Abstract:
A contact-making method for a semiconductor material contains the method steps of forming a diffusion barrier which promotes electrical contact and adhesion on at least one portion of a surface of a semiconductor and forming a metallization on the diffusion barrier. The diffusion barrier being formed by applying a metalliforous paste to at least one portion of the semiconductor surface or to at least one portion of a layer covering the semiconductor surface, and a semiconductor component with a diffusion barrier which is arranged in the surface of the semiconductor and which promotes electrical contact between the semiconductor material and a metallization. The metallization is applied to the diffusion barrier. The diffusion barrier is formed by a sintered metalliforous paste applied to at least one portion of the semiconductor surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a continuing application, under 35 U.S.C. § 120, of copending international application No. PCT/EP2007/001636, filed Feb. 26, 2007, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German patent application No. DE 10 2006 013 336.6, filed Mar. 21, 2006; the prior applications are herewith incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a contacting method for a semiconductor material by forming an electrically contacting and bonding diffusion barrier on at least part of the surface of the semiconductor material and forming a metallization on the diffusion barrier. 
         [0004]    In the production of semiconductor devices, or at the latest when they are integrated in an electrical or electronic circuit, it is necessary to form an electrical contact between the semiconductor material and a metallic conductor. In particular, the production of solar cells requires the electrical contacting of the semiconductor material for the purpose of removing charge carriers generated in the semiconductor material. It is generally intended for the contacting to be permanently reliable, so it must bond to the semiconductor material. Moreover, it is intended for the material used to have an electrical conductivity that is as good as possible; on the one hand to involve the lowest possible requirement for material, on the other hand to need only little surface area for the forming of an electrical supply or discharge with sufficiently high electrical conductivity. 
         [0005]    Electrical contacting methods of varying degrees of complexity are known. If there is no available metal that bonds adequately well to the semiconductor material used and at the same time has the required electrical conductivity, it is possible for example for a layer of a first metal, which bonds sufficiently on the semiconductor material, first to be vapor-deposited onto it. Then a further layer of a second metal, which bonds well on the first layer and has greatest possible electrical conductivity, is applied onto the first metal. 
         [0006]    With vapor-deposited contacts, a high-quality contact can be realized with at the same time a low surface-area requirement, but its production is costly on account of the vacuum installations required. Moreover, in the case of silicon, as the most frequently used semiconductor material, the forming of a satisfactory contact requires materials that are rare, and consequently expensive, such as titanium and palladium in conjunction with silver, which stands in the way of large quantities being used. 
         [0007]    In addition, in many cases the range of usable metals is restricted by the requirement that they must not have any harmful influence on the respectively used properties of the semiconductor material. Therefore, the use of some materials is ruled out in many cases, since, in a subsequent thermal treatment, they diffuse too quickly into the volume of a semiconductor material, where they form for example recombination centers for charge carriers, which in turn adversely affects the function of the device produced from it. 
         [0008]    Furthermore, it is known, inter alia, to contact semiconductor materials by applying and sintering in metal-containing pastes or the like. In particular in the area of solar cell production, there have been developed for this purpose pastes which are applied onto semiconductor materials by printing techniques, such as screen or stamp printing, and after thermal treatment that is often referred to as sintering or contact sintering form an ohmic contact with respect to the semiconductor material. Silver and/or aluminum are often used in this case as metals. 
         [0009]    With such methods it is possible to carry out comparatively low-cost contacting of semiconductor materials. However, the surface area requirement is comparatively great. For example in the case of solar cells, this has the effect of reducing the semiconductor area that is available for power generation. This is not necessarily caused by the printing techniques that are used requiring certain minimum dimensioning, but in many cases by the fact that the pastes used can only be applied in a controlled manner up to a certain thickness by a printing operation. However, thickening by multiple printing makes it already necessary to position and align the printing device that is used, such as a screen or stamp, and the semiconductor material that has already been printed. 
         [0010]    Moreover, additional thermal treatments between the individual printing operations are required to stabilize the layers already printed, which moreover can contribute to deterioration of the material properties. For example, this may lead to increased diffusion of impurities into the volume of the semiconductor material, which in turn can adversely affect the properties of the semiconductor device produced, even to the extent that it is unusable. 
       SUMMARY OF THE INVENTION 
       [0011]    It is accordingly an object of the invention to provide a contacting method for a semiconductor material and a semiconductor device that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, by which reliable, low-cost contacting of semiconductor material can be realized with at the same time a lowest possible surface area requirement and a greatest possible electrical conductivity being achieved. 
         [0012]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method for contacting a semiconductor material. The method includes the steps of: forming an electrically contacting and bonding diffusion barrier on at least part of a surface of the semiconductor material by applying a metal-containing paste onto at least part of the surface of the semiconductor material or to at least part of a layer covering the surface of the semiconductor material; and forming a metallization on the diffusion barrier. 
         [0013]    The method can be advantageously used for the contacting of semiconductor material of a semiconductor device, in particular a semiconductor device of silicon. 
         [0014]    Furthermore, the invention is based on the problem of providing a semiconductor device containing a semiconductor material that can be contacted reliably and at low cost, the contacting of which has a low surface area requirement and at the same time high electrical conductivity. 
         [0015]    The concept on which the invention is based is that of forming an electrically contacting and bonding diffusion barrier on at least part of the surface of a semiconductor by applying a metal-containing paste onto at least part of the semiconductor surface or to at least part of a layer covering the semiconductor surface and forming a metallization on the diffusion barrier. For the purposes of the present invention, a metal-containing paste is understood as meaning a paste which contains at least one pure metal and/or a metal alloy and/or a metal compound, in particular at least one metal oxide. 
         [0016]    The diffusion barrier acts here as a bonding layer between the semiconductor material and the metallization and counteracts the diffusion of metals from the metallization into the semiconductor material and the possibly accompanying adverse effect on the properties of the semiconductor material and properties of the semiconductor device, so that metals available in large quantities can be increasingly used in spite of their characteristic of diffusing comparatively quickly in the semiconductor material used. At the same time, the use of metal-containing pastes makes it possible to apply these pastes by low-cost printing techniques. The metallization applied onto the diffusion barrier for its part makes it possible to obtain greatest possible electrical conductivity of the contacting. 
         [0017]    In this way, a reliable, low-cost contacting of semiconductor material can be realized with at the same time low surface area requirement and great electrical conductivity, or a semiconductor device formed in such a way. 
         [0018]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0019]    Although the invention is illustrated and described herein as embodied in a contacting method for a semiconductor material and a semiconductor device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0020]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0021]      FIG. 1  is a flow chart showing a first exemplary embodiment of a method according to the invention; 
           [0022]      FIG. 2  is a flow chart showing a second exemplary embodiment of the method according to the invention; 
           [0023]      FIG. 3  is a diagrammatic, sectional view of a first exemplary embodiment of a semiconductor device according to the invention; 
           [0024]      FIG. 4  is a diagrammatic, sectional view of a second exemplary embodiment of the semiconductor device according to the invention; and 
           [0025]      FIG. 5  is a diagrammatic, perspective view of a solar cell as a third exemplary embodiment of the semiconductor device according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a first exemplary embodiment of the method according to the invention. In the method, first a metal-containing paste is applied onto the surface of a semiconductor material by a screen printing technique, step  10 . This is then alloyed into the semiconductor material in a thermal step, which is often referred to as sintering or contact sintering, step  12 . In the process, the metal from the paste forms an ohmic contact together with the semiconductor material. In the case of silicon as the semiconductor material that is used, this may cause, for example, the formation of metal silicides. In addition, it goes without saying that all other elementary semiconductors, such as for example germanium or compound semiconductor materials such as, inter alia, gallium arsenide or copper indium diselenide, may be used as semiconductor materials. 
         [0027]    The metal-containing paste and its constituents are in this case chosen such that they act as a diffusion barrier for the metallization subsequently applied. In the case of silicon as the semiconductor material used, pastes containing silver and/or nickel and/or molybdenum and/or palladium and/or chromium and/or aluminum may be used for this, for example, it being possible for the materials mentioned also in each case to take the form of an alloy or a compound. Once they have been alloyed in, there forms a layer that acts as a diffusion barrier, in particular for a metallization containing silver and/or tin and/or copper. The use of copper is particularly advantageous for this, since it is available in large quantities, whereas silver is much more rare. 
         [0028]    As a result of the diffusion barrier that is formed, in the example mentioned the stated materials can be deposited on the diffusion barrier in a subsequent electrodepositing operation  14 , without a relevant risk of degradation of the semiconductor material existing as a result of diffusion of the deposited materials into the semiconductor material. The materials used for the metallization and the diffusion barrier must in this case obviously be made to suit one another. The materials stated for the case where silicon is used as the semiconductor material may, however, also be used in the case of the other materials that are common in semiconductor technology. Instead of electrodepositing, it is also possible to use electroless depositing technology or plating technology, known per se. If a number of different materials are deposited, it is possible moreover to provide a combination of different depositing technologies. 
         [0029]      FIG. 2  shows a further exemplary embodiment of the method according to the invention. In it, silicon nitride is first deposited as a dielectric layer on the surface of the semiconductor material, step  16 , before application, step  17 , of the metal-containing paste onto the silicon nitride layer is performed by screen printing. The use of a screen printing technique is not absolutely necessary here. Alternatively, other techniques for applying the paste, in particular screen printing, spray printing, web printing, stencil printing or stamp printing, may obviously be used in all the embodiments. Application of the metal-containing paste is followed by the sintering step  12 , already known from the exemplary embodiment of  FIG. 1 . 
         [0030]    Following this, the metallization in the exemplary embodiment of  FIG. 2  is applied onto the diffusion barrier in the form of a further metal-containing paste  15 . This may in turn be performed, for example, by a printing technique, in particular by screen printing. In addition, however, all other printing processes of the known methods of paste application are also conceivable in particular. In the exemplary embodiment of  FIG. 2 , in a subsequent step the further metal-containing paste is sintered, step  19 , in order in this way to drive out solvent and form an ohmic contact between the diffusion barrier and the metallization. 
         [0031]    In addition, however, there is also the possibility of combining the sintering operations, steps  12  and  19 , in one sintering step. In this case, there is no sintering operation after the screen printing of the metal-containing paste onto the dielectric layer, in the present case the silicon nitride layer. Instead, the metal-containing paste is applied for the metallization. The forming of an ohmic contact between the semiconductor material and the diffusion barrier and also between the diffusion barrier and the metallization is then performed in a common, subsequent sintering step. 
         [0032]    It is also conceivable in the exemplary embodiment of  FIG. 1  to form the metallization by applying a metal-containing paste. As already stated at the beginning, an exact alignment of the semiconductor material and of the device used for the paste application, for example the screen of a screen printing device, is necessary here. Otherwise, some of the further metal-containing paste for the metallization will be applied alongside the diffusion barrier directly onto the semiconductor surface, from where it can diffuse unhindered into the volume of the semiconductor material and adversely affect the properties thereof, in particular in a thermal treatment of the semiconductor material. In many cases, it may therefore be expedient not to provide full-area application of the metallization onto the diffusion barrier but to make the surface-area extent of the metallization smaller than that of the diffusion barrier. As a result, with optimum application of the metallization, the diffusion barrier protrudes out from under the latter. There is then a tolerance range, which, given a limited inaccuracy of the positioning of the metallization on the diffusion barrier, prevents direct contact of the paste with the surface of the semiconductor material. 
         [0033]    If a dielectric layer is first applied onto the semiconductor area, as in the case of the exemplary embodiment of  FIG. 2 , inaccurate alignment of the metallization, or a metallization protruding beyond the diffusion barrier, is uncritical, since it does not come into contact directly with the surface of the semiconductor material but with the dielectric layer, which prevents the diffusion of impurities into the semiconductor material. This is illustrated by the exemplary embodiment that is schematically represented in  FIG. 4  and explained in more detail further below of a semiconductor device according to the invention, a metallization  34  of which protrudes beyond the surface-area extent of a diffusion barrier  32  and partially covers a adjacent dielectric layer  31 . 
         [0034]    The described configuration of the metallization may also be used for the purpose of increasing the electrical conductivity in the contacting formed by the diffusion barrier and the metallization by increasing the conducting cross section, by a metallization that protrudes at least partially beyond the dimensions of the diffusion barrier. 
         [0035]    The exemplary embodiments described and the method according to the invention itself can be advantageously used for the contacting of solar cells, in particular the sides thereof that are facing the light. In addition, they can be used in the case of all semiconductor devices in which a semiconductor material is to be contacted in an electrically conducting manner. 
         [0036]      FIG. 3  schematically shows in a sectional representation a first exemplary embodiment of a semiconductor device according to the invention, which has electrically conducting contacting of a semiconductor material  20 . Arranged in the surface of the semiconductor material  20  is a diffusion barrier  22 , on which a metallization  24  is provided. The diffusion barrier  22  thereby provides an electrically conducting contact between the semiconductor material  20  and the metallization  24 . Moreover, the diffusion barrier  22  is formed by a metal-containing paste applied onto at least part of the semiconductor surface and sintered in. 
         [0037]    In the exemplary embodiment of  FIG. 3 , the metallization  24  may be formed, for example, by electrodepositing of metal or a metal alloy, in particular silver or copper. 
         [0038]    The semiconductor device could have been produced, for example, by the method from exemplary embodiment 1. 
         [0039]      FIG. 4  shows a further exemplary embodiment of the semiconductor device according to the invention. By contrast with the exemplary embodiment from  FIG. 3 , here the dielectric layer  31  is provided adjacent the diffusion barrier  32 . The diffusion barrier  32  as well as the dielectric layer  31  are arranged on the surface of a semiconductor material  30 . As already described in connection with the method according to the invention, any elementary semiconductor or compound semiconductor may be concerned here. 
         [0040]    A metallization  34  is again arranged on the diffusion barrier  32 . In the exemplary embodiment of  FIG. 4 , the metallization extends beyond the surface-area extent of the diffusion barrier  32  and at least partially covers the adjacent dielectric layer  31 . As already explained in connection with the exemplary embodiment of the method according to the invention from  FIG. 2 , this is of advantage in particular whenever the metallization is applied in the form of a metal-containing paste, since alignment errors or tolerances cannot lead to paste getting onto the surface of the semiconductor material and being able to diffuse from there into the volume of the semiconductor material. The metallization  34  is therefore preferably formed from metal-containing and sintered paste. 
         [0041]    Nevertheless, a semiconductor device in which the metallization partially covers the dielectric layer in a way corresponding to the exemplary embodiment from  FIG. 4  may also be realized by other ways of forming the metallization. For example, it may have been applied by electrodepositing or by an electroless depositing or coating technology. A combination of different depositing technologies is also conceivable. 
         [0042]    The exemplary embodiments of semiconductor devices according to the invention of  FIGS. 3 and 4  may be considered on the one hand as semiconductor devices in the sense of a metal-semiconductor contact or as details of other semiconductor devices known per se, such as for example diodes, transistors, thyristors, microprocessors, sensors, microswitches, solar cells, detectors and the like, in which a semiconductor material has the respectively represented electrical contact. 
         [0043]      FIG. 5  shows a schematic representation of a solar cell  40  as a semiconductor device, fingers  62  of a front metallization of which are formed according to the invention, and for which  FIG. 4  consequently represents the drawing of a detail, if a first dielectric layer  56  and a second dielectric layer  58  of the solar cell are considered to be a dielectric layer  31  in the sense of  FIG. 4 . 
         [0044]    The solar cell  40  in  FIG. 5  is a p-type solar cell with a p-doped semiconductor material  50  as the starting material, in particular with p-doped silicon as the starting material. However, the contacting according to the invention can also be used in the same way for n-type solar cells or n-doped semiconductor materials. 
         [0045]    The fingers  62  of the front metallization are formed according to the invention and have a diffusion barrier  52 , which is formed from a metal-containing and sintered-in paste. A metallization  54 , which in the present case is formed by electrodepositing a metal, preferably silver or copper, has been applied onto the diffusion barrier  52 . However, as described above, the metallization  54  may also be applied in some other way. 
         [0046]    Formed on the upper side of the solar cell  40  in a way known per se is an n-doped emitter  60 . The emitter  60  is particularly sensitive to the ingress of impurities from the metallization, since on the one hand this may cause conducting connections through the emitter  60  that short-circuit the solar cell, and greatly reduce the conversion efficiency of the solar cell  40 , and on the other hand such impurities may represent recombination centers for the charge carriers generated in the volume of the semiconductor material  50 , which in turn leads to a reduced current yield. The risk of these adverse effects is all the greater the faster impurities, in particular metals, diffuse from the metallization in the semiconductor material  50  that is used, which is of significance in particular in thermal treatments of the solar cell  40  during its production. 
         [0047]    However, according to the invention, the diffusion of impurities from the metallization  54  into the volume of the semiconductor material of the solar cell  40  is hindered or even prevented by the applied diffusion barrier  52 . As a consequence, commonly occurring metals such as copper and/or nickel or alloys of these materials may be used for example for the metallization  54  of a silicon solar cell  40 , without this causing any adverse effect on efficiency in subsequent thermal treatment of the solar cell  40 . 
         [0048]    In the exemplary embodiment of  FIG. 5 , a back surface field  68 , which is formed by stronger p doping in comparison with the volume of the semiconductor material (in the case of n-type solar cells correspondingly stronger n doping) is provided on the back side of the solar cell. The back surface field  68  reduces the recombination of generated charge carriers, and thereby increases the conversion efficiency of the solar cell  40 . In principle, it is also possible to dispense with the formation of a full-area back surface field or to provide it only locally. 
         [0049]    The contacting of the back side of the solar cell, i.e. in effect of the volume of the semiconductor material  50 , takes place using back-side contacts  66 , if appropriate by way of the back surface field  68  represented in  FIG. 5 . In the exemplary embodiment of  FIG. 5 , the back-side contacts  66  of the solar cell  40  are not formed according to the invention. Here, no diffusion barrier has been provided between the semiconductor material  50  and the metallization of the back-side contacts  66 . Accordingly, suitable materials must be used and thermal treatment of the solar cell  40  during production must be performed at lowest possible temperatures after application of the back-side contacts. In principle, however, the back-side contacts  66  can likewise be readily formed according to the invention with a diffusion barrier. 
         [0050]    This is of advantage in particular whenever, for production reasons, the type of solar cell that is produced is exposed to high temperatures after application of the back-side metallization. 
         [0051]    A width in a range from 10 to 100 μm has proven to be favorable for the width of the fingers of the front metallization  62 . With preference, the width lies in a range from 30 to 70 μm and, with particular preference, is 30 μm. In this way, least possible shading of the active surface of the solar cell  40  with regard to power generation is achieved. The electrical conductivity of the fingers  62  that is necessary for the current to be optimally led away is in this case ensured by the choice of a material for the metallization  54  that has the best possible electrical conduction and choice of a corresponding cross section of the fingers  62 . With a reduced finger width, the required finger cross section is compensated by a thicker metallization  54  in the direction normal to the surface of the solar cell  40 , and consequently an increased finger thickness. 
         [0052]    In the exemplary embodiment of  FIG. 5 , to obtain a further increase in efficiency of the solar cell  40 , the latter is also provided with a surface texturing  64  on its front side. This may in particular have been chemically or mechanically applied in a way known per se. This illustrates that the form of semiconductor devices according to the invention, in particular solar cells, and the use of the method according to the invention are not restricted to semiconductor devices with an even semiconductor surface. Rather, comparatively uneven semiconductor surfaces may also be provided with an electrical contact according to the invention, which is advantageous in particular in the case of solar cells of novel materials such as for example types of silicon ribbon, which generally does not have an even surface. This is made possible by the way in which the diffusion barrier is formed according to the invention by applying the metal-containing paste, which can be realized with a comparatively high tolerance with respect to irregularities of the surface of the semiconductor material.