Abstract:
The present invention relates to a method for fabricating a diffusion-barrier cap on a Cu-containing interconnect element that has crystallites of at least two different crystal orientations, comprises selectively incorporating Si into only a first set of crystallites with at least one first crystal orientation, employing first process conditions, and subsequently selectively forming a first adhesion-layer portion comprising CuSi and a first diffusion-barrier-layer portion only on the first set of crystallites, thus forming a first barrier-cap portion, and subsequently selectively incorporating Si into only the second set of crystallites, employing second process conditions that differ from the first process conditions, and forming a second barrier-cap portion comprising a Si-containing second diffusion-barrier layer portion on the second set of crystallites of the interconnect element. The processing improves the properties of the diffusion-barrier cap and secures a continuous formation of a diffusion-barrier layer on the interconnect element.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for fabricating a diffusion-barrier cap on a Cu-containing interconnect element that has crystallites of at least two different crystal orientations in an interconnect stack of a semiconductor device. 
       BACKGROUND OF THE INVENTION 
       [0002]    The semiconductor industry has achieved a steady increase of the integration density of semiconductor devices with integrated circuits. Also a rapid increase of the frequencies, at which circuit elements may be operated, has been achieved. Both developments have been enabled, among others, by the replacement of aluminum and aluminum alloys as conductive materials for interconnect elements by copper. Copper has a lower electric resistance than aluminum and aluminum alloys and is more resistant to electromigration than these materials. 
         [0003]    The interconnect elements are typically arranged in a stacked layer architecture referred to as interconnect stack and form conductive lines, plugs, or vias of an interconnect network, according to the specific requirements of a particular integrated circuit. The interconnect elements of an interconnect stack are typically separated from each other by layers of a dielectric material. 
         [0004]    The introduction of copper (Cu) in the fabrication of interconnect elements for integrated circuits requires barriers to prevent a diffusion of Cu from the interconnect element into a respective adjacent dielectric layer or into the semiconductor substrate. A diffusion of Cu would give rise to a rapid degradation of interconnect structures. This in turn affects the performance of active devices of an integrated circuit or leads to an early. 
         [0005]    U.S. Pat. No. 6,339,025 B1 describes the fabrication of a capping layer as a diffusion barrier on a Cu interconnect element. In the described process flow, a copper interconnect element is provided that is laterally defined by lateral barrier layers preventing the diffusion of copper into a laterally surrounding dielectric layer. The structure is subjected to chemical mechanical polishing (CMP) to remove residual copper and barrier-layer material from the dielectric layer. On the exposed dielectric layer and the exposed interconnect elements, a silicon-rich silicon nitride layer is formed by plasma-enhanced chemical vapor deposition. In this step, silane and ammonium NH 3  are used as precursors. A sufficiently high ratio of the flow rates of silane and ammonium leads to a high silicon content in the silicon nitride layer. This favors a reaction of the deposited silicon with copper and results in a copper silicide layer, which is formed between the exposed copper interconnect element and the silicon nitride layer. The copper silicide layer enhances the adhesion of the silicon-rich nitride layer, which forms the capping layer. 
         [0006]    It has been observed that the SiN capping layer on top of the CuSi-layer built with such processing is often discontinuous in Cu-based interconnect elements with grains of different crystal orientations, resulting in a locally defective diffusion-barrier cap. This causes reliability problems such as undesired shortcuts in the interconnect stack and Cu-related deep defects in silicon, which lead to a corrupted device performance and a reduced device lifetime. 
       SUMMARY OF THE INVENTION 
       [0007]    It is desirable to provide an improved diffusion-barrier cap for preventing diffusion of copper from a Cu-containing interconnect element. 
         [0008]    According to a first aspect of the invention, a method for fabricating a diffusion-barrier cap on a Cu-containing interconnect element that has crystallites of at least two different crystal orientations in an interconnect stack of a semiconductor device is provided. The method comprises: 
         [0009]    selectively incorporating Si into only a first set of crystallites with at least one first crystal orientation, employing first process conditions, and subsequently selectively forming a first adhesion-layer portion comprising CuSi and a first diffusion-barrier-layer portion only on the first set of crystallites, thus forming a first barrier-cap portion and leaving a second set of crystallites of the interconnect element with at least one second crystal orientation exposed, or, in other words, uncovered, and subsequently 
         [0010]    selectively incorporating Si into only the second set of crystallites, employing second process conditions that differ from the first process conditions, and forming a second barrier-cap portion comprising a Si-containing second diffusion-barrier layer portion on the second set of crystallites of the interconnect element. 
         [0011]    The method of the invention overcomes the reliability problems caused by a discontinuous diffusion-barrier cap and enables the fabrication of reliable devices with a long lifetime. 
         [0012]    The method is based on the recognition that, under certain process conditions, an incorporation rate of silicon into a Cu-containing interconnect element depends on the crystal orientation of crystallites forming the surface of the interconnect element that is to be covered by the diffusion-barrier cap. A crystallite is a domain of solid-state matter that has the same structure as a single crystal. Crystallites are also referred to as grains. The first and second sets of crystallites are distinguished by crystal orientations, which give rise to an exposure of respective different lattice planes at the surface of the interconnect element. The respective crystal orientation of the respective first and second sets of crystallites influences their respective ability to incorporate silicon under certain process conditions. 
         [0013]    The term process conditions refers to the values of physical parameters characterizing an ambient atmosphere, typically a fluid, in particular a gaseous atmosphere, to which the interconnect element is exposed during respective process steps, a duration of particular steps of the processing, or the like. Crystallites with some specific first crystal orientations exhibit a high probability of Si incorporation under certain process conditions, which process conditions are also referred to as the first process conditions. These crystallites are referred to as the first set of crystallites. In contrast, under the first process conditions, other crystallites with some specific second crystal orientations exhibit an only small to vanishing probability for incorporation of Si atoms for the purpose of the adhesion-layer or diffusion-barrier-layer formation. 
         [0014]    In turn, this effect influences the formation rate of an adhesion layer containing CuSi on the different crystallites of different crystal orientation, and of a Si-containing diffusion-barrier layer portion on different crystallites of different crystal orientation. 
         [0015]    The method of the invention makes use of this effect to improve the control over the formation of a continuous diffusion-barrier cap. The method separates the formation of a continuous diffusion-barrier cap on the whole surface of the interconnect element into two separate phases. The first process conditions are employed to achieve a selective adhesion-layer formation only on the first set of crystallites of the interconnect element that have the first crystal orientation. There may be more than one crystal orientation that allows a selective Si incorporation under the first process conditions. However, due to the described selectivity of the Si incorporation, the first process conditions result in the formation of openings in the adhesion layer over the second set of crystallites of the interconnect element with at least one second crystal orientation. Therefore, when considering the whole surface to be covered by the diffusion-barrier cap, the Si incorporation into the crystallites of the interconnect elements, in particular into Cu crystallites, is deliberately kept discontinuous, and so is the formation of a diffusion barrier-cap after the Si incorporation. 
         [0016]    The formation of a second barrier-cap portion on the second set of crystallites is according to the invention performed in a subsequent processing phase that employs second process conditions differing from the first process conditions. 
         [0017]    The method of the invention thus turns away from the known concept of Si incorporation into the whole surface of the interconnect element in one single incorporation step. By individually performing the respective Si incorporation and the respective subsequent formation of the diffusion-barrier cap in two separate processing phases according to different crystal orientations, the method of the invention achieves the advantage that a control of the incorporation of silicon into the first and second sets of crystallites can be improved. 
         [0018]    In particular, the Si incorporation into the first, highly reactive set of crystallites can be controlled better. No care has to be taken in the first phase of the processing to also incorporate silicon into the second set of crystallites. As a consequence, the Si corporation into the first set of crystallites can be controlled very precisely. More specifically, the Si incorporation into the first set of crystallites can be limited to an amount that is really required. An increased incorporation of Si into the first set of crystallites of the interconnect element would lead to an increased electrical resistance, which is detrimental for many applications, including those requiring high-frequency operation. A higher resistance of the interconnect elements also increases the production of heat during operation, which causes problems in particular at high integration densities. 
         [0019]    The first barrier-cap portion on the first set of crystallites serves as a mask during the second processing phase, in which the second barrier-cap portion is fabricated on the second set of crystallites. This allows a selective incorporation of silicon into only the second set of crystallites of the interconnect element in this second processing phase under second process conditions that differ from the first process conditions. Due to the presence of the first-barrier cap portion during the second process phase, the second process conditions can be set suitably to achieve a desired Si incorporation into the second set of crystallites without taking a risk of incorporating an undesired high amount Si into the first set of crystallites at the same time. In turn, the control of Si incorporation into the second set of crystallites is also improved. 
         [0020]    The method of the invention therefore not only achieves the formation of a continuous diffusion-barrier cap on a Cu-containing interconnect element, but also avoids an incorporation of undesired, i.e., too large or too small amounts of Si into the different crystallites at the surface of interconnect element. 
         [0021]    A “Cu-containing interconnect element” for the purpose of the present application is an interconnect element that comprises at least a surface layer, which is made of Cu crystallites or crystallites of a Cu alloy. Note in this context that the first set of crystallites may contain crystallites with one or more different crystal orientations. Similarly, the second set of crystallites may contain crystallites with one or more different crystal orientations. Another component of the Cu alloy can for instance be Mg, Se, Zr, Hf, Nb, Ta, Cr, or Mo, or a combination of two or more of these elements. In one embodiment, the interconnect element consists of copper, except for some silicon that may be present in the interconnect element after the processing of the invention. Of course, silicon will also be present when using a Cu alloy. 
         [0022]    The processing of the method of the invention can be further differentiated to distinguish between separate subsequent processing phases with respective different process conditions for three or more sets of crystallites. 
         [0023]    In the following, embodiments of the method of the invention will be described. Unless described otherwise explicitly, the embodiments can be combined with each other. 
         [0024]    In one embodiment, forming the first barrier-cap portion comprises exposing a surface of the Cu-containing interconnect element containing the first and second sets of crystallites to a first ambient atmosphere that contains a silicon precursor and that has a first ambient pressure and a first ambient temperature, which is lower than a threshold temperature required for incorporation of Si into the second set of crystallites at the first ambient pressure. The first ambient temperature is suitably between 200 degrees Celsius and 425 degrees Celsius. In one embodiment, the first ambient temperature is 325 degrees Celsius. A silicon precursor is a silicon-containing compound, which is used as a source of silicon in the deposition process. Suitable silicon precursors are for instance silane, dichlorsilane, and trimethylsilane. In different alternative embodiments, the silicon incorporation employs a thermal or a plasma-enhanced process. The use of a plasma is believed to improve the efficiency of the silicon precursor and thus allows reducing the silicon-precursor flow without reducing the incorporation of silicon into the crystallites. 
         [0025]    In one embodiment, the step of forming the first barrier-cap portion comprises forming, on the first set of crystallites of the interconnect element, a first temporary layer stack comprising a first temporary layer, which contains Cu and Si chemically unbonded to each other, and a second temporary layer comprising copper silicide on the first temporary layer, and 
         [0026]    providing nitrogen to the first temporary layer stack in a first reactive annealing step, thus transforming the first temporary layer stack into the first barrier-cap portion that contains, on the first set of crystallites, a copper silicide (CuSi) layer, and a Si-containing and N-containing diffusion-barrier layer on the copper silicide layer. 
         [0027]    The use of a first barrier-cap portion that contains a copper silicide layer between the Si-containing diffusion-barrier layer and the first set of crystallites has the advantage of increasing the adhesion of the barrier material, which in one embodiment is silicon nitride. Other barrier materials can be used in combination with a CuSi layer. Suitable materials that prevent a diffusion of Cu and can be used in the context of the present invention are, beside silicon nitride, for instance silicon oxynitride, silicon carbide, hydrogenated oxidized silicon carbon (SiCOH), or CuSiN. Other diffusion-barrier materials can be employed in the structure and fabricated utilizing suitable precursors known in the art. 
         [0028]    A CuSi phase completion on the respective first or second set of crystallites can be achieved by different methods. In alternative embodiments, a nitrogen-providing ambient atmosphere contains either molecular nitrogen N 2  or NH 3  or both. In some embodiments, the annealing step is performed at an annealing temperature of 300 degrees Celsius or more than 300 degrees Celsius. 
         [0029]    In one of two alternative processing embodiments, forming the second barrier-cap portion comprises: 
         [0030]    forming, on the second set of crystallites of the interconnect element in the openings of the first layer stack, a second temporary layer stack with a layer sequence equal to that of the first temporary layer stack, and 
         [0031]    providing nitrogen to the second temporary layer stack in a second reactive annealing step, thus transforming the second temporary layer stack into the second barrier-cap portion that contains a copper silicide layer on the second set of crystallites of the interconnect element and a silicon nitride layer on the copper silicide layer. 
         [0032]    The formation of the second barrier-cap portion in the present embodiment resembles the formation of the first barrier-cap portion. It results in a diffusion-barrier layer on top of a CuSi layer in both the first and the second barrier-cap portions. 
         [0033]    Preferably, the step of forming the second temporary layer stack comprises exposing the surface containing the first barrier-cap portion and the second set of crystallites of the Cu-containing interconnect element to a second ambient atmosphere that has a second ambient pressure and that contains a silicon precursor, at a second ambient temperature higher than a threshold temperature for incorporation of Si into the second set of crystallites at the second ambient pressure and higher than the first ambient temperature. The second ambient pressure is in one embodiment the same as the first ambient pressure. 
         [0034]    The use of a second ambient temperature that is higher than the first ambient temperature and higher than a threshold temperature for incorporation of Si into the second set of crystallites at the second ambient pressure in this embodiment enables the selective incorporation of Si into the second set of crystallites and the subsequent completion of the CuSi layer on the whole surface of the interconnect element. In this embodiment, therefore, the adhesion of the diffusion-barrier layer on top of the CuSi layer is particularly high due to the presence of a continuous CuSi layer. 
         [0035]    In an alternative processing embodiment for the formation of the second barrier-cap portion, the following steps are performed: 
         [0036]    forming, on the second set of crystallites of the interconnect element in the openings of the first layer stack, only a third temporary layer, which contains Cu and Si chemically unbonded to each other; 
         [0037]    providing nitrogen to the third temporary layer in a second reactive annealing step, thus transforming the third temporary layer into a second barrier-cap portion that contains only a silicon nitride layer. 
         [0038]    This processing leads to a formation of the diffusion-barrier layer in between the discontinuous CuSi film on the first set of crystallites, which was fabricated during the previous first processing phase. This embodiments achieves a completion of the diffusion-barrier layer even though no CuSi layer is provided. However, the barrier properties are improved in comparison with a processing that leaves the second set of crystallites uncovered. 
         [0039]    In some embodiments that use the second alternative processing, forming the second temporary layer comprises exposing the second set of crystallites to a second ambient atmosphere that contains the silicon precursor under the following process conditions: 
         [0040]    a second temperature smaller than or equal to the first temperature, and 
         [0041]    a second ambient pressure that is higher than the first ambient pressure or 
         [0042]    a second exposure duration that is longer than the first exposure duration. 
         [0043]    This processing leads to a limited Si incorporation into the second set of crystallites and avoids the formation of chemical bonds between the Cu of the second set of crystallites and the incorporated Si. 
         [0044]    Further preferred embodiments of the invention are defined in the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings. 
           [0046]      FIGS. 1 to 5  show a schematic cross-sectional view of a section of an interconnect element during different phases of a fabrication of a diffusion-barrier cap on the interconnect element, according to a first embodiment; and 
           [0047]      FIGS. 6 and 7  show schematic cross-sectional views of a section of an interconnect element in an interconnect stack of an integrated-circuit device during different stages of the fabrication of a diffusion-barrier cap, according to a second embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0048]      FIGS. 1 to 5  show a schematic cross-sectional view of a section of an interconnect element during different phases of a fabrication of a diffusion-barrier cap on the interconnect element, according to a first embodiment. 
         [0049]      FIG. 1  shows a section of an interconnect stack  100  during its fabrication. At the processing stage displayed in  FIG. 1 , a Cu-containing interconnect element  102  has been embedded into a dielectric layer  104 . The dielectric layer  104  is covered by a dielectric liner  105 . The dielectric liner was introduced on top of  104  the dielectric layer  104  prior to a patterning of the features by lithography and etching, and prior to the formation of a diffusion barrier  106  and Cu filling. However, this is an option. The dielectric liner  105  is not necessarily required for the processing of the present embodiment. In one embodiment, the dielectric liner is not deposited at all. In another embodiment, it is completely removed from the wafer surface during a Mechanical polishing step preceding the processing stage shown in  FIG. 1 . 
         [0050]    A lateral section of a diffusion barrier  106  is arranged between the interconnect element  102  and the dielectric layer  104 . The diffusion barrier  106  is in one embodiment made of a TaN/Ta structure. 
         [0051]    Note that the present Figure and the following Figures only show a section of the interconnect stack  100  at serves to illustrate the processing of the present embodiment. For simplicity, the Figures do not show a complete interconnect element, nor a complete interconnect stack. It is to be understood that the interconnect element  102  is completely embedded into the dielectric layer  104 . The interconnect element is further separated from the dielectric layer  104  by the diffusion barrier  106 . The processing of the present embodiment is suitable for integration into a single or dual damascene process for the formation of an interconnect stack. 
         [0052]    The Cu-containing interconnect element  102  is in the present embodiment made of Cu. It contains crystallites or grains, which in  FIG. 1  are illustrated by grain boundaries. The grain boundaries  108  to  114  separate different crystallites  116  to  122  on the phase  124  of the interconnect element  102 . The graphical representation of the crystallites  116  to  122  is only schematic. The Cu crystallites in the interconnect element  102  typically have different crystal orientations. That means, lattice planes of the different crystallites  116  to  122  that form the surface  124  have two or more different Miller indices. 
         [0053]    Si incorporation into the different crystallites with different crystal orientation occurs at different rates for given process conditions. In the present embodiment, two different sets of crystallites are illustrated. The crystallites  116  and  120  form a first set of crystallites with a first crystal orientation. The crystallites  118  and  122  form a second set of crystallites with a second crystal orientation. For the purpose of the present description, it is assumed that the incorporation rate of silicon into the first set of crystallites is higher than incorporation rate of silicon into the second set of crystallites at first process conditions. The process conditions are characterized by a temperature T 0  of an ambient atmosphere containing a Si precursor, a pressure P 0  of the ambient atmosphere, and a duration t 0  of providing the ambient atmosphere containing the Si precursor. A suitable Si precursor is silane. Other suitable Si precursors are trimethylsilane and dichlorsilane. The process conditions are in one embodiment further characterized by RF power employed in a plasma-enhanced silicon incorporation process. 
         [0054]    The silicon incorporation into the interconnect element  102  leads to a formation of chemically unbonded Si into crystallites in a surface region of the interconnect element  102 . The process conditions are chosen to achieve a silicon incorporation into the first set of crystallites represented by the grains  116  and  120 , and not into the second set of crystallites  118 ,  122 . 
         [0055]    The result of this processing is shown in  FIG. 2 . In the crystallites  116  and  120 , which form the first set of crystallites, a layer stack has been created, which forms the first temporary layer stack  130  according to the claim language. The first temporary layer stack  130  contains a first temporary layer  126 , which contains Cu and Si chemically unbonded to each other, and a second temporary layer  128  comprising CuSi on the first temporary layer  126 . The first temporary layer structure  130  is only present in the first set of crystallites  116 ,  120  of this processing stage. 
         [0056]    Subsequently, the structure of  FIG. 2  is subjected to a reactive annealing step in a nitrogen-providing ambient atmosphere. The reactive annealing step leads to the formation of a silicon nitride layer  132  and completes the formation of the CuSi layer  128 . The silicon nitride layer  132  is only present on the first set of crystallites and thus forms a first diffusion-barrier-layer portion on top of the first adhesion-layer portion  128 . The second set of crystallites  118 ,  122  remains uncovered at this processing stage. 
         [0057]    The following processing for the completion of the diffusion-barrier cap can be performed according to two alternative methods. One alternative will be described in the following with reference to  FIGS. 4 and 5 . A second alternative method will be described thereafter with reference to  FIGS. 6 and 7 . 
         [0058]    In the first processing alternative, a temporary layer, which contains Cu and Si chemically unbonded to each other is formed on the crystallites  118  and  122 , i.e., the second set of crystallites. This is performed by a treatment with a silicon precursor, which can be one of those mentioned for the formation of the corresponding temporary layer  126  in the first set of crystallites  116 ,  120 . For the purpose of the present description, silane will be used as a Si precursor. The treatment with silane leads to the incorporation of Si into the second set of crystallites  118 ,  122  to form the temporary layer  134 . 
         [0059]    The silane treatment is performed with an ambient atmosphere that differs from that used for the Si incorporation into the first set of crystallites by the use of a second temperature that is smaller than the first temperature. Suitable temperature values lower than the first temperature should, however, range above 250° C. Furthermore, a second ambient pressure is used that is higher than the first ambient pressure used during the previous Si incorporation step into the first step of crystallites. Given theses parameters and otherwise identical parameters for the two different Si incorporation steps, the incorporation of silicon can be performed selectively, i.e., only into the second set of crystallites  118 ,  122 . Instead of using an increased second ambient pressure, the same effect can be achieved by increasing the exposure duration to the silicon precursor over the first exposure duration used for the Si incorporation into the first set of crystallites. In this alternative processing, the second ambient pressure is equal to the first ambient pressure. In another alternative processing, both the second ambient pressure and the second exposure duration are increased over the values of the corresponding quantities used during the first Si incorporation into the first set of crystallites. In one alternative embodiment, the second temperature is equal to the first temperature, whereas the second ambient pressure and/or the second exposure duration are increased over the values used during the first incorporation step. 
         [0060]    The processing up to this step has the advantage of limiting the incorporation of silicon and leading only to the presence of silicon solved in Cu crystallites. No CuSi layer formation is observed. 
         [0061]    Subsequently, nitrogen is provided to the temporary layer  134  in a second reactive annealing step, typically at a temperature of 300° C. or above. This way, the temporary layer  134  is transformed into a silicon nitride layer  136 . Thus, a second barrier-cap portion is formed by the silicon nitride layer  136 , which completes the formation of a diffusion-barrier cap  138  on the interconnect element  102 . The first barrier-cap portion  138 . 1  consists of a CuSi layer  128  on the interconnect element and of a silicon nitride layer  132  on the CuSi layer. The second barrier-cap  138 . 2  portion consists of a silicon nitride layer  136  only. 
         [0062]    The described processing leads to the formation of a self-aligned diffusion-barrier cap  138  on the interconnect element  102 . The reactive annealing steps performed during the formation of the first and second barrier-cap portions  138 . 1  and  138 . 2  and to a formation of a Si/O/N layer  105  on top of the dielectric layer  104 . 
         [0063]    In a second alternative processing, which forms an alternative to that described above with reference to  FIGS. 4 and 5  and which will now be described with reference to  FIGS. 6 and 7 , a second barrier-cap portion is fabricated, which has a layer structure equal to that of the first barrier-cap portion. The processing for the formation of the first barrier-cap portion is that described with reference to  FIGS. 1 to 3 . 
         [0064]    To form the second barrier-cap portion in this embodiment, the treatment of the surface  124  of the interconnect element  102  for an incorporation of Si into the second set of crystallites  118 ,  122  is modified in comparison with that described with reference to  FIGS. 4 and 5 . Specifically, the silane treatment includes a partial silicidation as during the first incorporation of silicon described with reference to  FIG. 2 . This way, a temporary layer structure  140  is fabricated, which has a CuSi layer  142  on the surface  124  of the interconnect element. Underneath the CuSi layer  142 , a layer  144  with Si in solution, i.e. Cu and Si chemically unbonded to each other, is formed. A suitable Si precursor can be selected from those mentioned earlier. A silane treatment is again used by way of example for the present embodiment. The silane treatment and the partial silicidation are performed at a second temperature that is higher than the first temperature used during the incorporation of Si into the first set of crystallites and higher than a threshold temperature for the formation of CuSi in the second set of crystallites, given the further parameter of ambient pressure and exposure duration identical to those used for the formation of the first temporary layer structure  130 . 
         [0065]    Subsequently, a plasma-enhanced treatment in a nitrogen-providing atmosphere is performed. The ambient atmosphere for instance contains NH 3 . This leads to the formation of a silicon nitride layer  146  on top of the CuSi layer  142 . This way, a second barrier-cap portion  138 . 2  is formed that has an identical layer structure as the first barrier-cap portion  138 . 1 . The diffusion-barrier cap  138  is thus formed by a continuous double-layer structure containing a CuSi adhesion layer and a silicon nitride diffusion-barrier layer. The adhesion layer is formed of a first adhesion-layer portion  128  on the first set of crystallites and a second adhesion-layer portion  142  on the second set of crystallites. The silicon nitride diffusion layer has a first diffusion-barrier-layer portion  132  on the first set of crystallites and a second diffusion-barrier-layer portion  146  on the second set of crystallites. 
         [0066]    Note that the thickness of the first diffusion-barrier-layer portion  132  is larger than the thickness of the second diffusion-barrier-layer portion  146  and  136  in both alternatives shown in  FIGS. 5 and 7 . However, the respective thicknesses can be tuned to desired values and can also be made equal, either within the framework of the previously described processing, or by subsequent further processing. 
         [0067]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 
         [0068]    Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 
         [0069]    In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. 
         [0070]    Any reference signs in the claims should not be construed as limiting the scope.