Method for producing metal contacts of an electronic component by means of photonic sintering

Abstract

The invention relates to a method for producing at least one metal contact in an electronic component, having the following steps: - applying a single- or multi-layer metal coating (4) onto the front face of a semiconductor substrate (1) by means of chemical vapor deposition, wherein the metal coating has, as the uppermost metal layer (3), a layer made of silver, aluminum, or copper or an alloy of the same metals, - printing a defined surface area of the uppermost metal layer of the metal coating with a paste (5) which contains metal particles and / or a metal particle precursor, - irradiating the printed paste with light so that a metal contact (7) which contains the metal particles sintered together is obtained by irradiating the paste (5), and - carrying out an etching or anodal oxidation of the exposed metal coating (4b).

Description

[0001] FRAUNHOFER SOCIETY FOR THE ADVANCEMENT OF APPLIED RESEARCH EV

[0002] Fraunhofer Society for the Advancement of Applied Research

[0003] Hansastraße 27c, 80686 Munich, Germany

[0004] Method for producing metallic contacts of an electronic component by photonic sintering

[0005] The present invention relates to a method for producing metallic contacts (for example in the form of electrical conductor tracks) of an electronic component by photonic sintering and to electronic components obtainable by this method.

[0006] For the use of electronic components, it is usually necessary that they have metallic contacts, e.g., in the form of electrical conductors. The electrical contacts serve, for example, to conduct current from the component or to tap voltage from it. If the electronic component is a solar cell, for example, the photocurrent generated in this component via the photovoltaic effect can be conducted via the metallic contacts.

[0007] In one known process, a paste containing metal particles is printed onto the electronic component (e.g., in the form of conductor tracks) and then thermally treated at a sufficiently high temperature to sinter the metal particles. This may require temperatures of at least 800°C. However, such high temperatures are unacceptable for many components. An example in this context is silicon heterojunction solar cells (SHJ solar cells). The SHJ solar cell is a wafer-based crystalline silicon solar cell with an emitter and a back or front surface field made of amorphous silicon. To prevent unwanted crystallization in the amorphous silicon layers of the SHJ solar cell, thermal treatment at temperatures above 250°C should be avoided.For other types of solar cells or other electronic components such as light-emitting diodes, it is also desirable to attach electrical contacts at the lowest possible temperature stress.

[0008] As an alternative to thermal sintering, the metal particles present in the printing paste can be sintered together by irradiation with light. Light-induced sintering is referred to, for example, as photonic sintering or intense pulsed light sintering (IPL sintering). While irradiation with light (e.g., pulsed light) generates a sufficiently high temperature in the printed paste for sintering of the metal particles, the short irradiation duration (e.g., light pulses with a pulse duration of a few milliseconds) keeps the thermal stress in the underlying semiconductor substrate of the electronic component relatively low.

[0009] H.-S. Kim et al., “Intense Pulsed Light Sintering of Screen-Printed Paste Electrode on Silicon Solar Cell for High Throughput and Cost-Effective Low Temperature Metallization," International Journal of Precision Engineering and Manufacturing - Green Technology”, 2022, 9, pp. 523-535, describe the metallization of a silicon heterojunction solar cell using photonic sintering. A layer of an electrically conductive transparent oxide (TCO layer) is printed in defined areas with a paste containing metal particles. The paste is irradiated with light pulses so that the metal particles present in the paste sinter together. The paste covers only a relatively small area of ​​the TCO layer, but the light pulses also hit the exposed surface of the TCO layer. This, in turn, can lead to heating and damage to semiconductor layers beneath the TCO layer.

[0010] EP 3 125 303 A1 describes a method for producing metallic contacts of a solar cell, wherein a paste containing metal particles is applied to a semiconductor substrate and the metal particles are subsequently sintered together by photonic sintering.

[0011] An overview of the use of photonic sintering in the production of solar cells can be found in the following publication: T. Druffel et al., "Intense pulsed light processing for photovoltaic manufacturing" , Solar Energy Materials and Solar Cells, 2018, 174, pp. 359-369.

[0012] For the application of metallic contacts to a substrate, instead of a paste containing metal particles, a paste containing a metal particle precursor can be used. This precursor decomposes under irradiation, forming metal particles that immediately sinter together. Examples in this context include: A. Yakushenko et al., "Self-Reducing Copper Precursor Inks and Photonic Additive Yield Conductive Patterns under Intense Pulsed Light," ACS Omega, 2017, 2, pp. 573-581.

[0013] The radiation used for sintering is typically not completely absorbed by the metal particles or metal particle precursors present in the printed paste, but also by underlying layers of the semiconductor component, so that these layers can also heat up. For example, a pulse duration that is too long and / or too many pulses can cause damage to the semiconductor material. As also mentioned above, the paste typically covers only a relatively small area of ​​the topmost substrate layer, but the light pulses also hit the exposed surface of the topmost substrate layer, which can lead to heating and damage to semiconductor layers beneath the topmost substrate layer.

[0014] It is known that metallic layers made of silver, aluminum, or copper have high reflectivity and are therefore used as a surface coating in optical mirrors. Furthermore, it is known that the reflectivity can be further improved if a very thin intermediate layer (e.g., with a layer thickness of 5 nm or less) is present between the highly reflective surface layer and the substrate. EP 3 779 526 A1 describes an optical element containing an aluminum layer acting as a mirror layer, with a seed layer (e.g., made of Cu, Ti, Fe, Zn, or Cr) having a maximum thickness of 5 nm being present between this aluminum layer and a substrate. One object of the present invention is to apply the metallic contacts of an electronic component by a method that minimizes the thermal stress on the component and is efficient to carry out.

[0015] The object is achieved by a method for producing at least one metallic contact in an electronic component, comprising the following steps:

[0016] Applying a single- or multi-layer metal coating by vapor deposition on a front side (and optionally also a back side) of a semiconductor substrate, wherein the metal coating has as the top metal layer a layer of silver, aluminum or copper or an alloy of one of these metals, printing a defined area of ​​the top metal layer of the

[0017] Metal coating with a paste containing metal particles and / or a metal particle precursor, so that a printed metal coating and an exposed metal coating are present,

[0018] Irradiating the printed paste with light, whereby the metal particles at least partially sinter together or, if the paste contains the metal particle precursor, the metal particle precursor decomposes to form metal particles and the metal particles at least partially sinter together, so that by irradiating the paste a metallic contact containing the sintered metal particles is obtained,

[0019] Etching or anodic oxidation of the exposed metal coating.

[0020] A layer of metallic silver, aluminum, or copper, or an alloy of one of these metals, exhibits high reflectivity. This high reflectivity of the Ag, Al, or Cu layer prevents, or at least significantly reduces, absorption of the light used for photonic sintering in deeper layers (particularly thermally sensitive semiconductor layers such as amorphous silicon). The risk of undesired and potentially damaging heat input during photonic sintering in deeper layers is significantly reduced. In addition, the presence of the highly reflective metal layer results in greater flexibility in the process parameters (such as irradiation duration and / or irradiation intensity) for photonic sintering. Through the vapor deposition of the single- or multi-layer metal coating, and thus of the Ag, Al, or Cu-containing layer, which forms the uppermost (i.e.last applied) metal layer of the metal coating, a high layer quality (e.g. low surface roughness, high density and defined layer thickness) and thus also a high reflectivity of the layer is possible.

[0021] The vapor deposition can be a physical or a chemical vapor deposition. The metal coating is preferably applied via physical vapor deposition.

[0022] The vapor deposition is preferably carried out in a vacuum.

[0023] For example, physical vapor deposition is carried out by sputtering (also known as cathode sputtering).

[0024] For example, the metal coating covers at least 80% of the area of ​​the front or back of the semiconductor substrate.

[0025] If the metal coating is single-layer, it consists of the top metal layer containing Ag, Al, or Cu. If the metal coating is multi-layer, there is at least one metallic intermediate layer between the top metal layer of silver, aluminum, or copper, or an alloy of one of these metals, and the semiconductor substrate.

[0026] When reference is made in the context of the present invention to an alloy of a metal M, this means that in this alloy the metal M is the main component, i.e. it is present in a higher concentration (in atomic%) than any other metal present in the alloy.

[0027] To simplify the process as much as possible, it may be preferable for the metal coating to be a single layer. Alternatively, the presence of one or more metallic intermediate layers with a specific functionality may be advantageous.

[0028] For example, the metallic intermediate layer can act as an adhesion promoter between the Ag-, Al- or Cu-containing top metal layer and the semiconductor substrate.

[0029] In order to further improve the reflectivity of the uppermost metal layer made of silver, aluminum, or copper, or an alloy of one of these metals, it may be advantageous to insert a very thin metal layer, preferably less than 5 nm thick (e.g., 2 nm to 4 nm), as a metallic intermediate layer and to apply the uppermost metal layer directly to this metallic intermediate layer. The very thin metallic intermediate layer is, for example, a layer made of Mo, W, Ti, Cr, Zn, or Cu, or an alloy of one of these metals. This very thin metallic intermediate layer is preferably present when the uppermost metal layer is a layer made of aluminum or an aluminum alloy.

[0030] Preferably, the layer thickness of the metal coating is not more than 100 nm, more preferably not more than 60 nm. For example, the metal coating has a layer thickness in the range of 5 nm to 100 nm, more preferably 10 nm to 60 nm.

[0031] If the metal coating has two or more metal layers (i.e. the top metal layer and at least one metallic intermediate layer), their respective layer thickness can be freely selected, but the layer thickness of the resulting metal coating is preferably not more than 100 nm, more preferably not more than 60 nm.

[0032] Preferably, the metal coating has no more than three metal layers (ie the top metal layer and optionally a maximum of two metallic intermediate layers) or no more than two metal layers (ie the top metal layer and optionally one metallic intermediate layer).

[0033] Particularly preferably, the metal coating is single-layer, i.e., the metal coating consists of a layer of silver, aluminum, copper, or an alloy of one of these metals. In a particularly preferred embodiment, the single-layer metal coating consists of a layer of silver or a silver alloy.

[0034] In an exemplary embodiment, the uppermost metal layer of the metal coating is a layer of aluminum or an aluminum alloy, and the metal coating contains, as a metallic intermediate layer, a layer of Mo, W, Ti, Cr, Zn, or Cu, or an alloy of one of these metals, wherein the uppermost metal layer is preferably present directly on the metallic intermediate layer. The metallic intermediate layer preferably has a layer thickness of less than 5 nm, e.g., 2 nm to 4 nm. The metallic intermediate layer made of Mo, W, Ti, Cr, Zn, or Cu, or an alloy of one of these metals, can be present, for example, directly on the semiconductor substrate. Alternatively, a further metallic intermediate layer can be present between the metallic intermediate layer made of Mo, W, Ti, Cr, Zn, or Cu, or an alloy of one of these metals, and the semiconductor substrate, which layer functions, for example, as an adhesion-promoting layer or diffusion barrier layer.

[0035] In a further exemplary embodiment, the uppermost metal layer of the metal coating is a layer of copper, silver, or an alloy of one of these metals, and the metal coating contains a layer of molybdenum or a molybdenum alloy as a metallic intermediate layer, wherein the uppermost metal layer is preferably present directly on the metallic intermediate layer. This metallic intermediate layer of molybdenum or a molybdenum alloy can function as an adhesion-promoting layer or as a diffusion barrier layer between the Cu or Ag layer and the semiconductor substrate, but can also be easily removed by etching, which is advantageous because, in a later optional process step, the metal coating in the unprinted (i.e., exposed) regions is removed again by etching. The metallic intermediate layer of molybdenum or a molybdenum alloy is preferably present directly on the semiconductor substrate.

[0036] In a further exemplary embodiment, the uppermost metal layer of the metal coating is a layer of aluminum or an aluminum alloy, and the metal coating contains a layer of silver or a silver alloy as a metallic intermediate layer, wherein the uppermost metal layer of aluminum or an aluminum alloy is preferably present directly on the metallic intermediate layer of silver or a silver alloy. Alternatively, for example, the uppermost metal layer of the metal coating is a layer of silver or a silver alloy, and the metal coating contains a layer of aluminum or an aluminum alloy as a metallic intermediate layer, wherein the uppermost metal layer of silver or a silver alloy is preferably present directly on the metallic intermediate layer of aluminum or an aluminum alloy.Silver metal layers exhibit very high reflectivity, particularly in the visible light wavelength range, while aluminum metal layers exhibit high reflectivity even in the UV radiation wavelength range. These two exemplary embodiments thus exhibit high reflectivity over a very broad wavelength range. This increases the scope for the light sources that can be used for photonic sintering or makes it possible to dispense with the use of optical filters (e.g., UV filters).

[0037] In another exemplary embodiment, the uppermost metal layer of the metal coating is a layer of copper, silver, or an alloy of one of these metals, and the metal coating contains a maximum of two metallic intermediate layers of tungsten, titanium, or a tungsten-titanium alloy. If two metallic intermediate layers are present, one of the two layers can be a layer of tungsten or a tungsten alloy, while the other layer is a layer of titanium or a titanium alloy. If only one metallic intermediate layer is present, it can be a layer of a tungsten-titanium alloy, for example.

[0038] The single- or multi-layer metal coating is applied to a front side (and optionally also a back side) of a semiconductor substrate.

[0039] A semiconductor substrate is a substrate that contains one or more semiconductor materials. Optionally, a semiconductor substrate can also contain one or more materials that are not semiconductors in addition to the semiconductor materials. Suitable semiconductor substrates that can be used for the production of electronic components such as solar cells are known to those skilled in the art. For example, the semiconductor substrate contains silicon as semiconductor material (e.g. undoped (amorphous or crystalline) silicon, n- or p-doped (amorphous or crystalline) silicon) and / or a transparent, electrically conductive oxide (hereinafter also referred to as TCO) such as indium tin oxide (ITO) or aluminum-doped zinc oxide (AZO). The TCO can, for example, be present on the front side of the semiconductor substrate and function as a contact and / or anti-reflective layer.As is known to those skilled in the art, the semiconductor substrate of an electronic component can have a multilayer structure, wherein the outermost layer on the front side (or back side) of the semiconductor substrate does not necessarily contain a semiconductor material. For example, the outermost layer on the front side of the multilayer semiconductor substrate is a layer of a transparent, electrically conductive oxide (which may have semiconducting properties) or a layer of a dielectric material (e.g., silicon nitride or aluminum oxide), and an inner layer of the multilayer semiconductor substrate contains silicon (e.g., undoped (amorphous or crystalline) silicon, n- or p-doped (amorphous or crystalline) silicon). Further components required for the function of the electronic component may optionally be present on the back side of the semiconductor substrate.The electronic component whose metallic contacts are produced by the method according to the invention is preferably a solar cell or precursor of a solar cell.

[0040] As already mentioned above, in the method according to the invention, a defined area of ​​the uppermost metal layer of the metal coating is printed with a paste containing metal particles and / or a metal particle precursor, so that a printed metal coating and an exposed metal coating adjacent thereto are present.

[0041] The geometry of the printed surface also determines the geometry of the metallic contacts. For example, the printed surface can be linear, grid-shaped, or comb-shaped.

[0042] Particularly when the electronic component is a solar cell or a precursor to a solar cell, the smallest possible area of ​​the top metal layer should be printed to minimize shading effects. Therefore, the printed area of ​​the top metal layer is preferably a maximum of 20%, more preferably a maximum of 10% (e.g., 1-20% or 1-10%) of the total area of ​​the top metal layer. It is therefore preferred that a maximum of 20%, more preferably a maximum of 10% (e.g., 1-20% or 1-10%) of the area of ​​the top metal layer is printed with the paste.

[0043] The defined area of ​​the top metal layer is printed using, for example, screen printing, dispensing, rotary printing, a laser-induced forward transfer (LIFT) process, or inkjet printing. These printing processes are known to those skilled in the art.

[0044] The printing of metal particle-containing pastes using the LIFT process is described, for example, by JM Fernandez-Pradas et al., “Laser-Induced Forward Transfer: A Method for Printing Functional Inks,” Crystals 2020, 10, 651 ; doi:10.3390 / cryst10080651 .

[0045] In the LIFT process, for example, the paste to be printed is first applied to a carrier material and then transferred to the defined area of ​​the top metal layer to be printed using focused laser radiation.

[0046] Information on dispensing can be found, for example, in the following publications: M. Pospischil et al., “Applications of Parallel Dispensing in PV Metallization”, AIP Conference Proceedings 2156, 020005 (2019), https: / / doi.Org / 10.1063 / 1.5125870; D. Erath et al., “Comparison of innovative metallization approaches for silicon heterojunction solar cells", Energy Procedia, 2017, 124, pp. 869-874.

[0047] Printable pastes containing metal particles are known to those skilled in the art and are commercially available. It may be preferable for the paste to contain the metal particles in a high concentration, for example, 60-90% by weight.

[0048] Optionally, the paste can contain a liquid dispersion medium (e.g. water and / or an organic solvent), an organic binder (e.g. a polymer) and / or a glass frit. The metal particles preferably contain copper, silver and / or nickel. For example, the metal particles can consist predominantly of at least one of these metals, so that other metals (i.e. metals that are not copper, silver or nickel) are present in the metal particles in quite low concentrations (e.g. in a concentration of at most 20 at%, more preferably at most 10 at%). The metal particles are, for example, metal particles with a copper concentration of at least 90 at%, metal particles with a silver concentration of at least 90 at% or core-shell metal particles whose core has a copper concentration of at least 90 at% and whose shell has a silver or nickel concentration of at least 90 at%.

[0049] Alternatively, a paste containing a metal particle precursor can be used to print the defined area of ​​the uppermost metal layer. As is known to those skilled in the art, a metal particle precursor is a compound of the metal (e.g. a salt of the metal or an organometallic compound) which decomposes under irradiation (e.g. with pulsed radiation) and forms metal particles. Suitable compounds that decompose under irradiation and form metal particles are known to those skilled in the art. For example, metal carboxylates (e.g. metal formates) can be used. Such printable pastes containing metal particle precursors are known, for example, under the name “metal-organic decomposition (MOD) inks”. By way of example, reference can be made in this context to the following publications: A. Yakushenko et al., „Self-Reducing Copper Precursor Inks and Photonic Additive Yield Conductive Patterns under Intense Pulsed Light’, ACS Omega, 2017, 2, S. 573-581 ; und Y. Piao et al., „Metal-Organic Decomposition Ink for Printed Electronics“ , Adv. Mater. Interfaces, 2019, 6, 1901002.

[0050] The printed paste is irradiated with light, whereby the metal particles at least partially sinter together or, if the paste contains the metal particle precursor, the metal particle precursor decomposes to form metal particles and the metal particles at least partially sinter together, so that irradiation of the paste creates a metallic contact containing the sintered metal particles. The light with which the paste is irradiated has, for example, wavelengths in the range from 100 nm to 1400 nm. In the context of the present invention, the term "light" encompasses not only electromagnetic radiation with a wavelength range of the electromagnetic spectrum visible to the human eye (approx. 400-780 nm), but also UV radiation (approx. 100 nm to <400 nm) and IR radiation (>780 nm, e.g. >780 nm to 1400 nm).If necessary, the wavelength range of the light with which the paste is irradiated can be adjusted by using optical filters.

[0051] Pulsed light is preferably used to irradiate the paste. A person skilled in the art can readily determine a suitable pulse duration, number of pulses, and pulse intensity for sintering the metal particles together and, if appropriate, decomposing the metal particle precursor to form metal particles based on their specialist knowledge. Suitable devices for generating pulsed light are known and commercially available. For example, a xenon flash lamp can be used.

[0052] The wavelength range of the light used for photonic sintering can, if necessary, be selected to match the reflection property of the top metal layer (optionally in combination with the reflection behavior of a metallic intermediate layer on which the top metal layer is present).

[0053] In an exemplary embodiment, the light with which the printed paste is irradiated has wavelengths in the UV radiation range (e.g., 100 nm to <400 nm), and the uppermost metal layer is a layer of aluminum or an aluminum alloy. In addition to the UV radiation, the light can optionally have wavelengths in the range of 400 nm to 1400 nm. For example, the light contains UV radiation in a proportion of at least 40%, more preferably at least 60%, or even at least 80%, based on the total irradiance of the light. The proportion of UV light can optionally be adjusted using suitable filters.

[0054] In a further exemplary embodiment, the light with which the printed

[0055] The paste being irradiated has wavelengths in the IR radiation range (e.g., >780 nm to 1400 nm), and the top metal layer is a layer of copper or a copper alloy. In addition to the IR radiation, the light can optionally have wavelengths in the range of 100 nm to 780 nm. For example, the light contains IR radiation in a proportion of at least 40%, more preferably at least 60%, or even at least 80%, based on the total irradiance of the light. The proportion of IR light can be adjusted, if necessary, using suitable filters.

[0056] There are cavities between the sintered metal particles of the metallic contact. The metallic contact is thus a porous metallic contact. Optionally, additional components of the printed paste, such as an organic binder and / or glass, can be present in these cavities between the sintered metal particles.

[0057] The metallic contact has, for example, a thickness of at least 200 nm, more preferably at least 500 nm, even more preferably at least 1 μm. For example, the thickness of the metallic contact is 200 nm to 20 μm, more preferably 500 nm to 15 μm, even more preferably 1 μm to 10 μm.

[0058] The metallic contact formed on the printed metal coating by irradiating the paste can function as an electrical conductor in the electronic component.

[0059] If the exposed metal coating is partially or completely removed by etching in a later process step, this etching could also attack the metallic contact. However, since the metallic contact is significantly thicker than the exposed metal coating, a sufficiently thick metallic contact remains even after complete removal of the exposed metal coating.

[0060] After the metallic contact is formed by irradiating the paste with light, the exposed metal coating is etched or anodically oxidized.

[0061] For example, etching the exposed metal coating exposes the underlying front side of the semiconductor substrate. According to another exemplary embodiment, an exposed metal oxide coating is obtained by anodically oxidizing the exposed metal coating. As is known to those skilled in the art, anodic oxidation is an electrolytic process for oxidizing a metal. The metal to be oxidized is brought into contact with an anodizing bath into which an electrode is immersed, and the metal to be oxidized is set to an anodic potential relative to the immersed electrode. Suitable anodizing baths are known to those skilled in the art and contain, for example, sulfuric acid, oxalic acid, citric acid, or chromic acid.For anodic oxidation, the exposed metal coating is brought into contact with an anodizing bath and set to an anodic potential relative to an electrode immersed in the anodizing bath. Since the anodizing bath comes into contact with the exposed metal coating but not with the printed metal coating, only the metal layers present in the exposed metal coating are oxidized, not the metal layers in the printed metal coating. In the electronic component obtained after anodic oxidation, the exposed metal oxide coating thus borders the printed metal coating, on which the porous metal coating is present.In this exemplary embodiment, in which the exposed metal coating is subjected to anodic oxidation, the uppermost metal layer is preferably aluminum or an aluminum alloy, since aluminum can be easily converted into an aluminum oxide via anodic oxidation. If the metal coating has a metallic intermediate layer, the metallic intermediate layer is preferably a layer of titanium or a titanium alloy, since titanium can be easily converted into a titanium oxide via anodic oxidation.

[0062] According to a further exemplary embodiment, the uppermost metal layer is removed by etching the exposed metal coating and an underlying metallic intermediate layer is exposed, and an exposed metal oxide coating is obtained by anodic oxidation of the exposed metallic intermediate layer. If one or more further metallic intermediate layers are present between the exposed metallic intermediate layer and the semiconductor substrate, these are also oxidized by the anodic oxidation. In this exemplary embodiment, the metal coating contains one or more (e.g., a maximum of two) metallic intermediate layers, each of these metallic intermediate layers preferably being a layer of aluminum or titanium or an alloy of one of these metals, since these metals can be very easily converted into the corresponding metal oxide by anodic oxidation.

[0063] Because the metal coating (and thus each of the metal layers present within the metal coating) is very thin (maximum layer thickness of the metal coating preferably < 100 nm), very thin metal oxide layers are present after anodic oxidation. Due to their thinness, these metal oxide layers exhibit high optical transparency, which has a very beneficial effect on the efficiency of the solar cell.

[0064] If anodic oxidation is performed, both the exposed metal coating and the metallic contact present on the printed metallic coating typically come into contact with the anodizing bath. To avoid or minimize oxidation of the metallic contact, it may be preferable for the metallic contact to be previously coated with a coating (e.g., an organic coating) to protect the surface. An organic surface protection agent to prevent the oxidation of metallic surfaces is known, for example, under the term "organic solderability preservative." Suitable organic compounds (e.g., nitrogen-containing heterocyclic compounds such as benzotriazole, imidazole, or benzimidazole) are known to those skilled in the art.A protective lacquer or a self-assembled monolayer (SAM) can also be used as a coating to protect the surface of the sintered metallic contact. If the exposed metal coating and the metallic contact present on the printed metal coating contain different metals, it may be advantageous to use an anodizing bath and / or anodizing conditions (e.g., a specific anodic potential) that selectively oxidize the exposed metal coating, while oxidizing the metallic contact only to a very limited extent or not at all. Suitable compositions for anodizing baths and suitable anodizing conditions for the anodic oxidation of specific metals are known to those skilled in the art or can be determined through routine testing.

[0065] The invention also relates, in a first embodiment, to an electronic component comprising a semiconductor substrate having a front side and a back side, wherein the front side of the semiconductor substrate has an exposed surface and a covered surface, a metal coating consisting of one or more metal layers, which is present on the covered surface of the front side of the semiconductor substrate and has been applied via a gas phase deposition, wherein the metal coating has, as the uppermost metal layer, a layer of silver, aluminum or copper or an alloy of one of these metals, a metallic contact which contains metal particles sintered together and is present on the metallic coating.

[0066] This electronic component is obtained using the method according to the invention when the underlying front side of the semiconductor substrate is exposed by etching the exposed metal coating. As already mentioned above, the printed metal coating, on which the metallic contact is present, remains on the semiconductor substrate after etching.

[0067] As already mentioned above, the electronic component is, for example, a solar cell or a precursor to a solar cell.

[0068] With regard to the preferred properties of the semiconductor substrate, reference can be made to the above statements.

[0069] Optionally, one or more (preferably no more than two) metallic intermediate layers are present between the uppermost metal layer and the semiconductor substrate. Regarding preferred metallic intermediate layers and preferred combinations of specific metallic intermediate layers with specific uppermost metal layers, reference can be made to the above explanations. In a preferred embodiment, the metal coating consists of only one metal layer, which metal layer is preferably a layer of silver or a silver alloy.

[0070] The top metal layer and, if present, the metallic intermediate layers of the metal coating are applied to the front side of the semiconductor substrate via a vapor deposition, preferably a physical vapor deposition. A metal deposited via a vapor deposition typically has very low porosity and thus a high density, which is very close to the theoretical density of the metal. Each of the metal layers of the metal coating applied via a vapor deposition therefore preferably has a density that is at least 90%, more preferably at least 95%, even more preferably at least 98% of its theoretical density p t h. The theoretical density p th of a metal can be taken from corresponding databases or calculated in a known manner. pth = [(number of atoms per unit cell) x atomic mass] / [(volume of the unit cell) x Avogadro number] Unit cell:

[0071] With regard to the sintered metal particles present in the metallic contact, reference can be made to the above statements.

[0072] As mentioned above, there are cavities between the sintered metal particles of the metallic contact. Optionally, additional components, such as an organic binder and / or glass, can be present in these cavities between the sintered metal particles.

[0073] Optionally, an organic protective film is present on the metallic contact. The organic protective film contains, for example, one or more nitrogen-containing heterocyclic compounds such as benzotriazole, imidazole, or benzimidazole. A protective lacquer or a self-assembled monolayer (SAM) can also be used as a coating to protect the surface of the metallic contact. In an alternative embodiment, the present invention relates to an electronic component comprising a semiconductor substrate having a front side and a back side, wherein the front side of the semiconductor substrate has a first coated area and a second coated area adjacent thereto, a metal coating consisting of one or more metal layers, which is present on the first coated area of ​​the front side of the semiconductor substrate and has been applied via physical vapor deposition,wherein the metal coating comprises, as the topmost metal layer, a layer of silver, aluminum, or copper or an alloy of one of these metals, an exposed metal oxide coating consisting of one or more metal oxide layers, which is present on the second coated surface of the front side of the semiconductor substrate, a metallic contact containing metal particles sintered together and which is present on the metallic coating.

[0074] This electronic component is obtained with the method according to the invention when the exposed metal coating is subjected to anodic oxidation and thereby transformed into an exposed metal oxide coating (ie each metal layer present in the exposed metal coating is oxidized to the corresponding metal oxide layer) or by etching the exposed metal coating, the uppermost metal layer is removed and an underlying metallic intermediate layer is exposed and by anodic oxidation of the exposed metallic intermediate layer (and optionally further metallic intermediate layers present between the exposed metallic intermediate layer and the semiconductor substrate) an exposed metal oxide coating is obtained.

[0075] As already mentioned above, the electronic component is, for example, a solar cell or a precursor to a solar cell.

[0076] Regarding the preferred properties of the semiconductor substrate, reference can be made to the above statements. Optionally, the metal coating contains one or more (preferably no more than two) metallic intermediate layers located between the topmost metal layer and the semiconductor substrate. Regarding preferred metallic intermediate layers and preferred combinations of specific metallic intermediate layers with specific topmost metal layers, reference can be made to the above statements.

[0077] The top metal layer and, if present, the metallic intermediate layers of the metal coating are applied to the front side of the semiconductor substrate via a vapor deposition, preferably a physical vapor deposition. A metal deposited via a vapor deposition typically has very low porosity and thus a high density, which is very close to the theoretical density of the metal. Each of the metal layers of the metal coating applied via a vapor deposition therefore preferably has a density that is at least 90%, more preferably at least 95%, even more preferably at least 98% of its theoretical density p t h. The theoretical density p t h of a metal can be taken from appropriate databases or calculated in a known manner.

[0078] Pth = [(number of atoms per unit cell) x atomic mass] / [(volume of the unit cell) x Avogadro number] Unit cell

[0079] Preferably, each layer present in the metal oxide coating is a layer containing aluminum oxide or titanium oxide.

[0080] For example, the metal coating and the metal oxide coating have the same number of layers (e.g., two layers each or only one layer each), with the top metal layer of the metal coating being a layer of aluminum or an aluminum alloy, and the metal oxide coating having a top metal oxide layer containing aluminum oxide, and optionally, the metal coating containing a layer of titanium or a titanium alloy as a metallic intermediate layer, and the metal oxide coating having an intermediate layer containing titanium oxide. Thus, if both the metal coating and the metal oxide coating each consist of two layers, the following layer sequences, for example, result when viewed from the front side of the semiconductor substrate:

[0081] Titanium (i.e. intermediate layer) -> Aluminium (i.e. top metal layer) in the metal coating and

[0082] Titanium oxide (i.e. intermediate layer) -> aluminum oxide (i.e. top metal oxide layer) in the metal oxide coating.

[0083] In a further exemplary embodiment, the metal coating has m metal layers and the metal oxide coating has n metal oxide layers, where m is 2 or 3, n is 1 or 2, with the proviso that m>n, wherein each of the m metal oxide layers of the metal oxide coating is an aluminum oxide-containing layer or a titanium oxide-containing layer, wherein the aluminum oxide-containing layer borders a metallic intermediate layer made of aluminum or an aluminum alloy in the metal coating, or the titanium oxide-containing layer borders a metallic intermediate layer made of titanium or a titanium alloy in the metal coating. The uppermost metal layer of the metal coating is, for example, a layer made of silver or copper or an alloy of one of these metals.

[0084] With regard to the sintered metal particles present in the metallic contact, reference can be made to the above statements.

[0085] As mentioned above, there are cavities between the sintered metal particles of the metallic contact. Optionally, additional components, such as an organic binder and / or glass, can be present in these cavities between the sintered metal particles.

[0086] Optionally, an organic protective film is present on the metallic contact. The organic protective film contains, for example, one or more nitrogen-containing heterocyclic compounds such as benzotriazole, imidazole, or benzimidazole. A protective lacquer or a self-assembled monolayer (SAM) can also be used as a coating to protect the surface of the metallic contact. Figures 1a-d and 2-4 illustrate exemplary embodiments of the present invention in more detail.

[0087] They show:

[0088] Figure 1 a-d shows the steps of the method according to the invention, in which a metal coating is applied to the semiconductor substrate by means of a vapor deposition, the metal coating is printed with a paste and then the metal particles of the paste are sintered together by irradiation with light;

[0089] Figure 2 shows an exemplary embodiment of the electronic component of the present invention obtained when, after photonic sintering of the paste, the exposed metal coating is completely removed by etching;

[0090] Figure 3 shows an exemplary embodiment of the electronic component of the present invention obtained when, after photonic sintering of the paste, the exposed metal coating is oxidized to a metal oxide coating by anodic oxidation;

[0091] Figure 4 shows an exemplary embodiment of the electronic component of the present invention, which is obtained when, after photonic sintering of the paste, the uppermost metal layer of the exposed metal coating is removed by etching and the remaining intermediate layer is oxidized to a metal oxide layer by anodic oxidation.

[0092] As shown in Figure 1a, a metal layer 2 and then a metal layer 3 are first applied to the front side of a semiconductor substrate 1 by physical vapor deposition (PVD). The metal layer 2 and the metal layer 3 together form the metal coating 4 (illustrated in Figure 1a by the curly bracket). In this metal coating 4, the metal layer 3 acts as the uppermost metal layer, while the metal layer 2 represents an intermediate layer. The uppermost metal layer 3 is a layer of silver, aluminum, or copper, or an alloy of one of these metals. The intermediate layer 2 is preferably equipped with a specific functionality.

[0093] For example, the intermediate layer 2 acts as an adhesion-promoting layer or as a diffusion barrier between the top metal layer 3 and the semiconductor substrate 1.

[0094] The intermediate layer 2 is selected, for example, so that its reflective properties complement the reflective properties of the uppermost metal layer 3. If the uppermost metal layer 3 is, for example, a layer of silver or a silver alloy (alternatively, a layer of aluminum or an aluminum alloy), it may be advantageous to use a layer of aluminum or an aluminum alloy (alternatively, a layer of silver or a silver alloy) as the metallic intermediate layer 2. Metal layers made of silver exhibit very high reflectivity, particularly in the wavelength range of visible light, while metal layers made of aluminum already exhibit high reflectivity in the wavelength range of UV radiation. These two exemplary embodiments thus exhibit high reflectivity over a very broad wavelength range.

[0095] After printing a defined area of ​​the topmost metal layer 3 with a paste 5 containing metal particles and / or a metal particle precursor, a printed metal coating 4a and an adjacent exposed metal coating 4b are present. This is illustrated in Figure 1b. In Figure 1b, the printed metal coating 4a and the exposed metal coating 4b are virtually demarcated from one another by vertical dashed lines. Both the printed metal coating 4a and the exposed metal coating 4b contain the topmost metal layer 3 and the metallic intermediate layer 2 (each illustrated by the dashed curly bracket).

[0096] The paste 5 is irradiated with light 6. This is illustrated in Figure 1c. Irradiation causes the metal particles present in the paste to sinter together, or the metal particle precursor present in the paste to decompose to form metal particles, and the resulting metal particles to sinter together. Irradiation of the paste 5 and the resulting photonic sintering produce a metallic contact 7 on the printed metal coating 4a, which contains the sintered metal particles. This is illustrated in Figure 1d. The paste is preferably irradiated with pulsed light, e.g., using a flash lamp.

[0097] After the formation of the metallic contact 7 by irradiating the paste with light, the exposed metal coating 4b is etched or anodically oxidized.

[0098] For example, by etching the exposed metal coating 4b, the underlying front side of the semiconductor substrate is exposed.

[0099] The structure of the resulting electronic component is illustrated in Figure 2.

[0100] The electronic component includes a semiconductor substrate 1 having a coated surface 1a and an exposed surface 1b on its front side. A metal coating 4a is present on the coated surface 1a, which has a metallic intermediate layer 2 and a topmost metallic layer 3 (illustrated by the curly bracket) and was applied via physical vapor deposition. The topmost metallic layer 3 is a layer of silver, aluminum, or copper, or an alloy of one of these metals. The metallic contact 7, which contains the sintered metal particles, is present on the metallic coating 4a.

[0101] As already mentioned above, the semiconductor substrate 1 can, for example, have a multilayer structure (not shown in Figure 2). For example, the outermost layer on the front side of the multilayer semiconductor substrate 1 is a layer of a transparent electrically conductive oxide (which may have semiconducting properties) or a layer of a dielectric material (e.g., silicon nitride or aluminum oxide), and an inner layer of the multilayer semiconductor substrate 1 contains silicon (e.g., undoped (amorphous or crystalline) silicon, n- or p-doped (amorphous or crystalline) silicon). The electronic component obtained when the exposed metal coating 4b is subjected to anodic oxidation in the method according to the invention is illustrated in Figure 3.

[0102] The electronic component contains a semiconductor substrate 1, which has at least one first coated area 1a on its front side and at least one second coated area 1c adjacent thereto. On the first coated area 1a there is a metal coating 4a, which consists of a metallic intermediate layer 2 and an uppermost metallic layer 3 (illustrated by a dashed curly bracket) and was applied via physical vapor deposition. The uppermost metallic layer 3 is, for example, a layer made of aluminium or an aluminium alloy, and the metallic intermediate layer is, for example, a layer made of titanium or a titanium alloy. The metallic contact ?, which contains the metal particles sintered together, is present on the metallic coating 4a.On the second coated surface 1c of the front side of the semiconductor substrate 1, there is a metal oxide coating 4b' obtained by anodic oxidation of the exposed metal coating 4b. The exposed metal oxide coating has an uppermost metal oxide layer 3' and a metal oxide intermediate layer 2' (illustrated by a curly bracket). If the uppermost metal layer 3 is, for example, a layer of aluminum or an aluminum alloy and the metallic intermediate layer is, for example, a layer of titanium or a titanium alloy, then the uppermost metal oxide layer 3' is an aluminum oxide-containing layer, and the metal oxide intermediate layer 2' is a titanium oxide-containing layer.

[0103] Figure 4 illustrates, by way of example, the structure of an electronic component that is obtained when, in the method according to the invention, the uppermost metal layer 3 is removed by etching the exposed metal coating 4b and an underlying metallic intermediate layer 2 is exposed, and an exposed metal oxide coating 4b' is obtained by anodic oxidation of the exposed metallic intermediate layer 2 (and optionally further metallic intermediate layers that are present between the exposed metallic intermediate layer 2 and the semiconductor substrate 1). The electronic component contains a semiconductor substrate 1 that has at least one first coated area 1a on its front side and, adjacent thereto, at least one second coated area 1c.On the first coated surface 1a, there is a metal coating 4a, which consists of a metallic intermediate layer 2 and an uppermost metallic layer 3 (illustrated by the dashed curly bracket) and was applied via physical vapor deposition. The uppermost metallic layer 3 is a layer of silver, aluminum, or copper, or an alloy of one of these metals, and the metallic intermediate layer 2 is, for example, a layer of titanium or a titanium alloy, or, if the uppermost metallic layer is not aluminum, a layer of aluminum or an aluminum alloy. On the metallic coating 4a, there is the metallic contact ?, which contains the metal particles sintered together. On the second coated surface 1c of the front side of the semiconductor substrate 1, there is a metal oxide coating 4b'.Since the top metal layer of the exposed metal coating was removed by etching prior to anodic oxidation, the metal oxide coating 4' has only a single metal oxide layer 2'. If the metallic intermediate layer 2 of the metal coating is, for example, a layer of titanium or a titanium alloy, the metal oxide layer 2' is a layer containing titanium oxide.

[0104] In the exemplary embodiments illustrated in Figures 1a-d, 2-4, the metal coating contains a metallic intermediate layer in addition to the topmost metal layer. As already mentioned above, the presence of the metallic intermediate layer can be linked to a specific functionality (e.g., adhesion-promoting layer or diffusion barrier between the topmost metal layer and the semiconductor substrate, improvement of reflectivity, etc.). However, to keep the process as simple as possible, it may be preferable for the metallic coating to be single-layered (i.e., consisting of the topmost metal layer).

Claims

Claims 1 . A method for producing at least one metallic contact in an electronic component, comprising the following steps: Applying a single- or multi-layer metal coating (4) by vapor deposition on a front side of a semiconductor substrate (1), wherein the metal coating (4) has as the top metal layer (3) a layer of silver, aluminum or copper or an alloy of one of these metals, Printing a defined area of ​​the uppermost metal layer (3) of the metal coating (4) with a paste (5) containing metal particles and / or a metal particle precursor, so that a printed metal coating (4a) and an exposed metal coating (4b) are present, Irradiating the printed paste (5) with light (6), wherein the metal particles at least partially sinter with one another or, if the paste (5) contains the metal particle precursor, the metal particle precursor decomposes to form metal particles and the metal particles at least partially sinter with one another, so that by irradiating the paste (5) a metallic contact (7) containing the sintered metal particles is obtained, Etching or anodic oxidation of the exposed metal coating (4b).

2. The method according to claim 1, wherein the metal coating (4) consists of the uppermost metal layer (3).

3. The method according to claim 1, wherein the metal coating (4) has one or more metallic intermediate layers (2) present between the uppermost metal layer (3) and the semiconductor substrate (1), wherein the metal coating (4) preferably contains no more than three metal layers (2, 3).

4. Method according to one of the preceding claims, wherein the metal coating (4) has a layer thickness of not more than 100 nm.

5. The method according to claim 3 and 4, wherein the uppermost metal layer (3) of the metal coating (4) is a layer of aluminum or an aluminum alloy, and the metal coating (4) contains a metallic intermediate layer (2) of silver or a silver alloy; or the uppermost metal layer (3) of the metal coating (4) is a layer of silver or a silver alloy, and the metal coating (4) contains a metallic intermediate layer (2) of aluminum or an aluminum alloy.

6. The method according to claim 3 or 4, wherein the uppermost metal layer (3) of the metal coating (4) is a layer of aluminum or an aluminum alloy and the metal coating (4) contains a metallic intermediate layer (2) of Ti, Cr, Zn, Cu, Mo or W or an alloy of one of these metals.

7. Method according to one of the preceding claims, wherein the light (6) with which the paste (5) is irradiated is a pulsed light.

8. Method according to one of the preceding claims, wherein the metallic contact (7) has a thickness of at least 200 nm.

9. Method according to one of the preceding claims, wherein the etching of the exposed metal coating (4b) exposes the underlying front side of the semiconductor substrate (1).

10. The method according to any one of claims 1 to 8, wherein an exposed metal oxide coating (4b') is obtained by anodic oxidation of the exposed metal coating (4b).

11. Method according to one of claims 3 to 8, wherein the uppermost metal layer (3) is removed by etching the exposed metal coating (4b) and an underlying metallic intermediate layer (2) is exposed, and an exposed metal oxide coating (4b') is obtained by anodic oxidation of the exposed metallic intermediate layer (2) and, if present, further metallic intermediate layers (2).

12. Electronic component, comprising a semiconductor substrate (1) having a front side and a back side, wherein the front side of the semiconductor substrate (1) has an exposed surface (1b) and a covered surface (1a), a metal coating (4a) consisting of one or more metal layers (3, 2) which is present on the covered surface (1a) of the front side of the semiconductor substrate (1) and has been applied via a gas phase deposition, wherein the metal coating (4a) has, as the uppermost metal layer (3), a layer of silver, aluminum or copper or an alloy of one of these metals, at least one metallic contact (7) which contains metal particles sintered together and is present on the metal coating (4a).

13. An electronic component comprising a semiconductor substrate (1) having a front side and a back side, wherein the front side of the semiconductor substrate (1) has a first coated surface (1a) and a second coated surface (1c) adjacent thereto, a metal coating (4a) consisting of one or more metal layers (3, 2) which is present on the first coated surface (1a) of the front side of the semiconductor substrate (1) and has been applied via physical vapor deposition, wherein the metal coating (4a) has, as the uppermost metal layer (3), a layer of silver, aluminum, or copper or an alloy of one of these metals, an exposed metal oxide coating (4b') consisting of one or more metal oxide layers (3', 2') which is present on the second coated surface (1c) of the front side of the semiconductor substrate (1), at least one metallic contact (7),which contains sintered metal particles and is present on the metal coating (4a).

14. Electronic component according to claim 13, wherein the metal coating (4a) contains no more than two metal layers (3, 2), the metal coating (4a) and the metal oxide coating (4b') have the same number of layers, the uppermost metal layer (3) of the metal coating (4a) being a layer of aluminum or an aluminum alloy and the metal oxide coating (4b') having a topmost Metal oxide layer (3') containing aluminum oxide, and optionally the metal coating (4a) contains a layer of titanium or a titanium alloy as a metallic intermediate layer (2) and the metal oxide coating (4b') has an intermediate layer (2') containing titanium oxide.

15. Electronic component according to claim 13, wherein the metal coating (4a) has m metal layers (3, 2) and the metal oxide coating (4b') has n metal oxide layers (3', 2'), where m is 2 or 3, n is 1 or 2, with the proviso that m > n, each of the n metal oxide layers (3', 2') of the metal oxide coating (4b') is an aluminum oxide-containing layer bordering a metallic intermediate layer (2) made of aluminum or an aluminum alloy in the metal coating (4a), or a titanium oxide-containing layer bordering a metallic intermediate layer (2) made of titanium or a titanium alloy in the metal coating (4a).

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