EP2368272A1 - Method for producing a metal contact on a semiconductor substrate provided with a coating - Google Patents

Abstract

The invention relates to a method for producing an electrically conducting metal contact on a semiconductor component that comprises a coating (20) on the surface of a semiconductor substrate (12). In order to keep transfer resistances low while maintaining good mechanical strength, the following method steps are proposed: - applying a particle-containing fluid onto the coating, wherein the particles contain at least metal particles and glass frits, - curing the fluid while simultaneously forming metal areas (26, 28) in the substrate (12) through heat treatment, - removing the cured fluid and the areas of the coating covered by the fluid, - depositing, for the purposes of forming the contact without using intermediate layers, electrically conducting material (18) from a solution onto areas of the semiconductor component in which the coating is removed while at the same time conductively connecting the metal areas (26, 28) present in said areas on the substrate (12).

Description

METHOD FOR PRODUCING METAL CONTACT ON A SEMI-FINISHED SEMICONDUCTOR SUBSTRATE

The invention relates to a method for producing a particular strip-shaped electrically conductive metal contact on a semiconductor device, such as a solar cell, the surface side on a substrate, a layer such as dielectric layer such as passivation of z. For example, silicon nitride, silica, titanium oxide.

More than 90% of the solar cells and solar modules currently available on the market rely on crystalline silicon as the solar cell material. The predominant part of these solar cells is based on methods in which firstly the near-surface region of a plate-shaped or disc-shaped substrate is doped with a diffusion method inversely to the doping of the base material (substrate) via a diffusion process as the starting wafer, which basically has a homogeneous output doping. Usually, p-doped Si starting wafers are n-doped with a phosphorus diffusion on at least one surface - usually the light-receiving side - or part of the surface. But also n-doped crystalline Si starting material is used as a solar cell material and p-doped with diffusion processes (for example with boron) or alloying processes (aluminum) on at least one of the surfaces or part of the surface. The vast majority of industrially produced silicon solar cells are based on metal-paste contacts, which tap the current on both sides of the semiconductor junction and transport them to collecting contacts - so-called beam / busbars or soldering pads / pads - which allow to solder solder joints and solder the solar cells to interconnect a solar module with each other electrically. The metal pastes used to contact the solar cells, for example, allow the contacting of silicon surfaces by dielectric layers such as SiN _x : H, SiO ₂ , TiO _x or others. For the most part, to date, use has been made of Ag pastes with a low proportion by weight of glass frits or in some cases other inorganic additives which are sintered at comparatively high temperatures (700 ^° C.-900 ^° C.). During the typically relatively short sintering process, the burning of organic constituents such as solvents and binders typically only very briefly passes through the maximum temperature of the sintering process ('peaking', 'sintering spike'). During the sintering process, the glass frit contained in the Ag paste, which contains metal oxides, first soft, then liquid and wets the surface of the solar cell. The glass frits and other inorganic additives are often chosen to etch through any dielectric layers present on the wafer surface. The metal oxides exchanges contained in the glass melt act in a redox reaction with the silicon surface. This produces SiO _x and reduced metal ions which are dissolved in the melt. The molten glass thus partially etches into the silicon surface and Ag is dissolved in the molten glass and the metal ions contained therein. During the cooling process, Ag is preferably epitaxially deposited on the silicon surface on <111> surfaces. Ag crystallites are formed in the original silicon surface, which after the cooling process are spatially and largely electrically separated from one another by the re-solidified glass intermediate layer. Above this glass layer, after sintering, there is a conductive structure of sintered-together Ag particles. The conductivity of the thus produced contact between silicon and Ag is limited by the limited conductivity of the glass layer between the Ag crystallites deposited in the silicon surface and the sintered together Ag particle structure. This conductivity is determined by the thickness of the glass layer and the metal precipitates dissolved therein, which exceed the tunneling probability. increase the probability of charge carriers. Contacts with silicon with other metal pastes can also be achieved.

Elements such as Ag, Ni, Pd, Zn, Ti, Pb, Sn, Sb, Bi, Al, B and others have been used for many years in pastes mostly as constituents in oxidic form, or within a glass matrix to make contact with silicon , However, if it is desired to contact through a dielectric layer, usually an advantageous passivation layer on the silicon surface, Ag pastes containing glass frits and baked at higher process temperatures (> glass melting point) are to be preferred, since this is a comparatively simple one Process with simultaneously achievable high throughput and industrial efficiency.

The density and size - lateral or penetration depth - of the resulting Ag crystallites in silicon and the thickness of the glass layer are strongly dependent on the temperature treatment or the glass frit used - type, amount, particle size distribution - and other inorganic additives and the furnace atmosphere during sintering / Firing the paste depending. It is aggravating that the optimum crystal density and the most conductive, ie thinnest glass layer can not necessarily be achieved at the same temperature treatment, so that a suitable temperature profile often represents a compromise of both requirements and must be achieved very reproducible in order to achieve optimal results. The process window is therefore quite small.

The previously developed Ag pastes and other metal pastes and the associated application, sintering and contacting methods, which are suitable for contacting industrial crystalline Si solar cells and are widely used, have a number of disadvantages. These disadvantages include:

o High contact contact resistance of the paste contacts to silicon, in particular for low-doped silicon surfaces ("10 ²⁰ dopant atoms / cm ³ ). o Relatively low conductivity of the paste contacts due to the porous contact structure and glass components in the contact structure. The conductivity is significantly reduced compared to pure metal layers. o A relatively high proportion of starting metals such as Ag is required to achieve sufficiently high conductivity of the contacts. Thus, increased consumption material costs and unnecessary shading losses are the consequences. Limitation of the minimum achievable contact line width by particle sizes in the pastes and required cross-sectional conductivity of the contact lines in conjunction with paste application method, such as screen printing, especially when narrow contact lines with high order strength are sought to reduce light shading on the solar cell front and / or emitter Contact with low transverse conductivity. o Limited adhesion on the solar cell surface and limited durability of the contacts. The metal structure or the glass layers of the contacts interact during cooling processes - after sintering or soldering - during soldering and during long-term interaction with substances produced in the module assembly. Here, thermal and mechanical influencing factors, such as different expansion coefficients of the material composite, as well as possible chemical interactions play a role. The influences mentioned can permanently adversely affect the performance of the solar cell and its contacts over the product life, ie solar module in the respective application environment. This results in necessary compromises that take into account the interaction with all candidate materials and materials during the manufacture and operation of the solar modules to ensure the required life and the maximum on solar cell level - at least temporarily - achievable efficiencies. So far commercially available Ag pastes are not suitable to contact phosphorus emitter with low surface dopant concentration so that low contact transition resistances can be achieved while high filling factors are achieved in solar cells. As a consequence, recombination losses result in the highly doped emitters (> 10 ²⁰ P atoms / cm ³ ) of the industrially used crystalline Si solar cells, which can be contacted with Ag pastes. As a result, the short-circuit current and efficiency of the solar cell is limited. o Use of heavy metal-containing glass frits such as PbO _x , CdO _x and thus harmful to health and environment constituents in the Ag pastes, which in the modular network may no longer meet future guidelines of the European electronics industry. o The contact structure of printed metal contacts on Si solar cells is porous and is therefore not fully suitable for aftertreatment in electroless or galvanic aqueous solutions for plating metals from the solution, since residues of the solutions used for depositing metals remain in the porous structure or can be included and later in the module network to damage the contacts and thus the solar modules can lead. Disadvantages resulting from this include loss of efficiency, delamination of the modules, discoloration of the contacts and solar cells.

From US-A-4,703,553 an extruding method for producing a solar cell is known. To form backside contacts, an aluminum containing paste is applied to a backside oxide layer. Subsequent heat treatment is intended to oxidize the Al particles on their surface, whereby the underlying oxide layer is to be removed in regions. Remaining aluminum particles form an alloy with the substrate material of the solar cell. By means of an HCl solution, the remaining on the substrate residues of aluminum and aluminum oxide are then removed. Below the areas thus exposed there is then a highly doped p ⁺ region.

The subject of US-A-2002/0153039 is the production of a solar cell, on the outer sides of which oxide layers are applied. P ₂ O ₅ - and on the back B ₂ θ ₃ material is then printed on the front side to form a phosphosilicate glass or borosilicate glass layer. As a result of thermal processes, highly doped n ⁺⁺ and p ⁺⁺ layers are formed in the front and rear areas, respectively. The phosphosilicate glass (PSG) or borosilicate glass layer (BSG) is then etched away by means of hydrofluoric acid. As a result, the adjacent areas of the oxide layers are partially attacked with. Subsequently, an application of electrically conductive Kon- contacts. Parts of the adjacent oxide layers should be retained after complete removal of the PSG and BSG layers.

It is an object of the present invention to develop a method for producing an electrically conductive metal contact on the surface of a semiconductor component, such as a solar cell, in such a way that low contact resistances occur with good mechanical strength. Also, the surface extension of the metal contact should be minimized to keep the shading as low as possible in solar cells as semiconductor devices. Furthermore, a secure contact in surface areas should be possible.

To achieve the object, the invention proposes a method for producing at least one in particular strip-shaped electrically conductive metal contact on a semiconductor component, such as a solar cell having on the surface side on a semiconductor substrate a layer, such as dielectric layer as Pas sivier layer, comprising the method steps,

line, strip and / or punctiform application of a fluid to the layer containing at least particles of metal and particles of glass frit, wherein surface of the particles is optionally coated or oxidized and / or the fluid optionally additionally contains metal oxide particles. Curing the fluid by heat treatment while forming:

- coherent metal structure by sintering the metal particles together

a glass layer between metal structure and semiconductor substrate and

Metal regions in the semiconductor substrate that are separated from the metal structure on the semiconductor substrate by the glass layer,

Removing the separating glass layer by etching and thus simultaneously removing the metal structure by undercutting, without removing the grown into the semiconductor substrate metal areas, to form the at least one electrically conductive contact, interlayer-free deposition of electrically conductive material from a solution onto the metal regions of the semiconductor component which have grown into the semiconductor component, above which the glass layer and metal structure layer have been removed, while at the same time electrically connecting the metal regions present in these regions in the substrate ,

It can be assumed that the metal areas during the heat treatment by redox reaction of the metal oxides containing glass melt with the semiconductor and Si material and etching of the semiconductor material and subsequent epitaxial deposition arise.

In particular, it is provided that the fluid used is one which additionally contains metal oxide.

As a fluid, in particular a paste, but also ink or an aerosol in question, the composition of which must ensure penetration through the layer, such as passivation layer.

However, the fluid is preferably a paste with a low weight fraction of glass frit or partially other inorganic additives, as are known from the prior art and etch through the layer as dielectric layer as the Pas sivier layer. Also, organic ingredients such as solvents and binders may be present in the paste.

Hereinafter, the existing on the semiconductor substrate layer - if one is present - basically referred to as a dielectric layer, without this being intended to limit the teaching according to the Invention.

The hitherto single-stage metallization contact for contacting semiconductor devices by Pastenaufbringverfahren with subsequent drying and sintering of the applied metal pastes is replaced by a multi-stage process, in which - according to the prior art - first a fluid such as the metal paste in narrow lines or stripes or applied selectively, dried and sintered. Then, however, the applied contact structure is removed to such an extent that the glass layer or metal oxide layer on which the metal contact structure is located largely detached and only the metal crystal regions remain on the semiconductor device surface, which directly the ohmic contact to the semiconductor, ie Form substrate, and thus make tunnel mechanisms for the contact resistance superfluous. After detachment of the porous metal structure and glass layers, an ohmic connection to metal layers to be applied to the crystallites is thus locally present at the locations where the contact lines or points were previously baked, via the crystallites (for example, epitaxially deposited Ag crystallite areas).

It is ensured that the existing on the substrate surface dielectric layer is locally opened by the occurring through the fluid, in particular metal paste, during the heat treatment as in melting etching effect, so that ensures about this for the subsequent metal layers of the desired electrically conductive contact with low contact resistance is. In particular, galvanically or electrolessly metal layers such as Ag, Ni, Cu, Pd, Ti, Sn, Al can be deposited. These can be deposited in the desired sequence and / or thickness and optionally in succession or one above the other.

Instead of producing an electrical contact to the semiconductor material such as silicon with metal pastes essentially by applying the paste and subsequent heat treatment or sintering according to the prior art, a multi-stage process is used according to the invention:

In a first stage, in a similar or adapted method according to the prior art, a fluid such as a metal-containing paste is applied to the desired areas of the semiconductor device such as solar cell surface and etched through the dielectric layer such as passivation layer in subsequent drying and sintering processes and metal areas such as metal in the silicon when cooling after the maximum sintering temperature epitaxially grown in the semiconductor material such as silicon.

During the cooling process, metal dissolved in the molten glass, such as Ag, is deposited using Ag paste. The deposition is preferably carried out at locations where the semiconductor as Si surface was etched by redox reaction. There it comes to epitaxial growth of metal and Ag crystals or crystallites.

Compared to the prior art, other fluid, such as paste compositions, other linewidths and application levels for the pastes, as well as other sintering conditions are contemplated since the resulting contact is only an intermediate to the final contact and thus can be otherwise optimized. According to the invention, a selective application of the fluid, in particular in rows of points, can also take place in order to subsequently produce the electrically conductive connection.

The heat treatment or the sintering is carried out at a temperature T _sint> T _schm with Tschm = melting temperature of the glass frit, whereby preferably 700 ⁰ C <T _smt <1000 ⁰ C.

In the 2nd stage, the contact thus formed is first partially removed in an additional etching step. In this case, for example, in hydrofluoric acid or other oxide-reducing solutions, the glass layer, which separates Ag crystallites and Ag particle structures from one another with sintered Ag paste contacts, is removed so that only the Ag crystallites which are present in the semiconductor and Si compounds are removed. Are epitaxially grown surface, remain in contact with the semiconductor such as silicon, while the remaining fluid and paste structure is removed.

For the metal and Ag particle structure, the following should be noted. Metal particles sinter together under the influence of temperature and melting of the glass frit. Among other things, the glass frit also reduces metals from the surface of the metal particles. After cooling, a coherent metal structure is formed, which, however, has a lower density and conductivity than a dense metal layer. In a further process step, a metallic layer with a higher conductivity and a lower contact contact resistance is deposited or applied directly in the third stage on the remaining structures of contiguous or closely adjacent Ag crystallites than is initially the case with the sintered metal contacts. Through the metal layer, the Ag crystallites are conductively connected together. Suitable for this purpose are all processes which achieve contacts with improved conductivity and / or long-term stability in the module assembly without significant financial or procedural additional effort and metallic contact between the ingrown in the semiconductor material metal areas (first stage of the process) and now deposited thereon metal layers. For example, processes which selectively deposit metal from chemical solutions on the metal regions (eg Ag crystallites) which have grown into the semiconductor in the case of customary Ag pastes and with those under the same conditions on semiconductor or Si surfaces or dielectric layers are suitable Layers no metal is deposited. These can be, for example, electroless or galvanic metal deposition processes in which metal is selectively deposited from aqueous solutions on the metal regions grown in the semiconductor, since there are selectively advantageous electrochemical potentials relative to the remaining regions of the solar cell surface. Since the typical growth of metal layers during deposition from chemical solutions is predominantly isotropic, closely adjacent crystallite metal-land islands grow together during the deposition process, so that overall a coherent conductive contact structure results in the desired regions. The contact contact resistance is significantly lower than that of the originally produced metal fluid as well as contact, since no tunneling mechanism is required within the separating glass layer, but the Ag crystallites are directly in good electrical contact with the semiconductor such as silicon and on the other hand are directly in metallic contact with the conductive layer of the contact lines. The line conductivity of the contact can be significantly improved with the same or smaller cross-section of the contacts, since it is no longer porous contact structure with glass components, but a solid, dense metallic layer can be deposited with high conductivity. In particular, it is provided that the conductive material having a width B with B <100 .mu.m, in particular with B <60 .mu.m, preferably with B <40 .mu.m, more preferably with B <20 microns is applied and / or that the conductive material with a Application height with H <15 microns, in particular with H <10 microns, preferably with H <5 microns, more preferably with H <1 micron is applied.

Deviating from the prior art, a direct contact between the semiconductor substrate and the in particular galvanically or electrolessly applied from a solution electrically conductive or the metal contacts forming material layer, so that there is a very low contact resistance compared to the prior art.

The existing according to the prior art insulating layer between the semiconductor substrate and the metal contacts forming electrically conductive layer is eliminated. In the case of silver, which form the crystallites in the semiconductor substrate, the prior art basically requires a glass matrix which has an insulating effect.

After the dissertation of G. Schubert, University of Konstanz (2006), "Thick Film Metallization of Crystalline Silicon Solar Cells Mechanisms, Models, Applications", an organic matrix is used, which also has an insulating effect in such a way that an undesirable contact resistance occurs Schubert does not focus on the production of solar cells with metal contacts, but rather investigates the extent to which an electrically conductive compound can be produced between the Ag crystallites forming in the substrate and the conductive silver applied to them However, it is not suitable for interconnecting solar cells, as this is not economically feasible due to its price and also neither selectively wets the Ag-crystallite, nor leads to contacts that can be contacted by the soldering used in the photovoltaic soldering to solar cell len permanently in the module assembly to connect electrically conductive. Furthermore, Schubert Leitsil- Be applied in a width that leads to shading, which would lead to a significant reduction in efficiency for solar cells.

The Ag or - generally - metal content in the fluids such as pastes and total amount of applied metals can be reduced in the multi-stage form of metal contact production, since the first application of the metal fluid such as paste contact a much smaller cross-section of the contact is sufficient. This contact only needs to produce suitable crystallite areas in the silicon and does not need to achieve a particularly conductive cross-section in the metal structure above the glass layer, so that on the one hand with a metal paste, the weight that is applied, can be significantly reduced and on the other hand their composition exclusively can be optimized with respect to metal area formation in the semiconductor device and the desired ideal sintering process. It is quite permissible that, for example, the proportion of glass in relation to the metal content in the paste is significantly increased, although this may mean that the separating glass layer is not particularly conductive. If this results in a high surface density of metal areas in the semiconductor device after the contact firing, this is to be seen as advantageous because the separating glass layer also easier in the second part process step can be removed and the higher glass content can form a larger area density of metal areas in the semiconductor material , During the deposition of the metal layer after the detachment of the glass layer (etching process), a denser metal layer with a higher conductivity is deposited, so that the required metal content can be lower overall than is the case with conventional metal paste contacts. This fact justifies, in addition to process advantages and improved solar cell efficiencies, the economic applicability of the additional processing steps.

Since only a small application height when applying the metal paste is necessary, additional methods for applying the metal pastes in question, which could not be used economically or advantageously in the industrial production of solar cells. These are, for example, pad printing or offset printing, inkjet methods, aerosoljet methods and other known methods. Since the application level when applying the paste is no longer for the subsequent conductivity of the pastes Significantly, even narrower contacts can be applied - even by screen printing using modified screens and / or pastes -. Even small breaks in the printed image - even at individual juxtaposed points - are acceptable, provided that they can be closed again in the subsequent construction of a conductive layer - after etching the glass layer and the metal particle structure. In the case of galvanic or electroless deposition of metals from solutions, this is done, for example, by the lateral growth of the deposited layer and coalescence of the individual metal crystallite regions.

For the preparation of the conductive layers that connect and directly contact the crystallite regions, there are a number of processes that are contemplated besides the deposition of metals from chemical solutions. Thus, there are also metal pastes which achieve conductive layers and good mechanical adhesion at comparatively low temperatures, without being able to etch through dielectric layers or to be able to contact silicon directly. In this case, a low ohmic contact contact resistance extends to the metal-crystallite regions which have already grown into the semiconductor component. Also, soldering methods such as thermal or ultrasonic methods and other connection methods for contacting metallic areas come into question. The selection of materials is not limited to the usual metals in the solar industry. Depending on the process, material composites are also suitable.

The use of this additional freedom grade leads to lower contact contact resistance, improved Linienleitfähigkeitswerten the metal contact lines, less Lichtab shading of the solar cells and better soldering properties of the contacts. The solar cells have a better efficiency and filling factor, a better durability in the module allied, reduced recombination on metal surfaces and optionally also lower production costs.

Furthermore, one of the biggest obstacles to currently manufactured crystalline Si solar cells with metal paste contacts can be overcome. The contact weakly doped emitter regions (phosphorus surface concentration in Si "10 ²⁰ P atoms / cm ³ ) is not currently possible advantageous. Low when trying To contact doped silicon surfaces requires higher process temperatures, more aggressive glass frits in the metal pastes, higher weight percent glass frits in the paste composition, or longer process times. As a result, the glass layer between the metal structure and the crystallite areas formed in the silicon is made stronger than in the case of conventional contacts. However, this causes the contact junction resistance to increase sharply and to limit the fill factor of the solar cell. However, if the glass layer can be completely removed after the sintering process and the contact is achieved directly on the ingrown metal areas, a low contact junction resistance can be achieved, provided that the fluid, like the paste, is optimized enough to produce close enough crystallite islands. to bring without impurities in the semiconductor junction. If necessary, the emitter area must be specially adapted for this purpose. According to the invention, it is also possible to suitably contact weakly doped, easily passivated emitter surfaces and to achieve high fill factor values in such solar cells. Together with reduced shading losses, the biggest differences between industrial solar cells and high-efficiency laboratory solar cells can therefore be bridged. As a result, significantly higher efficiencies are expected for solar cells, which are produced industrially by the method described here.

According to the invention, a metal contact can be applied to solar cells in which, for a p-type substrate, the emitter (n-type) has a concentration c of dopant atoms such as phosphorus with c <10 ²⁰ atoms / cm ³ , in particular c <5 • 10 ¹⁹ atoms / cm ³ , in particular c ~ 10 ¹⁹ atoms / cm ³ .

The inventive method is characterized in that the contact with the silicon predominantly takes place at an interface of epitaxially grown in the silicon metal crystallites and the crystallite regions are connected by a highly conductive layer with each other to interconnected contact structures, to the busbars or collecting latches (bars / busbars or soldering pads / pads) of the solar cells are coupled. If the method according to the invention has previously been explained primarily with respect to fluids containing Ag, such as pastes, this does not limit the teaching according to the invention. Rather, fluids such as pastes, ink, aerosol can be used, which as metal particles instead of or in addition to Ag, eg Ni, Cu, Pd, Ti, Sn, Al, combinations of these or alloys of these.

In particular, in the inventive method pastes are used, in addition to metal and / or metal atoms and / or glass frit and oxides of at least one element from the group Pb, Cd, Zn, Bi, Sn, Sb, Al, P, B, Ti, Pd, Tl, Zr, Li, Ga, Ni or Si included.

The method according to the invention is preferably used for solar cells whose contact-side passivation layer consists of or contains SiO _x , SiN _x : H, TiO _x , Al ₂ O ₃ , SiNO _x , SiC or other passivation layers customary in the field of semiconductor components.

The fluid is applied in particular by printing processes such as screen printing, offset printing, pad printing, transfer printing or dispensing methods, ink jet processes, aerosol jet processes, powder coating processes, as are known from copying technology, or other selective coating processes.

It is also possible to apply fluids in the first process step to the surface of the semiconductor component which contains Ag-containing, Ni-containing, Pd-containing, Ti-containing or other metal-containing powders or metal compounds in the form of particles, for example. B. from alloys or metal oxides containing mixtures.

The metal deposition on the metal layers or regions grown in the semiconductor by etching or reduction of oxide layers, in particular, can take place from solutions containing Ag, Ni, Cu, Pd, Ti, Al and / or Sn.

In the case of the galvanic metal deposition method for forming the contacts, in which individual metal areas which have grown into the semiconductor when the contact is fired are connected. can be beneficial, a light-induced influence of the galvanic potential. This makes it possible, for example, to control the deposition rate and the deposition selectivity (deposition rate at contacts to n regions in comparison to those in p regions) via the electrical voltage developing in the semiconductor component such as the solar cell and the corresponding photocurrent with appropriate illuminance Taxes. However, the electrically conductive connections to the metal regions can also be made via soldering methods, such as ultrasonic soldering or thermal soldering. Also methods such as flame spraying of metals are possible, provided that selectively the desired areas are electrically connected to each other and a direct ohmic contact between the grown in the semiconductor metal areas and the deposited metal layers.

Further details, advantages and features of the invention will become apparent not only from the claims, the features to be taken from them-alone and / or in combination, but also with reference to a preferred embodiment to be taken from the following drawing.

Show it:

Fig. 1 to 5 a method for producing a metal contact on a solar cell.

In the figures, a solar cell 10 is shown in principle as a semiconductor device, which is generally to represent a semiconductor device and exemplified a p-type silicon substrate 12, a back contact 14 and an n ⁺⁺ front region (emitter) 16 to form a np junction between the emitter 16 and the substrate 12 represent. This is necessary in order to separate the charge carriers generated by incident electromagnetic radiation and to be able to tap current or voltage via the back contact 14 and a front contact 18. The design of the front contact 18 will be described below with reference to FIGS. 1 to 5 in more detail. From the basic illustration of the solar cell 10, it is also clear that the emitter 16 is covered by a dielectric layer such as passivation layer 20, which may consist of SiN _x : H in the case of a silicon substrate.

On the Passivierschicht 20, which also performs the function of an anti-reflection layer, a fluid containing metal atoms is applied. Below this will be explained with reference to an Ag paste, without thereby the teaching of the invention is to be limited.

To form the metal contacts 18, the Ag paste in the form of strips 22, 24 is applied to the passivation layer 20 and dried. The silver paste contains u. a. Metal particles, glass particles and / or metal oxides, solvents, organic binders and additives. Then, a temperature treatment is performed, which is referred to in the production of metal contacts on solar cells, so the front contacts, as firing or sintering. During the corresponding temperature treatment, a glass matrix or metal oxide matrix wets both the Ag particle structure and the electrical passivation layer 20 and etches through this layer 20 locally (FIG. X). On cooling, Ag crystallites 26, 28 then precipitate in the emitter 16 (FIG. 3).

According to the invention, the originally applied metal structure, ie the constituents of the silver paste present after firing or sintering, and the regions of the passivation layer 20 present below the silver paste are then removed. This can be done by reducing or etching chemical treatment steps that substantially completely remove the resulting during firing or sintering on the emitter 16 glass or metal oxide layers, as is illustrated in FIG. 4. Preferably, the glass or metal oxide layer is undercut, so that it is removed with the metal contact structure located on it and the layer present on the substrate in the region of the glass or metal oxide layer. Only the crystallites 26, 28 that have grown into the emitter 16 remain, which protrude to the surface of the emitter 16 (region 30, 32). Optionally, oxide layers present on the surface of the crystallites 26, 28 are removed. removed. Then, preferably, galvanically or electrolessly, metal is deposited on the corresponding regions 30, 32 from a solution in order to form the metal or front contact 18 (FIG. 5). The deposited metal layers must enable good ohmic electrical conductivity to the remaining metal contacts, ie the crystallites 26, 28, and thus connect the crystallites in the silicon to one another in an electrically conductive manner. A present in the prior art intermediate layer between crystallites 26, 28 and the front contact is not present.

Preferred process parameters and materials in the formation of the metal front contacts 18 result from the following embodiment.

The invention will be explained in more detail below by way of example.

A preferred form of application of the teaching according to the invention is, for example, the application of a silver paste on the light-receiving side of a solar cell, which is provided with an n ⁺ -Emitter and an overlying SiNx: H Pas sivierungs layer. The silver paste is usually applied in line-like equidistant arrangements. For example, screen printing methods which apply Ag paste lines of a typical range of 40 μm to 140 μm at intervals of 1 mm to 3 mm are suitable for this purpose. Perpendicular to these lines usually elongated areas are applied with silver paste, which are significantly wider. These so-called "busbars" or collective contacts on the front side of the solar cell are typically between 0.5 mm and 3 mm wide and are printed symmetrically to the cell center as two or three collective contacts Solar modules can be electrically connected to each other.

The height of the printed Ag paste contacts can be chosen significantly lower in the method described here than in conventional methods in which only screen printing contacts are used, since the actual line conductivity of the contacts is generated by the galvanic reinforcement of the contact lines. Instead of typical application heights of approx. 10 μm to 15 μm for screen printing contacts So order heights of 1 micron to 10 microns are completely sufficient. After the Ag pastes have been applied and dried and optionally also printed on the back of the solar cell contacts, the contacts are sintered in a high-temperature firing step at typically 780 ⁰ C to 840 ⁰ C and through the silicon nitride Pas sivier- and antireflective layer therethrough Branded into the silicon of the emitter area. In this case, the silicon nitride layer is etched away below the contacts and parts of the emitter region are reduced by the glass molten metal. As the contacts cool, Ag deposits epitaxially from the melt in the silicon. The glass molten metal then solidifies and usually leaves Metallprezipitate in the glass layer back, which separates the ingrown epitaxially deposited Ag crystallites in the silicon from the contact structure of the originally applied Ag paste.

In a subsequent process step, the solar cells thus produced are preferably transported in a wet-chemical continuous process through an oxidic region and glass layers of the metal contacts reducing solution (for example buffered HF solution) on roller transports continuously for a predetermined process time (typically in the range of one minute) is sufficient to undercut the separating glass layer between metal structure and Ag crystallites. The solar cells are preferably processed with the front side down to ensure that the metal structure of the original contact lines due to the high density of Ag accumulates in the bottom of the wet-chemical continuous flow system and can be selectively removed there.

After a subsequent rinsing step, the solar cells continue to run into a subsequent wet-chemical plant for the light-galvanic deposition of Ag, as offered, for example, by Schmid in Freudenstadt. In this plant further Ag is deposited on the exposed Ag-crystallite areas ingrown in the silicon.

As the metal-to-salt divide proceeds almost isotropically out of the aqueous solution, adjacent crystallite regions grow together and again form a contiguous conductive contact along the originally applied contact surface. lines. The conductivity of these contacts is significantly better than that of screen-printed Ag contacts and can be determined by the application level of the galvanic reinforcement. The resulting Ag contact has a compact cross-section without appreciable porosity and thus almost the conductivity of Ag. The contact with silicon is improved by direct ohmic metallic contact between Ag crystallites and Ag deposited well above the original contact resistance of the screen printed contacts. Under certain circumstances, it may be useful to improve the contact properties in an additional treatment step under Formiergas atmosphere at about 250 ⁰ C to 450 ⁰ C for 10 min to 90 min further.

The method described here is by no means limited to this application example or front-side contacts of solar cells.

Also, the invention is not limited to solar cells, but rather the invention relates to all types of semiconductor devices on which an electrically conductive contact is to be applied. Overall, the term "solar cell" is to be understood as a synonym.

Furthermore, the teaching according to the invention is not abandoned even if a layer is not present on the semiconductor substrate, since the other method steps are insofar iron discoveries.

Claims

Claims: Method of making a metal contact

1. A method for producing at least one in particular strip-shaped, electrically conductive metal contact on a semiconductor component, such as a solar cell, having on the surface side on a semiconductor substrate a layer, such as a dielectric layer, such as a passivation layer, comprising the method steps,

line, strip and / or punctiform application of a fluid to the layer containing at least particles of metal and particles of glass frit,

Curing the fluid by heat treatment while forming:

coherent metal structure by sintering the metal particles together,

a glass layer between the metal structure and the semiconductor substrate and

Metal regions in the semiconductor substrate that are separated from the metal structure on the semiconductor substrate by the glass layer,

Removing the separating glass layer by etching and thus simultaneously removing the metal structure, without removing the ingrown into the semiconductor substrate metal regions to form the at least one electrically conductive contact interlayerless deposition of electrically conductive material from a solution on the ingrown into the semiconductor device metal portions of the semiconductor device, above the the glass layer and metal structure layer have been removed, while simultaneously electrically connecting the metal regions present in these areas in the substrate.

2. The method according to claim 1, characterized in that the metal regions are prepared by forming crystallites of the metal and / or metal atoms.

3. The method according to claim 1, characterized in that the fluid used is one which additionally contains metal oxide and / or surface of the metal particles is coated and / or oxidized.

4. The method according to claim 1, characterized in that a paste, an ink and / or an aerosol is used as the fluid.

5. The method according to at least claim 1, characterized in that as a layer, in particular dielectric layer, such is applied to the solar cell, which consists of at least one material of the group SiNχ: H, SiO ₂ , TiO _x , Al ₂ O ₃ , SiNO _x , SiC consists and / or contains.

6. The method according to at least one of the preceding claims, characterized in that the fluid such as the paste by printing processes, such as screen printing, offset printing, tampon printing and / or transfer printing, by dispensing method, ink-jet method, aerosol jet Method, powder coating method, is applied to the surface.

7. The method according to at least one of the preceding claims, characterized in that the fluid is applied strip, net or star shape.

8. The method according to at least one of the preceding claims, characterized in that the fluid is applied punctiform, in particular arranged in rows points.

9. The method according to at least one of the preceding claims, characterized in that as fluid is used, which is at least one oxide from the group Pb, Cd, Zn, Bi, Sn, Sb, Al, P, B, Ti, Pd, Tl, Zr, Li, Ga, Ni or Si.

10. The method according to at least one of the preceding claims, characterized in that as metal particles are used which contain at least one metal from the group Ag, Ni, Cu, Pb, Ti, Sn, Al or one or more alloys thereof.

11. The method according to at least one of the preceding claims, characterized in that the dried fluid is removed by etching, such as wet or dry chemical etching or plasma etching.

12. The method according to at least one of the preceding claims, characterized in that the electrically conductive material is applied to the surface of the semiconductor devices by galvanic or electroless metal deposition process.

13. The method according to at least one of the preceding claims, characterized in that instead of the deposition of the electrically conductive material from the solution, the electrically conductive material by soldering, such as ultrasonic soldering, or thermal brazing or by firing method, such as flame spraying is applied.

14. The method according to at least one of the preceding claims, characterized in that a metal paste is used as electrically conductive material.

15. The method according to at least one of the preceding claims, characterized in that the electrodeposition of the electrically conductive material is influenced light-induced.

16. The method according to at least one of the preceding claims, characterized in that the electrically conductive material having a width B with B <100 microns, in particular applied with B <60 microns, preferably with B <40 microns, more preferably with B <20 microns becomes.

17. The method according to at least one of the preceding claims, characterized in that the electrically conductive material with an application height with H <15 microns, in particular with H <10 microns, preferably with H <5 microns, more preferably applied with H <1 micron ,

18. Method according to claim 1, characterized in that a semiconductor substrate is used which, in its regions to be contacted with the electrically conductive material, has a dopant concentration C with C <10 ²⁰ atoms / cm ³ , preferably C <5 • 10 ¹⁹ Atoms / cm ³ , in particular C ~ 10 has ¹⁹ atoms / cm ³ .