Abstract:
A method for manufacturing a semiconductor package is disclosed. A wafer including a plurality of semiconductor chips is provided. Each chip has one or more mirrors mounted thereon. Further, a plurality of bond pads formed on a periphery of the chip. Next, a photoresist is formed over the one or more mirrors. Then, the semiconductor chips are singulated from the wafer. One ore more semiconductor chips are mounted on a base substrate. The bond pads of the semiconductor chip are electrically connected with the base substrate. The photoresist is then removed from the semiconductor chips.

Description:
[0001]    This application is a divisional of U.S. patent application Ser. No. 09/847,620 filed on May 2, 2001, now pending, which is herein incorporated by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method for manufacturing semiconductor device packages, and more particularly to a method for manufacturing digital micro-mirror device (DMD) packages.  
           [0004]    2. Description of the Related Arts  
           [0005]    In order to keep pace with the development of personal computers, a display has been developed from a cathode-ray tube type display into a liquid crystal display or a mirror type display. Especially, with the increasing demand for digital broadcasting appliances, a digital light processing (DLP) technology for high resolution becomes more and more important. A DMD, which is an essential component for the DLP technology, requires significant expertise in the manufacturing process for mirrors so that high reliability and low cost in the manufacturing process can be obtained.  
           [0006]    The DMD process involves driving the mirrors, and thus the proper driving of mirrors is very important. Further, moisture and dust within the packages affect the picture quality or resolution of the DMD as well as its reliability or durability. Therefore, during the fabrication of the DMD packages, the DMD packages themselves need to be protected from moisture and dust.  
           [0007]    [0007]FIG. 1 is a plan view showing a conventional semiconductor chip  12  for the DMD, and FIG. 2 is a cross-sectional view showing a DMD package  100  containing the semiconductor chip  12  of FIG. 1. With reference to FIG. 1 and FIG. 2, the semiconductor chip  12  is attached to an upper surface  21  of a base substrate  20  by interposing an Ag-epoxy adhesive  30  therebetween. The semiconductor chip  12  and the base substrate  20  are electrically interconnected to each other with one or more bonding wires  40 . In order to protect the semiconductor chip  12  from external environmental stresses, a metal sealing ring  24  with a predetermined height is provided at the periphery of the upper surface  21  of the base substrate  20 .  
           [0008]    The components, including the semiconductor chip  12 , are hermetically sealed up with a window lid  50 . A heat sink stud  60  is attached to the lower surface  23  of the base substrate  20 . The window lid  50  comprises a metal lid frame  52  contacting the metal sealing ring  24 , and a window  54 . A reflectance coating film  56  is applied to the lower surface of the window  54  along the periphery thereof. The metal sealing ring  24  and the base substrate  20  form a cavity  29 , and a moisture getter (absorbent)  58  is attached to the lower surface of the metal lid frame  52  of the window lid  50  within the cavity  29 . External terminals (not shown) are formed on the lower surface  23  of the base substrate  20 .  
           [0009]    A plurality of mirrors  16  (only a typical one of which is depicted in FIG. 2) are formed on the active surface of the semiconductor chip  12  at the center thereof, and one or more electrode pads  14  are formed on the active surface at the periphery thereof for interconnection via the one or more bonding wires  40 .  
           [0010]    [0010]FIG. 3 is a flow chart  90  describing a manufacturing process of the conventional DMD package  100 . Each step of the manufacturing process is described briefly below.  
           [0011]    A wafer comprising a plurality of the semiconductor chips  12  is prepared (step  71 ). Herein, a photoresist film is formed on the upper surface of the wafer in the predetermined portion. The photoresist film prevents damage to the mirrors  16  from the external environment by covering the mirrors  16 . The photoresist film is not formed on the electrode pads  14 .  
           [0012]    Prior to wafer-breaking, the wafer is half-cut (step  72 ). The photoresist film on the upper surface of the wafer is removed (step  73 ), and to shield the mirrors  16  from dust or moisture, a first anti-sticking film is formed thereon (step  74 ). The wafer is broken and separated into individual semiconductor chips  12  (step  75 ). A breaking means in a dome shape is brought into contact with to the back surface of the wafer and urged upwardly. As a result, the half-cut wafer is broken into a plurality of individual semiconductor chips  12 .  
           [0013]    In the wafer-breaking step, silicon particle scraps are generated. Therefore, the silicon particles are removed (step  76 ).  
           [0014]    The semiconductor chip  12  is attached to the upper surface  21  of the base substrate  20  by the Ag-epoxy adhesive  30  (step  77 ), and the Ag-epoxy adhesive  30  is cured (step  78 ). The semiconductor chip  12  is electrically interconnected to the base substrate  20  with the bonding wires  40  (step  79 ).  
           [0015]    The organic compounds remaining on the upper surface  21  of the base substrate  20 , the semiconductor chip  12  on the surface  21 , and the bonding wires  40  are removed (step  80 ). A second anti-sticking film is formed thereon (step  81 ).  
           [0016]    The metal sealing ring  24  is mounted on the upper surface  21  of the base substrate  20 , and the components are hermetically sealed by the window lid  50  having the moisture getter  58  attached thereon (step  82 ).  
           [0017]    The heat sink stud  60  is attached to the lower surface  23  of the base substrate  20  (step  83 ). The DMD package  100  is thus complete.  
           [0018]    The above-described method for manufacturing the conventional DMD packages has several problems as follows;  
           [0019]    The manufacturing process is very complicated. The major reason is that the manufacturing process for the conventional DMD package employs the wafer-breaking method for separating the wafer into individual semiconductor chips  12 . Since the wafer-breaking method comprises a first step of half-cutting the wafer and a second step of breaking the wafer, compared to the full-cutting method, which completely cuts the wafer at once, this method further involves an additional step, i.e. the wafer-breaking step.  
           [0020]    Even if the full-cutting method is employed to prevent this drawback, another problem occurs in the step of removing the photoresist after separating the wafer into the semiconductor chips by the full-cutting method. Conventionally, the wafer comprising separated semiconductor chips has the adhesive tape on its back surface. In the photoresist-removing step after the wafer-cutting step, the adhesive from the adhesive tape and the photoresist are unnecessarily removed together. Thus, the individual semiconductor chips can be undesirably detached from the adhesive tape. Therefore, the conventional manufacturing process normally cannot employ the full-cutting method.  
           [0021]    The mirrors within the semiconductor chip  12  can be easily damaged by the silicon particles generated in the wafer-breaking step. The silicon particles positioned between the mirrors  16  cannot be properly removed by the washing step. Since the wafer-breaking step is carried out after the step of removing the photoresist, damage to the mirrors  16  by the silicon particles commonly occurs.  
           [0022]    Since the Ag-epoxy adhesive is used to attach the semiconductor chip  12  to the base substrate  20 , moisture enters the package due to the hygroscopicity of the Ag-epoxy. Further, an exhaust gas generated during the curing of the Ag-epoxy adhesive contaminates the mirrors  16  on the active surface of the semiconductor chip  12 . Therefore, it is preferable to use solder as the adhesive means. However, with the use of the solder, damage such as the burning of the first anti-sticking film or the deformation of the mirrors can occur. In other words, to attach the semiconductor chip to the base substrate, the solder must be melted at a temperature of 150° C. or more. Such a high temperature causes the burning of the first anti-sticking film or the deformation of the mirrors  16  in the semiconductor chip  12 .  
         SUMMARY OF THE INVENTION  
         [0023]    Accordingly, an object of the present invention is to simplify the manufacturing process of the DMD packages.  
           [0024]    Another object of the present invention is to prevent failures generated in the sequence of steps including first half-cutting and second full-cutting the wafer. Still another object of the present invention is to prevent failures due to the use of the Ag-epoxy adhesive.  
           [0025]    In order to achieve the foregoing and other objects, a method for manufacturing digital micro-mirror device (DMD) packages comprises preparing a wafer including a plurality of DMD semiconductor chips, each chip having a plurality of mirrors formed on the center of an active surface, a plurality of electrode pads formed on the edges of the active surface, and a photoresist for protecting the mirrors. The method further comprises forming a metallic layer on a back surface of the wafer, said metallic layer being made of a metal having a low melting point. It further comprises separating the wafer into the individual semiconductor chips. It also comprises attaching each semiconductor chip to an upper surface of a base substrate with an adhesive made of a metal having a low melting point. The method then comprises the steps of interconnecting the electrode pads of the semiconductor chip to the base substrate with a bonding wire, removing the photoresist from the semiconductor chips, and forming an anti-sticking film on the active surface of the semiconductor chip for protecting the semiconductor chips from dust and moisture. Finally, the method comprises hermetically sealing the semiconductor chip and the bonding wires on the upper surface of the base substrate by using a window lid.  
           [0026]    It is preferable that the metallic layer is made of a metal having a low melting point selected from the group consisting of Va, Au, Ni, Ag, Cu, Al, Pb, Sn, Sb, Pd and metallic compounds thereof.  
           [0027]    The step of forming a metallic layer comprises lapping the back surface of the wafer and forming on the back surface a metallic layer made of a metal having a low melting point.  
           [0028]    Solder is preferably used as the metal adhesive having a low melting point.  
           [0029]    After the step of hermetically sealing the semiconductor chip and the bonding wires, the manufacturing method of the DMD packages further comprises attaching a heat sink stud to the lower surface of the base substrate. Further, it is preferable that the step of hermetically sealing the semiconductor chip and the bonding wires is carried out at a temperature which is no higher than the temperature of the step of attaching the semiconductor chip to the base substrate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    The various features and advantages of the present invention will be readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and, in which:  
         [0031]    [0031]FIG. 1 is a schematic plan view showing a conventional semiconductor chip for digital micro-mirror device (DMD);  
         [0032]    [0032]FIG. 2 is a cross-sectional view showing a conventional DMD package containing the semiconductor chip of FIG. 1;  
         [0033]    [0033]FIG. 3 is a flowchart describing a conventional manufacturing process of the DMD package in FIG. 2;  
         [0034]    [0034]FIG. 4 is a cross-sectional view showing a DMD package in accordance with an embodiment of the present invention;  
         [0035]    [0035]FIG. 5 is a flow chart describing a manufacturing process of the DMD package in FIG. 4;  
         [0036]    FIGS.  6  through FIG. 16 illustrate schematically each step of the manufacturing process in FIG. 5; wherein FIG. 6 is a schematic plan view that illustrates a wafer used in the DMD packages; FIG. 7 is a plan view that illustrates the manufactured wafer;  
         [0037]    [0037]FIG. 8 is a cross-sectional view taken along the line  8 - 8  in FIG. 7;  
         [0038]    [0038]FIG. 9 is a partial cross-sectional view showing back-lapping the wafer;  
         [0039]    [0039]FIG. 10 is a cross-sectional view that illustrates forming a metal layer on the back surface of the wafer;  
         [0040]    [0040]FIG. 11 is a cross-sectional view that illustrates cutting the wafer into individual semiconductor chip;  
         [0041]    [0041]FIG. 12 is a cross-sectional view that illustrates attaching a semiconductor chip to a base substrate;  
         [0042]    [0042]FIG. 13 is a cross-sectional view that illustrates wire-bonding;  
         [0043]    [0043]FIG. 14 is a cross-sectional view that illustrates removing the photoresist;  
         [0044]    [0044]FIG. 15 is a cross-sectional view that illustrates hermetically sealing the package with a window lid; and  
         [0045]    [0045]FIG. 16 is a cross-sectional view that illustrates attaching a heat sink stud on the lower surface of the base substrate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]    Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.  
         [0047]    [0047]FIG. 4 is a cross-sectional view showing a DMD package  200  in accordance with an embodiment of the present invention. With reference to FIG. 4, a semiconductor chip  112  is attached to an upper surface  121  of a base substrate  120  with a metallic adhesive  130  having a low melting point, and a metallic layer  115  made of a metal having a low melting point is formed on the back surface of the semiconductor chip  112 . The base substrate is preferably a ceramic board, a plastic board, or a printed circuit board. Herein, the metallic layer  115  enables the metallic adhesive  130  to be firmly attached to the semiconductor chip  112 . Other components are the same as those of the conventional DMD package  100  of FIG. 1. Referring to FIGS. 5 through 16, a manufacturing process of the DMD packages in accordance with an embodiment of the present invention is described below.  
         [0048]    [0048]FIG. 5 is a flow chart  190  illustrating a manufacturing process of the DMD package  200  in FIG. 4. FIGS. 6 through 16 show each step of the manufacturing process of FIG. 5.  
         [0049]    As shown in FIGS. 6 through 8, the manufacturing process starts with preparing the wafer  110  (step  191 ). The silicon wafer  110  comprises a plurality of mirror-driving integrated circuits (not shown) formed by conventional techniques. A plurality of semiconductor chips  112  is formed on the wafer  110 . Scribe lines  118  are also formed between the neighboring semiconductor chips  112 , where the circuits are not formed.  
         [0050]    The photoresist  113  is formed on a predetermined portion of the upper surface  10   a  of the wafer  110 . The photoresist  113  prevents damage to the mirrors  116  from the external environment. The photoresist  113  is not formed on the electrode pads  114 .  
         [0051]    A metallic layer  115  is formed on the back surface  110   b  of the wafer (step  192 ). The metallic layer  115  enables the metallic adhesive to be firmly attached to the back surface  110   b  of the wafer  110 . As shown in FIG. 9, the back surface  110   b  is lapped with a lapping device  180 . Because the silicon oxide layer is naturally formed on the back surface of the wafer  110 , if the metallic layer is formed on the back surface of the wafer  110  without any treatment, adhesion between the back surface of the wafer  110  and the metallic layer  115  can be undesirably weak.  
         [0052]    For this reason, in this embodiment, the back surface  110   b  is lapped with the lapping device  180 . However, the back surface may be lapped by any suitable conventional etching techniques. As shown in FIG. 10, the metallic layer  115  is formed on the lapped back surface  110   b  of the wafer  110 . With respect to the adhesive means and the temperature in the chip attachment process, it is preferable to use a metal having a low melting point as the metallic layer  115 . For example, the metal can be Va (Vanadium), Au (Gold), Ni (Nickel), Ag (Silver), Cu (Copper), Al (Aluminum), Pb (Lead), Sn (Tin), Sb (Stibium), Pd (Palladium) and metal-containing compounds thereof. Of course, the present invention is not limited to such metals and compounds. Those of ordinary skill in the art should also be aware the other suitable metals or metallic compounds are well within the broad scope of the present invention.  
         [0053]    As shown in FIG. 11, the wafer  110  is separated into individual semiconductor chips  112  by the full-cutting method (step  193 ). A scribe blade  170  saws the wafer  110  along the scribe lines  118  and thereby separates the wafer  110  into individual semiconductor chips  112 . This wafer-sawing step is carried out with the wafer  110  having the adhesive tape (not shown) attached to the back surface  10   b  of the wafer  110 . Then, the wafer-washing step is performed.  
         [0054]    Since the mirrors  116  of the semiconductor chips  112  are coated with the photoresist  113 , damage to the mirrors  116  by contaminants such as silicon particles during the wafer sawing process can be prevented.  
         [0055]    Conventionally, a step of removing the photoresist normally follows the washing step. However, with the conventional method, a delamination problem of the semiconductor chip from the adhesive tape occurs. In order to prevent this problem, in accordance with the embodiment of the present invention, as shown in FIG. 12, a chip attachment step (step  194 ) is followed. Each of the semiconductor chips  112  is separated from the wafer ( 110  in FIG. 11), and attached to the upper surface  121  of the base substrate  120  by interposing an adhesive  130  having a low melting point such as solder therebetween. Herein, the adhesive  130  is solidified at room temperature, and therefore the curing step for the Ag-epoxy adhesive is omitted. Since a metallic layer  115  is formed on the back surface of the semiconductor chip  112 , the adhesive  130  is more firmly attached to the semiconductor chip  112 . The adhesive  130  can be provided in various forms such as a ribbon, paste, wire or any other suitable patterns.  
         [0056]    If the adhesive  130  is used, the die-attaching step is carried out at higher temperature than if the Ag-epoxy adhesive is used. For example, with the solder, the die attaching step is processed at a temperature of approximately 150° C. or more. However, since the mirrors  116  of the semiconductor chip are coated with the photoresist  113 , although the die-attaching step is carried out at a high temperature, the mirrors  116  of the semiconductor chips are not damaged.  
         [0057]    Although this embodiment uses the base substrate  120  having a flat upper surface, other base substrates having a dented upper surface may be used. For the base substrate, however, a ceramic substrate having low hygroscopicity and high thermal conductivity preferably is used, although other plastic substrates or a printed circuit board may be used.  
         [0058]    As shown in FIG. 13, the wire-bonding step is carried out (step  195 ). Herein, the ball-bonding method using an Au bonding wire or the wedge-bonding method using an Al bonding wire may be alternatively employed. FIG. 13 shows the wedge-bonding method between the electrode pads  114  of the semiconductor chip  112  and the base substrate  120 .  
         [0059]    As shown in FIG. 14, the photoresist ( 113  in FIG. 13) is removed (step  196 ), and an anti-sticking film is formed (step  197 ). The photoresist  113  is not removed until after the wire-bonding step. This prevents the contamination of the mirrors  116  due to dust or moisture. However, after the wire-bonding step, the photoresist  113  on the mirrors  116  is removed, because the mirrors  116  in the semiconductor chip  112  are protected from the outside when sealing the components including the semiconductor chip with the window lid. Then, the anti-sticking film for preventing the sticking of dust or moisture is formed.  
         [0060]    The photoresist  113  is removed from the semiconductor chip  112  attached to the base substrate  120 . The embodiment of the present invention discloses the manufacturing process of the DMD packages, on which a single semiconductor chip  112  is mounted on the base substrate  120 . However, it still falls within the spirit and scope of the present invention that a plurality of the semiconductor chips  112  are mounted on the base substrate  120  in rows, and multiple packages are simultaneously manufactured. In such case, the photoresist  113  formed on a plurality of the semiconductor chips  112  are collectively removed.  
         [0061]    As shown in FIG. 15, the components including the semiconductor chip  112  are hermetically sealed (step  198 ). In order to protect the semiconductor chip  112  on the base substrate  120  and the bonding wire  140  from the external environment, the semiconductor chip  112  and the bonding wire  140  are hermetically sealed. A window lid  150  is attached to a metal sealing ring  124  on the periphery of the base substrate  120  by thermo-compression, and thereby the cavity ( 129  in FIG. 4) containing the semiconductor chip  112  is hermetically sealed.  
         [0062]    The window lid  150  comprises a metal lid frame  152  in contact with the metal sealing ring  124 , and a window  154  perforating the metal lid frame  152  on the center. A reflectance coating film  156  is formed on the lower surface of the window  154  on its periphery, and a moisture getter  158  is attached to a lower surface of the metal lid frame  152 .  
         [0063]    In order to prevent the bonding wires  140  from contacting the lower surface of the window lid  150  attached to the metal sealing ring  124 , it is preferable that a distance between the upper surface of the base substrate  120  and the lower surface of the window lid  150  is greater than the height of the bonding wire.  
         [0064]    When the metal lid frame  152  is attached to the metal sealing ring  124  by thermo-compression, a portion of the metal lid frame  152  attached to the metal sealing ring  124  has a thickness less than the thickness of the other portion of the metal lid frame  152 . This allows the effective heat transfer from a thermo-compression means through the upper surface of the metal lid frame  152 . An adhesive means having a lower melting point than that of the above-described metal adhesive  130  is used between the metal sealing ring  124  and the metal lid frame  152 . This prevents the conventional deformation problem that results from re-melting the metal adhesive  130 .  
         [0065]    As shown in FIG. 16, the heat sink stud  160  is attached (step  199 ). In order to effectively draw heat away from heat-generating semiconductor chip  112 , the heat sink stud  160  is attached to the lower surface  123  of the base substrate below the semiconductor chip  112 . The manufacture of the improved DMD package  200  is complete.  
         [0066]    Accordingly, in the manufacturing process of the present invention, since the photoresist is not removed immediately after the separation of the wafer into individual semiconductor chips, but is removed after the wire-bonding step, the present invention simplifies the manufacturing process of the DMD packages as follows:  
         [0067]    First, since the wafer is sawed by the full-cutting method, the present invention thus reduces the number of steps required for individual semiconductor chip  112  singulation. Second, because the mirrors  116  of the semiconductor chip  112  are protected with the photoresist  113 , the present invention can omit the conventional step of forming the first anti-sticking film. The present invention also omits the conventional step of removing undesirable organic particulate or compounds after the wire-bonding step. During the step for removing the photoresist, the present invention also removes any the organic compounds remaining on the upper surface of the base substrate, the semiconductor chip and the bonding wire.  
         [0068]    In the present invention, the mirrors  116  of the semiconductor chip  112  are protected by the photoresist  113 . Therefore, instead of the Ag-epoxy adhesive, a metal having a low melting point such as a solder can be used in the chip-attaching step. Although the chip attaching step is carried out at high temperatures, the mirrors  116  formed with the photoresist thereon thus are prevented from high temperature damage (e.g. deformation) that may otherwise occur. Accordingly, the present invention solves the affixation and out-gassing problems described above involving a metal adhesive with a low melting point and an Ag-epoxy adhesive (relating to the hygroscopicity of the Ag-epoxy adhesive and the exhaust gas generated during curing of the Ag-epoxy).  
         [0069]    Further, because the photoresist-removing step is performed with the semiconductor chip being mounted on the base substrate  120 , it is very easy to handle the inverted DMD semiconductor chip  112 .  
         [0070]    Although preferred embodiments of the present invention have been described in detail hereinabove, it should be understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the art will still fall within the spirit and scope of the present invention as defined in the appended claims.