Abstract:
A compact multi-chip module is provided. The multi-chip module may be built from a lead frame that does not have a die attach pad. Instead, the leads of the lead frame may define a central opening. A first semiconductor device extends across the central opening and is connected to the plurality of leads, with the leads being on a first side of the first semiconductor device. A second semiconductor is stacked on the first side of the first semiconductor device. A third semiconductor device is stacked on the second semiconductor device. A fourth semiconductor device is stacked on the third semiconductor device. The stack of semiconductor devices passes through the central opening formed by the plurality of leads. The stack of semiconductor devices is encapsulated with a thermoset plastic. The resulting final assembly may occupy no more volume than a typical single chip component.

Description:
The United States government has rights in this invention pursuant to Contract No. MDA904-98C-2159 between the National Security Agency and National Semiconductor Corporation. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor packaging. More specifically, the present invention relates to the packaging of semiconductors using multiple layers of dice. 
     BACKGROUND OF THE INVENTION 
     To facilitate discussion, FIG. 1 is a cross-sectional view of a molded integrated circuit (IC) package that may be provided by the prior art. In such an IC package  100 , an integrated circuit die  104  is mounted on a die attach pad  108 . Leads  112  of a lead frame extend to a location close to the IC die  104 . Wires  116  extend from die bond pads on the IC die  104  to the leads  112  to provide electrical connections between the IC die  104  and the leads. A thermoset plastic casing  120  encases and surrounds the IC die  104 , wires  116 , and parts of the leads  112  closest to the IC die  104 , leaving exposed parts of the leads  112  furthest from the IC die  104 . 
     FIG. 2 is a planar view of the lead frame  122  with leads  112  and the die attach pad  108  shown in FIG.  1 . The lead frame  122  further comprises a skirt  126  surrounding the periphery of the lead frame  122  and tie bars  130  that connect the skirt  126  to the die attach pad  108 . The skirt  126  may be used to support the leads  112 , which are attached to the skirt  126  and the die attach pad. During the manufacturing of the IC package, the skirt  126  is removed. 
     Such IC packages may require a footprint that is much greater than the footprint of the IC die. In view of the foregoing, it is desirable to provide an IC package that has a small footprint. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing and other objects and in accordance with the purpose of the present invention, a variety of techniques for providing a multi-chip module is described. Generally, a first side of a first semiconductor device is connected to a first side of the plurality of leads. A second semiconductor device is then connected to the first side of the first semiconductor. 
     These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
     FIG. 1 is a cross-sectional view of a molded integrated circuit used in the prior art. 
     FIG. 2 is a planar view of a lead frame used in the prior art. 
     FIG. 3 is a cross-sectional view of a multi-chip module. 
     FIG. 4 is a top view of the multi-chip module during the manufacturing process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well-known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention. 
     To facilitate discussion, FIG. 3 is a cross-sectional view of a preferred embodiment of the invention. A multi-chip module  300  comprises a plurality of leads  304  with a first semiconductor device  308  connected to the plurality of leads  304  with a first side and second side. The first semiconductor device  308 , which is a die, has a first side and a second side. The first side of the plurality of leads  304  is connected to the first side of the first semiconductor device  308 , so that the leads  304  are on the first side of the semiconductor device  308 . A first plurality of gold balls  312  on the first side of the first semiconductor device  308  is used to both physically and electrically connect the first side of the first semiconductor device  308  to first side of the plurality of leads  304 . Gold-ball thermocompression bonding is used to connect the first side of the first semiconductor device  308  to the plurality of leads  304 . The gold balls may be formed by a machine that forms gold wire bond balls. 
     A second semiconductor device  316 , with a first side and second side, is connected to and located on the first side of the first semiconductor device  308 . A second plurality of gold balls  320  is used to both physically and electrically connect a first side of the second semiconductor device  316  to the first side of the first semiconductor device  308 . Gold-ball thermocompression bonding is used to connect the first side of the second semiconductor device  316  to the first side of the first semiconductor device. Since the second semiconductor device  316  is stacked on the first side of the first semiconductor device, as shown in FIG. 3, the second semiconductor device  316  is placed between the plurality of leads, as shown in FIG.  3 . The first side of the second semiconductor device  316  is placed adjacent to the first side of the first semiconductor device  308 , separated from the first side of the first semiconductor device by the second plurality of gold balls  320 . 
     A third semiconductor device  324  is connected to a second side of the second semiconductor device  316 . An adhesive bond  328  is used to physically connect a first side of the third semiconductor device  324  to the second side of the second semiconductor device  316 , so that the first side of the third semiconductor  324  is adjacent to the second side of the second semiconductor  316 . Various known adhesives may be used for connecting the first side of the third semiconductor  324  to the second side of the second semiconductor  316 . A first side of a fourth semiconductor device  332  is connected to a second side of the third semiconductor device  324 . A third plurality of gold balls  336  is used to both physically and electrically connect a first side of the fourth semiconductor device  332  to the second side of the third semiconductor device  324 , so that the first side of the fourth semiconductor device  332  is adjacent to the second side of the third semiconductor device  324 . Gold-ball thermocompression bonding is used to connect the first side of the fourth semiconductor device  332  to the second side of the third semiconductor device  324 . 
     In the illustrated embodiment, conventional wire bonding  340  is used to electrically couple leads to associated contacts on the second side of the third semiconductor device  324 . Such wire connections may be placed between leads and any of the semiconductor devices or between semiconductor devices. In the preferred embodiment, wire connections are between leads and semiconductor devices that are not directly attached to the leads and between semiconductor devices that are not adjacent to each other. The wire connections  340  between the leads  304  and the third semiconductor device  324  may be on a second side of the leads  304  opposite from the first side of the leads  304 . The reason for connecting the wire connections  340  to the second side of the leads  304  is that since the stack of semiconductor devices  308 ,  316 ,  324 ,  332  passes between the center of the leads  304 , the third semiconductor device  324  is closest to the second side of the leads  304 . The first, second, third, and fourth semiconductor devices  308 ,  316 ,  324 ,  332 , the plurality of gold balls, and ends of the leads  304  adjacent to the semiconductor devices  308 ,  316 ,  324 ,  332  are encapsulated with a thermoset plastic casing through an operation called transfer molding. 
     In the manufacture of a multi-chip module, as illustrated in FIG. 3, a lead frame is provided. FIG. 4 is a top view of a lead frame  400 , which may be used in the production of the multi-chip module  300 . The lead frame  400  comprises the plurality of leads  304  and a skirt  404 . In the preferred embodiment of the invention, the lead frame  400  does not have a die attach pad and tie bars. The absence of a die attach pad creates a central opening  406  at the center of the lead frame  400  and between the plurality of leads  304 . The first semiconductor device  308  is placed across the central opening  406  and electrically and physically connected to the plurality of leads  304  by using gold-ball thermocompression bonding. As described above, the plurality of leads  304  is on the first side of the first semiconductor device  308 . The second semiconductor device  316  is connected to the first side of the first semiconductor device  308  using gold-ball thermocompression bonding in conventional flip-chip processes. As shown, the second semiconductor device  316  may be smaller than the first semiconductor device  308  to allow the second semiconductor device  316  to fit within the plurality of leads  304 . The third semiconductor device  324  is connected to the second semiconductor device  316  using conventional adhesive bond attachment processes. The fourth semiconductor device  332  is connected to the third semiconductor device  324  using gold-ball thermocompression bonding in conventional flip-chip processes. The first, second, third, and fourth semiconductor devices  308 ,  316 ,  324 ,  332  form a stack that passes through the center of the central opening between the plurality of leads  304 , wherein the first semiconductor device  308  extends across the central opening  406  and the second semiconductor device is within the central opening  406 , as shown. The plurality of wire bonding connections  340  is connected between leads and semiconductor devices, as shown. Facing the top, the second side of the leads  304 , the first side of the first semiconductor device  308 , the second side of the third semiconductor device  324 , and a second side of the fourth semiconductor device  332  may be seen in FIG.  4 . The first, second, third, and fourth semiconductor devices  308 ,  316 ,  324 ,  332  and parts of the plurality of leads closest to the first semiconductor device  308  are encapsulated with the thermoset plastic casing. The skirt  404  is then removed. 
     In the illustrated embodiment, the first, second, third, and fourth semiconductor devices  308 ,  316 ,  324 ,  332  have electrical connections only on one side. In such a situation the side with the electrical connections is designated as the front side and the side opposite the front side is designated as the back side. In the illustrated embodiment, the first side of the first semiconductor device  308 , the first side of the second semiconductor device  316 , the second side of the third semiconductor device  324 , and the first side of the fourth semiconductor device are the front sides. The second side of the second semiconductor device  316  and the first side of the third semiconductor device  324  are back sides. Since the second side of the second semiconductor device  316  and the first side of the third semiconductor device  324  are back sides, wire connections are used to provide an electrical connection to the third semiconductor device  324 . 
     Another preferred embodiment of the invention provides the same device as in the previous embodiment, except that the first side of the first semiconductor device is bonded to the plurality of leads by an anisotropic conductive adhesive. Anisotropic conductive adhesives are disclosed in U.S. Pat. No. 5,840,215 entitled “Anisotropic Conductive Adhesive Compositions” by Iyer et al., incorporated herein by reference. Anisotropic conductive adhesive compositions would provide conductive electrical paths, forming a plurality of conductive elements between contact pads on the first side of the first semiconductor device and a lead, while providing insulation in a transverse direction between leads and between contact pads. 
     In other embodiments of the invention, the gold balls may be replaced with other conductive elements such as conductive columns or pins, conductive adhesives, anisotropic conductive adhesives, or conductive balls. In other embodiments more semiconductor devices may be stacked together. Preferably, at least two semiconductor devices are stacked to form the multi-chip module. More preferably, at least three semiconductor devices are stacked to form the multi-chip module. Most preferably, at least four semiconductor devices are stacked to form the multi-chip module. In other embodiments, adhesive bonding may be substituted by conductive element arrays. In other embodiments, the third semiconductor device or fourth semiconductor device may be placed in the central opening instead of the second semiconductor device. In such cases, the stack of semiconductor devices extends through the central opening in the plurality of leads. Preferably, when the stack of semiconductor devices extends through the central opening, the first semiconductor device is not within the central opening of the plurality of leads, since the first semiconductor device extends across the central opening and is on the first side of the plurality of leads. Some or all of the semiconductor devices may be made thinner than standard semiconductor devices to meet a molded package thickness limit. Some or all of the semiconductor devices (dice or chips) may have active electrical connections on two sides. In other embodiments of the invention, the central opening may be closer to one side of the frame than another, so that when the semiconductor stack passes through the center of the central opening, the semiconductor stack may be closer to one side of the lead frame than another. 
     In another embodiment, the second semiconductor device has electrical connections on two opposite sides. One way of providing this is by providing a plurality of vias of electrically conducting material passing through the semiconductor device. In such an embodiment, the third semiconductor device may be connected to the second semiconductor device using gold-ball thermocompression or flip-chip processes. The third semiconductor device may also have electrical connections on two opposite sides, so that the fourth semiconductor device may be mounted on the third semiconductor device as a flip chip. In the alternative, the fourth semiconductor device may be held to the third semiconductor device using an adhesive bond, with wire connections providing electrical connections to the fourth semiconductor device. Another embodiment may provide a die attachment pad. 
     While this invention has been described in terms of several preferred embodiments, there are alterations, modifications, permutations, and substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.