Abstract:
A bonding pad structure. The bonding pad structure includes independently built current conduction structure and mechanical support structure between a bonding pad layer and a substrate. The current conduction structure is constructed using a plurality of serially connected conductive metallic layers each at a different height between the bonding pad layer and the substrate. The conductive metallic layers connect with each other via a plurality of plugs. At least one of the conductive metallic layers connects electrically with a portion of the device in the substrate by a signal conduction line. The mechanical support structure is constructed using a plurality of serially connected supportive metallic layers each at a different height between the bonding pad layer and the substrate. The supportive metallic layers connect with each other via a plurality of plugs. Furthermore, the mechanical support structure connects with a non-device section of the substrate so that stresses on the bonding pads are distributed evenly through the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application Ser. No. 90113549, filed on Jun. 5, 2001. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a bonding pad structure. More particularly, the present invention relates to a bonding pad structure having detached current conduction regions and mechanical support regions. 
     2. Description of Related Art 
     In the front stage of fabricating semiconductor devices, a plurality of optical masks is used to pattern out the active regions, gate structures, metallic layers, source/drain contacts, circuit pattern of multi-level interconnects and bonding pad windows. Due to the rapid increase in the level of integration of semiconductor devices, functional capacity and data processing speed, the number of signaling points on a semiconductor component increases considerably. As the number of contact points increases, the number of corresponding bonding pads required is also increased. After the formation of bonding pads, the integrated circuit chip must be packaged. In other words, the signal points and bonding pads on the silicon chip must connect electrically with a lead frame via metallic wires, a process known as wire-bonding. A wire-bonding operation links each bonding pad on a semiconductor chip with an inner lead of the lead frame using a fine metallic wire (30-50μm). Hence, electrical signal generated inside the semiconductor chip can be transmitted to circuits outside the package. The bonding pad on the semiconductor chip serves as a first bonding point while the inner lead of the lead frame serves as a second bonding point. During wire bonding, one end of a metallic wire is melted into a spherical blob and then the spherical blob is pressed onto the bonding pad to form a weld with the aid of an ultrasonic vibration. The metallic wire is pulled along a pre-defined path towards a corresponding inner lead position on the lead frame. Thereafter, the other end of the wire is bonded to the inner lead. Finally, excess metallic wire is pulled off from the bonded inner lead. By repeating the aforementioned wire bonding process, the entire package is connected. Because ultrasonic vibration is employed in the bonding of a metallic wire onto the bonding pad, passivation layer or dielectric layer surrounding the bonding pad regions may crack due to stress concentration. In addition, the difference in coefficient of thermal expansion (CTE) between epoxy resin and the silicon chip during subsequent packaging may cause a further widening of the cracks already formed in the passivation layer or the dielectric layer. 
     One method of reducing uneven stress distribution within a semiconductor package is to form a plurality of bonding pad metallic layers in the desired bonding pad locations. The bonding pad metallic layers are similar in shape to bonding pads during interconnect fabrication. Plugs having a circular, rectangular or other shape arranged in a pre-defined array pattern are used to connect between the bonding pad metallic layers or the bonding pad metallic layer and the bonding pad. The bottommost layer also connects with the silicon substrate so that stress is evenly distributed over the entire wafer by the silicon substrate. Hence, the silicon wafer is less vulnerable to damages during subsequent processing. 
     Conventionally, a metallic plug is formed by conducting a plasma etching operation to remove a portion of the insulation layer and expose a portion of the bonding pad metallic layer and then refilling the opening with a metal. During a plasma etching operation, a portion of the electrical charges in the plasma may be transferred to the bonding pad metallic layer. These electrical charges may be transmitted to the devices via the conductive path between the bonding pad metallic layer and the devices. 
     In addition, a fixed number of plugs must be provided between the bonding pads and the bonding pad metallic layers or between the bonding pad metallic layers to ensure sufficient mechanical support for the bonding pads. However, the conductive current transmitted to the devices resulting from electric charges in the plasma is proportional to the number of plugs used. In other words, the larger number of metallic plugs used, the larger will be the total amount of electric charges collected by various bonding pad metallic layers. The flow of a large conduction current into the device may lead to device failure and a lowering of production yield. 
     Nevertheless, reducing conduction current by using fewer plugs between the bonding pad and the silicon substrate often leads to a drop in mechanical strength of the bonding pad. Damages rendered by subsequent processing may result in a higher production cost. 
     A method capable of increasing mechanical strength of the bonding pad without increasing corresponding conductive current is unavailable at present. Hence, only a compromised solution involving a balance between an acceptable conduction current, a minimum mechanical support for bonding pad and production cost can be sought. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to provide a bonding pad structure having a detached current conduction structure and mechanical support structure. The detached current conduction structure and mechanical support structure reduces the quantity of electric charges flowing to devices during etching but increases the mechanical strength of the bonding pad. Ultimately, product yield is increased and failure rate of subsequently processed silicon wafer is reduced. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a bonding pad structure. The bonding pad structure includes independently built current conduction structure and mechanical support structure between a bonding pad layer and a substrate. The current conduction structure is constructed using a plurality of serially connected conductive metallic layers, each at a different height between the bonding pad layer and the substrate. The conductive metallic layers connect with each other via a plurality of plugs. At least one of the conductive metallic layers connects electrically with a portion of the device in the substrate by a signal conduction line. The mechanical support structure is constructed using a plurality of serially connected supportive metallic layers each at a different height between the bonding pad layer and the substrate. The supportive metallic layers connect with each other via a plurality of plugs. Furthermore, the mechanical support structure connects with a non-device section of the substrate so that stresses on the bonding pads are distributed evenly through the substrate. 
     In this invention, since cross-sectional area of the conductive metallic layer in the current conduction structure is smaller than the bonding pad layer, the number of plugs connected to various conductive metallic layers can be reduced. Hence, current transmitted to the device via the current conduction structure will not exceed the permitted charge current during plasma etching. In other words, device breakdown due to excess charge flow is prevented leading to a higher yield and a lower production cost. 
     In addition, mechanical strength of the bonding pads is increased because both the mechanical support structure connected to the bonding pad layer and the current conduction structure are used in this invention. Since the mechanical support structure and the current conduction structure are connected together via the bonding pad layer only, electric charges absorbed when forming the plugs above the supporting metallic layer will not transmit to the current conduction structure. In this way, plug density can be increased to improve supportive strength of bonding pads. In the meantime, excessive current flowing to devices leading to device failures can be prevented. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1 is a sketch showing a bonding pad structure fabricated according to a first preferred embodiment of this invention; 
     FIG. 2 is a cross-sectional view of the bonding pad structure as shown in FIG. 1; 
     FIGS. 3 is a sketch showing a bonding pad structure fabricated according to a second preferred embodiment of this invention; and 
     FIGS. 4 is a sketch showing a bonding pad structure fabricated according to a third preferred embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     FIG. 1 is a sketch showing a bonding pad structure fabricated according to a first preferred embodiment of this invention. As shown in FIG. 1, the bonding structure includes a substrate  100 , a mechanical support structure  112 , a current conduction structure  114  and a bonding pad layer  116 . The bonding pad layer  116  is above the substrate  100 . The current conduction structure  114  is attached to the bonding pad layer  116  between the bonding pad layer  116  and the substrate  100 . Similarly, the mechanical support structure  112  is also attached to the bonding pad layer  116  between the bonding pad layer  116  and the substrate  100 . The substrate  100  can be a semiconductor substrate or a substrate with multi-layered interconnects therein. 
     The current conduction structure  114  and the mechanical support structure  112  are two detached structures connected only by the bonding layer  116 . Furthermore, an insulation layer  106  is formed between the current conduction structure  114  and the mechanical support structure  112 . The insulation layer  106  comprises more than one insulating material layer and the insulating material is silicon nitride, silicon oxide or silicon oxynitride. 
     The current conduction structure  114  includes a plurality of serially connected metallic layers  104   a ,  104   b ,  104   c  each located at a different height level between the bonding pad layer  116  and the substrate  100 . The conductive metallic layers  104   a ,  104   b ,  104   c  are linked together via plugs  110   a  and  110   b , respectively. The conductive metallic layer  104   c  and the bonding pad layer  116  are linked together via plugs  110   c . The conductive metallic layer  104   a  is in contact with the substrate  100  so that the current conduction structure  114  actually connects the bonding pad layer  116  and the substrate  100  together. The conductive metallic layers  104   a ,  104   b ,  104   c  connect electrically with a signal line (not shown). Hence, the conductive metallic layers are electrically connected to a device section  118  on the substrate  100 . The conductive metallic layers can be local metallic interconnects, for example. 
     The mechanical support structure  112  includes a plurality of serially connected support metallic layers  102   a ,  102   b  and  102   c , each located at a different height level between the bonding pad layer  116  and the substrate  100 . The support metallic layers  102   a ,  102   b ,  102   c  are linked together via plugs  108   a  and  108   b . The support metallic layer  102   c  and the bonding pad layer  116  are linked together via plugs  108   c . The support metallic layer  102   a  and the substrate  100  are in contact with each other to form the mechanical support structure  112  between the bonding pad layer  116  and the substrate  100 . The support metallic layers can be local metallic interconnects, for example. 
     Since the bottom conductive metallic layer  104   a  of the current conduction structure  114  and the bottom support metallic layer  102   a  of the mechanical support structure  112  are formed on the substrate  100 , stress on the bonding pad layer  116  is transmitted to the conductive metallic layer  104   a  and the support metallic layer  102   a  via the current conduction structure  114  and the mechanical support structure  112 , respectively, and finally the stress is transmitted to the substrate  100  via the conductive metallic layer  104   a  and the support metallic layer  102   a . Ultimately, stress is evenly distributed across the entire substrate  100 . 
     Because cross-sectional area of the conductive metallic layers  104   a ,  104   b ,  104   c  in the current conduction structure  114  is smaller than that of a conventional bonding pad metallic layer, the number of plugs  110   a ,  110   b ,  110   c  attached to various conductive metallic layers  104   a ,  104   b ,  104   c  is smaller than the number of plugs attached to a conventional bonding pad metallic layer. Hence, overall in-processing current delivered to the device section of the substrate  100  via the current conduction structure  114  is greatly reduced. Ultimately, product yield is increased and production cost is lowered. 
     Besides the capacity to conduct current between the bonding pad layer  116  and the substrate  100 , the current conduction structure  114  also has some mechanical support capacity. Through a special patterning of the plugs, the current conduction structure  114  can increase plug density of the mechanical support structure  112  so that bonding pad  116  has the capacity to support a greater load. In addition, the current conduction structure  114  and the mechanical support structure  112  are detached structures. Hence, an increase in plug density for the mechanical support structure  112  will not lead to an increase in current transmitted to the device section during plasma processing. 
     Furthermore, cross-sectional profile of the bonding pad  116 , the conductive metallic layers  104   a ,  104   b ,  104   c  and the support metallic layers  102   a ,  102   b ,  102   c  can have any shape as corresponds to actual processing requirements. Similarly, cross-sectional profile of various plugs including  108   a ,  108   b ,  108   c ,  110   a ,  110   b , and  110   c  can have any shape. Moreover, the plugs can be arranged in whatever pattern is suitable for a particular application. 
     FIG. 2 is a cross-sectional view of the bonding pad structure as shown in FIG.  1 . As shown in FIG. 2, the bonding pad layer is rectangular, the plugs  208  and  210  have a circular top and the plugs  208  and  210  form a rectangular array. The aforementioned arrangement is used as an illustration only and is by no means to be construed as a limitation of this invention. 
     Because the conductive metallic layer  204  and the support metallic layer  202  correspond very much with the bonding pad layer, the number of plugs  210  distributed across the conductive metallic layers  204  is much smaller than the number of plugs  208  distributed across the support metallic layer  202  when the conductive metallic layer  204  has an area much smaller than the support metallic layer  202 . The number of paths available for charges to flow into the devices is greatly reduced in the process of forming the plugs  210 . Since the current flowing to the devices via the conductive metallic layer  204  will not exceed the capacity of the devices, device failure is reduced and product yield is increased. 
     In addition, area at the top of the support metallic layer  202  is only slightly smaller than that of a conventional bonding pad metallic layer. Moreover, the plugs  210  on the conductive metallic layer  204  have some capacity for supporting loads and hence the bonding pad stress supporting capacity in this invention is almost identical to that of a conventional design. Furthermore, the support metallic layer  202  has no direct contact with the device section on the substrate  200 . Therefore, the number of plugs  208  on the support metallic layer  202  can be increased to support higher stress at the bonding pad. 
     Shape and size of the conductive metallic layers and the support metallic layers can be different from the ones shown in FIG.  2 . For example, the conductive metallic layer can have an area greater than the support metallic layer or identical to the support metallic layer. 
     In the first embodiment, although a current conduction structure with three conductive metallic layers and a mechanical support structure with three support metallic layer are shown, there is no limitation to the total number of layers used. In general, a current conduction structure can have a multiple of conductive metallic layers and a mechanical support structure can have a multiple of support metallic layers between the bonding pad layer and the substrate. 
     FIGS. 3 is a sketch showing a bonding pad structure fabricated according to a second preferred embodiment of this invention. In the first embodiment, the substrate is in direct contact with a conductive metallic layer and a support metallic layer. In the second embodiment, however, the conductive metallic layer and the support metallic layer are in contact with the substrate via plugs. The following is a detailed description of the second embodiment of this invention. 
     As shown in FIG. 3, the bonding structure includes a substrate  300 , a mechanical support structure  312 , a current conduction structure  314  and a bonding pad layer  316 . The bonding pad layer  316  is above the substrate  300 . The current conduction structure  314  is attached to the bonding pad layer  316  between the bonding pad layer  316  and the substrate  300 . Similarly, the mechanical support structure  312  is also attached to the bonding pad layer  316  between the bonding pad layer  316  and the substrate  300 . The substrate  300  can be a semiconductor substrate or a substrate with multi-layered interconnects therein. 
     The current conduction structure  314  and the mechanical support structure  312  are two detached structures connected only by the bonding layer  316 . Furthermore, an insulation layer  306  is formed between the current conduction structure  314  and the mechanical support structure  312 . The insulation layer  306  comprises more than one insulating material layer and the insulating material is silicon nitride, silicon oxide or silicon oxynitride. 
     The current conduction structure  314  includes a plurality of serially connected metallic layers  304   a ,  304   b  and  304   c , each located at a different height level between the bonding pad layer  316  and the substrate  300 . The conductive metallic layers  304   a ,  304   b ,  304   c  are linked together via plugs  310   b  and  310   c , respectively. The conductive metallic layer  304   c  and the bonding pad layer  316  are linked together via plugs  310   d . The conductive metallic layer  304   a  and the substrate  300  are linked together via plugs  310   a  to form a current conduction structure  314  between the bonding pad layer  316  and the substrate  300 . The conductive metallic layers  304   a ,  304   b ,  304   c  connect electrically with a signal line (not shown). Hence, the conductive metallic layers are electrically connected to a device section (not shown) on substrate  300 . The conductive metallic layers  304   a ,  304   b  and  304   c  can be local metallic interconnects, for example. 
     The mechanical support structure  312  includes a plurality of serially connected support metallic layers  302   a ,  302   b  and  302   c , each located at a different height level between the bonding pad layer  316  and the substrate  300 . The support metallic layers  302   a ,  302   b ,  302   c  are linked together via plugs  308   b  and  308   c . The support metallic layer  302   c  and the bonding pad layer  316  are linked together via plugs  308   d . The support metallic layer  302   a  and the substrate  300  are linked together via plugs  308   a  to form a mechanical support structure  314  between the bonding pad layer  316  and the substrate  300 . The support metallic layers can be a local metallic interconnects, for example. 
     Since the plugs  310   a  of the current conduction structure  314  and the plugs  308   a  of the mechanical support structure  312  are formed on the substrate  300 , stress on the bonding pad layer  316  is transmitted to the plugs  310   a  and the plugs  308   a  via the current conduction structure  314  and the mechanical support structure  312 , respectively. Finally, the stress is transmitted to the substrate  300  via the plugs  310   a  and  308   a  so that stress is evenly distributed across the entire substrate  100 . 
     Because cross-sectional area of the conductive metallic layers  304   a ,  304   b ,  304   c  in the current conduction structure  314  is smaller than a conventional bonding pad metallic layer, the number of plugs  310   b ,  310   c ,  310   d  attached to various conductive metallic layers  304   a ,  304   b ,  304   c  is smaller than the number of plugs attached to a conventional bonding pad metallic layer. Hence, overall in-processing current delivered to the device section of the substrate  300  via the current conduction structure  314  is greatly reduced. Ultimately, product yield is increased and production cost is lowered. 
     Besides the capacity to conduct current between the bonding pad layer  316  and the substrate  300 , the current conduction structure  314  also has some mechanical support capacity. Through a special patterning of the plugs  310   a ,  310   b ,  310   c  and  310   d , the current conduction structure  314  can increase plug density of the mechanical support structure  312  so that bonding pad  316  has the capacity to support a greater load. In addition, the current conduction structure  314  and the mechanical support structure  312  are detached structures. Hence, an increase in plug density for the mechanical support structure  312  will not lead to an increase in current transmitted to the device section during plasma processing. 
     Furthermore, cross-sectional profile of the bonding pad  316 , the conductive metallic layers  304   a ,  304   b ,  304   c  and the support metallic layers  302   a ,  302   b ,  302   c  can have any shape according to actual processing requirements. Similarly, cross-sectional profiles of various plugs including  308   a ,  308   b ,  308   c ,  308   d ,  310   a ,  310   b ,  310   c  and  310   d  can have any shape. Moreover, the plugs can be arranged in whatever pattern is suitable for a particular application. 
     In the second embodiment, although a current conduction structure with three conductive metallic layers and a mechanical support structure with three support metallic layer are shown, there is no limitation to the total number of layers used. In general, a current conduction structure can have a multiple of conductive metallic layers and a mechanical support structure can have a multiple of support metallic layers between the bonding pad layer and the substrate. 
     FIGS. 4 is a sketch showing a bonding pad structure fabricated according to a third preferred embodiment of this invention. In the first embodiment, the number of conductive metallic layers and the number of support metallic layers are identical. In the third embodiment, however, the number of conductive metallic layers is different from the number of support metallic layers. The following is a detailed description of the second embodiment of this invention. 
     As shown in FIG. 4, the bonding structure includes a substrate  400 , a mechanical support structure  412 , a current conduction structure  414  and a bonding pad layer  416 . The bonding pad layer  416  is above the substrate  400 . The current conduction structure  414  is attached to the bonding pad layer  416  between the bonding pad layer  416  and the substrate  400 . Similarly, the mechanical support structure  412  is also attached to the bonding pad layer  416  between the bonding pad layer  416  and the substrate  400 . The substrate  400  can be a semiconductor substrate or a substrate with multi-layered interconnects therein. 
     The current conduction structure  414  and the mechanical support structure  412  are two detached structures connected only by the bonding layer  416 . Furthermore, an insulation layer  406  is formed between the current conduction structure  414  and the mechanical support structure  412 . The insulation layer  406  comprises of more than one insulating material layer and the insulating material is silicon nitride, silicon oxide or silicon oxynitride. 
     The current conduction structure  414  includes a plurality of serially connected metallic layers  404   a  and  404   b , each located at a different height level between the bonding pad layer  416  and the substrate  400 . The conductive metallic layers  404   a ,  404   b  are linked together via plugs  410   a . The conductive metallic layer  404   b  and the bonding pad layer  416  are linked together via plugs  410   b . The conductive metallic layer  404   a  is in contact with the substrate  400  so that the current conduction structure  414  actually connects the bonding pad layer  416  and the substrate  400  together. The conductive metallic layers  404   a ,  404   b  connect electrically with a signal line (not shown). Hence, the conductive metallic layers are electrically connected to a device section (not shown) on substrate  400 . The conductive metallic layers can be local metallic interconnects, for example. 
     The mechanical support structure  412  includes a plurality of serially connected support metallic layers  402   a ,  402   b ,  402   c  each located at a different height level between the bonding pad layer  416  and the substrate  400 . The support metallic layers  402   a ,  402   b ,  402   c  are linked together via plugs  408   a  and  408   b . The support metallic layer  402   c  and the bonding pad layer  416  are linked together via plugs  408   c . The support metallic layer  402   a  and the substrate  400  are in contact with each other to form the mechanical support structure  412  between the bonding pad layer  416  and the substrate  400 . The support metallic layers can be local metallic interconnects, for example. 
     Since the bottom conductive metallic layer  404   a  of the current conduction structure  414  and the bottom support metallic layer  402   a  of the mechanical support structure  412  are formed on the substrate  400 , stress on the bonding pad layer  416  is transmitted to the conductive metallic layer  404   a  and the support metallic layer  402   a  via the current conduction structure  414  and the mechanical support structure  412 , respectively. The stress is transmitted to the substrate  400  via the conductive metallic layer  404   a  and the support metallic layer  402   a . Ultimately, stress is evenly distributed across the entire substrate  400 . 
     When the cross-sectional area of the conductive metallic layers in the current conduction structure  414  is much smaller than the cross-sectional area of the support metallic layers in the mechanical support structure  412 , the number of conductive metallic layer in the current conduction structure  414  cannot be identical to the number of support metallic layer in the mechanical support structure  412 . This is because the purpose of the current conduction structure  414  is to provide an electrical path between the bonding pad layer and the substrate devices. The stress supporting capacity of the current conduction structure  414  is of secondary importance. The effect of having a number of conductive metallic layers in the current conduction structure that differs from the number of support metallic layer is relatively small. 
     Furthermore, cross-sectional profile of the bonding pad  416 , the conductive metallic layers  404   a ,  404   b  and the support metallic layers  402   a ,  402   b ,  402   c  can have any shape according to actual processing requirements. Similarly, cross-sectional profile of various plugs including  408   a ,  408   b ,  408   c ,  410   a , and  410   b  can have any shape. Moreover, the plugs can be arranged in whatever pattern suitable for a particular application. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.