Abstract:
A power semiconductor package has an ultra thin chip with front side molding to reduce substrate resistance; a lead frame unit with grooves located on both side leads provides precise positioning for connecting numerous bridge-shaped metal clips to the front side of the side leads. The bridge-shaped metal clips are provided with bridge structure and half or fully etched through holes for relieving superfluous solder during manufacturing process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Patent Application is a Divisional Application of a co-pending application Ser. No. 13/480,391 with attorney Docket# APOM076 and filed on May 24, 2012. Thus, this application claims the Priority Date of the co-pending application Ser. No. 13/480,391. Also, the Disclosures made in the co-pending application Ser. No. 13/480,391 are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a semiconductor power device and preparation method thereof. Particularly, this invention aims at providing a power device with a bottom source electrode and preparation method thereof. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    Power consumption of power devices is commonly very high. In the application of DC-DC power converter devices, some metal electrodes of the devices are usually exposed from plastic packaging material coating a semiconductor chip for improving the electrical connection and heat dissipation performance of the devices. For example, as shown in  FIG. 1 , US patent application publication U.S.2003/0132531A1 discloses a semiconductor packaging structure  24  with a bottom electrode of a semiconductor chip exposed and used for supporting surface mounting technology. Here, a power MOSFET  10  is arranged in an interior space of a cup-shaped metal can  12  and a drain electrode at one side of the MOSFET  10  is connected to the bottom of the interior space of the cup-shaped metal can  12  through a layer of conductive epoxy  14 , so that the drain electrode of the MOSFET  10  is electrically connected to an extruding edge  22  of the cup-shaped metal can  12 , while a source electrode  18  and a gate electrode (not shown) located at the other side of the MOSFET  10  become sub-flush with the surface of the extruding edge  22 . Low stress and high adhesion epoxy  16  is provided to fill in gaps in the interior space of the cup-shaped metal can  12  surrounding the MOSFET  10 . The semiconductor packaging structure  24  improves the heat dissipation performance. However, it is expensive to form the cup-shaped metal can  12  in actual production. In addition, both the source electrode and the gate electrode of the MOSFET  10  are fixed in the packaging structure  24 , as a result the contact surface of the gate electrode cannot be adjusted to level with the extruding edge  22 , thus it is hard to match the contact surface of the gate electrode with a pad on a PCB (Printed Circuit Board), which limits the application of the semiconductor packaging structure  24 . 
         [0004]    In addition, the resistance of a substrate in the chip of the power device is usually high, this makes the RDSon of the device correspondingly high; therefore, there is a need to reduce the resistance of the substrate of the chip. In a conventional wafer level chip scale packaging (WLCSP), packaging test is performed and ball placement on a wafer (for ball bonding) is carried out after the processing of all power devices in the whole wafer is completely finished, individual IC (Integrated Circuit) is then singulated with its size being same as the desired original chip. 
         [0005]    Given the above description of related prior arts, therefore, there is a need to manufacture ultra thin chips by WLCSP and to apply these chips in power devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The embodiment of the present invention is described more sufficiently through the drawings. However, the drawings are only used for explaining and illustrating rather than limiting the scope of the invention. 
           [0007]      FIG. 1  is a cross sectional schematic diagram of the semiconductor packaging structure of the prior art. 
           [0008]      FIGS. 2A-2E  are structural schematic diagrams of the power devices according to a first embodiment of the present invention. 
           [0009]      FIGS. 3A-3F  are schematic diagrams illustrating a process for preparing the primary packaging structure of the power devices of the present invention. 
           [0010]      FIGS. 4A-4C  are cross sectional schematic diagrams illustrating a process for preparing the power devices of the present invention. 
           [0011]      FIGS. 5A-5B  are cross sectional schematic diagrams illustrating the power devices according to a second embodiment of the present invention. 
           [0012]      FIGS. 6A-6D  are structural schematic diagrams illustrating the power devices according to a third embodiment of the present invention. 
           [0013]      FIGS. 7A-7C  are structural schematic diagrams illustrating the power devices according to a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]      FIG. 2A  and  FIG. 2B  are schematic diagrams showing a top view and a bottom view of the power device  100 A of a first embodiment of the present invention.  FIG. 2C  and  FIG. 2D-1  are cross sectional views of the power device  100 A along a plane AA and along a plane BB respectively shown in  FIG. 2B . The power device  100 A comprises a lead frame unit including a first base  111 , a second base  112 , a third base  113  and a fourth base  114  (as shown in  FIG. 2B ). The thicknesses of all the bases are basically the same and the bases are arranged on the same plane. The first base  111  preferably has a square shape. The third base  113  and the fourth base  114  are arranged at two opposite sides of the first base  111  respectively and extend along the edges of the first base  111 , and the second base  112  is positioned adjacent to the first base  111  between the third base  113  and the fourth base  114 . In one embodiment, the third base  113  and the fourth base  114  are symmetrical relative to the center of the first base  111 , thus the second base  112  is positioned on a line of symmetry of the third base  113  and the fourth base  114 . Usually, a lead frame strip comprises a plurality of such lead frame units with the bases connected to the lead frame strip through connecting bars (not shown). 
         [0015]    As shown in  FIG. 2C  and  FIG. 2D-1 , the power device  100 A also comprises a primary packaging structure  130  that is a completely packaged structure rather than an unpackaged original chip. The primary packaging structure  130  includes a semiconductor chip  131  that is flipped and attached onto the first base  111  and the second base  112 . Conductive epoxy (such as conductive solder or paste) is used for forming a plurality of balls or bumps  132   a - 1  and  132   b - 1  on the front surface of the primary packaging structure  130  for attaching it onto the second base  112  and the first base  111  respectively. As shown in  FIGS. 2C and 2D-1 , the primary packaging structure  130  comprises a semiconductor chip  131  and a top plastic packaging layer  134  covering the front surface of the chip  131 . The front surface of the chip  131  is provided with a plurality of metal pads. The solder bumps  132   a - 1  and  132   b - 1  are placed on the metal pads, which will be described in details later. The top plastic packaging layer  134  of the primary packaging structure  130  are only encapsulated on the side walls of the solder bumps  132   a - 1  and  132   b - 1 . A bottom metal layer  133  is formed at the back surface of the chip  131 . 
         [0016]    The power device  100 A also comprises a bridge-shaped metal clip  150  (also shown in  FIG. 2E ) attached to the bottom surface of the flipped primary packaging structure  130  or to the bottom surface of the bottom metal layer  133  of the flipped chip  131 . The bridge-shaped metal clip  150  is also attached to the third base  113  and the fourth base  114 . The bridge-shaped metal clip  150  comprises a top metal portion  151  and side metal portions  153   a  and  153   b  connected to two opposite sides of the top metal portion  151 . The side metal portions  153   a  and  153   b  are bent downwards as shown in  FIG. 2C . In particular, the side metal portions  153   a  and  153   b  are bent away from each other, so that the angles formed between the side metal portions  153   a  and  153   b  and the top metal portion  151  are obtuse angles. In one embodiment, the side metal portions  153   a  and  153   b  are symmetrically located relative to the center of the top metal portion  151 . In addition, a groove  113   a  is formed on the top surface of the third base  113  and a groove  114   a  is formed on the top surface of the fourth base  114 . As a result the side metal portion  153   a  can be attached and trapped in the groove  113   a,  likewise the side metal portion  153   b  can be attached and trapped in the groove  114   a.  A conductive material  140 , such as a conductive solder or paste, is applied to bond the bottom surface of the top metal portion  151  to the bottom metal layer  133 . The groove  113   a  and the groove  114   a  can be of many shapes, for example, the grooves  113   a  and  114   a  can be in V-shape, as shown in  FIG. 2C , for convenient engagement with the side metal portions  153   a  and  153   b.  The side metal portions  153   a  and  153   b  are respectively attached to the third base  113  and the fourth base  114  through conductive epoxy deposited in the groove  113   a  and the groove  113   b.    
         [0017]    Furthermore, the power device  100 A also comprises a plastic packaging body  160  encapsulating the lead frame unit, the primary packaging structure  130  and the bridge-shaped metal clip  150 . As shown in  FIG. 2B  where the power device  100 A is finally mounted on a PCB with the first base  111 , the second base  112 , the third base  113  and the fourth base  114  serving as electrical contacts directly connected to the pads on the PCB. Here, the respective bottom surfaces of the first base  111 , the second base  112 , the third base  113  and the fourth base  114  should be exposed from the bottom surface of the plastic packaging body  160 . Furthermore, as shown in  FIG. 2B , the third base  113  and the fourth base  114  usually include a plurality of pins, for example pins  113 ′ and pins  114 ′. Therefore, the bottom surfaces of the pins  113 ′ and the pins  114 ′ are also exposed from the bottom surface of the plastic packaging body  160  and serving as the electrical contacts of the third base  113  and the fourth base  114 . 
         [0018]    In the embodiment shown in  FIGS. 2C to 2D-1 , one or more through holes  152  are formed through the whole thickness of the top metal portion  151 .  FIG. 2E  shows a top view of the bridge-shaped metal clip  150  including a through hole  152 . The through hole  152  can be of a ‘cross’ shape as shown in  FIG. 2E  or can be of round, rectangle, polygon or any other suitable shapes. The through hole  152  is used for venting gas during the reflow of the conductive material  140  that attaches the top metal portion  151  to the bottom metal layer  133 . Additionally, any excess of the conductive material  140  deposited to form a conductive layer between the top metal portion  151  and the bottom metal layer  133  can be dredged into the through holes  152  so that the final layer thickness of the conductive material  140  is uniform. 
         [0019]    In another embodiment as shown in  FIG. 2D-2 , one or more clip grooves  152 ′ can be formed from a bottom surface of the top metal portion  151  with the bottom of the clip groove  152 ′ ended up inside the top metal portion  151 . The groove  152 ′ may be of many shapes similar to the through holes  152  as described above. Similar to the though hole  152 , the clip groove  152 ′ is used for venting gas during the reflow of the conductive material  140  for attaching the top portion  151  to the bottom metal layer  133  and for holding any excess of the conductive material  140  deposited to form a conductive layer between the top metal portion  151  and the bottom metal layer  133  thus improving the thickness uniformity of the conductive material  140 . 
         [0020]    Another difference between the embodiment of  FIG. 2D-1  and that of  FIG. 2D-2  is that the top surface of the top metal portion  151  as shown in  FIG. 2D-1  is not exposed from the plastic packaging body  160  whereas the top surface of the top metal sheet  151  as shown in  FIG. 2D-2  is exposed from the plastic packaging body  160 . To achieve a structure as shown in  FIG. 2D-2 , before depositing a plastic packaging material, such as epoxy resin, to form the plastic packaging body  160 , a resist film (not shown) can be applied to the inner surface of the top chase of the molding tool, which is then brought in contact with and covers the top surface of the top metal portion  151  thus preventing it from coverage by the plastic packaging materials. The plastic packaging body  160  encapsulates the lead frame unit, the primary packaging structure  130  and the side metal portions  153   a  and  153   b  of the bridge-shaped metal clip  150 . After the plastic packaging material is solidified, the resist film is peeled off from the top surface of the top metal portion  151 , thus the top surface of the top metal portion  151  is now exposed from the top surface of the plastic packaging body  160 . This plastic packaging process is usually completed at a wafer processing level (i.e., this technology is used in WLCSP), which is well known in the art. 
         [0021]    In another embodiment of the invention, recessed portions  154   a  and  154   b  formed on the top surface of the top metal portion  151  at the corner of the top metal portion  151  are configured to connect the side metal portions  153   a  and  153   b  thus forming a step structure.  FIGS. 2C-2E  illustrate the structure of the bridge-shaped metal clip  150 . Typically, the side metal portions  153   a  and  153   b  are originally formed on the same plane of the top metal portion  151 , then the side metal portions  153   a  and  153   b  are bent downward by an angle (through a stamping method), so that the angles formed between the side metal portions  153   a  and  153   b  and the top metal portion  151  are obtuse angles. However, the thus obtained final top metal portion  151  is not a flat plane and the edges of the top surface of the top metal portion  151  at the corner of the top metal portion  151  and the side metal portions  153   a    153   b  do not form a straight line. Therefore, the recessed portions  154   a  and  154   b  at the corner of the top metal portion  151  and the side metal portions  153   a    153   b  can beneficially buffer and stop the tension influences of the side metal portions  153   a  and  153   b  on the top metal portion  151  during the stamping step with the thus obtained top metal portion  151  free of deformation, in which case lines  151   a - 1  and  151   a - 2  at the two sides of the top surface of the top metal portion  151  are now straight lines and the top surface of the top metal portion  151  is now a flat rectangular plane. 
         [0022]      FIGS. 3A-3F  illustrate a method for preparing the primary packaging structure  130 . A wafer  1310  (shown in  FIG. 3C ) usually includes numerous semiconductor chips  131  (shown in  FIG. 3A ) formed at the top surface of the wafer and spaced-apart by scribe lines (not shown), which is well known in the art. The front surface of the chip  131  includes numerous metal pads  132 , such as aluminum-silicon pads, which serve as the electrodes of the chip or the terminals for off-chip signal transmission. In a preferred embodiment, the chip  131  is a vertical power metal oxide semiconductor field effect transistor (MOSFET). The metal pads  132  include metal pads  132   b  forming the first electrode (such as a source electrode) of the chip  131  and a metal pad  132   a  forming the second electrode (such as a gate electrode) of the chip  131 , while the drain electrode area of the chip  131  is formed at the back surface of the chip  131  (not shown). Firstly, numerous solder bumps are formed on the metal pads  132  by ball placement or plating and the likes. As shown in  FIG. 3B , a solder bump  132   a - 1  is formed on the metal pad  132   a  and a solder bumps  132   b - 1  are formed on the metal pads  132   b.  As the area of the metal pad  132   b  forming the source electrode is usually larger than that of the metal pad  132   a  forming the gate electrode, the size of the solder bumps  132   b - 1  is also larger than that of the solder bumps  132   a - 1  to carry large currents. Alternatively, numerous solder balls of smaller size than the solder bump  132   b - 1  can be placed on the metal pad  132   b  (not shown) and are closer to each other, so that the solder balls can be merged into one piece after being heated, softened and melted to form the solder bump  132   b - 1  of a larger size. As shown in  FIG. 3C , a plastic packaging layer  1340  is formed on the front surface of the wafer  1310  covering all the solder bumps  132   a - 1  and  132   b - 1 . Then the plastic packaging layer  1340  is ground until the solder bumps  132   a - 1  and  132   b - 1  are exposed through the plastic packaging layer  1340 . As shown in  FIG. 3D , the top surfaces of the solder bumps  132   a - 1  and  132   b - 1  and the top surface of the plastic packaging layer  1340  are co-planar. The plastic packaging layer  1340  physically supports the wafer  1310 . Therefore, when the wafer  1310  is ground and thinned, the wafer  1310  is not prone to crackage. This means that highly desirable ultra-thin chips with reduced substrate resistance can be made. As shown in  FIG. 3E , after the back surface of the wafer  1310  is ground and thinned, impurity ions can be heavily doped into the back surface of the thinned wafer  1310  (optionally), and then a metal layer  1330  can be deposited onto the back surface of the thinned wafer  1310  forming the drain electrode at the back surface of the chip. The wafer  1310 , the plastic packaging layer  1340  and the metal layer  1330  (as shown in  FIG. 3E ) are then cut apart to form individual primary packaging structures  130  (as shown in  FIG. 3F ), each of which includes a single chip  131  and a top plastic packaging layer  134  covering the front surface of the chip  131 . The top plastic packaging layer  134  only covers the side walls of the solder bumps  132   a - 1  and  132   b - 1  with the top surface of the solder bumps  132   a - 1  and  132   b - 1  exposed through the top plastic packaging layer  134  and is co-planar with the top surface of the top plastic packaging layer  134 . In this step, the metal layer  1330  is also cut apart into numerous bottom metal layers  133 , each of which covers the back surface of a chip  131  and is contact with the drain area at the back surface of the chip  131  forming the third electrode (such as the drain electrode) of the chip  131 . 
         [0023]    As shown in  FIG. 2D-1 , the solder bump  132   b - 1 , formed on the metal pad  132   b  forming the first electrode of the chip, is attached to the top surface of the first base  111 . As shown in  FIG. 2C , the solder bump  132   a - 1 , formed on the metal pad  132   a  forming the second electrode of the chip, is attached to the top surface of the second base  112 . As shown in  FIG. 2B , the surface area of the first base  111  forming the source electrode is usually larger than the surface area of the second base  112  forming the gate electrode. Therefore, the exposed area of the bottom surface of the first base  111  is larger than the exposed area of the bottom surface of the second base  112 , which also performs the function of heat dissipation. The third base  113  and the fourth base  114  are electrically connected to the drain electrode of the chip  131  through the bridge-shaped metal clip  150 . 
         [0024]      FIGS. 4A-4C  illustrate a method for preparing the power device  100 A shown in  FIG. 2D-1  along the line BB of  FIG. 2B . However, the preparation of the power device  100 A shown in  FIG. 2C  along the line AA of  FIG. 2B  is also described but not shown in  FIGS. 4A-4C . In  FIG. 4A , a lead frame unit is provided firstly. The lead frame unit includes the first base  111 , the second base  112 , the third base  113  and the fourth base  114 , all of which are separated from each other, with the third base  113  and the fourth base  114  respectively arranged at the two opposite sides of the first base  111  as described above. The primary packaging structure  130  is then attached on the first base  111  and the second base  112  of the lead frame unit by a conductive epoxy. In this step, the plurality of solder bumps  132   b - 1  and  132   a - 1  (see  FIG. 3F ) formed on the front surface of the primary packaging structure  130  are respectively attached to the first base  111  and the second base  112  by a conducting material, such as the conducting material  120   b  shown in  FIG. 4A . In  FIG. 4B , the bridge-shaped metal clip  150  is mounted atop the primary packaging structure  130 . The bridge-shaped metal clip  150  comprises the top metal portion  151  and the side metal portions  153   a  and  153   b  connected to two opposite sides of the top metal portion  151  and bent downwards. In this step, the top metal portion  151  is directly attached to the primary packaging structure  130 . The side metal portions  153   a  and  153   b  are respectively aligned and positioned in the groove  113   a  at the top surface of the third base  113  and the groove  114   a  at the top surface of the fourth base  114 . Conductive epoxy is deposited in the groove  113   a  and the groove  114   a  for attaching the side metal portions  153   a  and  153   b  of the bridge-shaped metal sheet  150  to the third base  113  and the fourth base  114  respectively. As such, the bridge-shaped metal clip  150  is precisely located in the groove  113   a  and the groove  114   a.  A bottom metal layer  133  at the back surface of the primary packaging structure  130  is connected to the bottom surface of the top metal portion  151  through the conductive material  140 . In  FIG. 4C , the plastic packaging material is deposited to form the plastic packaging body  160  encapsulating the lead frame unit, the primary packaging structure  130  and the bridge-shaped metal clip  150 . The bottom surfaces of the first base  111 , the second base  112 , the third base  113  and the fourth base  114  of the lead frame unit are exposed from the bottom surface of the plastic packaging body  160 , while the top surface of the top metal portion  151  can be selected whether to be exposed from the top surface of the plastic packaging body  160  or not. In  FIG. 4C , the top metal portion  151  is covered by the plastic packaging body  160  and the through hole  152  in the top metal portion  151  is filled with plastic packaging material. 
         [0025]      FIGS. 5A-5B  illustrate a structure of a power device  100 B according to another embodiment of the invention. The structure of power device  100 B is mostly similar as the structure of power device  100 A excepting the structure of the bridge-shaped metal clip  150 . As shown in these figures, the top metal portion  151  does not include a through hole. Instead it includes pluralities of dimples  155  formed on the bottom surface of the top metal portion  151 . 
         [0026]    The dimples  155  extrude from the bottom surface of the top metal portion  15  land are located between the bottom metal layer  133  and the bottom surface of the top metal portion  151  after the bridge-shaped metal clip  150  is mounted on the primary packaging structure  130 . With the dimples formed between the bottom metal layer  133  and the bottom surface of the top metal portion  151 , the thickness of the conducting material  140  is uniform. As shown in  FIG. 5B , the top surface of the top metal portion  151  of the power device  100 B is not exposed from the plastic packaging body  160 . Alternatively, the top surface of the top metal portion  151  can be exposed from the top surface of the plastic packaging body  160  (not shown). 
         [0027]      FIGS. 6A-6D  illustrate a power device  100 C of another embodiment of the invention with the structure and the position of a second base of the lead frame unit different from that in the power devices  100 A and  100 B.  FIGS. 6B and 6C  are cross sectional schematic diagrams along the dotted lines AA and BB in  FIG. 6A  respectively. As shown in  FIGS. 6A and 6B , the second base  212  includes a base extension  212   a  and an external pin  212   b  connected to the base extension  212   a.  The thickness of the base extension  212   a  is thinner than the thickness of the first base  111  and thus the base extension  212   a  is encapsulated inside the plastic packaging body  160 . Only the bottom surface of the external pin  212   b  is exposed from the bottom surface of the plastic packaging body  160 . 
         [0028]    As shown in  FIG. 6A , the length of the fourth base  214  is shorter than the length of the third base  113  and the external pin  212   b  is arranged on the same side as the fourth base  214 . Particularly the external pin  212   b  and a plurality of pins  214 ′ in the fourth base  214  are arranged on the same straight line. The base extension  212   a  extends under the primary packaging structure  130  until the solder bump  132   a - 1  on the front surface of the primary packaging structure  130  superimposed on the base extension  212   a.  As such, the conducting material  120   a  is deposited for attaching the solder bumps  132   a - 1  on the top surface of the base extension  212   a.  As shown in  FIGS. 6B-6C , the top surface of the base extension  212   a  and the top surface of the first base  111  are arranged on the same plane substantially, so that the primary packaging structure  130  is easily mounted on the first base  111  and the base extension  212   a  of the second base  212 . The thickness of the base extension  212   a  is thinner than the thickness of the first base  111  so that the extension base  212   a  is encapsulated inside the plastic packaging body  160  to avoid any negative effect on subsequent SMI technology. The external pin  212   b  and the fourth base  214  are arranged on the same straight line, therefore, to avoid a short circuit between the external pin  212   b  and the bridge-shaped metal clip  150 , as shown in  FIG. 6D , the bridge-shaped metal clip  150  includes a shorter side metal portion  153 ′b for connecting to the fourth base  214  without connecting to the external pin  212   b  of the second base  212 . Particularly, the width D 1  of the side metal portion  153 ′b is smaller than the width D 2  of the top metal portion  151 , while the width of the side metal sheet  153   a  is the same as the width D 2  of the top metal portion  151 . In the power device  100 C, the top surface of the top metal portion  151  is covered by the plastic packaging body  160 . 
         [0029]      FIGS. 7A-7C  illustrate a power device  100 D of another embodiment of the invention. The power device  100 D is similar to the power device  100 C, excepting that the top surface of the top metal portion  151  is exposed from the plastic packaging body  160 .  FIG. 7C  is a top view of the power device  100 D showing the top metal portion  151  is exposed from the plastic packaging body  160 , which is also used to improve the heat dissipation of the power device. 
         [0030]    The above detailed descriptions are provided to illustrate specific embodiments of the present invention and are not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. The present invention is defined by the appended claims.