Abstract:
An interconnect structure includes: a plurality of dielectric layers having aligned process control monitor (PCM) pads, and a conductive structure above a topmost one of the PCM pads. The conductive structure electrically connects the topmost PCM pad to a device under test above a level of the topmost PCM pad. The conductive structure is sized and shaped so as to leave a majority portion of the topmost PCM pad exposed for access by a test probe.

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 60/805,270, filed Jun. 20, 2006, which is incorporated by reference herein in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to semiconductor devices and fabrication techniques generally, and more particularly relates to interconnect structures and test methods. 
   BACKGROUND 
   In order to achieve high-integration and high-speed, dimensions of semiconductor integrated circuits have been reduced, and various materials and techniques have been proposed and used during fabrication. For example, dual damascene technology and copper conductors are applied to reduce resistances and resistance-capacitance (RC) delay of interconnect structures in ICs. As ICs are made smaller, and the distance between adjacent lines is reduced, low-k (low dielectric constant) dielectric materials are used in advanced copper interconnect technology to reduce these delays. 
   Interconnect structures of semiconductor ICs connect the various active devices and circuits of the IC to a plurality of conductive pads on the external surface of the die. Multi-level interconnect structures have been developed that accommodate the advances in active-device density by more effectively routing conductive paths between the active devices and the surface of the die. In typical IC designs, five or more individual interconnect levels of conductive paths may be used to accommodate the active-device density. Multi-level interconnect structures arrange the metallization lines in multiple layers. The metallization lines of each individual level are formed in an interlevel dielectric (ILD) material. The ILD electrically isolates the metallization lines from one another within each level of the interconnect structure and electrically isolates metallization lines in adjacent levels. 
   Damascene processes are routinely used in back-end-of-line (BEOL) processing for fabricating multi-level interconnect structures. In a damascene process, trenches and vias are etched in a layer of ILD and filled with a conductive material, such as copper (Cu) or a Cu-based alloy, to create conductive lines and vertical conductive paths between the interconnect lines in different levels. 
   The conductive paths of the multi-level interconnect structures terminate in bond pads at the surface of the IC. The bond pads are relatively large metal areas distributed about the die. Bond pads are used to establish electrical contact between the integrated circuits and either a package substrate of an IC package or a probe pin (that is used for wafer acceptance testing, or WAT). The pads used during WAT are also referred to as, “process control monitor (PCM) pads”. A probe makes an electrical contact between a probe pin and the bond pads, so voltage or current can be applied to test for device functionality and performance. Large bond pads allow longer probe needles, thus increasing parallel testing capability. The bond pads that are used for WAT may be distributed in the scribe lines between dies. These scribe lines are severed during the die singulation process, with the cuts passing through the bond pads. 
   A conventional approach for configuring the WAT bond pads of the interconnect structure is to include bond pads in the scribe line on each interconnect layer, aligned beneath the bond pads in scribe line of the top metal layer, and to include metal-filled vias connecting the bond pads in each of the interconnect levels, aligned beneath each bonding pad or probe pad in the top metal layer. The bond pads in the first interconnect (M1) layer may be used for in situ testing before the second (M2) through top metal (MT) layers are formed.  FIG. 1A  is a plan view showing a portion of a scribe line  102  between two IC&#39;s  100 , with a copper bond pad structure  104  (layers M1 to MT). 
   An aluminum pad  114  is formed over the pad  104  of the top metal (MT) layer. The aluminum pad  114  may be connected to a device under test (DUT)  116  in the aluminum (MT+1) layer. For the WAT or circuit probing (CP) test, the probe  118  is in direct contact on the Al Pad  114 .  FIG. 1B  is a plan view of the aluminum pad layer (MT+1) above the copper pad  104  shown in  FIG. 1A .  FIG. 1C  is a cross sectional view taken along section line IC-IC of  FIG. 1B . Thus, the PCM pad  120  includes the duplicated structure  104  (which may have width and length of 50 μm/70 μm, respectively) layer by layer in layers M1-M9 plus the Al pad  114 . 
   The dicing (or singulation) process can produce a large mechanical stress, which may be dependent on numerous conditions, including: cut width, die saw speed, die saw temperature, die saw pressure, etc. 
   Conventional multi-level interconnect structures have been susceptible to failure due to cracking when the PCM pad  120  is cut by a die saw. Experience has shown that during singulation, the dielectric in the scribe lines  102  may crack in the vicinity of the bond pads  120 , and that these cracks may propagate to the dies  100 . This problem becomes more acute when low-k dielectric materials (including extreme low k, ELK and ultra low k, ULK) are used for the ILD material, because low-k dielectric materials are more brittle than high k dielectrics. Cracking is severe in the aluminum layer. 
   It is desirable to decrease the failure rate during the die saw process, which in turn will result in a higher production yield. 
   SUMMARY OF THE INVENTION 
   In some embodiments, an interconnect structure comprises a plurality of dielectric layers having aligned process control monitor (PCM) pads in at least one scribe line. A conductive structure above a topmost one of the PCM pads electrically connects the topmost PCM pad to a device under test above a level of the topmost PCM pad. The conductive structure is sized and shaped so as to leave a majority portion of the topmost PCM pad exposed for access by a circuit probe. 
   In some embodiments, a wafer, comprises a substrate having a plurality of integrated circuits formed above the substrate, and at least one scribe line between two of the integrated circuits. A plurality of dielectric layers are formed in the at least one scribe line having aligned process control monitor (PCM) pads formed therein. A conductive structure above a topmost one of the PCM pads electrically connects the topmost PCM pad to a device under test above a level of the topmost PCM pad. The conductive structure is sized and shaped so as to leave a majority portion of the topmost PCM pad exposed for access by a circuit probe. 
   In some embodiments, a structure comprises a plurality of dielectric layers having aligned process control monitor (PCM) pads in at least one scribe line. The topmost PCM pad has a metallized region and a non-metallized region. A conductive structure above a topmost one of the PCM pads electrically connects the topmost PCM pad to a device under test above a level of the topmost PCM pad. The conductive structure overlies at least a portion of the metallized region, but does not overlie the non-metallized region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a plan view of conventional PCM pads in a scribe line. 
       FIG. 1B  is a plan view of a conventional aluminum pad, which overlies the top PCM pad of  FIG. 1A . 
       FIG. 1C  is a cross sectional view taken across section line  1 C- 1 C of  FIG. 1B . 
       FIGS. 2A-2C  show three substrates with scribe lines having respectively different pad densities. 
       FIG. 3  is an enlarged view of the bond pad shown in  FIG. 2B . 
       FIG. 4A  is a plan view of a PCM pads in a scribe line, according to an exemplary embodiment of the invention. 
       FIG. 4B  is a plan view of an aluminum pad, which overlies the top PCM pad of  FIG. 4A . 
       FIG. 4C  is a cross sectional view taken across section line  4 C- 4 C of  FIG. 4B . 
       FIG. 4D  is a plan view showing the aluminum pad structure of  FIG. 4B , overlying the PCM pads of  FIG. 4(   a ). 
       FIG. 5  is a variation of the embodiment of  FIG. 4B . 
       FIG. 6  is another variation of the embodiment of  FIG. 4B . 
   

   DETAILED DESCRIPTION 
   This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. 
   In the die saw process of low-K (LK) generation semiconductor products, cracking has been observed. The cracking mechanism is complex, but the inventor has determined that it is strongly related to the metal density when the die saw blade moves across the PCM pad in the scribe lines of the wafer during singulation. When the die saw blade moves across the larger metal structure of the PCM pads (having higher metal amount/density) the probability of crack formation is increased. In the scribe line structure, the PCM pad has a higher amount of metal and may result in crack damage in the dicing process. 
     FIGS. 2A-2C  show some examples of wafers having dies  200  with scribe lines  202  therebetween. These examples introduce the concept of pad density. The bond pad structures in  FIGS. 2A-2C  includes metal areas and non-metallized areas. The term, “pad density” is used herein to denote the ratio of the total metal area within a pad structure (as viewed from above in a plan view) to the total area of that pad structure. In  FIG. 2A , there are no PCM pads in the scribe line  202  (0% metal density). Cracking is not observed during die sawing. 
   In  FIG. 2B , the metal density of each bond pad  204  is about 64%. Within each bond pad  204 , each of the 12 non-metallized areas (“slots”) is about 40 μm by 2.6 μm. The remaining area has metal. The pad density equals [(70×50)−12×(40×2.6)]/(70×50)=0.64. Some cracking is observed. 
   In  FIG. 2C , the bond pads  206  are solid metal regions, with no non-metallized areas (i.e., pad density equals 100%). Cracking in the scribe line  202  is observed, and the cracks may propagate to the dies  200 . 
     FIG. 3  shows is an enlarged view of the pad configuration of  FIG. 2B , including a metallized region and a non-metallized region. This example includes a copper pad  204  having 64% pad density (because 36% of the area is occupied by 12 non-metallized 40×2.6 μm slots). In this example the non-metallized region comprises a plurality of distinct slots. In other embodiments, the non-metallized region may include a smaller number of wider slots, a single wider opening, or a larger number of smaller apertures. 
   In addition to the effects of individual bond pads  104  with high metal density, there is also a cumulative effect if many of the pad layers in the stacked configuration have a relatively high metal density. The term “cumulative density” is used herein to denote an average (e.g., arithmetic mean, median or mode) of the pad densities of the PCM pads  104 , in each level beginning at the first (M1) interconnect level and ending with the top metal (MT) level, and including the aluminum pad  114 . Due to the cumulative effect in a multi-level structure  120  ( FIG. 1C ) having a large cumulative pad density, the cracking failure rate is a problem, particularly if the cumulative pad density is about 65% or greater. Thus, the cumulative pad density can be calculated as the average of the pad densities of a sequence of aligned bond pads  104  in each level from M1 to MT, plus the Al level  114 . The likelihood of cracks during the die saw process is reduced when the cumulative pad density is about 50% or less. 
   As described above, the PCM pad structure  120  has a larger metal density and also results in the severe cracking issue in the dicing process. 
   Table 1 shows the volume of the metal in the PCM pad  120  in several of the layers of an interconnect structure for a 65 nm wafer (taking into account the respective metal thickness in each layer). 
   
     
       
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Layer Number 
               Metal Dimension (μm 3 ) 
             
             
                 
                 
             
           
           
             
                 
               4M 
               1882 
             
             
                 
               5M 
               2374 
             
             
                 
               6M 
               2867 
             
             
                 
               7M 
               3360 
             
             
                 
               8M 
               5376 
             
             
                 
               Al 
               4200 
             
             
                 
               8M + Al 
               9576 
             
             
                 
                 
             
           
        
       
     
   
   Table 2 shows the metal thickness for various layers (where Mx indicates an inter-metal layer, and MT indicates the top metal layer). Notably, the volume of the metal is greater in the upper layers, and layers 8M and Al have the greatest volume. 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
               Layer 
               Thickness (Å) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               M1 
               1800 
             
             
                 
               Mx(2-7) 
               2200 
             
             
                 
               MT(8) 
               9000 
             
             
                 
               Aluminum 
               12000 
             
             
                 
                 
             
           
        
       
     
   
   Based on the determination that the cracking problem is more severe when the bond pad density is greater, the inventor has further determined that the duplicated pad  114  in the aluminum layer occupies almost half the metal volume of the fully stacked PCM pad structure  120 . A major portion of the aluminum pad  114  duplicates the footprint of the top (MT) bond pad  104 , with which it is in conductive contact. Therefore, the PCM pad in the aluminum layer can be modified as shown in  FIGS. 4A-4D . 
   Instead of using an aluminum pad  114  with 100% density as shown in  FIGS. 1 and 3 , the aluminum pad  114  is replaced by a smaller block or contact  430  to reduce the fully stacked (cumulative) PCM metal density (volume) in this configuration. The contact  430  is above the topmost PCM pad (In this context, “above” refers to the level or height of the contact  430 , and does not require that the contact  430  covers the topmost PCM pad.). In  FIG. 4A , the top Cu layer  404  (M T ) can have the same configuration as described above with reference to  FIG. 1A . In  FIGS. 4B  and sectional view  4 C, the aluminum layer is patterned to provide a contact  436  extending downward (through the passivation layer  403 ) to provide direct conductive contact between the DUT  416  and the top Cu PCM pad  404  of layer M T . It will be understood by one of ordinary skill that as used herein, the term “device under test” (DUT) is not limited to a device that is currently under test, but also encompasses a device to be tested in the future. 
   In this configuration, the majority of the aluminum overlying the topmost PCM pad  404  is omitted. In some embodiments, the duplication of aluminum overlying copper is reduced by more than 50%. In some embodiments, the duplication of aluminum overlying copper is reduced by about 60% to about 85%. 
   In some embodiments, a conductive structure  430  contacts a first portion of the topmost PCM pad  404  near the device under test  416  without extending over a second portion of the topmost PCM pad distal from the device under test. 
   In  FIG. 4B , the Cu contact pads  404  of  FIG. 4A  are omitted from the drawing for clarity, but the relationship between the aluminum structure  430  and the top contact pad  404  is shown in plan view in  FIG. 4D . 
   This small (aluminum) block or contact  430  electrically connects the DUT  416  with the lower-level (M T ) pad  404  as shown in  FIG. 4C . By substituting the small block or contact  430  for the large aluminum pad  114  ( FIG. 1 ), it is possible to reduce the duplicated metal density (i.e., to reduce the aluminum overlying a similarly shaped copper pad). Preferably, the small (aluminum) block or contact  430  is as small as possible (e.g., a line width of about 3 μm or less). Also the contact extension lines  434  may be positioned at the edge of PCM pad  404  (to avoid the duplicated pad density shown in  FIG. 1C ). 
   As shown in the plan view of  FIGS. 4B and 4D , the aluminum layer may optionally include one or more contact extension lines  434 , which extend over the bond pad  404  of the M T  layer, ensuring a desired contact area. The shape, length and number of these contact extensions  434  may be varied. In some preferred embodiments, the contact extension lines  434  are parallel to the length of the scribe line  402 . In other embodiments (not shown), the contact extension lines are not parallel to the scribe line  402 , but they do not cross over the centerline of the scribe line (through which the die saw passes). 
     FIG. 4D  is a plan view showing the aluminum conductive structure or contact  430  and the underlying Cu (MT) layer  404 , which it contacts. In the example of  FIG. 4D , the contact extension lines  434  are placed on the aluminum layer  432  at or near the maximum displacement from the center of the scribe line  402 . In this example, because the die saw passes through the center of the scribe line  402 , the die saw only passes through the aluminum in the vertical contact portion  432 ,  436 , and does not pass through the contact extension lines  434 . 
     FIGS. 4C and 4D  show that the conductive structure  430  has a contact portion  436  extending substantially vertically from the device under test  416 , through a passivation layer  403 , to contact the topmost PCM pad  404 . The contact portion  436  has a width W 4  ( FIG. 4D ) that is small relative to a length W 3  of the topmost PCM pad  404 . For example, in some embodiments, the ratio of W 4 :W 3  may be about 0.2 or less. In some embodiments, the ratio of W 4 :W 3  may be about 0.15 or less. 
   In some embodiments, the topmost PCM pad  404  has a metallized region (e.g., the solid region at the perimeter) and a non-metallized region (e.g., some or all of the central region); the conductive structure  430  overlies at least a portion of the metallized region, but does not overlie the non-metallized region. In other embodiments, if the PCM pad has an alternative configuration (not shown) with a non-metallized region at a different location than the center, the conductive structure  430  can be modified to contact the metallized region, without overlying the non-metallized region. 
   In addition, the shape of conductive structure or contact  430  is not limited to the example in  FIGS. 4B-4D . For example, the distal ends of the extension lines may be connected to form a ring  530  around the perimeter of the top metal (MT) PCM pad  504 , as shown in  FIG. 5 . More generally, for PCM pads  504  having a variety of different shapes, the contact structure  530  may be shaped as a polygon extending around a perimeter of the topmost PCM pad  504 , with an opening that leaves an interior region of the topmost PCM pad exposed. Many other configurations are contemplated. 
   In some embodiments, the aluminum pattern is configured to provide the minimum conductive path between the DUT and the top (M T ) Cu PCM pad. For example, the contact extension lines  434  may be omitted, in which case the cross sectional view would appear as shown in  FIG. 4C , throughout the width of the bond pad  404 .  FIG. 6  shows an example in which the contact extension lines are omitted, but the width of the aluminum conductive structure  630  is increased slightly, to enhance the conductive contact between the aluminum conductive structure  630  and the top (M T ) Cu PCM pad  604 . 
   Alternatively, the contact extension lines  434  may be substantially shorter than those shown in  FIGS. 4B and 4D , with a minimum length to reduce a likelihood of delamination or loss of contact between the aluminum  436  and underlying copper PCM pad  404 . 
   Although an example is provided in which the PCM bond pads  404  are made of copper and the MT+1 layer  430  is aluminum, one of ordinary skill can readily apply the structures described above to other configurations, in which different metals are used. 
   Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.