Abstract:
The present invention relates to a method of making a robust wafer level chip scale package and, in particular, a method that prevents cracking of the passivation layer during solder flow and subsequent multiple thermal reflow steps. In one embodiment, a passivation layer that is formed using a highly compressive insulating material is used. In another aspect, another layer is applied over the passivation layer to assist with preventing cracking of the passivation layer.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of making a robust Wafer Level Chip Scale Package (WLCSP) that prevents cracking of the passivation layer during solder flow and subsequent multiple thermal reflow steps. 
       BACKGROUND OF THE INVENTION 
       [0002]    WLCSP technology is a commonly used wafer level packaging method for integrated circuits. Two commonly used WLCSP technologies are the solder “ball drop” method and the solder “screen print” method. Both of these technologies are routinely used in a standard manufacturing environment enabling the production of devices with I/O pitch finer than 0.4 mm. 
         [0003]    In the “ball drop” WLCSP process, as shown in  FIG. 1(   a ), on the wafer  100  each of the different areas  110  that will subsequently correspond to a chip. As shown in  FIG. 1(   b ), within an area  110 , there will exist a plurality of bond pads  120  (though only one is shown) used by each chip to establish electrical connections to the chip that are formed in a top layer of the metal, with the remainder of the integrated circuit below (not shown). Over the bond pads  120  is deposited a passivation layer  130 , that will then have formed passivation openings  140  (shown in the dotted line area) to expose the bond pads  120  below. After the passivation openings  140  are defined on the wafer  100 , a typical CSP process involves depositing and patterning an under bump material (UBM) layer  150  (which may have a plurality of different layers), followed by dropping a solder ball  160  in each of the passivation opening  140 . The wafer  100  is then subjected to a thermal cycle during which each of the solder balls  160  melts (also referred to as re-flowed) and then cools in a well defined shape on top of the UBM layer  150 . 
         [0004]    Alternately, in the “screen print” method, after defining the UBM layer, solder paste is squeezed through a mask and deposited on top of the UBM layer. The wafer is then subjected to a thermal reflow cycle during which the solder paste melts and then cools forming a well defined shape on top of the UBM layer. 
         [0005]    One drawback of these methods, as shown in  FIG. 1(   b ), is the formation of fractures or cracks  170  in the passivation layer  130  that can occur during the solder reflow process. The thermo-mechanical stress on the passivation layer  130  during the reflow process is significant enough to cause a complete fracture of the passivation film  130 . After the wafer  100  is sawed and the areas  110  are separated to become the individual integrated circuits (ICs)  200 , the cracks  170  remain in the passivation film  130 , thereby exposing the underlying circuits to the ambient environment. 
         [0006]    In most applications, the IC  200  is mounted on a printed circuit board (PC)  210  as shown in  FIG. 2 , using conventional solder flux, paste and another reflow step to melt the solder balls  160  and attach the IC  200  to the PC board  210 . During this assembly process, chemicals such as the solder flux can migrate through the passivation cracks  170  and attack the underlying circuits. The extent of damage depends on the type and concentration of the chemical as well as the exposure time. In many cases the flux will etch away the metal lines leading to open circuits. 
         [0007]    The passivation film is necessary to protect the underlying layers within an integrated circuit from the ambient environment. However, cracks in the passivation layer can easily expose the layers underneath to moisture, ionic contamination, etc., causing the circuit to malfunction. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention relates to a method of making a robust wafer level chip scale package and, in particular, a method that prevents cracking of the passivation layer during solder flow and subsequent multiple thermal reflow steps. 
         [0009]    In one embodiment, a passivation layer that is formed using a highly compressive insulating material is used. 
         [0010]    In a particular aspect, the method of making the wafer level chip scale package comprising initially forming a semiconductor wafer containing a plurality of circuits at each of a plurality of different chip areas, wherein the semiconductor wafer includes for each of the plurality of different chip areas a plurality of conductive bond pads connected to conductive wires; then forming a passivation layer over the plurality of conductive bond pads and the conductive wires, the passivation layer being formed using a highly compressive insulating material; removing contact areas from the passivation layer to expose certain ones of the plurality of conductive bond pads; applying an underbump material over each of the exposed conductive bond pads; placing a conductive bump over the underbump material associated with each of the certain ones of the plurality of conductive bond pads and attaching it to the underbump material layer using a thermal flow cycle; and dicing the wafer to obtain a plurality of bumped die. 
         [0011]    In another aspect, another layer is applied over the passivation layer to assist with preventing cracking of the passivation layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    These and other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein: 
           [0013]      FIG. 1  illustrates a conventional structure for a wafer level CSP solder bump; 
           [0014]      FIG. 2  illustrates a conventional attachment of a die having solder bumps to a PC board; 
           [0015]      FIG. 3  illustrates a flowchart of the method according to the present invention; 
           [0016]      FIGS. 4(   a ) and  4 ( b ) illustrate a conventional and preferred, respectively, layout of the top interconnect layer of a wafer at a bond pad connection area according to the present invention; 
           [0017]      FIG. 5  illustrates the preferred structure of the bond pad according to the present invention. 
           [0018]      FIG. 6  illustrates another embodiment of a structure to protect the passivation layer and the underlying layers of the wafer according to the present invention. 
           [0019]      FIG. 7  illustrates a passivation cracking vs passivation stress chart for various materials according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    The present invention describes a wafer level CSP process that substantially prevents cracking of the passivation layer during the solder flow and subsequent multiple reflow steps. 
         [0021]    The described invention is also implemented in a manner that minimizes the overall cost of manufacturing, and as such uses conventional manufacturing techniques and materials. It is understood, however, that the conventional techniques that the present invention uses, which are in the preferred embodiment described for a 0.5 mm pad pitch technology, also allow for implementation of the present invention, with advantageous results, in 0.4 mm pad pitch and smaller pad pitch CSP technologies. 
         [0022]    In certain embodiments, the resulting structure from implementation of the various aspects the methods according to the present invention appears the same as the conventional structure described in the background with respect to  FIG. 1 , except that the cracks  170  that appear in the conventional structure are eliminated.  FIG. 5  illustrates the structure of the present invention, which includes the passivation layer with compressive stress  130 A according to the present invention. 
         [0023]      FIG. 3  illustrates a flowchart that outlines one embodiment of the method  300  of the present invention. 
         [0024]    In step  310 , a wafer is conventionally processed, applying all layers below the last metal layer. 
         [0025]    In step  320  the last metal layer that includes the bond pads  120  is applied, and this last metal layer has been optimized to prevent fracture therein. In particular, as shown in  FIG. 4(   a ) at the stress point between the bond pad  120  conducting wire  122 , a stress point  124  has been determined to exist, and, as shown in  FIG. 4(   b ) a flaring  126  of the wire at this stress point is preferable in order to reduce the amount of stress at the joint between bond pad  120  and wire  122 . 
         [0026]    In step  330 , there is then applied the passivation layer  130 A, which passivation layer  130 A has also been optimized for thickness, composition, and/or compressive stress. 
         [0027]    In particular, optimization of the thickness requires optimizing the overall thickness, which optimization accounts for balancing two competing factors that have been determined: (1) increasing thickness of the passivation layer reduces stress at the junction between the bond pads and the bond wires; but (2) increasing the thickness of the passivation layer increases the overall stress. It has been determined, taking these competing interests into account, that a range of thickness of the passivation layer is 15-25 KA, with a preferred thickness of 17-20 KA. The thickness values are presented for a preferred embodiment in which a dual dielectric stack of silicon dioxide and silicon nitride is used as the passivation material. It should be clear to anyone skilled in the arts that changing the material and/or the stack will result in different thickness ranges. 
         [0028]    Optimization of the composition requires material composition of the passivation layer. It has been determined that a multi-layer passivation layer, with the layer directly underneath the underbump material layer  150  as the nitride film and an oxide layer underneath the nitride 
         [0029]    In a preferred embodiment, the compressive material is silicon nitride., is preferable. It is also preferred that the thickness of the nitride film be around 12 KA. Of course other passivation layer material compositions can be used within the scope of the present invention. The nitride and oxide compounds are routinely already used, and as such can be easily inserted into the overall processing. 
         [0030]    Optimization of the compressive stress in the passivation layer  130 A requires depositing the nitride passivation layer in a compressive manner. In particular, applying the passivation material with a compressive stress that is greater than about 5 E9 dynes/cm 2 . Further, if there is a multilayer passivation, compression of only the top layer is needed in order to achieve the advantages of the present invention. 
         [0031]      FIG. 7  shows the relationship between passivation cracks and the stress in the passivation layer  130 A. From this figure it is quite clear that compressive stress in excess of 5 E9 dynes/cm2 is necessary to eliminate cracks for a nitride film thickness of 12 KA and oxide film thickness of 6 KA. This value could be higher or lower for other combinations of passivation thickness and materials. 
         [0032]    In step  340 , subsequent conventional wafer processing steps occur, including passivation openings  140  to expose the bond pads  120 , applying and patterning the UBM layer  150 , dropping a solder ball  160  in each of the passivation opening  140 , subjecting the wafer  100  to a thermal cycle during which the each of the solder balls  160  melts and then cools in a well defined shape on top of the UBM layer  150 , and cutting the wafer  100 . Thereafter, die connection step  350  occurs, in which each of the die  200  are placed at the appropriate connection point, and then a thermal cycle occurs to reflow the solder balls  160  to establish connection to the PC board  210 . 
         [0033]    Not all of the steps described in  FIG. 3  are necessary in order to obtain certain of the advantageous features of the present invention. In particular, usage of any one of optimizations in step  330 , or any combination of them, will allow for the advantageous structure without the cracks  170  in the passivation layer  130 A. Further, the optimization of step  320 , while preferred, need not be used to obtain the advantages according to the present invention. 
         [0034]    An advantage of the present invention is that even if the passivation layer is subjected to a large number of thermal cycles (such as 10) for reflowing other integrated circuits on the same printed circuit board, that even with such a large number of thermal cycles the cracking of the passivation layer is completely eliminated. 
         [0035]    A further advantage of the present invention is that the usage of a passivation layer without any cracks prevents moisture, hydrogen, ionic contaminants and other foreign species from coming into contact with the conductive elements disposed adjacent and below the passivation layer. 
         [0036]    In an alternate embodiment, illustrated in  FIG. 6 , a thick layer of a protective film  180  (typically 5-10 um of polyimide] is deposited on top of the passivation layer  130  and then openings are made in the polyimide layer using conventional photolithographic techniques. The protective film, in order to perform appropriately and assist in providing a layer that will not crack, should have a coefficient of thermal expansion that is at least 5-10 times that of the passivation layer, and a Young&#39;s Modulus that is smaller than that of the passivation layer. In embodiments in which the passivation layer is silicon dioxide and/or nitride (which have a coefficient of thermal expansion of about 1-4 ppm/) deg K, a preferred protective layer is a polyimide film, which has a coefficient of thermal expansion that is about 30 ppm/deg K, as well as a Young&#39;s Modulus of 2-4 GPa, which is smaller that of the silicon nitride (which has a Young&#39;s Modulus of 290 GPa) and that of an oxide (which has a Young&#39;s Modulus of about 68 GPa). 
         [0037]    The protective film  180 , due to its large coefficient of thermal expansion can withstand the thermo-mechanical stress better and therefore does not crack. Even though the passivation layer  130  underneath might crack, the protective film  180  seals the IC circuitry from the environment. 
         [0038]    The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teachings.