Abstract:
Measures are provided for improving and simplifying metallic bonding processes which enable a reliable initiation of the bonding process and thus contribute to a uniform bonding. The present method provides a further option for using bonding layers. The method in the case of which the two semiconductor elements are bonded to one another via a bond of at least one metallic starting layer and at least one further starting layer provides that the two starting layers are structured in such a way that the layer areas which are assigned to one another have differently sized areal extents. Moreover, the layer thicknesses of the two starting layers should be selected in such a way that the layer areas which are assigned to one another meet the material ratio necessary for the bonding process.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a component including at least two semiconductor elements, which are bonded to one another via a structured bonding layer, this bonding layer being formed from at least one structured starting layer made of a metallic layer material and at least one further structured starting layer whose layer materials have been bonded in a bonding process under the application of pressure. Furthermore, the present invention relates to a method for manufacturing a component of this type. 
       BACKGROUND INFORMATION 
       [0002]    Bonds are used, for example, for constructing vertical hybrid integrated components. Components of this type generally include multiple MEMS and/or ASIC elements whose element functions complement each other. Vertical hybrid integrated sensor components which include a MEMS element and an ASIC element are important in particular. The micromechanical structure of the MEMS element is used to detect a measured variable which is converted into an electrical measuring signal with the aid of circuit elements of the MEMS element. These measuring signals are then processed and, if necessary, evaluated using the circuit functions of the ASIC element. 
         [0003]    The elements of a vertical hybrid integrated component are situated above one another in the form of a chip stack and are bonded to one another in such a way that further repackaging for the second level assembly on an application circuit board is not necessary. For this purpose, the mechanical connection between the individual elements must, on the one hand, be permanently stable and, on the other hand, also circumferentially tightly closed in order to protect the element functions against harmful environmental influences, e.g., dust particles, moisture, and gases. 
         [0004]    During the manufacture of vertical hybrid integrated components, the individual element components are not only generally produced in the wafer composite. Their assembly also takes place in the wafer composite during a wafer bonding process. The components are frequently even separated only after testing and calibration at the end of the manufacturing process. 
         [0005]    In practice, wafer bonding processes are mostly employed during which one or also both element surface(s) or wafer surface(s) to be bonded are provided with a structured bonding layer. These bonding processes include glass frit bonding, during which low melting lead glass is used as the bonding layer, and metallic bonding processes, such as the thermocompression bonding processes Au—Au bonding, Cu—Cu bonding and Al—Al bonding, eutectic bonding processes Au—Si bonding, Al—Ge bonding, Al—Au bonding, and Al—Si bonding as well as solid-liquid interdiffusion (SLID) bonding involving Cu/Sn and Au/Sn. 
         [0006]    Since the glass frit technique requires a relatively large chip area in order to ensure a hermetically sealed and permanently stable bond, metallic bonding processes are increasingly used. Although the space requirement for a hermetically sealed and mechanically solid connection is considerably smaller with this technique, it imposes significantly higher requirements on the wafer surfaces to be bonded, in particular on their planarity and purity. Since the wafer bond is established in this case with the aid of interdiffusion or by fusion of the bonding areas, the latter must be brought into intimate contact. For this purpose, very high contact pressures in the magnitude of up to 100 kN are applied during the entire bonding process and onto the entire bonding area. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides measures for improving and simplifying metallic bonding processes which enable a reliable initiation of the bonding process and thus contribute to a uniform bonding. Moreover, the present invention provides a further option for using bonding layers. 
         [0008]    The measures according to the present invention involve layout suggestions for the two starting layers which are converted into the bonding layer between the two elements during the bonding process. 
         [0009]    Thus, the method for manufacturing a component including at least two semiconductor elements provides that the two starting layers are structured in such a way that the layer areas which are assigned to one another have differently sized areal extents. Moreover, the layer thicknesses of the two starting layers should be selected in such a way that the layer areas which are assigned to one another meet the material ratio necessary for the bonding process. 
         [0010]    The present invention accordingly provides an asymmetric or noncongruent layout of the two starting layers for the bond. In this way, the contact area between the bonding materials is relatively small at the beginning of the bonding process. The applied pressure is accordingly relatively great at a given contact force, thus favoring the breaking-up of potentially present oxides and/or contaminants on the bonding surfaces and therefore ensuring the beginning of the bonding process. Alternatively, the contact force may also be reduced for this purpose as long as the minimum pressure necessary for the bonding process is applied at least on the contact point. In the course of the bonding process, the bond front expands laterally, which is accompanied by a height reduction of the two starting layers and is associated with an additional, vertical pulling effect. This pulling effect supports the contact force and facilitates a uniform compression and bonding of the layer materials. 
         [0011]    There are in principle many different possibilities for implementing the bonding concept according to the present invention. The layout of the starting layers for the bonding process, for example, also always depends on the type and function of the two elements to be bonded to one another. The various forms of implementation, however, do not only relate to the layout and thickness of the two starting layers, but also to the application of the layer material and the topography of the starting layers. 
         [0012]    The implementation of the different layouts for the two starting layers of the bonding process is particularly easy, when each one of the two element surfaces to be bonded is provided with a starting layer for the bonding process. Depending on the topography of the two element surfaces to be bonded, it may also be meaningful to apply both starting layers to one of the two element surfaces to be bonded. In this case, the first starting layer must then be structured prior to the application of the other starting layer. 
         [0013]    Depending on the size and shape of the bonding area between the elements, it may prove advantageous to assign to one contiguous layer area in the one starting layer multiple layer areas of the other starting layer. This procedure is suitable, in particular, for bonding areas which are not planarly closed, such as a circumferential bond frame. 
         [0014]    Depending on the layer material, the layer areas of the two starting layers may also be provided with a topography in order to thus influence the contact pressure in a targeted manner in the course of the bonding process at a given contact force and to distribute it in a suitably planar manner. 
         [0015]    It has furthermore been recognized according to the present invention that the noncongruent design of the layout of the two starting layers may be used for implementing structural elements having further functions such as an electrode or a standoff structure. A structural element of this type may be, as a matter of fact, easily structured out of one of the two starting layers next to the bonding area. The structural element, however, cannot be assigned a layer area in the other respective starting layer, so that a bond cannot form here during the bonding process. 
         [0016]    With the aid of a standoff structure of this type, it is very easily possible to delimit the thickness of the bonding layer, for example. However, it may also be used as a stop for a deflectable micromechanical structural component of one of the two elements. An electrode may be used depending on the application for signal detection or activation of micromechanical structural elements. 
         [0017]    As explained above, there are various possibilities of implementing and refining the present invention in an advantageous manner. For this purpose, reference is made, on the one hand, to the further subject matter which is subordinate to the main subject matter described herein, and, on the other hand, to the following description of multiple exemplary embodiments of the present invention on the basis of the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIGS. 1   a  through  1   e  show the bonding concept according to the present invention based on schematic sectional representations of two elements  10  and  20  during the bonding process. 
           [0019]      FIG. 2  shows a layout variant for a starting layer of this bonding process based on a schematic sectional representation. 
           [0020]      FIGS. 3   a  and  3   b  show other possibilities of applying the starting layers of the bonding process based on schematic representations. 
           [0021]      FIGS. 4   a  and  4   b  show starting layers for the bonding process which are provided with a topography. 
           [0022]      FIGS. 5   a  and  5   b  show the implementation of a standoff structure during the bonding process based on schematic representations. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In the exemplary embodiment illustrated in  FIGS. 1   a  through  1   e , the surfaces of the two elements  10  and  20  to be bonded are each provided with a starting layer, at least one of these starting layers being made of a metallic layer material. The exemplary embodiment refers to the eutectic material combination aluminum-germanium, but may also be transferred to other material systems such as gold-tin or gold-silicon. The layer materials of the two starting layers are supposed to bond during the bonding process, forming one bonding layer  30  for the two elements  10  and  20 . For many applications, the elements are supposed to be bonded to one another only in a subarea of the element surfaces, for example across a bond frame. For this purpose, the starting layers are structured even prior to the actual bonding process. 
         [0024]    In the exemplary embodiment described here, a layer area  11 , the areal extent of which essentially corresponds to the strived for bonding area, was structured out of the starting layer of element  10 . Layer area  11  is assigned a layer area  21  in the starting layer of element  20 , the areal extent of which is considerably smaller than that of layer area  11 . In turn, the starting layer of element  20  is, however, thicker than the starting layer of element  10  so that the quantitative ratio of the layer materials necessary for the eutectic bonding process is met.  FIG. 1   a  illustrates these proportions as well as the configuration of the two layer areas  11  and  21  with regard to one another. Layer area  11  has a thickness D 0 . Its diameter is identified by B. Since B also corresponds to the diameter of the bonding area to be generated, this parameter is predefined by the required stability of the bond. The effective contact pressure which acts on the contact area at least at the beginning of the bonding process at a given contact pressure may be influenced via diameter b of smaller layer area  21 . The smaller b is selected to be, the greater is the local contact pressure. Thickness d of layer area  21  must then be selected in such a way that a sufficient amount of layer material is available for the eutectic bond. Since thickness d is also a function of the areal extent and thus of b, b also determines the vertical shrinkage of the bond during the bonding process and thus the vertical “pull” which is applied to elements  10  and  20  in the process. 
         [0025]    The two elements  10  and  20  are brought into contact for the bonding process, which is illustrated in  FIG. 1   b . The bonding reaction has not started yet because the process temperature is still below the eutectic point, for example. The distance between elements  10  and  20  corresponds to the sum of the two layer thicknesses D 0 +d. The contact area corresponds to the area of layer area  21  and has diameter b. The smaller b is, the smaller is also the contact area and the larger is the contact pressure at the beginning of the bonding process at a given contact force. The comparably high contact pressure supports the targeted start of the bonding process if the conditions required therefor are met, i.e., the eutectic temperature is reached, for example. The higher the contact pressure, the easier it is to break through the bothersome layers on the bond surfaces which impede the bonding process such as surface oxides or contaminations. 
         [0026]      FIG. 1   c  shows the two elements  10  and  20  after the beginning of the bonding process during which the layer materials of the two starting layers  11  and  21  mix in a eutectic ratio. Bond front  31  expands laterally under the applied contact force. The material transportation associated with it results in a reduction of the overall thickness of the two starting layers. The interflowing layer material causes an additional force which contracts the two elements  10  and  20  and increases the applied contact force locally in the bonding area in this way. 
         [0027]    In the state of the bonding process illustrated in  FIG. 1   d , bond front  31  has progressed further laterally. The two elements  10  and  20  continue to be pressed together, original distance D 0 +d between elements  10  and  20  being further reduced. 
         [0028]    In  FIG. 1   e , the bonding process is completed. The entire layer material is distributed uniformly across entire bonding area  30  namely as far as areal extent and the eutectic composition are concerned which are required for the strived for stability of the bond. Bond  30  has thus reached its final areal extent and height in the present case. 
         [0029]      FIG. 2  shows a layout variant for the two starting layers of a eutectic bond between two semiconductor elements  10  and  20 , in this bond, a layer area  11  in the starting layer of element  10  being assigned multiple layer areas  21  in the starting layer on other element  20 . Each of the layer thicknesses of the two starting layers or layer areas  11  and  21  were also selected here in such a way that the layer materials are available in the quantitative ratio necessary for the bond. This layout variant, for example, lends itself when a starting layer is only manufacturable having a limited layer thickness and/or when the lateral mixing or interflowing of the layer materials is limited during the bonding process, but also when the geometry of the bonding area requires this, e.g., in the case of a circumferential bond frame. 
         [0030]    The two starting layers for the bond between two semiconductor elements  10  and  20  may also both be applied to one of the two element surfaces, which is illustrated by  FIGS. 3   a  and  3   b.    
         [0031]    In the case of  FIG. 3   a , a first thinner starting layer was initially deposited and structured on element  20 . The areal extent of resulting layer area  11  essentially corresponds in this case to the dimension of the bonding area to be generated. 
         [0032]    A second thicker starting layer was then deposited and structured over the thus structured first starting layer. Resulting layer area  21  is situated centrally on layer area  11 , but has a significantly smaller areal extent. 
         [0033]    In contrast thereto, the thicker starting layer was initially deposited and structured on element  20  in the case of  FIG. 3   b . Thinner starting layer  11  was then deposited and structured above thus generated layer area  21 . Since the element surface now has a not completely planar topography, the structuring of this thinner layer requires a little bit more effort. This variant is, however, advantageous when the material of lower layer  21  is slightly chemically corrosive, such as aluminum in the case of an Al—Ge bonding or silicon in the case of an Au—Si bond. With the aid of upper covering layer  11 , the formation of bothersome surface oxides may be prevented in this case. 
         [0034]    To reduce the contact area between the element surfaces to be bonded at the beginning of the bonding process, the flank profiles of the layer areas of the starting layers may also be influenced, for example, at least in the case of some layer materials, which is illustrated in  FIGS. 4   a  and  4   b.    
         [0035]      FIG. 4   a  shows a layer area  22  having beveled flanks, for example. This flank profile may, for example, occur when the starting layer was structured in an isotropic etching step. 
         [0036]    In the case of  FIG. 4   b , the flanks of layer area  23  are rounded. This flank profile may, for example, form when the starting layer was scraped in a screen printing process and the layer material then contracts due to surface tension, or when the structured starting layer is subsequently reshaped with the aid of fusion and solidification. 
         [0037]    As already mentioned above, the bonding layer generated according to the present invention is generally thinner than the sum of the layer thicknesses of the two starting layers. In one refinement of the present invention, this fact is used to provide the two element surfaces to be bonded to one another with structural elements which are situated very closely to the respective other element function and interact with it. These structural elements are simply formed in a starting layer for the bond, namely outside a bonding area. Depending on the material, electrodes for activation or the signal detection or also a standoff structure may be implemented, for example, as an overload protection, which is illustrated in  FIGS. 5   a  and  5   b.    
         [0038]    In this exemplary embodiment described here, a layer area  21 , the areal extent of which essentially corresponds to the strived for bonding area, was structured out of the starting layer of element  20 . This layer area  21  is assigned a layer area  11  in the starting layer of element  10 , the areal extent of which is considerably smaller than that of layer area  21 . In turn, the starting layer of element  10  is, however, thicker than the starting layer of element  20  so that the quantitative ratio of the layer materials necessary for the eutectic bonding process is provided. Next to layer area  11  and outside the bond to be generated, a further layer area  12  was structured out of the starting layer of element  10 .  FIG. 5   a  illustrates the configuration of the two layer areas  11  and  12  as well as of layer area  21  and the corresponding layer thicknesses. 
         [0039]      FIG. 5   b  shows the two elements  10  and  20  after the bonding process. The layer materials of layer areas  11  and  21  form bond  30 , the thickness of this bonding layer  30  being smaller than the sum of the layer thicknesses of the two starting layers  11  and  21 . The material and the thickness of layer area  12  on element  10  have not changed during the bonding process. This layer area  12  is used as a standoff structure which ensures a minimum distance between the two elements  10  and  20  and prevents a further compression of the bonding material.