Abstract:
A method of forming an ultra-thin wafer level stack package and structure thereof are provided. The method includes providing a first wafer having a plurality of base chips thereon, selectively binding the first wafer to a second substrate, lapping the first wafer to reduce its thickness, dicing the lapped first wafer, bonding a plurality stack chips to each base chip and packaging the base chip with the bonded stack chips to form an IC package. Thus, each IC package comprises at least a base chip and a stack chip. The IC package has a size almost identical to the base chip and a thickness a little larger than the combined thickness of the base chip and the stack chip. If a known good die inspection of the base chips and stack chips are carried out prior to wafer level packaging, overall yield of the IC package is increased.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority benefit of Taiwan application serial no. 93102569, filed on Feb. 5, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a semiconductor chip packaging technology. More particularly, the present invention relates to a method of forming a stack package and structure thereof.  
         [0004]     2. Description of the Related Art  
         [0005]     In recent years, the technology developments and the popularity of portable, handheld and consumer electronic products has almost overshadowed the conventional personal computer (PC) products. To facilitate the manufacturing of these electronic products, most devices are designed towards higher storage capacity and smaller line width to increase the packing density, the operational frequency, to reduce the power consumption and to achieve the integration of multi-functions. In the packaging technology of the integrated circuits (ICs), the chip scale package (CSP) and the wafer level package have been invented to meet the requirements for higher input/output pin count, higher heat-dissipating capacity and reduction of the package size. Furthermore, associated packaging techniques for reducing the weight and cost are also being developed.  
         [0006]     In the development of the chip scale package (CSP), a variety of techniques such as the single chip package, the stack chip package and the planar multi-chip package (MCM) are developed. The aforementioned techniques can reduce the dimension of a package to a size only slightly larger than the original size of the chip. However, the technology of the stack chip packages and the planar multi-chip packages must be combined with the known good die (KGD) technique to produce a high yield.  
         [0007]     Unlike the conventional chip scale package (CSP) method, the waver level package or wafer level chip scale package (WL-CSP) method packages an entire wafer before dicing up the wafer. Hence, the WL-CSP method can eliminate many process steps such as underfilling, assembling, substrate processing, chip attaching and wire bonding so that the overall fabrication cost can be substantially reduced. In general, the wafer can be packaged regardless of the size of the chip or the pin count. In other words, the wafer level packaging is able to reduce the process steps to thereby shorten the fabrication cycle time, to improve the performance and to lower down the cost. In addition, the amount of saving increases correspondingly with the size of the wafer. Therefore, the wafer level packaging method is particularly advantageous to the wafer processing plants shifting from 8-inch wafer production to 12-inch wafer production.  
         [0008]     System on chip (SOC) and system in a package (SIP) are regarded as two principal techniques for producing miniaturized and multi-functional semiconductor devices in the future. In particular, the system on chip (SOC) technique has some promising applications in manufacturing digital information products. At present, the multi-chip package modules with high operating frequency, low cost, small size and short fabrication cycle are the dominant packaging type. For example, a drawing chip or a memory chip is often fabricated by the multi-chip package technology to achieve the high processing frequency, super-fast processing speed and the capacity of integration of multi-functions. Therefore, the known good die technique is important in the packaging process of the multi-chip package technology. After a number of chips are packaged, and the electrical properties of each packaged chip is tested. The chips that fail the test are immediately discarded and the chips that pass the test are integrated by attaching to a packaging product. In this way, the area of the printed circuit board of the package system is reduced and the yield of a conventional multi-chip package is increased.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, at least one object of the present invention is to provide a method of forming an ultra-thin wafer level stack package capable of simplifying the packaging process and increasing overall yield and throughput of the package.  
         [0010]     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention provides a method of forming an ultra-thin wafer level stack package and package structure thereof. The package structure of the present invention comprises an independent chip set. The independent chip set is obtained by dicing an selectively adhered wafer. The selectively adhered wafer comprises a first wafer and a second substrate attached to the first wafer by using a plurality of adhesives, wherein the second substrate may comprise a plurality of chips, and the positions of the chips are matched with the first wafer. The first wafer has a plurality of base chips formed thereon. The first wafer separates from the second substrate by a distance equal to the thickness of the adhesive glue layer. Accordingly, each independent chip set comprises a base chip and a portion of the second substrate. In addition, a method of forming the adhesives may comprise, for example, a dispensing method. Moreover, a method of forming the adhesives may comprise forming a double side tape having a region comprising the adhesives. Thereafter, after the selectively adhered wafer is obtained and before the independent chip sets are formed, a thermal curing step may further be performed for curing the adhesives.  
         [0011]     The method of forming an ultra-thin wafer level stack packages, providing a first wafer having a plurality of base chips thereon, and a second substrate. Next, a first surface of the first wafer is bonded to a first surface of the second substrate by using a plurality of adhesives to form an selectively adhered wafer, wherein a distance between the first wafer and the second substrate is equal to a thickness of the adhesives. Next, a plurality of independent chip sets are formed, wherein each of the independent chip sets comprises the base chip and a portion of the second substrate. The independent chip set is formed by cutting a second surface of the first wafer of the selectively adhered wafer and cutting a second surface of the second substrate of the selectively adhered wafer. In addition, a method of cutting the selectively adhered wafer may comprise a diamond blade cutting method or a laser cutting method. Finally, the independent chip sets are packaged to achieve a packaged IC or chip.  
         [0012]     In a preferred embodiment of the present invention, a process of detecting a known good die (KGD) of the base chips of the first wafer is carried out before the commencement of the packaging.  
         [0013]     In another preferred embodiment of the present invention, one or more stacked chips are bonded to the base chip after the step of forming the independent chip sets but before the step of packaging the independent chip sets.  
         [0014]     In another preferred embodiment of the present invention, the second surface of the first wafer is polished after the step of forming the selectively adhered wafer but before the step of cutting the second surface of the first wafer of the selectively adhered wafer.  
         [0015]     According to an aspect of the present invention, at least a stack chip is also attached to the base chip of each independent chip set.  
         [0016]     According to another aspect of the present invention, the first wafer has a thickness between 200 μm to 500 μm.  
         [0017]     According to another aspect of the present invention, a polishing process is performed over the surface of the first wafer after producing the selectively adhered wafer but before dicing a surface of the first wafer in the selectively adhered wafer.  
         [0018]     According to another aspect of the present invention, the first wafer has a thickness between 30 μm to 250 μm after polishing the first wafer in the selectively adhered wafer.  
         [0019]     According to another aspect of the present invention, the first wafer preferably has a thickness between 30 μm to 80 μm after polishing the first wafer in the selectively adhered wafer.  
         [0020]     According to another aspect of the present invention, a ‘known good die’ (KGD) inspection of the stack chip or base chip is performed.  
         [0021]     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0023]     FIGS.  1  to  8  are top views and cross-sectional views illustrating a process of forming an ultra-thin wafer level stack package and package structure according to the preferred embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.  
         [0025]     FIGS.  1  to  8  are top views and cross-sectional views illustrating the process of forming an ultra-thin wafer level stack package according to a preferred embodiment of the present invention. First, referring to  FIG. 1 , a first wafer  102  having a plurality of base chips  112  thereon is provided. As shown, an area marked by the intersection of a solid horizontal line and a solid vertical line defines each base chip  112 . A second substrate  104  is also provided, wherein the second substrate  104  may comprise no chip, or one or more chips previously formed thereon, and the size and position of the chips previously formed has been preset to match with the first wafer  102 . The second substrate  104  can be a transparent or a non-transparent glass substrate comprised of, for example, but not limited to, a silicon wafer substrate, a plastic substrate, an acrylic substrate or a polymer substrate. Thereafter, the second substrate  104  is adhered to the surface of the first wafer  102  with the base chips  112  through a plurality of adhesives  106  to form a selectively adhered wafer  108 . Referring to  FIG. 1 , only a few specific areas on the first wafer  102  are adhered to the adhesive  106 . The first wafer  102  separates from the second substrate  104  by a distance equal to the thickness of the adhesives  106 . In one embodiment of the present invention, the method of forming the adhesives  106  comprises, for example, dispensing method. In another embodiment of the present invention, a specific double side tape, for example, having a shape the same as the first wafer  102  may be provided. In addition, the adhesives  106  may be disposed on a portion of the specific double side adhesive tape, for example, the portion corresponding to the first wafer  102 . Therefore, after the first wafer  102  is adhered to the second substrate  104  by the adhesives  106 , a thermal curing process may further be performed for curing the adhesives  106 .  
         [0026]     In one preferred embodiment of this invention, a known good die (KGD) inspection of the base chips  112  on the first wafer  102  is carried out prior to the commencement of the packaging process.  
         [0027]     After attaching the first wafer  102  to the second substrate  104  using the adhesives  106 , the exposed surface of the first wafer  102  of the selectively adhered wafer  108  facing the direction indicated by the cutting direction  222  in  FIG. 2  is polished. The original thickness of the first wafer  102  is in a range of about 200 μm to about 500 μm. After the polishing process, the first wafer  102  of the selectively adhered wafer  108  has a thickness in a range of about 30 μm to about 250 μm. Preferably, the thickness of the first wafer  102  after the polishing process is in a range of about 30 μm to about 80 μm.  
         [0028]     Referring to  FIG. 2 , the first wafer  102  of the selectively adhered wafer  108  is polished before proceeding with the following steps. However, it is to be noted that even if the polishing step is skipped, the following process steps are the same.  
         [0029]     Next, the first wafer  102  of the selectively adhered wafer  108  is diced along the direction indicated by the cutting direction  222 . The depth of the cut is greater than the thickness of the polished first wafer  102  but smaller than the total thickness of the first wafer  102  and the adhesive  106 . Hence, the saw in the dicing process is prevented from cutting into the second substrate  104 . Thereafter, the structure as shown in  FIG. 3A  and  FIG. 3B  may be obtained. In one embodiment of the present invention, the method of dicing the selectively adhered wafer  108  may comprise a diamond blade cutting method or a laser cutting method.  
         [0030]     After the dicing process, the first wafer  102  is cut into a plurality of first base chips  302  as shown in  FIGS. 3A and 3B . Thereafter, the second substrate  104  is diced along the direction indicated by the cutting direction  324  in  FIGS. 3A and 3B . The depth of the cut is greater than the thickness of the second substrate  104  but smaller than the total thickness of the second substrate  104  and the adhesive  106 . Thus, the saw in the dicing process is prevented from cutting into the first base chips  302 . Referring to  FIGS. 4A and 4B , after the second substrate  104  is diced up, the selectively adhered wafer  108  is cut into a plurality of independent chip sets, wherein each independent chip set comprising at least a first base chip  302 , an adhesive  106  and a portion of the second substrate  404 .  
         [0031]     In a preferred embodiment of the invention, as shown in  FIG. 5A , after the second substrate  104  is diced up along the cutting direction as shown in  FIGS. 3A and 4A , an external stack chip  502  may be bound to the surface of the first base chip  302  of each or some of the independent chip sets. Thereafter, the whole structure as shown in  FIG. 5A  may be wired and packaged, wherein every packaged integrated circuits (IC) may comprise, the first base chip  302  and the stack chip  502  stacked on the first base chip  302 . In one preferred embodiment of the present invention, a known good die (KGD) inspection of the stack chip  502  can be performed before the stack chip  502  is bound to the independent chip set. In addition, the thickness of the IC package is substantially similar to the whole thickness of the second substrate  404  and the first base chip  302 .  
         [0032]     As shown in  FIG. 6A , after binding the stack chip  502  to the first base chip  302  of the independent chip set, the independent chip set may be wired and packaged to form an integrated circuit (IC) package that at least comprises a first base chip  302 , a stack chip  502  stacked on the first base chip  302 , and one or more chips  522  on the cut second substrate  404 . In addition, the thickness of the IC is substantially similar to the whole thickness of the second substrate  404  and the first base chip  302 .  
         [0033]     In an alternative embodiment as shown in  FIG. 5B , a plurality of stack chips may bind to the surface of the first base chip  302  of the independent chip sets after the second substrate  104  is diced up. Here, only two stack chips  512  and  514  are shown, however, more than two stack chips can be provided in the present invention. In one preferred embodiment of this invention, a known good die (KGD) inspection of the stack chips  512  and  514  can be carried out before the stack chips are bound to the independent chip set. In addition, the thickness of the IC is substantially similar to the whole thickness of the second substrate  404  and the first base chip  302 .  
         [0034]     After binding the stack chips  512  and  514  to the first base chip  302  of the independent chip set, the excess portion of the second substrate  404  is removed to form a second base plate  604  as shown in  FIG. 6B . Thereafter, the independent chip set is packaged to form an integrated circuit (IC) package at least comprising a first base chip  302 , a plurality of stack chips  512  and  514 , and one or more chips  522  on the cut second substrate  404 . In addition, the thickness of the IC package is substantially similar to the whole thickness of the second substrate  404  and the first base chip  302 .  
         [0035]     In another embodiment as shown in  FIGS. 7 and 8 , a plurality of independent chip sets may be obtained after the surface of the second wafer  104  of the selectively adhered wafer is diced along the directions shown in  FIGS. 3B and 4B . In every independent chip set, the overlapped region between the second substrate  404  and the bas chip  302  may be dependent on the demand of the process. In addition, whether the chips  522 , the stack chips  512  and  514  are disposed on the second substrate  404  may be dependent on the demand of the process.  
         [0036]     In summary, the present invention provides an ultra-thin wafer level stack packaging structure as shown in  FIGS. 7 and 8 . The package structure comprises a base chip  302 , a second base plate  604  and a stack chip  502  or a plurality of stack chips  512  and  514 . The base chip  302  has a plurality of areas with the second base plate  604  bonded to one area of the base chip  302  by an adhesive  106 , and with the stack chip  502  or the stack chips  512 ,  514  bonded to another area of the base chip  302 . In addition, one or more chips  522  may be disposed on the second substrate  404 . Moreover, the thickness of the IC package is substantially similar to the whole thickness of the chip  522 , the second substrate  404  and the base chip  302 .  
         [0037]     This invention provides a method of forming an ultra-thin wafer level stack package. The advantages of the invention is that the selectively adhering of a second substrate to a first wafer having a plurality of base chips thereon can provide the reduction of the thickness of the first wafer, wherein the second substrate may be pre-designed to match with the first wafer. The thickness of the second substrate need not be too thick but need to be thick enough to maintain the selectively adhered wafer of the first wafer and prevent the second substrate from any deformation during the dicing process of the first wafer. Thus, for example, the second substrate may have a thickness close to that of a conventional wafer. By dicing the first wafer of the selectively adhered wafer, the problem caused from cutting a thin polished first wafer can be avoided. After binding a stack chip to the base chip, the entire assembly is packaged to form an integrated circuit (IC) package. The IC package may comprise at least a base chip and one or more stack chips, wherein the size of the IC package is substantially similar to the whole size of the base chip and the stack chip(s). In addition, the package has a size similar to the base chip and a thickness close to the combined thickness of the second substrat, the polished base chip and the adhesive. In other words, the package has a thickness close to a conventional wafer. Therefore, the package method of the present invention may provide a stack chip package structure.  
         [0038]     Furthermore, through the packaging method of the present invention, chips fabricated by different processes can be integrated to form a single package. The circuits that can be fabricated by the same processing steps can be fabricated on the same chip to shorten the fabrication cycle thereof. In addition, chips manufactured by different processes may be packaged together, therefore, the packaged structure is light, thin, and small. Furthermore, if a ‘known good die’ inspection is incorporated to check the stack chips or the base chips, the yield will be improved significantly and the cost will be lowered considerably.  
         [0039]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.