Abstract:
A method for reducing the tilt of an optical unit during manufacture of an image sensor includes the steps of: providing a semimanufacture of the image sensor, carrying out a preheating process, carrying out an adhesive application process, carrying out an optical unit mounting process, and carrying out a packaging process. Due to the preheating process, the semimanufacture will be subjected to a stabilized process environment during the adhesive application process and the optical unit mounting process, so as for the optical unit to remain highly flat once attached to the semimanufacture. The method reduces the chances of tilt and crack of the optical unit and thereby contributes to a high yield rate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part (CIP) of U.S. patent application Ser. No. 13/403,159, filed on Feb. 23, 2012, which claims priority from the U.S. Provisional Application No. 61/446,355, filed Feb. 24, 2011, entitled “Method for Reducing Tilt of Transparent Window during Manufacturing of Image Sensor”. The disclosure of the related applications is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a method for reducing the tilt of an optical unit during manufacturing of an image sensor, and more particularly, to a method for reducing the tilt of an optical unit during manufacturing of an image sensor that can improve the yield rate by carrying out a preheating process to stabilize the process environment. 
         [0004]    2. Description of Related Art 
         [0005]      FIG. 1A  is a schematic view illustrating a structure of a conventional image sensor.  FIG. 1B  is a schematic view illustrating tilting and consequent fracture of a transparent plate during a process of manufacturing the conventional image sensor.  FIG. 1C  is a schematic view illustrating tilting of the transparent plate and consequent overflow of an adhesive during the process of manufacturing the conventional image sensor. 
         [0006]    As shown in  FIG. 1A , the conventional image sensor  100  substantially includes a circuit substrate  10  (e.g., a printed circuit board; PCB), an image sensor die  20 , a transparent plate  30  and an encapsulant  40 . The image sensor die  20  is disposed on the circuit substrate  10  and is electrically connected to circuits on the circuit substrate  10  via metal wirings  25  through wire bonding, and the transparent plate  30  is disposed above a photosensitive region (not shown) of the image sensor die  20  by means of an adhesive  26  such as an epoxy resin, and then the metal wirings  25  and side edges of the image sensing die  20  and the transparent plate  30  are encapsulated by the encapsulant  40  through molding. 
         [0007]    However, as shown in  FIG. 1B , if the adhesive  26  is applied uneven, and then the transparent plate  30  adhered above the photosensitive region (not shown) of the image sensing die  20  is placed in an out-of-level state (e.g., inclined in the lateral direction) during the molding process, the tilt of the transparent plate  30  with respect to the image sensor die  20  or the circuit substrate  10  will become overly large to decrease the sensing quality and this makes it easy to cause fracture of the transparent plate  30  when the mold  50  is pressed downwards during the molding process, which decreases the yield rate of image sensors. 
         [0008]    Additionally, as shown in  FIG. 1C , the air in a space enclosed by the transparent plate  30 , the image sensor die  20  and the adhesive  26  during the molding process tends to expand non-uniformly when heated by a high environmental temperature. This will not only push the transparent plate  30  to cause tilting of the transparent plate  30  but also push the adhesive  26  outwards to cause overflow of the adhesive  26 , thus degrading the yield rate of image sensors. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention is a method for reducing the tilt of an optical unit during manufacture of an image sensor. According to the method, a preheating process is carried out to release the air pressure in the vicinity of an image sensor die so that the closed space formed by mounting the optical unit will not expand due to the high temperature of subsequent processes. Additionally, the closed space formed by mounting the optical unit may be formed with a gap in communication with the outside to lower the pressure inside the closed space, thereby preventing the optical unit from being out-of-level. Now that the tilt, if any, of the optical unit is reduced, and the optical unit is kept from fracture during the optical unit mounting process, an increased yield rate is achievable. 
         [0010]    The present invention provides a method for reducing the tilt of an optical unit during manufacturing of an image sensor, comprising the following steps: providing a semimanufacture of the image sensor, which comprises a circuit substrate and an image sensor die, wherein the circuit substrate has a supporting surface and a bottom surface, a plurality of first conductive contacts are provided on the supporting surface, and the image sensor die comprises: a first surface attached to the supporting surface; a second surface having a photosensitive region; and a plurality of second conductive contacts disposed outside the photosensitive region and electrically connected to the first conductive contacts via metal wirings respectively; carrying out a preheating process by placing the semimanufacture into an environment at a specific temperature; carrying out an adhesive application process by, after the preheating process, applying an adhesive onto the second surface around the photosensitive region without covering the photosensitive region; carrying out an optical unit closing process by, after the adhesive application process, placing an optical unit on the adhesive and curing the adhesive to fix the optical unit onto the second surface and to form an air chamber between the image sensor die and the optical unit; and carrying out a packaging process by packaging the semimanufacture and the optical unit with an encapsulant. 
         [0011]    Through implementation of the present invention, at least the following effects can be achieved: 
         [0012]    1. Environment factors for the semimanufacture during the adhesive application process can be made stable to reduce the tilt of the optical unit after the optical unit closing process; 
         [0013]    2. The optical unit is prevented from tilting which may otherwise occur if the air in the air chamber formed by the optical unit mounting process expands. 
         [0014]    3. The adhesive will not overflow after the optical unit mounting process. 
         [0015]    4. A balance between the air pressure in and outside the air chamber formed by the optical unit mounting process can be reached. 
         [0016]    The detailed features and advantages of the present invention will be described in detail with reference to the preferred embodiment so as to enable persons skilled in the art to gain insight into the technical disclosure of the present invention, implement the present invention accordingly, and readily understand the objectives and advantages of the present invention by perusal of the contents disclosed in the specification, the claims, and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0017]    The invention as well as a preferred mode of use and advantages thereof will be best understood by referring to the following detailed description of an illustrative embodiment in conjunction with the accompanying drawings, wherein: 
           [0018]      FIG. 1A  is a schematic view illustrating a structure of a conventional image sensor; 
           [0019]      FIG. 1B  is a schematic view illustrating tilting and consequent fracture of a transparent plate during a process of manufacturing the conventional image sensor; 
           [0020]      FIG. 1C  is a schematic view illustrating tilting of the transparent plate and consequent overflow of an adhesive during the process of manufacturing the conventional image sensor; 
           [0021]      FIG. 2  is a flowchart diagram of a method for reducing the tilt of an optical unit during manufacturing of an image sensor according to an embodiment of the present invention; 
           [0022]      FIG. 3  is a schematic view of a semimanufacture of an image sensor according to an embodiment of the present invention; 
           [0023]      FIG. 4A  is a top view of a semimanufacture of an image sensor obtained after an adhesive application process according to an embodiment of the present invention; 
           [0024]      FIG. 4B  is a top view of a semimanufacture of an image sensor obtained after an adhesive application process according to another embodiment of the present invention; 
           [0025]      FIG. 4C  is a top view of a semimanufacture of an image sensor obtained after an adhesive application process according to yet another embodiment of the present invention; 
           [0026]      FIG. 4D  is a schematic view of a semimanufacture of an image sensor obtained after an adhesive application process according to an embodiment of the present invention; 
           [0027]      FIG. 5  is a schematic view of a semimanufacture of an image sensor obtained after an optical unit mounting process according to an embodiment of the present invention; 
           [0028]      FIG. 6A  is a perspective view of an intermediate layer according to an embodiment of the present invention; 
           [0029]      FIG. 6B  is an assembled perspective view of an intermediate layer and a transparent plate according to an embodiment of the present invention; 
           [0030]      FIG. 6C  is a schematic view of a semimanufacture of an image sensor obtained after an optical unit mounting process according to another embodiment of the present invention; 
           [0031]      FIG. 6D  is a top view of  FIG. 6C ; 
           [0032]      FIG. 7A  illustrates the first aspect of an image sensor obtained after a packaging process according to an embodiment of the present invention; 
           [0033]      FIG. 7B  illustrates the second aspect of an image sensor obtained after a packaging process according to an embodiment of the present invention; 
           [0034]      FIG. 8A  is a cross-sectional view illustrating a large-scale packaging mold in combination with semimanufactures of image sensors according to an embodiment of the present invention; and 
           [0035]      FIG. 8B  is a partially enlarged view of  FIG. 7A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    As shown in  FIG. 2 , this embodiment is a method for reducing the tilt of an optical unit during manufacturing of an image sensor, which comprises the following steps of: providing a semimanfuacture (step S 10 ); carrying out a preheating process (step S 20 ); carrying out an adhesive application process (step S 30 ); carrying out an optical unit closing process (step S 40 ); and carrying out a packaging process (step S 50 ). 
         [0037]    As shown in  FIG. 3 , a semimanufacture is provided (step S 10 ). The semimanufacture  200  provided in this embodiment is a semimanufacture of an image sensor, which comprises a circuit substrate  10  and an image sensor die  20 . 
         [0038]    The circuit substrate  10  has a supporting surface  11  and a bottom surface  14 . A plurality of first conductive pads  12  is disposed on the supporting surface  11  for electrical connection in the wire bonding process and the plurality of first conductive pads  12  is electrically connected to circuits on the supporting surface  11 . Additionally, a drive integrated circuit (IC) and passive elements  13  may also be optionally disposed on the supporting surface  11  and electrically connected to the circuits on the supporting surface  11 . 
         [0039]    The image sensor die  20  may be a complementary metal oxide semiconductor (CMOS) image sensor die or a charge coupled device (CCE), and it comprises: a first surface  21 ; a second surface  22 ; and a plurality of second conductive contacts  24 . 
         [0040]    The first surface  21  is the lower surface of the image sensor die  20 , and is attached to the supporting surface  11  through use of an adhesive so that the image sensor die  20  is attached to the circuit substrate  10 . The second surface  22  is the upper surface of the image sensor die  20 , and has a photosensitive region  23  for receiving and sensing light rays. The second conductive contacts  24  are disposed outside the photosensitive region  23  and electrically connected to the first conductive contacts  12  on the first surface  21  by metal wirings  25  respectively. Thus, the image sensor die  20  can be electrically connected to the drive IC and the passive elements  13  through the circuits on the supporting surface  11 . 
         [0041]    A preheating process is carried out (step S 20 ) by placing the semimanufacture  200  into an environment at a specific temperature. The environment at the specific temperature may be an oven, and the specific temperature may range between 35° C. and 45° C. By preheating the semimanufacture  200 , the air temperature around the second surface  22  and the supporting surface  11  can be increased to a temperature range which is the same as the environment temperature in the subsequent adhesive application process and the subsequent optical unit closing process so as to release the air pressure in the space around the second surface  22  and the supporting surface  11 . This can prevent the air around the second surface  22  and the supporting surface  11  from being influenced by the temperature rise in the subsequent adhesive application process or the subsequent optical unit closing process to cause non-uniform expansion and consequent tilting of the optical unit (not shown). This can also prevent the enclosed space formed in the optical unit closing process from expanding due to the temperature rise to cause unevenness of the optical unit, and prevent occurrence of overflow of the adhesive. 
         [0042]    As shown in  FIG. 4A , an adhesive application process is carried out (step S 30 ) after the preheating process (step S 20 ) by applying an adhesive  26  onto the second surface  22  around the photosensitive region  23  without covering the photosensitive region  23 . During the adhesive application process (step S 30 ), the environment temperature may still be maintained at the specific temperature which is the same as that of the preheating process (e.g., between 35° C. and 45° C.). The adhesive  26  may be applied in regions between the photosensitive region  23  and the second conductive contacts  24  to form a closed pattern, which looks like a frame-shaped pattern. Thereby, the photosensitive region  23  after being packaged can be accommodated in the space formed by the adhesive  26  and the transparent plate (not shown) to prevent the photosensitive region  23  from being influenced by external factors. 
         [0043]    In addition to carrying out step S 20  to prevent pressure from building up in the aforesaid closed space due to a subsequent temperature rise, a gap may be formed to bring the closed space into communication with the outside and thereby reduce the pressure in the closed space. In the adhesive application process (step S 30 ), referring to  FIG. 4B , the adhesive  26  may be applied to an area between the photosensitive region  23  and the second conductive contacts  24  in such a way that the adhesive  26  not only does not cover the photosensitive region  23 , but also forms a generally C-shaped pattern. Thus, the adhesive  26  has a gap  28  at the opening of the generally C-shaped pattern. This allows the air in the space enclosed by the adhesive  26 , the optical unit (not shown), and the image sensor die  20  to communicate with the air outside the gap  28 , thereby balancing the pressure inside and outside the gap  28 . 
         [0044]    As shown in  FIG. 4C , the adhesive application process (step S 30 ) may also be carried out in such a manner that the adhesive  26  is applied to an area between the photosensitive region  23  and the second conductive contacts  24  and forms two L-shaped patterns that face each other. Thus, a hollow square pattern with a gap  28  in each of two opposite corners is formed. The two gaps  28  are located in two opposite right-angled corners of the hollow square pattern respectively such that the air in the space enclosed by the adhesive  26 , the optical unit (not shown), and the image sensor die  20  can communicate with the air outside the gaps  28  to achieve a balance between the pressure inside and outside the gaps  28 , thereby preventing unevenness of the optical unit and overflow of the adhesive, both of which may otherwise result from a pressure rise in the closed space formed by the optical unit mounting process (step S 40 ). 
         [0045]    As shown in  FIG. 4D , a plurality of ball spacers  27  may be further added into the adhesive  26  to keep a most appropriate distance between the optical unit (e.g., transparent plate  30 ) and the image sensing die  20  so that the tilt of the optical unit is controlled to be within a reasonable range. 
         [0046]    Then as shown in  FIG. 5 , an optical unit closing process (step S 40 ) is carried out. The optical unit can be a transparent plate  30  made of glass. The optical unit closing process is carried out after the adhesive application process (step S 30 ) by placing the optical unit (e.g., transparent plate  30 ) on the adhesive  26  and curing the adhesive  26  to fix the optical unit onto the second surface  22  and to form an air chamber  31  between the image sensor die  20  and the optical unit. During the optical unit closing process (step S 40 ), the environment temperature may also be maintained at the specific temperature which is the same as that of the preheating process (e.g., between 35° C. and 45° C.). 
         [0047]    The adhesive  26  may be a photocurable adhesive, and particularly an ultraviolet (UV) curable adhesive; and in the optical unit closing process (step S 40 ), the UV curable adhesive is cured through irradiation of UV light rays. 
         [0048]    As shown in  FIG. 4D  through  FIG. 6B , in cases where no ball spacers  27  are added into the adhesive  26 , the optical unit may include an intermediate layer  32  in addition to the transparent plate  30  in order to keep the transparent plate  30  properly spaced from the image sensor die  20  and thereby control the tilt of the transparent plate  30  within a reasonable range. The intermediate layer  32  has a fixed height and serves to control the spacing between the transparent plate  30  and the image sensor die  20 . 
         [0049]    The intermediate layer  32  is a hollow square structure and therefore will not cover the photosensitive region  23  when aligned with and adhesively attached to the adhesive  26  in the optical unit mounting process (step S 40 ). Furthermore, a frame-shaped groove  33  may be formed on the inner side of an upper surface  35  of the intermediate layer  32  so that the transparent plate  30  can be adhesively attached to the frame-shaped groove  33 . The intermediate layer  32  may be made of glass, ceramic, a liquid crystal polymer, a molding compound, a siloxane-based polymer, a photosensitive dry film, or a solder mask. 
         [0050]    Referring also to  FIG. 6C  and  FIG. 6D , while the foregoing processes are designed to render the pressure in the air chamber  31  as close to the ambient pressure as possible to prevent unevenness of the optical unit, the inner side of the intermediate layer  32  may further have a recess  34  so that, when the transparent plate  30  is attached to the intermediate layer  32  (e.g., adhesively attached to the frame-shaped groove  33 ) in the optical unit mounting process (step S 40 ), a gap  28 ′ is formed outside the transparent plate  30  to prevent complete air-tightness. The gap  28 ′ enables air circulation into and out of the air chamber  31  and ensures secure adhesion of the optical unit, which secure adhesion is difficult to achieve if the pressure in the air chamber  31  is greater than that outside the air chamber  31 . 
         [0051]    As shown in  FIG. 2 , the method S 100  for reducing the tilt of an optical unit during manufacture of an image sensor further includes a gap sealing process (step S 45 ). The gap sealing process (step S 45 ) is carried out after the optical unit mounting process (step S 40 ) by sealing the gap(s)  28 / 28 ′ with a sealant so as to protect the photosensitive region  23  from influences of external factors. 
         [0052]    Finally, a packaging process (step S 50 ) is carried out by packaging the semimanufacture and the optical unit with an encapsulant  40  through use of a molding process or a dispensing technology. 
         [0053]    As shown in  FIG. 7A , the encapsulant  40  may be applied to cover side edges of the semimanufacture, the adhesive  26 , and the optical unit (e.g., transparent plate  30 ). More specifically, a space formed by side edges of the optical unit and a bottom surface of the optical unit, side edges of the circuit substrate  10  and a top surface of the circuit substrate  10 , and the periphery of the closed pattern region applied with the adhesive  26  may be encapsulated by the encapsulant  40 . Thus, by using the encapsulant  40  to cover the side edges of the circuit substrate  10 , the side edges of the circuit substrate  10  can be prevented from being damaged due to impacts. 
         [0054]    Further, as shown in  FIG. 7B , the encapsulant  40  may also be disposed on the circuit substrate  10  and cover the side edges of the image sensor die  20 , the adhesive  26 , and the optical unit (e.g., transparent plate  30 ). More specifically, a space formed by side edges of the transparent plate  30  and the bottom surface of the transparent plate  30 , the top surface of the circuit substrate  10  (but except for the side edges of the circuit substrate  10 ) and the periphery of the closed pattern region applied with the adhesive  26  may be encapsulated by the encapsulant  40 . 
         [0055]    As shown in  FIG. 8A  and  FIG. 8B , in this embodiment, semimanufactures  200  that have been subjected to the optical unit closing process (step S 40 ) may also be arranged on a base  62  of a large-scale packaging mold  600 , and then a top cover  61  of the large-scale packaging mold  600  is joined with the base  62  to carry out the packaging process (step S 50 ) so as to achieve the purpose of mass production. 
         [0056]    The features of the present invention are disclosed above by the preferred embodiment to allow persons skilled in the art to gain insight into the contents of the present invention and implement the present invention accordingly. The preferred embodiment of the present invention should not be interpreted as restrictive of the scope of the present invention. Hence, all equivalent modifications or amendments made to the aforesaid embodiment should fall within the scope of the appended claims.