Method for cleaning semiconductor substrates for solar cells and corresponding cleaning system - Patents.com

Abstract

A method for cleaning a semiconductor substrate for a solar cell, the method including providing a semiconductor substrate, pre-oxidizing the substrate with a pre-oxidizing solution, oxidizing the substrate with the oxidizing solution to form an oxide on a surface of the substrate, and removing the oxide from the surface of the substrate with an oxide removal solution, the pre-oxidizing solution configured to remove metal ions from the surface of the substrate prior to formation of an oxide on the surface of the substrate, the pre-oxidizing solution being an acid solution including hydrogen chloride and no other acid forming components.

Description

[Technical field]

[0001] The present disclosure relates to a method for cleaning a semiconductor substrate for a solar cell and a cleaning system for cleaning a semiconductor substrate for a solar cell. [Background technology]

[0002] A solar module for providing electrical energy from sunlight comprises an array of solar cells, each comprising a substrate formed from a semiconductor material such as silicon (e.g., a silicon wafer). Additional layers of semiconductor material are deposited on a surface of the substrate to form the solar cells.

[0003] A general objective for solar cell development is to achieve high conversion efficiency balanced with the need for reduced production costs. Despite careful fabrication procedures, it is known that the surface of semiconductor substrates can be contaminated with organic and / or metallic contaminants (e.g., dust, dirt, anions, cations, and other particles, etc.).

[0004] One way to improve the performance of solar cells is to remove as many contaminants as possible from the surface of the substrate before depositing the other semiconductor layers of the device. Attempts to achieve this have focused on cleaning the semiconductor substrate using wet bench chemical techniques and methodologies.

[0005] One approach to removing surface contaminants is to immerse the substrate in "ozonated water," which has a very high oxidation potential that allows it to oxidatively decompose organic materials. It is also known to use aqueous solutions containing hydrogen fluoride (i.e., HF acid) to remove contaminants by lifting off the thin native oxide in which the contaminants are embedded.

[0006] Despite these attempts, there remains a need for improved methods of removing contaminants from substrate surfaces to improve the performance of fabricated solar cells. Summary of the Invention

[0007] It is an object of the present invention to provide a method for cleaning a semiconductor substrate, such as a silicon wafer, to remove contaminants from the surface of the substrate.

[0008] According to a first aspect of the present invention, there is provided a method of cleaning a semiconductor substrate for a solar cell, comprising providing a semiconductor substrate, pre-oxidizing the substrate with a pre-oxidizing solution, oxidizing the substrate with the oxidizing solution to form an oxide on a surface of the substrate, and removing the oxide from the surface of the substrate with an oxide removal solution, wherein the pre-oxidizing solution is configured to remove metal ions from the surface of the substrate prior to formation of an oxide on the surface of the substrate, and the pre-oxidizing solution is an acid solution comprising hydrogen chloride and no other acid forming components.

[0009] It will be understood that a pre-oxidation method step defines a method (e.g., procedure / process) that is performed prior to an oxidation method step. A pre-oxidation method step is not an oxidation method step, as opposed to an oxidation method step that is explicitly configured to oxidize the surface of a substrate. Thus, a pre-oxidation solution, as opposed to an oxidizing solution, is not configured to oxidize the surface of a substrate.

[0010] It has been found that the process of the present invention provides an enhanced method of cleaning semiconductor substrates. In particular, the pre-oxidation step complements the oxidation and oxide removal steps in removing contaminants from the surface of the substrate. In particular, the pre-oxidation step is particularly useful in removing contaminant metal ions that are deposited on the surface of the substrate during early cutting and texturing of the substrate.

[0011] Removal of metal ions from the surface of the substrate reduces metal contamination of the solutions used in subsequent oxidation and oxide removal steps, which can result in reduced concentrations of hydrogen chloride in the subsequent oxidation and oxide removal solutions.

[0012] In an exemplary method, at least one or each of the oxidizing solution and the oxide removal solution may contain hydrogen chloride. In this situation, the presence of hydrogen chloride in the pre-oxidizing solution means that the concentration of hydrogen chloride in the other solution can be reduced. Surprisingly, this means that the use of hydrogen chloride in the cleaning method can be reduced overall compared to a cleaning method that does not include a hydrochloric acid pre-oxidation step. Furthermore, the present invention means that all other acid-forming components can be excluded from the pre-oxidizing method step (e.g., the pre-oxidizing solution may be configured such that it also includes any other acid-forming component). In particular, the pre-oxidizing solution may be configured to be free of hydrogen fluoride (HF). HF is very dangerous to humans, so the removal of HF from the pre-oxidizing method step according to the present invention may advantageously reduce the risk of causing harm or injury to users of the substrate cleaning method.

[0013] In a further exemplary configuration, the substrate may be intended for use in the fabrication of solar cells. In this case, the invention leads to improved performance of solar cells fabricated using substrates cleaned according to the above method. For example, it has been shown that substrates cleaned by this method produce solar cells that operate more efficiently than comparable solar cells produced using a cleaning method that does not include a pre-oxidation step.

[0014] In particular, solar cells comprising substrates cleaned according to the above methods are shown to exhibit increased open circuit voltage (Voc) compared to devices fabricated with substrates cleaned according to methods that do not include a pre-oxidation process step.

[0015] The following are optional features, which can be applied alone or in any combination with any aspect.

[0016] The described embodiments are directed to semiconductor substrates comprising silicon materials (e.g., silicon wafers). However, it should be clearly understood that the present invention is not limited to cleaned substrates made from silicon semiconductor materials. Also, any of the known semiconductor materials (e.g., germanium, etc.) that have surface properties similar to silicon can be cleaned by the described methods.

[0017] The method may include providing a textured surface to a substrate. The textured surface may include a plurality of pyramid structures (e.g., a plurality of upright pyramid structures or inverted pyramid structures). The textured surface may be a surface on which an oxide has been formed and removed.

[0018] The method may include texturing the surface of the substrate (e.g., to form a plurality of pyramidal structures). Surface texturing may be achieved by any suitable surface texturing method, including the use of photolithographic, mechanical (e.g., sawing) and laser machining techniques. Alternatively, the method of texturing the surface of the substrate may include the use of a chemical etchant to selectively etch along a particular crystal plane of the material from which the substrate is formed. It will be appreciated that such etching techniques differ from the pre-oxidation method step of the present invention, as they require significant removal of material from the surface of the substrate. On the other hand, the pre-oxidation method step is advantageously configured to remove contaminants (e.g., metal ions) from the surface of the substrate.

[0019] The pre-oxidation solution defines an aqueous solution of hydrogen chloride (i.e., a hydrochloric acid solution). It will therefore be understood that the hydrogen chloride in the pre-oxidation solution is used as the acid-forming component of the solution. This is because, when hydrogen chloride is introduced into water, each HCl molecule and HO molecule combines to form hydronium cations HO+ (i.e., H+ ions) and chloride anions Cl- through a reversible chemical reaction. The pre-oxidation solution according to the present invention does not contain any other substances, elements, molecules, or components capable of reacting with water to form dissociated cations and anions.

[0020] The duration of the pre-oxidation process step may be configured to be at least 50 seconds and up to 250 seconds.

[0021] The method step of pre-oxidizing the substrate may include pre-cleaning the substrate with a pre-cleaning liquid. The method step of pre-oxidizing the substrate may include cleaning the substrate with a cleaning liquid. The method step of pre-oxidizing the substrate may include pre-cleaning the substrate with a pre-cleaning liquid and subsequently cleaning the substrate with a cleaning liquid. At least one or each of the pre-cleaning liquid and the cleaning liquid may comprise an acid solution containing hydrogen chloride and no other acid forming components. The pre-cleaning method step and the cleaning method step may thus define first and second pre-oxidation method steps, respectively, and the pre-cleaning liquid and the cleaning liquid may thus define first and second pre-oxidation solutions, respectively.

[0022] An exemplary method of the present invention may include providing a semiconductor substrate, pre-cleaning the substrate with a pre-clean solution, cleaning the substrate with a cleaning solution, oxidizing the substrate with an oxidizing solution to form an oxide on a surface of the substrate, and removing an oxide from the surface of the substrate with an oxide removal solution, where the pre-oxidizing solution is configured to remove metal ions from the surface of the substrate prior to formation of an oxide on the surface of the substrate, and where each of the pre-clean, cleaning, and pre-oxidizing solutions is an acid solution that includes hydrogen chloride and is free of other acid forming components.

[0023] The concentration of hydrogen chloride in the cleaning solution may be higher than the hydrogen chloride concentration of the pre-clean solution. Providing a pre-clean solution with a lower hydrogen chloride concentration reduces the cost of the cleaning process.

[0024] The pre-cleaning solution may include a hydrogen chloride concentration of at least 0.1% and / or at most 2.5% by weight. In an alternative, the hydrogen chloride concentration may be at least 0.4% and / or at most 2.0% by weight. According to a further alternative, the hydrogen chloride concentration may be approximately 0.8% by weight (e.g., 0.8% by weight). The remainder of the solution may be made up of water (e.g., deionized water).

[0025] The cleaning solution may include a hydrogen chloride concentration of at least 2.5% by weight and / or up to 10% by weight. In an alternative exemplary method, the hydrogen chloride concentration may be at least 3.0% by weight and / or up to 7.0% by weight. According to a further alternative method, the hydrogen chloride concentration may be approximately 5.0% by weight (e.g., 5% by weight). The remainder of the solution may be made up of water (e.g., deionized water).

[0026] It is believed that increasing the concentration of hydrogen chloride in the cleaning solution results in a greater amount of H+ and Cl- ions, which induce a polarization effect on the contaminants held on the substrate surface by strong valence bonds, which overcome the valence bonds, thereby removing the contaminants from the substrate surface.

[0027] The duration of the pre-clean method step may be longer than the duration of the cleaning method step, or alternatively, the duration of the pre-clean method step may be substantially the same as the other cleaning and / or rinsing steps to prevent bottlenecks in production.

[0028] The duration of the pre-clean method step may be configured to be at least 50 seconds and at most 250 seconds. The duration of the cleaning method step may be configured to be at least 50 seconds and at most 250 seconds.

[0029] The pre-clean liquid may be at a temperature that is substantially the same (e.g., equal to) or higher than the temperature of the cleaning liquid. The cleaning liquid may be at a higher temperature than the temperature of the pre-clean liquid. A higher temperature of the pre-clean liquid and / or cleaning liquid acts to increase the rate of the respective pre-clean and / or cleaning steps.

[0030] The pre-wash liquid may be heated to a temperature of at least 20° C. and / or up to 60° C. The wash liquid may be at a temperature of at least 15° C. and / or up to 25° C. In an alternative exemplary method, the wash liquid may be at a temperature of approximately 20° C. (e.g., 20° C.). Thus, the wash method may include not heating the wash liquid, i.e., the wash liquid may be maintained at room temperature.

[0031] It is believed that increasing the temperature of the pre-clean step increases the velocity of the H+ ions, which can migrate faster to the contaminants. The H+ ions can exchange themselves with the contaminants and thus attach themselves firmly to the anionized portions of the surface of the substrate. If the sites where migration occurs are negatively charged, these sites are neutralized and the positively charged contaminants are liberated from the surface of the substrate.

[0032] The oxidizing solution may include an oxidizing agent or oxidizing compound. The remainder of the oxidizing solution may consist of deionized water. The oxidizing agent may be ozone (O3) or hydrogen peroxide (H2O2). The oxidizing solution may thus define an ozone-containing solution. The purpose of the oxidizing solution is to oxidize organic elements remaining from additives used during texturing of the surface of the substrate (e.g., to form pyramidal structures). Ozone has a very high oxidation potential that allows it to oxidatively decompose organic materials. The ozone also oxidizes the surface of the substrate (i.e., the outer exposed surfaces of the pyramidal structures).

[0033] The oxidizing solution may include a hydrogen chloride concentration of at least 0.001% and / or at most 0.1% by weight. In the alternative, the hydrogen chloride concentration may be at least 0.005% and / or at most 0.05% by weight. The remainder of the solution may be made up of water (e.g., deionized water).

[0034] As mentioned above, the pre-oxidation method step means that the hydrogen chloride concentration of the oxidizing solution can be reduced. Advantageously, the hydrogen chloride concentration of the oxidizing solution may be lower than that required for an equivalent method that does not include a pre-oxidation step (i.e., a method that does not expose the substrate to a pre-oxidizing solution prior to oxidizing the surface of the substrate, as defined by the present invention).

[0035] The step of oxidizing the substrate may include forming an oxidizing solution (i.e., an ozone-containing solution) with hydrogen fluoride. The oxidizing solution may include a hydrogen fluoride concentration of up to 0.03% by weight. The hydrogen fluoride simultaneously removes oxides formed on the surface of the substrate by an oxidizing agent in the oxidizing solution.

[0036] The duration of the oxidation method step may be configured to be at least 50 seconds and at most 250 seconds. The duration of the oxide removal method step may be configured to be at least 50 seconds and at most 250 seconds.

[0037] The oxide removal solution may include a mixture of hydrogen fluoride and hydrogen chloride. The removal method steps may include forming the oxide removal solution with hydrogen fluoride, hydrogen chloride, and deionized water. Hydrogen fluoride may be included in a concentration of at least 3% and / or up to 9% by weight. Hydrogen chloride may be included in a concentration of at least 0.2% and / or up to 4% by weight. The remainder of the solution may be made up of water (e.g., deionized water).

[0038] The method may include directing a rinsing fluid to the substrate (e.g., immersing the substrate in the rinsing fluid), which may define a rinsing method step of the cleaning method. The cleaning fluid may be configured to remove any remaining active solution (e.g., any one of a pre-oxidation (e.g., pre-clean and / or clean), oxidation, or oxide removal solution) from the substrate (e.g., a surface of the substrate).

[0039] Thus, the rinsing method step may be configured to prevent continuing chemical reactions after the substrate is removed from the activation solution. The cleaning method step reduces contamination between different activation solutions. The rinsing method step also prevents contamination of subsequent solar cell fabrication processes.

[0040] The duration of the rinse method step may be configured to be at least 50 seconds and up to 250 seconds.

[0041] The rinsing fluid may comprise a liquid (i.e., not a solid or a gas). For example, the rinsing fluid may comprise water, such as deionized water. The rinsing fluid may be a homogenous solution of deionized water (i.e., substantially no other liquids or solid particles are present in the solution). The rinsing fluid may be comprised at a temperature of at least 15° C. and / or up to 25° C. In an alternative exemplary method, the rinsing fluid may be comprised at a temperature of approximately 20° C. (e.g., 20° C.). Thus, the rinsing method step may include not heating the rinsing fluid, i.e., the rinsing solution may be configured to be maintained at room temperature.

[0042] The method may include a rinsing step (e.g., a further rinsing step) performed between the pre-oxidation and oxidation method steps. The method may include a rinsing step performed between the oxidation and removal method steps. At least one or each of the rinsing steps may be configured to remove excess or residual solutions (e.g., pre-cleaning solution, cleaning solution, pre-oxidation solution, oxidation solution, and / or oxide removal solution) from the surface of the substrate to prevent cross-contamination of the solutions.

[0043] The method may include a rinsing step (e.g., a further rinsing step) performed between the pre-cleaning and cleaning method steps, which may include directing a rinsing fluid to the surface of the substrate to remove excess pre-cleaning liquid from the surface of the substrate, thereby preventing contamination of different solutions.

[0044] The or each rinsing step may include immersing (e.g., dipping and removing) the substrate in a reservoir filled with the rinsing fluid. Alternatively, the rinsing step may include directing (e.g., spraying) the rinsing fluid from a rinsing fluid dispensing assembly to the substrate. The rinsing fluid dispensing assembly may be fluidly coupled (e.g., by a fluid conduit) to a source of the rinsing fluid, such as a pressurized pressure vessel. Alternatively, the rinsing fluid may be pumped to the fluid dispensing assembly by a controllable fluid pump, as will be appreciated by those skilled in the art.

[0045] The method may include directing a drying fluid to the substrate, which may define a drying method step of the cleaning method.

[0046] The drying fluid may be a liquid (e.g., water) or a vapor (e.g., nitrogen gas). The drying method steps may include a liquid drying method step that may involve directing a liquid drying fluid to a surface of the substrate. The liquid drying method step may be performed after the oxide removal method step and / or before a subsequent vapor drying method step. The liquid drying method step may include immersing the substrate in a container (e.g., a tank) filled with a rinsing fluid (e.g., a bath). The substrate may be immersed for a period of time (e.g., 50-250 seconds) after which the substrate may be removed (e.g., slowly removed) from the rinsing fluid. The liquid drying fluid may include deionized water. The liquid drying fluid may be provided at ambient room temperature (e.g., near 20° C.). The liquid drying method step may be configured to partially dry the substrate and provide a substantially uniformly wet surface due to the slow removal of the substrate from the liquid drying fluid.

[0047] The drying method step may include a vapor drying method step (e.g., where the drying fluid is a vapor). The drying fluid (e.g., a vapor drying fluid) may be configured to remove any remaining liquid (e.g., pre-clean liquid, cleaning liquid, pre-oxidation solution, oxidation solution, oxide removal solution, rinsing fluid, and / or liquid drying fluid) from the substrate. The vapor drying step reduces residual liquid left on the substrate surface that may attract particles and / or contaminants to the substrate. The drying fluid may be an inert gas, such as nitrogen gas. Alternatively, other suitable inert gases may be used, including, for example, argon.

[0048] The duration of the drying process step (eg, steam drying process step) may be configured to be at least 600 seconds and at most 800 seconds.

[0049] As part of the drying process step, the vapor (e.g., nitrogen gas) may be heated to a temperature of at least 50° C. and / or up to 90° C. The elevated temperature of the drying fluid increases the rate at which the substrate is dried, thereby shortening the overall duration of the cleaning process.

[0050] A vapor drying method step may include directing (e.g., spraying) a vapor drying fluid from a vapor drying fluid dispensing assembly to a substrate. The vapor drying fluid dispensing assembly may be fluidly coupled (e.g., by a fluid conduit) to a source of drying fluid, such as a pressurized pressure vessel. The flow of the vapor drying fluid may be controlled by a variable valve disposed along the fluid conduit between the vapor drying fluid source and the vapor drying fluid dispensing assembly, as will be understood by one skilled in the art.

[0051] At least one or each of the pre-oxidation, oxidation, and oxide removal method steps may include immersing the substrate in a respective solution. According to an alternative method, at least one or each of the pre-oxidation (e.g., pre-cleaning and / or cleaning), oxidation, and oxide removal method steps may include coating the substrate with a film of the associated solution and then spinning the substrate to centrifugally remove the solution.

[0052] In an exemplary method, the pre-oxidation method step may include coating the substrate with a film of a pre-oxidation solution and then spinning the substrate and centrifugally removing the solution, and / or the oxidation method step may include coating the substrate with a film of an oxidizing solution and then spinning the substrate and centrifugally removing the solution, and / or the removal step may include coating the substrate with a film of an oxide removal solution and then spinning the substrate and centrifugally removing the solution.

[0053] At least one of the pre-oxidation (e.g., pre-cleaning and / or cleaning), oxidation, and removal method steps may include immersing the substrate in an associated solution. In an exemplary method, the pre-oxidation method step may include immersing the substrate in a pre-oxidation solution, and / or the oxidation method step may include immersing the substrate in an oxidizing solution, and / or the removal step may include immersing the substrate in an oxide removal solution.

[0054] The method may be applied to a batch immersion cleaning process, such as may be performed using a wet bench cleaning system. In this situation, at least one, or each, of the method steps may be performed using multiple containers (i.e., to hold different solutions, etc.). One or more substrates may be immersed in each of the solutions according to a predetermined sequence of the cleaning method steps.

[0055] Alternatively, two or more of the cleaning method steps may be performed in a single container, whereby each of the solutions (e.g., pre-clean solution, cleaning solution, pre-oxidation solution, oxidation solution, and / or oxide removal solution) is directed (e.g., sprayed) onto one or more substrates in turn according to a predetermined sequence of the cleaning method steps. For example, this method may be used in a single substrate cleaning process, such as may be performed using a spin-coating cleaning system.

[0056] According to a second aspect of the present invention, there is provided a system for cleaning a semiconductor substrate for a solar cell, the cleaning system configured to clean the substrate according to any one of the preceding methods.

[0057] According to a third aspect of the present invention, there is provided a cleaning system for cleaning a semiconductor substrate for a solar cell, the system comprising: a pre-oxidizer configured to direct a pre-oxidation solution over the substrate; an oxidizer configured to direct the oxidization solution over the substrate to form an oxide on a surface of the substrate; and an oxide remover configured to direct an oxide removal solution over the substrate to remove an oxide from the surface of the substrate, the pre-oxidation solution configured to remove metal ions from the surface of the substrate prior to forming an oxide on the surface of the substrate, the pre-oxidation solution being an acid solution comprising hydrogen chloride and no other acid forming components.

[0058] The pre-oxidizer may include a pre-cleaner configured to direct a pre-cleaning solution onto the substrate. The pre-oxidizer may include a scrubber configured to direct a cleaning solution onto the substrate. At least one or each of the pre-cleaning solution and the cleaning solution may include hydrogen chloride and be free of other acid forming components.

[0059] At least one or each of the pre-oxidizer (e.g., pre-cleaner and / or cleaner), oxidizer, and oxide remover may include a container of the wet bench cleaning system. The container may be configured to receive a substrate such that the substrate can be immersed in an associated solution, as will be appreciated by those skilled in the art. The container may be configured to receive multiple substrates simultaneously, for example, when multiple substrates are supported by a substrate holder or cradle.

[0060] At least one or each of the pre-oxidizer, oxidizer, and oxide remover may comprise components of a spin-coating cleaning system. For example, the oxidizer may comprise an outlet fluidly coupled to a reservoir configured to hold a volume of the pre-oxidation solution. A pump in the pre-oxidizer may be configured to direct the pre-oxidation fluid from the reservoir to the outlet and onto the substrate. The cleaning system may comprise a rotatable substrate support (or clamp) configured to rotate the substrate to centrifugally remove the pre-oxidation solution from the surface of the substrate, as will be appreciated by those skilled in the art.

[0061] Those skilled in the art will understand that, unless mutually exclusive, a feature or parameter described in connection with any one of the above embodiments may be applied to any other embodiment. Further, unless mutually exclusive, any feature or parameter described herein may be applied to any embodiment and / or may be combined with any other feature or parameter described herein. [Brief description of the drawings]

[0062] Embodiments will now be described, by way of example only, with reference to the drawings in which:

[0063] [Figure 1] 1A-1D are schematic diagrams of a substrate cleaning system at different stages of a substrate cleaning method. [Diagram 2] 2A-2C are schematic diagrams of a substrate cleaning system at different stages of a substrate cleaning method. [Diagram 3] 3A-3C are schematic diagrams of a substrate cleaning system at different stages of a substrate cleaning method. [Figure 4] 4A-4D are schematic diagrams of a substrate cleaning system at different stages of a substrate cleaning method. [Diagram 5] 5 is a schematic diagram of a substrate cleaning system at different stages of a substrate cleaning method. [Figure 6] 6A-6C are schematic diagrams of a substrate cleaning system at different stages of a substrate cleaning method. [Figure 7]7 is a flow chart illustrating a method for cleaning a substrate as shown in FIGS. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] Aspects and embodiments of the present disclosure will now be discussed with reference to the accompanying drawings. Further aspects and embodiments will be apparent to those skilled in the art.

[0065] 1-6 illustrate a substrate cleaning system 10 for cleaning a semiconductor substrate 20 in accordance with one embodiment of the present invention. The substrate 20 is formed of a semiconductor material, such as silicon. The substrate 20 is intended for use as a component of a solar cell, as will be understood by those skilled in the art. The substrate 20 may include a silicon wafer, although it will be understood that the cleaning method may be applied to other suitable semiconductor materials without departing from the scope of the present invention.

[0066] Prior to using a substrate in the fabrication of a solar cell, the surface of the substrate must be cleaned to remove adhering particles and organic / inorganic impurities. Also, the native silicon oxide surface layer must be removed. Contaminants on the surface of the substrate can be present as adsorbed ions and elements, thin films, discrete particles, particulate matter (e.g., particles), and adsorbed gases.

[0067] The cleaning system 10 includes four containers 12, 14, 16, 18, or vessels, each configured to hold a liquid solution, into each of which a substrate 20 is submerged in accordance with the cleaning method of the present invention. The system further includes a drying assembly 50 configured to remove the liquid solutions from the substrate 20 and to dry the substrate 20.

[0068] The cleaning system 10 also includes a rinse assembly (not shown) that includes a rinse tank containing a rinsing fluid (e.g., deionized water). The substrate 20 is received in the rinse tank where it is submerged in the rinsing fluid to remove any residual solution from the surface of the substrate. As will be appreciated by those skilled in the art, separate rinse assemblies may be provided for use after each cleaning step.

[0069] The drying assembly 50 includes a liquid drying tank 32 (e.g., a water dryer), as shown in FIG. 5. The substrate 20 is received in the liquid drying tank 32 where the substrate is submerged in a liquid drying fluid 42 to remove any residual solution from the surface of the substrate. After being submerged in the liquid drying fluid 42 for a period of time (e.g., 50-250 seconds), the substrate 20 is slowly removed from the liquid drying fluid 42 to partially dry the substrate 20 and provide a substantially uniformly wetted surface. The liquid drying fluid 42 is deionized water, which is held at room temperature (e.g., near 20° C.). The liquid drying fluid 42 is substantially the same as the rinsing fluid used in the rinsing assembly.

[0070] The drying assembly 50 also includes a drying tank 34 and a vapor drying fluid dispensing outlet 38, or nozzle, as shown in Figure 6. The substrate 20 is received in the drying tank 34 and a drying fluid 44 is directed (e.g., blown) from the dispensing outlet 38 towards the surface of the substrate to remove any residual liquid, thereby drying the substrate 20. The drying assembly 50 is located at the bottom of the drying tank 34 and fluid flow is directed upward and away from the substrate 20 via a drain 40 (e.g., a fluid outlet) located at the top of the drying tank 34 to allow liquid to drain.

[0071] Each of the containers 12, 14, 16, 18, and the rinse and dry tanks 32, 34 are made from a chemically inert material, such as polypropylene (PP) or polyvinylidene fluoride (PVDF). The containers configured to hold ozone may preferably be made from PVDF, as PVDF is more chemically inert. Thus, the containers and the rinse / dry tanks are each configured to not react with cleaning fluids that may be encountered during the cleaning process.

[0072] Each of the containers 12, 14, 16, 18 is equipped with a heating system (not shown) configured to control the temperature of the liquid solution contained therein. The heating systems comprise electrical resistive heating elements, although it will be understood that other types of heating systems may be used without departing from the scope of the invention.

[0073] The heating system includes a temperature controller configured to regulate the operation of the heating element, thereby regulating the temperature of the solution in the container. The temperature controller includes a temperature sensor (e.g., a thermocouple) configured to detect the temperature of the solution. The controller is configured to receive an input signal (e.g., indicative of the temperature of the liquid solution) from the temperature sensor and output a control signal (e.g., a current / voltage signal) to control the heating element to heat the container depending on the input signal. Thus, the controller can control the heating element to maintain the temperature of the solution at a predetermined temperature, or within a temperature range, as required by the cleaning method.

[0074] The containers 12, 14, 16, 18 are sized so that they can hold enough solution to allow the substrate 20 to be fully immersed (e.g., fully submerged). Each of the rinsing tank 32 and drying tank 34 are all sized so that they can receive the substrate 20 while also reducing the risk of any residual solution and / or rinsing / drying fluid splashing out of the respective tanks.

[0075] 1, the first container 12 contains a pre-cleaning solution 22 (i.e., a first solution) that includes hydrogen chloride and deionized water. The first container 12 thus defines a pre-cleaning solution of the cleaning system 10. Hydrogen chloride is included in the pre-cleaning solution 22 at a concentration of at least 0.4% and at most 2% by weight of the solution. The temperature of the pre-cleaning solution 22 is configured to be at least 20° C. and at most 60° C. In an exemplary configuration of the cleaning system 10, the pre-cleaning solution 22 is heated to a temperature of at least 30° C. and at most 45° C., optionally around 30° C. (e.g., 30° C.).

[0076] The second container 14 contains a cleaning solution 24 (i.e., a second solution) that includes hydrogen chloride and deionized water, as shown in Figure 2. The second container 24 thus defines the cleaning vessel of the cleaning system 10. Hydrogen chloride is included in the cleaning solution 24 at a concentration of at least 3% and at most 7% by weight of the solution. The temperature of the cleaning solution 24 is controlled to be approximately room temperature (i.e., the ambient air temperature surrounding the cleaning system), such as approximately 20°C (e.g., 20°C).

[0077] According to the cleaning method of the present invention, both the pre-cleaning solution and the cleaning solution 22, 24 define a pre-oxidizing solution since the substrate is immersed in each of the solutions before the substrate is oxidized by immersing the substrate in the oxidizing solution. Thus, the first and second containers 12, 14 each define a pre-oxidizing solution of the cleaning system 10.

[0078] 3, the third container 16 contains an oxidizing solution 26 (i.e., a third solution) that includes ozone, hydrogen fluoride, hydrogen chloride, and deionized water. The hydrogen fluoride is included in the oxidizing solution 26 at a concentration of at least 0% and at most 0.03% by weight of the solution. The hydrogen chloride is included in the oxidizing solution 26 at a concentration of at least 0.005% and at most 0.05% by weight of the solution.

[0079] Ozone is contained (i.e., dissolved) in the third solution 26 at a concentration of at least 1 and at most 100 ppm. In alternative configurations, the ozone concentration is configured to be at most 50 ppm, or alternatively, at most 20 ppm. The ozone in the oxidizing solution is configured to oxidize the surface of the substrate 20. Thus, the third container 26 defines an oxidizer of the cleaning system 10.

[0080] The temperature of the oxidizing solution 26 is configured to be at least 20° C. and at most 40° C. In an exemplary configuration of the cleaning system 10, the oxidizing solution 26 is heated to a temperature of at least 20° C. and at most 35° C., optionally approximately 23° C. (e.g., 23° C.).

[0081] 4, the fourth container 18 contains an oxide removal solution 28 (i.e., a fourth solution) that includes hydrogen fluoride, hydrogen chloride, and deionized water. The hydrogen fluoride is included in the oxide removal solution 28 at a concentration of at least 3% and at most 9% by weight of the solution. The hydrogen chloride is included in the oxide removal solution 28 at a concentration of at least 0.2% and at most 4% by weight of the solution. The temperature of the oxide removal solution 28 is controlled to be room temperature.

[0082] The hydrogen fluoride in the oxide removal solution 28 is configured to remove oxides formed on the substrate surface by the ozone in the oxidizing solution 26. Thus, the fourth container 28 defines an oxide remover of the cleaning system 10.

[0083] Each of the solutions 22, 24, 26, 28 is premixed separately and stored in a suitable storage container. Each of the premixed solutions 22, 24, 26, 28 is transferred directly from the solution storage container to the respective container 12, 14, 16, 18 prior to the start of the substrate cleaning process. Alternatively, each of the solutions can be mixed in situ within the housing of the cleaning system, as will be appreciated by those skilled in the art. The solutions are formed using ultra-high purity semiconductor grade reagents, including a source of ultra-pure water (e.g., deionized water).

[0084] Ozone is dissolved in the oxidizing solution 26 by directing a flow of an ozone-containing gas (e.g., ozone gas, O3) into the third container 16. The cleaning system 10 includes a gas delivery device (not shown) configured to deliver the ozone-containing gas from an ozone gas source to the third container 16. The ozone gas is bubbled through the oxidizing solution 26, as would be understood by one skilled in the art.

[0085] The ozone gas source is an electrolytic ozone gas generating assembly, although any suitable source of ozone gas may be used. For example, the ozone gas supply is generated from an ozone generator that is supplied with an oxygen (O2) supply gas. The gas delivery device includes a gas regulator (e.g., a controllable valve) configured to control the supply of ozone gas to the third container 16. This allows the ozone concentration in the third solution 26 to be precisely adjusted to a predetermined concentration or range of concentrations that can be maintained throughout the cleaning process.

[0086] The vapor drying fluid dispensing outlet 38 of the drying assembly 50 is positioned to direct a flow of vapor drying fluid 44 toward the substrate 20 when the substrate 20 is disposed within the drying tank 34, as shown in FIG. 6. The vapor drying fluid 44 is an inert gas, such as nitrogen gas. The vapor drying fluid dispensing outlet 38 is fluidly coupled to a source of nitrogen gas, such as a pressurized gas storage vessel or tank, via a separate fluid conduit (not shown). The flow of nitrogen gas to the outlet 38 is controlled by a variable valve (not shown), thereby controlling the flow of gas directed toward the substrate 20. The variable valve can be configured to only supply gas to the dispensing outlet 38 when the substrate 20 is positioned within the drying tank 34, as shown in FIG. 6.

[0087] Drying system 50 includes a dryer assembly heating system configured to control the temperature of the drying fluid dispensed from outlet 38. In particular, the heating system includes an electrical resistance heating element conductively coupled to a gas conduit that supplies nitrogen gas to second dispensing outlet 38. The heating element is operable to heat the nitrogen gas flowing through the conduit, as will be understood by those skilled in the art.

[0088] The drying assembly heating system includes a temperature controller as described above in connection with the cleaning fluid heating system. For example, the heating system includes a temperature sensor configured to determine the temperature of the gas being directed toward the substrate 20. The controller controls the operation of the heating element in response to the sensed gas temperature signal. In this manner, the controller is configured to maintain the temperature of the nitrogen gas at a predetermined temperature.

[0089] The drying assembly heating system is configured to heat the drying fluid 44 to a temperature of at least 50° C. and up to 90° C. In an exemplary configuration of the cleaning system 10, the drying fluid 44 is heated to a temperature of at least 65° C. and up to 75° C., optionally approximately 70° C. (e.g., 70° C.).

[0090] In an alternative configuration of the cleaning system 10, the dryer assembly heating system is configured to heat the drying tank 34 to control the temperature of the drying fluid 44. In this case, the drying tank 34 is fitted with a heating element as described above in connection with the cleaning fluid heating system.

[0091] The containers 12, 14, 16, 18, the rinse tanks and the drying tanks 32, 34 each define a different cleaning station or cleaning area of ​​the substrate cleaning system 10. All of the cleaning stations are located within a housing (not shown) configured to prevent contamination of the substrate 20 and the cleaning equipment. The housing is configured with a number of openings to allow access to a central interior of the housing in which the cleaning stations are located. The openings allow a user to move the substrate 20 between the different cleaning stations according to the cleaning method of the present invention.

[0092] The substrate 20 may be handled manually using a substrate tool such as a wafer handling bar or tongs. Alternatively, the system 10 may comprise a substrate handling robot for holding and lifting the substrate 20 in the housing. The substrate handling robot may thus be disposed in the housing and configured to move the substrate 20 between the different cleaning stations. By using such a robot, the housing may be closed during the cleaning method, which helps to reduce the risk of particles and other airborne contaminants entering the solutions 22, 24, 26, 28 and / or settling on the substrate surface.

[0093] 1-6, it will be appreciated that the cleaning system 10 may be configured to simultaneously clean multiple substrates 20. The multiple substrates 20 may be placed in a substrate carrier or cradle configured to support each of the substrates 20 in position while the substrates 20 are lifted between different cleaning stations. It will be appreciated that each of the containers 12, 14, 16, 18, the rinsing tank and the drying tank 32, 34 may be configured to receive a substrate carrier supporting multiple substrates 20.

[0094] The substrate carrier may also be formed of the same material as the container (e.g., PTFE), as will be appreciated by those skilled in the art. The substrate carrier may be configured to support the substrates 20 such that the substrates 20 are spaced apart from one another, thereby allowing the solution to reach the surfaces of the substrates. Thus, each of the substrates in the carrier is aligned substantially parallel to the other substrates.

[0095] The substrate 20 or a number of substrates are moved between the different cleaning stations in a predetermined sequence in a cleaning method 200 according to the present invention. The method for cleaning the substrate 20 will now be described with reference to the flow chart shown in Figure 7 and with reference to the cleaning system 10 shown in Figures 1-6.

[0096] The following description is directed to a method for cleaning a single substrate, however, it should be understood that the same method steps may be equally applied to cleaning multiple substrates 20 without departing from the invention.

[0097] The method begins with a first step 202 which includes providing a semiconductor substrate 20 to be cleaned. This first method step 202 also includes providing solutions 22, 24, 26, 28, 42 in their respective containers 12, 14, 16, 18, 34 as shown in Figures 1-5. It further includes providing a drying assembly 50 as shown in Figures 5 and 6.

[0098] In a subsequent method step 204, the substrate is immersed in the pre-cleaning solution 22 for 50-250 seconds (e.g., 215 seconds). As described above, the pre-cleaning solution 22 is an acid solution containing only hydrogen chloride and deionized water, with an acid concentration of at least 0.4% and at most 2%. As part of method step 204, the acid solution is heated to a temperature of at least 20°C and at most 60°C.

[0099] The cleaning method then proceeds to method step 204, which includes immersing the substrate 20 in the cleaning solution 24 for 50 to 250 seconds (e.g., 215 seconds). The cleaning solution 24 is also an acid solution containing only hydrogen chloride and deionized water. The acid concentration of the cleaning solution 24 is at least 3% and at most 7%. Method step 206 does not include heating the cleaning solution 24, which is instead maintained at ambient room temperature.

[0100] A further method step 208 involves immersing the substrate in the oxidizing solution 26 for 50-250 seconds (e.g., 215 seconds). The oxidizing solution 26, as described above, includes ozone, hydrogen fluoride, hydrogen chloride, and deionized water. Method step 208 includes bubbling ozone gas through the oxidizing solution 26 to form an "ozone solution." As part of method step 208, the third solution 26 is heated to a temperature of at least 20°C and up to 40°C.

[0101] The purpose of this ozone cleaning is to oxidize the organic elements remaining from the additives used during the texturing of the surface of the substrate to form the pyramidal structures. The ozone also oxidizes the surface of the substrate (i.e. the surface of the pyramidal structures).

[0102] The hydrogen fluoride in the oxidizing solution 26 removes the oxides formed by the ozone. This process of simultaneously removing the simultaneously formed oxides is referred to as "rounding off." In an alternative exemplary method, no rounding occurs during method step 208 due to the absence of hydrogen fluoride in the oxide removal solution 28. In this situation, the oxides formed by the ozone are removed during a subsequent oxide removal step, as described below.

[0103] The method then proceeds to method step 210, which includes immersing substrate 20 in oxide removal solution 28 for 50-250 seconds. Oxide removal solution 28, as described above, includes hydrogen fluoride, hydrogen chloride, and deionized water. Method step 210 does not include heating oxide removal solution 28; rather, the solution is maintained at ambient room temperature.

[0104] Following method step 210, the cleaning process proceeds to method step 212, where the substrate 20 is moved to a rinse tank (not shown) containing a rinsing fluid, which may rinse the substrate for 50-250 seconds. The rinsing method step 212 involves immersing / submerging the substrate 20 in the rinsing fluid to remove any residual solution from the surface of the substrate. This rinsing step may also be incorporated into each of method steps 204, 206, and 208. Thus, after immersing the substrate 20 in each of the pre-cleaning solution, cleaning solution, and oxidizing solutions 22, 24, and 26, the substrate 20 may be immersed in a rinsing fluid for a period of time (e.g., 50-250 seconds). After each rinse, the substrate 20 proceeds to the next method step in the sequence.

[0105] The rinsing fluid (e.g., deionized water) is configured to quickly stop any chemical reactions caused by the solutions 22, 24, 26, 28. The rinsing fluid also removes any chemical residues from the substrate 20 that may contaminate other solutions.

[0106] The cleaning process continues with a liquid drying method step 214, which includes immersing the substrate 20 in a liquid drying fluid 42 (e.g., deionized water) contained in a liquid drying tank 32, as shown in Figure 5. Once the substrate 20 has been immersed in the liquid drying fluid 42 for a period of time (e.g., 50-250 seconds), it is slowly removed from the tank 32, providing the substrate with a uniformly wetted surface.

[0107] Each of the pre-clean, clean, oxidize, strip, and rinse method steps (eg, 204, 206, 208, 210, 212, 214) are performed for the same duration so that there are no bottlenecks in the cleaning process.

[0108] Once substrate 20 has been rinsed (e.g., method step 212) and liquid dried (e.g., method step 214), substrate 20 is transferred, in method step 216, to drying tank 34 whereby substrate 20 is dried for 600-800 seconds with a rapid flow of vapor drying fluid 44. Method step 216 also includes heating vapor drying fluid 44, and thus drying tank 34, to at least 50°C and up to 90°C.

[0109] Once the cleaning method 200 is completed for the first substrate 20 (or first set of substrates), the method then returns to the first method step 202 (e.g., including providing a further substrate 20, or set of substrates) before proceeding in the same order as described above.

[0110] The parameters for each of the pre-clean, cleaning, oxidation, stripping, rinsing, and drying steps (204, 206, 208, 210, 212, 214, 216) are summarized below in Table 1. Method steps 204, 206 precede the main oxidation and stripping cleaning method steps 208 and 210, and thus, these method steps define the pre-oxidation method step of the cleaning method 200. [Table 1]

[0111] The process of the present invention has been found to provide an enhanced method of cleaning a semiconductor substrate 20. Each of the pre-clean and cleaning method steps 204, 206 (i.e., pre-oxidation method steps) are configured to complement the oxidation and oxide removal steps 208, 210 to remove contaminants from the surface of the substrate.

[0112] The pre-clean and cleaning solutions 22, 24 are configured to remove metal ions from the surface of the substrate 20, thereby reducing metal contamination of the solutions used in the subsequent oxidation and oxide removal method steps 208, 210 (i.e., the oxidizing and oxide removal solutions 26, 28). As a result, the concentration of hydrogen chloride in the oxidizing and oxide removal solutions 26, 28 can be reduced. Surprisingly, this means that the total hydrogen chloride usage for the cleaning method 200 can be reduced compared to a cleaning method that does not include an acid pre-oxidation step.

[0113] The benefits of the present invention are also demonstrated by the improved performance characteristics of solar cells fabricated using substrates 20 cleaned using cleaning method 200. In particular, substrates cleaned by this method have been shown to produce solar cells that operate more efficiently than solar cells produced using comparable cleaning methods that do not include a pre-oxidation method step (i.e., a method that does not include either the pre-cleaning method step or the cleaning method steps 204, 206, as defined above).

[0114] The device parameters of some exemplary solar cell devices A, B, C, D, and E are shown in Table 2 below. Each of Devices A-E is a crystalline silicon heterojunction solar cell (HJT). Each of the substrates of Devices A-E undergoes a different cleaning process before fabricating the devices. The performance parameters shown in Table 2 are normalized with respect to Device A to show the relative differences in device performance. For example, each of the parameter values ​​of Devices B, C, D, and E is shown as a percentage difference (+ / -%) with respect to the corresponding parameter value of Device A. Thus, the parameter values ​​of Device A are all shown as 0.0%.

[0115] Devices A and B were made from substrates cleaned according to a cleaning method that did not include a pre-oxidation method step. The substrate for Device A was cleaned for a total of 180 seconds, whereas the substrate for Device B was cleaned for a total of 215 seconds.

[0116] Solar cells with substrates 20 cleaned according to the cleaning method 200 of the present invention are identified as Devices C, D, and E in Table 2. The substrate of Device C was cleaned using an exemplary cleaning method of the present invention in which cleaning step 206 was omitted (i.e., the method includes only pre-cleaning method step 204). In this case, the total duration of cleaning method 200 was 180 seconds.

[0117] The substrates used for Devices D and E were both cleaned according to an exemplary method of the present invention including pre-clean and cleaning method steps 204, 206. The substrate for Device D was cleaned for a total of 180 seconds, whereas the substrate for Device E was cleaned for a total of 215 seconds.

[0118] For each of the substrates used in Devices A-E, the remaining process steps of the corresponding cleaning method (eg, oxidation, oxide removal, rinsing, and drying steps) were substantially the same. [Table 2]

[0119] Referring to Table 2, it can be seen that Devices C, D, and E (i.e., having substrates cleaned according to the method of the present invention) each exhibit an increased open circuit voltage (Voc) compared to Devices A and B (i.e., having substrates cleaned according to a method that does not include a pre-oxidation process step).

[0120] The results in Table 2 show that the conversion efficiencies (CE) and fill factors (FF) of solar cells C to E are slightly lower than those of devices A and B. This was attributed to minor variations in the solar cell fabrication process (e.g., cell printing).

[0121] In addition to the above, a surprising and unexpected result of the present invention is believed to be the simplicity of the methodology. For example, the method achieves enhanced cleaning of semiconductor substrates without the need to repeat any of the individual method steps. This means that the cleaning method can be completed quickly and reduces contamination between different solutions, thereby reducing waste. Thus, the cleaning method thereby reduces the overall substrate cleaning costs and improves the operating parameters of the resulting solar cell devices.

[0122] The above description outlines a cleaning method and cleaning system according to the present invention with a specific concentration of the solution, a specific processing duration, a specific frequency of immersing the substrate during processing, a specific period for rinsing, and a specific period for drying (e.g., liquid drying and vapor drying). However, the present invention is not so limited. Various modifications may be made, taking into account the degree of surface contamination, the size and amount of semiconductor substrates to be cleaned, and the degree of cleanliness required. The described method may also be combined with other cleaning techniques.

[0123] The cleaning methods described above include batch immersion cleaning processes, such as those that may be performed using a wet bench cleaning setup, however, it will be appreciated that the cleaning method 200 may include the use of alternative cleaning systems and apparatus that may be configured to direct the same solutions 22, 24, 26, 28 at the substrate 20 according to a prescribed method.

[0124] For example, each of the method steps 204, 206, 208, and 210 (i.e., the pre-clean, clean, oxidize, and remove method steps) may be performed using a spin-coating cleaning system. In this exemplary configuration, the substrate 20 may be rigidly fixed to a rotating platform, with the substrate positioned in a substantially horizontal orientation. The surface of the substrate is then coated with a film of the respective solution (e.g., pre-clean, clean, oxidize, or oxide remover solutions 22, 24, 26, 28) before the substrate is rotated to centrifugally remove the solution from its surface. Similarly, the rinse and / or liquid drying method steps may be performed using such a spin-coating cleaning system, as would be understood by one skilled in the art.

[0125] It will be understood that the present invention is not limited to the embodiments described above, and various modifications and improvements can be made without departing from the concepts described herein. Any feature can be used separately or in combination with any other feature, except where mutually exclusive, and the present disclosure extends to and includes all combinations and subcombinations of one or more features described herein. [Explanation of symbols]

[0126] 10 Cleaning System 12 First container 14 Second container 16 Third container 18 No. 4 Containment Unit 20 Substrate 22 Pre-cleaning solution 24 Cleaning Solution 26 Oxidizing Solution 28 Oxide removal solution 30 Liquid Drying Assembly 32 Liquid Drying Tank 34 Steam drying tank 36, 38 Fluid dispensing outlet 40 Steam Drying Tank Drain 42 Rinse Fluid 44 Drying fluid 50 Steam Drying Assembly 200 Cleaning Method 202, 204, 206, 208, 210, 212, 214, 216 Cleaning method steps

Claims

1. 1. A method for cleaning a semiconductor substrate for a solar cell, comprising: providing a semiconductor substrate; pre-oxidizing the substrate with a pre-oxidizing solution; oxidizing the substrate with an oxidizing solution to form an oxide on the surface of the substrate; removing the oxides from the surface of the substrate with an oxide removal solution; The method of claim 1, wherein the pre-oxidation solution is configured to remove metal ions from the surface of the substrate prior to the formation of the oxide on the surface of the substrate, and the pre-oxidation solution is an acid solution containing hydrogen chloride and no other acid-forming components.

2. 2. The method of claim 1, wherein the step of pre-oxidizing the substrate comprises the steps of pre-cleaning the substrate with a pre-cleaning solution, and after the pre-cleaning step, rinsing the substrate with a cleaning solution, each of the pre-cleaning solution and the cleaning solution being an acid solution containing hydrogen chloride and no other acid-forming components.

3. 3. The method of claim 2, wherein the concentration of hydrogen chloride in the cleaning solution is greater than in the pre-cleaning solution.

4. 3. The method of claim 2, wherein the pre-cleaning solution comprises a hydrogen chloride concentration of at least 0.1 wt. % and / or at most 2.5 wt. %, optionally at least 0.4 wt. % and / or at most 2.0 wt. %, further optionally at least 0.8 wt. %.

5. 3. The method of claim 2, wherein the cleaning solution comprises a hydrogen chloride concentration of at least 2.5 wt.% and / or at most 10 wt.%, optionally at least 3.0 wt.% and / or at most 7.0 wt.%, further optionally at least 5.0 wt.%.

6. The method of claim 2 , wherein the pre-wash liquid is at a temperature substantially equal to or higher than the temperature of the wash liquid.

7. 3. The method of claim 2, wherein the pre-wash liquid is heated to a temperature of at least 20°C and / or at most 60°C.

8. 3. The method of claim 2, wherein the cleaning solution is at a temperature of at least 15°C and / or at most 25°C, optionally 20°C.

9. 3. The method of claim 2, wherein the method includes rinsing the substrate between pre-cleaning with the substrate pre-cleaning solution and cleaning with the substrate cleaning solution, and the rinsing of the substrate includes directing deionized water over the substrate.

10. 2. The method of claim 1, wherein the oxidizing solution comprises a hydrogen chloride concentration of at least 0.001 wt. % and / or at most 0.1 wt. %, optionally at least 0.005 wt. % and / or at most 0.05 wt. %.

11. The method of claim 1 , wherein the method includes directing a drying fluid to the substrate, the drying fluid configured to remove any remaining solution from the surface.

12. The method of claim 11 , wherein the drying fluid is an inert gas, optionally nitrogen gas.

13. 12. The method of claim 11, wherein the drying fluid is heated to a temperature of at least 50°C and / or at most 90°C.

14. The method of claim 11 , wherein the method includes a rinsing step prior to directing the drying fluid to the substrate, the rinsing step including directing deionized water to the substrate.

15. 15. The method of claim 14, wherein the deionized water is at a temperature of at least 15°C and / or at most 25°C, optionally 20°C.

16. 10. The method of claim 1, wherein the method includes a rinse step performed between the pre-oxidation method step and the oxidation method step and / or between the oxidation method step and the removal method step.

17. The method of claim 1 , wherein oxidizing the substrate comprises using hydrogen fluoride to form the oxidizing solution.

18. 18. The method of claim 17, wherein the oxidizing solution comprises a hydrogen fluoride concentration of up to 0.03 wt.%.

19. The method of claim 1 , wherein at least one of the pre-oxidizing, oxide forming, and oxide removing steps comprises immersing the substrate in an associated solution.

20. 10. The method of claim 1, wherein at least one of the pre-oxidizing, oxide forming, and oxide removing steps comprises coating the substrate with a film of an associated solution, spinning the substrate, and centrifuging off the solution.

21. A cleaning system for cleaning semiconductor substrates for solar cells, the cleaning system being configured to clean the substrates according to the method of claim 1.

22. 1. A cleaning system for cleaning semiconductor substrates for solar cells, comprising: a pre-oxidizer configured to direct a pre-oxidation solution onto the substrate; an oxidizer configured to direct an oxidizing solution onto the substrate to form an oxide on a surface of the substrate; an oxide remover configured to direct an oxide removal solution onto the substrate to remove the oxides from the surface of the substrate; 1. A cleaning system comprising: a pre-oxidation solution configured to remove metal ions from the surface of the substrate prior to the formation of the oxide on the surface of the substrate; and a pre-oxidation solution comprising hydrogen chloride and no other acid-forming components.

Images