Abstract:
A structure for reducing electromigration cracking and extrusion effects in semiconductor devices includes a first metal line formed in a first dielectric layer; a cap layer formed over the first metal line and first dielectric layer; a second dielectric layer formed over the cap layer; and a void formed in the second dielectric layer, stopping on the cap layer, wherein the void is located in a manner so as to isolate structural damage due to electromigration effects of the first metal line, the effects including one or more of extrusions of metal material from the first metal line and cracks from delamination of the cap layer with respect to the first dielectric layer.

Description:
BACKGROUND 
     The present invention relates generally to semiconductor device manufacturing processes and, more particularly, to a structure and method of reducing electromigration cracking and extrusion effects in semiconductor devices. 
     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. 
     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 unidirectional or DC 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 accumulation of metal at the anode end of the interconnect material significantly increases the local mechanical stress in the system. This in turn and may lead to delamination, cracking, and even metal extrusion from the metal wire, thereby causing an electrical short to adjacent interconnects. Electromigration becomes increasingly more significant in integrated circuit design, as relative current densities through metallization lines continue to increase as the linewidth dimensions shrink. 
     For example,  FIG. 1  illustrates a scanning electron micrograph (SEM) cross-sectional image of a test structure  100  taken near the anode end of a failed interconnect line  102  included therein, as a result of electromigration stress. The current carrying interconnect line  102  is disposed between the adjacent “extrusion monitor” lines  104   a ,  104   b , that do not carry current. As indicated above, there are two phenomena associated with the illustrated electromigration fail. First, a metal/cap layer interface is delaminated by the high stress, with the delamination spanning across the gap between adjacent wire  104   b . Second, metal extrusion of line  102  occurs and reaches the adjacent wire  104   a , causing an electrical short. 
     Although electromigration-induced extrusion failure is not particularly prevalent in previous technologies using silicon dioxide (SiO 2 ) and dense SiCOH (carbon doped oxide) as dielectric materials (and was generally treated as irrelevant since extrusion typically occurs long after an initial electromigration failure, defined by 20% resistance increase), this phenomenon has been more frequently observed during the evaluation of advanced technologies using ultra low-K dielectrics. A low-k dielectric material is one in which the relative dielectric constant is less than 4, while an ultra low-k dielectric is one in which the relative dielectric constant is less than 3. Accordingly, it would be desirable to be able to minimize the adverse impacts of extrusion/delamination related damage associated with the electromigration phenomenon. 
     SUMMARY 
     The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated, in an exemplary embodiment, by a structure for reducing electromigration cracking and extrusion effects in semiconductor devices, including a first metal line formed in a first dielectric layer; a cap layer formed over the first metal line and first dielectric layer; a second dielectric layer formed over the cap layer; and a void formed in the second dielectric layer, stopping on the cap layer, wherein the void is located in a manner so as to isolate structural damage due to electromigration effects of the first metal line, the effects including one or more of extrusions of metal material from the first metal line and cracks from delamination of the cap layer with respect to the first dielectric layer. 
     In another embodiment, a semiconductor device structure includes a first wiring level, the first wiring level comprising a first metal line formed in a first dielectric layer, and a second metal line formed in the first dielectric layer, adjacent the first metal line; a first cap layer formed over the first wiring level; and a second wiring level formed over the first cap layer, the second wiring level comprising a second dielectric layer, a void formed in the second dielectric layer, stopping on the cap layer, and sealing dielectric material formed over the second cap layer, the sealing dielectric material configured to pinch off upper portions of the void while maintaining lower portions of the void; wherein the void is located in a manner so as to isolate structural damage due to electromigration effects of the first metal line, the effects including one or more of extrusions of metal material from the first metal line and cracks from delamination of the first cap layer with respect to the first dielectric layer. 
     In another embodiment, a method of reducing electromigration cracking and extrusion effects in semiconductor devices includes forming a first metal line in a first dielectric layer; forming a cap layer over the first metal line and first dielectric layer; forming a second dielectric layer over the cap layer; and forming a void formed in the second dielectric layer, stopping on the cap layer, wherein the void is located in a manner so as to isolate structural damage due to electromigration effects of the first metal line, the effects including one or more of extrusions of metal material from the first metal line and cracks from delamination of the cap layer with respect to the first dielectric layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
         FIG. 1  is a scanning electron micrograph (SEM) cross-sectional image of a test structure taken near the anode end of a failed interconnect line, as a result of electromigration stress; 
         FIG. 2(   a ) is a cross-sectional view of a structure for reducing electromigration cracking and extrusion effects in semiconductor devices, in accordance with an embodiment of the invention; 
         FIG. 2(   b ) illustrates the operation of the structure of  FIG. 2(   a ), with respect to containment of an electromigration induced extrusion and delamination defect; 
         FIG. 3  is a top view of the structure of  FIG. 2(   a ); and 
         FIGS. 4(   a ) through  4 ( d ) are a sequence of cross-sectional views illustrating an exemplary process flow for forming the structure of  FIG. 2(   a ), in accordance with a further embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein is a structure and method of reducing electromigration cracking and extrusion effects in semiconductor devices. Briefly stated, a small opening is intentionally formed within an insulating dielectric layer between interconnects (specifically, near the anode region of the interconnects), which serves as a local crack stop to avoid crack propagation, as well as a reservoir to accommodate metal extrusion induced by electromigration. In so doing, any such extrusions are contained within the gap and thus prevented from creating shorts to adjacent interconnect lines. Further, any delamination/cracking at the anode end of the interconnect metal is contained at the gap, and prevented from being spread any further into the dielectric material. 
     Referring now to  FIG. 2(   a ), there is shown a cross-sectional view of a semiconductor device  200 , including a structure for reducing electromigration cracking and extrusion effects in semiconductor devices, in accordance with an embodiment of the invention. As shown therein, a first dielectric layer  202  (e.g., SiO 2 , SiCOH, low-K material, etc.) has metal interconnect lines  204   a ,  204   b  formed therein, representing a lower wiring level. The interconnect lines  204   a ,  204   b  include a suitable semiconductor metal wiring material such as, for example, aluminum, copper, gold, silver and alloys thereof, and is also surrounded on the bottom and sides thereof with one or more diffusion barrier layers  206  (e.g., TaN, Ta, TiN, Ti, Ru, RuTa, etc.). The interconnect lines  204   a ,  204   b , and first dielectric layer  202  are further covered with a dielectric cap material  208  (e.g., Si 3 N 4 , SiC, SiCN, SiCH, etc.). To this point, the semiconductor structure  200  is formed in accordance with processing techniques known in the art. 
     In the example illustrated, only the interconnect line  204   a  is assumed to be susceptible to electromigration induced damage (i.e., conducts current in a single direction, where the anode end of the line  204   a  is shown in the figure). A second dielectric layer  210  is formed over the cap layer  208 , representing an upper wiring level. For purposes of simplicity, no interconnect lines are depicted in the second dielectric layer  210 , although it will be appreciated that such lines may be formed in that layer. Because line  204   a  in the first dielectric layer  202  is susceptible to electromigration induced extrusions and cracking, a void (gap)  212  is intentionally formed within dielectric layer  210  around the anode end of line  204   a , the void  212  landing on top of the cap layer  208 . At least a portion of the void  212  is located between interconnect  204   a  and adjacent interconnect  204   b.    
     As discussed in further detail herein, the void  212  may be lined with an optional liner/pore-sealing material  214  once the void  212  has been initially patterned after an upper level wiring cap layer  216  is formed over dielectric  210 . Then, a sealing dielectric material  218  is used to pinch off the upper portions of the void  212  before additional layers are formed in accordance with existing processes of record. 
       FIG. 2(   b ) illustrates the operation of the structure of  FIG. 2(   a ), with respect to containment of an electromigration induced extrusion and delamination defect. As is shown, electromigration-induced extrusion  220  is depicted in the anode region of interconnect  204   a , extending in the direction of adjacent interconnect wire  204   b.  However, the metal extrusion  220  is contained within the left portion of the void  212  shown in  FIG. 2(   b ) and does not short to the adjacent  204   b . Moreover, a delamination  222  of the interconnect metal and cap layer  208  is also confined by the right portion of the void  212  around the stressed wire  204   a  and does not propagate further into any adjacent wires within the first dielectric layer  202 . 
     Referring now to  FIG. 3 , there is shown a top view of the exemplary structure of  FIG. 2(   a ). In the example depicted, the void  212  is formed locally around the ends of the interconnect structure  204   a  where extrusions occur, so as minimize space restrictions. For ease of illustration, the cap layer  208  is not depicted in  FIG. 3 . In addition,  FIG. 3  also illustrates the formation of vias  224  formed above (or below) the interconnect  204   a , for connection to upper (or lower) wiring levels. In the event that it is known ahead of time which end of the line  204   a  is the anode end, then the void  212  can be even further localized to that specific end. Alternatively, the void could  212  also be formed on the cap layer (not shown in  FIG. 3)  above the interconnect line  204   a  in a manner so as to surround the interconnect line  204   a.    
     Finally,  FIGS. 4(   a ) through  4 ( d ) are a sequence of cross-sectional views illustrating an exemplary process flow for forming the structure of  FIG. 2(   a ), in accordance with a further embodiment of the invention. In  FIG. 4(   a ), the lower wiring level (dielectric layer  202 , interconnect lines  204   a ,  204   b , diffusion barrier  206  and cap layer  208 ) is formed, followed by the upper wiring level (dielectric layer  210 , associated metal lines (not shown) and cap layer  216 ). The dielectric layer  210  in the upper metal level may be the same material as the dielectric layer  202  in the lower wiring level and can be applied, for example, by chemical vapor deposition (CVD) or a spin-on technique. Then, in  FIG. 4(   b ), the void  212  around the anode end of the interconnect  204   a  is patterned in the dielectric layer through lithography and etching process, landing on the cap layer  208 . 
     As then shown in  FIG. 4(   c ), an optional pore-sealing liner material  214  is deposited over the upper wiring level and the sidewalls and bottom of the void  212 . The pore-sealing liner material  214  may be desirable where the dielectric layer  210  is a porous material, such as porous SiCOH, for example. The pore-sealing liner material  214  may include the same material as used for the metal line diffusion barrier  206  (e.g., TaN, TiN, etc.) or could also be an oxide material. In any case, the sealing dielectric material  218  is then deposited in  FIG. 4(   d ) in order to pinch off the upper portions of the voids  212 . The sealing dielectric material  218  is deposited in a manner that maintains the integrity of the bottom of the voids  212  (i.e., does not completely fill the voids  212  with dielectric material). One suitable deposition method in this regard is through chemical vapor deposition (CVD). 
     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.