the tantalum wafer. Further, the present invention relates to a corresponding treatment liquid and a tantalum wafer processed by the preparation method or the treatment liquid.

In order to greatly increase the efficiency of solar energy, it is necessary to convert as much incident light as possible into a current. The focus is on making the surface of the solar cell anti-reflective so that incident light that is reflected off without being used is minimized.

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

The roughened surface can better couple the beam into the solar cell compared to a smooth surface. This method is called surface texturing.

In view of cost reasons, the wet chemical etching method is mainly used to carry out the texturing treatment of the twinned solar cell.

Polycrystalline germanium is commonly used as a so-called acid-etching texturing agent (saure Isotextur, a mixture of hydrofluoric acid and nitric acid). Such a texturing agent can etch a grooved structure on the surface of the crucible.

Single crystal germanium wafers are typically subjected to a structuring process in an alkaline etching mixture. Here, the different crystal orientations of the crucibles use different etching rates. The face with the highest density, the (111) facet, has the slowest etch rate and the (100) face is much faster. If the surface of the wafer is etched with a <100> crystal orientation, a pyramid structure will be formed up <111>, which will make the surface Excellent anti-reflection ability.

A solution containing NaOH or KOH is usually used, but other basic compounds can also be used.

The higher the pyramid structure density of the surface, the lower the reflectivity of the incident light, and the better the optical coupling effect of the germanium wafer, the higher the energy production of the solar cell made of the germanium wafer.

Low reflectivity (reflection data is related to the intensity distribution of the solar spectrum wavelength) and macroscopic homogeneity throughout the wafer (ie, uniform visual appearance) have a significant impact on the use of solar cells.

In order to reduce the manufacturing cost, it is necessary to increase the processing amount of the processing method. To do this, the processing time must be shortened.

There are a variety of conventional alkali corrosion texturing solutions for single crystal germanium wafers. The most common is a mixture of water, KOH (or NaOH) and isopropanol. At a temperature of about 80 ° C, the general action time is 30 minutes.

Isopropanol plays an important role as an etch rate reducer here. Its efficacy is to slow down the etching reaction and provide more seed for pyramid etching. In addition, isopropyl alcohol reduces the surface tension of the solution, thereby improving the wetting effect of the etching solution on the wafer. This increases the surface pyramid density, reduces reflection, and improves wafer homogeneity. However, the properties of isopropyl alcohol have several fundamental disadvantages during use.

The maximum processing temperature is limited by the boiling point of isopropanol (82 ° C). However, it is desirable to achieve higher processing temperatures because the reaction rate achieved at temperatures around 80 °C is only moderately high for industrial applications. Especially for continuous technology In the art system (one-line system), the action time required for about 30 minutes when using isopropyl alcohol is too long.

Increasing the etching rate by increasing the lye concentration and/or isopropyl alcohol is less helpful because the increase in the etch rate is excessively large as the hydroxide concentration is increased, which adversely affects the quality of the velvet.

However, in order to obtain as short a treatment as possible, it is usually operated near the boiling point of isopropanol, which causes isopropanol to produce a higher evaporation rate. In this case, a large amount of feed must be added, and the disposal of organic solvents and their vapors is costly. In addition, preventive measures such as fire and explosion protection should be considered in the technical system.

Therefore, finding an alternative to isopropanol is the focus of finding a simpler, lower cost, lint blend that provides the same or better texturing effect.

For example, WO 2008/145231 describes an aqueous treatment solution for the addition of water-soluble hydroxycarbonyl, hydroxyaromatic, aliphatic or alicyclic hydroxy compounds. WO 2009/071333 describes the addition of carboxylic acids, polyols and hydroxyaromatic compounds.

However, the shortcomings of the above-mentioned major types of substances are that many of the specific compounds of these substances have toxicity and/or environmental compatibility problems, thereby causing waste disposal problems.

Surfactants are also used as additives (see, for example, EP 1 890 338 A1). However, surfactants often have a large number of blistering defects. In addition, such substances remain as surface-active substances even after thorough rinsing. On the surface. This point will affect subsequent processes.

JP 2005-19605 A proposes the use of cellulose acetate (a cellulose ester) as an etch rate reducing agent. However, the cellulose ester is insoluble in water, and an organic solvent is required for the treatment, resulting in the same problem as isopropanol.

It is also within the skill of the art to use a slurry with an additive instead of a treatment liquid and a method of treating the solution.

An etching paste for use as a structured treatment is described in WO 2004/032218 or WO 2004/023567. The disadvantage of these slurries is that their structural depth is limited to hundreds of nanometers. Deeper structured etching on the order of a few microns required for texturing is not achieved.

The slurries are used to form thin layers in a locally structured manner, such as solar cell emitters, for example for selective emitter technology or edge isolation. Therefore, the pastes provide an alternative to other masking methods such as laser processing or photolithography.

The above slurry contains a thickening agent for adjusting the viscosity, specifically cellulose and cellulose ether. These sizing agents have a low etchant content and a small coating thickness. Therefore, a large depth of etching cannot be achieved. Even increasing the amount of slurry applied does not solve this problem. Material diffusion is slow and it is difficult to achieve a large distance of material exchange during the production technology cycle.

Since the diffusion of the material in the slurry is slower than, for example, an aqueous solution, the rate of etchant depletion of the slurry at the interface to the crucible will be relatively fast. Therefore, the etching reaction rate is slowed down until it is completely stopped. It also increases the difficulty of removing the etching product from the interface.

In view of the above, it is an object of the present invention to provide a method superior to the foregoing, a corresponding treatment liquid, and a tantalum wafer treated with the treatment liquid, in order to avoid the problems of the prior art. The present invention dispenses with organic solvents and is intended to achieve homogeneous texture with low reflection.

The solution for achieving the above object of the present invention is a method having the features recited in claim 1 of the patent application, a treatment liquid having the features recited in claim 7 and a method of claim 9 Features a silicon wafer. Advantages and preferred technical solutions of the present invention can be found in other claims. Some of the features described below are only relevant to the method or the treatment fluid when described, but the features are applicable to both, as well as to the tantalum wafer. The words in the scope of the patent application become the contents of the specification by explicit reference.

The method of processing a germanium wafer is preferably used to process a single crystal germanium wafer.

The treatment liquid is preferably composed of the components contained therein.

The aqueous treatment solution of the present invention or the aqueous treatment solution used in the method of the present invention specifically has potassium hydroxide and/or sodium hydroxide and/or one or more amines such as ammonia, triethanolamine, ethylenediamine, ethanolamine, and tetra Methyl ammonium hydroxide is used as an alkaline etching component. In addition to the above-mentioned salts which function as an alkaline etching component, the treatment solution is preferably a metal salt which does not contain an inorganic anion.

The content of the alkaline etching component in the treatment solution is preferably from 1 wt% to 10 wt%, particularly preferably from 2 wt% to 5 wt%.

This treatment method is based on water. Organic solvents can be discarded.

The cellulose ether used as an etching additive in the present invention is soluble in water.

The specific characteristics of the cellulose ether are related to the type of substituent, the number of substituted hydroxyl groups, and the molecular weight of the cellulose chain. Its water solubility is related to the type and amount of ether groups added. Hydroxyethyl cellulose is still completely soluble in water in the case of low substitution, and ethyl cellulose is no longer soluble in water after complete ethoxylation. In the case where the cellulose ether itself is insoluble in water, a water-soluble salt of a cellulose ether such as a sodium salt may be used as the case may be. Thus, in the present patent application, the concept of cellulose ether also includes salts of cellulose ethers.

By using a polymer cellulose chain having a concentrated molecular weight distribution, it is possible to obtain uniform material properties as much as possible.

The cellulose ether used is non-toxic.

Preference is given to using sodium hydroxymethylcellulose and/or hydroxyethylcellulose and/or hydroxypropylcellulose and/or sodium carboxymethylhydroxyethylcellulose and/or sodium carboxymethylcellulose.

Preferably, hydroxymethylcellulose and/or hydroxyethylcellulose are used.

In order to carry out the process, the content of the cellulose ether in the treatment liquid may be between 10 ppm and 1%, preferably 100 ppm to 300 ppm of cellulose ether is added to the treatment liquid.

Preferably, the process is carried out at a temperature above the temperature of the treatment liquid. The treatment liquid must be heated for this purpose. It is preferred to heat it to between 80 ° C and 100 ° C, and it is preferred to heat it between 85 ° C and 98 ° C. Thereby the etching effect is improved to speed up the etching.

Preferably, the method is carried out in a manner such that the surface of the wafer is treated with the treatment fluid for a period of between 5 minutes and 30 minutes, more preferably between 10 minutes and 20 minutes.

During this time, an effective texturing process that is at least comparable to the prior art in terms of the number and depth of pyramid structures can be completed.

At least wafers used to fabricate single-faced solar cells require only one-sided texturing. The surface is completely wetted using a treatment liquid such as by spraying or dipping, and the surface to be treated is preferably upward. Both sides of the tantalum wafer may also be textured, or in the manner described above, or the entire tantalum wafer may be entirely immersed in a bath containing the processing liquid.

In order to improve the etching result while accelerating the etching speed, the processing liquid can be renewed or replaced at a relatively fast speed on the surface of the wafer. For this purpose, the treatment liquid target can be flowed explicitly to the crucible wafer, for example in the form of eddy currents or so-called surges. This can be achieved by a dedicated nozzle or a so-called surge device (for example, opening a gap near the surface of the crucible wafer), or in a full tank (Vollbad) or by moving the treatment fluid (for example, by stirring). Implemented.

Further, in carrying out the method of the present invention, the tantalum wafer can be oriented horizontally and preferably continuously during processing of the processing liquid. This particular horizontal processing technique has several advantages over the prior art vertical immersion method that is implemented in a periodic manner with such a treatment fluid: horizontal positioning improves the processing fluid exchange on the wafer surface. Continuous processing speeds up processing. Therefore, the method of the present invention is preferably carried out continuously.

The method of treating the wafer by the above treatment liquid can perform structured etching of several micrometers to 20 μm, thereby achieving effective texturing. Through this structured etch, a weighted reflection of 11% up to 12.5% can be achieved in the range of 400 nm to 1000 nm.

The wafer has macroscopic homogeneity throughout the textured region, i.e., has a uniform visual appearance.

Increasing the processing temperature shortens the processing time, thereby increasing the throughput, which is particularly advantageous for continuous processing in the industrial field. The use of a pure aqueous solution, ie the use of isopropanol and any other organic solvent, eliminates the need to replenish the application of isopropanol and does not require the disposal of isopropanol, which greatly simplifies the process.

The concentration of cellulose ether used as an etch additive is low, but sufficient to achieve the same or better texturing results as isopropyl alcohol or other etchants, which can significantly reduce the cost of the treatment liquid.

The invention will be more clearly illustrated by the following two examples.

Example 1:

Example 1 A 156 mm × 156 mm (100) Cz unpolished p-type single crystal germanium wafer was etched at 88 ° C for 20 minutes using an aqueous texturing solution. The aqueous texturing solution was made up of 4 wt% KOH (high). Pure or electronic grade) formed with 100 ppm hydroxyethyl cellulose (average molecular weight of 1,500,000) dissolved in demineralized water.

result:

A macroscopic homogeneous texture is obtained with an average basic length of the pyramid structure of 3 μm to 6 μm (see Figure 1).

The minimum reflection of this texture occurs at 960 nm, which is 9.6% (see Figure 2).

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

(Measured with Varian's Cary5000 UV-VIS spectrometer and the calibrated Weiß standard)

Example 2:

Example 2 was a 156 mm × 156 mm (100) Cz unpolished p-type single crystal germanium wafer etched at 95 ° C for 12 minutes using an aqueous texturing solution, the aqueous texturing solution being 6 wt% KOH ( High purity or electronic grade) formed with 150 ppm hydroxyethyl cellulose (average molecular weight of 1,500,000) dissolved in demineralized water.

result:

A macroscopic homogeneous texture is obtained with an average basic length of the pyramid structure of 2 μm to 5 μm.

The minimum reflection of this texture occurs at 965 nm, which is 9.8%.

The weighted reflection from 400 nm to 1000 nm is 11.9%.

Claims (9)

A method of processing a germanium wafer, which is specifically used as a substrate in solar cell manufacturing, wherein a surface of the germanium wafer is wetted using a processing liquid and the surface of the germanium wafer is textured. The treatment liquid comprises water, at least one alkaline etching component, and at least one etching additive selected from the group consisting of cellulose ethers. The method of claim 1, wherein the alkaline etching component is sodium hydroxide and/or potassium hydroxide. The method of claim 1 or 2, wherein the etching additive is hydroxymethylcellulose and/or hydroxyethylcellulose. The method of any of the preceding claims, wherein the etching additive is present in the treatment liquid in an amount between 10 ppm and 1%, preferably between 100 ppm and 300 ppm. The method of any one of the preceding claims, wherein the temperature of the treatment liquid is between 80 ° C and 100 ° C, preferably between 85 ° C and 98 ° C. The method of any one of the preceding claims, wherein the treatment with the treatment liquid lasts between 5 minutes and 30 minutes, preferably between 10 minutes and 20 minutes. A processing liquid for processing a germanium wafer, which is specifically applied to solar cell manufacturing, using the processing liquid to wet the surface of the germanium wafer and The surface of the tantalum wafer is textured, characterized in that the treatment liquid comprises water, at least one alkaline etching component, and at least one etching additive selected from the group consisting of cellulose ethers. The treatment liquid of claim 7, wherein the additive is contained in the treatment liquid at a content of between 10 ppm and 1%, preferably between 100 ppm and 300 ppm. A germanium wafer, in particular for solar cell manufacturing, characterized in that the germanium wafer has been subjected to the method of any one of claims 1 to 6 or the use of claim 7 or 8 The treatment liquid has been treated.