Abstract:
A wafer-level CSP ( 200 ) includes at least one die ( 202 ) from a wafer. The wafer-level CSP has a plurality of solder ball pads ( 206 ), a solder ball ( 308 ) at each solder ball pad and a polymer collar ( 310 ) around each solder ball. A moat ( 204 ) is formed in the surface of a polymer layer ( 412 ) disposed on the wafer during manufacturing of the wafer-level CSP. A temporarily liquified residual ( 502 ) from the polymer collar, which occurs while the wafer is heated to the reflow temperature of the solder ball, flows from the polymer collar. The moat acts as a barrier to material flow, limiting the distance that the residual spreads while liquified. The residual from the polymer collar remains within a region ( 314 ) defined by the moat. A full-depth moat ( 312 ) extends completely through the polymer layer. Alternatively, a partial-depth moat ( 712  and  912 ) extends partially through the polymer layer. The abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims pursuant to 37 C.F.R. §1.72(b).

Description:
RELATED APPLICATION 
     This application is related to application having Ser. No. 10/672,201 entitled FORMING PARTIAL DEPTH STRUCTURES IN POLYMER FILM, filed on even date with this application, assigned to the same assignee as the assignee of this application, which is hereby fully incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to wafer-level chip scale packages, and more particularly to forming a moat-like structure in a semiconductor wafer to restrict flow of a liquid prior to solidification of the liquid. 
     2. Description of the Related Art 
     A wafer-level chip scale package (CSP) is a package for an integrated circuit that is substantially the size of the integrated circuit or of a flip chip, which uses a wafer-level processing technique. Unlike a flip chip, the wafer-level CSP has one or more passivation layers on the active side of the die. Each passivation layer typically comprises a layer of photo-imageable polymer film. The wafer-level CSP is smaller than a standard ball grid array (BGA), typically uses metal traces of a re-distribution layer (RDL) to route solder ball pads to standard pitches, and uses CSP-size solder balls on the re-routed pads. A wafer-level CSP uses a standard surface mount technology assembly process that is also used for BGAs, and does not require underfill. 
     The use of a polymer collar around a solder ball, or solder bump, to support the solder ball in a wafer-level CSP is well known. When a semiconductor wafer, or wafer, is heated to the reflow temperature of the solder ball, some of the polymer collar material, which is very viscous at room temperature, becomes much less viscous, or liquefies. At times, the liquefied polymer collar material will disadvantageously flow farther from the solder ball than is desirable; occasionally merging with polymer collar material from an adjacent solder ball pad. Also, a solder ball disadvantageously tends to float on the liquefied polymer collar material. A solder ball will sometimes float to an adjacent solder ball pad, thereby creating a short. At times, a larger polymer collar is useful, but cannot be implemented with prior art wafer-level CSPs because a larger polymer collar would disadvantageously allow more liquefied polymer collar material to flow away from the polymer collar, thereby resulting in an undesirable appearance. 
     U.S. Pat. No. 6,437,434 entitled SEMICONDUCTOR DEVICE AND SEMICONDUCTOR DEVICE MOUNTING INTERCONNECTION BOARD, issued Aug. 20, 2002 to Sugizaki, discloses an interconnection board that has a moat etched in silicon around a BGA pad in order to release the BGA pad from stress. The moat is purposefully pre-filled with an elastomer. However, Sugizaki does not disclose a moat formed in a photo-imageable polymer film, does not disclose a moat on an integrated circuit, and does not disclose any means for stopping the spread of polymer collar material. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide a wafer-level CSP that overcomes the disadvantages of the prior art, and more particularly, to provide a wafer-level CSP that does not allow residual material from a polymer collar to flow beyond a predetermined distance from each solder ball. 
     It is another object of the present invention to provide a larger polymer collar without the detrimental effects of more polymer collar material flowing more than a predetermined distance from the solder ball. 
     It is still another object of the present invention to reduce any distance a solder ball can float on liquefied polymer collar residual material. 
     It is yet another object of the present invention to provide a wafer-level CSP with an enhanced cosmetic appearance. 
     These and other objects of the present invention will become apparent to those skilled in the art as the description thereof proceeds. 
     SUMMARY OF THE INVENTION 
     Briefly described, and in accordance with a preferred embodiment thereof, the present invention relates to a method of using a full-depth or partial-depth moat in a passivation polymer layer to confine or contain a subsequently applied liquid polymer material for cosmetic or structural purposes, prior to the liquid polymer material being cured into a solid. 
     Preferably, one aspect of the invention relates to a chip scale package of an integrated circuit, which includes at least one solder ball pad and a moat around each solder ball pad. 
     Another aspect of the present invention relates to a wafer for a chip scale package that has at least one solder ball pad. The wafer includes a solder ball at each solder ball pad, a polymer collar around the solder ball, and a moat around each solder ball pad. 
     A further aspect of the invention relates to a method of manufacturing a wafer-level chip scale package, comprising the steps of (a) providing a wafer, (b) disposing a passivation layer on the wafer; (c) forming, in the passivation layer, a central feature for a solder ball; and (d) forming, in the passivation layer, a moat around the central feature. 
     Other aspects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only and various modifications may naturally be performed without deviating from the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described with greater specificity and clarity with reference to the following drawings, in which: 
         FIG. 1  is a top view of a simplified prior art wafer-level CSP; 
         FIG. 2  is a top view of a simplified wafer-level CSP constructed in accordance with the preferred embodiments of the invention, showing a moat around each solder ball pad; 
         FIG. 3  is an enlarged simplified top view of area A of a wafer used to form the wafer-level CSP shown in  FIG. 2 , showing a solder ball with a polymer collar, surrounded by a full-depth moat, prior to heating of the wafer; 
         FIG. 4  is a cross-sectional view of  FIG. 3  through cut-line  4 — 4 ; 
         FIG. 5  is an enlarged simplified top view of area A of the wafer used to form the wafer-level CSP shown in  FIG. 2 , showing the solder ball with the polymer collar, surrounded by the full-depth moat, subsequent to heating of the wafer; 
         FIG. 6  is a cross-sectional view of  FIG. 5  through cut-line  6 — 6 ; 
         FIG. 7  is a simplified top view of area B of the wafer used to form the wafer-level CSP shown in  FIG. 2 , showing a first embodiment of a partial-depth moat formed by a plurality of lines; 
         FIG. 8  is a cross-sectional view of  FIG. 7  through cut-line  8 — 8 ; 
         FIG. 9  is a simplified top view of area C of the wafer used to form the wafer-level CSP shown in  FIG. 2 , showing a second embodiment of the partial-depth moat formed by a multiplicity of circles; 
         FIG. 10  is a cross-sectional view of  FIG. 9  through cut-line  10 — 10 ; 
         FIG. 11  is a photomicrograph of a portion of a prior art wafer showing the solder ball and the polymer collar following heating of the wafer; 
         FIG. 12  is a photomicrograph of a portion of a wafer in accordance with the invention showing the solder ball and the polymer collar following heating of the wafer; 
         FIG. 13  is a photomicrograph of a portion of a wafer in accordance with the invention, showing the partial-depth moat formed by a plurality of lines around each solder ball pad; 
         FIG. 14  is a photomicrograph of a portion of a wafer in accordance with the invention, showing the partial-depth moat formed by a multiplicity of circles around each solder ball pad; 
         FIG. 15  is a photomicrograph of a portion of a wafer in accordance with the invention, with the partial-depth moat formed by a plurality of lines around each solder ball pad, showing the solder ball and the polymer support collar following heating of the wafer; 
         FIG. 16  is a photomicrograph of a cross-section of the partial-depth moat shown in  FIG. 15 ; 
         FIG. 17  is a photomicrograph of a cross-section of a wafer in accordance with the invention, with the partial-depth moat formed by a multiplicity of circles; and 
         FIG. 18  is a photomicrograph of a portion of a wafer in accordance with the invention, showing the full-depth moat around the solder ball pad, and in which the full-depth moat is interrupted by a metal trace. 
     
    
    
     For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques are omitted to avoid unnecessarily obscuring the invention. Additionally, elements in the drawing figures are not necessarily drawn to scale. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It should be understood that the embodiments discussed below are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in the plural and vice versa with no loss of generality, for example, “one die”, “two die”. The terms first, second, third, and the like, in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. The terms top, front, side, and the like, in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing relative positions. All measurements are approximate, for example, “30 microns” means “30 microns, more or less”. 
       FIG. 1  is a top view of a simplified prior art wafer-level CSP  100  comprising a prior art integrated circuit, or die,  102  and a plurality of solder ball pads  106 . 
       FIG. 2  is a top view of a simplified wafer-level CSP  200  constructed in accordance with the preferred embodiments of the invention, comprising a single integrated circuit, or die,  202  with a moat  204  around each solder ball pad  206 . The die  202  is one of a plurality of die from a larger semiconductor wafer or “wafer” (not shown). Typically, there are 200–700 die per wafer. A wafer-level CSP design is described in U.S. Pat. No. 6,287,893 entitled METHOD FOR FORMING CHIP SCALE PACKAGE, issued Sep. 11, 2001, to Elenius et al., assigned to the assignee of the present invention, which is hereby fully incorporated herein by reference. The moat  204  is a ring-shaped (when seen in a top view) via formed in the surface of a passivation layer disposed on the wafer during a wafer-level processing step. By “wafer-level processing” it is meant, for example, that the moats  204  are formed in each die  202  prior to the die being cut from the wafer. Preferably, the passivation layer is a photo-imageable polymer film. The photo-imageable polymer film is typically benzocyclobutene (BCB), although the invention is useful with photo-imageable films of other materials. The moat  204  is used to confine and contain some material of the polymer collar that temporarily becomes much less viscous, or “liquefies”, when the wafer is heated to the reflow temperature of solder ball metal during a subsequent wafer-level CSP  200  processing step. 
       FIG. 3  is an enlarged simplified top view of portion  300 , indicated by area A of  FIG. 2 , of a wafer used to form the wafer-level CSP  200 , showing a solder ball  308  with a polymer collar  310  at a central feature  414  (see  FIG. 4 ), surrounded by a full-depth moat  312 , prior to heating of the wafer. Preferably, the polymer collar material is XNF-1502 manufactured by Ablestik Laboratories, of Rancho Dominguez, Calif. Alternatively, another material is used for the polymer collar  310 . The use of a polymer collar around a solder ball is described in U.S. Pat. No. 6,578,755 entitled POLYMER COLLAR FOR SOLDER BUMPS, issued Jun. 17, 2003, to Elenius et al., assigned to the assignee of the present invention, which is hereby fully incorporated herein by reference. The central feature  414  has a diameter  313  of two hundred eighty (280) microns. The full-depth moat  312  defines a region  314  within the moat, and a region  316  without, or outside of, the moat. The full-depth moat  312  has a width  315  of thirty (30) microns. A distance  317  between the inside edge of full-depth moat  312  and the outside edge of the central feature  414  is seventy-five (75) microns. The solder ball  308  has a diameter  416  (see  FIG. 4 ) of 300–350 microns. 
       FIG. 4  is a cross-sectional view of  FIG. 3  through cut-line  4 — 4 . The wafer typically comprises at least one layer of silicon, although the invention is also useful with wafers comprising other semiconductor materials. The one silicon layer  402  represents the wafer semiconductor substrate and all its layers, ready for CSP manufacturing. For simplification, the details of the wafer are not shown. The silicon layer  402  typically is coated with silicon nitride or silicon dioxide, dielectrics that generally do not conduct electricity, as a thin passivation layer (not shown), with openings over selected aluminum bond pads (not shown) of the integrated circuits of the wafer. The silicon nitride or silicon dioxide thin passivation layer is typically not placed on the wafer during CSP manufacturing, but is part of the wafer as it exists prior to CSP manufacturing. All other layers illustrated in  FIG. 4  are typically placed on the wafer in the course of manufacturing the wafer-level CSP from the wafer. 
     A first polymer layer  404  of photo-imageable polymer film is disposed over the thin passivation layer. The first polymer layer  404  is typically 4–5 microns thick. A metalization layer is disposed on the first polymer layer  404 , and over any exposed aluminum bond pads. The metalization layer includes an under bump metalization (UBM) area, or solder ball pad,  206  and a re-distribution layer (RDL)  406 . The RDL comprises metal traces that form a conductive path between each solder ball pad  206  and any associated aluminum bond pad not positioned at the same x-y coordinates as the solder ball pad. A second polymer layer  412  of photo-imageable polymer film is disposed on the first polymer layer  404  and the metalization layer. The second polymer layer  412  is typically 4–5 microns thick. The first polymer layer  404  and the second polymer layer  412  are typically of the same material, preferably a CYCLOTENE™ 4022-35 BCB passivation polymer, manufactured by Dow Chemical Company of Midland, Mich., as in a standard, two-layer ULTRA CSP® package. Alternatively, another one material is used for both photo-imageable polymer layers. As a further alternative, different materials are used for each photo-imageable polymer layer. The full-depth moat  312  is also used on wafer-level CSPs  200  with a single polymer layer of 4–5 microns in thickness. Typically, when a single polymer layer is used, no RDL is necessary. 
     The central feature  414  is a via that penetrates completely through both the first polymer layer  404  and the second polymer layer  412  of a finished wafer-level CSP. Using photo-imaging means well known to those skilled in the art, the central feature  414  is formed (i.e., opened) in the first polymer layer  404  prior to deposition of the second polymer layer  412 , thereby exposing any associated aluminum bond pad positioned at the same x-y coordinates as the solder ball pad  206 . The silicon nitride or silicon dioxide thin passivation layer is exposed at the bottom of the central feature  414  in designs wherein the associated aluminum bond pad is not positioned at the same x-y coordinates as the solder ball pad  206 . The first polymer layer  404  is then cured by baking in an oven at a temperature and for a period required for the polymer to polymerize. The metalization layer, which typically comprises layers of aluminum, nickel vanadium and copper, is sputtered over selected portions of the first polymer layer  404 , over any exposed aluminum bond pads, and over the silicon nitride or silicon dioxide at the bottom of the central feature  414 . Next, the second polymer layer  412  is disposed on the first polymer layer  404  including on the portions of the first polymer layer having metalization. Using photo-imaging means well known to those skilled in the art, full-depth moat  312  is formed in the wafer completely through the second polymer layer  412 , and, at the same time, the central feature  414  is re-opened down to the metalization layer, or solder ball pad  206 . The first polymer layer  404  is exposed at the bottom of full-depth moat  312 . The full-depth moat  312  does not overlie the RDL  406 , as shown in  FIG. 4 . Preferably, full-depth moat  312  is used in cases where the moat does not overlie the RDL  406 . 
       FIG. 5  is an enlarged simplified top view of the portion  300 , showing the solder ball  308  with the polymer collar  310 , surrounded by full-depth moat  312 , subsequent to heating of the wafer. As the solder ball  308  is reflowed, some liquefied material of the polymer collar  310  spreads out, but advantageously, only into region  314  within full-depth moat  312 . The full-depth moat  312  confines and contains the liquefied polymer collar material, and advantageously prevents it from spreading beyond the moat into region  316  outside the moat. During later stages of the reflow process, the liquefied polymer collar material that flowed into region  314  becomes much more viscous and hardens or “solidifies”, and forms a residual  502 .  FIG. 5  shows that most of the region  314  within full-depth moat  312  contains residual  502  of polymer collar material. The residual  502  is semi-transparent. The residual  502  does not necessarily completely fill the region  314  within full-depth moat  312  (though it may), nor does it necessarily spread out equally in all directions from the main portion of the polymer collar  310  (though it may). Therefore, there might be some random-looking appearance of the residual material as  FIG. 5 , however, the residual  502  is contained/controlled by full-depth moat  312  and the spread of the residual therefore is limited by the moat. In some instances the residual  502  may completely cover the bottom surface of full-depth moat  312 . 
       FIG. 6  is a cross-sectional view of  FIG. 5  through cut-line  6 — 6 . The full-depth moat  312  retains residual  502  of polymer collar material that spreads out along the surface of the second polymer layer  412  away from the polymer collar  310 . The flow of the residual  502  occurs prior to, and during solder reflow. Without the presence of full-depth moat  312 , the residual  502  flows out in a random pattern and for a greater distance, and, as a result, is cosmetically unacceptable. The purpose of full-depth moat  312  is to contain the flow of the residual and prevent/minimize its flow beyond the moat, thereby enhancing the cosmetic appearance of the wafer-level CSP. As a result of the presence of full-depth moat  312 , residual  502  flows a shorter distance from the polymer collar, the extent of residual flow is more nearly uniform in all directions, and the outer edge of the flow is thereby more nearly circular. The full-depth moat  312  surrounds the central feature  414 ; alternatively, the full-depth moat is a stand-alone feature. 
       FIG. 7  is a simplified top view of the portion  300 , indicated by area B of  FIG. 2 , of the wafer used to form the wafer-level CSP  200 , showing a first embodiment of a partial-depth moat  712  formed by a plurality of lines  701 ,  702  and  703  around the central feature  414 , preferably using the photo-imaging means in accordance with the Related Application. The partial-depth moat  712  has a width  715  of twenty-three (23) microns. A distance  717  between the inside edge of partial-depth moat  712  and the outside edge of the central feature  414  is seventy-five (75) microns. It should be noted that the invention is not limited to using three (3) lines. Any number of lines greater than one (1) can be used. 
       FIG. 8  is a cross-sectional view of  FIG. 7  through cut-line  8 — 8 . Using means well known to those skilled in the art, the central feature  414  is formed completely through both the second polymer layer  412  and the first polymer layer  404 . The solder ball pad  206  is exposed at the bottom of the central feature  414 . The partial-depth moat  712  is formed in the wafer partially through the second polymer layer  412 . The second polymer layer  412  is exposed at the bottom of partial-depth moat  712 . The partial-depth moat  712  does not penetrate to the first polymer layer  404 . The partial-depth moat  712  has a moat depth  801  of 1–99% of the thickness of the second polymer layer  412 . Alternatively, the partial-depth moat  712  is used on wafer-level CSPs  200  having a single polymer layer of 4–5 microns in thickness. In such case, partial-depth moat  712  has a moat depth  801  of 1–99% of the thickness of the single polymer layer. In  FIG. 8 , the partial-depth moat  712  overlies the RDL  406 . The RDL  406  is not exposed through partial-depth moat  712 . Advantageously, the partial-depth moat  712  may cross underlying metal traces without exposing the RDL  406 . 
       FIG. 9  is a simplified top view of the portion  300 , indicated by area C of  FIG. 2 , of the wafer used to form the wafer-level CSP  200 , showing a second embodiment of the partial-depth moat. Partial-depth moat  912  is formed by a multiplicity of circles  913  around the central feature  414 , preferably using photo-imaging means in accordance with the Related Application. The multiplicity of circles  913  are in the form of four (4) concentric rows  901 – 904  of closely-packed circles. The partial-depth moat  912  has a width  915  of twenty-eight (28) microns. A distance  917  between the inside edge of partial-depth moat  912  and the outside edge of the central feature  414  is seventy-five (75) microns. It should be noted that the invention is not limited to using four (4) concentric rows of circles. Any number of rows can be used, provided that there is a plurality of circles. Partial-depth moats  712  and  912  surround the central feature  414 ; alternatively, the partial-depth moats are stand-alone features. 
       FIG. 10  is a cross-sectional view of  FIG. 9  through cut-line  10 — 10 . Using means well known to those skilled in the art, the central feature  414  is formed in the wafer completely through both the second polymer layer  412  and the first polymer layer  404 . The solder ball pad  206  is exposed at the bottom of the central feature  414 . The partial-depth moat  912  is formed in the wafer partially through the second polymer layer  412 . The partial-depth moat  912  does not penetrate to the first polymer layer  404 , therefore, the second polymer layer  412  is exposed at the bottom of partial-depth moat  912 . The moat depth  801  of partial-depth moat  912  is 1–99% of the thickness of the second polymer layer  412 . Alternatively, the partial-depth moat  912  is used on wafer-level CSPs  200  having a single polymer layer of 4–5 microns in thickness. In such case, partial-depth moat  912  has a moat depth  801  of 1–99% of the thickness of the single polymer layer. The partial-depth moat  912  overlies the RDL  406 . As shown in  FIG. 10 , the RDL  406  is not exposed through partial-depth moat  912 . Advantageously, partial-depth moat  912  may cross underlying metal traces without exposing the RDL  406 . 
       FIG. 11  is a photomicrograph of a portion of a prior art wafer showing the solder ball  308  and the polymer collar  310  following heating of the wafer. The residue  502  of polymer collar material extends an irregular distance from the solder ball  308 . After the central features  414  are formed in the wafer, a polymer collar  310 , which is a fluxing polymer material, is applied to the central feature  414 , and then solder balls  308  are placed onto the fluxing polymer spots. The wafer is subsequently processed through reflow and cure processes where the polymer collar  310  softens and has a tendency to flow, and then cure. As can be seen in  FIG. 11 , without the moats  204 , the final appearance of the residual  502  is random and uncontrolled. 
       FIG. 12  is a photomicrograph of a portion of a wafer in accordance with the invention, showing the solder ball  308  and the polymer collar  310  following heating of the wafer.  FIG. 12  illustrates the results of the same processing steps and materials used on the prior art wafer in  FIG. 11 , but with moats  204 .  FIG. 12  shows that the moat  204  confines and contains the residual  502  within the moat confines. The moat  204  assists in creating a concentric/uniformly shaped, cured fluxing polymer, and the moat inhibits random flow of the residual  502  from the polymer collar  310 . 
       FIG. 13  is a photomicrograph of a portion of a wafer in accordance with the invention, showing partial-depth moat  712  formed by the plurality of lines  701 ,  702  and  703  around each solder ball pad  206 . There is no solder ball or polymer collar on the wafer shown in  FIG. 13 . 
       FIG. 14  is a photomicrograph of a portion of a wafer in accordance with the invention, showing partial-depth moat  912  formed by a multiplicity of circles  913  around each solder ball pad  206 . There is no solder ball or polymer collar on the wafer shown in  FIG. 14 . 
       FIGS. 15–17  are photographs made with a scanning electron microscope.  FIG. 15  is a photomicrograph of a portion of a wafer in accordance with the invention, with partial-depth moat  712  formed by the plurality of lines  701 ,  702  and  703  around the central feature  414 , showing the solder ball  308  and the polymer collar  310  following heating of the wafer. 
       FIG. 16  is a photomicrograph of an enlarged cross-section of partial-depth moat  712  shown in  FIG. 15 . The partial-depth moat  712  shown in  FIGS. 15 and 16  is produced by a photomask having three (3) concentric seven (7) micron wide chrome lines  701 ,  702  and  703  separated by one (1) micron wide spaces, using the method in accordance with the Related Application. Although produced by three lines, a single, partial-depth moat is formed, as shown in  FIGS. 15 and 16 . The partial-depth moat  712  of  FIGS. 15 and 16  is twenty-three (23) microns wide and has a moat depth of 2.1 microns, which is about 60% through the second polymer layer  412 . 
       FIG. 17  is a photomicrograph of a cross-section of a wafer in accordance with the invention, showing partial-depth moat  912  formed by a multiplicity of circles  913 . The partial-depth moat  912  shown in  FIG. 17  is produced by a photomask having four (4) rows of closely-packed seven (7) micron diameter chrome circles, using the method in accordance with the Related Application. Although produced by a multiplicity of circles, a single, partial-depth moat is formed, as shown in  FIG. 17 . The partial-depth moat  912  shown in  FIG. 17  is twenty-eight (28) microns wide and has a moat depth of 2.2 microns, which is about 64% through the second polymer layer  412 . 
       FIG. 18  is a photomicrograph of a portion of a wafer in accordance with the invention, showing full-depth moat  312  around the solder ball pad  206 , and in which the full-depth moat is interrupted by a metal trace. One of the partial-depth moats  712  and  912  is preferably used where a moat overlies a metal trace. Alternatively, the full-depth moat  312  is used, and the full-depth moat is preferably interrupted at the metal trace, as shown in  FIG. 18 , so as not to expose the metal trace. As a further alternative (not shown), when exposing a particular metal trace is not deleterious, full-depth moat  312  crosses a metal trace, thereby exposing the RDL  406 . 
     The invention advantageously keeps the applied material in a concentric shape/volume for either structural and/or cosmetic purposes. The ability of the moat  204 ,  312 ,  712  and  912  to confine the residual  502  depends upon the volume of the moat, the depth of the moat, and the distance  317 ,  717  and  917  from central feature  414 . Advantageously, it is easier to perform automatic optical inspection of bumped wafers when the spread of the residual  502  is controlled by a moat. 
     While the present invention has been described with respect to preferred embodiments thereof, such description is for illustrative purposes only, and is not to be construed as limiting the scope of the invention. Various modifications and changes may be made to the described embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims. For example, although the second polymer layer  412  is preferably photo-imageable, full-depth moat  312  and partial-depth-moats  712  and  912  are preferably formed used photo-lithographic means; alternatively, they are formed using other means, such as by using a laser or by mechanical means. Furthermore, the layer in which the moats  204  are formed can be of a material that is not photo-imageable. The shape of the moat  204  is not limited to being circular, but can be of any shape, including, for example, square. Furthermore, the invention is not limited to wafer-level CSPs, but can be extended to CSPs, in general. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 LIST OF REFERENCE NUMERALS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 100 
                 Prior Art Wafer-Level CSP 
               
               
                   
                 102 
                 Prior Art Die 
               
               
                   
                 106 
                 Prior Art Solder Ball Pads 
               
               
                   
                 200 
                 Wafer-Level CSP 
               
               
                   
                 202 
                 Die 
               
               
                   
                 204 
                 Moat 
               
               
                   
                 206 
                 Solder Ball Pad 
               
               
                   
                 300 
                 Portion of Wafer 
               
               
                   
                 308 
                 Solder Ball 
               
               
                   
                 310 
                 Polymer Collar 
               
               
                   
                 312 
                 Full-depth Moat 
               
               
                   
                 313 
                 Diameter of Central Feature 
               
               
                   
                 314 
                 Region within Moat 
               
               
                   
                 315 
                 Width of Full-Depth Moat 
               
               
                   
                 316 
                 Region without Moat 
               
               
                   
                 317 
                 Distance 
               
               
                   
                 402 
                 Silicon 
               
               
                   
                 404 
                 First Polymer Layer 
               
               
                   
                 406 
                 Re-Distribution Layer (RDL) 
               
               
                   
                 412 
                 Second Polymer Layer 
               
               
                   
                 414 
                 Central Feature 
               
               
                   
                 416 
                 Diameter of Solder Ball 
               
               
                   
                 502 
                 Residual 
               
               
                   
                 701–703 
                 Lines 
               
               
                   
                 712 
                 Partial-Depth Moat 
               
               
                   
                 715 
                 Width of Partial-Depth Moat 
               
               
                   
                 717 
                 Distance 
               
               
                   
                 801 
                 Moat Depth 
               
               
                   
                 901–904 
                 Rows 
               
               
                   
                 912 
                 Partial-Depth Moat 
               
               
                   
                 913 
                 Multiplicity of Circles 
               
               
                   
                 915 
                 Width of Partial-Depth Moat 
               
               
                   
                 917 
                 Distance