Patent Publication Number: US-9406634-B2

Title: Package structure and method of forming the same

Description:
PRIORITY CLAIM 
     The present application is a divisional of U.S. application Ser. No. 13/610,050, filed Sep. 11, 2012, which is a continuation application of U.S. application Ser. No. 13/095,185, filed Apr. 27, 2011, now U.S. Pat. No. 8,288,871, issued Oct. 16, 2012, which are incorporated herein by reference in their entireties. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to U.S. application Ser. No. 13/035,586, entitled “EXTENDING METAL TRACES IN BUMP-ON-TRACE STRUCTURES,” filed on Feb. 25, 2011, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Bump-on-Trace (BOT) structures have been used in flip chip packages, wherein metal bumps are bonded onto narrow metal traces in package substrates directly, rather than bonded onto metal pads that have greater widths than the respective connecting metal traces. The BOT structures require smaller chip areas, and the manufacturing cost of the BOT structures is relatively low. However, there are technical challenges related to BOT structures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A and 1B  illustrate cross-sectional views of a package structure in accordance with an embodiment. 
         FIG. 1C  shows a top view of a copper post, in accordance with some embodiments. 
         FIG. 1D  illustrates an exemplary perspective view of the metal package structure of  FIGS. 1A and 1B , in accordance with some embodiments. 
         FIG. 1E  shows the package structure of  FIGS. 1A and 1B  with an axis pointing to the center O of a die, in accordance with some embodiments. 
         FIG. 1F  shows a cross-sectional view of the package structure of  FIG. 1E , in accordance with some embodiments. 
         FIG. 2A  shows a number of bump-on-trace (BOT) structures on a die, in accordance with some embodiments. 
         FIG. 2B  shows a table of data comparing the highest stresses in a dielectric layer and the metal trace of each BOT structure of  FIG. 2A , in accordance with some embodiments. 
         FIGS. 3A and 3B  illustrate cross-sectional views of another package structure in accordance with an embodiment. 
         FIG. 3C  shows a top view of the BOT structure of  FIGS. 3A and 3B , in accordance with some embodiments. 
         FIG. 3D  shows the BOT structure of  FIGS. 3A and 3B  with an axis of the metal trace pointing to the center of a die, in accordance with some embodiments. 
         FIG. 3E  shows a table of normalized stress simulation results comparing two BOT structures, in accordance with some embodiments. 
         FIG. 4A  shows 2 BOT structures on two locations on a die, in accordance with some embodiments. 
         FIG. 4B  shows a table of stress simulation results comparing stresses of BOT structures at different locations, in accordance with some embodiments. 
         FIG. 5A  shows a metal bump with an axis at an angle with the axis of a metal trace, in accordance with some embodiments. 
         FIG. 5B  shows examples of different shapes of elongated metal bumps over a metal trace, in accordance with some embodiments. 
         FIG. 6  shows a process flow of reducing stresses of BOT structures on a packaged substrate, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative, and do not limit the scope of the disclosure. 
     A package structure comprising a Bump-on-Trace (BOT) structure is provided in accordance with an embodiment. The variations of the embodiment are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements. 
       FIG. 1A  illustrates a cross-sectional view of a package structure (or bump-on-trace structure)  150  in accordance with an embodiment. The package structure  150  includes work piece  100  bonded to work piece  200 . Work piece  100  may be a device die that includes active devices such as transistors (not shown) therein, although work piece  100  may also be an interposer that does not have active devices therein. In an embodiment wherein work piece  100  is a device die, substrate  102  may be a semiconductor substrate such as a silicon substrate, although it may include other semiconductor materials. Interconnect structure  104 , which includes metal lines and vias  106  formed therein and connected to the semiconductor devices, is formed on substrate  102 . Metal lines and vias  106  may be formed of copper or copper alloys, and may be formed using damascene processes. Interconnect structure  104  may include a commonly known inter-layer dielectric (ILD, not shown) and inter-metal dielectrics (IMDs)  108 . IMDs  108  may comprise low-k dielectric materials, and may have dielectric constants (k values) lower than about 3.0. The low-k dielectric materials may also be extreme low-k dielectric materials having k values lower than about 2.5. 
     Work piece  100  may further include under-bump metallurgy (UBM) layer  110  and a copper post  112  on UBM layer  110 . Throughout the description, the copper post  112  is also referred to as a copper-containing bump or metal bump. Although copper post  112  is used as an example in the description here and below, other types of metal bumps, such as solder bumps, may also be used in place of copper post  112 . The UBM layer  110  is disposed on a metal pad  105 , which is part of interconnect structure  104 . Between the interconnect structure  104  and the UBM layer  110  not contacting the metal pad  105 , there is a passivation layer  107 . In some embodiments, the passivation layer  107  is made of polyimide. 
     Work piece  200  may be a package substrate, although it may be other package components such as interposers, for example. Work piece  200  may include metal lines and vias  202  connecting metal features on opposite sides of work piece  200 . In an embodiment, metal trace(s)  210  on the topside of work piece  200  are electrically connected to ball grid array (BGA) balls  212  on the bottom side of work pieces  200  through metal lines and vias  202 . Metal lines and vias  202  may be formed in dielectric layers  214 , although they may also be formed in a semiconductor layer (such as a silicon layer, not shown) and in the dielectric layers that are formed on the semiconductor layer. 
     Metal trace  210  is formed over a top dielectric layer in dielectric layers  214 . Metal trace  210  may be formed of substantially pure copper, aluminum copper, or other metallic materials such as tungsten, nickel, palladium, gold, and/or alloys thereof.  FIG. 1A  shows that the copper post (or metal bump)  112  has a length of L 1 , in accordance with some embodiments.  FIG. 1A  also shows that the metal trace  210  has a length L 2 , in accordance with some embodiments. 
     Work pieces  100  and  200  are bonded to each other through solder bump  220 , which may be formed of a lead-free solder, a eutectic solder, or the like. Solder bump  220  is bonded to, and contacts, the top surfaces of metal trace  210  and copper post  112 , 
       FIG. 1B  illustrates a cross-sectional view of the package structure  150  shown in  FIG. 1 , wherein the cross-sectional view is obtained from the plane crossing line  2 - 2  in  FIG. 1A . As shown in  FIG. 1B , solder bumps  220  may also contact the sidewalls of metal trace  210 . After the bonding of work pieces  100  and  200 , a mold underfill (MUF) (not shown) may be filled into the space between work pieces  100  and  200 , in accordance with some embodiments. Accordingly, a MUF may also be filled into the space between neighboring metal traces  210 . Alternatively, no MUF is filled, while air fills the space between work pieces  100  and  200 , and fills the space between neighboring metal traces  210 .  FIG. 1B  shows that the copper post (or metal bump)  112  has a width of W 1 , in accordance with some embodiments.  FIG. 1B  also shows that the metal trace  210  has a width W 2 , in accordance with some embodiments. 
       FIG. 1C  shows a top view of the copper post  112 , in accordance with some embodiments. The copper post  112  has a shape of an oval with a width W 1  and a length L 1 . In some other embodiments, the ratio of L 1 /W 1  is greater than 1. In some embodiments, the ratio of L 1 /W 1  is equal to or greater than about 1.2. In some embodiments, the L 1  is in a range from about 10 μm to about 1000 μm. In some embodiments, W1 is in a range from about 10 μm to about 700 μm. 
       FIG. 1D  illustrates an exemplary perspective view of metal trace  210 , the overlying copper post  112 , and solder bump  220 , in accordance with some embodiments. The metal trace  210  has a width W 2 , and a length L 2 . In some other embodiments, the ratio of L 2 /W 2  is greater than 1. In some embodiments, the ratio of L 2 /W 2  is greater than about 1.2. In some embodiments, the L 2  is in a range from about 10 μm to about 10,000 μm. In some embodiments, W 2  is in a range from about 10 μm to about 500 μm. The structure as shown in  FIG. 1D  is referred to as being a BOT structure, because solder bump  220  is formed directly on the top surface and sidewalls of metal trace  210 , and not on a metal pad that has a width significantly greater than width W 2  of metal trace  210 . In some embodiments, the ratio of W 1 /W 2  is in a range from about 0.5 to about 5. 
       FIG. 1D  also shows that the metal trace  210  has an axis line X-X, in accordance with some embodiments. Axis X-X is defined along the length of the metal trace. The copper post  112  has an axis line X′-X′, in accordance with some embodiments. Axis X′-X′ is defined along the length of the copper post  112 . As shown in  FIG. 1D , axis X′-X′ is substantially parallel to axis X-X. Therefore, the BOT structure  150  of  FIG. 1D  has an axis X-X. The direction the BOT structures, such as BOT structure  150 , point to the center of the die and the relative locations of the BOT structures to the center of the die can affect the stress exerted on the structures. 
       FIG. 1E  shows the BOT structure  150  with the axis X-X pointing to the center O of a die, in accordance with some embodiments.  FIG. 1F  shows a cross-sectional view of BOT structure  150 , in accordance with some embodiments. Stress simulation (or mechanical analysis) results show high stress at location M of the dielectric sub-layer (noted as layer  108 ′ in  FIG. 1F ), which is a sub-layer of the IMDs  108  and contacts the metal pad  105 , as shown in  FIGS. 1E and 1F . The stress simulation (or mechanical analysis) is performed by using ANSYS 12.1 simulator, which is made by ANAYS, Inc. of Canonsburg, Pa. As mentioned above, IMDs  108  may comprise low-k dielectric materials, and may have dielectric constants (k values) equal to or lower than about 3.0 or may also be extreme low-k (ELK) dielectric materials having k values equal to or lower than about 2.5. As a result, the dielectric sub-layer  108 ′ contacting the metal pad  105  may be made of a material having a dielectric constant (k value) equal to or lower than about 3.0 or may also be an extreme low-k (ELK) dielectric material having a k value equal to or lower than about 2.5. A porous extreme low-k (ELK) material with a k value of about 2.5 is used in the simulation. The stress of the dielectric sub-layer  108 ′ at location M is highest due to location M being farthest away from the die center “O.” Stress simulation results also show high stress at location N of metal trace  210 , as shown in  FIGS. 1E and 1F . Location N of metal trace  210  is closest to the center of the die. 
     The axis direction and relative position of a BOT structure affects the stress on the BOT structure. For example, if the axis of a BOT structure is pointed perpendicularly to the center of a die, the stresses on the dielectric layer  108 ′ and on metal trace  210  are higher than the stresses of the BOT structure shown in  FIG. 1E .  FIG. 2A  shows BOT structures,  251 - 256 , on a die  250 , in accordance with some embodiments. These BOT structures  251 - 256  all have structures similar to them that are nearby and they are not isolated structures. Further, there are other structures in the remaining areas of die  250  that are not shown in  FIG. 2A . BOT structures  251 - 256  are placed on locations A-F of die  250  respectively. Both locations A and B are placed near two of the far corners of die  250 . Locations C-F are placed on 4 corners of a center region  258 . The axes of BOT structure  251  at location A, BOT structure  253  at location C, BOT structure  256  at location F all point toward the center P of die  250 . In contrast, the axes of BOT structure  252  at location B, BOT structure  254  at location D, and BOT structure  255  at location E are all pointing perpendicularly to the center P of die  250 . The BOT structures  251 - 256  of  FIG. 2A  are all similar to BOT structure  150  of  FIG. 1D  with the axes of the metal bumps substantially parallel to the axes of the metal traces. 
       FIG. 2B  shows a table of data comparing the highest stresses in the dielectric layer  108 ′ and the metal trace  201  of each BOT structure of  FIG. 2A , in accordance with some embodiments.  FIG. 2B  shows that the dielectric stress for BOT structure  251  at location A being the highest (110.0 MPa), which is followed by the BOT structure  252  at location B. The highest dielectric stresses of BOT structures  253 - 256  at locations C-F, which are closer to the center P of die  250  are all smaller than (or about 60% of) the stresses of edge BOT structures  251  and  252 .  FIG. 2B  also shows that stress at the metal trace is highest for BOT structure  252  at corner B (185.0 MPa), which is followed by the stress for BOT structure  251  at location A. Stress of the metal trace of BOT structure  252  at location B is about 40% higher than the stress of the metal trace of BOT structure  251  at location A. Because both structures are placed near the corner edges, the much higher stress is due to the orientation of the BOT structures. The axis of BOT structure  252  (at location B) is pointed about perpendicularly to the center P of die  250 . In contrast, the axis of BOT structure  251  is pointed toward (or about parallel) to the center P of die  250 . The different orientations of the axes of these two structures contribute to the significant difference in stresses of metal traces. The data collected in  FIG. 2B  assume the solder bumps  220  have been reflowed at 250° C. and then cooled to room temperature (25° C.). 
     The higher stresses on the metal traces of BOT structures with axes pointing in directions substantially perpendicular to the center P of die  250  compared to BOT structures with axes pointing toward the center P of die  250  are also supported by data of structures  253 - 256  at locations C-F. The axes of BOT structures  254  and  255  at locations D and E respectively point perpendicularly to the center P of die  250 . In contrast, the axes of BOT structures  253  and  256  at locations C and F respectively point toward the center P of die  250 . The stress results show that the highest stresses on the metal traces of BOT structures  254  and  255  are higher (about 12% higher) than the highest stresses on the metal traces of BOT structures  253  and  256 . Therefore, the metal trace stress results of BOT structures  253 - 256  also support the effect of orientation of axes of BOT structures. 
     Further, the results in  FIG. 2B  show that the stresses (both the dielectric stress and stress on metal traces) are higher when the BOT structures are farther away from the die center. The stress results of BOT structures  251  and  252 , which are placed at corners farthest from the center P of die  250 , are higher than the stress results of BOT structures  253 - 256 , which are placed closer to the center P of die  250  compared to BOT structures  251  and  252 . The extreme high stress on metal trace of BOT structure  252  at location B could cause the metal trace, which is similar to metal trace  210  of  FIGS. 1A, 1B, 1D and 1F , to be lifted off from the substrate surface (delamination) and to disrupt electrical connection. Similarly, the stress on the metal trace of BOT structure  251  at location A is also quite high and may also cause delamination of the metal trace. Further, the stress at the dielectric layer of BOT structures  251  and  252  are also high relative to the stress at the other BOT structures. High stress at the dielectric layer, which is similar to dielectric layer  108 ′ of  FIG. 1F , of BOT structures could also cause interface delamination, which could be a reliability and/or yield issue. Therefore, it&#39;s important to seek solutions to reduce stresses at dielectric layer over the metal pad and at metal traces for BOT structures. 
       FIGS. 3A and 3B  illustrate cross-sectional views of the package structure  150 ′ in accordance with an embodiment. The substrates  100 ′ and  200 ′ in  FIGS. 3A and 3B  are similar to substrate  100  and substrate  200  of  FIGS. 1A-1D , with the exception that the orientation of the copper post  112 ′, which is similar to copper post  112 , and its corresponding UBM layer  110 ′ and solder bump  220 ′. All are turned 90 degrees (i.e., 90°). Copper post  112 ′ is one type of metal bump. Other types of metal bumps, such as a solder bump, may also be used in place of copper post  112 ′.  FIG. 3A  shows that the width W 1  of the copper post  112 ′ is in the same direction as the length L 2  of the metal trace  210 .  FIG. 3B  shows that the length L 1  of the copper post  112 ′ is in the same direction as the width W 2  of the metal trace  210 . 
       FIG. 3C  shows a top view of BOT structure  150 ′, in accordance with some embodiments. The metal trace  210  has a width W 2 , and a length L 2 .  FIG. 3C  shows that the copper post  112 ′, which has a width W 1  and a length L 1  and an axis Y-Y, is disposed perpendicularly above metal trace  210 . Axis Y-Y is substantially perpendicular to axis X-X of metal trace  210 .  FIG. 3D  shows the BOT structure  150 ′ with the axis X-X of the metal trace pointing to the center O′ of a die, in accordance with some embodiments.  FIG. 3E  shows a table of normalized stress simulation results comparing BOT structure  150  of  FIGS. 1D-1F  and BOT structure  150 ′ of  FIGS. 3C-3D , in accordance with some embodiments. The stress results of BOT structure  150  are normalized to be 1 and the stress results of BOT structure  150 ′ are compared against the corresponding stresses of BOT structure  150 . The results in  FIG. 3E  show that the peeling stress and total stress of BOT structure  150 ′ at the dielectric layer  108 ′ are about 32% lower than BOT structure  150 . The results also show that the peeling stress of the metal trace  210  for BOT structure  150 ′ is lower than the BOT structure  150  by about 56%. The simulation results show a drastic reduction in stresses at the dielectric layer and at the metal trace. As mentioned above, high stress at the metal trace can cause poor or no electrical contact and reduce yield. In addition, high stress at the dielectric layer  108 ′ can result in reliability issue, which can also degrade yield. By orienting the axis of the copper post  112  to be perpendicular to the axis of the metal trace  210 , the maximum stresses at the dielectric layer  108 ′ and metal trace  120  can be significantly reduced. 
       FIG. 4A  shows 2 BOT structures,  251 ′ on location A′ and  252 ′ on location B′, on a die  250 ′, in accordance with some embodiments. These 2 BOT structures  251 ′,  252 ′, and locations A′, B′ are similar to structures  251 ,  252 , and locations A, B of  FIG. 2A  respectively, with the exception that the metal bumps of BOT structures  251 ′ and  252 ′ are oriented to be perpendicular to the metal traces of these two BOT structures and also the metal trace of BOT structure  252 ′ is reoriented toward the center P′ of die  250 ′.  FIG. 4B  shows a table of stress simulation results comparing stresses of structures  251 ′,  252 ′,  251 , and  252 , in accordance with some embodiments. The results show an about 30% reduction in dielectric stress and an about 40% reduction stress in metal stress for BOT structure  251 ′ at location A′ compared to BOT structure  251  at location A. The reduction is attributed to the change in the orientation of the metal bump, such as copper post  112 , from being parallel to being perpendicular to the metal trace. The results also show an about 14% reduction in dielectric stress and an about 35% reduction stress in metal stress for BOT structure  252 ′ at location B′ compared to BOT structure  252  at location B. The reduction is attributed to the change in the orientation metal trace pointing toward the center P′ of die  250 ′ and the metal bump, such as copper post  112 , being changed from being parallel to being perpendicular to the metal trace. 
     The results in  FIGS. 3E and 4B  indicate significant benefits in stress reduction by placing the metal bumps perpendicularly to the underlying metal trace and also by pointing the axes of metal traces toward the center of die. Such arrangement is especially needed for BOT structures near the edges or corners of the die, such as structures  251  and  252 , which have higher stress and are more likely to delaminate or have reliability issues. The BOT structures  251 - 256  and  251 ′- 252 ′ are all structures surrounded by other similar structures. Isolated bump structures and BOT structures are known to have higher stresses, compared to structures surrounded by other structures. Therefore, isolated structures can also benefit from the BOT structure design described above. 
     One potential downside of such BOT design is the larger surface area (or real-estate) required. With the lengthy side of the copper post (or metal bump), such as copper post  112 , instead of the narrower width of the copper post, being perpendicular to the metal trace ( 210 ), additional space (or surface area) is needed. Therefore, such design requires additional surface area. If higher density of BOT structures is needed, the design can be applied on BOT structures that are most at risk of delamination or reliability issues. For examples, BOT structures near the edge of the die or isolated BOT structures have higher stresses than other BOT structures. 
     The new BOT structures described  FIGS. 3A-3D  show that the axis of the copper post  112 ′ is substantially perpendicular to the axis the metal trace  210 ′. In some embodiments, the length L 1  of copper post  112 ′ is evenly divided on metal trace  210 ′ (or the center of copper post  112 ′ is aligned with the axis of metal trace  210 ′. However, the axis of the copper post  112 ′ does not need to be substantially perpendicular to the axis of the metal trace  210 ′ to reduce the stress.  FIG. 5A  shows a top view of a copper post  112 ″ over a metal trace  210 ″. As described above, the copper post  112 ″ is at an angle “α” of about 90°, as shown in  FIG. 5A  in accordance with some embodiments. In some embodiments, the angle “α” is in a range from about 30° to about 150°. In some embodiments, the angle “α” is in a range from about 45° to about 135°. In some other embodiments, the angle “α” is in a range from about 60° to about 120°. 
     The BOT structures described above show that the top view of the metal bumps and their associated UBM layers are in the shape of an oval. Alternatively, the top view of the metal bumps can be in other elongated shapes, such as a track-field-shaped oval (an oval with two parallel sides), a rectangle, a parallelogram, a trapezoid, or a triangle, etc, as shown in  FIG. 5B  in accordance with some embodiments. In some embodiments, the corners of the metal bumps are rounded to reduce stress. Any elongated metal bumps could have the benefit of lowering stresses by placing the long axis not-parallel to the axis of metal trace. 
       FIG. 6  shows a process flow  600  of reducing stresses of BOT structures on a packaged substrate, in accordance with some embodiments. The package substrate is a packaged die. At operation  601 , isolated BOT structures and BOT structures at and/or near the edges or corners of the packaged substrate are identified. Such BOT structures have high delamination risks due to high stress at the metal trace interface and the dielectric interface with the metal pad. At operation  603 , the axes of metal traces of the BOT structures identified in operation  601  are aligned to point toward the center of the package structure (or packaged die) and/or the metal bumps of these BOT structures are designed to have their axes non-parallel to the axes of the metal traces on which they are placed. In some embodiments, the axes of the metal bumps are substantially perpendicular to the axes of the metal traces to reduce the stresses of the BOT structures. 
     The embodiments of bump-on-trace (BOT) structures and their layout on a die described above reduce stresses on the dielectric layer on the metal pad and on the metal traces of the BOT structures. By orienting the axes of the metal bumps in non-parallel relation to the metal traces, the stresses can be reduced, which can reduce the risk of delamination of the metal traces from the substrate and the dielectric layer from the metal pad. Further, the stresses of the dielectric layer on the metal pad and on the metal traces may also be reduced by orienting the axes of the metal traces toward the center of the die. As a result, the yield can be increased. 
     One aspect of this description relates to a package. The package includes a first work piece with a metal trace on a surface of the first work piece, wherein the metal trace has a first axis, wherein the first work piece is rigid, and an entirety of the metal trace is on the first work piece. The package further includes a second work piece with a plurality of elongated bumps, wherein at least one of the plurality of elongated metal bumps has a second axis and at least another of the plurality of elongated metal bumps has a third axis, wherein the second and the third axes are not the same and the second axis is at a non-zero angle from the first axis, wherein the plurality of elongated bumps are electrically connected to the metal trace. 
     Another aspect of this description relates to a method of forming a package. The method includes forming a metal trace on a surface of a first work piece, wherein the metal trace has a first axis, wherein the first work piece is rigid, and an entirety of the metal trace is on the first work piece. The method further includes forming a plurality of elongated bumps on a second work piece, wherein at least one of the plurality of elongated metal bumps has a second axis and at least another of the plurality of elongated metal bumps has a third axis, wherein the second and the third axes are not the same and the second axis is at a non-zero angle from the first axis. The method further includes bonding the first work piece to the second work piece to electrically connect at least one elongated bump of the plurality of elongated bumps to the metal trace. 
     Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure.