TW201631121A - Agent for increasing etching rates - Google Patents

Abstract

The present invention relates to an etching composition for the etching of surfaces consisting of Si, SiO2, SiNx or transparent conductive oxides (TCO), comprising at least one aqueous phase and at least an agent for increasing etch rates, and to the use of said composition in production processes in electronic industry.

Description

Etch rate increase agent

The present invention relates to an etching composition for etching a surface composed of Si, SiO ₂ , SiN _x or a transparent conductive oxide (TCO), comprising at least one aqueous phase and at least one etch rate increasing agent, and the present invention is Use of the composition in the production process in the electronics industry.

The production of simple solar cells or solar cells, currently represented by the market share in the market, includes the basic production steps outlined below:

1. Separation damage etching and texturing

Tantalum wafers (single crystal, polycrystalline or quasi-single crystal, p or n-type base doped) are protected from the damage of the adhesion caused by the etching process and are typically "simultaneously" textured in the same etching bath. In this case, texturing is considered to mean a surface (property) that is preferentially aligned due to an etching step or simply an intentional rather than a specific alignment of the wafer surface. As a result of the texturing, the surface of the wafer now acts as a diffuse reflector and thereby reduces directional reflection depending on the wavelength and angle of incidence, ultimately resulting in an increase in the absorption ratio of light incident on the surface, and thus resulting in the same solar cell. Conversion efficiency increases.

In the case of a single crystal wafer, the above-mentioned etching solution for treating a germanium wafer is usually composed of a dilute potassium hydroxide solution to which isopropanol has been added as a solvent. Alternatively, other alcohols having a higher vapor pressure or higher boiling point than isopropanol may be added if this results in the desired etching result. The desired etch result is usually configured by random Or more precisely the morphology of the pyramidal shape with a square base etched from the original surface. The density, height, and thus the substrate area of the pyramids may be affected in part by the proper selection of the above-mentioned components of the etching solution, the etching temperature, and the residence time of the wafer in the etching bath. The texturing of the single crystal wafer is typically carried out at a temperature ranging from 70 ° C to < 90 ° C, at which an etch removal rate of up to 10 μm per wafer side can be achieved.

In the case of a polycrystalline silicon wafer, the etching solution may consist of a potassium hydroxide solution having a moderate concentration (10% to 15%). However, this etching technique is still difficult to use in industrial practice. An etching solution composed of nitric acid, hydrofluoric acid, and water is used more frequently. This etching solution can be modified by various additives such as sulfuric acid, phosphoric acid, acetic acid, N-methylpyrrolidone, and surfactants which in particular achieve the wetting characteristics of the etching solution and which will also specifically affect the etching rate thereof. These acidic etching mixtures produce a pattern of nested etched trenches on the surface. The etching is usually carried out at a temperature in the range between 4 ° C and < 10 ° C, and the etching removal rate here is usually 4 μm to 6 μm.

Immediately after texturing, the silicon wafer is centrally cleaned with water and the germanium wafer is treated with dilute hydrofluoric acid to remove the chemical oxide layer and the absorbed contaminants formed by the foregoing processing steps and adsorbed in the germanium wafer and The contaminants adsorbed on the germanium wafer are prepared for subsequent high temperature processing.

2. Diffusion and doping

The wafer which has been etched and cleaned in the foregoing step (in this case, p-type base doping) is treated with a vapor composed of phosphorous oxide at a high temperature (usually between 750 ° C and < 1000 ° C). During this operation, the wafer was exposed to a controlled atmosphere consisting of dry nitrogen, dry oxygen, and phosphorous chloride in a quartz tube in a tube furnace. For this purpose, the wafer is introduced into a quartz tube at a temperature between 600 ° C and 700 ° C. The gas mixture is conveyed through a quartz tube. During the passage of the gas mixture through the tube which is heated vigorously, the phosphonium chloride is decomposed to obtain a vapor composed of phosphorus oxide (e.g., P ₂ O ₅ ) and chlorine. Phosphorus oxide vapors are especially deposited on the wafer surface (coating). At the same time, the surface of the crucible oxidizes at these temperatures, accompanied by the formation of a thin oxide layer. The precipitated phosphorous oxide is embedded in this layer such that a mixed oxide of cerium oxide and phosphorus oxide is formed on the surface of the wafer. This mixed oxide is called phosphosilicate glass (PSG). This PSG has different softening points and different diffusion constants with respect to phosphorus oxide depending on the concentration of phosphorus oxide present. The mixed oxide acts as a diffusion source for the germanium wafer, wherein the phosphorous oxide diffuses in the direction of the interface between the PSG and the germanium wafer during diffusion, wherein the phosphorous oxide reacts with the germanium on the surface of the wafer (heating) ) and reduced to phosphorus. The phosphorus formed in this way has a solubility in the crucible which is several orders of magnitude higher than the solubility in the glass matrix in which it is formed, and therefore the phosphorus is preferentially dissolved in the crucible due to the extremely high segregation coefficient. After dissolution, the phosphorus diffuses along the concentration gradient into the volume of the crucible in the crucible. In this diffusion process, a concentration of about ¹⁰⁵ is formed in a gradient of 10 ²¹ atoms / cm with a typical surface concentration of ^{2 1016} atoms / cm ² of the area in between the base doping. Typical diffusion depths are from 250 nm to 500 nm, and depending on the selected diffusion temperature (eg, 880 ° C) and the total exposure duration of the wafer in a vigorously warming atmosphere (heating and coating stages and drive-in stages and cooling) . During the coating phase, a PSG layer typically having a layer thickness of 40 nm to 60 nm is formed. Coating the wafer by PSG is followed by a drive-in phase during which diffusion into the volume of the crucible has also occurred. This can be separated from the coating stage, but in practice it is usually combined directly with the coating in terms of time and therefore usually also at the same temperature. The composition of the gas mixture herein is adapted such that further supply of phosphonium chloride is inhibited. During the implantation, the surface of the crucible is further oxidized by the oxygen present in the gas mixture, thereby causing the consumption of phosphorus oxide which also contains phosphorus oxide between the actual doping source, the phosphorous-rich PSG and the germanium wafer. The ruthenium dioxide layer. The growth of this layer is much faster than the mass flow rate of the dopant from the source (PSG), which accelerates oxide growth (acceleration by one to two orders of magnitude) due to the high surface doping of the wafer itself. This situation makes it possible to achieve the depletion or separation of the dopant source in a specific way, the permeation of the dopant source of the diffused phosphorous oxide thereon being influenced by the material flow, which depends on the temperature and thus on the diffusion coefficient. In this way, the doping of germanium can be controlled within certain limits. The typical diffusion duration consisting of the coating phase and the injection phase is, for example, 25 minutes. After this treatment, the tube furnace is cooled and the wafer can be removed from the processing tube at a temperature between 600 ° C and 800 ° C.

In the case of boron doping of a wafer in the form of an n-type base doping, a different method is performed, which will not be explained separately. Doping in such cases is carried out, for example, by boron trichloride or boron tribromide. Depending on the choice of composition of the gas atmosphere for doping, so-called boron skin formation on the wafer can be observed. This boron skin depends on a variety of influencing factors: the key is the doping atmosphere, temperature, doping duration, source concentration, and the combination (or linear combination) parameters mentioned above.

In such diffusion processes, it is self-evident that if the substrate has not been previously pre-treated (for example, with a layer that inhibits and/or suppresses diffusion and material structuring the substrate), the wafer used may not contain Any preferred diffused and doped regions (except for the resulting balloons formed by non-uniform gas flow and non-uniform compositions).

For the sake of completeness, it should also be noted here that other diffusion and doping techniques have been established to varying degrees in the production of germanium-based crystalline solar cells. Thus, mention may be made of: - ion implantation, - by means of APCVD, PECVD, MOCVD and LPCVD processes via mixed oxides such as PSG and BSG (boron silicate glass) of their mixed oxides) Vapor deposition-promoted doping, (co)spraying of mixed oxides and/or ceramic materials and hard materials (eg boron nitride), - vapor deposition of the last two, - self-solid dopant source Pure thermal vapor deposition starting with (for example, boron oxide and boron nitride), and liquid deposition of doping liquid (ink) and paste.

The latter is often used in so-called in-line doping, where the corresponding paste and ink are used It is applied to the side of the wafer to be doped in a suitable method. The solvent present in the composition for doping is removed by temperature and/or vacuum treatment after the coating or even during the coating. This leaves the actual dopant on the surface of the wafer. The liquid doping source which can be employed is, for example, a dilute solution of phosphoric acid or boric acid, and a sol-gel based system of a polymeric borazil compound or a solution thereof. The corresponding doping paste is actually the only feature that uses an additional thickening polymer and contains a suitable form of dopant. Evaporation of the solvent from the doping medium mentioned above is typically followed by a high temperature treatment during which undesired and interfering additives (but additives necessary for the formulation) are "burned" and/or pyrolyzed. Solvent removal and burnout can occur (but not necessarily) at the same time. The coated substrate is then typically passed through a belt furnace at a temperature between 800 ° C and 1000 ° C, wherein the temperature can be increased slightly compared to the gas phase diffusion in the tube furnace to shorten the passage time. The main gas atmosphere in the belt furnace may vary depending on the doping requirements and may consist of dry nitrogen, dry air, a mixture of dry oxygen and dry nitrogen, and/or depending on the design of the furnace to be passed. And one of the gas atmospheres or other areas. Other gas mixtures are conceivable, but are currently not critical in the industry. The characteristic of on-line diffusion is that the coating and driving of the dopants can in principle occur separately from one another.

3. Removal of dopant source and optional edge isolation as appropriate

The wafers present after doping are coated on both sides, with the glass being applied more or less on both sides of the surface. In this case, more or less refers to the modification that can be applied during the doping process: the two-sided diffusion is promoted relative to the back-to-back configuration of the two wafers in one position of the used process boat. Unilateral diffusion. Although the latter variant primarily achieves one-sided doping, it does not completely inhibit diffusion on the back side. In both cases, the presence of glass after doping from the surface by etching in dilute hydrofluoric acid is presently state of the art. For this purpose, the wafers are first reloaded batchwise into a wet process boat and impregnated with a solution of dilute hydrofluoric acid (usually 2% to 5%) and held therein until the surface is completely No glass or until process cycle duration (which indicates the duration of the necessary etch) The total parameters of the process automation between the machine and the machine are expired. Complete removal of the glass can be determined, for example, by completely resisting wetting of the wafer surface with a dilute aqueous solution of hydrofluoric acid. Complete removal of the PSG is achieved in these process conditions using, for example, a 2% hydrofluoric acid solution at room temperature in 210 seconds. The etching of the corresponding BSG is slower and requires longer process times, and may also require the use of higher concentrations of hydrofluoric acid. After etching, the wafer is rinsed with water.

On the other hand, etching of the glass on the surface of the wafer can also be performed in a horizontal operation process in which the wafer is introduced into the etcher at a constant flow rate, and the wafer is horizontally passed through the corresponding process slot in the etcher (online machine ). In this case, the wafer is conveyed on the drum through the process tank and the etching solution present therein, or the etched medium is delivered onto the wafer surface by means of roll coating. The typical residence time of the wafer during etching of the PSG is about 90 seconds, and the hydrofluoric acid used is slightly higher in concentration than in the batch process to compensate for the shorter retention due to the increased etch rate. time. The concentration of hydrofluoric acid is usually 5%. In addition, the bath temperature may be slightly elevated compared to room temperature (>25 ° C < 50 ° C).

In the process outlined in the previous overview, it has been determined that so-called edge isolation is performed simultaneously, resulting in a slightly modified process flow: edge isolation glass etching. Edge isolation is the necessity of process engineering caused by the inherent characteristics of the system of double-sided diffusion, and is also necessary for process engineering in the case of intentional one-sided back-to-back diffusion. A large area of parasitic p-n junction is present on the back side of the solar cell (the latter), which was partially removed during processing for process engineering reasons but not completely removed. Due to this situation, the front and back sides of the solar cell are short-circuited (tunneled contact) via parasitic and residual p-n junctions, which reduces the conversion efficiency of the latter solar cell. To remove this junction, one side of the wafer is passed over an etching solution consisting of nitric acid and hydrofluoric acid. The etching solution may comprise, for example, sulfuric acid or phosphoric acid as a secondary component. Alternatively, the etching solution is transferred (transferred) to the back side of the wafer via a roller. At temperatures between 4 ° C and 8 ° C, the etch rate typically achieved in this process is about 1 μm (including the glass layer present on the surface to be treated). In this process, the glass layer still present on the opposite side of the wafer acts as a mask that provides a certain degree of protection against etching erosion on this side. Protection. This glass layer is then removed by means of the described glass etching.

In addition, edge isolation can be performed by means of a plasma etching process. This plasma etch is typically performed prior to glass etching. For this purpose, a plurality of wafers are stacked one on top of the other and the outer edges are exposed to the plasma. The plasma is supplied by a fluorinated gas (for example, tetrafluoromethane). The reactive material that occurs when the plasma decomposes these gases etches the edges of the wafer. In general, the plasma etch is followed by a glass etch.

4. Coating the front side with an anti-reflective layer

After etching of the glass and optionally edge isolation, the front side of the subsequent solar cell is coated with an anti-reflective coating typically composed of amorphous and yttrium-rich yttrium nitride. Alternative anti-reflective coatings are contemplated. Possible coatings may be titanium dioxide, magnesium fluoride, tin dioxide, and/or consisting of corresponding stacked layers of cerium oxide and tantalum nitride. However, antireflective coatings having different compositions are also technically possible.

Coating the wafer surface with the tantalum nitride mentioned above essentially fulfills two functions: - On the one hand, the layer generates an electric field due to the large amount of positive charge incorporated, which can keep the charge carriers in the crucible away from The surface and the recombination rate (field effect passivation) of these charge carriers at the surface of the crucible can be significantly reduced. On the other hand, this layer produces a reflection reduction depending on its optical parameters such as refractive index and layer thickness. Attributes that help to make more light possible to bond to the next solar cell.

These two effects increase the conversion efficiency of the solar cell. A typical property of the layer currently in use is that the layer thickness is about 80 nm when exclusively using the above-mentioned tantalum nitride having a refractive index of about 2.05. The anti-reflection reduction is most clearly visible in the light wavelength region of 600 nm. The directional and non-directional reflections here exhibit values of about 1% to 3% of the original incident light (to the normal incidence perpendicular to the surface of the germanium wafer).

The tantalum nitride layer referred to above is currently deposited on the surface by means of a direct PFCVD process. For this purpose, a plasma in which decane and ammonia are introduced is ignited in an argon atmosphere. Decane The reaction with ammonia in the plasma is carried out by reacting ions with radicals to obtain tantalum nitride, and simultaneously deposited on the surface of the wafer. The properties of the layer can be adjusted and controlled, for example, via individual gas flows of the reactants. The deposition of the tantalum nitride layer mentioned above can also be carried out by means of hydrogen as a carrier gas and/or only by means of a reactant. Typical deposition temperatures range between 300 °C and 400 °C. Alternative deposition methods can be, for example, LPCVD and/or sputtering.

5. Manufacturing of front electrode grid

After depositing the antireflective layer, the front side electrode is defined on the surface of the wafer coated with tantalum nitride. In industrial practice, it has been established to fabricate electrodes using a metal sintered paste by means of a screen printing method. However, this is only one of many different possibilities for manufacturing the desired metal contacts.

In stencil printing metallization, high-concentration silver particles (silver content) are usually used. 80%) of the paste. The sum of the remaining components is produced by the rheology aids (such as solvents, binders, and thickeners) required for the formulation of the paste. In addition, the silver paste comprises a specific glass frit mixture, typically based on cerium oxide, borosilicate glass, and oxides of lead oxide and/or cerium oxide and mixed oxides. The frit basically fulfills two functions: on the one hand, it acts as a tackifier between the surface of the wafer and the mass of silver particles to be sintered; on the other hand, it causes penetration of the top layer of tantalum nitride to facilitate Direct ohmic contact with the lower layer. The permeation of tantalum nitride occurs via an etching process in which silver dissolved in the frit substrate is subsequently diffused into the crucible surface, thereby achieving ohmic contact formation. In practice, the silver paste is deposited on the surface of the wafer by screen printing and subsequently dried at a temperature of about 200 ° C to 300 ° C for a few minutes. For completeness, it should be mentioned that a double printing process is also used in the industry which enables the second electrode grid to be printed onto the electrode grid produced during the first printing step with precise registration. The thickness of the silver metallization is thereby increased, which can have a positive effect on the conductivity in the electrode grid. During this drying, the solvent present in the paste is ejected from the paste. The printed wafer is then passed through a belt furnace. This type of furnace typically has a plurality of heating zones that can be activated and temperature controlled independently of each other. The wafer is heated to a temperature of up to about 950 ° C during passage through the belt furnace. However, individual wafers typically only experience this peak temperature for a few seconds. The wafer has a temperature of 600 ° C to 800 ° C during the remainder of the transfer phase. At these temperatures, the organic accompanying material (such as a binder) present in the silver paste is burned out and the etching of the tantalum nitride layer is initiated. During the short time interval of the main peak temperature, contact formation with ruthenium occurs. The wafer is then cooled.

The contact formation process briefly outlined in this way is usually carried out simultaneously with the formation of two remaining contacts (cf. 6 and 7), which in this case also uses the term co-firing process.

The front electrode grid itself is made up of thin fingers (typical number >= 68) having a width of typically 80 μm to 140 μm and a width ranging from 1.2 mm to 2.2 mm (depending on the number, usually two to three) The bus bar is composed. Typical heights of the printed silver elements are typically between 10 μm and 25 μm. The aspect ratio is rarely greater than 0.3.

6. Manufacturing of back busbars

The back busbars are typically applied and defined by means of a screen printing process. For this purpose, a silver paste similar to the silver paste used for front metallization is used. This paste has a similar composition but contains an alloy of silver and aluminum, of which the proportion of aluminum usually constitutes 2%. In addition, this paste contains a lower frit content. A busbar (typically two cells) having a typical width of 4 mm is printed onto the back side of the wafer by screen printing and pressed and sintered, as described under point 5.

7. Manufacturing of the back electrode

The back electrode is defined after printing the bus bar. The electrode material consisted of aluminum, which is the reason for printing the aluminum-containing paste onto the remaining free areas of the back side of the wafer by screen printing, with the edges separated by <1 mm for defining the electrodes. Paste by 80% aluminum. The remaining components are those components (such as solvents, binders, etc.) that have been mentioned under point 5. The aluminum paste is bonded to the wafer during the co-firing period by the aluminum particles beginning to melt during the warming period and the enthalpy from the wafer being dissolved in the molten aluminum. The molten mixture acts as a dopant source and releases aluminum to the ruthenium (dissolution limit: 0.016 atoms per percent), wherein ruthenium is p ⁺ doped due to this alloying and inward diffusion. During the cooling of the wafer, a eutectic mixture of aluminum and ruthenium, which is cured at 577 ° C and having a composition of Si with a mole fraction of 0.12, is deposited on the surface of the wafer.

As the aluminum diffuses inward into the crucible, a highly doped p-type layer is formed on the back side of the wafer, which acts as a mirror ("electron microscopy") on a portion of the free charge carriers in the crucible. These charge carriers are unable to overcome this potential wall and are therefore extremely effective in moving them away from the back wafer surface, as is the case with the overall reduction in charge carriers at this surface. This potential wall is often referred to as the back surface field.

The order of the process steps described under points 5, 6 and 7 may, but need not, correspond to the order outlined herein. It will be apparent to those skilled in the art that, in principle, the order of the process steps outlined can be carried out in any conceivable combination.

8. Optional edge isolation as appropriate

If the edge isolation of the wafer has not been carried out as described under point 3, this is usually done by means of a laser beam method after co-firing. For this purpose, the laser beam is directed towards the front side of the solar cell, and the front p-n junction is divided by the energy coupled by the beam. Here, a cutting groove having a depth of at most 15 μm is produced due to the action of the laser. Here, the crucible is removed from the treated portion or ejected from the laser trench via the ablation mechanism. This laser trench typically has a width of from 30 μm to 60 μm and is about 200 μm from the edge of the solar cell.

After production, solar cells are characterized according to their individual performance and are categorized by individual performance categories.

Those skilled in the art are aware of solar cell architectures having both n-type and p-type substrate materials. These types of solar cells include, inter alia,

‧ERC solar cell

‧PERL solar cell

‧PERT solar cell

‧MMW-PERT and MWT-PERL solar cells derived from them

‧ double-sided solar cell

‧Back junction contact battery

‧With the interdigitated contact back contact surface battery

Purpose of the invention

Today, most manufacturers of displays or electronic devices are trying to reduce the consumption of chemicals and related total emissions to prevent environmental pollution. At the same time, it attempts to achieve the same or improved process results, for example, to produce deeper etched structures with higher resolution in shorter process times.

In this context, a major challenge in this development is the introduction of a transparent conductive oxide layer (TCO layer) and a rational patterning method for Si, SiO ₂ , SiN _x layers in mass production.

Accordingly, it is an object of the present invention to provide an inexpensive, simple and fast etching process for patterning Si, SiO ₂ , SiN _x or transparent conductive oxide layers, wherein the need for chemicals is reduced and the total amount of chemicals to the environment Emissions are reduced. Another object of the present invention is to provide a suitable method for patterning Si, SiO ₂ , SiN _x or transparent conductive oxide layers on an inorganic or flexible polymer substrate with high accuracy.

However, there is also a need for a process for reproducibly patterning a plurality of substrates having a lateral dimension of 80 μm or less, preferably less than 50 μm. The process should be low cost, highly reproducible and scalable.

Surprisingly, superabsorbent particles have been found to be useful in etching compositions having improved etch rates.

More particularly, the present invention relates to an etching composition for etching a surface composed of Si, SiO ₂ , SiN _x or a transparent conductive oxide (TCO), which comprises phosphoric acid, phosphate or phosphate in combination with a superabsorbent compound Phosphate adducts or mixtures of phosphoric acid, phosphate and/or phosphoric acid adducts and/or etchants of hydrochloric acid and/or hydrofluoric acid. Most preferably, the superabsorbent compound added is a porous particle and is based on crosslinked sodium polyacrylate. However, a superabsorbent compound may also be added as long as it is stable with respect to the etchant contained therein.

The invention is also directed to the use of such novel etching compositions.

In particular, the present invention also relates to the method claimed by the technical solutions 8 to 13.

Different Figures 1 through 5 schematically show different states of the treated substrate surface corresponding to successive process steps: Figure 1: ECV quantitative curve of the n-type wafer with phosphor backed front and back before and after etching.

Figure 2: n-type solar cell with a deeper uniform back surface field

Figure 3: Printing the etch paste on the substrate

Figure 4: Activating the etching process by heat and structuring P-BSF

Figure 5: Substrate with selective back surface field (high and low diffusion regions).

Figure 6: Screen-printed n-type solar cell with S-BSF

In general, if it is desired to increase the etch rate or depth, the etch temperature and/or time is increased. However, this measure is only hopefully successful to a certain extent. When all of the water contained in the etch paste is consumed during the etching process, the process will stop after a certain time. If the etching step is supported by heating, the etching step ends earlier due to water evaporation loss. This means that the water contained is one of the limiting factors in the etching ability of the etching paste.

Since it has not been possible to subsequently change the water content after coating the composition from the outside, in the past few years, progress has been made such that aqueous gels have been developed in which an inorganic or organic additive forming a network structure is used to change the composition. Thick and used to include as much water as possible.

In the past, attempts have also been made to extend the duration of the etching process by adding a low melting point resin that is compatible with the etch paste that floats during the heating step and forms a dense layer for water vapor and prevents it from coming during etching. Water loss from etching paste.

However, this disadvantage is that a complicated purification step is required after the etching step because the residue of the molten polymer resin must be removed from the treated surface.

It has now been found that etching and doping results can be greatly improved by the addition of specific polymers available in the form of superabsorbents.

Superabsorbent polymer

The term "superabsorbent polymer" means a polymer which is capable of spontaneously absorbing at least 20 times its own weight of aqueous fluid (particularly water and especially distilled water) in its dry form. Such superabsorbent polymers are described in the publication "Absorbent polymer technology, Studies in polymer science 8" published by Elsevier in 1990 by L. Brannon-Pappas and R. Harland.

These polymers have a large capacity for absorbing and retaining water and aqueous fluids. After absorbing the aqueous liquid, the polymer particles thus filled with the aqueous fluid remain insoluble in the aqueous fluid and thereby preserve their individualized particulate state.

Superabsorbents are commercially available polymers which are composed of polyacrylates in the form of non-toxic spherical particles capable of absorbing and retaining water which is many times their weight. It has unique surface cross-linking chemistry that prevents gel clogging and allows liquid to flow freely to the particles for efficient absorption. At the same time, it thickens the liquid and turns it into a solid gel, and swells upon contact with the liquid and exchanges ions during absorption. Currently, such superabsorbents are used, for example, in the production of baby diapers or to improve the poor quality and unfavorable environment of soil and water, wherein such superabsorbents are used to retain water and provide a uniform supply of water to plants.

Thus, it is known that the superabsorbent polymer may have 20 to 2000 times its own weight (i.e., 20 g to 2000 g of absorbed water per gram of the absorbent polymer), preferably 30 to 1500 times and more preferably. The water absorption capacity in the range of 50 times to 1000 times. These water absorption characteristics are defined for standard temperature (25 ° C) and pressure (760 mm Hg, ie 100000 Pa) and for distilled water.

The value of the water absorption capacity of the polymer can be determined by dispersing 0.5 g of the polymer in 150 g of the aqueous solution, waiting for 20 minutes, filtering the unabsorbed solution through a 150 μm filter paper for 20 minutes, and weighing the unabsorbed water. Superabsorbent polymer for use in the compositions of the present invention In the form of particles. Preferably, the superabsorbent polymer in a dry or non-hydrated state has an average size of less than or equal to 100 μm, preferably less than or equal to 50 μm, in the range of, for example, 10 μm to 100 μm, preferably 15 μm to 50 μm. More preferably, it is 20 μm to 30 μm.

The average particle size analysis or corresponding to those skilled in the art by weight of other known method of measuring the average diameter equivalent (D ₅₀₎ particle size by a laser.

These particles, upon hydration, swell and form soft particles having an average size ranging from 10 μm to 1000 μm.

Preferably, the superabsorbent polymer used in the present invention is in the form of spherical particles.

Superabsorbent polymers may be selected from the following: - the crosslinked polyacrylic sodium, e.g., Avecia ^® under the name Octacare ^® X100, X110 and RM100 is crosslinked sodium polyacrylate of sale, the company SNF Flocare ^®, GB300 and 500. Flosorb name sold by crosslinking sodium polyacrylate, BASF under the name ^{Luquasorb ® 1003, Luquasorb ® 1010,} Luquasorb ® 1280 and Luquasorb ^® 1110 or the Artic Gel®, Evonik company name FAVOR ^® sold by crosslinking of polyacrylic acid Sodium, and Grain Processing Company sold under the name of Water Lock ^® G400 and G430 (INCI name: acrylamide/sodium acrylate copolymer) or crosslinked sodium polyacrylate sold under the name Aquakeep® 10 SH NF by Sumitomo Seika - a starch grafted with an acrylic polymer (copolymer) and especially with sodium polyacrylate, such as Sanyo Chemical Industries under the name Sanfresh ^® ST-100MC or Daito Kasei under the name Makimousse ^® 25 or Makimousse ^® 12 Starch (INCI name: sodium polyacrylate starch), using acrylic polymers (homopolymers or copolymers) and especially acrylonitrile propylene Amine/sodium acrylate copolymer grafted hydrolyzed starch, such as by Water Processing Corporation under the names Water Lock ^® A-240, A-180, B-204, D-223, A-100, C-200 and D-223 Hydrolyzed starch for sale (INCI name: starch/acrylamide/sodium acrylate copolymer), - a polymer based on starch, gum and cellulose derivatives, such as starches sold by Lysac under the name Lysorb ^® 220, Products of guar gum and sodium carboxymethylcellulose, and mixtures thereof.

The superabsorbent polymer used in the present invention may be crosslinked or non-crosslinked. It is preferably selected from the group of crosslinked polymers.

The preparation of the superabsorbent acrylic polymer is achieved by polymerization of acrylic acid with a "crosslinking agent" (crosslinking agent) (i.e., a compound having two or more double bonds). This is most often carried out in solution polymerization. Redox systems such as K2S2O8/Na2S2O5 or hydrogen peroxide/ascorbate are common and inexpensive initiators for this polymerization. Crosslinking between individual polymer chains is caused by the added crosslinking agent, which causes the polymer to be insoluble. Preferably, diacrylate, allyl methacrylate, triallylamine or tetraallyloxyethane is used as the crosslinking agent. Other suitable crosslinking agents are (i.e., ethylene glycol diacrylate or 1,1,1-trimethylolpropane-triacrylate). When sufficient chain bonds are joined together to form a macromolecular network, the system self-adhesive mass transforms into an elastic solid ("gel point"). The amount of cross-linking material determines the water absorption capacity of the gel. However, high crosslink density limits the possibility of polymer backbone expansion. The insoluble superabsorbent particles are in the form of porous particles after synthesis. Although these particles have a high affinity for water, they are non-absorbent, so their storage and transportation are not a problem.

Thus, as already mentioned, preferably, the superabsorbent polymer used in the present invention is a crosslinked acrylic acid homopolymer or copolymer which is preferably neutralized and in particulate form. Preferably, the superabsorbent polymer is selected from the group consisting of crosslinked sodium polyacrylates, preferably in the form of particles, more preferably in the form of spherical particles having an average size (or average diameter) of less than or equal to 100 microns. . These polymers preferably have from 10 g/g to 100 g/g (preferably 20 A water absorption capacity of from g/g to 80 g/g and more preferably from 40 g/g to 80 g/g.

The superabsorbent polymer may be present in a concentration ranging from 0.05% to 8% by weight, preferably from 0.05% to 5% by weight and preferably from 0.05% to 1% by weight, based on the total weight of the composition. In the etching composition according to the invention.

These preferred superabsorbents have a glass transition temperature of about 140 °C. If it is heated to a higher temperature, it decomposes. The latter property is advantageous for etching compositions used in etching and doping in one process step.

In accordance with the present invention, the superabsorbent is water saturated, and then a superabsorbent is added to the etch paste to act as a water consuming agent during the etching process. The duration of the actual etching process can be increased, which results in a significantly increased etch depth. The paste screen was printed on a tantalum wafer and subsequently heated up to 450 ° C for 2 minutes. After the heating step, the wafer can be cleaned with deionized water. Another option is to increase the temperature for a short period of time at which the treated surface is doped. Although the superabsorbent particles initially added consume their moisture for an increased duration of etching, the particles decompose at higher temperatures and, if desired, additional doping can be treated at the region where the etch paste is applied. After etching and doping, the surface is only cleaned with water.

Another embodiment of the fabrication process of the present invention is to increase the cell efficiency of a double-sided solar cell and to produce a selective back surface field (S-BSF). S-BSF can be produced by structuring an n-type wafer having a backside with deep phosphor diffusion, which is selectively back etched with an etch paste. This method has been used to form a selective emitter on the front side, but only 30 nm to 70 nm must be removed on the front side. In order to form a selective back surface field on the back side, this layer over 150 nm must be removed. However, this is not possible with current standard etch pastes because of the insufficient etch rate in the crucible.

Advantageously, the etch paste according to the present invention can also be used in other applications in semiconductor and display production steps, where a high etch rate is necessary, ie for a structured SiN _x layer, a SiO ₂ layer, a passivation layer or a TCO layer necessary. Furthermore, the compositions as disclosed herein support printing with high resolution by various methods because the added particulate superabsorbent acts as a thickener and acts as a water absorbing agent for the etching reaction. Furthermore, it is possible to perform different etch depths by using the same base paste in the same etching time by the composition of the invention, so that different settings of the process equipment used can be omitted where appropriate.

In order to demonstrate the utility of the paste of the present invention, the paste has been compared to conventional compositions. These results clearly demonstrate the improved etch depth achieved by an etch composition comprising superabsorbent particles.

For example, Figure 1 shows an electrochemical capacitance voltage (ECV) quantitative curve of an n-type wafer with a phosphor-diffused back surface before and after etching. In this figure, a corresponding diffusion amount curve of a substrate (green) etched without etching (green), a substrate etched with a standard etch paste, and an etch paste etched with a superabsorbent saturated with water. The curve shown in Figure 1 clearly shows that the etch depth is increased by 90 nm when the superabsorbent is added to the paste composition.

Sheet resistivity measurement

In Table 1 , it is shown that if the etching paste according to the present invention is used to carry out the method for structuring the surface of the substrate, the sheet resistivity is remarkably increased from 38 Ohm/sq to 64 Ohm/sq.

As has been described above, the methods disclosed herein can be performed on both industrial scale and minor amounts. The manner in which the user performs the etching method is free. It is self-evident that, depending on the nature of the surface, it may be advantageous to apply an etching composition of a different composition, and one or the other of the added superabsorbents in the form of porous polymer particles may be combined with the selected etching composition. of. Here, experts can be made of sodium polyacrylate, polyvinylpyrrolidone, natural polymers (such as amylopektin, starch, guar gum, gelatin, fiber The porous polymer made of the sodium and sodium carboxymethylcellulose is selected from the porous polymers as described above and which are crosslinked or non-crosslinked. Because of the existence of commercially available different superabsorbents in the form of particulate or spherical particles, one of ordinary skill in the art can readily identify the most suitable superabsorbent beads for producing the etching compositions most suitable for the desired application.

This description is made to enable the person of ordinary skill in the art to practice the invention. Therefore, it is assumed that even if there are no other annotations, those skilled in the art will be able to utilize the above description in the broadest scope.

In the event that any content is unclear, it should be understood that the publications and patent documents known to the skilled person should be consulted. Accordingly, the cited documents are considered a part of the disclosure of this description.

For a better understanding and to illustrate the invention, the following examples are presented within the scope of the invention. These examples are also used to illustrate possible variations.

Moreover, it is self-evident to those skilled in the art that, in both the established examples and the remainder of the description, the amount of components present in the composition is always only a total of 100% by weight of the composition as a whole. Or mol%, and this percentage cannot be exceeded, even if a higher value is produced by the indicated percentage range. Therefore, unless otherwise indicated, the % data is therefore % by weight or mol%, in addition to the ratios shown in the volume data.

Instance Example 1

5 parts phosphoric acid (85%)

3 parts of diethylene glycol monoethyl ether (DEGMEE)

2 parts of dimethyl sulfoxide

1 part of polyvinylpyrrolidone

1 part deionized water

Add 1% by weight of superabsorbent (saturated with deionized water) and 3 parts to this mixture Ceridust 9202 F. The entire mixture was then stirred for 2 hours.

Example 2

8 parts of phosphoric acid (85%)

3 parts of diethylene glycol monoethyl ether (DEGMEE)

2 parts of dimethyl sulfoxide

1 part of polyvinylpyrrolidone

1 part deionized water

To this mixture was added 1% by weight of superabsorbent (saturated with deionized water) and 3 parts of Ceridust 9202 F. The entire mixture was then stirred for 2 hours.

Example 3

6 parts phosphoric acid (85%)

3 parts of diethylene glycol monoethyl ether (DEGMEE)

3 parts of dimethyl sulfoxide

1 part of polyvinylpyrrolidone

1 part deionized water

To this mixture was added 1% by weight of superabsorbent (saturated with deionized water) and 3 parts of Ceridust 9202 F. The entire mixture was then stirred for 2 hours.

Example 4

6 parts phosphoric acid (85%)

3 parts of diethylene glycol monoethyl ether (DEGMEE)

3 parts of dimethyl sulfoxide

1 part of polyvinylpyrrolidone

2 parts deionized water

To this mixture was added 0.5% by weight of superabsorbent (saturated with deionized water) and 3 parts of Ceridust 9202 F. The entire mixture was then stirred for 2 hours.

Example 5

6 parts phosphoric acid (85%)

3 parts of diethylene glycol monoethyl ether (DEGMEE)

3 parts of dimethyl sulfoxide

1 part of polyvinylpyrrolidone

To this mixture was added 5% by weight of superabsorbent (saturated with deionized water) and 3 parts of Ceridust 9202 F. The entire mixture was then stirred for 2 hours.

Example 6

6 parts phosphoric acid (85%)

3 parts of diethylene glycol monoethyl ether (DEGMEE)

3 parts of n-methylpyrrolidone

1 part of polyvinylpyrrolidone

To this mixture was added 5% by weight of superabsorbent (saturated with deionized water) and 3 parts of Ceridust 9202 F. The entire mixture was then stirred for 2 hours.

Structure of the substrate

This finished etch paste was printed on a phosphor-diffused germanium wafer by a screen printer (Fig. 3). The wafer was then heated on a hot plate at a temperature of about 450 ° C for 2 minutes (Figure 4). After the heating step, the paste is removed by rinsing with deionized water followed by drying with compressed air. These process steps produce a structured substrate with S-BSF (Fig. 6).

The process is illustrated by the structuring results of the process steps in the fabrication of n-type solar cells with reference to the above figures.

Claims (13)

An etching composition for etching a surface composed of Si, SiO ₂ , SiN _x or a transparent conductive oxide (TCO), comprising phosphoric acid, phosphate or phosphate adduct or phosphoric acid, phosphoric acid in combination with a superabsorbent compound A mixture of salts and/or phosphate adducts and/or an etchant of hydrochloric acid and/or hydrofluoric acid. An etching composition according to claim 1, which comprises a porous microparticle superabsorbent compound based on sodium polyacrylate. The etching composition of claim 1 or 2, wherein the etchant is based on potassium hydroxide. The etching composition of claim 1 or 2, wherein the etchant is based on hydrochloric acid or hydrofluoric acid. The etching composition of any one of claims 1 to 4, comprising at least one solvent, optionally organic and/or inorganic thickeners and/or binders and/or crosslinkers, and selected from defoaming An additive as appropriate in the group of agents, shakers, wetting agents, deaerators, ammonium salts (eg, triethylene ammonium chloride). Use of an etching composition according to any one of claims 1 to 5, in a method for structuring a surface of a substrate, wherein the composition is passed through a through hole, sprayed, screen printed, spin coated, mobile printed, sprayed Ink printing, stencil printing or dispensing is applied to the surface. A use of an etching composition according to any one of claims 1 to 5, in a method for structuring a surface of a substrate, wherein the composition is applied to the surface by screen printing. A method for etching a surface of a substrate composed of Si, SiO ₂ , SiN _x or a transparent conductive oxide (TCO), wherein the etching composition according to any one of claims 1 to 5 is coated on the entire surface or selected The substrate is applied to the surface and then heated via a hot plate, IR radiation, microwave oven, convection oven or UV irradiation. The method of claim 7, which is for producing a solar cell, wherein the etching composition according to any one of claims 1 to 5 is coated on the entire surface or selectively coated with ruthenium, iridium oxide or ruthenium. On the surface of the composition. The method of claim 7 or claim 8, which is used for manufacturing a passivation layer in an electronic device, wherein the etching composition according to any one of claims 1 to 5 is coated with SiN _x or a transparent metal oxide (such as AlO _x ) is composed on the surface. The method of claim 8, wherein the etching composition according to any one of claims 1 to 4 is coated with a transparent conductive oxide (TCO). On the surface. The method of claim 8 or claim 9, which is for producing a display, wherein the etching composition according to any one of claims 1 to 4 is applied to Si, SiO ₂ , SiN _x or a transparent conductive oxide (TCO) ) on the surface of the composition. The method of claim 8 or 9, which is for producing a solar cell, wherein the etching composition according to any one of claims 1 to 4 is applied to a surface composed of SiN _x or a transparent metal oxide such as AlO _x Used for structuring and passivation.