Abstract:
A semiconductor structure comprises a top metal layer, a bond pad formed on the top metal layer, a conductor formed below the top metal layer, and an insulation layer separating the conductor from the top metal layer. The top metal layer includes a sub-layer of relatively stiff material compared to the remaining portion of the top metal layer. The sub-layer of relatively stiff material is configured to distribute stresses over the insulation layer to reduce cracking in the insulation layer.

Description:
This application is a continuation of prior U.S. patent application Ser. No. 13/717,942 filed on Dec. 18, 2012, which is a continuation of prior U.S. patent application Ser. No. 13/532,843 filed on Jun. 26, 2012, which is a continuation of prior U.S. patent application Ser. No. 12/825,030 filed Jun. 28, 2010, which is a continuation of U.S. patent application Ser. No. 11/737,392 filed Apr. 19, 2007, now U.S. Pat. No. 7,795,130, which is a continuation of Ser. No. 11/305,987, filed Dec. 19, 2005, now U.S. Pat. No. 7,224,074, which is a divisional of prior patent application Ser. No. 10/698,184, filed Oct. 31, 2003, now U.S. Pat. No. 7,005,369, which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 60/496,881, filed Aug. 21, 2003, and U.S. Provisional Application Ser. No. 60/507,539, filed Sep. 30, 2003, all of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to the formation of semiconductor devices and in particular a formation of active circuits under a bond pad. 
     BACKGROUND 
     Integrated circuits comprise two or more electronic devices formed in and/or on a substrate of semi-conductive material. Typically, the integrated circuits include two or more metal layers that are used in forming select devices and interconnects between said devices. The metal layers also provide electrical paths to input and output connections of the integrated circuit. Connections to the inputs and outputs of the integrated circuit are made through bond pads. Bond pads are formed on a top metal layer of the integrated circuit. A bonding process (i.e. the bonding of a ball bond wire to the bond pad) can damage any active circuitry formed under the metal layer upon which the bonding pad is formed. Therefore, present circuit layout rules either do not allow any circuitry to be formed under the bonding pad or only allow limited structures that have to be carefully tested. 
     Damage under bonding pads can be caused by many reasons but mainly it is due to the stresses which have occurred during bond wire attachment process and the subsequent stresses after packaging. For example, temperature excursions after packaging exert both lateral and vertical forces on the overall structure. The metal layers of integrated circuit are typically made of soft aluminum that are separated from each other by harder oxide layers. The soft aluminum tends to give under the forces while the harder oxide layers do not. This eventually leads to cracks in the oxide layers. Once an oxide layer cracks, moisture can enter causing corrosion of the aluminum layers and eventually failure of the circuit function. Therefore, the bonding process typically requires the real estate below the bond pad serve only as a buffer against damage that occurs during the bonding process. However, as chip designers try and reduce the size of chips it would be desired to able to use the real estate under the bonding pad for active circuits or interconnects. 
     For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved integrated circuit that effectively allows for use of the real estate under bonding pads for active circuits and interconnects. 
     SUMMARY 
     The above-mentioned problems of current systems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification. The following summary is made by way of example and not by way of limitation. It is merely provided to aid the reader in understanding some of the aspects of the invention. 
     In one embodiment, a semiconductor structure is provided. The structure comprises a top metal layer, a bond pad formed on the top metal layer, a conductor formed below the top metal layer, and an insulation layer separating the conductor from the top metal layer. The top metal layer includes a sub-layer of relatively stiff material compared to the remaining portion of the top metal layer. The sub-layer of relatively stiff material is configured to distribute stresses over the insulation layer to reduce cracking in the insulation layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the description of the preferred embodiments and the following figures in which: 
         FIG. 1  is a partial cross-sectional view of an integrated circuit of one embodiment of the present invention; 
         FIG. 2  is a top view of a portion of a metal layer with gaps of one embodiment of the present invention; and 
         FIGS. 3A through 3G  are partial cross-sectional side views of one method of forming an integrated circuit in one embodiment of the present invention. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the present invention. Reference characters denote like elements throughout Figures and text. 
     DETAILED DESCRIPTION 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the claims and equivalents thereof. 
     In the following description, the term substrate is used to refer generally to any structure on which integrated circuits are formed, and also to such structures during various stages of integrated circuit fabrication. This term includes doped and undoped semiconductors, epitaxial layers of a semiconductor on a supporting semiconductor or insulating material, combinations of such layers, as well as other such structures that are known in the art. Terms of relative position as used in this application are defined based on a plane parallel to the conventional plane or working surface of a wafer or substrate, regardless of the orientation of the wafer or substrate. The term “horizontal plane” or “lateral plane” as used in this application is defined as a plane parallel to the conventional plane or working surface of a wafer or substrate, regardless of the orientation of the wafer or substrate. The term “vertical” refers to a direction perpendicular to the horizontal. Terms, such as “on”, “side” (as in “sidewall”), “higher”, “lower”, “over,” “top” and “under” are defined with respect to the conventional plane or working surface being on the top surface of the wafer or substrate, regardless of the orientation of the wafer or substrate. 
     Embodiments of the present invention provide a method and structure of an integrate circuit that allows the use of real estate under bonding pads for active devices and interconnects. Moreover, embodiments of the present invention provide a structure that can use all the metal layers below the bond pad for functional interconnections of the device. In addition, embodiments of the present invention also provide a structure that allows submicron interconnects lines with a TiN top layer and relatively wide lines capable of carrying high currents to exist simultaneously under a bond pad. 
       FIG. 1  illustrates a partial cross-section view of an integrated circuit  100  of one embodiment of the present invention. In this embodiment, the part of the integrated circuit  100  shown includes a N-channel MOS power device  102 , a N-DMOS device  104  and a NPN bipolar device  106 . As  FIG. 1  also illustrates three conductive layers, which in this embodiment includes a first metal layer M1  108 , a second metal layer M2  110  and a third metal layer M3  112 . The metal layers  108 ,  110 , and  112  can be made of conductive material such as aluminum, copper and the like. Moreover, in another embodiment, at least one of the metal layers  108 ,  110  and  112  is made by a sub-micron process that forms many sub-layers of alternating conductive layers. The third metal layer M3  112  can be referred to as the top metal layer  112 . As illustrated, a bond pad  130  is formed on a surface of the third metal layer M3  112  by patterning a passivation layer  132 . A ball bond wire  114  (bond wire  114 ) can be coupled to the bonding pad  130  to provide an input or output to the integrated circuit  100 . Although, this embodiment, only illustrates three metal layers  108 ,  110  and  112 , other embodiments have more or less metal layers. For example, in an embodiment with more than three metal layers, additional metal layers are formed between metal layers  108  and  110 . Each interconnect metal layer  108 ,  110  and  112  is formed by conventional methods known in the art such as depositing and patterning. 
     As illustrated in  FIG. 1 , vias  116  selectively couple the interconnect metal layers  110  and  108  to form electrical connections between devices  102 ,  104  and  106  of the integrated circuit  100 . Further shown are vias  118  that provide electrical connections to elements of the devices  102 ,  104  and  106  and the first metal layer  108 . 
     In one embodiment, the sub-micron process is used to form metal layer M2  110  and metal layer M3  112 . The sub-micron process uses many sub-layers to form a metal layer. In one embodiment, the sub-layers are alternating layers of Ti, TiN and Al alloys. Further in one embodiment, the top layer of the sub-layers of metal layer  110  (i.e. the sub-layer facing metal  112 ) is a TiN layer  120 . The TiN layer  120  is used in this location because of its low reflective properties that aid in the pattering of metal layer  110 . However, the presence of sub-layer  120  tends to increase the probability that cracks will form in an oxide layer separating the metal layer  110  from metal layer  112 . In particular, because the TiN layer tends to be hard it doesn&#39;t yield when stress is applied. As a result, lateral stresses on the separating oxide tend to form cracks in the separating oxide layer. Further in another embodiment, a layer of TiW forms sub-layer  120 . 
     Embodiments, of the present invention reduce the probability of the cracks forming in the separating oxide layer  122 . In one embodiment, the separating oxide layer  122  (i.e. the oxide layer that separates metal layer  110  from metal layer  112 ) is formed to be relatively thick. In one embodiment, the separating oxide layer  122  is formed to be at least 1.5 um thick. The use of a separating oxide layer  122  that is relatively thick reduces the probabilities of crack forming in the oxide layer  122 . In further another embodiment, the separating oxide layer is generally a dielectric or insulating layer. 
     Moreover in one embodiment, the third metal layer M3  112  includes a relatively hard sub layer  126  of very stiff and hard material. The hard sub-layer  126  is formed adjacent the separating oxide layer  122  and opposite a side of the third metal layer M3 forming the bond pad  114 . The hard sub layer  126  is very stiff and hard compared to aluminum. The hard sub layer distributes lateral and vertical stresses over a larger area of the oxide  122  thereby reducing the propensity of cracking in the oxide  122 . In one embodiment, the material used for the hard sub-layer  126  is TiN. This is due to the compatibility of TiN with conventional sub-micron deposition and etch techniques. In yet another embodiment, the hard sub-layer  126  is a layer of nitride. In one embodiment, the hard sub-layer  126  is approximately 80 nm in thickness. In further other embodiments, materials such as TiW are used for the hard sub-layer  126 . 
     In further another embodiment, the second metal layer M2  110  is formed to have gaps  124  in selected areas. Very wide (lateral widths) of the second metal layer  110  tend to weaken the structure thus creating a higher chance that cracks will occur in the separating oxide layer  122 . In this embodiment, the gaps  124  tend to strengthen the structure by providing pillars of harder oxide. The impact of the gaps  124  on the function of the integrated circuit is minimized by the proper layout. That is, the density of the gaps may be minimized so that a layout design is not constrained significantly. In one embodiment, the gaps  124  take no more than 10% of the total area of the second metal layer M2  110  under the bond pads. In another embodiment, the gaps are oriented such that the impact on current flow through the second metal layer M2  110  is minimized. An example of gaps  124  formed to minimize the impact on the current flow in the second metal layer M2 is illustrated in  FIG. 2 .  FIG. 2 , also illustrates the third metal layer  112 . 
       FIGS. 3A through 3G  illustrates the forming of relevant aspects of one embodiment of the present invention.  FIG. 3A  illustrates a partial cross-sectional side view of the start of the formation of an integrated circuit  300  on a substrate  301 . The partial cross-sectional side view illustrates that integrated circuit  300  in this embodiment includes a N-Channel MOS  302 , a N-DMOS  304  and a NPN device  306 . It will be understood in the art that other types of devices can be formed in the integrated circuit  300  and that the present invention is not limited to only integrated circuits with N-Channel MOS, a N-DMOS and NPN devices. Since the formation of the devices  302 ,  304  and  306  are not a critical part of the present invention,  FIG. 3A  illustrates that they are already formed. These devices  302 ,  304  and  306  are formed by techniques known in the art such as deposition, etching masking and implantation. A first insulating layer  308  is formed overlaying devices  302 ,  304  and  306 . In one embodiment, the insulating layer  308  is a layer of first oxide layer  308 . Vias  310  are formed by techniques known in the art such as masking and etching. The vias  310  are then filled with conductive material to form contacts with the first metal layer  312  and elements of the devices  302 ,  304  and  306 . The first metal layer  312  is formed by first depositing a metal layer and then patterning the first metal layer  312  to form select interconnects. A second insulating layer  314  is then formed overlaying the first metal layer M1  312  and exposed areas of the first oxide layer  308 . In one embodiment, the second insulting layer  314  is a second oxide layer  314 . Vias are formed in the second layer of oxide  314  by masking a surface of the second layer of oxide and etching the vias  316  down to select portions of the patterned first metal layer  312 . The vias  316  are then filled with conductive material. 
     Referring to  FIG. 3B , a second metal layer M2  318  is deposited on a surface of the second oxide layer. In one embodiment, the second metal layer  318  is formed by a sub-micron process comprising a plurality of alternating layers of different metals. In one embodiment, the alternating layers of metal are Ti, TiN and Al alloys. A top sub layer  320  of the second metal layer M2  318  is made of TiN which aids in the pattering of the second metal layer M2  318 . The top sub layer  320  is illustrated in  FIG. 3C . As illustrated in  FIG. 3C , in this embodiment, the second metal layer  318  is then patterned to form gaps  322 . The gaps  322  strengthen the structure by providing pillars of hard oxide. A third insulating layer  324  is then formed overlaying the second metal layer M2. This is illustrated in  FIG. 3D . In one embodiment, the third insulating layer  324  is a third oxide layer  324 . The third oxide layer  324  also fills in the gaps  322 . In one embodiment, the third oxide layer  324  (separating oxide layer  324 ) is formed to be relatively thick. Moreover, in one embodiment the thickness of the separating oxide layer  324  is at least 1.5 um. 
     A layer of relatively stiff and hard metal layer  326  is then formed on the surface of the separating oxide layer  324 . This is illustrated in  FIG. 3E . This hard layer  326  distributes both lateral and vertical stress is over a larger area of the separating oxide layer  324 . Some embodiments of the hard layer  326  are formed by a layer of nitride such as TiN or SiN. In yet another embodiment the hard layer  326  is formed by a layer of TiW. Moreover, in one embodiment, the hard layer  326  is formed to be approximately 80 nm in thickness. Referring to  FIG. 3F  the third metal layer M3  328  is formed overlaying hard layer  326 . In one embodiment, the hard layer  326  is a sub layer formed during the formation of the third metal layer M3  328  by conventional sub-micron deposition and etch techniques. In still another embodiment (not shown), the hard layer  326  is a sub layer of the third metal layer M3  328  formed near the separating oxide layer  324 . A bond pad  330  is then formed on an upper surface of the third metal layer M3  328  by patterning a deposited passivation layer  332 . This is illustrated in  FIG. 3G . Further as illustrated in  FIG. 3G , a ball bond wire  334  is then coupled to the bond pad  330 . Although, not shown in the Figures, vias are formed in the relatively thick oxide  324  so that the top metal layer  328  can also be used to interconnect devices. Moreover, it will be understood in the art that a single integrated circuit may have multiple bond pads and the present invention is not limited to a single bond pad. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.