Abstract:
A packaged semiconductor device (a wafer-level chip scale package) containing a conductive adhesive material as an electrical interconnect route between the semiconductor die and a patterned conductive substrate is described. The patterned conductive substrate acts not only as a substrate, but also as a redistribution layer that converts the dense pad layout of the die to a larger array configuration of the solder balls in the circuit board. Using the invention allows the formation of a lower priced chip scale package that also overcomes the restriction of the die size used in die-sized chip packages and the input-output pattern that can be required by the printed circuit board. Thus, the invention can provide a familiar pitch (i.e., interface) to the printed circuit board for any small die.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 10/852,732, which is a continuation-in-part of priority of U.S. patent application Ser. No. 10/731,453, which is a continuation-in-part of Ser. No. 10/618,113, which is a continuation in part of U.S. patent application Ser. No. 10/295,281, which claims priority of Korean Patent Application No. KR 01-71043, the entire disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to methods for fabricating integrated circuits (ICs) and semiconductor devices and the resulting structures. Specifically, the invention relates to semiconductor packages and methods for fabricating and using such packages. More particularly, the invention relates to wafer level chip scale packages and methods for fabricating and methods for using such packages. 
     BACKGROUND OF THE INVENTION 
     Recent advancements in the electronics industry, especially with personal computers (PC), mobile phones, and personal data assistants (PDA), have triggered a need for light, compact, and multi-functional power systems that can process large amounts of data quickly. These advancements have also triggered a reduction in the size of semiconductor chips and the packaging used for theses chips. 
     The semiconductor chips typically have conductive pads formed at the top surface of the silicon substrate containing the IC. Wire bonding is used to connect the conductive pads on the substrate to corresponding pads on a package substrate. The increasing complexity of the circuitry in the IC has required the conductive pads to be formed closer together. With the bond pads narrower, the length of the wire (in the wire bonding) needs to be longer and width narrower which unfortunately induces a greater amount of inductance and thereby reduces the speed of the circuitry. 
     One type of packaging that has been recently used is wafer-level chip size packaging (WLCSP). See, for example, U.S. Pat. Nos. 6,187,615 and 6,287,893, the disclosures of which are incorporated herein by reference. 
     In general, to fabricate WLCSP, a wafer is processed and then packaged by a photolithography process and a sputtering process. This method is easier than general packaging processes that use die bonding, wire bonding, and molding. Processes for WLCSP also have other advantages when compared to general packaging processes. First, it is possible to make solder bumps for all chips formed on a wafer at a single time. Second, a wafer-level test on the operation of each semiconductor chip is possible during WLSCP processes. For these—and other reasons—WLCSP can be fabricated at a lower cost than general packaging. 
       FIGS. 1-3  illustrate several known wafer-level chip scale packages. As shown in  FIG. 1 , chip pads  40  are formed of a metal such as aluminum on a silicon substrate  5 . A passivation layer  10  is formed to expose a portion of each of the chip pads  40  on the silicon substrate  5  while protecting the remainder of the silicon substrate  5 . A first insulating layer  15  is formed over the passivation layer  10  and then a re-distribution line (RDL) pattern  20  (which re-distributes electrical signals from the bond pad  40  to solder bump  35 ) is formed over portions of the first insulating layer  15  and the exposed chip pads  40 . A second insulating layer  25  is formed on portions of the RDL pattern  20  while leaving portions of the RDL pattern  20  exposed. Under bump metals (UBM)  30  are formed between solder bumps  35  and the exposed portions of the RDL pattern  20 . The RDL pattern  20  contains inclined portions on the first insulating layer  15  near the chip pads  40 . In these areas, short circuits can occur and the pattern  20  can crack and deform in these areas due to stresses. 
     As depicted in  FIG. 2 , package  50  contains an RDL pattern  54  that adheres to a solder connection  52  in a cylindrical band. Such a configuration has several disadvantages. First, the contact area between the RDL pattern  54  and the solder connection  52  is minimal, thereby deteriorating the electrical characteristics between them. Second, short circuits may occur due to the stresses in the contact surface between the RDL pattern  54  and the solder connection  52 . Third, the solder connection  52 —which is connected with a solder bump  58  formed on a chip pad  56 —is exposed to the outside of the package  50 , i.e., to air. Thus, there is a higher possibility that moisture penetrates into the solder connection  52  and decreases the reliability of the solder connection  52 . Fourth, the package  50  is completed only by carrying out many processing steps and, therefore, manufacturing costs are high. 
     As shown in  FIG. 3 , package  60  contains a RDL pattern  76  that is electrically connected with a chip pad  72  via a connection bump  74 . The RDL pattern  76  is, however, inclined on the connection bump  74 , causing cracks therein due to stresses as described above. As well, the connection bump  74  is made by a plating process and is formed of aluminum, copper, silver, or an alloy thereof. Accordingly, the package  60  is not easy to manufacture. 
     Other problems exist with conventional WLSCP. Often, such packaging uses UMB (i.e., layer  30  in  FIG. 1 ) and two insulating layers (i.e., layers  15  and  25  in  FIG. 1 ) that are made of polymeric materials such as polyimide and benzocyclobutene (BCB). Such structures are complicated to manufacture and very expensive because of materials and equipment used. As well, the coefficient of thermal expansion (CTE) between the various layers can induce thermal stresses into the ICs and damage the ICs during high temperature curing of these polymeric materials. 
     As well, conventional packaging methods have used a conductive film or paste in flip chip packaging. See, for example, U.S. Pat. Nos. 5,9494,142, 6,509,634, and 6,518,097, the disclosures of which are incorporated herein by reference. Generally, these methods used a gold bump on a silicon die and then bonded it to a substrate (usually ceramic) using the conductive film or paste using ultrasonic bonding. Such methods, however, suffer from a high cost and poor reliability. 
     Further, the trend of semiconductor packaging including WL-CSP is to use smaller, lighter and thinner form factors that enable the manufacture of smaller semiconductor devices. The use of smaller form factors in a WL-CSP packaging with small die and large I/O, however, could result in manufacturing challenges. One of these challenges is the alignment of solder balls on a die  501  (i.e., small pitch) to the alignment of lands/pads on the printed circuit board  502  (i.e. large pitch) as illustrated in  FIG. 31 . 
     SUMMARY OF THE INVENTION 
     The invention provides a packaged semiconductor device (a wafer-level chip scale package) containing a conductive adhesive material as an electrical interconnect route between the semiconductor die and a patterned conductive substrate. The patterned conductive substrate acts not only as a substrate, but also as a redistribution layer that converts the dense pad layout of the die to a larger array configuration of the solder balls in the circuit board. Using the invention allows the formation of a lower priced chip scale package that also overcomes the restriction of the die size used in die-sized chip packages and the input-output pattern that can be required by the printed circuit board. Thus, the invention can provide a familiar pitch (i.e., interface) to the printed circuit board for any small die. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-39  are views of one aspect of the devices and methods of making the devices according to the invention, in which: 
         FIG. 1  is a cross-sectional view of a conventional wafer-level chip scale package; 
         FIG. 2  is a cross-sectional view of another conventional wafer-level chip scale package; 
         FIG. 3  is a cross-sectional view of another conventional wafer-level chip scale package; 
         FIG. 4  is a cross-sectional view showing a stage in a method of fabricating a wafer-level chip scale package according to an aspect of the invention; 
         FIG. 5  is a cross-sectional view showing a stage in a method of fabricating a wafer-level chip scale package according to an aspect of the invention; 
         FIG. 6  is a cross-sectional view showing a stage in a method of fabricating a wafer-level chip scale package according to an aspect of the invention; 
         FIG. 7  is a cross-sectional view showing a stage in a method of fabricating a wafer-level chip scale package according to an aspect of the invention; 
         FIG. 8  is a cross-sectional view showing a stage in a method of fabricating a wafer-level chip scale package according to an aspect of the invention; 
         FIG. 9  is a cross-sectional view showing a stage in a method of fabricating a wafer-level chip scale package according to an aspect of the invention; 
         FIG. 10  is a cross-sectional view showing a stage in a method of fabricating a wafer-level chip scale package according to an aspect of the invention; 
         FIG. 11  is a cross-sectional view of a wafer-level chip scale package according to one aspect of the invention; 
         FIGS. 12-15  illustrate stages in a method of fabricating a wafer-level chip scale package in one aspect of the invention; 
         FIG. 16  depicts another stage in a method of fabricating a wafer-level chip scale package in one aspect of the invention; 
         FIG. 17  depicts a process for making a wafer-level chip scale package in another aspect of the invention; 
         FIGS. 18-25  illustrate stages in a method of fabricating a wafer-level chip scale package in one aspect of the invention; 
         FIG. 26  depicts conductive particles that can be used in one aspect of the invention; 
         FIG. 27  depicts a wafer-level chip scale package in one aspect of the invention; 
         FIG. 28  shows stages in a method of fabricating a wafer-level chip scale package in one aspect of the invention; 
         FIGS. 29-30  illustrate stages in a method of fabricating a wafer-level chip scale package in one aspect of the invention; 
         FIG. 31  illustrates a problem with conventional wafer-level chip scale packaging; 
         FIGS. 32-33  illustrate stages in a method of fabricating a wafer-level chip scale package in one aspect of the invention; 
         FIG. 34  illustrates a potion of a wafer-level chip scale package in one aspect of the invention; 
         FIGS. 35-36  illustrate stages in a method of fabricating a wafer-level chip scale package in one aspect of the invention; 
         FIG. 37  illustrates a potion of a wafer-level chip scale package in one aspect of the invention; 
         FIG. 38  illustrates a stage in a method of fabricating a wafer-level chip scale package in one aspect of the invention; and 
         FIG. 39  illustrates a wafer-level chip scale package in one aspect of the invention. 
     
    
    
       FIGS. 1-39  presented in conjunction with this description are views of only particular—rather than complete—portions of the devices and methods of making the devices according to the invention. Together with the following description, the Figures demonstrate and explain the principles of the invention. In the Figures, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. The same reference numerals in different drawings represent the same element, and thus their descriptions will be omitted. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention now will be described more fully with reference to the accompanying drawings, in which one aspect of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the aspects set forth herein. Rather, these aspects are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art. Although the invention is described with respect to IC chips, the invention could be used for other devices where packaging is needed, i.e., silicon MEMS devices, LCD displays, optoelectonics, and the like. 
       FIGS. 4 through 10  illustrate one aspect of the invention for fabricating a wafer-level chip scale package containing a re-distributed line (RDL) pattern that is not inclined between the bottom of a solder bump and the top surface of a chip pad. Referring to  FIG. 4 , a substrate (or chip)  100  is prepared on which a passivation layer  110  and a chip pad  115  are formed. The substrate  100  can be any known semiconductor substrate known in the art, including “compound” semiconductors and single crystal silicon. The passivation layer  110  can be made of any dielectric material known in the art, such as silicon nitride, silicon oxide, or SOG. 
     Then, the chip pad  115  is formed on the upper surface of substrate  100 . First, a portion of passivation layer in this area is removed by a conventional masking and etching process. Then, the metal for the chip pad  115  is blanket deposited and the portions of the metal layer not needed for the bond pad are removed by etching or planarization. The chip pad  115  can be made of conductive material, such as metals and metal alloys. In one aspect of the invention, the chip pad comprises aluminum. 
     A wire  120  is next attached to the chip pad  115  using a capillary  130 . As shown in  FIG. 5 , the bottom of the wire  120  is bonded to the chip pad  115 . Then a coining process is performed to press the wire  120  under a predetermined pressure, thereby forming a coined stud bump  125 . By using the capillary  130 , the coined stud bump  125  can be formed with a simple structure and with a simple manufacturing process. 
     As depicted in  FIG. 6 , a first insulating layer  135  is then deposited to cover the coined stud bump  125  and passivation layer  110 . In this aspect of the invention, the first insulating layer  135  is formed of a dielectric polymer material such as BCB, polyimide (PI), and epoxy molding compound (EMC). As illustrated in  FIG. 7 , the first insulating layer  135  and the coined stud bump  125  are planarized using conventional processing. In the planarization process, a stud bump  125 ′ and a first insulating layer  135 ′ as formed. In one aspect of the invention, a chemical mechanical polishing (CMP) process is used to planarize the first insulating layer  135  and the stud bump  125 . 
     As shown in  FIG. 8 , a re-distributed line (RDL) pattern  140  is formed on the stud bump  125 ′ and the first insulating layer  135 ′. The RDL pattern  140  electrically connects the stud bump  125 ′ and the solder bump that is formed during subsequent processing (as described below). The RDL pattern is formed by blanket depositing a metal layer and then removing—typically by masking and etching—the portions of the metal layer not needed for the RDL pattern  140 . The RDL pattern  140  can contain any electrically conductive material, such as metals and metal alloys. Examples of such metal and metal alloys include Cu, Al, Cr, NiV, and Ti. In one aspect of the invention, the RDL comprises a composite layer of Cu, Al, Cr, and Cu, or a material selected from NiV and Ti. In conventional wafer-level chip scale package as shown in  FIG. 1 , the RDL pattern  20  was formed of Al, NiV, Cu, NiV, and Cu that are sequentially deposited on the chip pad  40 . Such a configuration has poor adhesive characteristics and reliability, is not easy to fabricate, and high manufacturing costs. 
     As depicted in  FIG. 9 , a second insulating layer  150  is then formed to cover the RDL pattern  140  and the first insulating layer  135 ′. A portion of the second insulating layer  150  is removed—typically by masking and etching—to expose a portion of the RDL pattern  140  to which a solder bump is later attached. As shown in  FIG. 10 , a solder bump  160  is then attached to the exposed portion of the RDL pattern  140  as known in the art. 
     The stud bump comprises any conductive material such as metal and metal alloys. In one aspect of the invention, the stud bump comprises gold (Au) or copper (Cu). 
     The wafer-level chip scale package  1000  is illustrated in  FIG. 10 . The silicon substrate  100  contains an IC (not shown) and chip pad  115  which extends into the passivation layer  110  and is encircled by the passivation layer  110 . Electrical signals from the IC contained in substrate  100  are transmitted through chip pad  115 , through RDL pattern  140 , to solder bump  160 , and then to the outside of the packaged semiconductor device (i.e., to a circuit board). 
     In the device of  FIG. 10 , the first insulating layer  135 ′ encircles and covers the stud bump  125 ′. Since the top surface of the first insulating layer  135 ′ and stud bump  125 ′ are coplanar in this aspect of the invention, the RDL pattern  140  may be formed as a substantially planar layer without an inclined portion. Therefore, cracks in the RDL pattern  140  due to stresses are prevented. 
     The RDL pattern  140  shown in  FIG. 10  is illustrated as on only a portion of the upper surface of the stud bump  125 ′. In another aspect of the invention, the RDL pattern can be formed to cover the entire stud bump  125 ′, thus enhancing the electrical characteristics and reliability of the wafer-level chip scale package  1000 . 
     The RDL pattern  20  of  FIG. 1  contains an inclined portion in the conventional wafer-level chip scale package. Accordingly, it is extremely difficult to form a thick first insulating layer  15  in  FIG. 1 . In this aspect of the invention, however, the first insulating layer  135 ′ in  FIG. 10  is formed as thick layer. 
       FIG. 11  illustrates another aspect of the invention where a wafer-level chip scale package has a two-layer RDL pattern. A wafer-level chip scale package  2000  contains: a substrate (or chip)  100 ; a passivation layer  110 ; chip pads  115 ; stud bumps  125 ′ that are formed on chip pads  115  and are encircled by a first insulating layer  135 ′; intermediate RDL pattern  210  that connects the stud bumps  125 ′ and intermediate stud bumps  220 ; an intermediate insulating layer  230  that insulates the intermediate RDL pattern  210 ; RDL pattern  140  that connects the intermediate stud bumps  220  and solder bumps  160 ; a second insulating layer  150  that insulates the RDL patterns  140 ; and solder bumps  160  that are attached to a portion of each of the RDL pattern  140 . 
     Components not described in  FIG. 11  are the same as those components explained with reference to  FIG. 10 . The same reference numerals in  FIGS. 10 and 11  denote the same elements that have substantially the same functions and are formed of the same materials and in substantially the same manner. The structure, functions, materials, and effects of the intermediate stud bumps  220 , the intermediate RDL pattern  210  and the intermediate insulating layer  230  are substantially the same as those of the stud bump  125 , the RDL pattern  140 , and the second insulating layer  150 , respectively. The intermediate stud bumps  220  connect the intermediate RDL pattern  210  and the RDL pattern  140 . Each intermediate RDL pattern  210  is formed at the bottom of each intermediate stud bump  220 . The intermediate insulating layer  230  exposes a portion of the intermediate RDL pattern  210  so it can be connected with the intermediate stud bumps  220 . 
     In another aspect of the invention, additional intermediate stud bumps, intermediate RDL patterns, and intermediate insulating layers may be formed to make three (or more) layer RDL pattern rather than the two layer RDL pattern illustrated in  FIG. 11 . 
     In the aspects of the invention described above, it is possible to reduce or prevent an inclined portion of a RDL pattern in the art between a solder bump and a chip pad. Such a configuration suppresses cracks in the RDL pattern, even where an underlying insulating layer has a large thickness. Further, a stud bump can be easily and inexpensively formed using a capillary. 
     In another aspect of the invention, the wafer level chip scale package is manufactured in the manner depicted in  FIGS. 12-17  so as to not contain a UBM between the chip pad the RDL pattern and to contain a single non-polymeric insulating layer. In this aspect of the invention, and as depicted in  FIG. 17 , the bond pads are first redistributed (as depicted in more detail in  FIGS. 12-15 ). Then, the stud bumps are formed on the wafer (as depicted in more detail in  FIG. 16 ). The solder balls are then attached to the stud bumps, either directly or by using solder paste, and the solder balls are re-flowed. The resulting packaged semiconductor device can then be mounted on a circuit board as known in the art. 
     In this aspect of the invention, and as illustrated in  FIGS. 12-13 , a substrate (or chip)  300  (substantially similar to substrate  100 ) containing IC  305  is obtained. A passivation layer  310  (substantially similar to passivation layer  110 ) is then formed on substrate  300 . A portion of the passivation layer is then removed and a chip pad  315  (substantially similar to chip pad  115 ) is formed in that exposed portion. The methods used for these processes are substantially similar to those described above. 
     Next, as depicted in  FIG. 14 , a re-distributed (RDL) pattern  340  is formed on directly on the chip pad  315  and the passivation layer  310 . The RDL pattern  340  electrically connects the chip pad  315  and the solder bump  365  that is formed during subsequent processing (as described below). The RDL pattern  340  is formed by blanket depositing a metal layer and then removing—typically by masking and etching—the portions of the metal layer not needed for the RDL pattern  340 . The RDL pattern  340  can contain any electrically conductive material, such as metals and metal alloys. Examples of such metal and metal alloys include Cu, Al, Cr, NiV, and Ti. In one aspect of the invention, the RDL pattern comprises Al. 
     Next, as shown in  FIG. 15 , an insulating layer  350  is formed to cover the RDL pattern  340 . In this aspect of the invention, the material for the insulating layer is blanket deposited on the RDL pattern  340 . A masking and etching process is then used to remove a portion of this insulating material in the area of region  375  (where stud bumps  365  will later be formed). 
     The material for the insulating layer  350  does not comprise a polymer material like BCB, PI, and EMC. As described above, such materials are often used in conventional WLCSP. To form such layers, however, the structure containing the material is subjected to a high temperature heating process. This heating is necessary to cure the polymer material. Unfortunately, such a high temperature heating process damages the structure underlying the polymeric material including the IC  305  in substrate  300 . 
     In this aspect of the invention, the insulating layer  350  is not made of polymeric materials. Rather, the insulating layer  350  is made of dielectric non-polymeric materials. Examples of such non-polymeric dielectric materials include silicon nitride, silicon oxide, and silicon oxynitride. Such materials can be deposited by any known method in the art. 
     In this aspect of the invention, only a single layer is used as the redistribution layer. In the aspect of the invention shown in  FIGS. 4-10 , a UBM and a metal layer are used to redistribute the electrical signal from the chip pad  115  to the stud bump  160 . By using only a metal layer in this aspect of the invention, the cost of the manufacturing the UBM can be eliminated. Thus, this aspect of the invention uses only a single conductive layer as the RDL pattern in the WLSCP. 
     As depicted in  FIG. 16 , the stud bumps are then formed on the exposed portion of the RDL pattern  340  (in the area  375 ). The stud bumps  365 A can be formed by electroplating the material for the stud bumps and with a cladding as known in the art. In this aspect of the invention, the material for the study bumps is Cu and the cladding is a Ni/Au alloy. 
     Alternatively, the stud bumps  365 B can be formed by a wire bonding process. In this aspect of the invention, a coated wire  380  is attached to the RDL pattern  340  using a capillary  385 . As shown in  FIG. 16 , the bottom of the wire  380  is first bonded to the metal of the RDL pattern  340 . Then a coining process is performed to press the wire  380  under a predetermined pressure to form a coined stud bump  365 B. By using the capillary, the coined stud bump  365 B can be formed with a simple structure and with a simple manufacturing process. In one aspect of the invention, the material for the wire comprises Cu and the coating comprises Pd. 
     Finally, as shown in  FIG. 17 , the solder balls are then attached to the stud bumps, either directly or by using solder paste, and the solder balls are re-flowed. Both of these processes are performed using conventional processing that is known in the art. 
     In yet another aspect of the invention, the wafer level chip scale package is manufactured in the manner depicted in  FIGS. 18-30 . Using this process eliminates the steps of dispensing the solder and reflowing the solder bumps, and optionally eliminates the use of a redistribution trace. In this aspect of the invention, an adhesive film or paste is used between the chip and the substrate. 
     In this aspect of the invention, and as illustrated in  FIGS. 18-19 , a substrate (or chip)  400  (substantially similar to substrate  100 ) containing IC  405  is provided. A passivation layer  410  (substantially similar to passivation layer  110 ) is then formed on chip  400 . A portion of the passivation layer is then removed and a chip pad  415  (substantially similar to chip pad  115 ) is formed in that exposed portion. The methods used for these processes are substantially similar to those described above. 
     Next, as depicted in  FIG. 20 , a re-distributed (RDL) pattern  440  is optionally formed on directly on the chip pad  415  and the passivation layer  410 . The semiconductor package can be made with or without the RDL pattern  440  depending on whether re-distribution is necessary. When used, the RDL pattern  440  electrically connects the chip pad  415  and the solder bump  465  that is formed during subsequent processing (as described below). The RDL pattern  440  is formed by blanket depositing a metal layer and then removing—typically by masking and etching—the portions of the metal layer not needed for the RDL pattern  440 . The RDL pattern  440  can contain any electrically conductive material, such as metals and metal alloys. Examples of such metal and metal alloys include Cu, Al, Cr, NiV, and Ti. In one aspect of the invention, the RDL pattern comprises Al. 
     Next, when the RDL pattern  440  is used, an insulating layer  450  is formed to cover the RDL pattern  440  as shown in  FIG. 20 . In this aspect of the invention, the material for the insulating layer is blanket deposited on the RDL pattern  440 . A masking and etching process is then used to remove a portion of this insulating material in the area of region  475  (where stud bump  465  will later be formed). 
     The material for the insulating layer  450  does not comprise a polymer material like BCB, PI, and EMC. As described above, such materials are often used in conventional WLCSP. To form such layers, however, the structure containing the material is subjected to a high temperature heating process. This heating is necessary to cure the polymer material. Unfortunately, such a high temperature heating process damages the structure underlying the polymeric material including the IC  405  in substrate  400 . 
     In this aspect of the invention, the insulating layer  450  is not made of polymeric materials. Rather, the insulating layer  450  is made of dielectric non-polymeric materials. Examples of such non-polymeric dielectric materials include silicon nitride, silicon oxide, and silicon oxynitride. 
     Then studs (or stud bumps)  465  are formed on the structures depicted in  FIG. 19  (without a redistribution layer) and  FIG. 20  (with a redistribution layer). As depicted in  FIGS. 21 and 22 , the studs  465  are respectively formed on the chip pad  415  and the exposed of the RDL pattern  440  (in the area  475 ). The stud bumps  465  can be formed by electroplating the material for the stud bumps with a cladding as known in the art. In one aspect of the invention, the material for the stud bumps is Cu and the cladding is Pd. Alternatively, the stud bumps  465  can be formed by a wire bonding process as described above. 
     Next, as shown in  FIGS. 23 and 24 , an adhesive layer  458  containing conductive particles  459  is applied to the structures of  FIGS. 21 and 22 . The adhesive layer  458 , as described herein, attaches the chip  400  and the substrate  101  while serving as a limited conductor. Any material functioning in this manner can be used as the adhesive layer  458 , including an adhesive material with conductive particles therein. In one aspect of the invention, the adhesive layer  458  comprises an ACF (anisotropic conductive film), an ACP (anisotropic conductive paste) or ICP (isotropic conductive paste). 
     The adhesive layer  458  can be applied using any known mechanism in the art. For example, when ACP is used as the adhesive, the layer  458  can be applied by stencil printing. As another example, when ACF is used as the adhesive, the layer  458  can be applied by a film attach process. 
     The conductive particles  459  can be any known in the art that can be used with the material of the adhesive. Examples of conductive particles that can be used in adhesive layer  458  are illustrated in  FIG. 26 . Conductive particle  459   a  comprises a polymer particle with a metal layer surrounded by an insulating layer. Conductive particle  459   b  comprises a metal particle surrounded by an insulating layer. The insulating layers in the conductive particles are broken—thereby creating a conductive path—when there is contact between the stud bumps and the substrate (as described below). 
     Next, substrate  101  with bond pads  201  (also called electrode pads) is provided. The bond pad  201  is that portion through which the substrate  101  is attached to the chip  400  containing studs  465 . The bond pads  201  can be provided on the substrate  101  as known in the art. In one aspect of the invention, the bond pads are provided by a conventional deposition and etching process. The substrate  101  can be made of any suitable material. One example of a suitable material for the substrate is high glass-transition materials like bis-malesimide triazine (BT) epoxy. 
     Next, any know flip chip procedure is used to attach the chip  400  and the substrate  101 . In one aspect of the invention, chip  400  containing studs  465  is flipped and placed on the substrate  101  containing the adhesive  458 . Alternatively, as depicted in  FIG. 25 , the adhesive layer  458  could be placed on the chip  400  and the substrate  101  flipped and placed on the chip  400 . In yet another aspect of the invention, the adhesive layer can be formed on both the chip  400  and the substrate  100  before they are attached. When contacting the substrate  101  and the chip  400 , the bond pads  201  and the studs  465  should be substantially aligned as known in the art. 
     Next, pressure is applied while the adhesive material is pre-cured, thereby preliminarily connecting chip  400  and substrate  101 . The pressure in this process need only be enough to keep the chip  400  and substrate  101  together while the adhesive layer  458  is pre-cured. The pressure that is applied generally can range from about 2 to about 3 Kgf/cm 2  generally for about 0.2 to about 5 seconds. 
     The adhesive material is then finally cured by any mechanism in the art, which will depend on the material used. Generally, light and/or heat can be applied to cure the adhesive layer  458 . In one aspect of the invention, the adhesive is cured by heating for a sufficient time (greater than about 20 seconds) and at a sufficient temperature (in the range of about 180 degrees Celsius) to finish the curing process. 
     The adhesive layer  458  contains conductive particles  459  that will become positioned at intervals inside the adhesive layer  458 . Thus, as illustrated in  FIG. 27 , when the chip  400  and the substrate  101  are attached, at least one conductive particle becomes located between the stud bumps  465  and the bond pads  201 . Because the bulk of the adhesive layer  458  is not a conductive material, the only conduction between the chip  400  and the substrate  101  is through the conductive particles located between the stud bumps  465  and the bond pads  201 . 
     After the chip and the substrate have been attached to each other, the resulting structure is as depicted in  FIG. 27 . Then, this structure is encapsulated through any procedure known in the art. In one aspect of the invention, the encapsulation is carried out, as illustrated in  FIG. 28 , by first applying a support film  501  to the backside of the substrate  201 . In one aspect of the invention, the support film is a polyimide (PI) film. Next, the molding compound  502  is applied by any known means, e.g., by transfer molding using an epoxy molding compound, by an applied liquid molding compound in a strip form, or by an array molding. After the molding compound is applied, the support film  501  is removed using any known process in the art. 
     After the molding process, the non-singulated semiconductor packages may be electrically tested. Parametric testing is performed while the semiconductor packages are in the form of a strip. After electrical testing, the molded molding material in the semiconductor packages may be laser marked. After laser marking, the semiconductor packages in the array of semiconductor package are singulated using any suitable process, such as by sawing and scribing. 
       FIGS. 18-28  depicts the use of chip pad  415  in the WLCSP. In one aspect of the invention, the chip pad  415  can be eliminated. The chip pad is typically used to protect the chip (IC  405 ) during subsequent processing. Such a function can also be accomplished by the adhesive layer  458 . Thus, in this aspect of the invention, the chip pad  415  can be eliminated as depicted in  FIGS. 29-30 . 
     In this aspect of the invention, the semiconductor packages have the following advantages. First, the semiconductor packages are more reliable. Known semiconductor packages made using by a flip chip method with an ACF were prone to fail for two reasons. First, formation of non-conductive film on either the contact area or on the conductive particles. Second, there was a loss of mechanical contact between the conductive elements due to either loss of adherence or relaxation of the compressive force. In the invention, these failure mechanisms are reduced or eliminated by encapsulation. The encapsulation reduces moisture attacks and oxidation of the conductive particles. The encapsulation also provides compressive residual stress on the ACF and reduces creep at high temperatures/times. 
     A second advantage is that the adhesive material (ACF and ACP) does not contain substantial amounts of lead and are, therefore, more environmentally friendly than solder. A third advantage is that the semiconductor packages of the invention offer higher resolution capability than those currently using solder paste because of the smaller particle size. A fourth advantage is that the semiconductor packages of the invention are cured at much lower temperatures than those required for soldering, thus reducing thermal stress and is better for thermally sensitive components and the substrate. A final advantage is that less process steps are needed as compared to soldering process, e.g., the flux and flux cleaning processes are not needed. 
     In yet another aspect of the invention, the wafer level chip scale package is manufactured in the manner depicted in  FIGS. 32-39 . In this aspect of the invention, and as illustrated in  FIG. 32 , a conductive substrate  515  (also referred to as “substrate” or “substrate  515 ”) is provided. In this aspect of the invention, conductive substrate  515  serves as a substrate for the adhesive layer as described below and comprises a material that is later used to form conductive signal traces as described below, as well as a substrate. Thus, the substrate  515  can be formed using any material that serves these functions. In one aspect of the invention, the substrate  515  is formed from a conductive material like metallic foil or metallic alloy foil, such as Cu or Al. In one aspect of the invention, the substrate  515  comprises Cu foil. In another aspect of the invention, the Cu foil is a Cu foil with a weight sufficient for handling and stability and that is able to carry a reasonable amount of current. Generally, the CU foil can be about 0.5 to about 2 oz. In one aspect of the invention, the Cu foil is a 2 oz foil. Optionally, the handling and stability of the Cu foil can be increased by using a polymeric adhesive such as a B-stage polymer on the back side of the Cu foil. In another aspect of the invention, the substrate could comprise a plurality of metallic foil layers. 
     The conductive substrate  515  can have any thickness consistent with its functions described above. Generally, the thickness of the substrate can range from about 70 to about 300 micrometers. In one aspect of the invention, such as when Cu foil is used, the thickness of the substrate  515  is about 70 micrometers. 
     As shown in  FIG. 32 , an adhesive layer  510  containing conductive particles  559  is applied to the conductive substrate  515 . The adhesive layer  510 , as described herein, attaches the substrate  515  to the die  500  (as described below) while also serving as a limited conductor. Any material functioning in this manner can be used as the adhesive layer  510 , including an adhesive material with conductive particles therein, as well as those materials described above for adhesive layer  458 . In one aspect of the invention, the adhesive material can have the form of a liquid (such as a paste like an ACP or ICP) or a solid (such as a film like ACF). Both the liquid form and the solid form of the adhesive material contain substantially the same composition (in terms of conductive particles, resin, hardener, etc. . . . ), but have differing amounts of diluent (i.e., low amounts of diluent for the solid and higher amounts for the liquid). Since the diluent evaporates during subsequent processing, both the liquid form and solid form of the adhesive material can be used in the invention. 
     The adhesive layer  510  can be applied using any known mechanism in the art. For example, when ACP is used as the adhesive, the layer  510  can be applied by stencil printing. As another example, when ACF is used as the adhesive, the layer  510  can be applied by a film attach process. 
     The conductive particles  459  can be any known in the art that can be used with the material of the adhesive. Examples of conductive particles that can be used in adhesive layer  510  are illustrated in  FIG. 26 . Conductive particle  459   a  comprises a polymer particle with a metal layer surrounded by an insulating layer. Conductive particle  459   b  comprises a metal particle surrounded by an insulating layer. The insulating layers in the conductive particles are broken—thereby creating a conductive path—when there is contact between the stud bumps and the substrate (as described above). Generally, the conductive particles comprise about 1 to about 40 wt % of the adhesive material. In one aspect of the invention, the conductive particles comprise about 4 to about 20 wt % of the adhesive material. 
     The content, size, and specie of conductive particle—and therefore the amount of conductivity in the adhesive layer  510 —can be designed for the specific type of device that is desired. Thus, the insulation resistance of the adhesive layer  510  can be adjusted from about 10 8  cmΩ (for particles containing about 20% Au with a pitch of less than 30) to about 10 15  cmΩ or more (for particles containing about 4% Ni). 
     The adhesive layer  510  can have any thickness consistent with its functions described above. Generally, the thickness of the adhesive layer  510  can range from about 5 to about 200 micrometers. In one aspect of the invention, such as when ACF made of epoxy or an acryl based material containing conductive filler is used, the thickness of the adhesive layer  510  can range from about 25 to about 50 micrometers. The adhesive layer should match the height of the bumps on the die. 
     As shown in  FIG. 32 , a die  500  is then provided. Die (or chip)  500  can be any conventional die as known in the art. An optional passivation layer (substantially similar to passivation layer  110 ) can then be formed on die. When used, a portion of the passivation layer is then removed in the area of integrated circuit  508  and a chip pad  512  is formed in that exposed portion. The methods used for these processes are substantially similar to those described above. 
     Then studs (or stud bumps)  505  are formed. As depicted in  FIG. 32 , the studs  505  are respectively formed on the chip pad  512 . The studs  505  can be formed by electroplating the material for the studs with a cladding as known in the art. In one aspect of the invention, the material for the stud bumps is Cu and the cladding is Pd. Alternatively, the studs  505  can be formed by a wire bonding process as described above. 
     Next, any know flip chip procedure is used to attach the die  500  (containing studs  505 ) and the conductive substrate  515  with the adhesive layer  510  thereon. In one aspect of the invention, die  500  containing studs  505  is flipped and placed on the substrate  515  containing the adhesive layer  510 . Alternatively, as described above, the adhesive layer  510  could be placed on the die  500  containing the studs  505  and the substrate  515  flipped and placed thereon. In yet another aspect of the invention, the adhesive layer  510  can be formed on both the die  500  and the substrate  515  before they are attached. 
     Next, pressure is applied while the adhesive material is pre-cured, thereby preliminarily connecting die  500  and substrate  515 . The pressure in this process need only be enough to keep these two components together while the adhesive layer  510  is pre-cured. The pressure that is applied generally can range from about 2 to about 3 Kgf/cm 2  and generally is applied for about 0.2 to about 5 seconds. 
     The adhesive material is then finally cured by any mechanism in the art, which will depend on the material used. Generally, light and/or heat can be applied to cure the adhesive layer  510 . In one aspect of the invention, the adhesive is cured by heating for a sufficient time (greater than about 20 seconds) and at a sufficient temperature (in the range of about 180 degrees Celsius) to finish the curing process. 
     After this thermo-compression bonding process, the die  500  and substrate  515  are connected both electrically and mechanically as shown in  FIG. 33 . The adhesive layer  510  contains conductive particles  459  that will become positioned at intervals inside the adhesive layer  458 . Thus, as illustrated in  FIG. 34 , when the die  500  and the substrate  515  are attached, at least one conductive particle becomes located between the studs  505  and the substrate  515 . Because the bulk of the adhesive layer  510  is not a conductive material, substantially the only conduction between the die  500  and the substrate  515  is through the conductive particles. 
     After the die and the substrate have been attached to each other, the resulting structure is as depicted in  FIG. 33 . Then this structure is encapsulated through any procedure known in the art. In one aspect of the invention, the encapsulation is carried out, as illustrated in  FIG. 35 , by any overmolding process known in the art. See, for example, the process described in U.S. Pat. No. 6,537,853, the disclosure of which is incorporated herein by reference. In one aspect of the invention, the overmolding process includes those molding processes described above. 
     After the molding process that forms encapsulation  517 , the substrate  515  is then etched to form the electrical signal traces. Any etching process known in the art can be used to etch the substrate  515  to form the signal traces  516 . In one aspect of the invention, the etching is performed by conventional process which comprise of photoresist coating, developing, etching and stripping. In another aspect of the invention, the etching process is performed so the patterned substrate acts a redistribution layer that converts the dense pad layout formed on the die  500  to a larger pitch lay out needed for the circuit board. As shown in  FIG. 37 , the patterned substrate redistributes the electrical signal from the die pad locations  505  to the locations  525  where the solder balls will be located and used for mounting on a circuit board. 
     In one aspect of the invention, the traces  516  comprising the patterned substrate are then plated. This plating process places a conductive layer over the traces to protect the patterned substrate from oxidation. Any conductive material that serves this purpose can be used in the invention, such as Au, Ni, Pd, and combinations or alloys thereof. In one aspect of the invention, a Ni/Pd alloy is used as the material for the plating. 
     Next, a solder resist layer  530  is optionally formed on the resulting structure. Because the adhesive layer  510  could have a high moisture absorption, the solder resist  530  layer can protect the adhesive layer  510  from moisture and the outside impact could be printed on the patterned substrate  515  to exclude the region for solder ball attach. If desired, the solder balls  535  can then be attached through any process known in the art, as well as those described above. 
     Next, the non-singulated semiconductor packages may be electrically tested. Parametric testing is performed while the semiconductor packages are in the form of a strip. After electrical testing, the molded molding material in the semiconductor packages may be laser marked. After laser marking, the semiconductor packages in the array of semiconductor package are singulated as shown in  FIG. 39  using any suitable process, such as by sawing and scribing. 
     In this aspect of the invention, the semiconductor packages have the following advantages. First, the semiconductor packages are formed using a simple process. Second, the conductive substrate attached to the die also serves as a redistribution layer. Third, the same die can be used both in wire bonding packages and in flip chip bonding packaging without any modification. Fourth, the cost for production (especially for assembly) is much lower than the cost for conventional WLCSP. Finally, the decrease in the size of the package is quite substantial. For example, die sizes of 1.9 mm×2.5 mm typically require a package with a size of about 4 mm×4 mm. Using the invention, the same die size would only require package size of about 3 mm×2.5 mm or less. 
     Having described these aspects of the invention, it is understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description, as many apparent variations thereof are possible without departing from the spirit or scope thereof.