DE102019101921A1 - Method for connecting two workpieces by means of soldering and device with a first and a second workpiece, which are connected by means of a solder - Google Patents

Abstract

The invention relates to a method for connecting two workpieces by means of soldering, comprising the steps of applying a solder to at least one soldering side of a first workpiece and melting the solder by means of heat before and / or after arranging the second workpiece on the soldering side of the first workpiece characterized in that before the solder is applied, a layer system is applied to the soldering side of the workpiece by means of physical vapor deposition, which comprises at least one solderable layer and at least one protective layer, the protective layer being applied directly or indirectly to the side of the solderable layer facing away from the first workpiece, wherein the protective layer with a mass fraction of at least 0.8 contains one or more substances from the group {precious metals, copper}, the solderable layer with a mass fraction of at least 0.8 contains one or more substances from the group of transition metals that the Protective layer and the solderable layer are each formed with a thickness in the range 10 nm to 100 nm, the sum of both layer thicknesses being less than 100 nm and the solder being heated to a temperature of less than 350 ° C. during the soldering process. The invention further relates to a device with a first and a second workpiece, the first and second workpiece being connected by means of a solder.

Description

The invention relates to a method for connecting two workpieces by means of soldering, as well as a device with a first and a second workpiece, the first and second workpieces being connected by means of a solder, in accordance with the preamble of claim 14.

In a large number of applications, workpieces are firmly bonded by means of soldering. For this purpose, a solder is applied to at least one solder side of a first workpiece. The solder can be in the form of solder paste, solder wire, as a coating on one or both workpieces or in another form. By melting the solder by the action of heat before and / or after arranging the second workpiece on the solder side of the first workpiece, a liquid phase of the solder is achieved, so that after the solder has cooled and solidified, a cohesive connection is formed between the first and second workpiece. One of the prerequisites for this is the extensive removal of all oxides on the workpiece to be soldered, which is achieved in the joining technology with the aid of a flux. This is applied to the workpiece to be soldered before the soldering process and activated by the temperature input during the soldering process. Alternatively, solder and / or flux can also be supplied directly during the soldering process. For special applications, e.g. the residues of the flux disturb after the soldering process, flux-free soldering processes are also used. Particular care must be taken to ensure that the oxide thicknesses on the surfaces of the workpieces are minimal.

Such integral connections are used to mechanically connect workpieces to one another. In a large number of applications, in addition to the mechanical connection, an electrically conductive connection between the first and second workpiece is formed by means of soldering.

With some workpieces to be connected, there is the problem that the desired solder does not wet the soldering side of the workpiece and, as a result, there is no cohesive connection or only in parts of the surface of the soldering side, so that no or only a poor mechanical and / or electrical connection is formed.

It is known, for example, that an aluminum oxide layer forms on an aluminum workpiece or with an aluminum coating on the surface as soon as the workpiece is exposed to an oxygen-containing atmosphere. This takes place in particular during the heating during the soldering process. Even if such a layer typically only has a thickness in the range of 3 nm to 5 nm, this layer nevertheless has a high stability and prevents a high-quality solder connection, unless very aggressive fluxes are used. The latter are difficult to dispose of, among others, and residues remaining on the workpiece can lead to corrosion on the workpiece.

To avoid this problem, soldering processes at high temperatures above 600 ° C with a long soldering time (over 30 minutes) have been developed (Humpston G., Jacobson DM, Principles of Soldering, ASM International, 2004). However, due to the soldering time, such processes are not suitable for industrial production. In addition, the high temperatures can adversely affect the workpieces and the solder joint.
Another approach to avoiding the problems of oxidation during the soldering process is to carry out the soldering in a vacuum or in an oxygen-free atmosphere. However, this results in a significantly higher expenditure on equipment.

The rear electrical contact of photovoltaic solar cells is off DE 10 2013 219 560 A1 a method is known in which a contacting layer made of aluminum is applied to the rear of the solar cell, a diffusion barrier layer is applied to the side of the aluminum layer facing away from the solar cell and a solderable layer, for example a silver layer, is applied to the diffusion barrier layer. The diffusion barrier layer and contacting layer are each applied by means of physical vapor deposition (PVD, Physical Vapor Deposition). Between the diffusion barrier layer and the solderable layer, another layer is inserted as an adhesion promoter, which, however, makes the coating process less economically advantageous. Due to the diffusion barrier layer, diffusion of the non-solderable aluminum into the silver layer is avoided. Due to the use of the PVD process for applying the layer system, this process forms the basis for use in industrial processes. However, studies show that for a high-quality and permanent formation of the layer structure, high layer thicknesses greater than 100 nm and furthermore an additional introduction of oxygen into the diffusion barrier layer (“oxygen stuffing”) is necessary for sufficient quality (see Kumm, Developement of temperature-stable , solderable pvd rear metallization for industrial silicon solar cells). For a large number of processes, in particular for large-area use in photovoltaic solar cells, this process therefore has disadvantages in terms of economy.

The invention is therefore based on the object of providing a method for connecting two workpieces by means of soft soldering and an apparatus having a first and a second workpiece connected by means of solder, which avoid the aforementioned disadvantages and in particular enable a more cost-effective but nevertheless permanent soldered connection.

This object is achieved by a method according to claim 1 and a device according to claim 14. Advantageous refinements can be found in the dependent claims.

The method according to the invention is preferably designed to form the device according to the invention, in particular an advantageous embodiment thereof. The device according to the invention is preferably formed by means of the method according to the invention, in particular a preferred embodiment thereof.

• Ein Lot und bevorzugt zusätzlich ein Flussmittel werden auf zumindest eine Lötseite eines ersten Werkstücks aufgebracht. Mittels Wärmeeinwirkung wird das Lot vor und/oder nach Anordnen eines zweiten Werkstücks an der Lötseite des ersten Werkstücks aufgeschmolzen.

The process according to the invention has the following process steps:

• A solder and preferably additionally a flux are applied to at least one solder side of a first workpiece. By means of the action of heat, the solder is melted before and / or after arranging a second workpiece on the soldering side of the first workpiece.

It is essential that at least on the soldering side of the first workpiece, prior to applying the solder, a layer system is applied by means of physical vapor deposition, which comprises at least one solderable layer and at least one protective layer, the protective layer being directly or indirectly on the side of the solderable layer facing away from the first workpiece is applied.

An essential aspect is therefore based on the knowledge that the solderable layer on the solder side, that is to say on the side on which the solder is applied, is protected by means of an additional layer.

Here, the protective layer with a mass fraction of at least 0.8 contains one or more substances from the group {precious metals, copper}. With a mass fraction of at least 0.8, the solderable layer contains one or more substances from the group of transition metals. The term “mass fraction” is defined in a conventional manner, that is, with a mass fraction of 1.0 of a substance, the element consists exclusively of this substance.

It is also essential that the solderable layer (3) is applied directly to the surface of the first workpiece to be soldered or that only layers with a total thickness of less than 20 nm are arranged between the surface to be soldered and the solderable layer and that the protective layer and the solderable layer are each formed with an average thickness in the range from 2 nm to 98 nm, the sum of the two layer thicknesses being less than 100 nm, preferably less than 50 nm, in particular less than 25 nm, and the solder heating to a temperature of less than 350 ° C. during the soldering process becomes.

If a sum of the layer thicknesses of a layer system and preferred layer thicknesses or preferred mean layer thicknesses of individual layers of the system are specified, the individual layers thus always have a preferred maximum layer thickness which corresponds to the sum of the minimum layer thicknesses of the other layers of the system. With the above advantageous sum of the layer thicknesses, the minimum average thickness of the layers is thus 2 nm: Sum of both layer thicknesses average thickness of protective layer and solderable layer in the area 100 nm 2 nm to (100nm-2nm =) 98 nm 50 nm 2 nm to (50nm-2nm =) 48 nm 25 nm 2 nm to (25nm-2nm =) 23 nm This applies accordingly to the following layer systems.

Within the scope of this invention, a solderable layer or another layer, in particular a thin layer with a thickness of less than 10 nm, cannot be formed by non-closed layers, the individual have connected adjacent islands, be formed. In this case, the average thickness denotes the total material volume of this layer divided by the area to be soldered. The layers, in particular the solderable layer, are preferably designed as continuous layers.

By using physical vapor deposition (PVD) to apply the solderable layer and protective layer, the method according to the invention enables use in industrial production lines, in particular since such processes for layer deposition are often already provided. Due to the small overall thickness, the layer system consisting of solderable layer and protective layer can be applied in a shorter time compared to previously known processes, so that lower process and material costs arise and a higher throughput is possible. The protective layer is preferably produced without a vacuum break in the PVD process and is preferably used to prevent oxidation of the underlying solderable layer and thus to maintain the solderability.

The use of a soldering temperature of less than 350 ° C also means less energy and less heat in the workpieces, so that damage is avoided or at least reduced compared to soldering processes at higher temperatures. The soldering process therefore falls into the soft soldering category.

The principle of operation of soft soldering is based on the partial erosion of the base material, typically the metal of the solderable layer, in the solder bath. To form the desired solder joint, the formation of a diffusion zone between the metal of the solderable layer and the solder is necessary. Typically, metallurgical reactions occur which lead to heterogeneous phases of the solder joint (see Humpston, Jacobsen, op. Cit., P. 78). Individual crystal phases in the solder joint are usually many micrometers in size.

So far, it has therefore been assumed that with only a few nanometer thin layers, sufficient formation of alloy phases cannot occur without the metallic layer being completely dissolved and thus the solder being de-wetted. Accordingly, there has been a teaching so far that permanent mechanical stability can only be formed with considerably thicker layers.

However, investigations by the applicant show that, contrary to the previous assumptions, a permanent soldered connection can also be formed with the aforementioned layer system of the method according to the invention, which has a considerably smaller thickness. It can be assumed that a higher solubility of the substances contained in the protective layer at least with a mass fraction of 0.8 in the solder compared to a lower solubility of the substances contained at least with a mass fraction of 0.8 in the solderable layer and further in combination with the Soft soldering at a temperature of less than 350 ° C prevents the metallic layer from completely dissolving and thus prevents the solder from dewetting.

In particular, according to the applicant's knowledge, it is not necessary to arrange a diffusion barrier layer between the surface of the workpiece to be soldered and the solderable layer. The solderable layer is therefore preferably arranged directly on the surface to be soldered. Alternatively, a thin layer or a thin layer system with a total thickness of less than 20 nm, preferably less than 15 nm, in particular less than 10 nm, can be arranged between the solderable layer and the surface of the workpiece to be soldered, in particular an adhesive layer as described below.

The method according to the invention thus makes it possible to apply a layer system by means of physical vapor deposition, the layer system comprising at least the aforementioned protective layer and the aforementioned solderable layer, the total thickness of which is less than 100 nm, preferably less than 50 nm, in particular less than 25 nm and mechanically stable connection of two workpieces by means of soldering.

The object stated at the outset is further achieved by a device having a first and a second workpiece, the first and second workpiece being connected by means of a solder. It is essential that a layer system is arranged between the first workpiece and solder, which comprises at least one solderable layer and at least one protective layer, the protective layer being arranged indirectly or directly on the side of the solderable layer facing away from the first workpiece, the protective layer having a mass fraction of at least 0.8 contains one or more substances from the group {precious metals, copper}, the solderable layer with a mass fraction of at least 0.8 contains one or more substances from the group of transition metals and that the solderable layer is directly on the one to be soldered Surface of the first Workpiece is arranged or only layers with a total thickness less than 20 nm are arranged between the surface to be soldered and the solderable layer and that the protective layer and the solderable layer are each formed with an average thickness in the range 2 nm to 98 nm, the sum of both Layer thicknesses is less than 100 nm. This results in the advantages mentioned above in the method according to the invention.

In order to avoid dewetting of the solder particularly efficiently, the protective layer preferably has a solubility in solder greater than 50 nm / s and the solderable layer has a solubility in solder less than 50 nm / s @ 250 ° C. in PbSn solder (40 mass percent Sn) on. It is particularly advantageous that the protective layer has a solubility in solder greater than 100 nm / s and the solderable layer has a solubility in solder less than 10 nm / s.

The solubility is determined here in a manner known per se: for example, the layer whose solubility is to be determined can be applied to a glass substrate. The solder is applied to the side of the layer facing away from the glass substrate. This is followed by heating so that the solder melts and the layer erodes. After a certain time, the experimental set-up cools down. In a sectional image, for example, a scanning electron microscope can be used to measure how deep the solder has penetrated into the layer. The solubility in nanometers per second is finally determined by carrying out several such measurements for different periods of time.

The protective layer is advantageously formed with a mass fraction of at least 0.8 containing one or more substances from the group {Ag, Cu}. For typical soft solders, particularly preferably PbSn, SnAgCu or SnBi, these substances have a high solubility. A mass fraction of at least 0.8 Ag is particularly advantageous.

Here copper (Cu) has the advantage that it is a comparatively inexpensive metal. To save costs, copper from the aforementioned group is therefore chosen in particular.

Silver (Ag), on the other hand, has the advantage as a precious metal that it forms no or only a very thin oxide layer in air.

Both Ag and Cu have a particularly low specific line resistance. For applications in which not only a mechanical but also an electrical connection with low electrical resistance is advantageous, silver or copper is therefore advantageously selected from the aforementioned group. In addition, Ag is much cheaper than the protective layers made of Au, Pt and Pd that are often used.

The protective layer serves in particular to prevent or at least to reduce oxidation of layers underneath. Silver is known to be highly permeable to oxygen, especially at higher temperatures. However, investigations by the applicant show that, surprisingly, the use of silver with a mass fraction of at least 0.8 for the protective layer also leads to a high-quality result in the process according to the invention. It can therefore be assumed that with the parameters of the method according to the invention, even when silver is used in the protective layer, oxidation of the layers underneath is avoided or sufficiently reduced for good solderability.

So far it has been assumed that thin Ag layers are not effective as a diffusion barrier for oxygen, see Principles of soldering, op. Cit., P. 148]: "The porosity of thin electroplated or evaporated coatings is frequently overlooked []. Further proof of the ineffectiveness of noble metal coatings in preventing oxidation of an underlying base metal comes from the beneficial exploitation of their use to grow oxide films of controlled thickness in the manufacture of solar cells. In particular, 5 um of silver, applied by thermal evaporation, to a clean copper surface permits the growth of Cu2O at the common interface at a rate of roughly 0.1 um / h at 500 C [Rosenstock and Riess 2000] “.

With a mass fraction of at least 0.8, the solderable layer preferably has one or more substances from the group {Ni, Cr, V, NiCr, NiV, W, Mo}. The substances of the aforementioned group have a comparatively low solubility for typical solders.

A soft solder known per se can be used as the solder. In particular, the use of a solder is advantageous which, with a mass fraction of at least 0.9, comprises one or more substances from the group {Pb, Sn, Bi, Ag}, e.g. Sn60Pb40, Sn96Ag4, Sn96.5Ag3.0Cu0.5, Sn42Bi58, Sn43Pb43Bi14.

In order to enable soft soldering in the method according to the invention, the solder used preferably has a melting point of less than 350 ° C.

By means of the method according to the invention, a previously non-solderable surface of a workpiece can be supplied to a soldered connection, in particular wettability by solder is achieved. The thicknesses of the solderable layer and protective layer in total preferably have a thickness of less than 100 nm (particularly preferably <50 nm, very particularly preferably <25 nm), so that a cost-effective application is possible in a PVD process. In particular, due to the small layer thickness, only smaller amounts of expensive metals are used compared to previously known processes.

For some processes, it is desirable that after the layer system having the solderable layer and the protective layer has been applied, stability of the layer system is ensured over a longer period of time and / or at higher temperatures. This is the case, for example, if heat treatment of the workpiece, in particular at temperatures greater than 400 ° C., is desirable after the layer system has been applied and before the soldering process is carried out.

In an advantageous embodiment of the method according to the invention, the protective layer is formed as an alloy, in particular an alloy comprising copper, preferably as a silver-copper alloy. Investigations by the applicant show that a protective layer formed as an alloy prevents agglomeration of Ag even at higher temperatures or over longer periods of time (in particular over periods of longer than 6 months). Such an agglomeration would prevent or at least significantly reduce wetting of the soldering side of the first workpiece with solder.

In a further advantageous embodiment, the layer system has an anti-agglomeration layer, which is applied indirectly or preferably directly to the side of the protective layer facing away from the first workpiece, at least partially covering, preferably completely covering. In this advantageous embodiment, an anti-agglomeration layer is thus provided in addition to the protective layer in order to ensure particularly high robustness over a long period of time and / or at high temperatures.

The anti-agglomeration layer is preferably formed with a mass fraction of at least 0.5 containing one or more substances from the group {copper, silver}. The anti-agglomeration layer is advantageously formed with a thickness of less than 10 nm, preferably less than 5 nm. This ensures high cost efficiency.

The layer system comprising solderable layer, protective layer and anti-agglomeration layer preferably has a total thickness of less than 100 nm, preferably less than 50 nm, in particular less than 25 nm.

In an advantageous embodiment, the layer system is formed with an adhesive layer, which is formed indirectly or preferably directly between the solderable layer and the first workpiece with a material composition different from the solderable layer. Investigations by the applicant show that the mechanical stability can be considerably improved by means of such an adhesive layer. The adhesive layer with a mass fraction greater than 0.8 preferably has one or more substances from the group of transition metals, in particular from the group {Cr, Ni, V, Ti}. The adhesive layer is preferably formed with a thickness of less than 15 nm (particularly preferably <10 nm) in order in particular to enable high cost efficiency. It is therefore particularly advantageous that the layer system comprising the adhesive layer, solderable layer and protective layer has an overall thickness of less than 100 nm, preferably less than 50 nm, in particular less than 25 nm, in particular a layer system of adhesive layer, solderable layer, protective layer and anti-agglomeration layer has an overall layer thickness less than 100 nm, preferably less than 50 nm, in particular less than 25 nm.

After application to the first workpiece and before application of the solder, the layer system is advantageously heated to a temperature of at least 300 ° C. for at least 10 s. This has the advantage that defects are healed.
To reduce costs, the layer system applied to the solder side is preferably formed with a total thickness of 100 nm, preferably less than 50 nm, in particular less than 25 nm.

The soldering process is preferably carried out under an oxygen-containing atmosphere (in particular ambient air). This has the advantage that the mechanical outlay and the operating costs are significantly lower and the throughput is higher than with soldering in a low-O2 atmosphere or in a vacuum.

Investigations by the applicant show that for the soldering process the solder is preferably melted for a period of less than 30 s, in particular less than 10 s, preferably less than 3 s. As a result, loosening of the substances of the solderable layer in the solder is further reduced and, in addition, impairment of the workpieces due to the action of heat is avoided or at least reduced.

The surface of the workpiece to be soldered is the surface that is to be connected to the other workpiece by means of soldering.

The method according to the invention is particularly suitable for connecting temperature-sensitive workpieces. Glass, ceramics (Al ₂ O ₃ , ZrO ₂ or others), as well as metal-ceramic composites (TiC, or others) can be coated using the method according to the invention and made accessible to a soldering process. The surface to be soldered is typically the immediate surface of the material described. Furthermore, by means of the method according to the invention, surfaces with metals which are difficult to solder, such as Ti, W, Cr, Zr, can be made accessible using soldering methods known per se using solders known per se. The surface to be soldered is accordingly the surface of the metal layer described.

In particular, the method according to the invention finds an advantageous application when connecting a workpiece by means of soldering, the workpiece having an aluminum layer on the soldering side. The surface to be soldered is therefore the aluminum layer. In a large number of applications, in particular electrical and electronic applications, the components have aluminum surfaces which are to be electrically and mechanically conductively connected to a second workpiece by means of soldering, in particular for the electrical supply and / or discharge of charges, via the second workpiece. The method according to the invention is particularly suitable for this.

The layer system of the method according to the invention can be applied to various aluminum surfaces (aluminum surfaces produced using screen printing paste, aluminum surfaces produced using PVD, aluminum foils, aluminum sheets, aluminum bodies or aluminum components and tools). There are therefore only minor restrictions if the surface of the workpiece to be soldered can be coated using a PVD process. This results in particular in the advantageous possibility of contacting aluminum surfaces of a workpiece under soft soldering conditions. This in turn enables further process simplifications, such as soldering under normal pressure and normal atmosphere, soldering at lower temperatures instead of high temperatures (in particular instead of brazing), as well as the elimination of aggressive chemicals that are used in previously known processes in order to enable an aluminum surface to be wetted with solder .

The method according to the invention is particularly suitable for electrically contacting photovoltaic solar cells by means of soldering, in particular for electrically connecting the solar cell to an adjacent solar cell or an external circuit. For many solar cell structures, in particular silicon-based solar cell structures, it is advantageous to provide an aluminum-containing or aluminum-containing contact structure on the rear. In the case of such solar cells in particular, the method according to the invention is advantageously used for making electrical contact with the aluminum surface.

Particularly cost-effective methods for producing a solar cell are known, in which an aluminum foil is used to form a metallic contact structure for a semiconductor component, in particular for a silicon-based photovoltaic solar cell. Such a process is in DE 10 2006 044 936 A1 described.

The use of an aluminum foil for contacting a semiconductor component and in particular a photovoltaic solar cell leads to a simplification of materials and processes and thus to a reduction in costs. To connect such a solar cell in the module, however, such a solar cell structure poses great challenges to the connection technology, since the aluminum foil has a small thickness and typically has a poorly wettable surface with a solder. In an advantageous embodiment, the method according to the invention is therefore used for electrically contacting a metallic contact structure of a semiconductor component, in particular a photovoltaic solar cell, which metallic contact structure is formed by means of an aluminum foil. Such a contact structure differs from other metallic contact structures in that no metallic layer is produced, in particular vapor-deposited, but an aluminum foil is arranged on the semiconductor component to form the metallic contact structure.

The method according to the invention thus preferably forms an electrically conductive connection between the first and second workpieces, in particular a metallic contacting structure of a photovoltaic solar cell is preferably connected to a metallic cell connector structure.

In the device according to the invention, the first workpiece is therefore preferably designed as a semiconductor component, in particular as a solar cell, preferably as a photovoltaic solar cell. The second workpiece is preferably designed as a line element, in particular as a cell connector. The solder forms an electrically conductive connection between the first and second workpiece.

• 1 ein erstes Ausführungsbeispiel mit einem 3-Schichtsystem mit Haftschicht,
• 2 ein zweites Ausführungsbeispiel mit einem 2-Schichtsystem ohne Haftschicht und
• 3 ein drittes Ausführungsbeispiel mit einem 3-Schichtsystem mit Anti-Agglomerationsschicht.

Further advantageous features and embodiments are explained below using exemplary embodiments and the figures. It shows:

• 1 a first embodiment with a 3-layer system with adhesive layer,
• 2nd a second embodiment with a 2-layer system without adhesive layer and
• 3rd a third embodiment with a 3-layer system with anti-agglomeration layer.

The figures show schematic representations that are not to scale. The same reference symbols in the figures denote the same or equivalent elements.

In all figures, a first workpiece 1 is shown below, which in the present case is designed as a photovoltaic silicon solar cell. The back of the solar cell is in each case at the top, the back of the solar cell having an aluminum foil as a metallic contacting structure, which according to the method DE 10 2006 044 936 A1 was trained.

The aluminum foil thus represents the soldering side of the first workpiece 1, which is thus shown at the top in the figures.

A layer system is applied to the soldering side using an exemplary embodiment of a method according to the invention, in order to subsequently apply a soft solder, preferably a tin-containing solder, to the side of the layer system facing away from the first workpiece and in a soldering process at a temperature of 250 ° C. for a period of time of a few seconds, in particular less than 5 seconds, to form a mechanical and electrically conductive connection with a (not shown) metallic cell connector as a second workpiece. The solder can be applied both to the surface to be soldered or to a connecting piece, such as the cell connector.

Each figure thus shows a preliminary stage of an exemplary embodiment of a device according to the invention, on which a second workpiece is arranged in a soldering process for completion by means of solder.

• Auf der Lötseite des ersten Werkstücks 1 werden nacheinander eine Haftschicht 2, eine lötbare Schicht 3 und eine Schutzschicht 4 aufgebracht. Die Haftschicht 2 ist als Cr-Schicht ausgebildet. Zuvor wird in einem Plasmaätzvorgang die Lötseite des ersten Werkstücks 1 konditioniert. Die Dicke der Haftschicht liegt im Bereich 1 nm bis 30 nm, vorliegend 10 nm. Die Haftschicht dient als Haftvermittler zwischen dem ersten Werkstück und den weiteren Schichten. In alternativen Ausführungsbeispielen kann die Haftschicht als CrON-Schicht, Ti-Schicht oder NiCr-Schicht ausgebildet sein.

This in 1 The first exemplary embodiment shown is generated by means of a first exemplary embodiment of a method according to the invention. Here, a multi-layer system is applied to the first workpiece 1 by means of physical vapor deposition:

• On the solder side of the first workpiece 1, an adhesive layer 2, a solderable layer 3 and a protective layer 4 are applied in succession. The adhesive layer 2 is designed as a Cr layer. The soldering side of the first workpiece 1 is conditioned beforehand in a plasma etching process. The thickness of the adhesive layer is in the range 1 nm to 30 nm, in the present case 10 nm. The adhesive layer serves as an adhesion promoter between the first workpiece and the further layers. In alternative exemplary embodiments, the adhesive layer can be designed as a CrON layer, Ti layer or NiCr layer.

In the first exemplary embodiment of a method according to the invention for forming the structure according to 1 After application of the adhesive layer 2, a solderable layer 3 is applied, which in the present case is designed as a Cu layer. The solderable layer has an average thickness in the range from 2 nm to 98 nm, in the present case 10 nm.

However, the provision of an adhesive layer is not absolutely necessary. In a modification of the first embodiment according to 2nd only a two-layer system is produced by the method as described above, but without the application of an adhesive layer.

In the present case, the protective layer 4 is designed as an Ag layer. In an alternative exemplary embodiment, it is also possible to use a different noble metal. The protective layer 4 - as in the Figures can be seen - always the top layer on which the solder is applied for connection to the second workpiece. The protective layer 4 has an average thickness in the range from 2 nm to 98 nm, in the present case 30 nm. The protective layer serves to avoid oxidation of the solderable layer. The protective layer must also dissolve quickly enough in the solder to enable soft soldering.

• In einer ersten vorteilhaften Abhandlung des erfindungsgemäßen Verfahrens wird eine Legierung ausgebildet, welche die Bildung von Agglomeraten vermeidet. In diesem Fall wird die Schutzschicht 4 als Legierungsschicht ausgebildet.
• In einem abgewandelten Ausführungsbeispiel wird die Schutzschicht 4 als Silber-Kupfer-Legierung ausgebildet, mit einem Massenanteil von Kupfer im Bereich 0,02 bis 0,15, vorliegend 0,08. Silber weist einen Massenanteil im Bereich von 0,98 bis 0,85, vorliegend 0,92, auf. Das Ausbilden einer Legierung vermeidet einen diffusionsgesteuerten Agglomerationsprozess, sodass die Stabilität der Schutzschicht 4 auch bei einer Wärmebehandlung wie zuvor beschrieben, insbesondere bei einem Annealing gewährleistet ist.

For some manufacturing processes, in particular photovoltaic solar cells, it is advantageous to heat the metallic contact structure, in particular in a so-called annealing. At the in DE 10 2013 219 560 A1 described solar cell, an annealing step at a temperature of 400 ° C for 10 s is advantageous. Performing such an annealing would, however, lead to an agglomeration of the protective layer 4 described above. Nevertheless, the method according to the invention can be combined with upstream heat treatment steps, in particular annealing steps, preferably according to one of the two following variants:

• In a first advantageous treatment of the method according to the invention, an alloy is formed which avoids the formation of agglomerates. In this case, the protective layer 4 is formed as an alloy layer.
• In a modified exemplary embodiment, the protective layer 4 is formed as a silver-copper alloy, with a mass fraction of copper in the range 0.02 to 0.15, in the present case 0.08. Silver has a mass fraction in the range from 0.98 to 0.85, in the present case 0.92. The formation of an alloy avoids a diffusion-controlled agglomeration process, so that the stability of the protective layer 4 is also ensured in the case of a heat treatment as described above, in particular in the case of annealing.

In a second variant of the above-described exemplary embodiment of the method according to the invention, an anti-agglomeration layer is additionally applied to the protective layer 4. In the present case, a Cu or Ni layer is applied to the previously described protective layer 4, which is designed as an Ag layer. Studies show that it is not necessary for the anti-agglomeration layer 5 to completely cover the protective layer 4. In many cases, non-contiguous layers with an average thickness of 0.5-4.5 nm are sufficient. The anti-agglomeration layer 5 avoids the diffusion-controlled agglomeration and thus leads to a stability of the protective layer 4 even during thermal pretreatment, in particular annealing. Such a layer system, in the present case without the provision of an adhesive layer, is shown in 3rd shown. Likewise, in a modification, the provision of an adhesive layer between the first workpiece 1 and the solderable layer 3 is also possible.

However, the use of an anti-agglomeration layer 5 or the formation of the protective layer as an alloy are not only advantageous if an annealing is carried out as described above. Basically, such layers are advantageous if the layer system is to have increased stability, in particular over a longer period of time. As described above, thin layers of silver (especially with a layer thickness of less than 50 nm) are unstable when exposed to ambient air. By using anti-agglomeration layers or a protective layer formed as an alloy as described above, the time period in which the solderable layer is exposed to ambient air can be considerably extended.

The table below shows advantageous configurations for layer systems according to the invention in one column each with the material of the layer (underlined), layer thicknesses in nm (thick pressure) and, if appropriate, with alloy proportions in the alloys mentioned using “:” (italics). Advantageous value ranges are given in brackets: layer material if necessary, alloy content in wt% layer thickness Adhesive layer (2) Cr - Cr Cr - - 10 nm (2 nm -20 nm) 10 nm (2 nm-20 nm) 10 nm (2 nm -20 nm) Solderable layer (3) NiCr NiCr NiCr NiCr NiCr NiCr 10 nm (2 nm -98 nm) 10 nm (2nm - 98 nm) 10 nm (2-98 nm) 10 nm (2-98 nm) 10 nm (2-98 nm) 10 nm (2-98 nm) Protective layer (4) Ag Ag Ag Ag: Cu Ag Ag: Cu 30 nm (2nm - 98 nm) 30 nm (2 nm-98 nm) 30 nm (2 nm - 98 nm) 92: 8 (85: 15-98: 2) 30 nm (2 nm-98 nm) 30 nm (2 nm - 98 nm) 92: 8 (85: 15-98: 2) 30 nm (2 nm - 98 nm) Anti-agglomeration layer (5) - - Cu - Cu - 2 nm (0.25nm - 10nm) 2 nm (0.25nm-10nm)

Reference list

1

first workpiece

2nd

Adhesive layer

3rd

solderable layer

4th

Protective layer

5

Anti-agglomeration layer

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102013219560 A1 [0007, 0066]
• DE 102006044936 A1 [0052, 0058]

Claims (15)

Method for connecting two workpieces by means of soldering, comprising the steps of applying a solder to at least one soldering side of a first workpiece and melting the solder by means of heat before and / or after arranging the second workpiece on the soldering side of the first workpiece, characterized in that on the soldering side At least of the first workpiece, before applying the solder, a layer system is applied by means of physical vapor deposition, which comprises at least one solderable layer (3) and at least one protective layer (4), the protective layer (4) indirectly or directly on the side of the workpiece facing away from the first workpiece solderable layer is applied, the protective layer (4) with a mass fraction of at least 0.8 containing one or more substances from the group {precious metals, copper}, the solderable layer (3) with a mass fraction of at least 0.8 one or more Substances from the group of transition metals contains that the solderable right layer (3) is applied directly to the surface of the first workpiece to be soldered or only layers with a total thickness of less than 20 nm are arranged between the surface to be soldered and the solderable layer such that the protective layer (4) and the solderable layer (3) are each formed with an average thickness in the range from 2 nm to 98 nm, the sum of both layer thicknesses being less than 100 nm and the solder being heated to a temperature of less than 350 ° C. during the soldering process. Procedure according to Claim 1 , characterized in that the protective layer (4) made of a material with a solubility in solder greater than 50 nm / s, preferably greater than 100 nm / s, in each case @ 250 ° C. in PbSn solder (40 mass percent Sn) and the solderable layer ( 3) is formed from a material with a solubility in solder less than 50 nm / s, preferably less than 10 nm / s, in each case @ 250 ° C. in PbSn solder (40 mass percent Sn). Method according to one of the preceding claims, characterized in that the protective layer (4) is formed with a mass fraction of at least 0.8 containing one or more substances from the group {Ag, Cu}, in particular with a mass fraction of at least 0.8 Ag becomes. Method according to one of the preceding claims, characterized in that the solderable layer (3) with a mass fraction of at least 0.8 one or more substances from the group {Ni, Cr, V, NiCr, NiV, W, Mo, Ti, Fe } is trained. Method according to one of the preceding claims, characterized in that the protective layer (4) is designed as an alloy, in particular an alloy comprising copper, preferably as a silver-copper alloy. Method according to one of the preceding claims, characterized in that the layer system is formed with an anti-agglomeration layer (5) which indirectly or preferably directly on the side of the protective layer (4) facing away from the first workpiece at least partially covering, preferably completely covering, is applied. Procedure according to Claim 6 , characterized in that the anti-agglomeration layer (5) is formed with a mass fraction of at least 0.5 containing one or more substances from the group {copper, silver} and that the anti-agglomeration layer (5) with an average thickness of less than 10 nm, in particular less than 5 nm. Method according to one of the preceding claims, characterized in that the layer system is formed with an adhesive layer (2) which is formed indirectly or preferably directly between the solderable view and the first workpiece with a material composition that is different from the solderable view, in particular that the adhesive layer (2 ) with a mass fraction greater than 0.8 contains one or more substances from the group of transition metals, in particular from the group {Cr, Ni, V, Ti}, and / or that the adhesive layer (2) with a thickness of less than 15 nm is preferred less than 10 nm. Method according to one of the preceding claims, characterized in that after application of the layer system and before application of the solder, the layer system is heated to a temperature of at least 300 ° C for at least 10s. Method according to one of the preceding claims, characterized in that the layer system is formed with a total thickness of less than 100 nm, preferably less than 50 nm, particularly preferably less than 25 nm. Method according to one of the preceding claims, characterized in that the soldering process is carried out under an oxygen-containing atmosphere. Method according to one of the preceding claims, characterized in that for the soldering process the solder is melted for a period of less than 30 s, in particular less than 10 s, preferably less than 3 s. Method according to one of the preceding claims, characterized in that an electrically conductive connection is formed between the first and second workpiece by means of the method, in particular that a metallic contacting structure of a solar cell is connected to a metallic cell connector structure by means of the method. Device with a first and a second workpiece, the first and second workpiece being connected by means of a solder, characterized in that a layer system is arranged between the first workpiece and the solder, which comprises at least one solderable layer (3) and at least one protective layer (4) , wherein the protective layer (4) is arranged indirectly or directly on the side of the solderable layer facing away from the first workpiece, the protective layer (4) containing at least 0.8% by mass of one or more substances from the group {precious metals, copper} , the solderable layer (3) with a mass fraction of at least 0.8 contains one or more substances from the group of transition metals and that the solderable layer (3) is arranged directly on the surface of the first workpiece to be soldered or only layers with a total thickness less than 20 nm are arranged between the surface to be soldered and the solderable layer that the protective layer ht (5) and the solderable layer (3) are each formed with an average thickness in the range 2 nm to 100 nm, the sum of both layer thicknesses being less than 100 nm. Device after Claim 14 , characterized in that the first workpiece is designed as a semiconductor component, in particular as a solar cell, that the second workpiece is designed as a line element, in particular as a cell connector, and that an electrically conductive connection between the first and second workpiece is formed by means of the solder.

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