Abstract:
A thermally conductive adhesive tape and method for its use in packaging integrated circuits fabricated on semiconductor material. The thermally conductive adhesive tape includes a thermally conductive base upon which an adhesive layer is laminated or coated onto at least one side of the thermally conductive base. Thermal energy generated by operating the integrated circuit may be transferred from the integrated circuit via the thermally conductive adhesive tape to a medium to which the semiconductor material is attached. As a result, any excessive heat that may negatively affect the performance of the integrated circuit is dissipated through the medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of pending U.S. patent application Ser. No. 09/327,692, filed Jun. 8, 1999 now U.S. Pat. No. 6,359,334. 
    
    
     TECHNICAL FIELD 
     The present invention relates to semiconductor device packaging, and more particularly, to attaching a semiconductor die in a device package. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices are typically fabricated on thin wafers of silicon. Several die are produced on each wafer, with each die representing a single semiconductor device. Each die on a wafer is tested for gross functionality, and sorted according to whether the die passes or fails the gross functionality test. After being sorted according to gross functionality, the wafers are cut using a wafer saw, and the individual die are singulated. The die determined to be non-functional are scrapped. The functional die are packaged and further tested to ensure that each packaged device satisfies a minimum level of performance. Typically, the functional devices are permanently packaged by encapsulating the die in a plastic package. Packaging of the functional devices facilitates handling of the devices and also protects the die from damage during the manufacture of circuits using the packaged devices. 
     There are several conventional structures and methods for packaging singulated die. For example, more common package types include small outline j-bend (SOJ) packages, thin small outline packages (TSOP), and zigzag in-line packages (ZIP). The finished packaged devices are often mounted onto a substrate to form a module. A singulated die is packaged in the aforementioned package types by attaching the die to a lead frame paddle and electrically coupling exposed bond pads of the die to metal leads. The lead frame, die, and a portion of the metal leads are subsequently encapsulated by a plastic resin to protect the integrated circuit from damage. The encapsulated device is then trimmed from the lead frame and the metal leads formed to the correct shape. 
     An alternative lead frame structure, known as lead on chip (LOC) may be employed instead of the structure having a lead frame paddle. In an LOC structure, individual metal leads are typically attached to the surface of the die using double-sided adhesive tape having a polyimide base coated on both sides with adhesive material. The metal leads and die are then heated to melt the adhesive material. The bond pads of the semiconductor die are subsequently wire bonded to a respective metal lead to electrically connect the semiconductor die to receive electrical signals applied to the conductive leads. The LOC lead frame and die are then encapsulated in a plastic resin, then followed by a trim and form process. The LOC structure and packaging process are described in U.S. Pat. No. 4,862,245 to Pashby et al., issued Aug. 29, 1989, and U.S. Pat. No. 4,916,519 to Ward, issued Apr. 10, 1990, which are incorporated herein by reference. 
     Recently, semiconductor manufacturers have developed a package structure where unpackaged die are mounted directly onto a substrate, for example, a printed circuit board, thus allowing modules to be designed with increased device density. The devices are mounted onto the substrate and are electrically coupled by wire bonding the bond pads of the die to conductive traces formed on the surface of the substrate. The die are typically attached to the substrate by using strips of single or double-sided adhesive tape that are sandwiched between the substrate and the die. Following attachment, the substrate and die are heated to cure the adhesive in order to firmly fix the die. 
     As described above, many of the current methods of packaging semiconductor die involve attaching the die to a lead frame or a substrate using a single or double-sided adhesive tape. However, a problem with the conventional die attachment structures is that they do not facilitate the transmission or dissipation of heat generated by the device while in operation. The heat generated by an integrated circuit fabricated on the semiconductor die may adversely affect the performance of the device if not dissipated. The problems associated with the heat generated by the individual devices have been precipitated by the demand for high speed memory systems. 
     Consequently, the individual devices generate more heat than when operated at slower speeds. For example, in a RAMBUS memory architecture, or other high-speed memory application, the heat generated by the individual packaged devices may operate at temperatures as high as 100° C. In the case where the generated heat is not dissipated, the temperature may be great enough to cause memory system errors. Therefore, there is a need for a means to dissipate excessive heat generated by an integrated circuit in a packaged semiconductor device when being operated. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a thermally conductive adhesive tape and method for its use in packaging integrated circuits fabricated on semiconductor material. The thermally conductive adhesive tape includes a thermally conductive base upon which an adhesive layer is laminated or coated onto at least one side of the thermally conductive base. Thermal energy may be dissipated by transferring the heat through the thermally conductive adhesive tape from the integrated circuit to the medium to which the integrated circuit is attached. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded isometric view of an thermally conductive adhesive tape according to an embodiment of the present invention. 
     FIG. 2 is an exploded isometric view of a Board On Chip (BOC) type package using the thermally conductive adhesive tape of FIG.  1 . 
     FIG. 3 is an exploded isometric view of a Lead On Chip (LOC) type package using the thermally conductive adhesive tape of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A thermally conductive adhesive tape may be used to attach a semiconductor die in those package types requiring adhesive tapes. The heat generated by an integrated circuit fabricated on the semiconductor die during its operation will be transferred through the thermally conductive adhesive tape to a medium to which the semiconductor die is attached. Thus, the medium may act as a heat sink for the semiconductor die and dissipate any excessive heat generated by the integrated circuit. 
     Shown in FIG. 1 is a thermally conductive adhesive tape  10  according to an embodiment of the present invention. An adhesive layer  12  is laminated or coated to one side of a thermally conductive carrier layer  14 . Such adhesives for use in semiconductor device packaging are well known in the art and will not be discussed in detail herein in the interests of brevity. Some examples of well known adhesives are epoxy, NBR-phenol resin, and polyimide, to name a few. Another adhesive layer  16  is laminated or coated to the opposite side of the thermally conductive carrier  14 . However, the thermally conductive adhesive tape  10  may have an adhesive laminated or coated to only one side of the thermally conductive carrier  14 , and still remain within the scope of the present invention. The side without adhesive material would be attached to an object in a conventional manner, for example, using epoxy or other adhesive. Such methods of attachment are well known in the art and will not be discussed in detail herein for the sake of brevity. Whether the thermally conductive adhesive tape  10  is single-sided or double-sided depends on the particular application of the thermally conductive adhesive tape  10 . 
     As a person of ordinary skill in the art will appreciate, the material from which the thermally conductive carrier is formed will be also determined from the particular application of the thermally conductive adhesive tape  10 . Some typical examples of materials are platinum, aluminum, gold, copper, and silver. An advantage to using a copper foil as the thermally conductive carrier  14  is that copper foil is commercially available in the market in 5, 9, 12, and 18 μm thicknesses. Additionally, slitting technology for copper foil is already known, and working copper foil material, for example punching the copper foil, does not appear to be problematic. 
     The material from which the thermally conductive carrier  14  is formed also includes non-metallic materials that have sufficient thermal conductivity. For reasons of comparison, the thermal conductivity of platinum, aluminum, gold, copper, and silver at 400° K. are, 0.722, 2.40, 3.12, 3.92, and 4.20 Watts per centimeter-degree Kelvin (W/cm° K.), respectively. Non-metallic materials may have thermal conductivities greater or lower these values, but should have higher values of thermal conductivity than the conventional thermally resistant base, for example, a polyimide base, which has a thermal conductivity of approximately 0.002 W/cm° K. 
     FIG. 2 illustrates an example of an application of the adhesive material in a Board On Chip (“BOC”) type package  20 . A semiconductor die  38  is attached to a substrate  24  in a position represented by outline  28 . The substrate  24  may be a rigid organic substrate, such as BT resin, or FR-4 or FR-5 material, or a flexible substrate, such as polyimide. A person of ordinary skill in the art will appreciate that the substrate  24  may formed from materials other than those provided herein. Consequently, the type of material used for the substrate  24  does not limit the scope of the present invention. The substrate  24  has an opening  26  through the substrate which exposes a portion of the semiconductor die  38  when positioned according to the outline  28 . Strips of thermally conductive adhesive tape  10   a  and  10   b  are adhered to the surface of the substrate  24  adjacent to the opening  26 , as indicated by outlines  34   a  and  34   b,  respectively. The thermally conductive adhesive tape illustrated in FIG. 2 has adhesive material laminated or coated to both sides of the thermally conductive carrier. A singulated semiconductor integrated circuit, or die  38 , is positioned with respect to the substrate  24  as shown in FIG. 2, and attached to the substrate  24  by pressing the die  38  against the strips of thermally conductive adhesive tape  10   a  and  10   b.  The entire BOC package  20  is subsequently heated to melt the adhesive layers of the tape  10   a  and  10   b.    
     The surface of the die  38  contacting the strips of thermally conductive adhesive tape  10   a  and  10   b  typically has a protective layer of polyimide or SiON to prevent the device from being damaged during the die singulating process or the die attachment process. Consequently, the die  38  will not be damaged when attached to the substrate  24 . The die  38  has bond pads (not shown) formed essentially along its longitudinal axis that are exposed through the protective layer of polyimide or SiON. When the die  38  is mounted onto the substrate  24  in the position shown by the outline  28  the bond pads will remain exposed through the opening  26 . The die  38  is electrically connected to electrically conductive traces (not shown) formed on the surface of the substrate  24 , opposite of the one to which the die  38  is attached, by bond wires bonded to the bond pads of the die  38 . That is, the bond wires extend from the bond pads of the die  38 , through the opening  26 , and to the electrically conductive traces formed on the surface of the substrate  24 . The resulting structure is then substantially encapsulated to protect it from damage. Types of encapsulant materials are well known in the art, it will not be described in detail herein for the sale of brevity. 
     The BOC type structure illustrated in FIG. 2 is constructed in a manner such that the heat generated by the semiconductor die  38  during its operation is transmitted to the substrate  24  through the thermally conductive carrier of the thermally conductive adhesive tape  10   a  and  10   b.  Thus, the substrate  24  acts as a heat sink to draw heat away from the die  38  and dissipate the generated heat. In contrast, the conventional adhesive tape constructed from a heat resistant base cannot efficiently transmit the heat generated by the die  38  to the substrate  24 . Consequently, and as mentioned previously, the performance of the integrated circuit fabricated on the semiconductor die  38  may be adversely affected by the excessive heat when the device is used in high-speed memory applications. 
     Shown in FIG. 3 is another example where the thermally conductive adhesive tape  10  (FIG. 1) may be used for packaging integrated circuits. A packaged semiconductor device is formed using a Lead On Chip (“LOC”) type structure  50 . A lead frame  54  is formed having lead fingers  56   a  and  56   b  to which a die  58  is attached and electrically bonded through bond wires. Strips of thermally conductive adhesive tape  10   a  and  10   b  are adhered to the surface of the lead fingers  56   a  and  56   b,  respectively. Typically, the lead frame  54  comes formed in strips having several lead frames, and having the strips of thermally conductive adhesive tape  10   a  and  10   b  pre-attached to the lead fingers  56   a  and  56   b  in order to facilitate the die attach process. However, additional lead frames have been omitted from FIG. 3 to simplify the explanation of the LOC type structure  50 . 
     The die  58  is formed with bond pads  60   a-c  located substantially along the longitudinal axis of the die  58 . The die  58  is attached to the lead frame  54  by pressing the die  58  against the lead frame  54  and the strips of thermally conductive adhesive tape  10   a  and  10   b.  The lead frame  54 , the strips of thermally conductive adhesive tape  10   a  and  10   b,  and the die  58  are heated to cure the adhesive and firmly attach the die  58  to the lead frame  54 . The die  58  is subsequently electrically coupled to the respective lead fingers  56   a  and  56   b  through bond wires (not shown) bonded to the bond pads  60   a-c  and the respective lead fingers  56   a  and  56   b.  The LOC structure  50  will eventually be encapsulated in a plastic package to protect the die  58  from damage and to facilitate handling. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, the thermally conductive adhesive tape  10  has been shown with regards to the specific applications of a BOC and a LOC type package. However, the use of the thermally conductive adhesive tape  10  is not limited to only these applications, but includes any package type where an adhesive tape is required for die attachment. Accordingly, the invention is not limited except as by the appended claims.