Abstract:
A two-stage packaging method of image sensors is disclosed. The packaging method includes the following steps: providing a substrate, fixing an image sensor chip on the substrate, fixing a transparent board on the image sensor chip, electrically connecting the image sensor chip and the substrate, forming a first encapsulant lay, and forming a second encapsulant layer. The two-stage packaging method prevents excessive pressure from being generated by formation of the encapsulant layers during the image sensor packaging process. Such excessive pressure, if generated, may result in position shift of the image sensor chip or damage of the bonding wires. The two-stage packaging method can increase the yield of the image sensor packaging process as well as the sensitivity of image sensors, thereby improving the quality and production of image sensor packaging while lowering the manufacturing costs.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a packaging method of image sensors and, more particularly, to a two-stage packaging method of image sensors. 
         [0003]    2. Description of Related Art 
         [0004]    Conventionally, the packaging of an image sensor involves forming an encapsulant layer on the image sensor chip as well as on the bonding wires that connect the image sensor chip to the substrate, in order to protect both the image sensor chip and the bonding wires, wherein the encapsulant layer is formed by a molding process. 
         [0005]    However, during the conventional encapsulant forming process, the pressure resulting from the encapsulant is likely to result in position shift of the image sensor chip or breakage of the bonding wires, and the encapsulant may contaminate the image sensor if passing through the gaps between the image sensor chip and the transparent board serving to protect it. Should the image sensor chip, particularly its sensing area, be contaminated, the functions of the image chip will be affected, and the yield of the image sensor packaging process, reduced. 
         [0006]    Therefore, it is important for the semiconductor and image sensor industries to develop an image sensor packaging method which can effectively reduce the molding pressure of the encapsulant. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The present invention discloses a two-stage packaging method of image sensors. The two-stage packaging method prevents excessive pressure from being generated by formation of the encapsulant layers during the image sensor packaging process so that the image sensor chip will not be shifted and the bonding wires which serve the purpose of electrical connection will not be damaged. Hence, the two-stage packaging method can increase the yield of the image sensor packaging process&#39; and improve the sensitivity of image sensors thus made. 
         [0008]    It is an object of the present invention to provide a two-stage packaging method of image sensors. The two-stage packaging method includes the steps of: providing a substrate having a first surface and a second surface, wherein the second surface faces away from the first surface; fixing an image sensor chip on the substrate, wherein the image sensor chip is fixed on the first surface and has a sensing area; fixing a transparent board on the image sensor chip, wherein the transparent board is connected to the image sensor chip via an intermediate so as to form a sealed space between the transparent board and the image sensor chip, the intermediate surrounding the boundary of the sensing area such that the sensing area is enclosed in the sealed space; electrically connecting the substrate and the image sensor chip, wherein the substrate is electrically connected to the image sensor chip by a plurality of bonding wires, and the bonding wires are located outside the intermediate; forming a first encapsulant layer, wherein a first encapsulant covers the bonding wires, the top surface of the image sensor chip outside the intermediate, and the first surface excluding the area occupied by the image sensor chip, and the first encapsulant is subsequently cured to form the first encapsulant layer, the first encapsulant layer covering a portion of the lateral sides of the transparent board and having a top surface lower than the top surface of the transparent board; and forming a second encapsulant layer, wherein a second encapsulant covers an outer portion of the first encapsulant layer and is subsequently cured to form the second encapsulant layer, the second encapsulant layer covering the remaining portion of the lateral sides of the transparent board and having a top surface as high as and coplanar with the top surface of the transparent board. 
         [0009]    The two-stage packaging method of the present invention makes at least two improvements over its prior art counterparts: 
         [0010]    1. effectively reducing the impact of the molding pressure of the encapsulant on the image sensor chip and the bonding wires; and 
         [0011]    2. increasing the yield, quality, and production of image sensor packaging while reducing manufacturing costs. 
         [0012]    The features and advantages of the present invention are detailed hereinafter with reference to the preferred embodiments. The detailed description is intended to enable a person skilled in the art to gain insight into the technical contents disclosed herein and implement the present invention accordingly. In particular, a person skilled in the art can easily understand the objects and advantages of the present invention by referring to the disclosure of the specification, the claims, and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]    The structure as well as a preferred mode of use, further objects, and advantages of the present invention will be best understood by referring to the following detailed description of some illustrative embodiments in conjunction with the accompanying drawings, in which: 
           [0014]      FIG. 1  is a flowchart of steps showing a two-stage packaging method of image sensors according to an embodiment of the present invention; 
           [0015]      FIG. 2A  is a cross-sectional view showing a substrate according to an embodiment of the present invention; 
           [0016]      FIG. 2B  is a cross-sectional view showing how an image sensor chip is fixed on the substrate according to an embodiment of the present invention; 
           [0017]      FIG. 2C  is a cross-sectional view showing how a transparent board is fixed on the structure depicted in  FIG. 2B  according to an embodiment of the present invention; 
           [0018]      FIG. 2D  is a cross-sectional view showing how electrical connection is made between the image sensor chip and the substrate of the structure depicted in  FIG. 2C ; 
           [0019]      FIG. 2E  is a cross-sectional view showing how a first encapsulant layer is formed on the structure depicted in  FIG. 2D ; 
           [0020]      FIG. 2F  is a cross-sectional view showing how a second encapsulant layer is formed on the structure depicted in  FIG. 2E , thus the cross-sectional view also showing an image sensor package formed by the two-stage packaging method of image sensors according to an embodiment of the present invention; and 
           [0021]      FIG. 3  is a cross-sectional view showing another image sensor package formed by the two-stage packaging method of image sensors according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Referring to  FIG. 1  for an embodiment of the present invention, a two-stage packaging method S 100  of image sensors includes the steps of: providing a substrate (S 10 ); fixing an image sensor chip on the substrate (S 20 ); fixing a transparent board on the image sensor chip (S 30 ); electrically connecting the image sensor chip and the substrate (S 40 ); forming a first encapsulant layer (S 50 ); and forming a second encapsulant layer (S 60 ). 
         [0023]    Referring to  FIG. 1 ,  FIG. 2A ,  FIG. 2F , and  FIG. 3 , the two-stage packaging method begins by providing a substrate (S 10 ), wherein the substrate  10  has a first surface  11  and a second surface  12 , and the second surface  12  faces away from the first surface  11 . The substrate  10  is made of plastic or ceramic. The substrate  10  is further provided with a plurality of electrical contacts  13  fixed on the second surface  12 . The electrical contacts  13  enable the circuit on the substrate  10  or on the image sensor chip  20  to make electrical connection with components outside the substrate  10 . 
         [0024]    Referring to  FIG. 1 ,  FIG. 2B ,  FIG. 2F , and  FIG. 3 , the step of fixing an image sensor chip on the substrate (S 20 ) is carried out as follows. The image sensor chip  20  is fixed on the first surface  11  of the substrate  10 . The image sensor chip  20  has a sensing area  21  as an input area for signals sensed by the image sensor chip  20 . The image sensor chip  20  is one of a complementary metal-oxide-semiconductor (CMOS) image sensor chip and an integrated circuit having a CMOS image sensor chip. 
         [0025]    Referring to  FIG. 1 ,  FIG. 2C ,  FIG. 2F , and  FIG. 3 , the step of fixing a transparent board on the image sensor chip (S 30 ) involves connecting a transparent board  30  to the image sensor chip  20  via an intermediate  40  so as to form a sealed space  50  between the transparent board  30  and the image sensor chip  20 . The intermediate  40  surrounds the boundary of the sensing area  21  such that the sensing area  21  is enclosed in the sealed space  50 . 
         [0026]    Referring to  FIG. 2C-2F  and  FIG. 3 , the transparent board  30  is formed of one of transparent glass, transparent acrylic, and transparent plastic. A signal sensed by the image sensor chip  20  penetrates the transparent board  30  and is received by the sensing area  21 . The intermediate  40  is formed of one of glass, plastic, a polyimide, and an amide resin, or the intermediate  40  is formed by molding a liquid crystal polymer (LCP). If the intermediate  40  is formed of any of the foregoing materials, the upper end and the lower end of the intermediate  40  can be tightly connected, through an adhesive  41 , to the transparent board  30  and the image sensor chip  20  respectively. 
         [0027]    As shown in  FIG. 3 , the intermediate  40  may alternatively be one of a heat-curable adhesive, a UV-curable adhesive, an adhesive polyimide, and an adhesive amide resin. In that case, the transparent board  30  can be adhered to the image sensor chip  20  through the curing process of the intermediate  40 . If the intermediate  40  is one of a heat-curable adhesive and a UV-curable adhesive, the intermediate  40  and the image sensor chip  20  can be tightly connected without application of the adhesive  41 . 
         [0028]    Referring to  FIG. 2C ,  FIG. 2F , and  FIG. 3 , the height H of the sealed space  50  formed by the intermediate  40 , the transparent board  30 , and the image sensor chip  20  is the vertical distance between the transparent  30  and the image sensor chip  20 . The height H of the sealed space  50  can be determined according to the sensing requirement of the image sensor chip  20  and be achieved by selecting an intermediate  40  of the appropriate size in the step of fixing a transparent board on the image sensor chip (S 30 ). 
         [0029]    Referring to  FIG. 2C  and  FIG. 2F , the height H of the sealed space  50  may range from 100 um to 500 um (um=micro meter; i.e., 10 −6  meter). 
         [0030]    Reference is now made to  FIG. 1 ,  FIG. 2D ,  FIG. 2F , and  FIG. 3 . To perform the step of electrically connecting the substrate and the image sensor chip (S 40 ), the two connecting ends  61  of each of a plurality of bonding wires  60  are respectively connected to the substrate  10  and the image sensor chip  20  so as to electrically connect the substrate  10  and the image sensor chip  20 . The bonding wires  60  and the connecting ends  61  are located outside the intermediate  40 ; that is to say, the bonding wires  60  and the connecting ends  61  are located outside the sealed space  50 . The bonding wires  60  are electric wires made of gold (Au), a mixture of gold and silver (Ag), or a mixture of gold and copper (Cu). 
         [0031]    Referring to  FIG. 1 ,  FIG. 2E ,  FIG. 2F , and  FIG. 3 , the step of forming a first encapsulant layer (S 50 ) is executed in the following manner. A first encapsulant covers the bonding wires  60 , the connecting ends  61 , the top surface of the image sensor chip  20  outside the intermediate  40 , and the first surface  11  excluding the area occupied by the image sensor chip  20 . The first encapsulant is then cured to form a first encapsulant layer  70 , wherein the first encapsulant layer  70  covers a portion of the lateral sides of the transparent board  30  and has a top surface lower than the top surface of the transparent board  30 . 
         [0032]    The first encapsulant may be an epoxy. The first encapsulant is applied by a dispensing process such that the first encapsulant covers the bonding wires  60 , the connecting ends  61  at both ends of each bonding wire  60 , the top surface of the image sensor chip  20  outside the intermediate  40 , and the first surface  11  excluding the area occupied by the image sensor chip  20 . As the first encapsulant maintains a liquid and gluey substance during its application, and the epoxy is not required to be of the same height as the transparent board  30 , the pressure resulting from the first encapsulant is relatively low, and because of the fluid property, the impact of the pressure on the bonding wires  60 , the two connecting ends  61  of each bonding wire  60 , or the image sensor chip  20  is relatively small. After the first encapsulant is cured and forms the first encapsulant layer  70 , all the components in first encapsulant layer  70  are protected, wherein the components refer to the bonding wires  60 , the connecting ends  61  of each bonding wire  60 , the top surface of the image sensor chip  20  outside the intermediate  40 , and the first surface  11  excluding the area occupied by the image sensor chip  20 . 
         [0033]    Refer again to  FIG. 1 ,  FIG. 2F , and  FIG. 3 . To perform the step of forming a second encapsulant layer (S 60 ), a second encapsulant is used to cover an outer portion of the first encapsulant layer  70 . The second encapsulant is then cured to form a second encapsulant layer  80 , wherein the second encapsulant layer  80  covers the remaining portion of the lateral sides of the transparent board  30 . Moreover, the top surface of the second encapsulant layer  80  is as high as and coplanar with the top surface of transparent board  30 . 
         [0034]    Referring to  FIG. 2E ,  FIG. 2F , and  FIG. 3 , the lateral sides of the first encapsulant layer  70  and the lateral sides of second encapsulant layer  80  are made flush with the lateral sides of the substrate  10 . 
         [0035]    As the second encapsulant covers the aforesaid outer portion of the first encapsulant layer  70  only after the first encapsulant is cured and forms the first encapsulant layer  70 , the components covered and protected by the first encapsulant layer  70  will not be affected by the pressure resulting from the step of forming a second encapsulant layer (S 60 ). 
         [0036]    The second encapsulant may be a heat-curable encapsulant and may vary in material or color depending on application requirements. When implementing the two-stage packaging method S 100  of image sensors, the second encapsulant may be a black or opaque material in order not to interfere with sensing. 
         [0037]    In the two-stage packaging method S 100  of image sensors, the two-stage formation of the first encapsulant layer  70  and the second encapsulant layer  80  greatly reduces the adverse effects of the encapsulant forming pressure on the bonding wires  60  or the image sensor chip  20 . Consequently, not only can the yield, quality, and production of the image sensor packaging process be increased, but also the manufacturing costs can be lowered. 
         [0038]    The embodiments described above are intended only to demonstrate the technical concept and features of the present invention so as to enable a person skilled in the art to understand and implement the contents disclosed herein. It is understood that the disclosed embodiments are not to limit the scope of the present invention. Therefore, all equivalent changes or modifications based on the concept of the present invention should be encompassed by the appended claims.