Abstract:
The invention is directed to the application of viscous materials, such as the adhesives used in LOC die attach processes, to a lead frame by forming a film of viscous material and then bringing a portion of the lead frame and the film of viscous material into contact with one another. In one exemplary embodiment of the method of the invention, the viscous material is drop dispensed, sprayed, pumped or otherwise placed on a carrier surface, the material is spread to a uniform film thickness and then brought into contact with the die attach portion of the lead frame. One embodiment of the apparatus for applying the viscous material includes (1) a carrier surface, (2) a plurality of orifices in fluid communication with the carrier surface, and (3) a pump for pumping the viscous material through the orifices to the carrier surface. The apparatus may also include a metering blade for spreading the material to a uniform film thickness over the carrier surface.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to semiconductor assembly processes and equipment and, more particularly, to the application of adhesives and other viscous materials to components of a semiconductor device lead frame.  
         BACKGROUND OF THE INVENTION  
         [0002]    Higher performance, lower cost, increased miniaturization of semiconductor components, and greater packaging density of integrated circuits are goals of the computer industry. One way to reduce the overall cost of a semiconductor component is to reduce the manufacturing cost of that component. Lower manufacturing costs can be achieved through faster production and/or reduction in the amount of materials used in fabricating the semiconductor component.  
           [0003]    One area where faster production and reduction in material usage can be achieved is lead frame attachment to semiconductor dice. U.S. Pat. No. 5,286,679 issued Feb. 15, 1994 to Farnworth et al. (“the &#39;679 patent”), assigned to the assignee of the present invention and incorporated herein by reference, teaches attaching leads to a semiconductor device with adhesive material in a “lead-over-chip” (“LOC”) configuration. The &#39;679 patent teaches applying a patterned thermoplastic or thermoset adhesive layer to a semiconductor wafer. The adhesive layer is patterned to keep the “streets” on the semiconductor wafer clear of adhesive for saw cutting and to keep the wire bonding pads on the individual dice clear of adhesive for wire bonding. Patterning of the adhesive layer is generally accomplished by hot or cold screen/stencil printing or dispensing by roll-on. Following the printing and baking of the adhesive layer on the semiconductor wafer, the individual dice are cut from the semiconductor wafer. During packaging, each adhesive coated die is attached to lead fingers of a lead frame by heating the adhesive layer and pressing the lead fingers onto the adhesive. If the adhesive layer is formed of a thermoset material, a separate oven cure is required. Furthermore, the adhesive layer may be formulated to function as an additional passivating/insulating layer or alpha barrier for protecting the packaged die.  
           [0004]    Although the teaching of the &#39;679 patent is an effective method for attaching leads in a LOC configuration, it is sometimes difficult to achieve an adequate profile on the adhesive such that there is sufficient area on the top of the adhesive to attach the lead fingers. The process disclosed on the &#39;679 patent is illustrated in FIGS.  15 - 21 . FIG. 15 illustrates a cross sectional view of a semiconductor substrate  302  with a bond pad  304 , wherein a stencil or a screen print template  306  has been placed over the semiconductor substrate  302 , generally a silicon wafer. The stencil or screen print template  306  is patterned to clear the area around the bond pads  304  and to clear street areas  308  for saw cutting (i.e., for singulating the substrate into individual dice). An adhesive material  310  is applied to the stencil or screen print template  306 , as shown in FIG. 16. Ideally, when the stencil or screen print template  306  is removed, adhesive prints  312  are formed with vertical sidewalls  314  and a planar upper surface  316 , as shown in FIG. 17. However, since the adhesive material  310  must have sufficiently low viscosity to flow and fill the stencil or screen print template  306 , as well as allow for the removal of the stencil or screen print template  306  without the adhesive material  310  sticking thereto, the adhesive material  310  of the adhesive prints  312  may spread, sag, or flow laterally under the force of gravity after the removal of the stencil or screen print template  306 , as shown in FIG. 18. This post-application flow of adhesive material  310  can potentially cover all or a portion of the bond pads  304  or interfere with the singulating of the semiconductor wafer by flowing into the street areas  308 .  
           [0005]    Furthermore, and of even greater potential consequence than bond pad or street interference is the effect that the lateral flow or spread of adhesive material  310  has on the adhesive material upper surface  316 . As shown in FIG. 19, the adhesive material upper surface  316  is the contact area for lead fingers  318  of a lead frame  320 . The gravity-induced flow of the adhesive material  310  causes the once relatively well-defined edges  322  of the adhesive material to curve, resulting in a loss of surface area  324  (the ideal shape is shown with dotted lines) for the lead fingers  318  to attach. This loss of surface area  324  is particularly problematical for the adhesive print material upper surface  316  at the longitudinal ends  326  (seen in FIG. 20). At the adhesive material longitudinal ends  326 , the adhesive material flows in three directions (to both sides as well as longitudinally) causing a severe curvature  328 , as shown in FIGS. 20 and 21. The longitudinal ends of the adhesive print on patch flow in a 180° flow front resulting in blurring of the print boundaries into a curved perimeter. This curvature  328  results in complete or near complete loss of effective surface area on the adhesive material upper surface  316  for adhering the outermost lead finger closest to the adhesive material end  326  (lead finger  330 ). This results in what is known as a “dangling lead.” Since the lead finger  330  is not adequately attached to the adhesive material end  326 , the lead finger  330  may move or bounce when a wire bonding apparatus attempts to attach a bond wire between the lead finger  330  and its respective bond pad  304 . This movement can cause inadequate bonding or non-bonding between the bond wire and the lead finger  330 , resulting in the failure of the component due to a defective electrical connection.  
           [0006]    LOC attachment can also be achieved by attaching adhesive tape, preferably insulative, to an active surface of a semiconductor die, then attaching lead fingers to the insulative tape. As shown in FIG. 22, two strips of adhesive tape  410  and  410 ′ are attached to an active surface  412  of a semiconductor die  404 . The two adhesive tape strips  410 ,  410 ′ run parallel to and on opposing sides of a row of bond pads  406 . Lead fingers  402 ,  402 ′ are then attached to the two adhesive tape strips  410 ,  410 ′, respectively. The lead fingers  402 ,  402 ′ are then electrically attached to the bond pads  406  with bond wires  408 . Although this method is effective in attaching the lead fingers  402 ,  402 ′ to the semiconductor die  404 , this method is less cost effective than using adhesive since the cost of adhesive tape is higher than the cost of adhesive material. The higher cost of the adhesive tape is a result of the manufacturing and placement steps which are required with adhesive tapes. The individual tape segments are generally cut from a larger tape sheet. This cutting requires precision punches with extremely sharp and accurate edges. These precision punches are expensive and they wear out over time. Furthermore, there is always waste between the segments which are punched out, resulting in high scrap cost. Moreover, once punch out is complete, the tape segments are placed on a carrier film for transport to the die-attach site. Thus, there are problems with placement, alignment, and attachment with film carriers, plus the cost of the film carrier itself.  
           [0007]    LOC attachment can also be achieved by placing adhesive material on the lead fingers of the lead frame rather than on the semiconductor substrate. As shown in FIG. 23, the adhesive material  502  may be spray applied on an attachment surface  504  of lead fingers  506 . However, the viscous nature of the adhesive material  502  results in the adhesive material  502  flowing down the sides  508  of the lead finger  506  and collecting on the reverse, bond wire surface  510  of the lead finger  506 , as shown in FIG. 24. The adhesive material  502 , which collects and cures on the bond wire surface  510 , interferes with subsequent wire bonding, which in turn, can result in a failure of the semiconductor component. The flow of adhesive material  502  for the attachment surface  504  to the bond wire surface  510  can be exacerbated if the lead fingers  506  are formed by a stamping process rather than by etching, the other widely employed alternative. The stamping process leaves a slight curvature  512  to edges  514  of at least one surface of the lead finger  506 , as shown in FIG. 25. If an edge curvature  512  is proximate the lead finger attachment surface  504 , the edge curvature  512  results in less resistance (i.e., less surface tension) to the flow of the adhesive material  502 . This, of course, results in the potential for a greater amount of adhesive material  502  to flow to the bond wire surface  510 .  
           [0008]    Furthermore, present methods of adhesive material application on a surface (whether the semiconductor die or the lead fingers) tend to waste adhesive material. For example, spray application loses a great deal of adhesive material because not all of the sprayed adhesive material attaches to the target surface. As another example, the patterning of an adhesive layer on a semiconductor die, such as described in the &#39;679 patent, results in a substantial area of the adhesive pattern not being utilized to attach leads.  
         SUMMARY OF THE INVENTION  
         [0009]    The invention is directed to the application of viscous materials, such as the adhesives used in LOC die attach processes, to a lead frame by forming a film of viscous material and then bringing a portion of the lead frame and the film of viscous material into contact with one another. In one exemplary embodiment of the method of the invention, the viscous material is drop dispensed, sprayed, pumped or otherwise placed on a carrier surface, the material is spread to a uniform film thickness and then brought into contact with the die attach portion of the lead frame. One embodiment of the apparatus for applying the viscous material includes (1) a carrier surface, (2) a plurality of orifices in fluid communication with the carrier surface, and (3) a pump for pumping the viscous material through the orifices to the carrier surface. The apparatus may also include a metering blade for spreading the material to a uniform film thickness over the carrier surface.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0010]    While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which:  
         [0011]    [0011]FIG. 1 is a top plan view of a lead frame strip;  
         [0012]    [0012]FIGS. 2 and 3 are schematic representations of one embodiment of the invention in which the lead fingers are biased against the adhesive film;  
         [0013]    [0013]FIG. 4 is a schematic representation of another embodiment of the invention in which the adhesive film is biased against the lead fingers;  
         [0014]    [0014]FIG. 5 is an elevation view showing the application of adhesive to offset lead fingers;  
         [0015]    FIGS.  6 - 8  are elevation views showing the application of adhesive to planar lead fingers;  
         [0016]    [0016]FIG. 9 is a detail cross sectional view of a lead finger after application of adhesive;  
         [0017]    [0017]FIG. 10 is a cross sectional view taken along the line  10 - 10  in FIG. 9;  
         [0018]    [0018]FIGS. 11 and 12 are schematic representations of one system for applying a film of adhesive to a carrier plate;  
         [0019]    [0019]FIG. 13 is a schematic representation of another system for applying a film of adhesive to a carrier plate;  
         [0020]    [0020]FIG. 14 is a schematic representation of a height detection and control loop for maintaining the height of the film of adhesive on the carrier plate;  
         [0021]    FIGS.  15 - 21  are side cross-sectional views of a prior art technique of forming adhesive areas on a substrate for LOC attachment;  
         [0022]    [0022]FIG. 22 is a top view of a prior art technique of LOC attachment using adhesive tape; and  
         [0023]    FIGS.  23 - 25  are cross sectional views of a prior art technique of forming adhesive areas on lead fingers for LOC attachment.  
         [0024]    [0024]FIG. 26 is an isometric exploded view illustrating an LOC die attachment.  
     
    
       [0025]    The figures are not actual views of semiconductor devices, components or processing equipment. Rather, the figures are idealized representations used to better illustrate different embodiments of invention in conjunction with the following detailed description.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    [0026]FIG. 1 illustrates a portion of an exemplary lead frame strip  100 . Individual lead frames  102 , each including a plurality of lead fingers  104 , are formed in a long, thin strip of conductive material  106 , such as copper, copper alloy, or the like. The lead frames  102  are generally formed by a stamping process or an etching process. The lead frames  102  are formed side-by-side along the conductive material strip  106  wherein the conductive material strip  106  includes a plurality of indexing holes  107 ,  107 ′ on opposing lengthwise edges  109 ,  109 ′, respectively, of the conductive material strip  106 . The indexing holes  107 ,  107 ′ are used to move the lead frame strip  100  and align the lead frames  102  throughout a process of attaching the lead frames  102  to semiconductor dice.  
         [0027]    Referring to FIGS. 2 and 3, a lead frame strip  100  such as illustrated in FIG. 1 is fed from a source  108 , such as a spool, to an adhesive carrier plate  110 . As the lead fingers (not shown) are aligned over carrier plate  110 , lead frame strip  100  is biased downward as shown in FIG. 3, such as by hydraulic, pneumatic, or electrically-powered biasing mechanism  116 , to contact a film of adhesive material  114  on the surface  112  of carrier plate  110 . The adhesive material may be any viscous adhesive material including but riot limited to thermoplastics, thermoset resins, flowable pastes, and B-stage adhesive materials. Preferred adhesive materials include polyimide, epoxy, bismaleimide and cyanate ester.  
         [0028]    [0028]FIG. 4 illustrates another embodiment, similar to that shown in FIGS. 2 and 3, except that in the embodiment of FIG. 4 the biasing mechanism is an elevator  117  that moves carrier plate  110  up to contact the lead fingers rather than moving the lead frame strip down to contact the carrier plate.  
         [0029]    As an alternative to the biasing mechanisms  116  and  117  shown in FIGS.  2 - 4 , or in combination with those biasing mechanism, lead fingers  104  may be offset as shown in FIG. 5 so that only the ends  124  of lead fingers  104  contact the film of adhesive material  114  as the lead frame strip  100  moves over carrier plate  110 . Using the offset lead fingers shown in FIG. 5 helps ensure that adhesive is applied to only those portions of the lead fingers that attach to the semiconductor dice. As another alterative, planar or offset lead fingers  104  may be brought into close proximity to adhesive film  114  and then additional adhesive material may be pumped or otherwise delivered to the surface  112  of carrier plate  110  to raise the level of adhesive film  114  to contact lead fingers  104 . A system for pumping adhesive to surface  112  is shown in FIG. 13.  
         [0030]    FIGS.  6 - 8  illustrate side views of planar lead fingers  104  being brought into contact with the adhesive material  114  and being retracted therefrom. As shown in FIG. 6, lead fingers  104  are positioned over carrier plate  110 . The lead fingers  104  are lowered onto or proximate the film of the adhesive material  114  as shown in FIG. 7. When the bottom surface  124  of the lead fingers  104  comes in contact with adhesive film  114 , the adhesive material wets out across the bottom surface  124  of the lead finger  104 . As shown in FIG. 8, when lead fingers  104  are retracted from the adhesive material, the cohesion of the adhesive material with the lead fingers  104  pulls some of the adhesive material from film  114  to form an adhesive film  126  on the bottom surface  124  of the lead finger  104 . The thickness of the adhesive film  126  can range from 0.1 to 15 millimeters depending on the viscosity of the adhesive material. Changing the shape of lead finger  104 , changing the rheology of the adhesive material, pre-coating lead finger  104  with a surfactant, such as NMP, or placing a solvent in the adhesive material to improve wetting, and/or adding adhesion promoters, such as silane, siloxane, or polyimide siloxane, to the adhesive material will also change the thickness and/or pattern of adhesive film  126  on lead finger  104 . It is desirable that the adhesive material flow easily to form a uniform film  114  across carrier plate  110  and adhere to lead fingers  104 , but not run off lead fingers  104  as they are removed from contact with adhesive film  114 . Carrier plate  110  and adhesive film  114  can be shaped and positioned such that when lead fingers  104  contact film  114  the exposed surface  122  of the adhesive material  114 , the adhesive material  114  attaches to only specific, desired portions of the lead fingers  104 .  
         [0031]    [0031]FIG. 9 is a cross sectional view of a lead finger  104  after application of the adhesive material. FIG. 10 is a cross sectional view of lead finger  104  taken along the line  10 - 10  in FIG. 9. As shown in FIGS. 9 and 10, by only contacting bottom surface  124  of lead finger  104  to adhesive film  114 , the adhesive material will not wet sides  128  of the lead finger  104  and, of course, will not collect on bond wire surface  130  of lead finger  104  (the bond wire surface  130  is the lead finger surface where a bond wire is subsequently attached during further processing). Since the adhesive material does not collect on bond wire surface  130 , there will be no adhesive material to interfere with a wire bonding step subsequent to LOC attachment of lead fingers  104  to an active surface of a semiconductor die.  
         [0032]    It is desireable that adhesive film  114  be as level as possible. If film  114  is not substantially level, lead fingers  104  may dip too deeply into the adhesive material  114 . When this occurs, the adhesive material  114  may wet the lead finger sides  128  and may even wet the lead finger bond wire surface  130 . If the adhesive material wets the bond wire surface  130 , the adhesive material may interfere with wire bonding subsequent to LOC attachment, as mentioned above.  
         [0033]    A variety of techniques may used to form adhesive film  114  on carrier plate  110 . In FIGS. 11 and 12, adhesive material is sprayed onto carrier plate  110  through a centrally located dispensing nozzle  150 . The adhesive is then spread evenly into film  114  using a metering blade  138 . Alternatively, the adhesive material may be pumped up through an array of orifices  152  on to surface  112  of carrier plate  110 , as shown in FIG. 13. The adhesive material might also be machine or hand troweled on to carrier plate  110 . Metering blade  138  can be used to maintain the desired thickness and uniformity of adhesive film  114 .  
         [0034]    Furthermore, a variety of feed back and feed forward control schemes may be used to control the thickness and uniformity of film  114 . One such control scheme is shown in FIG. 14. A detection mechanism, shown as a transmitter  140  and a receiver  142 , is used to determine the thickness and/or the uniformity of the surface of film  114 . A control signal  144  triggers pump  132  to pump or stop pumping to achieve the desired thickness. The control signal can also be used to trigger metering blade  138  to move across film  114  to maintain thickness and uniformity. The transmitter  140  and receiver  142  may be a light (preferably a laser) transmitter and receiver. When a light beam (not shown) from the transmitter  140  is altered by the exposed surface  122 , the receiver  142  detects the discontinuation of light transmission and generates the control signal  144 . Additionally, the transmitter  140  and receiver  142  may be an ultrasonic transmitter and receiver. When an ultrasonic sound wave (not shown) from the transmitter  140  is altered by the exposed surface  122 , the receiver  142  detects the change in transit time or phase shifts of the ultrasonic sound wave and generates the control signal  144 .  
         [0035]    It is desireable, of course, to precisely control the relative positions of lead frames  102  and adhesive film  114  to better control the depth to which lead fingers  104  are dipped into film  114 .  
         [0036]    The invention contemplates the use of a single carrier plate or a series or array carrier plates as necessary or desireable to accommodate the LOC die attach process. Once the adhesive material has been applied to lead fingers  104 , lead frame strip  100  may, optionally, be fed to a curing oven  118 , shown in FIGS.  2 - 4 , to set the adhesive material. Preferably, the adhesive material is cured with the lead frame in a downward facing position. The advantages of placing viscous material, such as an adhesive material, in a downward facing position are described in U.S. patent application Ser. No. 08/709,182 by Tongbi Jiang and Syed S. Ahmad filed Sep. 6, 1996, assigned to the assignee of the present invention and hereby incorporated herein by reference. Rather than gravitational forces causing the viscous material to flow and expand as when on top of the lead frame, the gravitational forces on the inverted lead frame maintain the shape and boundary definition of the viscous material. The adhesive should be compatible with the lead finger material so as to adhere thereto and must not be of such a low viscosity that it drips when the lead fingers are removed from contact with the adhesive film. Preferably, the viscous materials have viscosities between about 1000 cps and 500,000 cps. Furthermore, with regard to drying or curing an adhesive material, the lead frame need only be inverted until the viscous adhesive material has stabilized sufficiently to maintain its shape and boundary definition. Depending on the particular viscous adhesive material used, the minimum inversion time could be the time required to cure the outer surfaces of the viscous adhesive material such that a film is formed which contains the as yet uncured viscous adhesive material therein, or the minimum inversion time could be the time required to completely dry or cure of the viscous adhesive material.  
         [0037]    Referring to FIG. 26, a semiconductor die  154  is attached to lead frame  102 . In a typical LOC die attach process, adhesive film  126  on lead fingers  104  is heated by heating lead frame  102  and then the lead fingers  104 , adhesive film  126  and die  154  are pressed together. The adhesive film  126  on lead fingers  104  may also be heated by pressing the adhesive coated lead fingers in to a heated die  154 . Once die  154  is attached to lead frame  102 , the assembly is encapsulated according to conventional semiconductor packaging process steps.  
         [0038]    The present invention is not limited to the above detailed description. The structures coated are not limited to lead frames and can include TAB structures and the like. The lead frames may not be limited to delivery in strips, but can be delivered individually or in sheets. The adhesive material can be applied to a structure under at least a partially-evacuated chamber such that lower-viscosity materials applied to the structure would be at least partially dried or cured prior to removal from the chamber. Further, the invention is not limited to only adhesives applied to lead fingers. Other viscous materials could be applied using the invented method and apparatus. It is also contemplated that viscous materials may be applied to bus bars, die attach paddles, or other components of a lead frame. Moreover, the carrier plate may embodied in different designs. One example of an alternative design is illustrated in U.S. patent application attorney docket no. 3089US(96-0734) filed Aug. 5, 1997 by Moden et al. and assigned to the assignee of the present invention. FIG. 25 and the accompanying text of the Moden et al. patent application, which is incorporated herein by reference, shows a plate-type reservoir (150) in which a thin layer of viscous material (152) is delivered across plate (154) from an inlet (156) to an opposing outlet (158).  
         [0039]    Having thus described in detail the preferred embodiments of the invention, it is to be understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description as variations thereof are possible without departing from the spirit or scope thereof.