1. Field of the Invention
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The method can be advantageously used for the contacting of semiconductor material of a semiconductor device, in particular a semiconductor device of silicon.
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.
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.
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.
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.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
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.
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.
Referring now to the figures of the drawing in detail and first, particularly, to
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.
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.
Following this, the metallization in the exemplary embodiment of
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.
It is also conceivable in the exemplary embodiment of
If a dielectric layer is first applied onto the semiconductor area, as in the case of the exemplary embodiment of
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.
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.
In the exemplary embodiment of
The semiconductor device could have been produced, for example, by the method from exemplary embodiment 1.
A metallization 34 is again arranged on the diffusion barrier 32. In the exemplary embodiment of
Nevertheless, a semiconductor device in which the metallization partially covers the dielectric layer in a way corresponding to the exemplary embodiment from
The exemplary embodiments of semiconductor devices according to the invention of
The solar cell 40 in
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.
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.
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.
In the exemplary embodiment of
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
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.
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.
In the exemplary embodiment of
Number | Date | Country | Kind |
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10 2006 013 336.6 | Mar 2006 | DE | national |
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.
Number | Date | Country | |
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Parent | PCT/EP2007/001636 | Feb 2007 | US |
Child | 12235264 | US |