Abstract:
The present invention disclosed a wafer level chip size packaged chip device with a double-layer lead structure and methods of fabricating the same. The double-layer lead is designed to meet a tendency of increasing quantity per area of peripheral arrayed compatible pads on a semiconductor chip, and also to save more space for layout of lead on the chip bottom surface for avoiding potential short inbetween which happen in increasing probability with increasing quantity per area on the condition of one-layer lead.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a wafer level chip size packaging technology, and more particularly to a wafer level chip size packaged chip device with a double-layer lead package structure rather than a one-layer external lead package structure in standard Shellcase type wafer level chip size packaging technology, and to methods for the manufacture thereof. 
       BACKGROUND OF THE INVENTION 
       [0002]    With the miniaturization of electronic devices and increasing circuit density in semiconductor industry, chip size package (CSP) is developed, where the package size is similar to the semiconductor chip encased within the package. Conventional packaging technologies, wire bonding, tape automatic bonding (TAB) and flip chip, have their own disadvantages. In wire bonding and TAB, a semiconductor package has a footprint much larger than that of the primitive chip. Flip chip package involves a direct electrical connection of face down electronic components onto substrates/carriers via conductive solder balls/bumps of the chip. The flip-chip package encounters a problem, namely, cracking of solder ball joint due to large thermal expansion mismatch between a wafer and a substrate. Chip size package is manufactured either in the form of individual chips diced from a wafer, or in a wafer form and then the individual chip size packages are singulated from the wafer. The latter is referred to as a wafer level chip size package (hereinafter WLCSP). For WLCSP, generally a plurality of compatible pads formed in a peripheral arrayed type on semiconductor chips are redistributed through conventional redistribution processes involving a redistribution layer into a plurality of metal pads, sometimes called solder bumps, in an area array type. The solder bumps on a WLCSP surface are much larger in diameter and much farer inbetween, and the printed circuit board assembly of a WLCSP is more robust. This kind of WLCSP technique has superior electrical performance and lower manufacturing costs than other packaging types, so it will play an important role in the production of future electronics. 
         [0003]    Shellcase Co. Israel developed its unique and advanced WLCSP technology, classified ShellOP, ShellOC, and ShellUT, to package optical and image sensors, e.g., charge-coupled devices (CCD) and/or CMOS imagers integrated on a silicon wafer. Currently, CCD and CMOS imagers are explosively used in electronic products. Unlike many packaging methods, the Shellcase process requires no lead frames, or wire bonding. Briefly, ShellOP utilizes a glass/silicon/glass sandwich structure to enable image-sensing capabilities through the actual packaging structure and to protect the sensors from being contaminated by external environment. ShellOC adopts the same sandwich structure, but extra cavities are configured on a first glass which is bonded to a silicon wafer with integrated circuits on for accommodating the above imagers. Also, cavities enable the use of micro-lenses for enhanced image quality. ShellOC is thus the packaging solution of choice for image sensors with micro-lenses. In the ShellUT package, cavities are still kept but a second glass is removed so that the associated package height is reduced. It is expected that ShellUT package should be a mainstream technology among Shellcase type packaging technology in the future. U.S. Pat. Nos. 6,646,289, 6,777,767 and 6,972,480 are considered to be relevant. 
         [0004]      FIG. 1  is a typical cross-section of prior art ShellOC packaged chip device with a one-layer lead structure and T-shape junction thereof. As shown in  FIG. 1 , a first/top glass  5  with cavity walls  10  thereon covers compatible pads  15  furnished silicon chip  20 . An epoxy  25  is used to bond a second/bottom glass  30  to the chip  20  on which a portion of compatible pads have been exposed before by means of photolithography and plasma etching techniques. After a barrier solder mask  35  is coated on the glass  30 , notching is performed so that inverted leads  40 , via sputtering deposition, connect electrically to the compatible pads  15  in the form of so-called T-shape junction as marked by circle. The leads  40  are coated with a protective solder mask  45  thereon. The solder-mask  45  is a dielectric material that electrically isolates the leads  40  from external contact, and protects the lead surface against corrosion. Solder bumps  50  are attached to the bottom end of leads  45 , and are suitable for printed circuit board (PCB) mounting by known methods. Solder bumps  50  may be formed by known methods such as screen printing, and may be suitably shaped for PCB mounted. 
         [0005]    In the foregoing Shellcase type WLCSP technology, a one-layer lead is only incorporated in a whole package process, therefore, the quantity of per area of compatible pads is limited due to limited space for layout of lead on the chip bottom surface. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is intended to improve the quantity of per area of compatible pads and the electrical connection in wafer level chip size packaging technology. 
         [0007]    In accordance with one embodiment of the present invention, a wafer level chip size packaged chip device with a double-layer lead structure comprises: 
         [0008]    a substrate having formed thereon a silicon chip, with a plurality of compatible pads disposed at the periphery of said chip on said substrate; 
         [0009]    a first glass disposed above said chip and said substrate; 
         [0010]    a layer of insulating material disposed at the back of said substrate; 
         [0011]    a first metal layer disposed at the back of said layer of insulating material, with a through hole at the center of said first metal layer; 
         [0012]    a first solder mask disposed at the back of said first metal layer, with at least one opening on said first solder mask so as to enable part of said first metal layer exposed through said opening of said first solder mask; 
         [0013]    a patterned second metal layer disposed at the sides of said chip device and on the back of said first solder mask; 
         [0014]    a plurality of solder bumps each attached to a bottom end of said second metal layer so as to enable electrical connection between said compatible pads and said solder bumps via said second metal layer and said first metal layer. 
         [0015]    In accordance with another embodiment, a cavity is formed between said substrate and said first glass so as to receive therein said chip on said substrate. 
         [0016]    In accordance with another embodiment, a second glass is disposed between said layer of insulating material and said first metal layer. 
         [0017]    Alternatively, a third solder mask is disposed between said second glass and said first metal layer, acting as a stress buffer and enhance adhesion of said first metal layer. 
         [0018]    Wherein, said packaged chip device has T-shape junction between said compatible pads and said second metal layer, and has L-shape and U-shape junction between said first metal layer and said second metal layer, such that, each of said compatible pads is electrically connected with a solder bump via T-, L-, U-shape junctions. 
         [0019]    Alternatively, said packaged chip device has T-shape junction between said compatible pads and said second metal layer, and has T-shape and 2U-shape junctions between said first metal layer and said second metal layer, such that, each of said compatible pads is electrically connected with a solder bump via 2T-, 2U-shape junctions. 
         [0020]    The present invention further provides a method for fabricating the wafer level chip size packaged chip device with a double-layer lead structure, comprising following steps: 
         [0021]    providing a wafer, which has plurality of substrates having formed thereon a silicon chip, with plurality of compatible pads disposed at the periphery of said chip on said substrate; 
         [0022]    disposing a first glass above said chip and said substrate; 
         [0023]    disposing a layer of insulating material at the back of said substrate; 
         [0024]    disposing a first metal layer at the back of said layer of insulating material, with a through hole at the center of said first metal layer; 
         [0025]    disposing a first solder mask at the back of said first metal layer, with at least one opening on said first solder mask so as to enable part of said first metal layer exposed through said opening of said first solder mask; 
         [0026]    disposing a second metal layer at the sides of said chip device and on the back of said first solder mask; 
         [0027]    patterning said second metal layer, leaving enough space for layout of parts of said second metal layer; 
         [0028]    disposing plurality of solder bumps each attached to a bottom end of said second metal layer so as to enable electrical connection between said compatible pads and said solder bumps via said second metal layer and said first metal layer; 
         [0029]    cutting said wafer so as to form individual chip size packaged chip device. 
         [0030]    With such a double-layer lead, it is possible to realize application of considerable quantity per area of peripheral arrayed compatible pads on the silicon chip, improvement of electrical connection, and layout of lead on the chip bottom surface due to more space available. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a typical cross-section of prior art ShellOC packaged chip device with a one-layer lead structure and T-shape junction thereof; 
           [0032]      FIGS. 2A to 2L  show the schematic package process flow for fabricating the ShellOC packaged chip device with a double-layer lead structure and T-, L-, U-shape junctions thereof according to embodiment 1 of the present invention; 
           [0033]      FIG. 3  is a three-dimension cross-section of the ShellOC packaged chip device with a double-layer lead structure and 2T-, 2U-shape junctions thereof according to embodiment 2 of the present invention; 
           [0034]      FIGS. 4A to 4D  show the schematic package process flow for fabricating the ShellOC packaged chip device as shown in  FIG. 3 ; 
           [0035]      FIG. 5  is a typical cross-section of the ShellUT packaged chip device with a double-layer lead structure and 2T-, 2U-shape junctions thereof according to embodiment 3 of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    It should be pointed out that all figures in the invention are not drawn in a right proportion. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
         [0037]    We follow a whole package process flow, although some process steps of which are exactly the same as that of the standard Shellcase packaging technology, to present the current invention. In this way, it will help one to understand the present invention well. Still, the difference between the invention and the standard Shellcase packaging technology will be stressed therein. 
       Embodiment 1 
       [0038]    According to embodiment 1 of the present invention, ShellOC packaged chip device with a double-layer lead structure has T-, L-, U-shape junctions. As shown in  FIGS. 2A to 2L , the package process flow for fabricating the same are as follows: on a first glass  5 , cavity walls  10 , are formed by means of photolithography technique ( FIG. 2A ). With the aid of a high-temperature epoxy, the glass  5  with cavity walls  10  formed thereon is applied to cover the silicon chip  20  with optical or image sensors at its center and peripheral arrayed compatible pads  15  on ( FIG. 2B ), wherein an optical or imaging component (as shadowed at the center) is encased within a cavity, thereby preventing the optical/imaging component from being contaminated by outside environment. Following this, the chip  20  is first thinned at its non-active surface by using mechanical grinding and plasma technique in sequence, and further selectively etched by means of photolithography and plasma techniques, thus a portion of compatible pads  15  ( FIG. 2C ) being exposed through trench formation therein. An insulating material  25 , e.g., epoxy, is employed to fully fill the trench and therefore covers the silicon slope and the exposed compatible pads  15 . Afterwards, a second glass  30  is bonded to the silicon chip  20  ( FIG. 2D ). Until now, a so-called glass/silicon/glass doubly bonded sandwich structure has been produced in the terms of ShellOC technology. As a mechanical buffer layer for later notching, an insulating material  351 , solder mask, is coated on the glass  30  ( FIG. 2E ). Next, metal deposition  401  instead of notching in the standard process flow is conducted with sputtering deposition technique ( FIG. 2F ). Following this metal layer deposition, openings are constructed on the metal layer  401  through photolithography ( FIG. 2G ). Thereafter, a light sensitive solder mask  352  is coated onto the openings-formed metal layer  401 , where the metal layer is split and isolated by the openings filled with the solder mask  352  ( FIG. 2H ). The solder mask  352  is processed to make through photolithography predetermined openings, thereby exposing a portion of the metal layer  401  ( FIG. 2I ). Notching is executed along scribe lines so that the lateral side of compatible pads  15  are exposed and V-shape trenches are formed ( FIG. 2I ). Then, a second metal layer  402  is, again through sputtering technique, deposited on the solder mask  352  with the openings and the V-shape trenches such that the compatible pads  15  and the first metal  401  are both electrically connected to the second metal layer  402  ( FIG. 2J ). Note that the second metal layer  402  is further patterned through lithography technique, leaving enough space for layout of parts of the second metal layer  402  which only directly electrically connect to the compatible pads  15  as in standard Shellcase type packaging process (see  FIG. 1 ). Up to now, a so-called double-layer lead package structure has been constructed. In other words, by rerouting and redistribution of electrical connection lead via the above double-layer structure, some compatible pads  15  directly electrically connects along the notching edge surface to the end terminal of the second metal layer  402  whereas the rest of compatible pads  15  does via the first metal layer  401 . One may notice that the latter electrical connection is realized through T-, L-, and U-shape junctions in sequence as marked by circle. We might as well call it the double-layer lead structure with T-, L-, U-shape junctions. Obviously, the reliability of electrical connection with this structure is better than that of  FIG. 1 . For the following package steps, as in standard Shellcase type process, formation of under ball metallurgy on the second metal  402 , coating solder mask  45 , and BGA  50  printing are carried out in turn. 
         [0039]    Wherein the glass  5  is transmissive for light; the thermal expansion coefficient of the glass  5  and the glass  30  are preferred to be similar to that of the material of the chip; both of solder mask  351  and  352  are heat resistant; the metal layer is made of but not limited to Aluminium. 
       Embodiment 2 
       [0040]    To further improve electrical connection reliability, the ShellOC packaged chip device has a double-layer lead structure and 2T-, 2U-shape junctions thereof as shown in  FIG. 3 . 
         [0041]    Relative to the above embodiment 1, the completely same procedures as shown in  FIG. 2A-2H  are performed in this embodiment. But thereafter, openings on the solder mask  352  are constructed in a different structure ( FIG. 4A , 2U-shape) from the counterpart (1U-shape) shown in  FIG. 2I . With the procedures as in  FIG. 4B-4D  being finished, the double-layer lead structure with 2T-, 2U-shape junctions are finally formed. At the bottom edge, if T-shape junction is broken, the electrical connection is possibly okay. 
       Embodiment 3 
       [0042]    It should be noted that the present double-layer lead structure with T-, L-, U-shape and with 2T-, 2U-shape junctions is applicable to ShellUT and ShellOP.  FIG. 5 , as an example, shows the double-layer lead structure with 2T-, 2U-shape junctions associated with ShellUT, wherein cavities are still kept but the second glass is removed so that the associated package height is reduced. Herein relevant package processes are not detailed as they are very similar to embodiment 2. 
         [0043]    The embodiments shown and described above are preferred and illustrative but not restrictive, and other embodiments may include the same concept, scope and spirit of the invention. Some variations or modifications in other embodiments could be clear to those skilled in the art.