Abstract:
A test structure is disclosed for locating electromigration voids in a semiconductor interconnect structure having an interconnect via interconnecting a lower metallization line with an upper metallization line. In an exemplary embodiment, the test structure includes a via portion the top of the interconnect via at the upper metallization line. In addition, a line portion extends from the via portion, wherein the line portion connects to an external probing surface, in addition to a probing surface on the lower metallization line, thereby allowing the identification of any electromigration voids present in the interconnect via.

Description:
BACKGROUND  
         [0001]    The present invention relates generally to the manufacture of integrated circuit devices and, more particularly, to a test structure for locating electromigration voids in dual damascene interconnects.  
           [0002]    Integrated circuits are typically fabricated with multiple levels of patterned metallization lines, electrically separated from one another by interlayer dielectrics containing vias at selected locations to provide electrical connections between levels of the patterned metallization lines. As these integrated circuits are scaled to smaller dimensions in a continual effort to provide increased density and performance (e.g., by increasing device speed and providing greater circuit functionality within a given area chip), the interconnect linewidth dimension becomes increasingly narrow, which in turn renders them more susceptible to deleterious effects such as electromigration.  
           [0003]    Electromigration is a term referring to the phenomenon of mass transport of metallic atoms (e.g., copper or aluminum) which make up the interconnect material, as a result of electrical current conduction therethrough. More specifically, the electron current collides with the metal ions, thereby pushing them in the direction of current travel. Over an extended period of time, the vacated atoms tend to cause void formations typically at one end of a line, whereas the accumulation of atoms at the other end of the line tend to cause hillock formations. Such deformation degrades line resistance and, in some instances, leads to open circuits, short circuits and device failure. This phenomenon becomes increasingly more significant in integrated circuit design, as relative current densities through metallization lines continue to increase as the linewidth dimensions shrink.  
           [0004]    In dual damascene interconnects, electromigration-induced voiding may occur in either the via portion or the line portion of the dual damascene structure. However, the root cause(s) of the electromigration voiding may differ, depending upon the specific location of the void. For example, a void located near the bottom of a via usually indicates defects in the via, or perhaps poor coverage of liner material at the bottom of the via. On the other hand, voiding in the line may suggest a problem at the interface between the capping layer and the metallization. As a result, it is desirable to distinguish between the two failure locations in order to identify the root cause of electromigration fails, and to modify the fabrication processes for reliability improvement.  
           [0005]    Unfortunately, conventional probing structures presently in existence do not allow for a distinction to be made between the two types of failure mechanisms discussed above, since electromigration tests for a via void and a line void yield the same electrical failure signature. Thus, to correctly determine the void location in a dual damascene structure, a failure analysis of a cross-sectional portion of the structure by scanning electron microscope (SEM) may be necessary. Such an analysis, however, is both costly and time consuming.  
         BRIEF SUMMARY  
         [0006]    The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by an electromigration test structure with an interconnect via added to the top of the interconnect of interest. In an exemplary embodiment, the test structure includes a via portion on the top of the interconnect via at the upper metallization line. In addition, a line portion extends from the via portion, wherein the line portion connects to an external probing surface, in addition to a probing surface on the lower metallization line, thereby allowing the identification of any electromigration voids present in the interconnect via.  
           [0007]    In a preferred embodiment, the semiconductor interconnect structure is a dual damascene structure. The via portion overlaps the interconnect via, wherein the via portion is centered over a top corner of the interconnect via.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures:  
         [0009]    [0009]FIG. 1 is a cross-sectional view of a dual damascene structure, wherein a conventional electromigration probe test is performed thereupon;  
         [0010]    [0010]FIG. 2 is a cross-sectional view of an interconnect structure used for locating electromigration voids in dual damascene structures, in accordance with an embodiment of the invention;  
         [0011]    [0011]FIG. 3 is a top view of the interconnect structure shown in FIG. 2; and  
         [0012]    [0012]FIG. 4 is a cross-sectional view of an additional interconnect structure added to the structure of FIG. 2, in accordance with an alternative embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    Referring initially to FIG. 1, there is shown an exemplary dual damascene structure  10 , as may be commonly found within an integrated circuit device. The dual damascene structure  10  includes interconnect vias  12   a ,  12   b , that serve to interconnect an upper level metallization line  14  with lower level metallization lines  16   a ,  16   b . For ease of illustration, the interlevel dielectrics that insulate the upper and lower level metallization layers from one another are not shown in the Figures.  
         [0014]    As can be seen in FIG. 1, the dual damascene structure  10  has a pair of electromigration induced voids therein, as a result of a generally unidirectional current traveling therethrough (indicated by the e −  arrow). A first void  18  (hereinafter referred to as the via void) is formed at the bottom of interconnect via  12   a , while a second void  20  (hereinafter referred to as the line void) is formed within upper metallization line  14 . In conventional electromigration testing, a technique such as a four-point probe is used to supply a current through the dual damascene structure  10  through a first pair of probes  22   a ,  22   b . A voltmeter is then used to measure the voltage across the structure  10  through a second pair of probes  24   a ,  24   b . Through Ohm&#39;s law the resistivity of the structure  10  is determined, thereby determining the presence of electromigration voids.  
         [0015]    However, it will be appreciated that when both the force (current) and sense (voltage) lines are wired out across both the interconnect vias  12   a ,  12   b  and the metallization line  14 , as shown in FIG. 1, it is impossible to distinguish between the via void  18  and the line void  20 , since both the voids are located between the voltage and current probes. Therefore, in accordance with an embodiment of the invention, there is disclosed an interconnect structure that provides an additional wireout location upon which to apply probes for electromigration testing. In this manner, both the vias and the metallization lines in a dual damascene structure may be tested independently to localize any voids discovered therein.  
         [0016]    Referring now to FIG. 2, there is a shown a cross-sectional view of the dual damascene structure of FIG. 1, this time with the addition of an interconnect test structure  30 . The interconnect test structure  30  includes a via portion  32  disposed above interconnect via  12   a , and is maintained in electrical contact therewith. Interconnect test structure  30  also includes a line portion  34 , extending from the via portion  32 , and is preferably disposed within the same metallization level as upper metallization line  34 .  
         [0017]    As shown in the top view of FIG. 3, the via portion  32  of interconnect test structure  30  is oriented in an overlapping configuration with respect to interconnect via  12   a . More specifically, the center of via portion  32  is aligned over one of the comers of interconnect via  12   a . In so doing, several advantages are realized. First, the offset disposition of via portion  32  results in less of an impact of the total volume of the metallization (e.g., copper) over the interconnect via  12   a . Second, any additional stresses introduced over interconnect via  12   a  due to thermal expansion mismatch is reduced, since most of the volume of via portion  32  is located over inter level dielectric (ILD) material. Third, the offset relationship between the two via will ensure that the liners thereof will connect. Thus, the structure will function even if a stress void is generated under via portion  32 .  
         [0018]    Thus configured, the interconnect test structure  30  provides an additional wire-out location for the upper metallization line  14 . As a result, the resistivity of the upper metallization line  14  may be tested independently of, for example, interconnect via  12   a , and vice versa. By placing one of each of the force and sensing probes  22   a ,  24   a  on lower metallization line  16  and the other of the force and sensing probes  22   b ,  24   b  on line portion  34  (in FIG. 2), it can be seen that the resistivity of interconnect via  12   a  may be measured independently, thereby locating any electromigration voids present therein. In the example depicted, such a testing configuration would allow for the isolated detection of first void  18 .  
         [0019]    In another embodiment, an additional interconnect test structure  40  may be provided at the opposite end of upper metallization line, as shown in FIG. 4. As with interconnect test structure  30 , the additional interconnect test structure  40  includes a via portion  42  disposed above interconnect via  12   b , and is maintained in electrical contact therewith. Interconnect structure  40  also includes line portion  44 . Naturally, the additional interconnect test structure  40  allows for isolated electromigration testing of interconnect via  12   b  in a manner similar to the testing of interconnect via  12   a . In addition, the upper metallization line  14  may be isolated from interconnect vias  12   a  and  12   b  for electromigration testing and the detection of second void  20 . Force probe  22   a  and sense probe  24   a  would contact line portion  34 , while force probe  22   b  and sense probe  24   b  would contact line portion  44 .  
         [0020]    Through the use of the above described test structure embodiments, the different failure mechanisms as between via voiding and line voiding may be determined by isolating the locations of the voids themselves. The line portions of each interconnect test structure are used as additional wire outs for the four-point probe structures. Accordingly, individual vias may be singled out for electromigration testing, as well as the metallization line in between vias. Once again, the offset nature of the via portions of the test structures reduces the total volume of conductive fill (e.g., copper) over the interconnect vias, as well as reduces the mechanical stress thereupon in view of thermal expansion mismatch. Furthermore, the offset between the two vias will ensure the contact between the conductive liners of the two vias.  
         [0021]    It should also be pointed out that although the test structure embodiments described herein provide additional wire out locations, vias and metallization lines for electromigration testing purposes, it is not necessarily intended for these structures to increase the overall number of signal communication pathways formed within the integrated circuit device. By the same token, however, it is contemplated that such a testing structure could be adapted for an alternative or an additional use than originally intended, such as a redundant communication pathway.  
         [0022]    While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.