Process for producing textures or structures on the surface of silicon

Abstract

A method for producing textures or structures on a silicon surface by subjecting the surface to an etching process using a water-containing or aqueous mixture as an etching solution, wherein the etching solution comprises hydrofluoric acid (HF), bromine (Br2) and, in addition, a bromine source, wherein the etching solution has the following concentrations of hydrofluoric acid and bromine: c(HF) = 0.1 to 10 mol / L, and c(Br2) = 0.01 to 0.2 mol / L.

Description

[0001] The invention relates to a method for producing textures or defined structures on the surface of silicon, in particular by isotropic or anisotropic etching processes.

[0002] Conventionally, silicon wafers, such as crystalline silicon wafers, are subjected to wet-chemical etching during micromachining to remove layers from the surface of the silicon wafer. During wet-chemical etching, the surface of the silicon wafer can be cleaned and / or modified using a chemical reaction.

[0003] For wet-chemical etching, monocrystalline silicon wafers can be treated with alkaline media, such as aqueous potassium hydroxide, sodium hydroxide, or tetramethylammonium hydroxide solutions. The silicon atoms are removed from different crystal planes at different rates, resulting in an anisotropic etching process. This anisotropy has the following effects when etching differently oriented silicon wafers: • Silicon(111) surfaces are attacked only slowly or hardly at all. • Pyramids with a square base are formed on silicon (100) surfaces, which minimize light reflection in silicon solar cells, for example. • Rectangular trenches are formed on silicon(110) surfaces, as they are used in micromechanics and microfluidics.

[0004] To ensure uniform texturing, additives, usually i-propanol (IPA), are added to the alkaline solutions.

[0005] Processes for producing silicon surfaces with a pyramidal texture, e.g., DE 10 2008 014 166 B3, EP 2 605 289 A2, describe the use of alkaline solutions for treating monocrystalline silicon wafers. A disadvantage of alkaline etching baths is their low reactivity towards silicon at room temperature. To achieve economical removal rates, an increase in temperature and thus a high energy input is necessary. Due to the boiling point of IPA, the maximum temperature is approximately 80 °C. Cleaning of the silicon wafer surface, e.g., from metal contamination, using alkaline solutions is only partially effective. This necessitates additional process steps for cleaning and removing the metal contamination.

[0006] Traditionally, anisotropic etching processes for the treatment of silicon wafers were carried out using alkaline solutions. Acidic solutions for the creation of such textures were long excluded by experts, e.g. Seidel, H. [et al.]: Anisotropic Etching of Crystalline Silicon in Alkaline Solutions: I. Orientation Dependence and Behavior of Passivation Layers, . In: J. Electrochem. Soc. , Vol. 137, 1990, pp. 3612-3626 . As a rule, an isotropic etching process is present in acidic etching systems (HF-HNO3). These are therefore unsuitable for the creation of pyramids on monocrystalline silicon wafers. On the other hand, acidic solutions are preferred for the creation of textures and polishes on polycrystalline silicon wafers, some of which contain other mineral acids or other oxidizing agents (e.g. US 2003 / 0 119 332 A1). In some cases, surface-active substances are also added to increase the wetting of the silicon surface (e.g. US 2013 / 0 130 508 A1).The disadvantages are the enormous toxicity of the solutions and the release of nitrous gases (NO. x ). This makes costly processing necessary.

[0007] Publications on investigations of acidic solutions of hydrofluoric acid (HF), hydrochloric acid (HCl) and added oxidizing agents, e.g. DE 43 25 543 A1, showed the cleaning effect and the basic suitability for etching, but not for texturing silicon surfaces, especially the creation of pyramid structures.

[0008] Furthermore, an aqueous mixture of hydrofluoric acid (HF), hydrochloric acid (HCl), and oxidizing agents, e.g. in DE 10 2014 001 363 B3, was described as an etching solution that can be used for the isotropic and anisotropic etching of silicon surfaces. Depending on their composition, the etching solutions can be used for texturing monocrystalline silicon wafers, as well as for polishing or removing saw damage. Alternatively, chlorine gas was used instead of hydrochloric acid (HCl) and oxidizing agent. Due to the poor solubility of chlorine gas in the solution and its constant consumption, the chlorine gas should be continuously introduced into the solution, for example because permanent work with a saturated solution may be necessary to keep the reactivity of the etching solution approximately constant (reproducible) over many etching steps.In addition, the use of chlorine gas is associated with risks, for example due to its toxicity, its high corrosiveness (e.g., chlorine gas is extremely corrosive in the presence of traces of moisture, for example to stainless steel), and the need for storage or transport under high pressure.

[0009] This process, which uses chlorine and / or a chlorine source, can have other disadvantages. For example, the oxidizing agent (i.e. the combination of hydrochloric acid (HCl) and oxidizing agent, or chlorine gas) is continuously consumed, so that a constant supply of chlorine gas is necessary to ensure constant reactivity of the oxidizing agent. Furthermore, chlorine gas is produced as exhaust gas during the process, for example because a significant excess of chlorine gas can be discharged from the plant unused due to the constant supply. This means that a significant portion of the chlorine gas cannot be used for the reaction but goes directly into the exhaust gas. This chlorine gas as exhaust gas can be extremely corrosive and polluting and / or harmful to the environment and must therefore be separated and disposed of.

[0010] Given that oxidative treatment of silicon wafers in an acidic environment is a highly complex process that is, in most cases, not yet fully understood, it can be very difficult to find a suitable (e.g., efficient) oxidant for wet chemical etching, especially in combination with hydrofluoric acid (HF). Above all, the oxidant or the overall process should achieve a high removal rate, be safe from an occupational health and safety perspective, and / or eliminate the other disadvantages mentioned above.

[0011] For less complex etching systems, it can be assumed that better material removal can be achieved using a stronger oxidizing agent. Starting with chlorine, for example, one would turn to stronger oxidizing agents in order to achieve a higher etching rate. However, extensive research has shown that this is not the case with silicon wafer etching. Evidence of the complexity of silicon wafer etching systems can be found, for example, in Gondek C. [et al.]: Etching Silicon with HF-H2O2-Based Mixtures: Reactivity Studies and Surface Investigations, . In: J. Phys. Chem. C, Vol. 118, 2014, pp. 2044-2051, and e.g. in Gondek C. [et al.]: Etching Silicon with Aqueous Acidic Ozone Solutions: Reactivity Studies and Surface Investigations, . In: J. Phys. Chem. C, Volume 120, 2016, p.22349-22357 It was also found that the oxidizing agents commonly used in inorganic chemistry, which are classified as stronger oxidizing agents than chlorine, for example peroxides (e.g. hydrogen peroxide), ozone, peroxodisulfates, permanganate salts, dichromate salts, perchloric acid and perchlorates, do not work efficiently in combination with hydrofluoric acid, ie the surface of the silicon wafers is not etched or is etched much too slowly for industrial applications and / or texturing of the surface of the silicon wafers is not achieved or is not achieved evenly.

[0012] Further processes are known from: Schubert, N. [et al.]: HF-HBr-Br2-Mixtures for Wet Chemical Etching of Silicon Wafers - Reactivity Studies, Analytics of Etching Solutions and Surface Morphologies. Poster at the 8th WCPEC, 26-30 Sept. 2022, Milan, Italy; DE 12 87 405 A; US 3 272 748 A; US 2 619 414 A; US 2018 / 0 040 748 A1; and US 2004 / 0 076 813 A1

[0013] The invention is set out in the appended claims.

[0014] Various aspects of the present invention are directed to providing a method for producing textures, structures or polishes on the surface of silicon, for example monocrystalline silicon wafers, in which inexpensive, easily accessible non-toxic raw materials can be used, which can be carried out in an energy-saving manner near room temperature, in which the cleaning and texturing or polishing of the wafer surface takes place simultaneously, in which fewer or no toxic exhaust gases are released and smaller or no amounts of waste water are produced, with which a sufficiently high removal rate can be achieved so that in-line processing is possible, and which produces comparable textures on both SiC slurry-sawed and diamond wire-sawed or otherwise pretreated silicon wafers.

[0015] According to various embodiments, it has been found that when using water-containing or aqueous mixtures comprising bromine (Br2) together in combination with hydrofluoric acid (HF) and hydrogen bromide (HBr) as etching solution (also referred to herein as solution for short) in the treatment of monocrystalline (100) silicon wafers, pyramids with a square base can be produced (i.e. without masking or lithographic processes), and the etching rates are very high (in particular higher than with analogous chlorine-containing solutions) although the oxidizing power of the bromine (E 0 = 1 V) is significantly weaker than the oxidizing power of chlorine (E 0 = 1.36 V) or as the oxidizing power of the oxidizing agents commonly used in inorganic chemistry.

[0016] According to a first aspect, a method for producing textures, structures or polishes on the surface of silicon, for example monocrystalline silicon wafers, can be provided by subjecting the surface to an etching process, wherein a water-containing or aqueous mixture is used as etching solution in the method, wherein the etching solution comprises hydrofluoric acid (HF) and bromine (Br2).

[0017] The use of bromine-containing oxidants enables an efficient process for creating textures, structures, or polishes on the surface of silicon. This can be achieved with high removal rates and can lead, for example, to silicon wafers with high performance, such as higher light absorption. In relation to the finished solar cell, the efficiency of the finished solar cell can be increased through the texture. By using bromine (Br2), for example, impurities can be removed from the surface of the silicon workpieces in parallel with texturing, structuring, or polishing. This leads to highly precise texturing or polishing of the surface of silicon, for example, monocrystalline silicon wafers.

[0018] In addition, bromine (Br2) exhibits high solubility in the etching solution, for example, compared to some known oxidizing agents. This can enable high removal rates (e.g., higher removal rates than with the oxidizing agent chlorine). Due to the high removal rates that can be achieved by the method according to the present invention, efficient in-line processing is possible, for example.

[0019] A further advantage is that bromine (Br2) can be introduced into the etching solution in liquid form. Handling the oxidizing agent is simplified, for example, because bromine (Br2) is a liquid and, due to its non-volatility, is easy to dose in the etching solution. Furthermore, unlike the reaction products of known oxidizing agents, the consumed bromine remains in the solution (i.e., little or no offgas is released). This can increase the safety of the process during silicon wafer processing.

[0020] According to various embodiments, the etching solution may have the following concentrations of hydrofluoric acid (HF) and bromine (Br2): • c(HF) = 0.1 to 10 mol / L, for example 1 to 7 mol / L, for example 1.5 to 6 mol / L; and • c(Br2) = 0.01 to 0.2 mol / L, for example 0.03 to 0.15 mol / L, for example 0.06 to 0.15 mol / L.

[0021] The removal rate of the etching solution depends on the bromine concentration. For example, the removal rate can increase with increasing bromine concentration in the etching solution.

[0022] A complete understanding of the reaction mechanisms is still largely lacking. The basic processes, including the chemical reaction steps and mechanisms of the wet-chemical treatment of silicon, are very complex and not yet fully understood. During the etching process, bromine (Br2) can oxidize the silicon, forming bromide (Br -) can be formed (equation (1)). According to various embodiments, bromide can be present in equilibrium with various species in the etching solution, such as hypobromite, bromite, bromate, and / or bromine oxides. Subsequently, hydrofluoric acid (HF) or fluoride ions can act as complexing agents to transport the silicon into the liquid phase, causing the coordination and dissolution of the oxidized silicon species (equation (2)). Si + 2 Br2 → Si 4+ + 4 Br - (1) Si 4+ + 6 F - → SiF6 2- (2)

[0023] According to various embodiments, the consumed bromine, for example in the form of bromide (Br -) from the reaction of bromine (Br2) with silicon, unlike other etchants, can be regenerated. This is possible because the spent bromine can remain in the etching solution. For example, the spent bromine can be regenerated by adding an oxidizer to the etching solution to regenerate the spent bromine in the form of bromide (Br - ) is added back into bromine (Br2). Thus, according to various embodiments, a catalytic amount of bromine (Br2) can be used in the etching solution due to the regeneration potential of the consumed bromine. For example, the ratio of the concentration of hydrofluoric acid (c(HF)) to the concentration of bromine (c(Br2)) can be in a range from 1000:1 to 10:1, for example from 500:1 to 50:1. By using bromine in a catalytic amount, the process can remain cost-effective. This can be of particular relevance, for example, due to the high price of bromine (Br2).

[0024] According to various embodiments, the concentration of oxidant in the etching solution may be in a range from 0.01 mol / L to 10 mol / L. According to various embodiments, bromine (Br2) may be obtained from bromide (Br - ) can be regenerated in-situ using a small amount of oxidant or electrochemically, for example, starting at an oxidant concentration of 0.1 mol / L. It should be noted in this regard that an accurate determination of the oxidant concentration in the etching solution can be difficult, as it is directly consumed in the etching solution. Consequently, the oxidant concentration present in the etching solution can be very close to 0 mol / L. In the gas stream (O3 in O2), for example, ozone in oxygen can be used at concentrations ranging from 1 wt% to 20 wt%.

[0025] For example, hydrogen peroxide (H2O2) or ozone (O3), or ozone-oxygen or ozone-air mixtures, can be used as oxidants to regenerate the bromine consumed during the process. The following oxidant concentrations can be used, for example: • V(O3) = 0.1 to 10 L / h per 100 mL etching solution, for example 2 to 5 L / h per 100 mL etching solution; • V(H2O2) = 0.01 mol to 0.1 mol per 100 mL etching solution for an etching time in a range of 1 to 30 minutes.

[0026] In the case where hydrogen peroxide (H2O2) is used, for example, bromide (Br - ) can be regenerated into bromine (Br2) according to equation (3). H2O2 + 2Br - + 2H + → 2H2O + Br2 (3)

[0027] Additional oxidants can be used to regenerate the spent bromine, as described below.

[0028] According to various embodiments, the bromine (Br2) can be added as such (CAS number: 7726-95-6), for example in liquid form, to the etching solution. Alternatively or additionally, the bromine (Br2) can be generated by a homogeneous and / or heterogeneous chemical reaction in the etching solution and / or on the silicon surface. For example, the bromine (Br2) can be generated from a bromine source, for example by means of an oxidizing agent, or by other chemical reactions (e.g., from hypobromites, bromites, bromates, or perbromates) or electrochemically in the etching solution. The bromine source can provide a bromide, which can be oxidized to bromine (Br2) by means of the oxidizing agent. According to various embodiments, the bromine source can oxidize the surface of the silicon without reacting with an oxidizing agent, for example, when bromate is used.For example, the addition of an oxidizing agent may be unnecessary or even avoided. According to various embodiments, when the Br2 is generated from a bromine-oxygen species, the reactant is a reducing agent.

[0029] According to various embodiments, the water-containing or aqueous mixture (also referred to as etching solution) may further comprise a bromine source (for example, in addition to bromine (Br2) as such). As a result, the etching solution may, for example, lead to an anisotropic etching process of the surface of the silicon wafers.

[0030] For example, according to one embodiment, if no bromine source other than bromine (Br2) is added to the etching solution, the etching process can be carried out isotropic. The etching solution can be used, for example, for cleaning silicon fragments, polishing, or removing saw damage on silicon, for example, monocrystalline or multicrystalline silicon wafers. Alternatively, if the etching solution contains a bromine source other than bromine (Br2), the etching process can be carried out anisotropic, for example, for texturing silicon, for example, monocrystalline or multicrystalline silicon wafers.

[0031] According to various embodiments, the bromine source can be provided by means of a bromide (Br -), for example by means of hydrobromic acid (HBr) and / or a bromide-containing salt, and at least one oxidizing agent (e.g. which can also be used for the regeneration of the spent bromine). In other words, the bromine source can comprise a bromide and at least one oxidizing agent. For example, a chemical reaction of the bromide with an oxidizing agent can provide at least bromine (Br2). In other words, the solution can comprise bromide and oxidizing agent. The bromide can be generated from the bromine (Br2) by means of the reaction of the bromine (Br2) with the silicon, and / or can be provided from a bromine source.

[0032] The bromine and / or the bromine source can act as a bromine donor (also referred to as a bromine donor), which is able to release bromine and / or bromide ions (e.g., hypobromite, bromite, bromate, and perbromate, as well as bromine oxides) and, in a chemical reaction between the solution and the silicon surface (e.g., with silicon atoms on the silicon surface), provide bromine and / or bromide ions. For example, parts of the bromine source can react, e.g., oxidize, to form bromine. Bromine (Br2) can also be generated, for example, through comproportionation reactions (e.g., also referred to as synproportionations) by reacting bromide (Br - ) or bromine compounds, in which bromine has the oxidation number -1, with hypobromite (BrO - ), bromite (BrO2 - ), bromate (BrO3 - ) or perbromate (BrO4 -) or bromine oxides (BrOx) or other compounds in which bromine is present in a positive oxidation state (e.g., BrClx or BrFx). This can result in, for example, two bromine sources being present to generate the bromine (Br2). The bromine and / or the bromine source can, for example, be capable of transferring at least bromine and / or bromide to a reactant (e.g., the silicon surface).

[0033] According to various embodiments, the etching solution may have the following concentrations: • c(HF) = 0.1 to 10 mol / L, for example 1 to 7 mol / L; • c(HBr) = 0 to 10 mol / L, for example 0 to 9 mol / L; and • c(Br2) = 0.01 to 0.2 mol / L, for example 0.06 to 0.15 mol / L.

[0034] In addition, the etching solution may contain an oxidizing agent, for example in a concentration in a range of 0.1 mol / L to 10 mol / L.

[0035] According to a second aspect, a method for producing textures, structures, or polishes on the surface of silicon, for example, monocrystalline silicon wafers, can be provided by subjecting the surface to an etching process. The method uses a water-containing or aqueous mixture as the etching solution, which mixture comprises hydrofluoric acid (HF) and a bromine source, for example, without the addition of bromine (Br2) as such (CAS number: 7726-95-6). The bromine source can be as defined in the first aspect; for example, the bromine source can be provided by means of a bromide, for example, hydrobromic acid (HBr) and / or a bromide-containing salt, and at least one oxidizing agent.

[0036] According to various embodiments, the etching solution may have the following concentrations: • c(HF) = 0.1 to 10 mol / L, for example 1 to 7 mol / L; • c(HBr) = 1 to 10 mol / L, for example 1 to 9 mol / L; and • c(oxidizing agent) = 0.1 to 10 mol / L, for example 0.1 to 7 mol / L.

[0037] According to various embodiments of the first and second aspects, the etching solution can contain ammonium peroxodisulfate ((NH4)2S2O8), sodium peroxodisulfate (Na2S2O8), hydrogen peroxide (H2O2), ozone (O3), oxygen (O2), nitric acid (HNO3), potassium permanganate (KMnO4), lead(IV) oxide (PbO2), chlorine (Cl2), sodium chlorate (NaClO3), vanadates, cerium dioxide, dichromates, halogen oxides such as ClO2, or a mixture thereof as oxidizing agents. According to various embodiments, for example, a mixture of these and other oxidizing agents can be used. In other words, the solution can contain a mixture of several oxidizing agents.

[0038] According to various embodiments of the first and second aspects, the solution can be provided as a water-containing or aqueous mixture. For example, water can be added to the solution.

[0039] According to various embodiments of the first and second aspects, at least one bromide-containing salt, for example sodium bromide (NaBr), potassium bromide (KBr) and / or ammonium bromide (NH4Br), can be added to the etching solution, for example in addition to the bromine source, which for example comprises hydrobromic acid (HBr).

[0040] According to various embodiments of the first and second aspects, a surface-active substance (component), for example a substance from the group of surfactants, can be added to the etching solution.

[0041] According to various embodiments of the first and second aspects, the process can be carried out at a pH in a range of less than 5, for example at a pH of 1 or less or, for example, at a pH of 0 or less, for example at a pH in a range of -3 to 5, -2 to 1, or -1 to 0. Because the pH is in the acidic range, for example, the equilibrium Br2 + H2O → HBr + HBrO can be very far to the left.

[0042] According to further aspects, a method for processing a silicon surface may be provided, the method comprising: providing a solution comprising hydrofluoric acid (HF), bromine (Br2) and / or a bromine source, and optionally an oxidizing agent; and applying the solution to the silicon surface.

[0043] The solution can be an etching solution corresponding to one of the etching solutions described in connection with the method according to the first or second aspect. According to various embodiments, the etching solution can be characterized by means of UV-Vis and Raman spectroscopy or iodometric titration, whereby the presence of bromine (Br2) can be detected, in a manner analogous to that described, for example, in Stapf A. [et al.]: On The Mechanism of the Anisotropic Dissolution of Silicon in Chlorine Containing Hydrofluoric Acid Solutions . In: J. Electrochem. Soc. , 165(4), 2018, H3045-H3050.

[0044] According to various embodiments, the solution can be applied to a silicon surface with a monocrystalline crystal lattice (also referred to as a single-crystal crystal lattice or single crystal). According to various embodiments, the solution can be applied to or brought into contact with a silicon surface with a polycrystalline crystal lattice.

[0045] According to various embodiments, the solution can be applied to the silicon surface of a silicon wafer, e.g., to the surface of a monocrystalline silicon wafer, to a surface of a silicon wafer created by sawing the silicon wafer, or to an already processed silicon surface, e.g., a silicon surface that has already been etched, ground, polished, cleaned, or otherwise processed. For example, a solution according to various embodiments can be applied to a silicon surface multiple times, e.g., consecutively, e.g., with different compositions. This allows different surface morphologies to be superimposed on one another.

[0046] According to various embodiments, the silicon surface can be subjected to an etching process by applying the solution. In other words, by applying the solution, the silicon surface can be etched, e.g., ablated, structured, polished, and / or textured in combination with other (lithographic) processes. In other words, the solution can act or be used as an etchant (also referred to as an etching solution).

[0047] According to various embodiments, the silicon surface can be textured, structured, and / or polished (e.g., smoothed) by applying the solution. In other words, a texture, a structure, and / or a polish can be created on the silicon surface by applying the solution. For this purpose, the solution can react chemically with the silicon surface, for example, with silicon atoms of the silicon surface. In this process, silicon can be removed from the silicon surface, and a texture and / or a structure and / or a polish can be created by removing silicon.

[0048] By texturing the silicon surface, a structure can be created on the silicon surface, for example, a structure with typical patterns, depressions, or protrusions (in other words, elevations), e.g., a so-called pyramid structure. For example, depressions such as trenches or openings, e.g., pyramid-shaped openings or openings of other shapes, can be etched into the silicon surface, or projections such as ridges, points, pyramids, or other shaped islands can be left behind during etching, or an overlay of these structures (e.g., depressions and protrusions) can be created.

[0049] By structuring the silicon surface, a structure of the silicon surface can be obtained by anisotropic wet chemical etching, which is not textured but is structured with specific structures (often in combination with lithographic processes), e.g. for AFM cantilevers and AFM tips or loudspeaker membranes or structures for microfluidics.

[0050] By polishing the silicon surface, the silicon surface can be smoothed, allowing a silicon surface with low roughness to be created. For example, a silicon surface with a roughness of a few nanometers (nm) can be created, e.g., less than approximately 10 nm, e.g., less than approximately 1 nm.

[0051] According to various embodiments, solids, e.g., hypobromites, bromates, perbromates, or bromide-containing salts, can be added to the solution. The solid can, for example, completely dissolve in the solution, partially dissolve, and / or decompose and / or chemically react upon contact with the solution, releasing bromine or bromide and thus acting as a bromine source. Depending on the surface morphology to be created, a suitable solid can be added at an appropriate rate and / or quantity, releasing a specific amount of bromine or bromide. For example, low removal rates can be achieved by adding hypobromite or perbromate. Alternatively, or in combination, several solids can be added.

[0052] According to various embodiments, providing the solution may comprise providing the bromine source by, for example, adding a liquid to the solution. In other words, liquids may be added to the solution which, for example, dissolve in the solution and / or, upon contact with the solution, decompose and / or chemically react, releasing bromine or bromide and thus acting as a bromine source. For example, metal and / or non-metal bromides, e.g. molecular non-metal bromides and / or alkali metal bromides, may be added to the solution, which act as a bromine source. Depending on which surface morphology is to be created, a suitable liquid can be added at an appropriate rate and / or amount, which releases a specific amount of bromine or bromide.

[0053] According to various embodiments, at least one solid, at least one liquid and / or at least one gas can be added to the solution in combination or alternatively to one another.

[0054] Bromine or bromide can be released, for example, through hydrolysis of a solid, liquid, and / or gas added to the solution. Illustratively, the solid, liquid, and / or gas can react chemically with the water present in the solution. For example, hydrolysis can release and / or generate hydrobromic acid (HBr).

[0055] According to various embodiments, the method can be carried out at a temperature in a range of 15°C to 30°C. An increase in temperature can lead to an increase in the removal rates.

[0056] According to various embodiments, the method according to the invention can be carried out even at room temperature, for example, at 21 °C. Energy input or output, for example, by heating or cooling, or energy supply, for example, ion bombardment, is not necessary for the production of textures, structures, or polishes on the surface of silicon, for example, monocrystalline silicon wafers, according to the method according to the invention.

[0057] According to various embodiments, a solution comprising hydrofluoric acid (HF), bromine (Br2) and / or a bromine source, and optionally an oxidizing agent, can be used for processing a silicon surface. For example, an etching solution can be used as a water-containing or aqueous mixture for processing, e.g., etching, texturing, structuring, or polishing, a silicon surface made of • Hydrofluoric acid (HF) and bromine (Br2), and for example an oxidizing agent, or • Hydrofluoric acid (HF), hydrobromic acid (HBr) and an oxidizing agent, or • Hydrofluoric acid (HF), hydrobromic acid (HBr) and bromine (Br2), and for example an oxidizing agent.

[0058] According to various further aspects, the use of an etching solution for generating textures, structures, or polishes on the surface of silicon, for example, monocrystalline silicon wafers, can be provided, wherein the etching solution comprises: hydrofluoric acid (HF); and bromine (Br2) and / or a bromine source, and optionally an oxidizing agent. For example, the etching solution can be used as a water-containing or aqueous mixture for processing, e.g., etching, texturing, structuring, or polishing, a silicon surface made of • Hydrofluoric acid (HF) and bromine (Br2), and for example an oxidizing agent, or • Hydrofluoric acid (HF), hydrobromic acid (HBr) and an oxidizing agent, or • Hydrofluoric acid (HF), hydrobromic acid (HBr) and bromine (Br2), and for example an oxidizing agent.

[0059] According to various embodiments, bath compositions are used which c(HF) = 0.1 to 10 mol / L, preferably 1 to 7 mol / L, c(HBr) = 0 to 10 mol / L, preferably 0 to 9 mol / L and c(Br2) = 0.01 to 0.2 mol / L, preferably 0.06 to 0.15 mol / L The etching solution may additionally contain bromide-containing salts such as sodium bromide (NaBr), potassium bromide (KBr) and / or ammonium bromide (NH4Br), hypobromite, bromite, bromate, or perbromate. Surface-active substances (e.g., from the group of surfactants) may also be added to influence the wettability of the silicon surface. Using a method according to various embodiments, monocrystalline silicon(100) wafers can be etched, regardless of their pretreatment. Specifically, as-cut diamond wire-sawed, as-cut SiC slurry-sawed, pre-cleaned, pre-textured, pre-structured, or pre-polished silicon(100) wafers can be used.

[0060] The solution can be applied to the silicon surface using a bath (e.g., an etching bath). For example, a silicon wafer can be immersed in the solution. Depending on the intended surface morphology (texture, structure, and / or polish) to be created on the silicon wafer, the solution can have a specific composition (bath composition). This means that the solution contains the respective components (bath components, e.g., acids or oxidizing agents) in a specific concentration.

[0061] Furthermore, uniformly textured, structured or polished surfaces can be created by changing the proportions of the bath components.

[0062] The invention is further illustrated by the following embodiments: Example 1 - Texturing of monocrystalline silicon(100) surfaces

[0063] When using the following bath composition c(HF) = 1.5 mol / L, c(HBr) = 6.6 mol / L and c(Br2) = 0.03 mol / L and if the following process parameters are observed: treatment time t = 20 min, Treatment temperature ϑ = 20 °C average removal rates (a rate at which material is removed, e.g. etched) of r = 10 nm / s are obtained. Example 2 - Polishing of monocrystalline silicon(100) surfaces

[0064] When using the following bath composition c(HF) = 5.8 mol / L, and c(Br2) = 0.13 mol / L and if the following process parameters are observed: treatment time t = 20 min, Treatment temperature ϑ = 20 °C average removal rates of r = 15 nm / s are obtained.

[0065] By adjusting or regulating the amount of bromine and / or the bromine source and the associated oxidant, the removal rate and / or the resulting surface morphology can be influenced. In the case of hydrobromic acid (HBr), for example, the removal rate cannot be controlled solely by the amount of hydrobromic acid (HBr). Since the associated oxidant may be responsible for Br2 generation, its concentration can have a greater influence on the etch rate than the HBr concentration.

[0066] Further examples of the composition of the solution are summarized in the following table: c (HF)[mol / L] c (HBr)[mol / L] c (Br2)[mol / L] t[min] ϑ[°C] Wafer pretreatment r[nm / s] Preserved surface 6 0 0, 13 20 21 as-cut, DW (1) 15,3 polish 3 0 0,03 15 21 as-cut, DW (1) 4, 4 polish 3 0 0,06 5 21 as-cut, DW (1) 11,7 polish 3 0 0, 12 5 21 as-cut, DW (1) 18,2 TransitionTexture Polish 3 0 0,24 15 21 as-cut, DW (1) 21, 6 TransitionTexture Polish 1,5 8, 4 0, 03 5 21 as-cut, DW (1) 1, 6 texture 1,5 5,7 0, 03 5 21 as-cut, DW (1) 9, 4 texture 1,5 3,1 0, 03 5 21 as-cut, DW (1) 6, 9 texture 1,5 0, 4 0, 03 5 21 as-cut, DW (1) 3, 8 texture 1,5 0, 4 0, 06 5 21 as-cut, DW (1) 13,4 texture 6 7 0, 03 5 21 as-cut, DW (1) 4, 6 texture 6 5,3 0, 03 5 21 as-cut, DW (1) 9, 3 texture 6 3,5 0,03 5 21 as-cut, DW (1) 7, 8 texture 6 1, 8 0, 03 5 21 as-cut, DW (1) 4, 3 texture 1,5 3,5 0,03 5 21 as-cut, DW (1) 11, 6 texture 1,5 3,5 0,03 10 21 as-cut, DW (1) 4, 7 texture 1,5 3,5 0,03 15 21 as-cut, DW (1) 3, 0 texture (1) DW - Diamond Wire, diamond wire sawn

[0067] In some embodiments, at least one oxidizing agent may be added to the etching solution containing HBr, e.g., hydrogen peroxide (H2O2) at a concentration of 0.1 to 2 mol / L to generate bromine (Br2). Alternatively, ozone (O3), for example, may also be used as the oxidizing agent, which is introduced into the etching solution in gaseous form. Examples of HF-HBr oxidizers c (HF)[mol / L] c (HBr)[mol / L] c (H2O2)[mol / L] t[min] ϑ[°C] Wafer pretreatment r[nm / s] Preserved surface 1,5 3, 1 0, 03 20 21 as-cut, DW (1) 5, 1 texture 3 5, 7 0, 12 5 21 as-cut, DW (1) 8, 2 texture 6 5, 3 0, 03 5 21 as-cut, DW (1) 7, 9 texture

[0068] In the following, various examples are described which relate to what has been described above and shown in the figures.

[0069] Example 1 is a method for treating silicon by subjecting the silicon material to an etching process, characterized in that a water-containing or aqueous mixture of hydrofluoric acid (HF) and bromine and / or a water-containing or aqueous mixture of hydrofluoric acid (HF) and a bromine source is used as the etching solution.

[0070] In Example 2, the process according to Example 1 may further comprise that the bromine source comprises at least one bromide and at least one oxidizing agent.

[0071] In Example 3, the process according to Example 1 or 2 may further comprise that the bromine source comprises at least hydrobromic acid and at least one oxidizing agent.

[0072] In Example 4, the method according to Example 2 or 3 may further comprise that the etching solution contains as oxidizing agent ammonium peroxodisulfate ((NH4) 2S2O8), sodium peroxodisulfate (Na2S2O8), hydrogen peroxide (H2O2), ozone (O3), nitric acid (HNO3), potassium permanganate (KMnO4), lead(IV) oxide (PbO2), chlorine (Cl2), sodium chlorate (NaClO3), vanadates, cerium dioxide, oxygen, dichromates, halogen oxides such as ClO2 or a mixture of several of these oxidizing agents.

[0073] In Example 5, the process according to Example 1 may further comprise that the etching solution contains the following concentrations of hydrofluoric acid and bromine: c(HF) = 0.1 to 10 mol / L, and c(Br2) = 0.01 to 0.2 mol / L.

[0074] In Example 6, the process according to Example 3 may further comprise that the etching solution contains the following concentrations of hydrofluoric acid and bromine: c(HF) = 0.1 to 10 mol / L, c(HBr) = 1 to 10 mol / L, and c(oxidizer) = 0.1 to 8 mol / L.

[0075] In Example 7, the method according to any one of Examples 1 to 6 may further comprise adding at least bromine-containing salts to the etching solution.

[0076] In Example 8, the method according to any one of Examples 1 to 7 may further comprise adding at least sodium bromide (NaBr), potassium bromide (KBr) and / or ammonium bromide (NH4Br) to the etching solution.

[0077] In Example 9, the method according to any one of Examples 1 to 8 may further comprise adding at least one surface-active substance to the etching solution.

[0078] In Example 10, the process according to Example 9 may further comprise that the surface-active substance is a substance from the group of surfactants.

[0079] In Example 11, the method according to Example 1 may further comprise that the silicon material is in the form of fragments or wafers.

[0080] In Example 12, the method according to Example 11 may further comprise that the wafers are monocrystalline oriented.

[0081] Example 13 is an etching solution for treating silicon, the etching solution comprising: hydrofluoric acid and bromine and / or a bromine source.

[0082] Example 14 is using an etching solution to treat silicon, the etching solution comprising: hydrofluoric acid and bromine and / or a bromine source.

[0083] A water-containing mixture described herein may contain water but additionally another solvent (e.g. another polar solvent, e.g. methanol or ethanol) in relevant amounts, e.g. even more other solvent than water.

[0084] In contrast, an aqueous mixture can be understood to mean that water is the main solvent, e.g. it forms the majority or the entirety of the solvent.

Claims

[1] Method for producing textures or structures on a silicon surface by subjecting the surface to an etching process using a water-containing or aqueous mixture as the etching solution, wherein the etching solution comprises hydrofluoric acid (HF), bromine (Br2) and additionally a bromine source, wherein the etching solution has the following concentrations of hydrofluoric acid and bromine: c(HF) = 0.1 to 10 mol / L, and c(Br2) = 0.01 to 0.2 mol / L. [2] Method according to claim 1, wherein the bromine (Br2) is added as such to the etching solution. [3] Method according to claim 1 or 2, wherein the bromine (Br2) is generated from a bromine source in the etching solution and / or on the silicon surface. [4] Method according to claim 3, wherein the bromine source is provided by means of liquid hydrobromic acid (HBr), and / or wherein the bromine source is provided by means of at least one bromide salt. [5] Method according to any one of claims 1 to 4, wherein the etching solution further comprises an oxidizing agent for regenerating spent bromine, wherein the spent bromine is in a reduced form which is produced by the reaction of the bromine (Br2) with the silicon. [6] Method according to any one of claims 1 to 4, wherein the etching solution is bromide (Br - ) which is produced from the bromine (Br2) by the reaction of the bromine (Br2) with the silicon and / or is provided from a bromine source, and wherein the etching solution further comprises an oxidizing agent for oxidizing the bromide (Br - ) in bromine (Br2). [7] Method according to any one of claims 1 to 6, wherein the etching solution has the following concentrations of hydrofluoric acid (HF), hydrobromic acid (HBr) and bromine (Br2): c(HF) = 0.1 to 10 mol / L, c(HBr) = 0 to 10 mol / L, and c(Br2) = 0.01 to 0.2 mol / L. [8] Method according to any one of claims 1 to 7, wherein the bromine (Br2) is added to the etching solution in a catalytic amount or is generated by chemical reactions. [9] Method according to any one of claims 1 to 8, wherein the etching solution further comprises an oxidizing agent. [10] Method according to any one of claims 1 to 9, wherein at least one surfactant is added to the etching solution. [11] Method according to any one of claims 1 to 10, wherein the silicon is in the form of fragments or wafers. [12] Etching solution for producing textures or structures on the surface of silicon, wherein the etching solution comprises hydrofluoric acid (HF), bromine (Br2) and a bromine source, wherein c(HF) = 0.1 to 10 mol / L, and c(Br2) = 0.01 to 0.2 mol / L. [13] Method according to any one of claims 1 to 11, wherein further components selected from polar solvents, acids, dissolved uncharged molecular compounds, soluble salts, insoluble components or gases are added to the aqueous etching solution. [14] Using an etching solution to produce textures or structures on the surface of silicon, wherein the etching solution comprises hydrofluoric acid (HF), bromine (Br2) and a bromine source, wherein c(HF) = 0.1 to 10 mol / L, and c(Br2) = 0.01 to 0.2 mol / L.